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Energies, Volume 5, Issue 5 (May 2012), Pages 1292-1704

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Research

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Open AccessArticle Heat Transfer Analysis of Methane Hydrate Sediment Dissociation in a Closed Reactor by a Thermal Method
Energies 2012, 5(5), 1292-1308; doi:10.3390/en5051292
Received: 15 March 2012 / Revised: 13 April 2012 / Accepted: 18 April 2012 / Published: 2 May 2012
Cited by 22 | PDF Full-text (7933 KB) | HTML Full-text | XML Full-text
Abstract
The heat transfer analysis of hydrate-bearing sediment involved phase changes is one of the key requirements of gas hydrate exploitation techniques. In this paper, experiments were conducted to examine the heat transfer performance during hydrate formation and dissociation by a thermal method [...] Read more.
The heat transfer analysis of hydrate-bearing sediment involved phase changes is one of the key requirements of gas hydrate exploitation techniques. In this paper, experiments were conducted to examine the heat transfer performance during hydrate formation and dissociation by a thermal method using a 5L volume reactor. This study simulated porous media by using glass beads of uniform size. Sixteen platinum resistance thermometers were placed in different position in the reactor to monitor the temperature differences of the hydrate in porous media. The influence of production temperature on the production time was also investigated. Experimental results show that there is a delay when hydrate decomposed in the radial direction and there are three stages in the dissociation period which is influenced by the rate of hydrate dissociation and the heat flow of the reactor. A significant temperature difference along the radial direction of the reactor was obtained when the hydrate dissociates and this phenomenon could be enhanced by raising the production temperature. In addition, hydrate dissociates homogeneously and the temperature difference is much smaller than the other conditions when the production temperature is around the 10 °C. With the increase of the production temperature, the maximum of ΔToi grows until the temperature reaches 40 °C. The period of ΔToi have a close relation with the total time of hydrate dissociation. Especially, the period of ΔToi with production temperature of 10 °C is twice as much as that at other temperatures. Under these experimental conditions, the heat is mainly transferred by conduction from the dissociated zone to the dissociating zone and the production temperature has little effect on the convection of the water in the porous media. Full article
(This article belongs to the Special Issue Natural Gas Hydrate 2011)
Open AccessArticle Evidence of Gas Hydrates in Block 26—Offshore Trinidad
Energies 2012, 5(5), 1309-1320; doi:10.3390/en5051309
Received: 15 March 2012 / Revised: 26 March 2012 / Accepted: 22 April 2012 / Published: 3 May 2012
Cited by 3 | PDF Full-text (1535 KB) | HTML Full-text | XML Full-text
Abstract
Natural gas hydrates are increasingly viewed as a potential economic resource as energy demands rise. In this study, three-dimensional seismic data for Block 26 in the Atlantic Continental Margin offshore Trinidad were evaluated to determine if there is the potential for oceanic [...] Read more.
Natural gas hydrates are increasingly viewed as a potential economic resource as energy demands rise. In this study, three-dimensional seismic data for Block 26 in the Atlantic Continental Margin offshore Trinidad were evaluated to determine if there is the potential for oceanic hydrate-bearing sediments. The seismic dataset covered an area of approximately 1210 km2 of the continental slope. A bottom simulating reflector which generally ran parallel to the sea floor and cut the dominant stratigraphy was observed and mapped over approximately 43% of the study area. Full article
(This article belongs to the Special Issue Natural Gas Hydrate 2011)
Open AccessArticle An Improved Maximum Power Point Tracking Method for Wind Power Systems
Energies 2012, 5(5), 1339-1354; doi:10.3390/en5051339
Received: 6 April 2012 / Revised: 1 May 2012 / Accepted: 2 May 2012 / Published: 7 May 2012
Cited by 23 | PDF Full-text (2131 KB) | HTML Full-text | XML Full-text
Abstract
This paper proposes an improved maximum power point tracking (MPPT) method for wind power systems. The proposed method combines hysteresis control with tip speed ratio (TSR) control using a power coefficient curve. It has fast dynamic characteristics with the TSR control using [...] Read more.
This paper proposes an improved maximum power point tracking (MPPT) method for wind power systems. The proposed method combines hysteresis control with tip speed ratio (TSR) control using a power coefficient curve. It has fast dynamic characteristics with the TSR control using data obtained from an anemometer. Moreover it can track the maximum power point (MPP) with hysteresis control even when there is incomplete data. Since the proposed method selects the operating mode according to the operation state of the generator of three control modes, the controllers do not interfere with each other and they provide excellent performance. The effectiveness of the proposed algorithm is verified by simulation and experiments based on a 3 kW wind turbine system. Full article
(This article belongs to the Special Issue Wind Turbines)
Open AccessArticle Short-Term Solar Irradiance Forecasting Model Based on Artificial Neural Network Using Statistical Feature Parameters
Energies 2012, 5(5), 1355-1370; doi:10.3390/en5051355
Received: 15 February 2012 / Revised: 22 April 2012 / Accepted: 27 April 2012 / Published: 9 May 2012
Cited by 33 | PDF Full-text (356 KB) | HTML Full-text | XML Full-text
Abstract
Short-term solar irradiance forecasting (STSIF) is of great significance for the optimal operation and power predication of grid-connected photovoltaic (PV) plants. However, STSIF is very complex to handle due to the random and nonlinear characteristics of solar irradiance under changeable weather conditions. [...] Read more.
Short-term solar irradiance forecasting (STSIF) is of great significance for the optimal operation and power predication of grid-connected photovoltaic (PV) plants. However, STSIF is very complex to handle due to the random and nonlinear characteristics of solar irradiance under changeable weather conditions. Artificial Neural Network (ANN) is suitable for STSIF modeling and many research works on this topic are presented, but the conciseness and robustness of the existing models still need to be improved. After discussing the relation between weather variations and irradiance, the characteristics of the statistical feature parameters of irradiance under different weather conditions are figured out. A novel ANN model using statistical feature parameters (ANN-SFP) for STSIF is proposed in this paper. The input vector is reconstructed with several statistical feature parameters of irradiance and ambient temperature. Thus sufficient information can be effectively extracted from relatively few inputs and the model complexity is reduced. The model structure is determined by cross-validation (CV), and the Levenberg-Marquardt algorithm (LMA) is used for the network training. Simulations are carried out to validate and compare the proposed model with the conventional ANN model using historical data series (ANN-HDS), and the results indicated that the forecast accuracy is obviously improved under variable weather conditions. Full article
Open AccessArticle Characteristic Evaluation on the Cooling Performance of an Electrical Air Conditioning System Using R744 for a Fuel Cell Electric Vehicle
Energies 2012, 5(5), 1371-1383; doi:10.3390/en5051371
Received: 6 February 2012 / Revised: 4 April 2012 / Accepted: 8 May 2012 / Published: 11 May 2012
Cited by 5 | PDF Full-text (570 KB) | HTML Full-text | XML Full-text
Abstract
The objective of this study was to investigate the cooling performance characteristics of an electrical air conditioning system using R744 as an alternative of R-134a for a fuel cell electric vehicle. In order to analyze the cooling performance characteristics of the air [...] Read more.
The objective of this study was to investigate the cooling performance characteristics of an electrical air conditioning system using R744 as an alternative of R-134a for a fuel cell electric vehicle. In order to analyze the cooling performance characteristics of the air conditioning system using R744 for a fuel cell electric vehicle, an electrical air conditioning system using R744 was developed and tested under various operating conditions according to both inlet air conditions of the gas cooler and evaporator and compressor speed. The cooling capacity and coefficient of performance (COP) forcooling of the tested air conditioning system were up to 6.4 kW and 2.5, respectively. In addition, the electrical air conditioning system with R744 using an inverter driven compressor showed better performance than the conventional air conditioning system with R-134a under the same operating conditions. The observed cooling performance of the developed electrical air conditioning system was found to be sufficient for cooling loads under various real driving conditions for a fuel cell electric vehicle. Full article
Open AccessArticle Evaluation of Net Energy Obtainable from Combustion of Stabilised Olive Mill By-Products
Energies 2012, 5(5), 1384-1397; doi:10.3390/en5051384
Received: 22 March 2012 / Revised: 23 April 2012 / Accepted: 10 May 2012 / Published: 11 May 2012
Cited by 15 | PDF Full-text (574 KB) | HTML Full-text | XML Full-text
Abstract
This work is aimed at calculating the energy content of the residues from olive oil production. Olive pulp, olive husk and sludge (a mixture of olive pulp and husk) have been analyzed separately. Olive Mill Effluents (OME) are normally a problem for [...] Read more.
This work is aimed at calculating the energy content of the residues from olive oil production. Olive pulp, olive husk and sludge (a mixture of olive pulp and husk) have been analyzed separately. Olive Mill Effluents (OME) are normally a problem for olive mill farms, yet they may be used as feedstock for biomass-fuelled power plants. Nonetheless, OMEs are characterized by a relatively high humidity content and are produced only during the olive season. Thus, OME need a stabilization process to be employed as a solid biofuel throughout the year. The analyses conducted attempt an evaluation of the energy consumption of a three-stage stabilization process: drying, milling and pelletising. The net electrical energy available from OME is then calculated as a difference between gross energy available and energy consumed for stabilization. The gross available electrical energy was calculated based on direct energy conversion of the stabilized feedstock on a small scale direct combustion and Organic Rankine Cycle (ORC) turbine. Results show that OME are suitable for energy production. Approximately 4500 kJ/kg of net electrical energy may be obtained out of olive sludge or olive pulp, while olive husk shows a potential gross energy of 3400 kJ/kg. Full article
(This article belongs to the Special Issue Biomass and Biofuels 2012)
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Open AccessArticle A Novel Maximum Power Point Tracking Control for Permanent Magnet Direct Drive Wind Energy Conversion Systems
Energies 2012, 5(5), 1398-1412; doi:10.3390/en5051398
Received: 20 March 2012 / Revised: 9 May 2012 / Accepted: 11 May 2012 / Published: 15 May 2012
Cited by 15 | PDF Full-text (1150 KB) | HTML Full-text | XML Full-text
Abstract
This paper proposes a novel optimal current given (OCG) maximum power point tracking (MPPT) control strategy based on the theory of power feedback and hill climb searching (HCS) for a permanent magnet direct drive wind energy conversion system (WECS). The presented strategy [...] Read more.
This paper proposes a novel optimal current given (OCG) maximum power point tracking (MPPT) control strategy based on the theory of power feedback and hill climb searching (HCS) for a permanent magnet direct drive wind energy conversion system (WECS). The presented strategy not only has the advantages of not needing the wind speed and wind turbine characteristics of the traditional HCS method, but it also improves the stability and accuracy of MPPT by estimating the exact loss torque. The OCG MPPT control strategy is first carried out by simulation, then an experimental platform based on the dSPACE1103 controller is built and a 5.5 kW permanent magnet synchronous generator (PMSG) is tested. Furthermore, the proposed method is compared experimentally with the traditional optimum tip speed ratio (TSR) MPPT control. The experiments verify the effectiveness of the proposed OCG MPPT strategy and demonstrate its better performance than the traditional TSR MPPT control. Full article
(This article belongs to the Special Issue Wind Turbines)
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Open AccessArticle Finding Multiple Optimal Solutions to Optimal Load Distribution Problem in Hydropower Plant
Energies 2012, 5(5), 1413-1432; doi:10.3390/en5051413
Received: 29 March 2012 / Revised: 6 May 2012 / Accepted: 8 May 2012 / Published: 15 May 2012
Cited by 7 | PDF Full-text (3255 KB) | HTML Full-text | XML Full-text
Abstract
Optimal load distribution (OLD) among generator units of a hydropower plant is a vital task for hydropower generation scheduling and management. Traditional optimization methods for solving this problem focus on finding a single optimal solution. However, many practical constraints on hydropower plant [...] Read more.
Optimal load distribution (OLD) among generator units of a hydropower plant is a vital task for hydropower generation scheduling and management. Traditional optimization methods for solving this problem focus on finding a single optimal solution. However, many practical constraints on hydropower plant operation are very difficult, if not impossible, to be modeled, and the optimal solution found by those models might be of limited practical uses. This motivates us to find multiple optimal solutions to the OLD problem, which can provide more flexible choices for decision-making. Based on a special dynamic programming model, we use a modified shortest path algorithm to produce multiple solutions to the problem. It is shown that multiple optimal solutions exist for the case study of China’s Geheyan hydropower plant, and they are valuable for assessing the stability of generator units, showing the potential of reducing occurrence times of units across vibration areas. Full article
Open AccessArticle Numerical Modelling of Mutual Effect among Nearby Needles in a Multi-Needle Configuration of an Atmospheric Air Dielectric Barrier Discharge
Energies 2012, 5(5), 1433-1454; doi:10.3390/en5051433
Received: 29 March 2012 / Revised: 23 April 2012 / Accepted: 2 May 2012 / Published: 15 May 2012
Cited by 1 | PDF Full-text (3996 KB) | HTML Full-text | XML Full-text
Abstract
A numerical study has been conducted to understand the mutual effect among nearby needles in a multi-needle electrode dielectric barrier discharge. In the present paper, a fluid-hydrodynamic model is adopted. In this model, the mutual effect among nearby needles in a multi-needle [...] Read more.
A numerical study has been conducted to understand the mutual effect among nearby needles in a multi-needle electrode dielectric barrier discharge. In the present paper, a fluid-hydrodynamic model is adopted. In this model, the mutual effect among nearby needles in a multi-needle configuration of an atmospheric air dielectric barrier discharge are investigated using a fluid-hydrodynamic model including the continuity equations for electrons and positive and negative ions coupled with Poisson’s equation. The electric fields at the streamer head of the middle needle (MN) and the side needles (SNs) in a three-needle model decreased under the influence of the mutual effects of nearby needles compared with that in the single-needle model. In addition, from the same comparison, the average propagation velocities of the streamers from MN and SNs, the electron average energy profile of MN and SNs (including those in the streamer channel, at the streamer head, and in the unbridged gap), and the electron densities at the streamer head of the MN and SNs also decreased. The results obtained in the current paper agreed well with the experimental and simulation results in the literature. Full article
Open AccessArticle Online Estimation of Peak Power Capability of Li-Ion Batteries in Electric Vehicles by a Hardware-in-Loop Approach
Energies 2012, 5(5), 1455-1469; doi:10.3390/en5051455
Received: 5 April 2012 / Revised: 9 May 2012 / Accepted: 11 May 2012 / Published: 15 May 2012
Cited by 28 | PDF Full-text (347 KB) | HTML Full-text | XML Full-text
Abstract
Battery peak power capability estimations play an important theoretical role for the proper use of the battery in electric vehicles. To address the failures in relaxation effects and real-time ability performance, neglecting the battery’s design limits and other issues of the traditional [...] Read more.
Battery peak power capability estimations play an important theoretical role for the proper use of the battery in electric vehicles. To address the failures in relaxation effects and real-time ability performance, neglecting the battery’s design limits and other issues of the traditional peak power capability calculation methods, a new approach based on the dynamic electrochemical-polarization (EP) battery model, taking into consideration constraints of current, voltage, state of charge (SoC) and power is proposed. A hardware-in-the-loop (HIL) system is built for validating the online model-based peak power capability estimation approach of batteries used in hybrid electric vehicles (HEVs) and a HIL test based on the Federal Urban Driving Schedules (FUDS) is used to verify and evaluate its real-time computation performance, reliability and robustness. The results show the proposed approach gives a more accurate estimate compared with the hybrid pulse power characterization (HPPC) method, avoiding over-charging or over-discharging and providing a powerful guarantee for the optimization of HEVs power systems. Furthermore, the HIL test provides valuable data and critical guidance to evaluate the accuracy of the developed battery algorithms. Full article
(This article belongs to the Special Issue Vehicle to Grid)
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Open AccessArticle Efficiency of Small Scale Manually Fed Boilers —Mathematical Models
Energies 2012, 5(5), 1470-1489; doi:10.3390/en5051470
Received: 27 February 2012 / Revised: 17 April 2012 / Accepted: 3 May 2012 / Published: 15 May 2012
Cited by 1 | PDF Full-text (358 KB) | HTML Full-text | XML Full-text
Abstract
This study reviews test results for a biomass-fired hot water boiler with a nominal boiler thermal power of 120 kW. In the experiments, prismatic wheat straw bales were used as biomass. The impact of the quantity (220, 290, 360 and 430 m [...] Read more.
This study reviews test results for a biomass-fired hot water boiler with a nominal boiler thermal power of 120 kW. In the experiments, prismatic wheat straw bales were used as biomass. The impact of the quantity (220, 290, 360 and 430 m3 h−1) of inlet air fed to the boiler firebox was continuously monitored. This was to examine the influence of the quantity of inlet air and recirculation (0, 16.5 and 33%) of combustion products on the boiler thermal power and boiler energy efficiency. Thus, the following mathematical models and formulas were presented: correlation between boiler thermal power and bale residence time; bale mass loss during the combustion process; correlation between boiler energy efficiency and bale residence time. Mathematical models were obtained by using experimental data and by applying nonlinear regression analysis. Adjustment evaluation of mathematical models with experimental data was performed based on the determination coefficient, t-test and F-test. Increase the amount of air throughout the firebox produced boiler thermal power increase and bale residence time decrease. It was shown that combustion products recirculation of 16.5% partly improved boiler characteristics, while the recirculation of 33% did not, comparing with the case without recirculation. Full article
Open AccessArticle A Transformer Partial Discharge Measurement System Based on Fluorescent Fiber
Energies 2012, 5(5), 1490-1502; doi:10.3390/en5051490
Received: 28 December 2011 / Revised: 9 May 2012 / Accepted: 9 May 2012 / Published: 16 May 2012
Cited by 4 | PDF Full-text (450 KB) | HTML Full-text | XML Full-text
Abstract
Based on the physical phenomena of optical effects produced by the partial discharge (PD) and on the characteristics of fluorescent fiber sensing of weak fluorescent signals, a PD measurement system using a fluorescent fiber sensor was designed. The main parameters of the [...] Read more.
Based on the physical phenomena of optical effects produced by the partial discharge (PD) and on the characteristics of fluorescent fiber sensing of weak fluorescent signals, a PD measurement system using a fluorescent fiber sensor was designed. The main parameters of the sensing system were calculated, an experimental testing platform for PD simulation in the lab was established, and PD signals were then detected through ultra-high frequency (UHF) and optical methods under a needle-plate discharge model. PD optical pulses in transformer oil contained signal-peak and multi-peak pulse waveforms. Compared with UHF detection results, the number of PD pulses and the elapsed PD pulse phase time revealed a good corresponding relationship. However, PD signal amplitudes presented the opposite, thus indicating that PD UHF signals reflected pulse amplitude value, whereas PD optical signals reflected pulse energy magnitude. The n-u-φ three-dimensional distributions indicated that most of the PD signals concentrated in the nearby industrial frequency voltage peak value. Overall, the proposed fluorescent fiber sensing system design can be used successfully in transformer PD signal detection. Full article
Open AccessArticle Probabilistic Method to Assess the Impact of Charging of Electric Vehicles on Distribution Grids
Energies 2012, 5(5), 1503-1531; doi:10.3390/en5051503
Received: 22 April 2012 / Revised: 5 May 2012 / Accepted: 9 May 2012 / Published: 18 May 2012
Cited by 6 | PDF Full-text (3851 KB) | HTML Full-text | XML Full-text
Abstract
This paper describes a grid impact analysis of charging electric vehicles (EV) using charging curves with detailed battery modelling. A probabilistic method using Monte Carlo was applied to a typical Spanish distribution grid, also using mobility patterns of Barcelona. To carry out [...] Read more.
This paper describes a grid impact analysis of charging electric vehicles (EV) using charging curves with detailed battery modelling. A probabilistic method using Monte Carlo was applied to a typical Spanish distribution grid, also using mobility patterns of Barcelona. To carry out this analysis, firstly, an IEEE test system was adapted to a typical distribution grid configuration; secondly, the EV and its battery types were modeled taking into account the current vehicle market and the battery characteristics; and, finally, the recharge control strategies were taken into account. Once these main features were established, a statistical probabilistic model for the household electrical demand and for the EV charging parameters was determined. Finally, with these probabilistic models, the Monte Carlo analysis was performed within the established scenario in order to study the lines’ and the transformers’ loading levels. The results show that an accurate model for the battery gives a more precise estimation about the impact on the grid. Additionally, mobility patterns have been proved to be some of the most important key aspects for these type of studies. Full article
Open AccessArticle High Lipid Induction in Microalgae for Biodiesel Production
Energies 2012, 5(5), 1532-1553; doi:10.3390/en5051532
Received: 30 March 2012 / Revised: 3 May 2012 / Accepted: 8 May 2012 / Published: 18 May 2012
Cited by 189 | PDF Full-text (349 KB) | HTML Full-text | XML Full-text
Abstract
Oil-accumulating microalgae have the potential to enable large-scale biodiesel production without competing for arable land or biodiverse natural landscapes. High lipid productivity of dominant, fast-growing algae is a major prerequisite for commercial production of microalgal oil-derived biodiesel. However, under optimal growth conditions, [...] Read more.
Oil-accumulating microalgae have the potential to enable large-scale biodiesel production without competing for arable land or biodiverse natural landscapes. High lipid productivity of dominant, fast-growing algae is a major prerequisite for commercial production of microalgal oil-derived biodiesel. However, under optimal growth conditions, large amounts of algal biomass are produced, but with relatively low lipid contents, while species with high lipid contents are typically slow growing. Major advances in this area can be made through the induction of lipid biosynthesis, e.g., by environmental stresses. Lipids, in the form of triacylglycerides typically provide a storage function in the cell that enables microalgae to endure adverse environmental conditions. Essentially algal biomass and triacylglycerides compete for photosynthetic assimilate and a reprogramming of physiological pathways is required to stimulate lipid biosynthesis. There has been a wide range of studies carried out to identify and develop efficient lipid induction techniques in microalgae such as nutrients stress (e.g., nitrogen and/or phosphorus starvation), osmotic stress, radiation, pH, temperature, heavy metals and other chemicals. In addition, several genetic strategies for increased triacylglycerides production and inducibility are currently being developed. In this review, we discuss the potential of lipid induction techniques in microalgae and also their application at commercial scale for the production of biodiesel. Full article
(This article belongs to the Special Issue Algae Fuel)
Open AccessArticle DC Flashover Performance of Various Types of Ice-Covered Insulator Strings under Low Air Pressure
Energies 2012, 5(5), 1554-1576; doi:10.3390/en5051554
Received: 9 April 2012 / Revised: 15 May 2012 / Accepted: 15 May 2012 / Published: 21 May 2012
Cited by 1 | PDF Full-text (1378 KB) | HTML Full-text | XML Full-text
Abstract
In this study, icing flashover performance tests of typical DC porcelain, glass, and composite insulators are systematically carried out in a multifunction artificial climate chamber. The DC icing flashover voltages of seven typical insulators under various conditions of icing thickness, pollution severity [...] Read more.
In this study, icing flashover performance tests of typical DC porcelain, glass, and composite insulators are systematically carried out in a multifunction artificial climate chamber. The DC icing flashover voltages of seven typical insulators under various conditions of icing thickness, pollution severity before icing, string length, and atmospheric pressure are obtained. The relationships between icing thickness, salt deposit density as well as atmospheric pressure and the 50% icing flashover voltage are analyzed, and the formulas are obtained by regression method. In addition, the DC icing flashover voltage correction method of typical porcelain, glass, and composite insulator in the coexisting condition of high altitude, contamination, and icing is proposed. Full article
Open AccessArticle Seasonal Variation of Lipids and Fatty Acids of the Microalgae Nannochloropsis oculata Grown in Outdoor Large-Scale Photobioreactors
Energies 2012, 5(5), 1577-1592; doi:10.3390/en5051577
Received: 12 March 2012 / Revised: 3 May 2012 / Accepted: 9 May 2012 / Published: 21 May 2012
Cited by 45 | PDF Full-text (1110 KB) | HTML Full-text | XML Full-text
Abstract
While focus in oil-producing microalgae is normally on nutrient deficiency, we addressed the seasonal variations of lipid content and composition in large-scale cultivation. Lipid content, fatty acid profiles and mono- di- and triglycerides (MAGs, DAGs, and TAGs) were analyzed during May 2007–May [...] Read more.
While focus in oil-producing microalgae is normally on nutrient deficiency, we addressed the seasonal variations of lipid content and composition in large-scale cultivation. Lipid content, fatty acid profiles and mono- di- and triglycerides (MAGs, DAGs, and TAGs) were analyzed during May 2007–May 2009 in Nannochloropsis oculata grown outdoors in closed vertical flat panels photobioreactors. Total lipids (TL) ranged from 11% of dry weight (DW) in winter to 30% of DW in autumn. 50% of the variation in TL could be explained by light and temperature. As the highest lipid content was recorded during autumn indicating an optimal, non-linear, response to light and temperature we hypothesize that enhanced thylakoid stacking under reduced light conditions resulted in more structural lipids, concomitantly with the increase in glycerides due to released photo-oxidative stress. The relative amount of monounsaturated fatty acids (MUFA) increased during autumn. This suggested a synthesis, either of structural fatty acids as MUFA, or a relative increase of C16:1 incorporated into TAGs and DAGs. Our results emphasize the significant role of environmental conditions governing lipid content and composition in microalgae that have to be considered for correct estimation of algal oil yields in biodiesel production. Full article
(This article belongs to the Special Issue Algae Fuel)
Open AccessArticle A Power Smoothing Control Strategy and Optimized Allocation of Battery Capacity Based on Hybrid Storage Energy Technology
Energies 2012, 5(5), 1593-1612; doi:10.3390/en5051593
Received: 19 November 2011 / Revised: 12 April 2012 / Accepted: 3 May 2012 / Published: 21 May 2012
Cited by 12 | PDF Full-text (562 KB) | HTML Full-text | XML Full-text
Abstract
Wind power parallel operation is an effective way to realize the large scale use of wind power, but the fluctuations of power output from wind power units may have great influence on power quality, hence a new method of power smoothing and [...] Read more.
Wind power parallel operation is an effective way to realize the large scale use of wind power, but the fluctuations of power output from wind power units may have great influence on power quality, hence a new method of power smoothing and capacity optimized allocation based on hybrid energy storage technology is proposed in terms of the uncontrollable and unexpected characteristics of wind speed in wind farms. First, power smoothing based on a traditional Inertial Filter is introduced and the relationship between the time constant, its smoothing effect and capacity allocation are analyzed and combined with Proportional Integral Differential (PID) control to realize power smoothing control of wind power. Then wavelet theory is adopted to realize a multi-layer decomposition of power output in some wind farms, a power smoothing model based on hybrid energy storage technology is constructed combining the characteristics of the Super Capacitor (SC) and Battery Energy Storage System (BESS) technologies. The hybrid energy storage system is available for power fluctuations with high frequency-low energy and low frequency-high energy to achieve good smoothing effects compared with a single energy storage system. The power fluctuations filtered by the Wavelet Transform is regarded as the target value of BESS, the charging and discharging control for battery is completed quickly by Model Algorithm Control (MAC). Because of the influence of the inertia and the response speed of the battery, its actual output is not completely equal to the target value which mainly reflects in high-frequency part, the difference part uses SC to compensate and makes the output of battery and SC closer to the target value on the whole. Compared with the traditional Inertial Filter and PID control method, the validity of the model was verified by simulation results. Finally under the premise of power grid standards, the corresponding capacity design had been given to reduce the size of the energy storage devices as far as possible, which has a certain practical engineering value. Full article
(This article belongs to the Special Issue Wind Turbines)
Open AccessArticle Cup Anemometers’ Loss of Performance Due to Ageing Processes, and Its Effect on Annual Energy Production (AEP) Estimates
Energies 2012, 5(5), 1664-1685; doi:10.3390/en5051664
Received: 22 March 2012 / Revised: 30 April 2012 / Accepted: 16 May 2012 / Published: 23 May 2012
Cited by 8 | PDF Full-text (318 KB) | HTML Full-text | XML Full-text
Abstract
The deviation of calibration coefficients from five cup anemometer models over time was analyzed. The analysis was based on a series of laboratory calibrations between January 2001 and August 2010. The analysis was performed on two different groups of anemometers: (1) anemometers [...] Read more.
The deviation of calibration coefficients from five cup anemometer models over time was analyzed. The analysis was based on a series of laboratory calibrations between January 2001 and August 2010. The analysis was performed on two different groups of anemometers: (1) anemometers not used for any industrial purpose (that is, just stored); and (2) anemometers used in different industrial applications (mainly in the field—or outside—applications like wind farms). Results indicate a loss of performance of the studied anemometers over time. In the case of the unused anemometers the degradation shows a clear pattern. In the case of the anemometers used in the field, the data analyzed also suggest a loss of performance, yet the degradation does not show a clear trend. A recalibration schedule is proposed based on the observed performances variations. Full article
Open AccessArticle Study on the Development of an Optimal Heat Supply Control Algorithm for Group Energy Apartment Buildings According to the Variation of Outdoor Air Temperature
Energies 2012, 5(5), 1686-1704; doi:10.3390/en5051686
Received: 12 March 2012 / Revised: 4 May 2012 / Accepted: 18 May 2012 / Published: 24 May 2012
Cited by 1 | PDF Full-text (793 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
In the present study, we have developed an optimal heat supply algorithm which minimizes the heat loss through the distribution pipe line in a group energy apartment. Heating load variation of a group energy apartment building according to the outdoor air temperature [...] Read more.
In the present study, we have developed an optimal heat supply algorithm which minimizes the heat loss through the distribution pipe line in a group energy apartment. Heating load variation of a group energy apartment building according to the outdoor air temperature was predicted by a correlation obtained from calorimetry measurements of all households in the apartment building. Supply water temperature and mass flow rate were simultaneously controlled to minimize the heat loss rate through the distribution pipe line. A group heating apartment building located in Hwaseong city, Korea, which has 1473 households, was selected as the object building to test the present heat supply algorithm. Compared to the original heat supply system, the present system adopting the proposed control algorithm reduced the heat loss rate by 10.4%. Full article
(This article belongs to the Special Issue Energy Efficient Buildings and Green Buildings)

Review

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Open AccessReview The Smart Grids in China—A Review
Energies 2012, 5(5), 1321-1338; doi:10.3390/en5051321
Received: 2 March 2012 / Revised: 31 March 2012 / Accepted: 26 April 2012 / Published: 4 May 2012
Cited by 23 | PDF Full-text (347 KB) | HTML Full-text | XML Full-text
Abstract
The concept of the smart grid has been gaining more and more attention worldwide since it was proposed by the U.S. Electric Power Research Institute in 2001. Recently, it has been propelled again by the promotion of low carbon economies in developing [...] Read more.
The concept of the smart grid has been gaining more and more attention worldwide since it was proposed by the U.S. Electric Power Research Institute in 2001. Recently, it has been propelled again by the promotion of low carbon economies in developing countries. To satisfy the exponential increase in electricity demand and alleviate environmental degradation caused by fossil fuel-based power generation, China has made great efforts in constructing a smart grid as a substitution of traditional energy-intensive power grid. In the 12th Five-Year Plan in particular, it was stated that emphasis should be placed on the development of renewable energy and smart grids. The objective of this paper is to provide an insight into the current research on smart grids, and shed light on the development of smart grids in China, based on the analysis of which, the obstacles and barriers in the development process are identified. Finally, policy prospects on the construction of smart grids in China are proposed from the aspects of technology, administration and management. Full article
Open AccessReview Achieving a Green Solution: Limitations and Focus Points for Sustainable Algal Fuels
Energies 2012, 5(5), 1613-1647; doi:10.3390/en5051613
Received: 6 March 2012 / Revised: 29 April 2012 / Accepted: 9 May 2012 / Published: 21 May 2012
Cited by 14 | PDF Full-text (324 KB) | HTML Full-text | XML Full-text
Abstract
Research investigating the potential of producing biofuels from algae has been enjoying a recent revival due to heightened oil prices, uncertain fossil fuel sources and legislative targets aimed at reducing our contribution to climate change. If the concept is to become a [...] Read more.
Research investigating the potential of producing biofuels from algae has been enjoying a recent revival due to heightened oil prices, uncertain fossil fuel sources and legislative targets aimed at reducing our contribution to climate change. If the concept is to become a reality however, many obstacles need to be overcome. Recent studies have suggested that open ponds provide the most sustainable means of cultivation infrastructure due to their low energy inputs compared to more energy intensive photobioreactors. Most studies have focused on strains of algae which are capable of yielding high oil concentrations combined with high productivity. Yet it is very difficult to cultivate such strains in open ponds as a result of microbial competition and limited radiation-use efficiency. To improve viability, the use of wastewater has been considered by many researchers as a potential source of nutrients with the added benefit of tertiary water treatment however productivity rates are affected and optimal conditions can be difficult to maintain year round. This paper investigates the process streams which are likely to provide the most viable methods of energy recovery from cultivating and processing algal biomass. The key findings are the importance of a flexible approach which depends upon location of the cultivation ponds and the industry targeted. Additionally this study recommends moving towards technologies producing higher energy recoveries such as pyrolysis or anaerobic digestion as opposed to other studies which focused upon biodiesel production. Full article
(This article belongs to the Special Issue Algae Fuel)
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Open AccessTechnical Note A Smart Green Building: An Environmental Health Control Design
Energies 2012, 5(5), 1648-1663; doi:10.3390/en5051648
Received: 14 March 2012 / Revised: 11 May 2012 / Accepted: 14 May 2012 / Published: 21 May 2012
Cited by 2 | PDF Full-text (1579 KB) | HTML Full-text | XML Full-text
Abstract
This study proposes the establishment of an environmental health information management platform providing residential users with a comfortable, healthy indoor environment. Taking the S House as an example, the study: (1) assigned environmental health performance indicators, (2) established constraints to maintain environmental [...] Read more.
This study proposes the establishment of an environmental health information management platform providing residential users with a comfortable, healthy indoor environment. Taking the S House as an example, the study: (1) assigned environmental health performance indicators, (2) established constraints to maintain environmental conditions, and (3) provided optimized management control mechanisms and methods. The environmental health information management platform provides an optimized control and solution pathway ensuring the quality of the indoor health environment and equipment energy conservation. Full article
(This article belongs to the Special Issue Energy Efficient Buildings and Green Buildings)

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