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Energies, Volume 10, Issue 6 (June 2017)

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Editorial

Jump to: Research, Review, Other

Open AccessEditorial Recent Advances in Energy Time Series Forecasting
Energies 2017, 10(6), 809; doi:10.3390/en10060809
Received: 8 June 2017 / Revised: 12 June 2017 / Accepted: 12 June 2017 / Published: 14 June 2017
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Abstract
This editorial summarizes the performance of the special issue entitled Energy Time Series Forecasting, which was published in MDPI’s Energies journal. The special issue took place in 2016 and accepted a total of 21 papers from twelve different countries. Electrical, solar, or wind
[...] Read more.
This editorial summarizes the performance of the special issue entitled Energy Time Series Forecasting, which was published in MDPI’s Energies journal. The special issue took place in 2016 and accepted a total of 21 papers from twelve different countries. Electrical, solar, or wind energy forecasting were the most analyzed topics, introducing brand new methods with very sound results. Full article
(This article belongs to the Special Issue Energy Time Series Forecasting)
Open AccessEditorial Editorial Special Issue “Combustion and Propulsion”
Energies 2017, 10(6), 824; doi:10.3390/en10060824
Received: 17 April 2017 / Revised: 16 June 2017 / Accepted: 16 June 2017 / Published: 18 June 2017
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Abstract The increasing demand for socially and environmentally sustainable development requires appropriate use of energy resources, particularly in the transportation of people and goods.[...] Full article
(This article belongs to the Special Issue Combustion and Propulsion)

Research

Jump to: Editorial, Review, Other

Open AccessArticle The Influence of Small-Scale Power Plant Supporting Schemes on the Public Trader and Consumers
Energies 2017, 10(6), 800; doi:10.3390/en10060800
Received: 13 February 2017 / Revised: 2 June 2017 / Accepted: 6 June 2017 / Published: 12 June 2017
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Abstract
The mechanism of support schemes for achieving the required share of renewable energy sources (RES) was implemented into the energy sector. The issued amount of support requires state subsidies. The end-users of electricity are paying the mandatory procurement component taxes to cover these
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The mechanism of support schemes for achieving the required share of renewable energy sources (RES) was implemented into the energy sector. The issued amount of support requires state subsidies. The end-users of electricity are paying the mandatory procurement component taxes to cover these subsidies. The article examines the way of minimizing the influence of the existing RES supporting schemes on the consumers. The fixed purchased electricity price in the case of RES does not encourage producers to operate at hours of peak consumption or when the price is high. Modification of the RES support mechanisms at the legislative level, firstly, could minimize the influence of the mandatory procurement component on the end-users’ electricity price, and secondly, could provide a great opportunity for the public trader to forecast the operation of small power plants and their generation abilities. Numerical experiments with models of two types of power plants (biofuel and hydropower) prove the existence of a problem and the presence of a solution. This problem constitutes the main subject of the present paper. Full article
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Open AccessFeature PaperArticle Thermal Response Testing Results of Different Types of Borehole Heat Exchangers: An Analysis and Comparison of Interpretation Methods
Energies 2017, 10(6), 801; doi:10.3390/en10060801
Received: 13 April 2017 / Revised: 7 June 2017 / Accepted: 10 June 2017 / Published: 13 June 2017
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Abstract
The design phase of ground source heat pump systems is an extremely important one as many of the decisions made at that time can affect the system’s energy performance as well as installation and operating costs. The current study examined the interpretation of
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The design phase of ground source heat pump systems is an extremely important one as many of the decisions made at that time can affect the system’s energy performance as well as installation and operating costs. The current study examined the interpretation of thermal response testing measurements used to evaluate the equivalent ground thermal conductivity and thus to design the system. All the measurements were taken at the same geological site located in Molinella, Bologna (Italy) where a variety of borehole heat exchangers (BHEs) had been installed and investigated within the project Cheap-GSHPs (Cheap and efficient application of reliable Ground Source Heat exchangers and Pumps) of the European Union’s Horizon 2020 research and innovation program. The measurements were initially analyzed in accordance with the common interpretation based on the first-order approximation of the solution for the infinite line source model and then by utilizing the complete solutions of both the infinite line and cylinder source models. An inverse numerical approach based on a detailed model that considers the current geometry of the BHE and the axial heat transfer as well as the effect of weather on the ground surface was also used. Study findings revealed that the best result was generally obtained using the inverse numerical interpretation. Full article
(This article belongs to the Special Issue Low Enthalpy Geothermal Energy)
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Open AccessArticle Transmission Power and Antenna Allocation for Energy-Efficient RF Energy Harvesting Networks with Massive MIMO
Energies 2017, 10(6), 802; doi:10.3390/en10060802
Received: 22 February 2017 / Revised: 1 June 2017 / Accepted: 9 June 2017 / Published: 13 June 2017
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Abstract
The optimum transmission strategy for maximizing energy efficiency (EE) of a multi-user massive multiple-input multiple-output (MIMO) system in radio frequency energy harvesting networks is investigated. We focus on dynamic time-switching (TS) antennas, to avoid the practical problems of power-splitting antennas, such as complex
[...] Read more.
The optimum transmission strategy for maximizing energy efficiency (EE) of a multi-user massive multiple-input multiple-output (MIMO) system in radio frequency energy harvesting networks is investigated. We focus on dynamic time-switching (TS) antennas, to avoid the practical problems of power-splitting antennas, such as complex architectures, power loss and signal distortion when splitting the power of the received signal into power for information decoding (ID) and energy harvesting (EH). However, since a single TS antenna cannot serve ID and EH simultaneously, the MIMO system is considered in this paper. We thus formulate an EE optimization problem and propose an iterative algorithm as a tractable solution, including an antenna selection strategy to optimally switch each TS antenna between ID mode and EH mode using nonlinear fractional programming and the Lagrange dual method. Further, the problem is solved under practical constraints of maximum transmission power and outage probabilities for a minimum amount of harvested power and rate capacity for each user. Simulation results show that the proposed algorithm is more energy-efficient than that of baseline schemes, and demonstrates the trade-off between the required amount of harvested power and energy efficiency. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Autonomous Wireless Self-Charging for Multi-Rotor Unmanned Aerial Vehicles
Energies 2017, 10(6), 803; doi:10.3390/en10060803
Received: 25 April 2017 / Revised: 31 May 2017 / Accepted: 6 June 2017 / Published: 13 June 2017
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Abstract
Rotary-wing unmanned aerial vehicles (UAVs) have the ability to operate in confined spaces and to hover over point of interest, but they have limited flight time and endurance. Conventional contact-based charging system for UAVs has been used, but it requires high landing accuracy
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Rotary-wing unmanned aerial vehicles (UAVs) have the ability to operate in confined spaces and to hover over point of interest, but they have limited flight time and endurance. Conventional contact-based charging system for UAVs has been used, but it requires high landing accuracy for proper docking. Instead of the conventional system, autonomous wireless battery charging system for UAVs in outdoor conditions is proposed in this paper. UAVs can be wirelessly charged using the proposed charging system, regardless of yaw angle between UAVs and wireless charging pad, which can further reduce their control complexity for autonomous landing. The increased overall mission time eventually relaxes the limitations on payload and flight time. In this paper, a cost effective automatic recharging solution for UAVs in outdoor environments is proposed using wireless power transfer (WPT). This research proposes a global positioning system (GPS) and vision-based closed-loop target detection and a tracking system for precise landing of quadcopters in outdoor environments. The system uses the onboard camera to detect the shape, color and position of the defined target in image frame. Based on the offset of the target from the center of the image frame, control commands are generated to track and maintain the center position. Commercially available AR.Drone. was used to demonstrate the proposed concept which is equppied with bottom camera and GPS. Experiments and analyses showed good performance, and about 75% average WPT efficiency was achieved in this research. Full article
(This article belongs to the Special Issue Wireless Power Transfer)
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Open AccessArticle A Novel Decentralized Economic Operation in Islanded AC Microgrids
Energies 2017, 10(6), 804; doi:10.3390/en10060804
Received: 16 April 2017 / Revised: 27 May 2017 / Accepted: 9 June 2017 / Published: 13 June 2017
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Abstract
Droop schemes are usually applied to the control of distributed generators (DGs) in microgrids (MGs) to realize proportional power sharing. The objective might, however, not suit MGs well for economic reasons. Addressing that issue, this paper proposes an alternative droop scheme for reducing
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Droop schemes are usually applied to the control of distributed generators (DGs) in microgrids (MGs) to realize proportional power sharing. The objective might, however, not suit MGs well for economic reasons. Addressing that issue, this paper proposes an alternative droop scheme for reducing the total active generation costs (TAGC). Optimal economic operation, DGs’ capacity limitations and system stability are fully considered basing on DGs’ generation costs. The proposed scheme utilizes the frequency as a carrier to realize the decentralized economic operation of MGs without communication links. Moreover, a fitting method is applied to balance DGs’ synchronous operation and economy. The effectiveness and performance of the proposed scheme are verified through simulations and experiments. Full article
(This article belongs to the Special Issue Advanced Operation and Control of Smart Microgrids)
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Open AccessArticle An Investigation of the Composition of the Flow in and out of a Two-Stroke Diesel Engine and Air Consumption Ratio
Energies 2017, 10(6), 805; doi:10.3390/en10060805
Received: 11 March 2017 / Revised: 24 May 2017 / Accepted: 9 June 2017 / Published: 13 June 2017
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Abstract
The aim of this research was to investigate the mass, substance and energy flow through two-stroke low speed Diesel engines. For this reason, a zero-dimensional model of the combustion in the engine was developed with a calculated amount and composition of exhaust gases.
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The aim of this research was to investigate the mass, substance and energy flow through two-stroke low speed Diesel engines. For this reason, a zero-dimensional model of the combustion in the engine was developed with a calculated amount and composition of exhaust gases. Due to the large amount of oxygen in the exhaust gases, a ratio of real air consumption and stoichiometric amount of air required for combustion of injected fuel was set. The calculated ratio showed that the engine consumed four times more air than needed for combustion in AFRstoich. In this work, this was called the Air Consumption Factor or Ratio, and has not previously been mentioned in scientific literature. The air consumption ratio is defined as a factor of dry or humid air. To be more comprehensive, a modified diagram of the composition of the flow in and out of a two-stroke fuel injection engine and the cylinder was made. Full article
(This article belongs to the Section Energy Sources)
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Open AccessArticle Implementation of a Real-Time Microgrid Simulation Platform Based on Centralized and Distributed Management
Energies 2017, 10(6), 806; doi:10.3390/en10060806
Received: 26 February 2017 / Revised: 21 April 2017 / Accepted: 7 June 2017 / Published: 14 June 2017
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Abstract
Demand response and distributed generation are key components of power systems. Several challenges are raised at both technical and business model levels for integration of those resources in smart grids and microgrids. The implementation of a distribution network as a test bed can
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Demand response and distributed generation are key components of power systems. Several challenges are raised at both technical and business model levels for integration of those resources in smart grids and microgrids. The implementation of a distribution network as a test bed can be difficult and not cost-effective; using computational modeling is not sufficient for producing realistic results. Real-time simulation allows us to validate the business model’s impact at the technical level. This paper comprises a platform supporting the real-time simulation of a microgrid connected to a larger distribution network. The implemented platform allows us to use both centralized and distributed energy resource management. Using an optimization model for the energy resource operation, a virtual power player manages all the available resources. Then, the simulation platform allows us to technically validate the actual implementation of the requested demand reduction in the scope of demand response programs. The case study has 33 buses, 220 consumers, and 68 distributed generators. It demonstrates the impact of demand response events, also performing resource management in the presence of an energy shortage. Full article
(This article belongs to the Special Issue Distributed Energy Resources Management)
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Open AccessArticle Energy Modelling and Automated Calibrations of Ancient Building Simulations: A Case Study of a School in the Northwest of Spain
Energies 2017, 10(6), 807; doi:10.3390/en10060807
Received: 1 June 2017 / Revised: 12 June 2017 / Accepted: 12 June 2017 / Published: 14 June 2017
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Abstract
In the present paper, the energy performance of buildings forming a school centre in the northwest of Spain was analyzed using a transient simulation of the energy model of the school, which was developed with TRNSYS, a software of proven reliability in the
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In the present paper, the energy performance of buildings forming a school centre in the northwest of Spain was analyzed using a transient simulation of the energy model of the school, which was developed with TRNSYS, a software of proven reliability in the field of thermal simulations. A deterministic calibration approach was applied to the initial building model to adjust the predictions to the actual performance of the school, data acquired during the temperature measurement campaign. The buildings under study were in deteriorated conditions due to poor maintenance over the years, presenting a big challenge for modelling and simulating it in a reliable way. The results showed that the proposed methodology is successful for obtaining calibrated thermal models of these types of damaged buildings, as the metrics employed to verify the final error showed a reduced normalized mean bias error (NMBE) of 2.73%. It was verified that a decrease of approximately 60% in NMBE and 17% in the coefficient of variation of the root mean square error (CV(RMSE)) was achieved due to the calibration process. Subsequent steps were performed with the aid of new software, which was developed under a European project that enabled the automated calibration of the simulations. Full article
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Open AccessArticle Comprehensive Reactive Power Support of DFIG Adapted to Different Depth of Voltage Sags
Energies 2017, 10(6), 808; doi:10.3390/en10060808
Received: 9 May 2017 / Revised: 9 June 2017 / Accepted: 10 June 2017 / Published: 14 June 2017
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Abstract
The low voltage ride-through (LVRT) capability of the doubly-fed induction generator (DFIG) significantly impacts upon the integration of wind power into the power grid. This paper develops a novel comprehensive control strategy to enhance the LVRT and reactive power support capacities of the
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The low voltage ride-through (LVRT) capability of the doubly-fed induction generator (DFIG) significantly impacts upon the integration of wind power into the power grid. This paper develops a novel comprehensive control strategy to enhance the LVRT and reactive power support capacities of the DFIG by installing the energy storage system (ESS). The ESS is connected to the DC-link capacitor of the DFIG and used to regulate the DC-link voltage during normal or fault operations. The unbalanced power between the captured wind power and the power injected to the grid during the transient process is absorbed or compensated by the ESS. The rotor-side converter (RSC) is used to control the maximum power production and the grid-side converter (GSC) is used to control the reactive power before participating in the voltage support. When the supply voltage continues to drop, the rotor speed is increased by controlling the RSC to realize the LVRT capability and help the GSC further enhance the reactive power support capability. The capacity of the GSC is dedicated to injecting the reactive power to the grid. An auxiliary transient pitch angle controller is proposed to protect the generator’s over speed. Both RSC and GSC act as reactive power sources to further enhance the voltage support capability with serious voltage sags. Simulations based on a single-machine infinite-bus power system verify the effectiveness of the developed comprehensive control strategy. Full article
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Open AccessArticle Experimental Study on the Performance of Water Source Trans-Critical CO2 Heat Pump Water Heater
Energies 2017, 10(6), 810; doi:10.3390/en10060810
Received: 12 March 2017 / Revised: 2 May 2017 / Accepted: 3 June 2017 / Published: 14 June 2017
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Abstract
The effect of the discharge pressure on the performance of the trans-critical CO2 heat pump with a low gas-cooler outlet temperature is experimentally investigated on a test rig of water source heat-pump water heater. The optimal discharge pressure of the trans-critical CO
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The effect of the discharge pressure on the performance of the trans-critical CO2 heat pump with a low gas-cooler outlet temperature is experimentally investigated on a test rig of water source heat-pump water heater. The optimal discharge pressure of the trans-critical CO2 heat pump is investigated under different external operation conditions. When the tap-water temperature is low, the characteristic of the S-shape isotherm at the supercritical region has little effect on the occurrence of the optimal discharge pressure; while the mass flow rate of CO2, the suction pressure and the gas-cooler outlet temperature play a significant role in determining the emergence of the optimal discharge pressure. At the optimal discharge pressure, the COP reaches the peak; however, the corresponding heating capacity is still lower than its maximum, which is reached as the discharge pressure is slightly above the optimal discharge pressure. Reducing the tap-water flowrate or increasing the water-source temperature can increase the optimal discharge pressure. The COP is positively dependent on both the tap-water flowrate and the water-source temperature. In addition, the tap-water flowrate has a negligible influence on the maximum heating capacity while increasing the water-source temperature can greatly enhance the heating capacity. Full article
(This article belongs to the Section Energy Fundamentals and Conversion)
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Open AccessArticle Optimal Placement and Sizing of Renewable Distributed Generations and Capacitor Banks into Radial Distribution Systems
Energies 2017, 10(6), 811; doi:10.3390/en10060811
Received: 31 January 2017 / Revised: 9 June 2017 / Accepted: 11 June 2017 / Published: 14 June 2017
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Abstract
In recent years, renewable types of distributed generation in the distribution system have been much appreciated due to their enormous technical and environmental advantages. This paper proposes a methodology for optimal placement and sizing of renewable distributed generation(s) (i.e., wind, solar and biomass)
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In recent years, renewable types of distributed generation in the distribution system have been much appreciated due to their enormous technical and environmental advantages. This paper proposes a methodology for optimal placement and sizing of renewable distributed generation(s) (i.e., wind, solar and biomass) and capacitor banks into a radial distribution system. The intermittency of wind speed and solar irradiance are handled with multi-state modeling using suitable probability distribution functions. The three objective functions, i.e., power loss reduction, voltage stability improvement, and voltage deviation minimization are optimized using advanced Pareto-front non-dominated sorting multi-objective particle swarm optimization method. First a set of non-dominated Pareto-front data are called from the algorithm. Later, a fuzzy decision technique is applied to extract the trade-off solution set. The effectiveness of the proposed methodology is tested on the standard IEEE 33 test system. The overall results reveal that combination of renewable distributed generations and capacitor banks are dominant in power loss reduction, voltage stability and voltage profile improvement. Full article
(This article belongs to the Special Issue Electric Power Systems Research 2017)
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Open AccessArticle Accurate Short-Term Power Forecasting of Wind Turbines: The Case of Jeju Island’s Wind Farm
Energies 2017, 10(6), 812; doi:10.3390/en10060812
Received: 22 April 2017 / Revised: 7 June 2017 / Accepted: 13 June 2017 / Published: 15 June 2017
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Abstract
Short-term wind power forecasting is a technique which tells system operators how much wind power can be expected at a specific time. Due to the increasing penetration of wind generating resources into the power grids, short-term wind power forecasting is becoming an important
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Short-term wind power forecasting is a technique which tells system operators how much wind power can be expected at a specific time. Due to the increasing penetration of wind generating resources into the power grids, short-term wind power forecasting is becoming an important issue for grid integration analysis. The high reliability of wind power forecasting can contribute to the successful integration of wind generating resources into the power grids. To guarantee the reliability of forecasting, power curves need to be analyzed and a forecasting method used that compensates for the variability of wind power outputs. In this paper, we analyzed the reliability of power curves at each wind speed using logistic regression. To reduce wind power forecasting errors, we proposed a short-term wind power forecasting method using support vector machine (SVM) based on linear regression. Support vector machine is a type of supervised leaning and is used to recognize patterns and analyze data. The proposed method was verified by empirical data collected from a wind turbine located on Jeju Island. Full article
(This article belongs to the Special Issue Wind Generators Modelling and Control)
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Open AccessArticle Deriving Optimal End of Day Storage for Pumped-Storage Power Plants in the Joint Energy and Reserve Day-Ahead Scheduling
Energies 2017, 10(6), 813; doi:10.3390/en10060813
Received: 8 March 2017 / Revised: 8 June 2017 / Accepted: 9 June 2017 / Published: 15 June 2017
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Abstract
This paper presents a new methodology to maximise the income and derive the optimal end of day storage of closed-loop and daily-cycle pumped-storage hydropower plants. The plants participate in the day-ahead energy market as a price-taker and in the secondary regulation reserve market
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This paper presents a new methodology to maximise the income and derive the optimal end of day storage of closed-loop and daily-cycle pumped-storage hydropower plants. The plants participate in the day-ahead energy market as a price-taker and in the secondary regulation reserve market as a price-maker, in the context of the Iberian electricity system. The real-time use of the committed reserves is considered in the model formulation. The operation of the plants with the proposed methodology is compared to the ones that use an end of day storage of an empty reservoir or half of the storage capacity. Results show that the proposed methodology increases the maximum theoretical income in all the plants analysed both if they only participate in the day-ahead energy market and if they also participate in the secondary regulation service. It is also shown that the increase in the maximum theoretical income strongly depends on the size of the plant. In addition, it is proven that the end of day storages change notably in the new reserve-driven strategies of pumped-storage hydropower plants and that the proposed methodology is even more recommended if the secondary regulation service is considered. Full article
(This article belongs to the Special Issue Hydropower 2017)
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Open AccessArticle Economic Analysis of a Photovoltaic System: A Resource for Residential Households
Energies 2017, 10(6), 814; doi:10.3390/en10060814
Received: 28 March 2017 / Revised: 8 June 2017 / Accepted: 11 June 2017 / Published: 15 June 2017
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Abstract
New installed annual solar photovoltaic (PV) capacity was equal to 76.1 GW in 2016 (+49%), reaching the total of 305 GW around the world. PV sources are able to achieve a greater energy independence, to tackle the climate change and to promote economic
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New installed annual solar photovoltaic (PV) capacity was equal to 76.1 GW in 2016 (+49%), reaching the total of 305 GW around the world. PV sources are able to achieve a greater energy independence, to tackle the climate change and to promote economic opportunities. This work proposes an economic analysis based on well-known indicators: Net Present Value (NPV), Discounted Payback Time (DPBT) and Levelized Cost of Electricity (LCOE). Several case studies are evaluated for residential households. They are based on three critical variables: plant size (1, 2, 3, 4, 5 and 6 kW), levels of insolation (1350, 1450 and 1550 kWh/(m2×y)) and share of self-consumption (30%, 40% and 50%). The profitability is verified in all case studies examined in this work. The role of self-consumption, that is the harmonization between demanded and produced energy, is strategic in a mature market to improve financial performance. A sensitivity analysis, based on both electricity purchase and sales prices (critical variables), confirms these positive results. The Reduction in the Emissions of Carbon Dioxide (ERcd) signifies an environmental improvement when a PV system is used as an alternative to a mix of fossil fuels. Finally, a policy proposal is examined based on a fiscal deduction of 50% fixing the period of deduction equal to 5 years. Full article
(This article belongs to the Section Energy Sources)
Open AccessArticle A Non-Ventilated Solar Façade Concept Based on Selective and Transparent Insulation Material Integration: An Experimental Study
Energies 2017, 10(6), 815; doi:10.3390/en10060815
Received: 12 April 2017 / Revised: 29 May 2017 / Accepted: 8 June 2017 / Published: 15 June 2017
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Abstract
A new solar façade concept based on transparent insulation and a selective absorber is proposed, tested and compared with conventional insulation and a non-selective type of absorber, respectively. The presented study focuses on an experimental non-ventilated solar type of façade exposed to solar
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A new solar façade concept based on transparent insulation and a selective absorber is proposed, tested and compared with conventional insulation and a non-selective type of absorber, respectively. The presented study focuses on an experimental non-ventilated solar type of façade exposed to solar radiation both in the laboratory and in outdoor tests. Due to the high solar absorbance level of the façade, high- and low-emissivity contributions were primarily analysed. All of the implemented materials were contrasted from the thermal and optical point of view. An analysis was made of both thermodynamic and steady state procedures affecting the proposed solar façade concept. Experimental full scale tests on real building components were additionally involved during summer monitoring. An indicator of the temperature response generated by solar radiation exposure demonstrates the outdoor performance of the façade is closely related to overheating phenomena. From the thermal point of view, the proposed transparent insulation and selective absorber concept corresponds to the performance of conventional thermal insulation of identical material thickness; however, the non-selective prototype only provides 50% thermal performance. The results of the solar-based experiments show that with a small-scale experimental prototype, approximately no significant difference is measured when compared with a non-selective absorber type. The only difference was achieved at the maximum of 2.5 K, when the lower temperature was obtained in the solar selective concept. At the full-scale outdoor mode, the results indicate a maximum of 3.0 K difference, however the lower temperature achieves a non-selective approach. This solar façade can actively contribute to the thermal performance of building components during periods of heating. Full article
(This article belongs to the Special Issue Solar Energy Application in Buildings)
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Open AccessArticle A Multiconductor Model of Power Line Communication in Medium-Voltage Lines
Energies 2017, 10(6), 816; doi:10.3390/en10060816
Received: 26 May 2017 / Revised: 9 June 2017 / Accepted: 12 June 2017 / Published: 15 June 2017
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Abstract
Most power line communication (PLC) models are designed to date simulate power lines as two-wire lines. However, in alternating current (AC) electrical distribution, the two-wire option is seldom applied, and medium-voltage lines are most often based on the three-phase configuration. In this context,
[...] Read more.
Most power line communication (PLC) models are designed to date simulate power lines as two-wire lines. However, in alternating current (AC) electrical distribution, the two-wire option is seldom applied, and medium-voltage lines are most often based on the three-phase configuration. In this context, the influence of the ground, which constitutes another conductor with specific parameters, cannot be neglected. Two-wire models are characterized by limited accuracy, not allowing us to simulate certain major phenomena affecting PLC. This, for example, could embody the answer to the question of whether it is more advantageous to transmit a signal independently through each phase, reference the signal with respect to another phase or to use the ground as a reference. This paper discusses a multi-conductor model that eliminates the disadvantages outlined above; the proposed model exploits the multi-conductor telegrapher’s equations. In order to be able to include medium-voltage (MV)/ low-voltage (LV) transformers in medium-voltage network models, we constituted a transformer model. The designed models were validated on a real medium-voltage network. To be able to evaluate the suitability of the PLC, the noise in the medium voltage network was measured in order to determine the signal-to-noise ratio (SNR). Full article
(This article belongs to the collection Smart Grid)
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Open AccessArticle Overview of Real-Time Simulation as a Supporting Effort to Smart-Grid Attainment
Energies 2017, 10(6), 817; doi:10.3390/en10060817
Received: 29 March 2017 / Revised: 16 May 2017 / Accepted: 26 May 2017 / Published: 16 June 2017
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Abstract
The smart-grid approach undergoes many difficulties regarding the strategy that will enable its actual implementation. In this paper, an overview of real-time simulation technologies and their applicability to the smart-grid approach are presented as enabling steps toward the smart-grid’s actual implementation. The objective
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The smart-grid approach undergoes many difficulties regarding the strategy that will enable its actual implementation. In this paper, an overview of real-time simulation technologies and their applicability to the smart-grid approach are presented as enabling steps toward the smart-grid’s actual implementation. The objective of this work is to contribute with an introductory text for interested readers of real-time systems in the context of modern electric needs and trends. In addition, a comprehensive review of current applications of real-time simulation in electric systems is provided, together with the basis to understand real-time simulation and the topologies and hardware used to implement it. Furthermore, an overview of the evolution of real-time simulators in the industrial and academic background and its current challenges are introduced. Full article
(This article belongs to the Special Issue Energy Production Systems)
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Open AccessArticle Prototyping a Software Defined Utility
Energies 2017, 10(6), 818; doi:10.3390/en10060818
Received: 3 April 2017 / Revised: 7 June 2017 / Accepted: 13 June 2017 / Published: 16 June 2017
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Abstract
The smart grid can be seen as a hybrid system composed by many systems. From a large scale point of view, it combines the electric power system itself and a heterogeneous information and communication technology (ICT) infrastructure. Additionally, these systems are composed by
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The smart grid can be seen as a hybrid system composed by many systems. From a large scale point of view, it combines the electric power system itself and a heterogeneous information and communication technology (ICT) infrastructure. Additionally, these systems are composed by many building blocks that are designed and managed as separated systems which are hard to fully integrate between each other. Relying on the experiences arisen and the knowledge gathered from the partners during the development of the FP7 European projects INTEGRIS (intelligent electrical grid sensor communications) and FINESCE (future internet smart utility services), this paper presents the software defined utility (SDU) concept for the management of the smart grid and its security, which advocates for the migration of the utility infrastructure to software systems instead of relying on complex and rigid hardware based systems. Following this approach, SDU proposes the evolution of power systems’ ICT and the usage of programmable commodity hardware, low-cost sensors, and reliable high-speed IP-based communications underneath. More concretely, this paper proposes some building blocks for the deployment of the SDU (flexible data management infrastructure, context-aware security and web of things interface) and evaluates their functionalities and benefits for the smart grids of the future. Full article
(This article belongs to the Special Issue ICT for Energy)
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Open AccessArticle Performance Analysis of Conjugate Gradient Algorithms Applied to the Neuro-Fuzzy Feedback Linearization-Based Adaptive Control Paradigm for Multiple HVDC Links in AC/DC Power System
Energies 2017, 10(6), 819; doi:10.3390/en10060819
Received: 23 March 2017 / Revised: 24 May 2017 / Accepted: 5 June 2017 / Published: 16 June 2017
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Abstract
The existing literature predominantly concentrates on the utilization of the gradient descent algorithm for control systems’ design in power systems for stability enhancement. In this paper, various flavors of the Conjugate Gradient (CG) algorithm have been employed to design the online neuro-fuzzy linearization-based
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The existing literature predominantly concentrates on the utilization of the gradient descent algorithm for control systems’ design in power systems for stability enhancement. In this paper, various flavors of the Conjugate Gradient (CG) algorithm have been employed to design the online neuro-fuzzy linearization-based adaptive control strategy for Line Commutated Converters’ (LCC) High Voltage Direct Current (HVDC) links embedded in a multi-machine test power system. The conjugate gradient algorithms are evaluated based on the damping of electro-mechanical oscillatory modes using MATLAB/Simulink. The results validate that all of the conjugate gradient algorithms have outperformed the gradient descent optimization scheme and other conventional and non-conventional control schemes. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessFeature PaperArticle Recycling COR-TEN® Sea Containers into Service Modules for Military Applications: Thermal Analysis
Energies 2017, 10(6), 820; doi:10.3390/en10060820
Received: 1 June 2017 / Revised: 13 June 2017 / Accepted: 14 June 2017 / Published: 17 June 2017
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Abstract
It is thought that there are 14 million standardised sea containers out of order, equal to 23.3 million twenty-foot equivalent unit (TEU), which could be devoted for other purposes. The COR-TEN®steel containers are the best, due to the features of the
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It is thought that there are 14 million standardised sea containers out of order, equal to 23.3 million twenty-foot equivalent unit (TEU), which could be devoted for other purposes. The COR-TEN®steel containers are the best, due to the features of the materials, to be used in applications intended for the accommodation of people. One of the uses is framed in the area of humanitarian help, as service modules in areas of social emergency. Moreover, the deployment of these kinds of modules is quite simple, as the Army is generally able to transport containers in its units. In this project we have selected three typologies of service modules and we have designed modules based on sea containers which can fit into these typologies. The container chosen to be recycled is the 1 TEU, to which the closings and divisions are defined, which must reflect the different typologies of the service modules chosen. Afterwards, a study of thermal charges is carried out on three different settings of service modules located in five locations spread in different climate areas. Throughout a thermal simulation in Trnsys® (V17, Thermal Energy System Specialists, LLC, Madison, WI, USA), the consumption and maximum demand of heat and cold are analysed. Based on these results, the best equipment to cover the energy demand of the modules analysed in this study could be chosen. Moreover, the results can be used to estimate the operational costs of the deployment of the modules. Full article
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Open AccessArticle Large-Eddy Simulations of Two In-Line Turbines in a Wind Tunnel with Different Inflow Conditions
Energies 2017, 10(6), 821; doi:10.3390/en10060821
Received: 15 May 2017 / Revised: 10 June 2017 / Accepted: 14 June 2017 / Published: 17 June 2017
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Abstract
Numerical simulations reproducing a wind tunnel experiment on two in-line wind turbines have been performed. The flow features and the array performances have been evaluated in different inflow conditions. Following the experimental setup, different inlet conditions are obtained by simulating two grids upstream
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Numerical simulations reproducing a wind tunnel experiment on two in-line wind turbines have been performed. The flow features and the array performances have been evaluated in different inflow conditions. Following the experimental setup, different inlet conditions are obtained by simulating two grids upstream of the array. The increased turbulence intensity due to the grids improves the wake recovery and the efficiency of the second turbine. However, the inlet grid induces off-design operation on the first turbine, decreasing the efficiency and increasing fatigue loads. Typical grid flow patterns are observed past the rotor of the first turbine, up to the near wake. Further downstream, the signature of the grid on the flow is quite limited. An assessment of numerical modeling aspects (subgrid scale tensor and rotor parameterization) has been performed by comparison with the experimental measurements. Full article
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Open AccessArticle Multi-Frequency Control in a Stand-Alone Multi-Microgrid System Using a Back-To-Back Converter
Energies 2017, 10(6), 822; doi:10.3390/en10060822
Received: 4 May 2017 / Revised: 7 June 2017 / Accepted: 13 June 2017 / Published: 17 June 2017
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Abstract
A stand-alone multi-microgrid (MMG) system can be formed by connecting multiple stand-alone microgrids (MGs). In the stand-alone MMG system where the frequencies of each MG system are different, a back-to-back (BTB) converter can be used for interconnecting the adjacent MG system. The frequency
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A stand-alone multi-microgrid (MMG) system can be formed by connecting multiple stand-alone microgrids (MGs). In the stand-alone MMG system where the frequencies of each MG system are different, a back-to-back (BTB) converter can be used for interconnecting the adjacent MG system. The frequency control performance of the MMG system can be improved by designing the suitable controller of the BTB converter. This study proposes a multi-frequency control in the BTB converter to improve the performance of frequency regulation in the MMG system. Autonomous power sharing between each MG system is achieved by using the proposed multi-frequency control. The stand-alone MMG system where two stand-alone MG systems with different nominal frequencies are interconnected using the BTB converter is simulated in this study to show the feasibility of the proposed multi-frequency controller. Each stand-alone MG system consists of an inverter-based distributed generator (DG) that uses a grid-forming converter with a conventional frequency droop controller. The inverter-based DG is responsible for the primary frequency control in each MG system. To show the effectiveness of the proposed multi-frequency control, a comparison study of the multi-frequency control and the single frequency control is presented in this study. Simulation results show that the system stability can be improved by using the proposed multi-frequency controller. Full article
(This article belongs to the Special Issue Advanced Operation and Control of Smart Microgrids)
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Open AccessArticle CFD Analysis on the Air-Side Thermal-Hydraulic Performance of Multi-Louvered Fin Heat Exchangers at Low Reynolds Numbers
Energies 2017, 10(6), 823; doi:10.3390/en10060823
Received: 5 April 2017 / Revised: 7 May 2017 / Accepted: 13 June 2017 / Published: 18 June 2017
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Abstract
The air-side thermal-hydraulic performance of multi-louvered aluminium fin heat exchangers is investigated. A systematic numerical study has been performed to analyze the air-sde thermal hydraulic characteristics over a wide range of Reynolds number i.e., from 30 to 500. Air-side heat transfer coefficient and
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The air-side thermal-hydraulic performance of multi-louvered aluminium fin heat exchangers is investigated. A systematic numerical study has been performed to analyze the air-sde thermal hydraulic characteristics over a wide range of Reynolds number i.e., from 30 to 500. Air-side heat transfer coefficient and pressure drop were calculated and validated over the mentioned band of Reynolds numbers. The critical Reynolds number was determined numerically; and also the variation of flow pattern along with the air-side heat transfer coefficient and pressure drop in a multi-louvered heat exchanger associated with R e c r i has been reported. Moreover, a parametric study of the multi-louvered aluminium fin heat exchangers was also performed for 36 heat exchanger configurations with the louver angles (19–31°); fin pitches (1.0, 1.2, 1.4 mm) and flow depths (16, 20, 24 mm); and the geometric configuration exhibiting the highest air-side heat transfer coefficient was reported. The air-side heat transfer coefficient and pressure drop results for different geometrical configurations were presented in terms of Colburn j factor and Fanning friction factor f; as a function of Reynolds number based on louver pitch. Full article
(This article belongs to the Section Energy Fundamentals and Conversion)
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Open AccessArticle Flexibility-Based Evaluation of Variable Generation Acceptability in Korean Power System
Energies 2017, 10(6), 825; doi:10.3390/en10060825
Received: 28 March 2017 / Revised: 6 June 2017 / Accepted: 15 June 2017 / Published: 18 June 2017
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Abstract
This study proposes an evaluation method for variable generation (VG) acceptability with an adequate level of power system flexibility. In this method, a risk index referred to as the ramping capability shortage expectation (RSE) is used to quantify flexibility. The RSE
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This study proposes an evaluation method for variable generation (VG) acceptability with an adequate level of power system flexibility. In this method, a risk index referred to as the ramping capability shortage expectation (RSE) is used to quantify flexibility. The RSE value of the current power system is selected as the adequate level of flexibility (i.e., RSE criterion). VG acceptability is represented by the VG penetration level for the RSE criterion. The proposed evaluation method was applied to the generation expansion plan in Korea for 2029 in order to examine the validity of the existing plan for VG penetration. Sensitivity analysis was also performed to analyze the effects of changes in system uncertainty on VG acceptability. The results show that the planned VG penetration level for 2029 can improve by approximately 12% while securing flexibility. Full article
(This article belongs to the Special Issue Sustainable Energy Technologies)
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Open AccessArticle Performance Evaluation of Photovoltaic Solar System with Different Cooling Methods and a Bi-Reflector PV System (BRPVS): An Experimental Study and Comparative Analysis
Energies 2017, 10(6), 826; doi:10.3390/en10060826
Received: 6 April 2017 / Revised: 7 June 2017 / Accepted: 14 June 2017 / Published: 19 June 2017
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Abstract
Reducing the price of solar photovoltaic (PV) systems has been a constant challenge. Despite recent advances, solar PV systems are still more costly than conventional energy resources. For the first time, this study examines the effectiveness of three different structures/materials: (i) silvered glass
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Reducing the price of solar photovoltaic (PV) systems has been a constant challenge. Despite recent advances, solar PV systems are still more costly than conventional energy resources. For the first time, this study examines the effectiveness of three different structures/materials: (i) silvered glass plane mirror; (ii) convex spherical mirrors; and (iii) aluminum (Al) foil as reflector. Comparative analysis of four different cooling techniques, i.e., water sprinkling system, passive heat sink method, active air fan method, and closed loop method, for enhancement of output power was performed. A novel Bi reflector solar PV system (BRPVS) was suggested to control the working of the reflectors. The Al foil enhanced the power output compared to the others. In addition, the effect of using a reflector on the temperature of a solar PV system was studied. High operating temperatures resulted in a decrease in the maximum output power under the same solar radiation conditions. The combined enhancement of the output power by both Al foil BRPVS system and cooling system was almost 22.75–38.55%. An optimal control algorithm to use cooling and BRPVS in an efficient manner is described. Full article
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Open AccessArticle Separation of CO2 in a Solid Waste Management Incineration Facility Using Activated Carbon Derived from Pine Sawdust
Energies 2017, 10(6), 827; doi:10.3390/en10060827
Received: 30 March 2017 / Revised: 7 June 2017 / Accepted: 15 June 2017 / Published: 20 June 2017
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Abstract
The selective separation of CO2 from gas mixtures representative of flue gas generated in waste incineration systems is studied on two activated carbons obtained from pine sawdust and compared to a commercial activated carbon. Dynamic adsorption experiments were conducted in a fixed-bed
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The selective separation of CO2 from gas mixtures representative of flue gas generated in waste incineration systems is studied on two activated carbons obtained from pine sawdust and compared to a commercial activated carbon. Dynamic adsorption experiments were conducted in a fixed-bed adsorption column using a binary mixture (N2/CO2) with a composition representative of incineration streams at temperatures from 30 to 70 °C. The adsorption behavior of humid mixtures (N2/CO2/H2O) was also evaluated in order to assess the influence of water vapor in CO2 adsorption at different relative humidity in the feed gas: 22% and 60%. Moreover, CO2 adsorption was studied in less favorable conditions, i.e., departing from a bed initially saturated with H2O. In addition, the effect of CO2 on H2O adsorption was examined. Experimental results showed that the CO2 adsorption capacity can be reduced significantly by the adsorption of H2O (up to 60% at high relative humidity conditions). On the other hand, the breakthrough tests over the adsorbent initially saturated with water vapor indicated that H2O is little affected by CO2 adsorption. The experimental results pointed out the biomass based carbons as best candidates for CO2 separation under incineration flue gas conditions. Full article
(This article belongs to the Special Issue CO2 Capture)
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Open AccessArticle Natural Gas Hydrate as a Storage Mechanism for Safe, Sustainable and Economical Production from Offshore Petroleum Reserves
Energies 2017, 10(6), 828; doi:10.3390/en10060828
Received: 16 April 2017 / Revised: 6 June 2017 / Accepted: 11 June 2017 / Published: 20 June 2017
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Abstract
Century Fathom presents an innovative process to utilize clathrate hydrates for the production, storage and transportation of natural gas from off-shore energy reserves in deep ocean environments. The production scheme was developed by considering the preferred state of natural gas in the deep
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Century Fathom presents an innovative process to utilize clathrate hydrates for the production, storage and transportation of natural gas from off-shore energy reserves in deep ocean environments. The production scheme was developed by considering the preferred state of natural gas in the deep ocean and addressing the hazards associated with conventional techniques to transport natural gas. It also is designed to mitigate the significant shipping cost inherent with all methods. The resulting proposed scheme restrains transport in the hydrate form to the ocean and does not attempt to supply energy to the residential consumer. Instead; the target recipients are industrial operations. The resulting operational concept is intrinsically safer by design; environmentally sustainable and significantly cost-effective compared with currently proposed schemes for the use of natural gas hydrates and has the potential to be the optimal solution for new production of reserves; depending on the distance to shore and capacity of the petroleum reserve. A potential additional benefit is the byproduct of desalinated water. Full article
(This article belongs to the Special Issue Methane Hydrate Research and Development)
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Open AccessArticle A New Backward Euler Stabilized Optimum Controller for NPC Back-to-Back Five Level Converters
Energies 2017, 10(6), 735; doi:10.3390/en10060735
Received: 20 March 2017 / Revised: 5 May 2017 / Accepted: 9 May 2017 / Published: 23 May 2017
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Abstract
This paper presents a backward Euler stabilized-based control strategy applied to a neutral point clamped (NPC) back-to-back connected five level converters. A generalized method is used to obtain the back-to-back NPC converter system model. The backward Euler stabilized-based control strategy uses one set
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This paper presents a backward Euler stabilized-based control strategy applied to a neutral point clamped (NPC) back-to-back connected five level converters. A generalized method is used to obtain the back-to-back NPC converter system model. The backward Euler stabilized-based control strategy uses one set of calculations to compute the optimum voltage vector needed to reach the references and to balance the voltage of the DC-bus capacitors. The output voltage vector is selected using a modified cost functional that includes variable tracking errors in the functional weights, whereas in classic approaches, the weights are considered constant. The proposed modified cost functional enables AC current tracking and DC-bus voltage balancing in a wide range of operating conditions. The paper main contributions are: (i) a backward Euler stabilized-based control strategy applied to a double, back-to-back connected, five level NPC converter; (ii) the use of cost functional weight varying as a function of the controlled variable tracking errors to enforce the controlled variables and to balance the DC capacitor voltages; and (iii) the demonstration of system feasibility for this type of converter topology and control strategy, ensuring a high enough computational efficiency and extending the modulation index from 0.6 to 0.93. Experimental results are presented using a prototype of a five level NPC back-to-back converter. Full article
(This article belongs to the Special Issue Power Electronics in Power Quality)
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Open AccessArticle Numerical Analysis on the Formation of Fracture Network during the Hydraulic Fracturing of Shale with Pre-Existing Fractures
Energies 2017, 10(6), 736; doi:10.3390/en10060736
Received: 15 February 2017 / Revised: 12 May 2017 / Accepted: 19 May 2017 / Published: 23 May 2017
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Abstract
In this paper, configurations of pre-existing fractures in cubic rock blocks were investigated and reconstructed for the modeling of experimental hydraulic fracturing. The fluid-rock coupling process of hydraulic fracturing was simulated based on the displacement discontinuities method. The numerical model was validated against
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In this paper, configurations of pre-existing fractures in cubic rock blocks were investigated and reconstructed for the modeling of experimental hydraulic fracturing. The fluid-rock coupling process of hydraulic fracturing was simulated based on the displacement discontinuities method. The numerical model was validated against the related laboratory experiments. The stimulated fracture configurations under different conditions can be clearly shown using the validated numerical model. First, a dominated fracture along the maximum principle stress direction is always formed when the stress difference is large enough. Second, there are less reopened pre-existing fractures, more newly formed fractures and less shear fractures with the increase of the cohesion value of pre-existing fractures. Third, the length of the stimulated shear fracture decreases rapidly with the increase of the friction coefficient, while the length of the tensile fracture has no correlation to the fiction coefficient. Finally, the increase of the fluid injection rate is favorable to the formation of a fracture network. The unfavorable effects of the large stress difference and the large cohesion of pre-existing fractures can be partly suppressed by an increase of the injection rate in the hydraulic fracturing treatment. The results of this paper are useful for understanding fracture propagation behaviors during the hydraulic fracturing of shale reservoirs with pre-existing fractures. Full article
(This article belongs to the Section Energy Sources)
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Open AccessArticle Frequency-Adaptive Modified Comb-Filter-Based Phase-Locked Loop for a Doubly-Fed Adjustable-Speed Pumped-Storage Hydropower Plant under Distorted Grid Conditions
Energies 2017, 10(6), 737; doi:10.3390/en10060737
Received: 28 January 2017 / Revised: 11 May 2017 / Accepted: 19 May 2017 / Published: 23 May 2017
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Abstract
The control system of a doubly-fed adjustable-speed pumped-storage hydropower plant needs phase-locked loops (PLLs) to obtain the phase angle of grid voltage. The main drawback of a comb-filter-based phase-locked loop (CF-PLL) is the slow dynamic response. This paper presents a modified comb-filter-based phase-locked
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The control system of a doubly-fed adjustable-speed pumped-storage hydropower plant needs phase-locked loops (PLLs) to obtain the phase angle of grid voltage. The main drawback of a comb-filter-based phase-locked loop (CF-PLL) is the slow dynamic response. This paper presents a modified comb-filter-based phase-locked loop (MCF-PLL) by improving the pole-zero pattern of the comb filter, and gives the parameters’ setting method of the controller, based on the discrete model of MCF-PLL. In order to improve the disturbance resistibility of MCF-PLL when the power grid’s frequency changes, this paper proposes a frequency-adaptive modified, comb-filter-based, phase-locked loop (FAMCF-PLL) and its digital implementation scheme. Experimental results show that FAMCF-PLL has good steady-state and dynamic performance under distorted grid conditions. Furthermore, FAMCF-PLL can determine the phase angle of the grid voltage, which is locked when it is applied to a doubly-fed adjustable-speed pumped-storage hydropower experimental platform. Full article
(This article belongs to the Special Issue Hydropower 2017)
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Open AccessArticle Annual Assessment of Large-Scale Introduction of Renewable Energy: Modeling of Unit Commitment Schedule for Thermal Power Generators and Pumped Storages
Energies 2017, 10(6), 738; doi:10.3390/en10060738
Received: 31 March 2017 / Revised: 10 May 2017 / Accepted: 19 May 2017 / Published: 23 May 2017
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Abstract
The fast-increasing introduction of renewable energy sources (RESes) leads to some problems in electrical power network due to fluctuating generated power. A power system must be operated with provision of various reserve powers like governor free capacity, load frequency control and spinning reserve.
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The fast-increasing introduction of renewable energy sources (RESes) leads to some problems in electrical power network due to fluctuating generated power. A power system must be operated with provision of various reserve powers like governor free capacity, load frequency control and spinning reserve. Therefore, the generator’s schedule (unit commitment schedule) should include the consideration of the various power reserves. In addition, it is necessary to calculate the annual operational costs of electric power systems by solving the unit commitment per week of thermal power generators and pumped storages in order to compare and examine the variance of the operational costs and the operating ratio of the generators throughout the year. This study proposes a novel annual analysis for the thermal power generator and pumped storages under a massive introduction of RESes. A weekly unit commitment schedule (start/stop planning) for thermal power generator and pumped storages has been modeled and calculated for one year evaluation. To solve the generator start/stop planning problem, Tabu search and interior point methods are adopted to solve the operation planning for thermal power generators and the output decision for pumped storages, respectively. It is demonstrated that the proposed method can analyze a one-year evaluation within practical time. In addition, by assuming load frequency control (LFC) constraints to cope with photovoltaic (PV) output fluctuations, the impact of the intensity of LFC constraints on the operational cost of the thermal power generator has been elucidated. The increment of the operational cost of the power supply with increasing PV introduction amount has been shown in concrete terms. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Prediction of Steam Jacket Dynamics and Water Balances in Underground Coal Gasification
Energies 2017, 10(6), 739; doi:10.3390/en10060739
Received: 24 March 2017 / Revised: 18 May 2017 / Accepted: 19 May 2017 / Published: 23 May 2017
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Abstract
Underground coal gasification (UCG) converts coal to a high-calorific synthesis gas for the production of fuels or chemical feedstock. UCG reactors are generally operated below hydrostatic pressure to avoid leakage of UCG fluids into overburden aquifers. Additionally, fluid flow out of and into
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Underground coal gasification (UCG) converts coal to a high-calorific synthesis gas for the production of fuels or chemical feedstock. UCG reactors are generally operated below hydrostatic pressure to avoid leakage of UCG fluids into overburden aquifers. Additionally, fluid flow out of and into the reactor is also determined by the presence of the steam jacket, emerging in close reactor vicinity due to the high temperatures generated in UCG operation. Aiming at improving the understanding of the substantial role of the steam jacket in UCG operations, we employ numerical non-isothermal multiphase flow simulations to assess the occurring multiphase fluid flow processes. For that purpose, we first validate our modeling approach against published data on the U.S. UCG field trials at Hanna and Hoe Creek, achieving a very good agreement between our simulation and the observed water balances. Then, we discuss the effect of coal seam permeability and UCG reactor pressure on the dynamic multiphase flow processes in the reactor’s vicinity. The presented modeling approach allows for the quantification and prediction of time-dependent temperature and pressure distributions in the reactor vicinity, and thus steam jacket dynamics as well as reactor water in- and outflows. Full article
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Open AccessArticle The Effect of Embodied Impact on the Cost-Optimal Levels of Nearly Zero Energy Buildings: A Case Study of a Residential Building in Thessaloniki, Greece
Energies 2017, 10(6), 740; doi:10.3390/en10060740
Received: 22 March 2017 / Revised: 12 May 2017 / Accepted: 16 May 2017 / Published: 24 May 2017
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Abstract
Considering the nearly zero energy building (nZEB) and the increased measures for the improvement of the energy efficiency, the international literature indicates an extended use of specialized materials and technical installations. An increase in the embodied energy follows that use, with a final
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Considering the nearly zero energy building (nZEB) and the increased measures for the improvement of the energy efficiency, the international literature indicates an extended use of specialized materials and technical installations. An increase in the embodied energy follows that use, with a final share between 74% and 100% in the total life cycle energy of residential nZEBs. The current energy efficiency legislation considers only the impact from the operational phase and ignores the embodied impact from the remaining life cycle phases of the building. Nevertheless, the delegated regulation 244 of 2012 acknowledges the incompleteness of this assessment and provides an optional consideration of the embodied (“grey”) energy. The current study applies this option through the macroeconomic global cost calculations and the cost-optimal levels of nZEBs. The results indicate a limited effect of the embodied impact on the cost-optimal levels and in specific on extended calculation periods and in combination with other key parameters of the sensitivity analysis. An increase in the primary energy and a transposition to variants with lower use of materials and decreased embodied emissions follow this effect. Sensitivity analysis confirms the calculation period as a key parameter and indicates the need for further research. Full article
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Open AccessArticle Impact of Vibration on the Surface Film of Lithium-Ion Cells
Energies 2017, 10(6), 741; doi:10.3390/en10060741
Received: 24 March 2017 / Revised: 17 May 2017 / Accepted: 22 May 2017 / Published: 25 May 2017
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Abstract
Cylindrical 18650-type lithium-ion cells are being utilized more often for automotive applications. This introduces error in calculating expected lifetime due to varied usage conditions accelerating or reducing material damage. One such usage condition is vibration, which has been shown to impact the electrical
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Cylindrical 18650-type lithium-ion cells are being utilized more often for automotive applications. This introduces error in calculating expected lifetime due to varied usage conditions accelerating or reducing material damage. One such usage condition is vibration, which has been shown to impact the electrical performance over extended periods. Within this study X-ray photoelectron spectroscopy (XPS) has been performed on nickel manganese cobalt (NMC) cells subjected to vibration. This study found that vibration causes the removal of the selectively-formed surface film created during a cell’s first cycles and replaces it with the surface film from electrolyte decomposition. The surface films formed by vibration are composed of much higher concentrations of organic electrolyte decomposition products than the film from the control cell. The impact of this chemical mechanism is an increased level of cell degradation. This is exhibited in increased capacity fade and cell impedance. This is the first study presented within the academic literature which has identified an electro-mechanical mechanism responsible for the performance degradation in lithium-ion cells from vibration. Full article
(This article belongs to the Section Energy Sources)
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Open AccessArticle Five Megawatt Wind Turbine Power Output Improvements by Passive Flow Control Devices
Energies 2017, 10(6), 742; doi:10.3390/en10060742
Received: 24 February 2017 / Revised: 1 May 2017 / Accepted: 17 May 2017 / Published: 24 May 2017
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Abstract
The effects of two types of flow control devices, vortex generators (VGs) and Gurney flaps (GFs), on the power output performance of a multi-megawatt horizontal axis wind turbine is presented. To that end, an improved blade element momentum (BEM)-based solver has been developed
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The effects of two types of flow control devices, vortex generators (VGs) and Gurney flaps (GFs), on the power output performance of a multi-megawatt horizontal axis wind turbine is presented. To that end, an improved blade element momentum (BEM)-based solver has been developed and BEM-based computations have been carried out on the National Renewable Energy Laboratory (NREL) 5 MW baseline wind turbine. The results obtained from the clean wind turbine are compared with the ones obtained from the wind turbine equipped with the flow control devices. A significant increase in the average wind turbine power output has been found for all of the flow control device configurations and for the wind speed realizations studied in the present work. Furthermore, a best configuration case is proposed which has the largest increase of the average power output. In that case, increments on the average power output of 10.4% and 3.5% have been found at two different wind speed realizations. The thrust force and bending moment in the root of the blade have also been determined and compared with the values of the clean wind turbine. A residual increase in the bending moment of less than 1% has been found. Full article
(This article belongs to the Special Issue Wind Turbine 2017)
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Open AccessArticle Lifetime Assessment of Combined Cycles for Cogeneration of Power and Heat in Offshore Oil and Gas Installations
Energies 2017, 10(6), 744; doi:10.3390/en10060744
Received: 31 March 2017 / Revised: 12 May 2017 / Accepted: 19 May 2017 / Published: 25 May 2017
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Abstract
The utilization of combined cycles for offshore cogeneration of power and heat is an attractive option to reduce the CO2 emissions directly related to the oil and gas sector. Main challenges for their efficient implementation are the potentially large heat-to-power ratios and
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The utilization of combined cycles for offshore cogeneration of power and heat is an attractive option to reduce the CO2 emissions directly related to the oil and gas sector. Main challenges for their efficient implementation are the potentially large heat-to-power ratios and the variability of power and heat requirements throughout the different stages of a field’s lifetime. This paper aimed to provide the first elements for an assessment of the technology. Two combined cycle configurations were evaluated (backpressure and extraction steam turbine cycle), as well as different scenarios of power and heat requirements. The optimum design approach was firstly investigated. Designing the combined cycle at the end-life conditions, rather than at peak conditions, demonstrated to return better overall performance, when the entire plant’s lifetime is considered. A comparative analysis between the defined optimum designs was then carried out. Although the backpressure steam turbine cycle demonstrated to be feasible in all the cases analyzed, it showed to be effective only for offshore installations characterized by low temperature large process heat demands. On the other hand, the extraction steam turbine cycle could not meet large process heat demands but it was very attractive when the heat requirements were more limited, irrespective of the temperature at which this heat was requested. Full article
(This article belongs to the Section Energy Fundamentals and Conversion)
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Open AccessArticle Effect of Gas Recycling on the Performance of a Moving Bed Temperature-Swing (MBTSA) Process for CO2 Capture in a Coal Fired Power Plant Context
Energies 2017, 10(6), 745; doi:10.3390/en10060745
Received: 2 April 2017 / Revised: 18 May 2017 / Accepted: 20 May 2017 / Published: 25 May 2017
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Abstract
A mathematical model of a continuous moving-bed temperature-swing adsorption (MBTSA) process for post-combustion CO2 capture in a coal-fired power plant context has been developed. Process simulations have been done using single component isotherms and measured gas diffusion parameters of an activated carbon
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A mathematical model of a continuous moving-bed temperature-swing adsorption (MBTSA) process for post-combustion CO2 capture in a coal-fired power plant context has been developed. Process simulations have been done using single component isotherms and measured gas diffusion parameters of an activated carbon adsorbent. While a simple process configuration with no gas re-circulation gives quite low capture rate and CO2 purity, 86% and 65%, respectively, more advanced process configurations where some of the captured gas is recirculated to the incoming flue gas drastically increase both the capture rate and CO2 purity, the best configuration reaching capture rate of 86% and CO2 purity of 98%. Further improvements can be achieved by using adsorbents with higher CO2/N2 selectivity and/or higher temperature of the regeneration section. Full article
(This article belongs to the Special Issue CO2 Capture)
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Open AccessArticle Experimental Study of 3D Movement in Cushioning of Hydraulic Cylinder
Energies 2017, 10(6), 746; doi:10.3390/en10060746
Received: 26 March 2017 / Revised: 20 May 2017 / Accepted: 22 May 2017 / Published: 25 May 2017
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Abstract
A double acting cylinder operation has been fully monitored in its key functional parameters, focused on characterization of end-of-stroke cushioning and starting phases. Being the cylinder performance reliant in the piston constructive geometry, the number and location of piston circumferential grooves is a
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A double acting cylinder operation has been fully monitored in its key functional parameters, focused on characterization of end-of-stroke cushioning and starting phases. Being the cylinder performance reliant in the piston constructive geometry, the number and location of piston circumferential grooves is a significant parameter affecting the internal cushioning system performance. An eddy current displacement sensor assembled in the piston allows assessment of piston radial displacement inside the cylinder tube, which is directly related with the studied operating phases. Due to such 3D displacements, the piston becomes as an active and self-adjusting element along the functional cycle of the cylinder. Mechanical joints orientation and operating pressure are also relevant parameters affecting piston radial displacement and, thus, the cushioning and starting performance. Computational Fluid Dynamics (CFD) results confirm the observed functional role of the perimeter grooves; the flow and pressure distributions, where develops a significant radial force, are also in accordance with the registered radial displacement. Full article
(This article belongs to the Special Issue Energy Efficiency and Controllability of Fluid Power Systems)
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Open AccessArticle Comparison of Liquid Water Dynamics in Bent Gas Channels of a Polymer Electrolyte Membrane Fuel Cell with Different Channel Cross Sections in a Channel Flooding Situation
Energies 2017, 10(6), 748; doi:10.3390/en10060748
Received: 7 April 2017 / Revised: 15 May 2017 / Accepted: 19 May 2017 / Published: 26 May 2017
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Abstract
The transport characteristics of water slugs in a bent gas channel of a polymer electrolyte membrane (PEM) fuel cell are numerically studied using the volume of fluid (VOF) method. To investigate the effects of channel cross-sectional shape in a channel flooding situation, the
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The transport characteristics of water slugs in a bent gas channel of a polymer electrolyte membrane (PEM) fuel cell are numerically studied using the volume of fluid (VOF) method. To investigate the effects of channel cross-sectional shape in a channel flooding situation, the gas channels (GCs) with one rectangular and two trapezoidal cross sections are compared. Parametric studies are also conducted to evaluate the effects of the contact angle of the top and side walls, the contact angle of the gas diffusion layer (GDL) surface, and the air inlet velocity. Considering both of the water volume fraction (WVF) and GDL water coverage ratio (WCR), the trapezoidal channel with open angles of 60 degrees provides the most favorable performance in a channel flooding condition. Among all the top and side wall contact angles considered, the hydrophobic contact angle of 120 degrees shows the best results. Among the three GDL contact angles of 90, 110 and 140 degrees, the hydrophobic GDL contact angle of 140 degrees provides the most favorable water removal characteristics in a channel flooding situation. For all cross-sectional shapes, the water removal rate increases and the liquid water interface shows more complex patterns as the air inlet velocity increases. Full article
(This article belongs to the Section Energy Sources)
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Open AccessArticle Damping Force Modeling and Suppression of Self-Excited Vibration due to Magnetic Fluids Applied in the Torque Motor of a Hydraulic Servovalve
Energies 2017, 10(6), 749; doi:10.3390/en10060749
Received: 30 March 2017 / Revised: 6 May 2017 / Accepted: 22 May 2017 / Published: 27 May 2017
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Abstract
As a key component of hydraulic control systems, hydraulic servovalves influence their performance significantly. Unpredictable self-excited noise inside hydraulic servovalves may cause instability and even failure. Being functional, with higher saturation magnetization and increased viscosity when exposed to a magnetic field, magnetic fluids
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As a key component of hydraulic control systems, hydraulic servovalves influence their performance significantly. Unpredictable self-excited noise inside hydraulic servovalves may cause instability and even failure. Being functional, with higher saturation magnetization and increased viscosity when exposed to a magnetic field, magnetic fluids (MFs) have been widely used in dampers, sealing, and biomedical treatment. In this paper, magnetic fluids are applied in the torque motor of a hydraulic servovalve to exert damping and resistance for vibration and noise suppression. Construction of the torque motor armature with magnetic fluids is introduced and the forces due to magnetic fluids on the torque motor armature are studied. Based on a bi-viscosity-constituted relationship, a mathematical model of the damping force from magnetic fluids is built when magnetic fluids are filled in the working gaps of the torque motor. Measurements of the properties of an Fe3O4 composite magnetic fluid are carried out to calculate the parameters of this mathematical model and to investigate the influence of magnetic fluids on the vibration characteristics of the armature assembly. The simulated and tested harmonic responses of the armature with and without magnetic fluids show the good suppression effects of magnetic fluids on the self-excited noise inside the servovalve. Full article
(This article belongs to the Special Issue Energy Efficiency and Controllability of Fluid Power Systems)
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Open AccessArticle Measurement of Soot Volume Fraction and Temperature for Oxygen-Enriched Ethylene Combustion Based on Flame Image Processing
Energies 2017, 10(6), 750; doi:10.3390/en10060750
Received: 27 March 2017 / Revised: 28 April 2017 / Accepted: 18 May 2017 / Published: 27 May 2017
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Abstract
A method for simultaneously visualizing the two-dimensional distributions of temperature and soot volume fraction in an ethylene flame was presented. A single-color charge-coupled device (CCD) camera was used to capture the flame image in the visible spectrum considering the broad-response spectrum of the
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A method for simultaneously visualizing the two-dimensional distributions of temperature and soot volume fraction in an ethylene flame was presented. A single-color charge-coupled device (CCD) camera was used to capture the flame image in the visible spectrum considering the broad-response spectrum of the R and G bands of the camera. The directional emissive power of the R and G bands were calibrated and used for measurement. Slightly increased temperatures and reduced soot concentration were predicted in the central flame without self-absorption effects considered, an iterative algorithm was used for eliminating the effect of self-absorption. Nine different cases were presented in the experiment to demonstrate the effects of fuel mass flow rate and oxygen concentration on temperature and soot concentration in three different atmospheres. For ethylene combustion in pure-air atmosphere, as the fuel mass flow rate increased, the maximum temperature slightly decreased, and the maximum soot volume fraction slightly increased. For oxygen fractions of 30%, 40%, and 50% combustion in O2/N2 oxygen-enhanced atmospheres, the maximum flame temperatures were 2276, 2451, and 2678 K, whereas combustion in O2/CO2 atmospheres were 1916, 2322, and 2535 K. The maximum soot volume fractions were 4.5, 7.0, and 9.5 ppm in oxygen-enriched O2/N2 atmosphere and 13.6, 15.3, and 14.8 ppm in oxygen-enriched O2/CO2 atmosphere. Compared with the O2/CO2 atmosphere, combustion in the oxygen-enriched O2/N2 atmosphere produced higher flame temperature and larger soot volume fraction. Preliminary results indicated that this technique is reliable and can be used for combustion diagnosis. Full article
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Open AccessArticle Gas Flow Behavior of Nanoscale Pores in Shale Gas Reservoirs
Energies 2017, 10(6), 751; doi:10.3390/en10060751
Received: 5 April 2017 / Revised: 13 May 2017 / Accepted: 22 May 2017 / Published: 27 May 2017
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Abstract
The gas transport in shale nanopores is always one of the major concerns in terms of the development of shale gas reservoirs. In this study, the gas flow regimes in shale nanopores were classified and analyzed according to Knudsen number. Then the gas
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The gas transport in shale nanopores is always one of the major concerns in terms of the development of shale gas reservoirs. In this study, the gas flow regimes in shale nanopores were classified and analyzed according to Knudsen number. Then the gas flow model considering Darcy flow, slip flow, transition flow, molecular free flow and adsorption effect was proposed to evaluate the gas flow behavior in shale nanopores. The result shows that the contributions of Darcy flow, slip flow and transition flow in shale nanopores are reciprocal, and are mainly dominated by pore radius and pressure. The adsorption effect greatly influences the total mass flux. The total mass flux will increase as Langmuir pressure and temperature increase while it will decrease with reservoir pressure and the adsorption thickness. These results can provide insights for a better understanding of gas flow in the shale nanopores so as to optimize the production performance of shale gas reservoirs. Full article
(This article belongs to the Section Energy Fundamentals and Conversion)
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Open AccessArticle Control Method Based on Demand Response Needs of Isolated Bus Regulation with Series-Resonant Converters for Residential Photovoltaic Systems
Energies 2017, 10(6), 752; doi:10.3390/en10060752
Received: 24 April 2017 / Revised: 14 May 2017 / Accepted: 19 May 2017 / Published: 27 May 2017
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Abstract
Considering the effects of isolation and high efficiency, a series-resonant DC-DC converter (L-L-C type, with two inductors and a capacitor) has been introduced into a residential photovoltaic (PV) generation and storage system in this work, and a voltage gain curve upwarp drifting problem
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Considering the effects of isolation and high efficiency, a series-resonant DC-DC converter (L-L-C type, with two inductors and a capacitor) has been introduced into a residential photovoltaic (PV) generation and storage system in this work, and a voltage gain curve upwarp drifting problem was found. In this paper, the reason of upwarp drifting in the voltage gain curve is given, and a new changing topological control method to solve the voltage regulation problem under light load conditions is proposed. Firstly, the ideal and actual first harmonic approximation (FHA) models are given, and this drifting problem is ascribed to the multiple peaks of higher-order resonance between resonant tank and parasitic capacitors. Then the paper presents the pulse-frequency-modulation (PFM) driver signals control method to translate the full-bridge LLC into a half-bridge LLC converter, and with this method the voltage gain could easily be reduced by half. Based on this method, the whole voltage and resonant current sharing control methods in on-line and off-line mode are proposed. The parameters design and optimization methods are also discussed in detail. Finally, a residential PV system platform based on the proposed parallel 7-kW full-bridge LLC converter is built to verify the proposed control method and theoretical analysis. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Design and Implementation of Low-Power Analog-to-Information Conversion for Environmental Information Perception
Energies 2017, 10(6), 753; doi:10.3390/en10060753
Received: 22 March 2017 / Revised: 17 May 2017 / Accepted: 22 May 2017 / Published: 27 May 2017
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Abstract
Because sensing nodes typically have limited power resources, it is extremely important for signals to be acquired with high efficiency and low power consumption, especially in large-scale wireless sensor networks (WSNs) applications. An emerging signal acquisition and compression method called compressed sensing (CS)
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Because sensing nodes typically have limited power resources, it is extremely important for signals to be acquired with high efficiency and low power consumption, especially in large-scale wireless sensor networks (WSNs) applications. An emerging signal acquisition and compression method called compressed sensing (CS) is a notable alternative to traditional signal processing methods and is a feasible solution for WSNs. In our previous work, we studied several data recovery algorithms and network models that use CS for compressive sampling and signal recovery. The results were validated on large data sets from actual environmental monitoring WSNs. In this paper, we focus on the hardware solution for signal acquisition and processing on separate end nodes. We propose the paradigm of an analog-to-information converter (AIC) based on CS theory. The system model consists of a modulation module, filtering module, and sampling module, and was simulated and analyzed in a MATLAB/Simulink 7.0 environment. Further, the hardware design and implementation of an improved digital AIC system is presented. We also study the performances of three different greedy data recovery algorithms and analyze the system power consumption. The experimental results show that, for normal environmental signals, the new system overcomes the Nyquist limit and exhibits good recovery performance with a low sampling frequency, which is suitable for environmental monitoring based on WSNs. Full article
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Open AccessArticle Techno-Economic Investigation of Solar Powered Electric Auto-Rickshaw for a Sustainable Transport System
Energies 2017, 10(6), 754; doi:10.3390/en10060754
Received: 4 April 2017 / Revised: 4 May 2017 / Accepted: 24 May 2017 / Published: 28 May 2017
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Abstract
Technologies influencing alternative means of transportation have been expanding in recent years due to increasing urbanization and motorization. In this paper, a solar powered electric auto-rickshaw (SPEA) is designed and developed for Indian conditions. The vehicle developed is comprehensively analyzed techno-economically for its
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Technologies influencing alternative means of transportation have been expanding in recent years due to increasing urbanization and motorization. In this paper, a solar powered electric auto-rickshaw (SPEA) is designed and developed for Indian conditions. The vehicle developed is comprehensively analyzed techno-economically for its viability in the Indian market. The performance analysis of SPEA results in an optimal charging rate of 2 kWh per day with an average solar irradiance of 325 W/m2 on a typical sunny day. The discharging characteristics are studied based on different loading conditions. The vehicle achieved a maximum speed of 21.69 km/h with battery discharge rate of 296 W at 90 kg load and also reached a maximum discharge rate of 540 W at 390 kg loading with a maximum speed of 12.11 km/h. Environmental analysis of SPEA indicated that the yearly CO2 emissions of 1777 kg, 1987 kg and 1938 kg from using Compressed Natural Gas, Liquefied Petroleum Gas and gasoline engines respectively can be mitigated using SPEA. The financial analysis of SPEA concluded that the investor’s payback duration is 24.44% less compared to a gasoline-run vehicle. Socio-Economic analysis of SPEA discussed its significant advantages and showed 15.74% and 0.85% increase in yearly income over gasoline driven and battery driven vehicles. Full article
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Open AccessArticle Effect of Gas Velocity Distribution on Heat Recovery Process in Packed Bed of Plate-Shaped Slag
Energies 2017, 10(6), 755; doi:10.3390/en10060755
Received: 31 March 2017 / Revised: 22 May 2017 / Accepted: 23 May 2017 / Published: 28 May 2017
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Abstract
A new twin-roll continuous slag solidification process and heat recovery process from a slag packed bed was developed for utilization of the waste heat of steelmaking slag. Plate-shaped slag with the thickness about 7 mm was successfully produced in a pilot plant, and
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A new twin-roll continuous slag solidification process and heat recovery process from a slag packed bed was developed for utilization of the waste heat of steelmaking slag. Plate-shaped slag with the thickness about 7 mm was successfully produced in a pilot plant, and the sensible heat of the slag was recovered by blowing air into the slag chamber. However, the gas distribution inside the slag packed bed was unclear because of the unique shape of the slag plates, and this remained a concern for further scale-up designing of the slag chamber. Therefore, in order to estimate the gas distribution in the packed bed, a simple computational fluid dynamics (CFD) model which considers the wall effect around the inner wall of the chamber was developed, and this model was fitted to the results of laboratory-scale velocity distribution measurements. The results showed that the gas velocity distribution was properly estimated, and the intensity of the wall effect was similar in both cases. As the next step, the gas velocity distribution and its effect on the slag heat recovery process in a pilot-scale slag chamber were evaluated with the assistance of the CFD simulation model. The simulation results were compared with the measured data obtained in a pilot-scale test, and as the result, a similar wall effect was also observed in the pilot-scale chamber. However, the intensity of the wall effect was limited enough to prevent serious deterioration of the uniformity of the gas distribution. Full article
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Open AccessArticle Failure Mechanical Behavior of Australian Strathbogie Granite at High Temperatures: Insights from Particle Flow Modeling
Energies 2017, 10(6), 756; doi:10.3390/en10060756
Received: 30 November 2016 / Revised: 4 May 2017 / Accepted: 15 May 2017 / Published: 28 May 2017
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Abstract
Thermally induced damage has an important influence on rock mechanics and engineering, especially for high-level radioactive waste repositories, geological carbon storage, underground coal gasification, and hydrothermal systems. Additionally, the wide application of geothermal heat requires knowledge of the geothermal conditions of reservoir rocks
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Thermally induced damage has an important influence on rock mechanics and engineering, especially for high-level radioactive waste repositories, geological carbon storage, underground coal gasification, and hydrothermal systems. Additionally, the wide application of geothermal heat requires knowledge of the geothermal conditions of reservoir rocks at elevated temperature. However, few methods to date have been reported for investigating the micro-mechanics of specimens at elevated temperatures. Therefore, this paper uses a cluster model in particle flow code in two dimensions (PFC2D) to simulate the uniaxial compressive testing of Australian Strathbogie granite at various elevated temperatures. The peak strength and ultimate failure mode of the granite specimens at different elevated temperatures obtained by the numerical methods are consistent with those obtained by experimentation. Since the tensile force is always concentrated around the boundary of the crystal, cracks easily occur at the intergranular contacts, especially between the b-b and b-k boundaries where less intragranular contact is observed. The intergranular and intragranular cracking of the specimens is almost constant with increasing temperature at low temperature, and then it rapidly and linearly increases. However, the inflection point of intergranular micro-cracking is less than that of intragranular cracking. Intergranular cracking is more easily induced by a high temperature than intragranular cracking. At an elevated temperature, the cumulative micro-crack counts curve propagates in a stable way during the active period, and it has no unstable crack propagation stage. The micro-cracks and parallel bond forces in the specimens with elevated temperature evolution and axial strain have different characteristics than those at lower temperature. More branch fractures and isolated wider micro-cracks are generated with increasing temperature when the temperature is over 400 °C. Therefore, the total number of cracks is almost constant when the temperature is below 400 °C; next, it linearly increases when the temperature is over 400 °C. This trend is the same as that observed by experimentation. Full article
(This article belongs to the Special Issue Unconventional Natural Gas (UNG) Recoveries)
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Open AccessArticle Wind Turbine Wake Mitigation through Blade Pitch Offset
Energies 2017, 10(6), 757; doi:10.3390/en10060757
Received: 7 March 2017 / Revised: 18 May 2017 / Accepted: 26 May 2017 / Published: 29 May 2017
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Abstract
The reduction in power output associated with complex turbine-wake interactions in wind farms necessitates the development of effective wake mitigation strategies. One approach to this end entails the downregulation of individual turbines from its maximum power point with the objective of optimizing the
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The reduction in power output associated with complex turbine-wake interactions in wind farms necessitates the development of effective wake mitigation strategies. One approach to this end entails the downregulation of individual turbines from its maximum power point with the objective of optimizing the overall wind farm productivity. Downregulation via blade pitch offset has been of interest as a potential strategy, though the viability of this method is still not clear, especially in regard to its sensitivity to ambient turbulence. In this study, large-eddy simulations of a two-turbine arrangement, with the second turbine in the full wake of the first, were performed. The effects of varying the blade pitch angle of the upstream turbine on its wake characteristics, as well as the combined power of the two, were investigated. Of specific interest was the effect of turbulence intensity of the inflow on the efficacy of this method. Results showed enhanced wake recovery associated with pitching to stall, as opposed to pitching to feather, which delayed wake recovery. The increased wake recovery resulted in a noticeable increase in the power of the two-turbine configuration, only in conditions characterized by low turbulence in the incoming flow. Nevertheless, the low turbulence scenarios where the use of this method is favorable, are expected in realistic wind farms, suggesting its possible application for improved power generation. Full article
(This article belongs to the Section Energy Fundamentals and Conversion)
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Open AccessArticle Modified Synchronous Reference Frame Based Shunt Active Power Filter with Fuzzy Logic Control Pulse Width Modulation Inverter
Energies 2017, 10(6), 758; doi:10.3390/en10060758
Received: 22 February 2017 / Revised: 16 May 2017 / Accepted: 18 May 2017 / Published: 29 May 2017
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Abstract
Harmonic distortion in power networks has greatly reduced power quality and this affects system stability. In order to mitigate this power quality issue, the shunt active power filter (SAPF) has been widely applied and it is proven to be the best solution to
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Harmonic distortion in power networks has greatly reduced power quality and this affects system stability. In order to mitigate this power quality issue, the shunt active power filter (SAPF) has been widely applied and it is proven to be the best solution to current harmonics. This paper evaluates the performance of the modified synchronous reference frame extraction (MSRF) algorithm with fuzzy logic controller (FLC) based current control pulse width modulation (PWM) inverter of three-phase three-wire SAPF to mitigate current harmonics. The proposed FLC is designed with a reduced amount of membership functions (MFs) and rules, and thus significantly reduces the computational time and memory size. Modeling and simulations of SAPF are carried out using MATLAB/Simulink R2012a with the power system toolbox under steady-state condition, and this is followed with hardware implementation using a TMS320F28335 digital signal processor (DSP), Specrum Digital Inc., Stafford, TX, USA. The results obtained demonstrate a good and satisfactory response to mitigate the harmonics in the system. The total harmonic distortion (THD) for the system has been reduced from 25.60% to 0.92% and 1.41% in the simulation study with and without FLC, respectively. Similarly for the experimental study, the SAPF can compensate for the three-phase load current by reducing THD to 5.07% and 7.4% with and without FLC, respectively. Full article
(This article belongs to the Special Issue Power Electronics in Power Quality)
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Open AccessArticle Optimizing the Renewable Energy Potential: Myth or Future Trend in Romania
Energies 2017, 10(6), 759; doi:10.3390/en10060759
Received: 9 March 2017 / Revised: 10 May 2017 / Accepted: 25 May 2017 / Published: 29 May 2017
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Abstract
This paper investigates the potential of Romanian renewable energy across energy production and consumption definitions. Moreover, it presents a model that estimates the regional evolution of renewable energy diffusion, focusing primarily on hydro energy. Statistics showed that Romanian consumers have a key role
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This paper investigates the potential of Romanian renewable energy across energy production and consumption definitions. Moreover, it presents a model that estimates the regional evolution of renewable energy diffusion, focusing primarily on hydro energy. Statistics showed that Romanian consumers have a key role in developing the renewable energy market, while it is also very important to ensure the ability of renewable energy suppliers to accommodate this demand. Sample data and econometric analysis were applied to delineate factors influencing Romanian consumers’ awareness about renewable energy. The findings highlighted a strong connection between those who bear the cost of renewable energy development and the actual beneficiaries of a clean environment; in that sense, they identify the major drivers and barriers for renewable energy diffusion in Romania. The results of this research could be used for further research in the area of implementing renewable energy projects in the region. Full article
(This article belongs to the Special Issue Sustainable and Renewable Energy Systems)
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Open AccessArticle Robust Frequency and Voltage Stability Control Strategy for Standalone AC/DC Hybrid Microgrid
Energies 2017, 10(6), 760; doi:10.3390/en10060760
Received: 21 March 2017 / Revised: 18 May 2017 / Accepted: 25 May 2017 / Published: 30 May 2017
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Abstract
The microgrid (MG) concept is attracting considerable attention as a solution to energy deficiencies, especially in remote areas, but the intermittent nature of renewable sources and varying loads cause many control problems and thereby affect the quality of power within a microgrid operating
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The microgrid (MG) concept is attracting considerable attention as a solution to energy deficiencies, especially in remote areas, but the intermittent nature of renewable sources and varying loads cause many control problems and thereby affect the quality of power within a microgrid operating in standalone mode. This might cause large frequency and voltage deviations in the system due to unpredictable output power fluctuations. Furthermore, without any main grid support, it is more complex to control and manage the system. In past, droop control and various other coordination control strategies have been presented to stabilize the microgrid frequency and voltages, but in order to utilize the available resources up to their maximum capacity in a positive way, new and robust control mechanisms are required. In this paper, a standalone microgrid is presented, which integrates renewable energy-based distributed generations and local loads. A fuzzy logic-based intelligent control technique is proposed to maintain the frequency and DC (direct current)-link voltage stability for sudden changes in load or generation power. Also from a frequency control perspective, a battery energy storage system (BESS) is suggested as a replacement for a synchronous generator to stabilize the nominal system frequency as a synchronous generator is unable to operate at its maximum efficiency while being controlled for stabilization purposes. Likewise, a super capacitor (SC) and BESS is used to stabilize DC bus voltages even though maximum possible energy is being extracted from renewable generated sources using maximum power point tracking. This newly proposed control method proves to be effective by reducing transient time, minimizing the frequency deviations, maintaining voltages even though maximum power point tracking is working and preventing generators from exceeding their power ratings during disturbances. However, due to the BESS limited capacity, load switching (load shedding scheme) as last option is also introduced in this paper. Simulation results prove the effectiveness of the proposed control strategy from both frequency and voltage perspectives. Full article
(This article belongs to the Special Issue Advanced Operation and Control of Smart Microgrids)
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Open AccessArticle Policy and Environmental Implications of Photovoltaic Systems in Farming in Southeast Spain: Can Greenhouses Reduce the Greenhouse Effect?
Energies 2017, 10(6), 761; doi:10.3390/en10060761
Received: 21 January 2017 / Revised: 5 May 2017 / Accepted: 25 May 2017 / Published: 31 May 2017
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Abstract
Solar photovoltaic (PV) systems have grown in popularity in the farming sector, primarily because land area and farm structures themselves, such as greenhouses, can be exploited for this purpose, and, moreover, because farms tend to be located in rural areas far from energy
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Solar photovoltaic (PV) systems have grown in popularity in the farming sector, primarily because land area and farm structures themselves, such as greenhouses, can be exploited for this purpose, and, moreover, because farms tend to be located in rural areas far from energy production plants. In Spain, despite being a country with enormous potential for this renewable energy source, little is being done to exploit it, and policies of recent years have even restricted its implementation. These factors constitute an obstacle, both for achieving environmental commitments and for socioeconomic development. This study proposes the installation of PV systems on greenhouses in southeast Spain, the location with the highest concentration of greenhouses in Europe. Following a sensitivity analysis, it is estimated that the utilization of this technology in the self-consumption scenario at farm level produces increased profitability for farms, which can range from 0.88% (worst scenario) to 52.78% (most favorable scenario). Regarding the Spanish environmental policy, the results obtained demonstrate that the impact of applying this technology mounted on greenhouses would bring the country 38% closer to reaching the 2030 greenhouse gas (GHG) target. Furthermore, it would make it possible to nearly achieve the official commitment of 20% renewable energies by 2020. Additionally, it would have considerable effects on the regional socioeconomy, with increases in job creation and contribution to gross domestic product (GDP)/R&D (Research and Development), allowing greater profitability in agrifood activities throughout the entire region. Full article
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Open AccessArticle A Step towards the Hydrogen Economy—A Life Cycle Cost Analysis of A Hydrogen Refueling Station
Energies 2017, 10(6), 763; doi:10.3390/en10060763
Received: 8 April 2017 / Revised: 25 May 2017 / Accepted: 26 May 2017 / Published: 31 May 2017
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Abstract
This study was aimed to define a methodology based on existing literature and evaluate the levelized cost of hydrogen (LCOH) for a decentralized hydrogen refueling station (HRS) in Halle, Belgium. The results are based on a comprehensive data collection, along with real cost
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This study was aimed to define a methodology based on existing literature and evaluate the levelized cost of hydrogen (LCOH) for a decentralized hydrogen refueling station (HRS) in Halle, Belgium. The results are based on a comprehensive data collection, along with real cost information. The main results indicated that a LCOH of 10.3 €/kg at the HRS can be reached over a lifetime of 20 years, if an average electricity cost of 0.04 €/kWh could be achieved and if the operating hours are maximized. Furthermore, if the initial capital costs can be reduced by 80%, in the case of direct subsidy, the LCOH could even fall to 6.7 €/kg. Full article
(This article belongs to the Special Issue Sustainable and Renewable Energy Systems)
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Open AccessArticle Improved OCV Model of a Li-Ion NMC Battery for Online SOC Estimation Using the Extended Kalman Filter
Energies 2017, 10(6), 764; doi:10.3390/en10060764
Received: 22 March 2017 / Revised: 16 May 2017 / Accepted: 26 May 2017 / Published: 31 May 2017
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Abstract
Accurate modeling of the nonlinear relationship between the open circuit voltage (OCV) and the state of charge (SOC) is required for adaptive SOC estimation during the lithium-ion (Li-ion) battery operation. Online SOC estimation should meet several constraints, such as the computational cost, the
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Accurate modeling of the nonlinear relationship between the open circuit voltage (OCV) and the state of charge (SOC) is required for adaptive SOC estimation during the lithium-ion (Li-ion) battery operation. Online SOC estimation should meet several constraints, such as the computational cost, the number of parameters, as well as the accuracy of the model. In this paper, these challenges are considered by proposing an improved simplified and accurate OCV model of a nickel manganese cobalt (NMC) Li-ion battery, based on an empirical analytical characterization approach. In fact, composed of double exponential and simple quadratic functions containing only five parameters, the proposed model accurately follows the experimental curve with a minor fitting error of 1 mV. The model is also valid at a wide temperature range and takes into account the voltage hysteresis of the OCV. Using this model in SOC estimation by the extended Kalman filter (EKF) contributes to minimizing the execution time and to reducing the SOC estimation error to only 3% compared to other existing models where the estimation error is about 5%. Experiments are also performed to prove that the proposed OCV model incorporated in the EKF estimator exhibits good reliability and precision under various loading profiles and temperatures. Full article
(This article belongs to the Special Issue Advances in Electric Vehicles and Plug-in Hybrid Vehicles 2017)
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Open AccessFeature PaperArticle Treatment of Slaughterhouse Waste Water Mixed with Serum from Lacteal Industry of Extremadura in Spain to Produce Clean Energy
Energies 2017, 10(6), 765; doi:10.3390/en10060765
Received: 3 April 2017 / Revised: 12 May 2017 / Accepted: 24 May 2017 / Published: 31 May 2017
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Abstract
The problem of slaughterhouse waste water can be resolved by mixing it with serum from lacteal industry to produce a biogas. The effect of serum addition on the anaerobic co-digestion of solid and liquid slaughterhouse waste has been studied. The experimental device consisted
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The problem of slaughterhouse waste water can be resolved by mixing it with serum from lacteal industry to produce a biogas. The effect of serum addition on the anaerobic co-digestion of solid and liquid slaughterhouse waste has been studied. The experimental device consisted of a continuous digester by recirculation of biogas produced in the anaerobic digestion. The input effluent was a mixture of slaughterhouse waste from Badajoz city (Spain) and animal serum in a proportion of 20%. The anaerobic digestion was developed in a complete mixing continuous digester with a capacity of 6.2 L at 37 °C and a feed rate of 350 mL/day. From the results obtained for the co-digestion of the feeding effluent of the slaughterhouse waste, without and with serum added, in the same operating conditions, comparative data about the biological depuration and biogas production have been obtained. A 10 L biogas production was obtained with the slaughterhouse waste and 18 L with the slaughterhouse waste with serum added. In conclusion, the highest energetic yield (97.52% higher) was obtained in the second case, due to the positive action of catalytic enzymes present in the animal serum. Full article
(This article belongs to the Special Issue Energy and Water, Current and Future Crisis)
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Open AccessArticle Investigation on the Effect of the Gas Exchange Process on the Diesel Engine Thermal Overload with Experimental Results
Energies 2017, 10(6), 766; doi:10.3390/en10060766
Received: 30 March 2017 / Revised: 16 May 2017 / Accepted: 26 May 2017 / Published: 31 May 2017
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Abstract
In this paper, the influence of the gas exchange process on the diesel engine thermal overload is provided. Main components involved in the gas exchange process are discussed. The ambient conditions, the turbocharger performance, and the valve timing that affect the gas exchange
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In this paper, the influence of the gas exchange process on the diesel engine thermal overload is provided. Main components involved in the gas exchange process are discussed. The ambient conditions, the turbocharger performance, and the valve timing that affect the gas exchange process have been investigated. Experiments were conducted to simulate ambient conditions at different geographical locations and demonstrated a decrease in oxygen concentration in the exhaust as the humidity level in the air increased. Additionally, the effect of an inefficient turbocharger on an engine operating at part-load was also investigated. It was observed that an overly lean air/fuel mixture caused inefficient scavenging and the corresponding level of residual gas trapped in the cylinder increased. This resulted in partial combustion which could be observed as white smoke from the engine exhaust stack, therefore indicating the presence of unburnt fuel. Exhaust valve timing measurements showed that the cylinder with the highest wear rate had its valve closure timing 10 crank angle degrees after the cylinder with least wear rate. The exhaust valves were closed earlier than the designed condition which impaired the scavenging process and increased the level of residual gas trapped in the cylinder. This resulted in a reduction of the actual air-to-fuel ratio and high exhaust gas temperatures. Full article
(This article belongs to the Special Issue Internal Combustion Engines)
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Open AccessArticle Modeling and Scaling of the Viscosity of Suspensions of Asphaltene Nanoaggregates
Energies 2017, 10(6), 767; doi:10.3390/en10060767
Received: 27 February 2017 / Revised: 14 May 2017 / Accepted: 25 May 2017 / Published: 1 June 2017
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Abstract
The scaling and modeling of the viscosity of suspensions of asphaltene nanoaggregates is carried out successfully taking into consideration the solvation and clustering of nanoaggragates, and the jamming of the suspension at the glass transition volume fraction of asphaltene nanoaggregates. The nanoaggregates of
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The scaling and modeling of the viscosity of suspensions of asphaltene nanoaggregates is carried out successfully taking into consideration the solvation and clustering of nanoaggragates, and the jamming of the suspension at the glass transition volume fraction of asphaltene nanoaggregates. The nanoaggregates of asphaltenes are modeled as solvated disk-shaped “core–shell” particles taking into account the most recent small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and solid-state 1H NMR studies available on the size and structure of asphaltene nanoaggregates. This work is an extension of our earlier studies on modeling of asphaltene suspensions where solvation of asphaltene nanoaggregates was not considered. A new mathematical model is developed for estimating the aspect ratio (ratio of thickness to diameter of particle) and the corresponding intrinsic viscosity of suspension of solvated disk-shaped asphaltene nanoaggregates using the experimental relative viscosity data of suspensions at low asphaltene concentrations. The solvation of asphaltene nanoaggregates is found to be significant. The intrinsic viscosity increases with the increase in the degree of solvation of nanoaggregates. At high concentrations of asphaltenes, clustering of solvated nanoaggregates dominates resulting in large viscosities. A new scaling law is discovered to scale the viscosity data of different asphaltene suspensions. According to the new scaling law, a unique correlation is obtained, independent of the type of asphaltene system, when the data are plotted as ( η r 1 ) / [ η ] S versus ϕ S where η r is the relative viscosity of suspension, [ η ] S is the intrinsic viscosity of suspension of solvated nanoaggregates, and ϕ S is the volume fraction of solvated nanoaggregates. Twenty sets of experimental viscosity data on asphaltene suspensions gathered from different sources are used to verify and confirm the scaling law and the viscosity model proposed in this work. Based on the experimental data, the glass transition volume fraction of solvated asphaltene nanoaggregates where jamming of suspension, and hence divergence of viscosity, takes place is found to be approximately 0.4. The viscosity model proposed in this work can be used to predict the viscosity of a new asphaltene system over a broad range of asphaltene concentrations provided that the intrinsic viscosity of the suspension is obtained from viscosity measurements at very low asphaltene concentrations. Full article
(This article belongs to the Special Issue Oil and Gas Engineering)
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Open AccessArticle A Behavioral Economics Approach to Residential Electricity Consumption
Energies 2017, 10(6), 768; doi:10.3390/en10060768
Received: 20 April 2017 / Revised: 27 May 2017 / Accepted: 29 May 2017 / Published: 1 June 2017
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Abstract
Consumer behavior is complex and is difficult to represent in traditional economic theories of decision-making. This paper focuses on the development of an agent-based approach to analyze people’s behavior in consuming electricity using a behavioral economics framework, where the consumer is the main
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Consumer behavior is complex and is difficult to represent in traditional economic theories of decision-making. This paper focuses on the development of an agent-based approach to analyze people’s behavior in consuming electricity using a behavioral economics framework, where the consumer is the main agent of power systems. This approach may bring useful insights for distribution companies and regulatory agencies, helping to shift thinking to a more user-centric approach. The emergent properties of electricity consumption are modeled by the means of consumer’s heuristics, taking into account the electricity price, consumer’s satisfaction level, willingness to invest in new technologies, social interactions, and marketing strategies by the power utility. Analysis on the emergent behavior of this approach through simulation studies showed that it is indeed valuable, as does not require in-depth data of all details on human behavior. However, it contributes to the understanding of relations among various objects involved in electricity consumption. Full article
(This article belongs to the Special Issue Innovative Methods for Smart Grids Planning and Management)
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Open AccessArticle Physical and Numerical Modeling of the Stability of Deep Caverns in Tahe Oil Field in China
Energies 2017, 10(6), 769; doi:10.3390/en10060769
Received: 4 April 2017 / Revised: 26 May 2017 / Accepted: 27 May 2017 / Published: 1 June 2017
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Abstract
Cave collapses emerge during the process of oil reservoir development, seriously affecting oil production. To reveal the collapse and failure mechanism of the carbonate cavern with a buried depth of 5600 m in Tahe Oil Field, using a self-developed ultra-high pressure model test
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Cave collapses emerge during the process of oil reservoir development, seriously affecting oil production. To reveal the collapse and failure mechanism of the carbonate cavern with a buried depth of 5600 m in Tahe Oil Field, using a self-developed ultra-high pressure model test system with the intelligent numerical control function, the model simulation material of carbonate rocks developed to carry out the 3D geo-mechanical model test. The model test and numerical results indicate that: (1) collapse and failure mechanism of the deep-buried caves mainly involve the failure mode of tensile shear. The rupture plane on the side wall is approximately parallel to the direction of maximum principal compressive stress. The V-type tension and split rupture plane then emerges. (2) In the process of forming holes in the model caverns, micro cracks are generated at the foot of the left and right side walls of the caverns, and the roof panels are constantly moving downward. The shorter the distance to the cave wall, the severer the destructiveness of the surrounding rocks will be. (3) The displacement of the top of the model cavern is relatively large and uniform, indicating that the cave roof moves downward as a whole. The area of the cavity suffering damage is 2.3 times as large as the cave span. The research results in this paper lay a solid test basis for revealing the cave collapse and failure mechanism in super depth. Full article
(This article belongs to the Section Energy Sources)
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Open AccessArticle Wind Turbine Synchronous Reset Pitch Control
Energies 2017, 10(6), 770; doi:10.3390/en10060770
Received: 6 April 2017 / Revised: 22 May 2017 / Accepted: 27 May 2017 / Published: 1 June 2017
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Abstract
Reset controllers are commonly used to smooth the transient response of systems. We use this technique to improve a standard baseline pitch controller for offshore wind turbines (WTs). The introduction of this strategy enhances the overall performance of the WT. In particular, the
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Reset controllers are commonly used to smooth the transient response of systems. We use this technique to improve a standard baseline pitch controller for offshore wind turbines (WTs). The introduction of this strategy enhances the overall performance of the WT. In particular, the fore-aft and side-to-side accelerations of the WT tower are significantly reduced, whilst a steadier power output is obtained, in comparison to the standard baseline pitch controller. Furthermore, our designed pitch control’s main advantage, with respect to the baseline, is its ease of implementation and reduced complexity as it does not require a gain-scheduling technique, nor pitch position measurement (thus, it is insensitive to pitch sensor faults). The proposed approach has been simulated on the NREL 5-MW prototype offshore turbine model, mounted on a jacket support. The simulations are carried out using the aero-hydro-servo-elastic simulator FAST, and key observations are thoroughly discussed. Full article
(This article belongs to the collection Wind Turbines)
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Open AccessArticle Sizing Combined Heat and Power Units and Domestic Building Energy Cost Optimisation
Energies 2017, 10(6), 771; doi:10.3390/en10060771
Received: 17 March 2017 / Revised: 8 May 2017 / Accepted: 26 May 2017 / Published: 1 June 2017
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Abstract
Many combined heat and power (CHP) units have been installed in domestic buildings to increase energy efficiency and reduce energy costs. However, inappropriate sizing of a CHP may actually increase energy costs and reduce energy efficiency. Moreover, the high manufacturing cost of batteries
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Many combined heat and power (CHP) units have been installed in domestic buildings to increase energy efficiency and reduce energy costs. However, inappropriate sizing of a CHP may actually increase energy costs and reduce energy efficiency. Moreover, the high manufacturing cost of batteries makes batteries less affordable. Therefore, this paper will attempt to size the capacity of CHP and optimise daily energy costs for a domestic building with only CHP installed. In this paper, electricity and heat loads are firstly used as sizing criteria in finding the best capacities of different types of CHP with the help of the maximum rectangle (MR) method. Subsequently, the genetic algorithm (GA) will be used to optimise the daily energy costs of the different cases. Then, heat and electricity loads are jointly considered for sizing different types of CHP and for optimising the daily energy costs through the GA method. The optimisation results show that the GA sizing method gives a higher average daily energy cost saving, which is 13% reduction compared to a building without installing CHP. However, to achieve this, there will be about 3% energy efficiency reduction and 7% input power to rated power ratio reduction compared to using the MR method and heat demand in sizing CHP. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Comparative Energy Performance Analysis of Six Primary Photovoltaic Technologies in Madrid (Spain)
Energies 2017, 10(6), 772; doi:10.3390/en10060772
Received: 21 March 2017 / Revised: 21 March 2017 / Accepted: 9 May 2017 / Published: 1 June 2017
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Abstract
There are a wealth of studies on photovoltaic cell technologies, however their performance in different climatic or geographies over an extended period is not completely established. The objective of this paper is to add to this area of study with an analysis of
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There are a wealth of studies on photovoltaic cell technologies, however their performance in different climatic or geographies over an extended period is not completely established. The objective of this paper is to add to this area of study with an analysis of the principle photovoltaic technologies: monocrystalline silicon (mc-Si), polycrystalline silicon (pc-Si), tandem structure of amorphous silicon and microcrystalline silicon (a-Si/µc-Si tandem), cadmium telluride/cadmium sulfide (CdTe/CdS), copper-indium diselenide (CIS) and monocrystalline silicon with double contact back technology (mc-dc-Si), in the climatic conditions particular to the middle latitude urban environment of Madrid (Spain). To carry out this study six photovoltaic subsystems of peak power 1 kWp approximately have been installed for each selected technology on the roof of the Escuela Técnica Superior de Ingeniería y Diseño Industrial (ETSIDI) of the Universidad Politécnica de Madrid (UPM) in Spain. Each subsystem has an inverter of the same model and power for its connection to the internal electricity network of the university. This paper analyzes the energy performance of the six subsystems, calculating the reference production rates and losses, from February 2013 to December 2015. The result of the study is the extensive capture of data and detailed analysis of real time energy yields and performance ratios of key technologies resulting with patterns in line with those of other regions with comparable climatic conditions. Full article
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Open AccessArticle Thermal Cycle and Combustion Analysis of a Solar-Assisted Micro Gas Turbine
Energies 2017, 10(6), 773; doi:10.3390/en10060773
Received: 30 March 2017 / Revised: 24 May 2017 / Accepted: 28 May 2017 / Published: 2 June 2017
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Abstract
The authors discuss in this paper the potential of two power plant concepts for distributed generation, based on the integration of a cogenerating micro gas turbine with a solar panel array. The first one relies on the adoption of a parabolic trough network
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The authors discuss in this paper the potential of two power plant concepts for distributed generation, based on the integration of a cogenerating micro gas turbine with a solar panel array. The first one relies on the adoption of a parabolic trough network with an intermediate thermal carrier, while the second one considers the direct heating of the working air in a solar tower system. The first solution also includes a bottoming organic Rankine cycle (ORC) plant, so that it is mainly addressed to the power output increase. The second one involves a relevant temperature increase of the air entering the combustor, so allowing a direct fuel energy saving, whose amount is strongly variable with both the solar irradiance and the eventual part-load operation. In addition, the latter solar-assisted scheme involves noticeable variations in the conditions for the combustion development. This suggested the authors to proceed with a detailed CFD analysis of the combustion, after a preliminary thermal cycle study for highlighting the main benefits from the solar integration of the power plant. Full article
(This article belongs to the Section Energy Fundamentals and Conversion)
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Open AccessFeature PaperArticle A Generic Matrix Method to Model the Magnetics of Multi-Coil Air-Cored Inductive Power Transfer Systems
Energies 2017, 10(6), 774; doi:10.3390/en10060774
Received: 27 April 2017 / Revised: 25 May 2017 / Accepted: 30 May 2017 / Published: 3 June 2017
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Abstract
This paper deals with a generic methodology to evaluate the magnetic parameters of contactless power transfer systems. Neumann’s integral has been used to create a matrix method that can model the magnetics of single coils (circle, square, rectangle). The principle of superposition has
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This paper deals with a generic methodology to evaluate the magnetic parameters of contactless power transfer systems. Neumann’s integral has been used to create a matrix method that can model the magnetics of single coils (circle, square, rectangle). The principle of superposition has been utilized to extend the theory to multi-coil geometries, such as double circular, double rectangle and double rectangle quadrature. Numerical and experimental validation has been performed to validate the analytical models developed. A rigorous application of the analysis has been carried out to study misalignment and hence the efficacy of various geometries to misalignment tolerance. The comparison of single-coil and multi-coil inductive power transfer systems (MCIPT) considering coupling variation with misalignment, power transferred and maximum efficiency is carried out. Full article
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Open AccessArticle Influence of Stratigraphic Conditions on the Deformation Characteristics of Oil/Gas Wells Piercing Longwall Pillars and Mining Optimization
Energies 2017, 10(6), 775; doi:10.3390/en10060775
Received: 18 April 2017 / Revised: 26 May 2017 / Accepted: 31 May 2017 / Published: 3 June 2017
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Abstract
Hydrocarbon wells drilled vertically through longwall coal pillars are vulnerable to severe deformation and potential failure as a result of underground coal mining. The lithology of the host rocks play a critical role in well stability. In this study, a two dimensional finite
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Hydrocarbon wells drilled vertically through longwall coal pillars are vulnerable to severe deformation and potential failure as a result of underground coal mining. The lithology of the host rocks play a critical role in well stability. In this study, a two dimensional finite element method is employed to investigate the horizontal shear offset, vertical delamination, and compression at the weak interface between neighboring soft and stiff layers after the sequential extraction of longwall panels flanking the protective coal pillar. The influence of stratigraphic conditions, including the single rock layer thickness (SRLT), seam mining height (SMH), and seam dip angle (SDA), on deformation of hydrocarbon wells is explored. An optimization of mining sequence along strike and for panel advance direction along dip is also performed. Finally, some recommendations regarding coal mining and peripheral support measures are suggested. Full article
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Open AccessArticle Enhanced Microgrid Dynamic Performance Using a Modulated Power Filter Based on Enhanced Bacterial Foraging Optimization
Energies 2017, 10(6), 776; doi:10.3390/en10060776
Received: 24 December 2016 / Revised: 22 May 2017 / Accepted: 30 May 2017 / Published: 6 June 2017
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Abstract
This paper presents a design of microgrid (MG) with enhanced dynamic performance. Distributed energy resources (DER) are widely used in MGs to match the various load types and profiles. DERs include solar PV cells, wind energy sources, fuel cells, batteries, micro gas-engines and
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This paper presents a design of microgrid (MG) with enhanced dynamic performance. Distributed energy resources (DER) are widely used in MGs to match the various load types and profiles. DERs include solar PV cells, wind energy sources, fuel cells, batteries, micro gas-engines and storage elements. MG will include AC/DC circuits, developed power electronics devices, inverters and power electronic controllers. A novel modulated power filters (MPF) device will be applied in MG design. Enhanced bacterial foraging optimization (EBFO) will be proposed to optimize and set the MPF parameters to enhance and tune the MG dynamic response. Recent dynamic control is applied to minimize the harmonic reference content. EBFO will adapt the gains of MPF dynamic control. The present research achieves an enhancement of MG dynamic performance, in addition to ensuring improvements in the power factor, bus voltage profile and power quality. MG operation will be evaluated by the dynamic response to be fine-tuned by MPF based on EBFO. Digital simulations have validated the results to show the effectiveness and efficient improvement by the proposed strategy. Full article
(This article belongs to the Special Issue Energy Conservation in Infrastructures 2016)
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Open AccessArticle Design of the OffWindChina 5 MW Wind Turbine Rotor
Energies 2017, 10(6), 777; doi:10.3390/en10060777
Received: 5 March 2017 / Revised: 19 May 2017 / Accepted: 30 May 2017 / Published: 3 June 2017
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Abstract
The current article describes the conceptual design of a rotor for a 5 MW machine situated at an offshore site in China (OffWindChina). The OffWindChina 5 MW rotor design work was divided into two parts between the Technical University of Denmark (DTU) and
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The current article describes the conceptual design of a rotor for a 5 MW machine situated at an offshore site in China (OffWindChina). The OffWindChina 5 MW rotor design work was divided into two parts between the Technical University of Denmark (DTU) and the Chong Qing University (CQU). The two parts consist of the aeroelastic and structural design phases. The aeroelastic part determines the optimal outer blade shape in terms of cost of energy (COE), while the structural part determines the internal laminate layup to achieve a minimum blade mass. Each part is performed sequentially using in-house optimization tools developed at DTU and CQU. The designed blade yields a high energy output while maintaining the structural feasibility with respect to international standards. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Evaluation Methodology for Tariff Design under Escalating Penetrations of Distributed Energy Resources
Energies 2017, 10(6), 778; doi:10.3390/en10060778
Received: 22 March 2017 / Revised: 2 May 2017 / Accepted: 1 June 2017 / Published: 5 June 2017
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Abstract
As the penetration of distributed energy resources (DERs) escalates in distribution networks, new network tariffs are needed to cope with this new situation. These tariffs should allocate network costs to users, promoting an efficient use of the distribution network. This paper proposes a
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As the penetration of distributed energy resources (DERs) escalates in distribution networks, new network tariffs are needed to cope with this new situation. These tariffs should allocate network costs to users, promoting an efficient use of the distribution network. This paper proposes a methodology to evaluate and compare network tariff designs. Four design attributes are proposed for this aim: (i) network cost recovery; (ii) deferral of network reinforcements; (iii) efficient consumer response; (iv) recognition of side-effects on consumers. Through an analytical hierarchy process (AHP), the evaluation methodology is applied to compare traditional cost allocation methods, on the basis of 100% energy, 100% demand, and 50% energy-50% demand, with more advanced pricing methods based on distribution locational marginal prices in combination with cost-reflective network charges. Numerical results are obtained through a case study based on the IEEE 34-node test feeder with DER integration. The results illustrate the advantage of advanced pricing methods to promote an efficient integration of DER and demand price-response from consumers. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle A New Real Time Lyapunov Based Controller for Power Quality Improvement in Unified Power Flow Controllers Using Direct Matrix Converters
Energies 2017, 10(6), 779; doi:10.3390/en10060779
Received: 17 March 2017 / Revised: 22 May 2017 / Accepted: 1 June 2017 / Published: 6 June 2017
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Abstract
This paper proposes a Direct Matrix Converter operating as a Unified Power Flow Controller (DMC-UPFC) with an advanced control method for UPFC, based on the Lyapunov direct method, presenting good results in power quality assessment. This control method is used for real-time calculation
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This paper proposes a Direct Matrix Converter operating as a Unified Power Flow Controller (DMC-UPFC) with an advanced control method for UPFC, based on the Lyapunov direct method, presenting good results in power quality assessment. This control method is used for real-time calculation of the appropriate matrix switching state, determining which switching state should be applied in the following sampling period. The control strategy takes into account active and reactive power flow references to choose the vector converter closest to the optimum. Theoretical principles for this new real-time vector modulation and control applied to the DMC-UPFC with input filter are established. The method needs DMC-UPFC dynamic equations to be solved just once in each control cycle, to find the required optimum vector, in contrast to similar control methods that need 27 vector estimations per control cycle. The designed controller’s performance was evaluated using Matlab/Simulink software. Controllers were also implemented using a digital signal processing (DSP) system and matrix hardware. Simulation and experimental results show decoupled transmission line active (P) and reactive (Q) power control with zero theoretical error tracking and fast response. Output currents and voltages show small ripple and low harmonic content. Full article
(This article belongs to the Special Issue Power Electronics in Power Quality)
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