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Energies, Volume 8, Issue 12 (December 2015)

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Open AccessArticle
Life is Short: The Impact of Power States on Base Station Lifetime
Energies 2015, 8(12), 14407-14426; https://doi.org/10.3390/en81212435
Received: 13 November 2015 / Revised: 24 November 2015 / Accepted: 7 December 2015 / Published: 19 December 2015
Cited by 3 | Viewed by 1850 | PDF Full-text (797 KB) | HTML Full-text | XML Full-text
Abstract
We study the impact of power state transitions on the lifetime of base stations (BSs) in mobile networks. In particular, we propose a model to estimate the lifetime decrease/increase as a consequence of the application of power state changes. The model takes into [...] Read more.
We study the impact of power state transitions on the lifetime of base stations (BSs) in mobile networks. In particular, we propose a model to estimate the lifetime decrease/increase as a consequence of the application of power state changes. The model takes into account both hardware (HW) parameters, which depend on the materials used to build the device, and power state parameters, that instead depend on how and when power state transitions take place. More in depth, we consider the impact of different power states when a BS is active, and one sleep mode state when a BS is powered off. When a BS reduces the power consumption, its lifetime tends to increase. However, when a BS changes the power state, its lifetime tends to be decreased. Thus, there is a tradeoff between these two effects. Our results, obtained over universal mobile telecommunication system (UMTS) and long term evolution (LTE) case studies, indicate the need of a careful management of the power state transitions in order to not deteriorate the BS lifetime, and consequently to not increase the associated reparation/replacement costs. Full article
(This article belongs to the Special Issue Energy-Efficient and Sustainable Networking)
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Open AccessArticle
Integrated Assessment of Carbon Capture and Storage (CCS) in South Africa’s Power Sector
Energies 2015, 8(12), 14380-14406; https://doi.org/10.3390/en81212432
Received: 17 October 2015 / Revised: 29 November 2015 / Accepted: 7 December 2015 / Published: 18 December 2015
Cited by 6 | Viewed by 2369 | PDF Full-text (3551 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
This article presents an integrated assessment conducted in order to explore whether carbon capture and storage (CCS) could be a viable technological option for significantly reducing future CO2 emissions in South Africa. The methodological approach covers a commercial availability analysis, an analysis [...] Read more.
This article presents an integrated assessment conducted in order to explore whether carbon capture and storage (CCS) could be a viable technological option for significantly reducing future CO2 emissions in South Africa. The methodological approach covers a commercial availability analysis, an analysis of the long-term usable CO2 storage potential (based on storage capacity assessment, energy scenario analysis and source-sink matching), an economic and ecological assessment and a stakeholder analysis. The findings show, that a reliable storage capacity assessment is needed, since only rough figures concerning the effective capacity currently exist. Further constraints on the fast deployment of CCS may be the delayed commercial availability of CCS, significant barriers to increasing the economic viability of CCS, an expected net maximum reduction rate of the power plant’s greenhouse gas emissions of 67%–72%, an increase in other environmental and social impacts, and low public awareness of CCS. One precondition for opting for CCS would be to find robust solutions to these constraints, taking into account that CCS could potentially conflict with other important policy objectives, such as affordable electricity rates to give the whole population access to electricity. Full article
(This article belongs to the Special Issue Multi-Disciplinary Perspectives on Energy and Sustainable Development)
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Open AccessArticle
Investigation of a Co-Axial Dual-Mechanical Ports Flux-Switching Permanent Magnet Machine for Hybrid Electric Vehicles
Energies 2015, 8(12), 14361-14379; https://doi.org/10.3390/en81212434
Received: 31 July 2015 / Revised: 8 December 2015 / Accepted: 14 December 2015 / Published: 18 December 2015
Cited by 8 | Viewed by 2569 | PDF Full-text (7465 KB) | HTML Full-text | XML Full-text
Abstract
In this paper, a co-axial dual-mechanical ports flux-switching permanent magnet (CADMP-FSPM) machine for hybrid electric vehicles (HEVs) is proposed and investigated, which is comprised of two conventional co-axial FSPM machines, namely one high-speed inner rotor machine and one low-speed outer rotor machine and [...] Read more.
In this paper, a co-axial dual-mechanical ports flux-switching permanent magnet (CADMP-FSPM) machine for hybrid electric vehicles (HEVs) is proposed and investigated, which is comprised of two conventional co-axial FSPM machines, namely one high-speed inner rotor machine and one low-speed outer rotor machine and a non-magnetic ring sandwiched in between. Firstly, the topology and operation principle of the CADMP-FSPM machine are introduced; secondly, the control system of the proposed electronically-controlled continuously-variable transmission (E-CVT) system is given; thirdly, the key design specifications of the CADMP-FSPM machine are determined based on a conventional dual-mechanical ports (DMP) machine with a wound inner rotor. Fourthly, the performances of the CADMP-FSPM machine and the normal DMP machine under the same overall volume are compared, and the results indicate that the CADMP-FSPM machine has advantages over the conventional DMP machine in the elimination of brushes and slip rings, improved thermal dissipation conditions for the inner rotor, direct-driven operation, more flexible modes, lower cogging torque and torque ripple, lower total harmonic distortion (THD) values of phase PM flux linkage and phase electro-motive force (EMF), higher torque output capability and is suitable for the E-CVT systems. Finally, the pros and cons of the CADMP-FSPM machine are highlighted. This paper lays a theoretical foundation for further research on CADMP-FSPM machines used for HEVs. Full article
(This article belongs to the Special Issue Advances in Plug-in Hybrid Vehicles and Hybrid Vehicles)
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Open AccessArticle
Wind Power Grid Connected Capacity Prediction Using LSSVM Optimized by the Bat Algorithm
Energies 2015, 8(12), 14346-14360; https://doi.org/10.3390/en81212428
Received: 21 November 2015 / Revised: 14 December 2015 / Accepted: 15 December 2015 / Published: 18 December 2015
Cited by 7 | Viewed by 1936 | PDF Full-text (1878 KB) | HTML Full-text | XML Full-text
Abstract
Given the stochastic nature of wind, wind power grid-connected capacity prediction plays an essential role in coping with the challenge of balancing supply and demand. Accurate forecasting methods make enormous contribution to mapping wind power strategy, power dispatching and sustainable development of wind [...] Read more.
Given the stochastic nature of wind, wind power grid-connected capacity prediction plays an essential role in coping with the challenge of balancing supply and demand. Accurate forecasting methods make enormous contribution to mapping wind power strategy, power dispatching and sustainable development of wind power industry. This study proposes a bat algorithm (BA)–least squares support vector machine (LSSVM) hybrid model to improve prediction performance. In order to select input of LSSVM effectively, Stationarity, Cointegration and Granger causality tests are conducted to examine the influence of installed capacity with different lags, and partial autocorrelation analysis is employed to investigate the inner relationship of grid-connected capacity. The parameters in LSSVM are optimized by BA to validate the learning ability and generalization of LSSVM. Multiple model sufficiency evaluation methods are utilized. The research results reveal that the accuracy improvement of the present approach can reach about 20% compared to other single or hybrid models. Full article
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Open AccessArticle
Optimal Operation of Combined Heat and Power System Based on Forecasted Energy Prices in Real-Time Markets
Energies 2015, 8(12), 14330-14345; https://doi.org/10.3390/en81212427
Received: 4 October 2015 / Revised: 23 November 2015 / Accepted: 10 December 2015 / Published: 18 December 2015
Cited by 10 | Viewed by 1899 | PDF Full-text (1959 KB) | HTML Full-text | XML Full-text
Abstract
This paper develops a discrete operation optimization model for combined heat and powers (CHPs) in deregulated energy markets to maximize owners’ profits, where energy price forecasting is included. First, a single input and multi-output (SIMO) model for typical CHPs is established, considering the [...] Read more.
This paper develops a discrete operation optimization model for combined heat and powers (CHPs) in deregulated energy markets to maximize owners’ profits, where energy price forecasting is included. First, a single input and multi-output (SIMO) model for typical CHPs is established, considering the varying ratio between heat and electricity outputs at different loading levels. Then, the energy prices are forecasted with a gray forecasting model and revised in real-time based on the actual prices by using the least squares method. At last, a discrete optimization model and corresponding dynamic programming algorithm are developed to design the optimal operation strategies for CHPs in real-time. Based on the forecasted prices, the potential operating strategy which may produce the maximum profits is pre-developed. Dynamic modification is then conducted to adjust the pre-developed operating strategy after the actual prices are known. The proposed method is implemented on a 1 MW CHP on a typical day. Results show the optimized profits comply well with those derived from real-time prices after considering dynamic modification process. Full article
(This article belongs to the Special Issue Advanced Heating and Cooling Techniques)
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Open AccessArticle
Optimization of Train Operation in Multiple Interstations with Multi-Population Genetic Algorithm
Energies 2015, 8(12), 14311-14329; https://doi.org/10.3390/en81212433
Received: 29 September 2015 / Revised: 30 November 2015 / Accepted: 7 December 2015 / Published: 18 December 2015
Cited by 18 | Viewed by 2050 | PDF Full-text (736 KB) | HTML Full-text | XML Full-text
Abstract
Subway systems consume a large amount of energy each year. How to reduce the energy consumption of subway systems has already become an issue of concern in recent years. This paper proposes an energy-efficient approach to reduce the traction energy by optimizing the [...] Read more.
Subway systems consume a large amount of energy each year. How to reduce the energy consumption of subway systems has already become an issue of concern in recent years. This paper proposes an energy-efficient approach to reduce the traction energy by optimizing the train operation for multiple interstations. Both the trip time and driving strategy are considered in the proposed optimization approach. Firstly, a bi-level programming model of multiple interstations is developed for the energy-efficient train operation problem, which is then converted into an integrated model to calculate the driving strategy for multiple interstations. Additionally, the multi-population genetic algorithm (MPGA) is used to solve the problem, followed by calculating the energy-efficient trip times. Finally, the paper presents some examples based on the operation data of the Beijing Changping subway line. The simulation results show that the proposed approach presents a better energy-efficient performance than that with only optimizing the driving strategy for a single interstation. Full article
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Open AccessArticle
Study of the Impact of Initial Moisture Content in Oil Impregnated Insulation Paper on Thermal Aging Rate of Condenser Bushing
Energies 2015, 8(12), 14298-14310; https://doi.org/10.3390/en81212429
Received: 19 October 2015 / Revised: 9 December 2015 / Accepted: 11 December 2015 / Published: 18 December 2015
Cited by 4 | Viewed by 1794 | PDF Full-text (1965 KB) | HTML Full-text | XML Full-text
Abstract
This paper studied the impact of moisture on the correlated characteristics of the condenser bushings oil-paper insulation system. The oil-impregnated paper samples underwent accelerated thermal aging at 130 °C after preparation at different initial moisture contents (1%, 3%, 5% and 7%). All the [...] Read more.
This paper studied the impact of moisture on the correlated characteristics of the condenser bushings oil-paper insulation system. The oil-impregnated paper samples underwent accelerated thermal aging at 130 °C after preparation at different initial moisture contents (1%, 3%, 5% and 7%). All the samples were extracted periodically for the measurement of the moisture content, the degree of polymerization (DP) and frequency domain dielectric spectroscopy (FDS). Next, the measurement results of samples were compared to the related research results of transformer oil-paper insulation, offering a theoretical basis of the parameter analysis. The obtained results show that the moisture fluctuation amplitude can reflect the different initial moisture contents of insulating paper and the mass ratio of oil and paper has little impact on the moisture content fluctuation pattern in oil-paper but has a great impact on moisture fluctuation amplitude; reduction of DP presents an accelerating trend with the increase of initial moisture content, and the aging rate of test samples is higher under low moisture content but lower under high moisture content compared to the insulation paper in transformers. Two obvious “deceleration zones” appeared in the dielectric spectrum with the decrease of frequency, and not only does the integral value of dielectric dissipation factor (tan δ) reflect the aging degree, but it reflects the moisture content in solid insulation. These types of research in this paper can be applied to evaluate the condition of humidified insulation and the aging state of solid insulation for condenser bushings. Full article
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Open AccessArticle
Efficient Parallelization of the Stochastic Dual Dynamic Programming Algorithm Applied to Hydropower Scheduling
Energies 2015, 8(12), 14287-14297; https://doi.org/10.3390/en81212431
Received: 6 November 2015 / Revised: 4 December 2015 / Accepted: 10 December 2015 / Published: 18 December 2015
Cited by 3 | Viewed by 2133 | PDF Full-text (753 KB) | HTML Full-text | XML Full-text
Abstract
Stochastic dual dynamic programming (SDDP) has become a popular algorithm used in practical long-term scheduling of hydropower systems. The SDDP algorithm is computationally demanding, but can be designed to take advantage of parallel processing. This paper presents a novel parallel scheme for the [...] Read more.
Stochastic dual dynamic programming (SDDP) has become a popular algorithm used in practical long-term scheduling of hydropower systems. The SDDP algorithm is computationally demanding, but can be designed to take advantage of parallel processing. This paper presents a novel parallel scheme for the SDDP algorithm, where the stage-wise synchronization point traditionally used in the backward iteration of the SDDP algorithm is partially relaxed. The proposed scheme was tested on a realistic model of a Norwegian water course, proving that the synchronization point relaxation significantly improves parallel efficiency. Full article
(This article belongs to the Special Issue Hydropower)
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Open AccessArticle
Multi-Agent System-Based Microgrid Operation Strategy for Demand Response
Energies 2015, 8(12), 14272-14286; https://doi.org/10.3390/en81212430
Received: 14 September 2015 / Revised: 10 December 2015 / Accepted: 14 December 2015 / Published: 18 December 2015
Cited by 13 | Viewed by 2504 | PDF Full-text (8223 KB) | HTML Full-text | XML Full-text
Abstract
The microgrid and demand response (DR) are important technologies for future power grids. Among the variety of microgrid operations, the multi-agent system (MAS) has attracted considerable attention. In a microgrid with MAS, the agents installed on the microgrid components operate optimally by communicating [...] Read more.
The microgrid and demand response (DR) are important technologies for future power grids. Among the variety of microgrid operations, the multi-agent system (MAS) has attracted considerable attention. In a microgrid with MAS, the agents installed on the microgrid components operate optimally by communicating with each other. This paper proposes an operation algorithm for the individual agents of a test microgrid that consists of a battery energy storage system (BESS) and an intelligent load. A microgrid central controller to manage the microgrid can exchange information with each agent. The BESS agent performs scheduling for maximum benefit in response to the electricity price and BESS state of charge (SOC) through a fuzzy system. The intelligent load agent assumes that the industrial load performs scheduling for maximum benefit by calculating the hourly production cost. The agent operation algorithm includes a scheduling algorithm using day-ahead pricing in the DR program and a real-time operation algorithm for emergency situations using emergency demand response (EDR). The proposed algorithm and operation strategy were implemented both by a hardware-in-the-loop simulation test using OPAL-RT and an actual hardware test by connecting a new distribution simulator. Full article
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Open AccessArticle
Research on and Design of a Self-Propelled Nozzle for the Tree-Type Drilling Technique in Underground Coal Mines
Energies 2015, 8(12), 14260-14271; https://doi.org/10.3390/en81212426
Received: 25 October 2015 / Revised: 14 December 2015 / Accepted: 15 December 2015 / Published: 17 December 2015
Cited by 11 | Viewed by 1699 | PDF Full-text (1909 KB) | HTML Full-text | XML Full-text
Abstract
Due to the increasing depths of coal mines and the low permeability of some coal seams, conventional methods of gas drainage in underground mines are facing many problems. To improve gas extraction, a new technique using water jets to drill tree-type boreholes in [...] Read more.
Due to the increasing depths of coal mines and the low permeability of some coal seams, conventional methods of gas drainage in underground mines are facing many problems. To improve gas extraction, a new technique using water jets to drill tree-type boreholes in coal seams is proposed. A self-propelled water-jet drilling nozzle was designed to drill these boreholes. The configuration of the self-propelled nozzle was optimized by conducting drilling experiments and self-propelling force measurements. Experimental results show that the optimal self-propelled nozzle has a forward orifice axial angle at 25°, a radial angle at 90°, a center distance of 1.5 mm, and backward pointing orifices with an axial angle of 25°. The self-propelling force generated by the jets of the nozzle with 30 MPa pump pressure can reach 29.8 N, enough to pull the hose and the nozzle forward without any external forces. The nozzle can drill at speeds up to 41.5 m/h with pump pressures at 30 MPa. The radial angles of the forward orifices improve the rock breaking performance of the nozzle and, with the correct angle, the rock breaking area of the orifices overlap to produce a connecting hole. The diameter of boreholes drilled by this nozzle can reach 35.2 mm. The nozzle design can be used as the basis for designing other self-propelled nozzles. The drilling experiments demonstrate the feasibility of using the tree-type drilling technique in underground mines. Full article
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Open AccessArticle
A Study on Maximum Wind Power Penetration Limit in Island Power System Considering High-Voltage Direct Current Interconnections
Energies 2015, 8(12), 14244-14259; https://doi.org/10.3390/en81212425
Received: 7 September 2015 / Revised: 27 November 2015 / Accepted: 7 December 2015 / Published: 17 December 2015
Cited by 11 | Viewed by 2249 | PDF Full-text (2313 KB) | HTML Full-text | XML Full-text
Abstract
The variability and uncontrollability of wind power increases the difficulty for a power system operator to implement a wind power system with a high penetration rate. These are more serious factors to consider in small and isolated power systems since the system has [...] Read more.
The variability and uncontrollability of wind power increases the difficulty for a power system operator to implement a wind power system with a high penetration rate. These are more serious factors to consider in small and isolated power systems since the system has small operating reserves and inertia to secure frequency and voltage. Typically, this difficulty can be reduced by interconnection with another robust power system using a controllable transmission system such as a high-voltage direct current (HVDC) system. However, the reliability and stability constraints of a power system has to be performed according to the HVDC system implementation. In this paper, the method for calculation of maximum wind power penetration in an island supplied by a HVDC power system is presented, and the operational strategy of a HVDC system is proposed to secure the power system reliability and stability. The case study is performed for the Jeju Island power system in the Korean smart grid demonstration area. Full article
(This article belongs to the Special Issue Electric Power Systems Research) Printed Edition available
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Open AccessFeature PaperArticle
Wave Energy Resource along the Coast of Santa Catarina (Brazil)
Energies 2015, 8(12), 14219-14243; https://doi.org/10.3390/en81212423
Received: 24 September 2015 / Revised: 5 December 2015 / Accepted: 10 December 2015 / Published: 17 December 2015
Cited by 31 | Viewed by 2435 | PDF Full-text (7658 KB) | HTML Full-text | XML Full-text
Abstract
Brazil has one of the largest electricity markets in South America, which needs to add 6000 MW of capacity every year in order to satisfy growing the demand from an increasing and more prosperous population. Apart from biomass, no other renewable energy sources, [...] Read more.
Brazil has one of the largest electricity markets in South America, which needs to add 6000 MW of capacity every year in order to satisfy growing the demand from an increasing and more prosperous population. Apart from biomass, no other renewable energy sources, besides hydroelectricity, play a relevant role in the energy mix. The potential for wind and wave energy is very large. Brazil's Santa Catarina state government is starting a clean energy program in the state, which is expected to bring more than 1 GW of capacity. Assessment of wave energy resources is needed along the coastline. This work studied the potential wave energy along the north-central coasts of Santa Catarina, in Southern Brazil, by analysis of the hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF). The annual offshore wave power was found to be equal to 15.25 kW/m, the bulk of which is provided by southeastern waves. The nearshore energetic patterns were studied by means of a numerical coastal propagation model (Mike21 SW). The mean wave power of 20 m isobaths is 11.43 kW/m. Supplementary considerations are drawn on realistic perspectives for wave energy converters installations. Full article
(This article belongs to the Special Issue Tools and Techniques for Economic Delivery of Ocean Energy)
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Open AccessArticle
Designing an Incentive Contract Menu for Sustaining the Electricity Market
Energies 2015, 8(12), 14197-14218; https://doi.org/10.3390/en81212419
Received: 14 October 2015 / Revised: 4 December 2015 / Accepted: 7 December 2015 / Published: 16 December 2015
Cited by 3 | Viewed by 1578 | PDF Full-text (2231 KB) | HTML Full-text | XML Full-text
Abstract
This paper designs an incentive contract menu to achieve long-term stability for electricity prices in a day-ahead electricity market. A bi-level Stackelberg game model is proposed to search for the optimal incentive mechanism under a one-leader and multi-followers gaming framework. A multi-agent simulation [...] Read more.
This paper designs an incentive contract menu to achieve long-term stability for electricity prices in a day-ahead electricity market. A bi-level Stackelberg game model is proposed to search for the optimal incentive mechanism under a one-leader and multi-followers gaming framework. A multi-agent simulation platform was developed to investigate the effectiveness of the incentive mechanism using an independent system operator (ISO) and multiple power generating companies (GenCos). Further, a Q-learning approach was implemented to analyze and assess the response of GenCos to the incentive menu. Numerical examples are provided to demonstrate the effectiveness of the incentive contract. Full article
(This article belongs to the Special Issue Electric Power Systems Research) Printed Edition available
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Open AccessArticle
Waste-to-Energy in China: Key Challenges and Opportunities
Energies 2015, 8(12), 14182-14196; https://doi.org/10.3390/en81212422
Received: 15 September 2015 / Revised: 7 December 2015 / Accepted: 8 December 2015 / Published: 16 December 2015
Cited by 24 | Viewed by 3458 | PDF Full-text (3799 KB) | HTML Full-text | XML Full-text
Abstract
China—the largest developing country in the world—is experiencing both rapid economic maturation and large-scale urbanization. These situations have led to waste disposal problems, and the need to identify alternative energy sources. Waste-to-energy (WTE) conversion processes, a source of renewable energy, are expected to [...] Read more.
China—the largest developing country in the world—is experiencing both rapid economic maturation and large-scale urbanization. These situations have led to waste disposal problems, and the need to identify alternative energy sources. Waste-to-energy (WTE) conversion processes, a source of renewable energy, are expected to play an increasingly important role in China’s sustainable management of municipal solid waste (MSW). The purpose of this research is to investigate the key problems and opportunities associated with WTE, to provide recommendations for the government. This paper begins by describing China’s current MSW management situation and analyzing its waste disposal problems. The major challenges associated with China’s WTE incineration are then discussed from economic, environmental and social points of view. These include the high costs associated with constructing necessary facilities, the susceptibility of facilities to corrosion, the lower heating value of China’s MSW, air pollutant emissions and especially public opposition to WTE incineration. Since discarded waste can be used to produce energy for electricity and heat—thus reducing its volume and the production of greenhouse gas (GHG) emissions—with government policies and financial incentives, the use of WTE incineration as a renewable energy source and part of a sustainable waste management strategy will be of increasing importance in the future. The paper concludes by summarizing the management, economic and social benefits that could be derived from developing the country’s domestic capacity for producing the needed incineration equipment, improving source separation capabilities, standardizing regulatory and legal responsibilities and undertaking more effective public consultation processes. Full article
(This article belongs to the Special Issue Waste Energy Harvesting)
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Open AccessArticle
High Resolution Modeling of the Impacts of Exogenous Factors on Power Systems—Case Study of Germany
Energies 2015, 8(12), 14168-14181; https://doi.org/10.3390/en81212424
Received: 24 August 2015 / Revised: 24 November 2015 / Accepted: 10 December 2015 / Published: 16 December 2015
Cited by 1 | Viewed by 1777 | PDF Full-text (18410 KB) | HTML Full-text | XML Full-text
Abstract
In order to reliably design the planning and operation of large interconnected power systems that can incorporate a high penetration of renewables, it is necessary to have a detailed knowledge of the potential impacts of exogenous factors on individual components within the systems. [...] Read more.
In order to reliably design the planning and operation of large interconnected power systems that can incorporate a high penetration of renewables, it is necessary to have a detailed knowledge of the potential impacts of exogenous factors on individual components within the systems. Previously, the assessment has often been conducted with nodes that are aggregated at the country or regional scale; this makes it impossible to reliably extrapolate the impact of higher penetration of renewables on individual transmission lines and/or power plants within an aggregated node. In order to be able to develop robust power systems this study demonstrates an integrated framework that employs high resolution spatial and temporal, physical modeling of power generation, electricity transmission and electricity demand, across the scale of a continent or country. Using Germany as a test case, an assessment of the impacts of exogenous factors, including local changes in ambient weather conditions, effect of timely implementation of policy, and contingency for scenarios in 2020 are demonstrated. It is shown that with the increased penetration of renewables, while the power production opportunities of conventional power plants are reduced, these power plants are required during periods of low renewables production due to the inherent variability of renewables. While the planned reinforcements in Germany, including high voltage direct current lines, reduce congestion on the grid and alleviate the differentials in power price across the country, on the other hand the reinforcements make the interconnected transmission system more vulnerable as local perturbations have a more widespread impact. Full article
(This article belongs to the Special Issue Resilience of Energy Systems)
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Open AccessArticle
Optimal Design of an Axial-Flux Permanent-Magnet Middle Motor Integrated in a Cycloidal Reducer for a Pedal Electric Cycle
Energies 2015, 8(12), 14151-14167; https://doi.org/10.3390/en81212421
Received: 21 September 2015 / Revised: 30 October 2015 / Accepted: 10 December 2015 / Published: 16 December 2015
Cited by 4 | Viewed by 4403 | PDF Full-text (3043 KB) | HTML Full-text | XML Full-text
Abstract
This paper proposes an optimal design of a middle motor integrated into a mid-drive unit for pedal electric cycles. This middle motor is an axial-flux permanent magnet brushless DC motor with a stator plate. Facing this plate is a rotor plate that acts [...] Read more.
This paper proposes an optimal design of a middle motor integrated into a mid-drive unit for pedal electric cycles. This middle motor is an axial-flux permanent magnet brushless DC motor with a stator plate. Facing this plate is a rotor plate that acts as an eccentric component of a cycloidal reducer next to the stator. Both the middle motor and cycloidal reducer are easily installed on the same axle of the bike crankset. In the preliminary design, driving requirements are used to make the middle motor specifications. The numbers of stator slots and magnet poles on the rotor were chosen to achieve the best winding factor, and the winding layout was determined accordingly. A one-dimensional magnetic circuit model of the middle motor was built, and its shape was optimized utilizing a multifunctional optimization system tool. Finally, the resulting design was verified and refined by a finite element analysis. A prototype of the middle motor was fabricated with a stator made of a soft magnetic composite and integrated with a cycloidal reducer. Experimental results show that the motor has a sinusoidal back electromotive force. Additionally, it is easily controlled by sinusoidal currents with acceptable torque and speed curves. Full article
(This article belongs to the Special Issue Energy Efficient Actuators and Systems)
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Open AccessArticle
Optimization of Biodiesel-Diesel Blended Fuel Properties and Engine Performance with Ether Additive Using Statistical Analysis and Response Surface Methods
Energies 2015, 8(12), 14136-14150; https://doi.org/10.3390/en81212420
Received: 15 October 2015 / Revised: 29 November 2015 / Accepted: 2 December 2015 / Published: 16 December 2015
Cited by 20 | Viewed by 3094 | PDF Full-text (4217 KB) | HTML Full-text | XML Full-text
Abstract
In this study, the fuel properties and engine performance of blended palm biodiesel-diesel using diethyl ether as additive have been investigated. The properties of B30 blended palm biodiesel-diesel fuel were measured and analyzed statistically with the addition of 2%, 4%, 6% and 8% [...] Read more.
In this study, the fuel properties and engine performance of blended palm biodiesel-diesel using diethyl ether as additive have been investigated. The properties of B30 blended palm biodiesel-diesel fuel were measured and analyzed statistically with the addition of 2%, 4%, 6% and 8% (by volume) diethyl ether additive. The engine tests were conducted at increasing engine speeds from 1500 rpm to 3500 rpm and under constant load. Optimization of independent variables was performed using the desirability approach of the response surface methodology (RSM) with the goal of minimizing emissions and maximizing performance parameters. The experiments were designed using a statistical tool known as design of experiments (DoE) based on RSM. Full article
(This article belongs to the Special Issue Energy for Agriculture)
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Open AccessArticle
Energetic and Exergetic Analysis of Combined Cycle Power Plant: Part-1 Operation and Performance
Energies 2015, 8(12), 14118-14135; https://doi.org/10.3390/en81212418
Received: 12 August 2015 / Revised: 4 November 2015 / Accepted: 9 December 2015 / Published: 14 December 2015
Cited by 7 | Viewed by 2675 | PDF Full-text (4604 KB) | HTML Full-text | XML Full-text
Abstract
Energetic and exergetic analyses are conducted using operating data for Sabiya, a combined cycle power plant (CCPP) with an advanced triple pressure reheat heat recovery steam generator (HRSG). Furthermore, a sensitivity analysis is carried out on the HRSG using a recent approach to [...] Read more.
Energetic and exergetic analyses are conducted using operating data for Sabiya, a combined cycle power plant (CCPP) with an advanced triple pressure reheat heat recovery steam generator (HRSG). Furthermore, a sensitivity analysis is carried out on the HRSG using a recent approach to differentiate between the sources of irreversibility. The proposed system was modelled using the IPSEpro software and further validated by the manufacturer’s data. The performance of the Sabiya CCPP was examined for different climatic conditions, pressure ratios, pinch point temperatures, high-pressure steam, and condenser pressure values. The results confirmed that 60.9% of the total exergy destruction occurs in the combustion chamber, which constitutes the main source of irreversibilities within a system. The exergy destruction was significantly affected by both the pressure ratio and the high-pressure steam, where the relation between them was seen to be inversely proportional. The high-pressure stage contributes about 50% of the exergy destruction within the HRSG compared to other stages and the reheat system, due to the high temperature difference between the streams and the large number of components, which leads to high energy loss to the surroundings. Numerous possibilities for improving the CCPP’s performance are introduced, based on the obtained results. Full article
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Open AccessArticle
Provision of Supplementary Load Frequency Control via Aggregation of Air Conditioning Loads
Energies 2015, 8(12), 14098-14117; https://doi.org/10.3390/en81212417
Received: 9 October 2015 / Revised: 20 November 2015 / Accepted: 7 December 2015 / Published: 14 December 2015
Cited by 5 | Viewed by 1857 | PDF Full-text (13535 KB) | HTML Full-text | XML Full-text
Abstract
The integration of large-scale renewable energy poses great challenges for the operation of power system because of its increased frequency fluctuations. More load frequency control (LFC) resources are demanded in order to maintain a stable system with more renewable energy injected. Unlike the [...] Read more.
The integration of large-scale renewable energy poses great challenges for the operation of power system because of its increased frequency fluctuations. More load frequency control (LFC) resources are demanded in order to maintain a stable system with more renewable energy injected. Unlike the costly LFC resources on generation side, the thermostatically controlled loads (TCLs) on the demand side become an attractive solution on account of its substantial quantities and heat-storage capacity. It generally contains air conditioners (ACs), water heaters and fridges. In this paper, the supplementary LFC is extracted by the modeling and controlling of aggregated ACs. We first present a control framework integrating the supplementary LFC with the traditional LFC. Then, a change-time-priority-list method is proposed to control power output taking into account customers’ satisfaction. Simulations on a single-area power system with wind power integration demonstrate the effectiveness of the proposed method. The impact of ambient temperature changes and customer preferences on room temperature is also involved in the discussion. Results show that the supplementary LFC provided by ACs could closely track the LFC signals and effectively reduce the frequency deviation. Full article
(This article belongs to the Special Issue Microgrids 2016)
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Open AccessArticle
Modelling of Evaporator in Waste Heat Recovery System using Finite Volume Method and Fuzzy Technique
Energies 2015, 8(12), 14078-14097; https://doi.org/10.3390/en81212413
Received: 29 October 2015 / Revised: 27 November 2015 / Accepted: 7 December 2015 / Published: 12 December 2015
Cited by 8 | Viewed by 3451 | PDF Full-text (4514 KB) | HTML Full-text | XML Full-text
Abstract
The evaporator is an important component in the Organic Rankine Cycle (ORC)-based Waste Heat Recovery (WHR) system since the effective heat transfer of this device reflects on the efficiency of the system. When the WHR system operates under supercritical conditions, the heat transfer [...] Read more.
The evaporator is an important component in the Organic Rankine Cycle (ORC)-based Waste Heat Recovery (WHR) system since the effective heat transfer of this device reflects on the efficiency of the system. When the WHR system operates under supercritical conditions, the heat transfer mechanism in the evaporator is unpredictable due to the change of thermo-physical properties of the fluid with temperature. Although the conventional finite volume model can successfully capture those changes in the evaporator of the WHR process, the computation time for this method is high. To reduce the computation time, this paper develops a new fuzzy based evaporator model and compares its performance with the finite volume method. The results show that the fuzzy technique can be applied to predict the output of the supercritical evaporator in the waste heat recovery system and can significantly reduce the required computation time. The proposed model, therefore, has the potential to be used in real time control applications. Full article
(This article belongs to the Special Issue Waste Energy Harvesting)
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Open AccessArticle
Strategies for Reducing the Start-up Operation of Microbial Electrochemical Treatments of Urban Wastewater
Energies 2015, 8(12), 14064-14077; https://doi.org/10.3390/en81212416
Received: 14 October 2015 / Revised: 13 November 2015 / Accepted: 3 December 2015 / Published: 12 December 2015
Cited by 12 | Viewed by 2820 | PDF Full-text (1706 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Microbial electrochemical technologies (METs) constitute the core of a number of emerging technologies with a high potential for treating urban wastewater due to a fascinating reaction mechanism—the electron transfer between bacteria and electrodes to transform metabolism into electrical current. In the current work, [...] Read more.
Microbial electrochemical technologies (METs) constitute the core of a number of emerging technologies with a high potential for treating urban wastewater due to a fascinating reaction mechanism—the electron transfer between bacteria and electrodes to transform metabolism into electrical current. In the current work, we focus on the model electroactive microorganism Geobacter sulfurreducens to explore both the design of new start-up procedures and electrochemical operations. Our chemostat-grown plug and play cells, were able to reduce the start-up period by 20-fold while enhancing chemical oxygen demand (COD) removal by more than 6-fold during this period. Moreover, a filter-press based bioreactor was successfully tested for both acetate-supplemented synthetic wastewater and real urban wastewater. This proof-of-concept pre-pilot treatment included a microbial electrolysis cell (MEC) followed in time by a microbial fuel cell (MFC) to finally generate electrical current of ca. 20 A·m−2 with a power of 10 W·m−2 while removing 42 g COD day−1·m−2. The effective removal of acetate suggests a potential use of this modular technology for treating acetogenic wastewater where Geobacter sulfurreducens outcompetes other organisms. Full article
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Open AccessArticle
Solar Dryer Application for Olive Oil Mill Wastes
Energies 2015, 8(12), 14049-14063; https://doi.org/10.3390/en81212415
Received: 9 September 2015 / Revised: 4 December 2015 / Accepted: 7 December 2015 / Published: 12 December 2015
Cited by 6 | Viewed by 2149 | PDF Full-text (1551 KB) | HTML Full-text | XML Full-text
Abstract
Global waste production has raised recently due to numerous agricultural and industrial activities. Among other actions devoted to waste reduction, revaluation seems to be the most advantageous one from an environmental and economic point of view. In the olive oil sector, by-product management [...] Read more.
Global waste production has raised recently due to numerous agricultural and industrial activities. Among other actions devoted to waste reduction, revaluation seems to be the most advantageous one from an environmental and economic point of view. In the olive oil sector, by-product management (namely olive pomace, olive mill wastewater, and sludge residue) poses serious problems for companies, with the energy use of these wastes being a feasible option to solve these problems. Due to their high moisture content, drying constitutes the main stage for a possible bio-fuel conversion. This research work deals with the analysis of drying for the three main wastes from olive oil by using a prototype dryer. This equipment has different working regimes depending on the kind of convection (natural or forced for passive and active mode, respectively), the incidence of solar radiation on the product (indirect or mixed type) and the use of an additional system for energy supply (hybrid type). From the results, it could be said that all the wastes were suitable for solar drying. However, drying conditions were different for each case. Olive pomace and olive mill wastewater showed promising results for solar drying application in mixed active mode. Concerning sludge residue, its special physical structure could employ the use of turners for drying, in order to improve heat transfer to the product. Moreover, hybrid active mode enabled a considerable reduction in drying time, being an aspect to take into account for its use during low solar radiation or at night time. Full article
(This article belongs to the Special Issue Organic Waste/Resources to Energy and Value added Product)
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Open AccessArticle
Characteristics of Biochar Obtained by Hydrothermal Carbonization of Cellulose for Renewable Energy
Energies 2015, 8(12), 14040-14048; https://doi.org/10.3390/en81212412
Received: 31 August 2015 / Revised: 30 November 2015 / Accepted: 8 December 2015 / Published: 11 December 2015
Cited by 21 | Viewed by 2125 | PDF Full-text (1517 KB) | HTML Full-text | XML Full-text
Abstract
The effect of hydrothermal carbonization on the properties of cellulose present in lignocellulosic biomass was investigated for converting it into a renewable energy resource with high energy recovery efficiency. The biochar obtained from cellulose subjected to hydrothermal carbonization showed a significant increase in [...] Read more.
The effect of hydrothermal carbonization on the properties of cellulose present in lignocellulosic biomass was investigated for converting it into a renewable energy resource with high energy recovery efficiency. The biochar obtained from cellulose subjected to hydrothermal carbonization showed a significant increase in its carbon content and a calorific value. 13C NMR spectroscopy showed that when raw cellulose was subjected to hydrothermal carbonization above 220 °C, the resulting biochar had more aromatic and aliphatic fractions than those in raw cellulose. The resulting composition of the biochars was comparable to that of solid fuels and was between that of lignite and sub-bituminous coal. Therefore, cellulose, the main component of lignocellulosic biomass, was used to investigate the effects of varying the reaction temperature during hydrothermal carbonization. The energy recovery efficiency calculations showed that the optimum reaction temperature for the transformation of a mixture of cellulose was approximately 220 °C. Full article
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Open AccessArticle
Pareto-Optimal Evaluation of Ultimate Limit States in Offshore Wind Turbine Structural Analysis
Energies 2015, 8(12), 14026-14039; https://doi.org/10.3390/en81212414
Received: 29 October 2015 / Revised: 3 December 2015 / Accepted: 7 December 2015 / Published: 11 December 2015
Cited by 1 | Viewed by 2294 | PDF Full-text (1948 KB) | HTML Full-text | XML Full-text
Abstract
The ultimate capacity of support structures is checked with extreme loads. This is straightforward when the limit state equations depend on a single load component, and it has become common to report maxima for each load component. However, if more than one load [...] Read more.
The ultimate capacity of support structures is checked with extreme loads. This is straightforward when the limit state equations depend on a single load component, and it has become common to report maxima for each load component. However, if more than one load component is influential, e.g., both axial force and bending moments, it is not straightforward how to define an extreme load. The combination of univariate maxima can be too conservative, and many different combinations of load components can result in the worst value of the limit state equations. The use of contemporaneous load vectors is typically non-conservative. Therefore, in practice, limit state checks are done for each possible load vector, from each time step of a simulation. This is not feasible when performing reliability assessments and structural optimization, where additional, time-consuming computations are involved for each load vector. We therefore propose to use Pareto-optimal loads, which are a small set of loads that together represent all possible worst case scenarios. Simulations with two reference wind turbines show that this approach can be very useful for jacket structures, whereas the design of monopiles is often governed by the bending moment only. Even in this case, the approach might be useful when approaching the structural limits during optimization. Full article
(This article belongs to the Special Issue Wind Turbine 2015)
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Open AccessArticle
Voltage Control Method Using Distributed Generators Based on a Multi-Agent System
Energies 2015, 8(12), 14009-14025; https://doi.org/10.3390/en81212411
Received: 21 September 2015 / Revised: 22 November 2015 / Accepted: 3 December 2015 / Published: 11 December 2015
Cited by 9 | Viewed by 2404 | PDF Full-text (879 KB) | HTML Full-text | XML Full-text
Abstract
This paper presents a voltage control method using multiple distributed generators (DGs) based on a multi-agent system framework. The output controller of each DG is represented as a DG agent, and each voltage-monitoring device is represented as a monitoring agent. These agents cooperate [...] Read more.
This paper presents a voltage control method using multiple distributed generators (DGs) based on a multi-agent system framework. The output controller of each DG is represented as a DG agent, and each voltage-monitoring device is represented as a monitoring agent. These agents cooperate to accomplish voltage regulation through a coordinating agent or moderator. The moderator uses the reactive power sensitivities and margins to determine the voltage control contributions of each DG. A fuzzy inference system (FIS) is employed by the moderator to manage the decision-making process. An FIS scheme is developed and optimized to enhance the efficiency of the proposed voltage control process using particle swarm optimization. A simple distribution system with four voltage-controllable DGs is modeled, and an FIS moderator is implemented to control the system. Simulated data show that the proposed voltage control process is able to maintain the system within the operating voltage limits. Furthermore, the results were similar to those obtained using optimal power flow calculations, even though little information on the power system was required and no power flow calculations were implemented. Full article
(This article belongs to the Special Issue Electric Power Systems Research) Printed Edition available
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Open AccessArticle
Design and Optimization of Permanent Magnet Brushless Machines for Electric Vehicle Applications
Energies 2015, 8(12), 13996-14008; https://doi.org/10.3390/en81212410
Received: 19 September 2015 / Accepted: 2 November 2015 / Published: 10 December 2015
Cited by 15 | Viewed by 2845 | PDF Full-text (4782 KB) | HTML Full-text | XML Full-text
Abstract
In this paper, by considering and establishing the relationship between the maximum operating speed and d-axis inductance, a new design and optimization method is proposed. Thus, a more extended constant power speed range, as well as reduced losses and increased efficiency, especially [...] Read more.
In this paper, by considering and establishing the relationship between the maximum operating speed and d-axis inductance, a new design and optimization method is proposed. Thus, a more extended constant power speed range, as well as reduced losses and increased efficiency, especially in the high-speed region, can be obtained, which is essential for electric vehicles (EVs). In the first step, the initial permanent magnet (PM) brushless machine is designed based on the consideration of the maximum speed and performance specifications in the entire operation region. Then, on the basis of increasing d-axis inductance, and meanwhile maintaining constant permanent magnet flux linkage, the PM brushless machine is optimized. The corresponding performance of the initial and optimal PM brushless machines are analyzed and compared by the finite-element method (FEM). Several tests are carried out in an EV simulation model based on the urban dynamometer driving schedule (UDDS) for evaluation. Both theoretical analysis and simulation results verify the validity of the proposed design and optimization method. Full article
(This article belongs to the Special Issue Advances in Plug-in Hybrid Vehicles and Hybrid Vehicles)
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Open AccessArticle
Combustion of Microalgae Oil and Ethanol Blended with Diesel Fuel
Energies 2015, 8(12), 13985-13995; https://doi.org/10.3390/en81212409
Received: 12 October 2015 / Revised: 2 December 2015 / Accepted: 7 December 2015 / Published: 10 December 2015
Cited by 5 | Viewed by 2286 | PDF Full-text (2875 KB) | HTML Full-text | XML Full-text
Abstract
Using renewable oxygenated fuels such as ethanol is a proposed method to reduce diesel engine emission. Ethanol has lower density, viscosity, cetane number and calorific value than petroleum diesel (PD). Microalgae oil is renewable, environmentally friendly and has the potential to replace PD. [...] Read more.
Using renewable oxygenated fuels such as ethanol is a proposed method to reduce diesel engine emission. Ethanol has lower density, viscosity, cetane number and calorific value than petroleum diesel (PD). Microalgae oil is renewable, environmentally friendly and has the potential to replace PD. In this paper, microalgae oil (10%) and ethanol (10%) have been mixed and added to (80%) diesel fuel as a renewable source of oxygenated fuel. The mixture of microalgae oil, ethanol and petroleum diesel (MOE20%) has been found to be homogenous and stable without using surfactant. The presence of microalgae oil improved the ethanol fuel demerits such as low density and viscosity. The transesterification process was not required for oil viscosity reduction due to the presence of ethanol. The MOE20% fuel has been tested in a variable compression ratio diesel engine at different speed. The engine test results with MOE20% showed a very comparable engine performance of in-cylinder pressure, brake power, torque and brake specific fuel consumption (BSFC) to that of PD. The NOx emission and HC have been improved while CO and CO2 were found to be lower than those from PD at low engine speed. Full article
(This article belongs to the Special Issue Energy for Agriculture)
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Open AccessArticle
Electromagnetic Performance Analysis of Novel Flux-Regulatable Permanent Magnet Machines for Wide Constant-Power Speed Range Operation
Energies 2015, 8(12), 13971-13984; https://doi.org/10.3390/en81212407
Received: 27 August 2015 / Revised: 18 November 2015 / Accepted: 30 November 2015 / Published: 10 December 2015
Cited by 5 | Viewed by 1782 | PDF Full-text (5779 KB) | HTML Full-text | XML Full-text
Abstract
Two novel structures of permanent magnet (PM) machine, namely a hybrid excitation flux modulation machine (HEFMM) and a variable flux memory machine (VFMM), which have excellent field-weakening capability, are presented in this paper. The HEFMM incorporates the advantages of parallel hybrid excitation structure [...] Read more.
Two novel structures of permanent magnet (PM) machine, namely a hybrid excitation flux modulation machine (HEFMM) and a variable flux memory machine (VFMM), which have excellent field-weakening capability, are presented in this paper. The HEFMM incorporates the advantages of parallel hybrid excitation structure and flux modulation structure, so as to increase the torque density as well as increase the constant-power speed range. Inspired by the HEFMM, aiming to further improve the efficiency of machine, the VFMM with aluminum-nickel-cobalt (AlNiCo) PMs in the inner stator which can be magnetized by the current pulse of the direct current (DC) windings is developed. With double-stator structure, flux modulation effect in both machines can be employed to realize the hybrid excitation and regulate the air-gap flux density readily. The operation principle is illustrated and the static and steady performances of the machines are analyzed and compared with time stepping finite element analysis, which validates the effectiveness of the proposed designs. Full article
(This article belongs to the Special Issue Energy Efficient Actuators and Systems)
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Open AccessArticle
TOPSIS Multi-Criteria Decision Modeling Approach for Biolubricant Selection for Two-Stroke Petrol Engines
Energies 2015, 8(12), 13960-13970; https://doi.org/10.3390/en81212408
Received: 15 October 2015 / Revised: 10 November 2015 / Accepted: 25 November 2015 / Published: 9 December 2015
Cited by 8 | Viewed by 2516 | PDF Full-text (3816 KB) | HTML Full-text | XML Full-text
Abstract
Exhaust pollutants from two-stroke petrol engines are a problem for the environment. Biolubricants are a new generation of renewable and eco-friendly vegetable-based lubricants, which have attracted a lot of attention in recent years. In this paper, the applicability of the Technique for Order [...] Read more.
Exhaust pollutants from two-stroke petrol engines are a problem for the environment. Biolubricants are a new generation of renewable and eco-friendly vegetable-based lubricants, which have attracted a lot of attention in recent years. In this paper, the applicability of the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) method to support the process of building the scoring system for selecting an appropriate two-stroke lubricant has been analyzed. For this purpose, biolubricants (TMP-triesters) based on castor oil, palm oil, and waste cooking oil were produced and then utilized in a 200 cc two-stroke gasoline engine to investigate their effects on its performance and exhaust emissions. The results obtained from the use of the entropy technique in the TOPSIS algorithm showed that palm oil-based lubricant took up the greatest distance from the Negative Ideal Solution (NIS) and was selected as the most optimal lubricant for these types of engines. Full article
(This article belongs to the Special Issue Energy for Agriculture)
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Open AccessTechnical Note
A Simple Expression for the Tortuosity of Gas Transport Paths in Solid Oxide Fuel Cells’ Porous Electrodes
Energies 2015, 8(12), 13953-13959; https://doi.org/10.3390/en81212406
Received: 28 October 2015 / Revised: 23 November 2015 / Accepted: 1 December 2015 / Published: 9 December 2015
Cited by 7 | Viewed by 1869 | PDF Full-text (1282 KB) | HTML Full-text | XML Full-text
Abstract
Based on the three-dimensional (3D) cube packing model, a simple expression for the tortuosity of gas transport paths in solid oxide fuel cells’ (SOFC) porous electrodes is developed. The proposed tortuosity expression reveals the dependence of the tortuosity on porosity, which is capable [...] Read more.
Based on the three-dimensional (3D) cube packing model, a simple expression for the tortuosity of gas transport paths in solid oxide fuel cells’ (SOFC) porous electrodes is developed. The proposed tortuosity expression reveals the dependence of the tortuosity on porosity, which is capable of providing results that are very consistent with the experimental data in the practical porosity range of SOFC. Furthermore, for the high porosity (>0.6), the proposed tortuosity expression is also accurate. This might be helpful for understanding the physical mechanism for the tortuosity of gas transport paths in electrodes and the optimization electrode microstructure for reducing the concentration polarization. Full article
(This article belongs to the Special Issue Solid Oxide Fuel Cells)
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