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Energies, Volume 5, Issue 7 (July 2012), Pages 2071-2625

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Open AccessArticle On Space-Time Resolution of Inflow Representations for Wind Turbine Loads Analysis
Energies 2012, 5(7), 2071-2092; doi:10.3390/en5072071
Received: 13 April 2012 / Revised: 30 May 2012 / Accepted: 15 June 2012 / Published: 25 June 2012
Cited by 8 | PDF Full-text (3323 KB) | HTML Full-text | XML Full-text
Abstract
Efficient spatial and temporal resolution of simulated inflow wind fields is important in order to represent wind turbine dynamics and derive load statistics for design. Using Fourier-based stochastic simulation of inflow turbulence, we first investigate loads for a utility-scale turbine in the neutral
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Efficient spatial and temporal resolution of simulated inflow wind fields is important in order to represent wind turbine dynamics and derive load statistics for design. Using Fourier-based stochastic simulation of inflow turbulence, we first investigate loads for a utility-scale turbine in the neutral atmospheric boundary layer. Load statistics, spectra, and wavelet analysis representations for different space and time resolutions are compared. Next, large-eddy simulation (LES) is employed with space-time resolutions, justified on the basis of the earlier stochastic simulations, to again derive turbine loads. Extreme and fatigue loads from the two approaches used in inflow field generation are compared. On the basis of simulation studies carried out for three different wind speeds in the turbine’s operating range, it is shown that inflow turbulence described using 10-meter spatial resolution and 1 Hz temporal resolution is adequate for assessing turbine loads. Such studies on the investigation of adequate filtering or resolution of inflow wind fields help to establish efficient strategies for LES and other physical or stochastic simulation needed in turbine loads studies. Full article
(This article belongs to the Special Issue Wind Turbines)
Open AccessArticle CFD Simulation of a Concrete Cubicle to Analyze the Thermal Effect of Phase Change Materials in Buildings
Energies 2012, 5(7), 2093-2111; doi:10.3390/en5072093
Received: 3 May 2012 / Revised: 13 June 2012 / Accepted: 15 June 2012 / Published: 25 June 2012
Cited by 7 | PDF Full-text (816 KB) | HTML Full-text | XML Full-text
Abstract
In this work, a CFD-based model is proposed to analyse the effect of phase change materials (PCMs) on the thermal behaviour of the walls of a cubicle exposed to the environment and on the resistance of the walls to climate changes. The effect
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In this work, a CFD-based model is proposed to analyse the effect of phase change materials (PCMs) on the thermal behaviour of the walls of a cubicle exposed to the environment and on the resistance of the walls to climate changes. The effect of several days of exposure to the environment was simulated using the proposed method. The results of the simulation are compared with experimental data to contrast the models. The effects of exposure on the same days were simulated for several walls of a cubicle made of a mixture of concrete and PCM. The results show that the PCM stabilizes temperatures within the cubicle and decreases energy consumption of refrigeration systems. Full article
(This article belongs to the Special Issue Energy Efficient Buildings and Green Buildings)
Open AccessArticle Methane Production from Gas Hydrate Deposits through Injection of Supercritical CO2
Energies 2012, 5(7), 2112-2140; doi:10.3390/en5072112
Received: 5 April 2012 / Revised: 5 May 2012 / Accepted: 7 June 2012 / Published: 25 June 2012
Cited by 23 | PDF Full-text (1243 KB)
Abstract
The recovery of natural gas from CH4-hydrate deposits in sub-marine and sub-permafrost environments through injection of CO2 is considered a suitable strategy towards emission-neutral energy production. This study shows that the injection of hot, supercritical CO2 is particularly promising.
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The recovery of natural gas from CH4-hydrate deposits in sub-marine and sub-permafrost environments through injection of CO2 is considered a suitable strategy towards emission-neutral energy production. This study shows that the injection of hot, supercritical CO2 is particularly promising. The addition of heat triggers the dissociation of CH4-hydrate while the CO2, once thermally equilibrated, reacts with the pore water and is retained in the reservoir as immobile CO2-hydrate. Furthermore, optimal reservoir conditions of pressure and temperature are constrained. Experiments were conducted in a high-pressure flow-through reactor at different sediment temperatures (2 °C, 8 °C, 10 °C) and hydrostatic pressures (8 MPa, 13 MPa). The efficiency of both, CH4 production and CO2 retention is best at 8 °C, 13 MPa. Here, both CO2- and CH4-hydrate as well as mixed hydrates can form. At 2 °C, the production process was less effective due to congestion of transport pathways through the sediment by rapidly forming CO2-hydrate. In contrast, at 10 °C CH4 production suffered from local increases in permeability and fast breakthrough of the injection fluid, thereby confining the accessibility to the CH4 pool to only the most prominent fluid channels. Mass and volume balancing of the collected gas and fluid stream identified gas mobilization as equally important process parameter in addition to the rates of methane hydrate dissociation and hydrate conversion. Thus, the combination of heat supply and CO2 injection in one supercritical phase helps to overcome the mass transfer limitations usually observed in experiments with cold liquid or gaseous CO2. Full article
(This article belongs to the Special Issue Natural Gas Hydrate 2011)
Open AccessArticle Estimation of Power Packet Transfer Properties on Indoor Power Line Channel
Energies 2012, 5(7), 2141-2149; doi:10.3390/en5072141
Received: 3 May 2012 / Revised: 8 June 2012 / Accepted: 22 June 2012 / Published: 26 June 2012
Cited by 14 | PDF Full-text (346 KB) | HTML Full-text | XML Full-text
Abstract
The dispatching properties of power packets in indoor power line channel were investigated. A power packet is physically composed of a power payload and information signals. For evaluation, the arrival power ratio and the bit error rate were calculated by numerical simulation. The
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The dispatching properties of power packets in indoor power line channel were investigated. A power packet is physically composed of a power payload and information signals. For evaluation, the arrival power ratio and the bit error rate were calculated by numerical simulation. The results are important for the development and design of power packet transfer, based on the transmission frequency and bit energy parameters. Full article
(This article belongs to the Special Issue Electrical Power and Energy Systems)
Open AccessArticle Net Energy, CO2 Emission and Land-Based Cost-Benefit Analyses of Jatropha Biodiesel: A Case Study of the Panzhihua Region of Sichuan Province in China
Energies 2012, 5(7), 2150-2164; doi:10.3390/en5072150
Received: 16 November 2011 / Revised: 18 June 2012 / Accepted: 19 June 2012 / Published: 28 June 2012
Cited by 12 | PDF Full-text (357 KB) | HTML Full-text | XML Full-text
Abstract
Bioenergy is currently regarded as a renewable energy source with a high growth potential. Forest-based biodiesel, with the significant advantage of not competing with grain production on cultivated land, has been considered as a promising substitute for diesel fuel by many countries, including
[...] Read more.
Bioenergy is currently regarded as a renewable energy source with a high growth potential. Forest-based biodiesel, with the significant advantage of not competing with grain production on cultivated land, has been considered as a promising substitute for diesel fuel by many countries, including China. Consequently, extracting biodiesel from Jatropha curcas has become a growing industry. However, many key issues related to the development of this industry are still not fully resolved and the prospects for this industry are complicated. The aim of this paper is to evaluate the net energy, CO2 emission, and cost efficiency of Jatropha biodiesel as a substitute fuel in China to help resolve some of the key issues by studying data from this region of China that is well suited to growing Jatropha. Our results show that: (1) Jatropha biodiesel is preferable for global warming mitigation over diesel fuel in terms of the carbon sink during Jatropha tree growth. (2) The net energy yield of Jatropha biodiesel is much lower than that of fossil fuel, induced by the high energy consumption during Jatropha plantation establishment and the conversion from seed oil to diesel fuel step. Therefore, the energy efficiencies of the production of Jatropha and its conversion to biodiesel need to be improved. (3) Due to current low profit and high risk in the study area, farmers have little incentive to continue or increase Jatropha production. (4) It is necessary to provide more subsidies and preferential policies for Jatropha plantations if this industry is to grow. It is also necessary for local government to set realistic objectives and make rational plans to choose proper sites for Jatropha biodiesel development and the work reported here should assist that effort. Future research focused on breading high-yield varieties, development of efficient field management systems, and detailed studies lifecycle environmental impacts analysis is required to promote biologically and economically sustainable development of Jatropha biodiesel and to assist government agencies in setting realistic objectives and appropriate and advantageous policies for the regions and the country. Full article
(This article belongs to the Special Issue Low Carbon Transitions Worldwide)
Open AccessArticle Evaluating the Marginal Land Resources Suitable for Developing Pistacia chinensis-Based Biodiesel in China
Energies 2012, 5(7), 2165-2177; doi:10.3390/en5072165
Received: 29 April 2012 / Revised: 14 June 2012 / Accepted: 26 June 2012 / Published: 29 June 2012
Cited by 11 | PDF Full-text (498 KB) | HTML Full-text | XML Full-text
Abstract
Bio-energy from energy plants is expected to play an increasing role in the future energy system, with benefits in terms of reducing greenhouse gas emissions and improving energy security. Pistacia chinensis is believed to be one of the most promising non-food input for
[...] Read more.
Bio-energy from energy plants is expected to play an increasing role in the future energy system, with benefits in terms of reducing greenhouse gas emissions and improving energy security. Pistacia chinensis is believed to be one of the most promising non-food input for biodiesel production. This study focused on the marginal land availability for developing Pistacia chinensis-based bioenergy in China. The spatial distribution, quality and total amount of marginal land resources suitable for cultivating Pistacia chinensis were identified with multiple datasets (natural habitat, remote sensing-derived land use, meteorological and soil data) and geoinformatic techniques. The results indicate that the area of marginal land exploitable for Pistacia chinensis plantations in China is 19.90 million hectares, which may produce approximately 56.85 million tons of biodiesel each year. The spatial variation of both marginal land resources and biodiesel potential are also presented. The results can be useful for national and regional bio-energy planning. Full article
(This article belongs to the Special Issue Biomass and Biofuels 2012)
Open AccessArticle The Impact of Roof Pitch and Ceiling Insulation on Cooling Load of Naturally-Ventilated Attics
Energies 2012, 5(7), 2178-2196; doi:10.3390/en5072178
Received: 29 May 2012 / Revised: 29 June 2012 / Accepted: 29 June 2012 / Published: 4 July 2012
Cited by 2 | PDF Full-text (4048 KB) | HTML Full-text | XML Full-text
Abstract
A 2D unsteady computational fluid dynamics (CFD) model is employed to simulate buoyancy-driven turbulent ventilation in attics with different pitch values and ceiling insulation levels under summer conditions. The impacts of roof pitch and ceiling insulation on the cooling load of gable-roof residential
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A 2D unsteady computational fluid dynamics (CFD) model is employed to simulate buoyancy-driven turbulent ventilation in attics with different pitch values and ceiling insulation levels under summer conditions. The impacts of roof pitch and ceiling insulation on the cooling load of gable-roof residential buildings are investigated based on the simulation of turbulent air flow and natural convection heat transfer in attic spaces with roof pitches from 3/12 to 18/12 combined with ceiling insulation levels from R-1.2 to R-40. The modeling results show that the air flows in the attics are steady and exhibit a general streamline pattern that is qualitatively insensitive to the investigated variations of roof pitch and ceiling insulation. Furthermore, it is predicted that the ceiling insulation plays a control role on the attic cooling load and that an increase of roof pitch from 3/12 to 8/12 results in a decrease in the cooling load by around 9% in the investigated cases. The results suggest that the increase of roof pitch alone, without changing other design parameters, has limited impact on attics cooling load and airflow pattern. The research results also suggest both the predicted ventilating mass flow rate and attic cooling load can be satisfactorily correlated by simple relationships in terms of appropriately defined Rayleigh and Nusselt numbers. Full article
(This article belongs to the Special Issue Energy Efficient Buildings and Green Buildings)
Open AccessArticle The Characteristics of the Exergy Reference Environment and Its Implications for Sustainability-Based Decision-Making
Energies 2012, 5(7), 2197-2213; doi:10.3390/en5072197
Received: 4 May 2012 / Revised: 25 June 2012 / Accepted: 3 July 2012 / Published: 5 July 2012
Cited by 5 | PDF Full-text (285 KB) | HTML Full-text | XML Full-text
Abstract
In the energy realm there is a pressing need to make decisions in a complex world characterized by biophysical limits. Exergy has been promoted as a preferred means of characterizing the impacts of resource consumption and waste production for the purpose of improving
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In the energy realm there is a pressing need to make decisions in a complex world characterized by biophysical limits. Exergy has been promoted as a preferred means of characterizing the impacts of resource consumption and waste production for the purpose of improving decision-making. This paper provides a unique and critical analysis of universal and comprehensive formulations of the chemical exergy reference environment, for the purpose of better understanding how exergy can inform decision-making. Four related insights emerged from the analysis, notably: (1) standard and universal chemical exergy reference environments necessarily encounter internal inconsistencies and even contradictions in their very formulations; (2) these inconsistencies are a result of incompatibility between the exergy reference environment and natural environment, and the desire to model the exergy reference environment after the natural environment so as to maintain analytical relevance; (3) the topics for which exergy is most appropriate as an analytical tool are not well served by comprehensive reference environments, and (4) the inconsistencies point to a need for deeper reflection of whether it is appropriate to adopt a thermodynamic frame of analysis for situations whose relevant characteristics are non-thermodynamic (e.g., to characterize scarcity). The use of comprehensive reference environments may lead to incorrect recommendations and ultimately reduce its appeal for informing decision-making. Exergy may better inform decision-making by returning to process dependent reference states that model specific processes and situations for the purpose of engineering optimization. Full article
(This article belongs to the Special Issue Exergy Analysis of Energy Systems)
Open AccessArticle Transient Studies in Large Offshore Wind Farms Employing Detailed Circuit Breaker Representation
Energies 2012, 5(7), 2214-2231; doi:10.3390/en5072214
Received: 25 May 2012 / Revised: 18 June 2012 / Accepted: 29 June 2012 / Published: 5 July 2012
Cited by 3 | PDF Full-text (489 KB) | HTML Full-text | XML Full-text
Abstract
Switching overvoltages (SOV) are considered a possible source of component failures experienced in existing offshore wind farms (OWFs). The inclusion of sufficiently accurate and validated models of the main electrical components in the OWF in the simulation tool is therefore an important issue
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Switching overvoltages (SOV) are considered a possible source of component failures experienced in existing offshore wind farms (OWFs). The inclusion of sufficiently accurate and validated models of the main electrical components in the OWF in the simulation tool is therefore an important issue in order to ensure reliable switching operations. Transient measurement results in an OWF are compared with simulation results in PSCAD EMTDC and DigSILENT Power Factory. A user-defined model of the vacuum circuit breaker (VCB) is included in both tools, capable of simulating multiple prestrikes during the closing operation. An analysis of the switching transients that might occur in OWFs will be made on the basis of the validated model, and the importance of the inclusion of a sufficiently accurate representation of the VCB in the simulation tool will be described. The inclusion of the VCB model in PSCAD greatly improves the simulation results, whereas little improvement is found in DigSILENT. Based on the transient study it is found that the simulated SOV can be up to 60% higher at the sending end when using the detailed VCB representation compared to the built-in switch, which emphasises the need for accurate representation of the VCB for energisation studies. Full article
(This article belongs to the Special Issue Electrical Power and Energy Systems)
Open AccessArticle Accelerated Electromechanical Modeling of a Distributed Internal Combustion Engine Generator Unit
Energies 2012, 5(7), 2232-2247; doi:10.3390/en5072232
Received: 26 March 2012 / Revised: 7 June 2012 / Accepted: 29 June 2012 / Published: 6 July 2012
Cited by 6 | PDF Full-text (715 KB) | HTML Full-text | XML Full-text
Abstract
Distributed generation with a combustion engine prime mover is still widely used to supply electric power in a variety of applications. These applications range from backup power supply systems and combined wind-diesel generation to providing power in places where grid connection is either
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Distributed generation with a combustion engine prime mover is still widely used to supply electric power in a variety of applications. These applications range from backup power supply systems and combined wind-diesel generation to providing power in places where grid connection is either technically impractical or financially uneconomic. Modelling of such systems as a whole is extremely difficult due to the long-time load profiles needed and the computational difficulty of including small time-constant electrical dynamics with large time-constant mechanical dynamics. This paper presents the development of accelerated, reduced-order models of a distributed internal combustions engine generator unit. Overall these models are shown to achieve a massive improvement in the computational time required for long-time simulations while also achieving an extremely high level of dynamic accuracy. It is demonstrated how these models are derived, used and verified against benchmark models created using established techniques. Throughout the paper the modelling set as a whole, including multi level detail, is presented, detailed and finally summarised into a crucial tool for general system investigation and multiple target optimisation. Full article
(This article belongs to the Special Issue Electrical Power and Energy Systems)
Open AccessArticle Kinetics of the Formation and Dissociation of Gas Hydrates from CO2-CH4 Mixtures
Energies 2012, 5(7), 2248-2262; doi:10.3390/en5072248
Received: 6 February 2012 / Revised: 3 May 2012 / Accepted: 25 May 2012 / Published: 6 July 2012
Cited by 9 | PDF Full-text (8597 KB) | HTML Full-text | XML Full-text
Abstract
Sequestration of carbon dioxide (CO2) in the form of its hydrates in natural methane (CH4) hydrate reservoirs, via CO2/CH4 exchange, is an attractive pathway that also yields valuable CH4 gas as product. In this paper,
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Sequestration of carbon dioxide (CO2) in the form of its hydrates in natural methane (CH4) hydrate reservoirs, via CO2/CH4 exchange, is an attractive pathway that also yields valuable CH4 gas as product. In this paper, we describe a macroscale experiment to form CO2 and CH4-CO2 hydrates, under seafloor-mimic conditions, in a vessel fitted with glass windows that provides visualization of hydrates throughout formation and dissociation processes. Time resolved pressure and temperature data as well as images of hydrates are presented. Quantitative gas conversions with pure CO2, calculated from gas chromatographic measurements yielded values that range from 23 – 59% that correspond to the extent of formed hydrates. In CH4-rich CH4-CO2 mixed gas systems, CH4 hydrates were found to form preferentially. Full article
(This article belongs to the Special Issue Natural Gas Hydrate 2011)
Open AccessArticle Towards Energy Demand Reduction in Social Housing Buildings: Envelope System Optimization Strategies
Energies 2012, 5(7), 2263-2287; doi:10.3390/en5072263
Received: 4 May 2012 / Revised: 26 June 2012 / Accepted: 27 June 2012 / Published: 6 July 2012
Cited by 16 | PDF Full-text (1525 KB) | HTML Full-text | XML Full-text
Abstract
This work evaluates the potential for the reduction of energy demand in residential buildings by acting on the exterior envelope, both in newly constructed buildings and in the retrofitting of existing stock. It focuses on analysing social housing buildings in Mediterranean areas and
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This work evaluates the potential for the reduction of energy demand in residential buildings by acting on the exterior envelope, both in newly constructed buildings and in the retrofitting of existing stock. It focuses on analysing social housing buildings in Mediterranean areas and on quantifying the scope of that reduction in the application of different envelope design strategies, with the purpose of prioritizing their application based on their energy efficiency. The analyses and quantifications were made by means of the generation of energy models with the TRNSYS tool for simple or combined solutions, identifying possible potentials for reduction of the energy demand from 20% to 25%, basically by acting on the windows. The case study was a newly built social housing building of a closed block type located in Seville (Spain). Its constructive techniques and the insulation level of its envelope are standardized for current buildings widespread across Mediterranean Europe. Full article
(This article belongs to the Special Issue Energy Efficient Buildings and Green Buildings)
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Open AccessArticle Sliding Mode Control Strategy for Wind Turbine Power Maximization
Energies 2012, 5(7), 2310-2330; doi:10.3390/en5072310
Received: 15 March 2012 / Revised: 25 June 2012 / Accepted: 3 July 2012 / Published: 9 July 2012
Cited by 18 | PDF Full-text (399 KB) | HTML Full-text | XML Full-text
Abstract
The efficiency of the wind power conversions systems can be greatly improved using an appropriate control algorithm. In this work, a sliding mode control for variable speed wind turbine that incorporates a doubly fed induction generator is described. The electrical system incorporates a
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The efficiency of the wind power conversions systems can be greatly improved using an appropriate control algorithm. In this work, a sliding mode control for variable speed wind turbine that incorporates a doubly fed induction generator is described. The electrical system incorporates a wound rotor induction machine with back-to-back three phase power converter bridges between its rotor and the grid. In the presented design the so-called vector control theory is applied, in order to simplify the electrical equations. The proposed control scheme uses stator flux-oriented vector control for the rotor side converter bridge control and grid voltage vector control for the grid side converter bridge control. The stability analysis of the proposed sliding mode controller under disturbances and parameter uncertainties is provided using the Lyapunov stability theory. Finally simulated results show, on the one hand, that the proposed controller provides high-performance dynamic characteristics, and on the other hand, that this scheme is robust with respect to the uncertainties that usually appear in the real systems. Full article
(This article belongs to the Special Issue Wind Turbines)
Open AccessArticle Energy Storage System with Voltage Equalization Strategy for Wind Energy Conversion
Energies 2012, 5(7), 2331-2350; doi:10.3390/en5072331
Received: 21 March 2012 / Revised: 29 May 2012 / Accepted: 26 June 2012 / Published: 9 July 2012
Cited by 9 | PDF Full-text (1202 KB) | HTML Full-text | XML Full-text
Abstract
In this paper, an energy storage system with voltage equalization strategy for wind energy conversion is presented. The proposed energy storage system provides a voltage equalization strategy for series-connected lead-acid batteries to increase their total storage capacity and lifecycle. In order to draw
[...] Read more.
In this paper, an energy storage system with voltage equalization strategy for wind energy conversion is presented. The proposed energy storage system provides a voltage equalization strategy for series-connected lead-acid batteries to increase their total storage capacity and lifecycle. In order to draw the maximum power from the wind energy, a perturbation-and-observation method and digital signal processor (DSP) are incorporated to implement maximum power point tracking (MPPT) algorithm and power regulating scheme. In the proposed energy storage system, all power switches have zero-voltage-switching (ZVS) feature at turn-on transition. Therefore, the conversion efficiency can be increased. Finally, a prototype energy storage system for wind energy conversion is built and implemented. Experimental results have verified the performance and feasibility of the proposed energy storage system for wind energy conversion. Full article
(This article belongs to the Special Issue Wind Turbines)
Open AccessArticle Power System Stability of a Small Sized Isolated Network Supplied by a Combined Wind-Pumped Storage Generation System: A Case Study in the Canary Islands
Energies 2012, 5(7), 2351-2369; doi:10.3390/en5072351
Received: 17 April 2012 / Revised: 1 June 2012 / Accepted: 26 June 2012 / Published: 9 July 2012
Cited by 9 | PDF Full-text (1447 KB) | HTML Full-text | XML Full-text
Abstract
Massive integration of renewable energy sources in electrical power systems of remote islands is a subject of current interest. The increasing cost of fossil fuels, transport costs to isolated sites and environmental concerns constitute a serious drawback to the use of conventional fossil
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Massive integration of renewable energy sources in electrical power systems of remote islands is a subject of current interest. The increasing cost of fossil fuels, transport costs to isolated sites and environmental concerns constitute a serious drawback to the use of conventional fossil fuel plants. In a weak electrical grid, as it is typical on an island, if a large amount of conventional generation is substituted by renewable energy sources, power system safety and stability can be compromised, in the case of large grid disturbances. In this work, a model for transient stability analysis of an isolated electrical grid exclusively fed from a combination of renewable energy sources has been studied. This new generation model will be installed in El Hierro Island, in Spain. Additionally, an operation strategy to coordinate the generation units (wind, hydro) is also established. Attention is given to the assessment of inertial energy and reactive current to guarantee power system stability against large disturbances. The effectiveness of the proposed strategy is shown by means of simulation results. Full article
(This article belongs to the Special Issue Wind Turbines)
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Open AccessArticle Fuel Characteristics of Biodiesel Produced from a High-Acid Oil from Soybean Soapstock by Supercritical-Methanol Transesterification
Energies 2012, 5(7), 2370-2380; doi:10.3390/en5072370
Received: 29 May 2012 / Revised: 19 June 2012 / Accepted: 29 June 2012 / Published: 9 July 2012
Cited by 7 | PDF Full-text (370 KB) | HTML Full-text | XML Full-text
Abstract
A supercritical methanol transesterification method was applied to produce biodiesel from the high-acid oil of soybean soapstock. The fuel properties of biodiesel produced with various molar ratios of methanol to raw oil were analyzed and compared in this experimental study. Oleic acid (C18:1),
[...] Read more.
A supercritical methanol transesterification method was applied to produce biodiesel from the high-acid oil of soybean soapstock. The fuel properties of biodiesel produced with various molar ratios of methanol to raw oil were analyzed and compared in this experimental study. Oleic acid (C18:1), linoleic acid (C18:2), and palmitic acid (C16:0) were the three main compounds in the high-acid oil-biodiesel. The saturated fatty acid content of the high-acid oil increased significantly due to the supercritical-methanol transesterification reaction. The fuel characteristics of the resulting high-acid oil, including the specific gravity and kinematic viscosity, were also greatly improved. The saturated fatty acid content of the biodiesel produced from the high-acid oil was higher than that of biodiesel from waste cooking oil produced by the subcritical transesterification using a strongly alkaline catalyst. The high-acid oil-biodiesel that was produced with a molar ratio of methanol to raw oil of 42 had the best fuel properties, including a higher distillation temperature and cetane index and a lower kinematic viscosity and water content, among the biodiesels with different molar ratios. Full article
(This article belongs to the Special Issue Biomass and Biofuels 2012)
Open AccessArticle Parametric Study of Thermodynamics in the Mediterranean Courtyard as a Tool for the Design of Eco-Efficient Buildings
Energies 2012, 5(7), 2381-2403; doi:10.3390/en5072381
Received: 19 March 2012 / Revised: 29 May 2012 / Accepted: 26 June 2012 / Published: 10 July 2012
Cited by 2 | PDF Full-text (4811 KB) | HTML Full-text | XML Full-text
Abstract
Traditionally, people in the Mediterranean region knew that the temperatures in their courtyards were cooler in summer than outside temperature. This paper provides a quantitative study on the usefulness of Mediterranean courtyards as passive energy saving systems. This work is based on the
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Traditionally, people in the Mediterranean region knew that the temperatures in their courtyards were cooler in summer than outside temperature. This paper provides a quantitative study on the usefulness of Mediterranean courtyards as passive energy saving systems. This work is based on the creation of a Computational Fluid Dynamics (CFD) numerical model developed using the open source Freefem++ language. In this work, first the numerical model is tested using simplified-shape courtyards which have been previously studied both physically under controlled parameters, and mathematically through numerical simulations. We also study the most appropriate depth ratio for a courtyard, based on these simplified shapes, depending on the climate. Secondly, we apply the numerical model in a real geometry, the Monte Málaga hotel. We compare the numerical results with the monitored data of the temperature in the courtyard of the hotel. The numerical model takes into account precomputed solar radiation in the walls of the courtyard, the predominant wind and buoyancy effects. Full article
(This article belongs to the Special Issue Energy Efficient Building Design)
Open AccessArticle Reactivation System for Proton-Exchange Membrane Fuel-Cells
Energies 2012, 5(7), 2404-2423; doi:10.3390/en5072404
Received: 28 March 2012 / Revised: 27 June 2012 / Accepted: 9 July 2012 / Published: 13 July 2012
PDF Full-text (789 KB) | HTML Full-text | XML Full-text
Abstract
In recent years, Proton-Exchange Membrane Fuel Cells (PEMFCs) have been the focus of very intensive researches. Manufacturers of these alternative power sources propose a rejuvenation sequence after the FC has been operating at high power for a certain period of time. These rejuvenation
[...] Read more.
In recent years, Proton-Exchange Membrane Fuel Cells (PEMFCs) have been the focus of very intensive researches. Manufacturers of these alternative power sources propose a rejuvenation sequence after the FC has been operating at high power for a certain period of time. These rejuvenation methods could be not appropriate for the reactivation of the FC when it has been out of operation for a long period of time or after it has been repaired. Since the developed reactivation system monitors temperature, current, and the cell voltages of the stack, it could be also useful for the diagnostic and repairing processes. The limited number of published contributions suggests that systems developing reactivation techniques are an open research field. In this paper, an automated system for reactivating PEMFCs and results of experimental testing are presented. Full article
(This article belongs to the Special Issue Hydrogen Energy and Fuel Cells)
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Open AccessArticle Fault Detection of Wind Turbines with Uncertain Parameters: A Set-Membership Approach
Energies 2012, 5(7), 2424-2448; doi:10.3390/en5072424
Received: 27 April 2012 / Revised: 11 June 2012 / Accepted: 6 July 2012 / Published: 13 July 2012
Cited by 20 | PDF Full-text (573 KB) | HTML Full-text | XML Full-text
Abstract
In this paper a set-membership approach for fault detection of a benchmark wind turbine is proposed. The benchmark represents relevant fault scenarios in the control system, including sensor, actuator and system faults. In addition we also consider parameter uncertainties and uncertainties on the
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In this paper a set-membership approach for fault detection of a benchmark wind turbine is proposed. The benchmark represents relevant fault scenarios in the control system, including sensor, actuator and system faults. In addition we also consider parameter uncertainties and uncertainties on the torque coefficient. High noise on the wind speed measurement, nonlinearities in the aerodynamic torque and uncertainties on the parameters make fault detection a challenging problem. We use an effective wind speed estimator to reduce the noise on the wind speed measurements. A set-membership approach is used generate a set that contains all states consistent with the past measurements and the given model of the wind turbine including uncertainties and noise. This set represents all possible states the system can be in if not faulty. If the current measurement is not consistent with this set, a fault is detected. For representation of these sets we use zonotopes and for modeling of uncertainties we use matrix zonotopes, which yields a computationally efficient algorithm. The method is applied to the wind turbine benchmark problem without and with uncertainties. The result demonstrates the effectiveness of the proposed method compared to other proposed methods applied to the same problem. An advantage of the proposed method is that there is no need for threshold design, and it does not produce positive false alarms. In the case where uncertainty on the torque lookup table is introduced, some faults are not detectable. Previous research has not addressed this uncertainty. The method proposed here requires equal or less detection time than previous results. Full article
(This article belongs to the Special Issue Wind Turbines)
Open AccessArticle Methane Hydrate Pellet Transport Using the Self-Preservation Effect: A Techno-Economic Analysis
Energies 2012, 5(7), 2499-2523; doi:10.3390/en5072499
Received: 2 February 2012 / Revised: 9 May 2012 / Accepted: 28 June 2012 / Published: 16 July 2012
Cited by 36 | PDF Full-text (2400 KB) | HTML Full-text | XML Full-text
Abstract
Within the German integrated project SUGAR, aiming for the development of new technologies for the exploration and exploitation of submarine gas hydrates, the option of gas transport by gas hydrate pellets has been comprehensively re-investigated. A series of pVT dissociation experiments, combined with
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Within the German integrated project SUGAR, aiming for the development of new technologies for the exploration and exploitation of submarine gas hydrates, the option of gas transport by gas hydrate pellets has been comprehensively re-investigated. A series of pVT dissociation experiments, combined with analytical tools such as x-ray diffraction and cryo-SEM, were used to gather an additional level of understanding on effects controlling ice formation. Based on these new findings and the accessible literature, knowns and unknowns of the self-preservation effect important for the technology are summarized. A conceptual process design for methane hydrate production and pelletisation has been developed. For the major steps identified, comprising (i) hydrate formation; (ii) dewatering; (iii) pelletisation; (iv) pellet cooling; and (v) pressure relief, available technologies have been evaluated, and modifications and amendments included where needed. A hydrate carrier has been designed, featuring amongst other technical solutions a pivoted cargo system with the potential to mitigate sintering, an actively cooled containment and cargo distribution system, and a dual fuel engine allowing the use of the boil-off gas. The design was constrained by the properties of gas hydrate pellets, the expected operation on continental slopes in areas with rough seas, a scenario-defined loading capacity of 20,000 m3 methane hydrate pellets, and safety as well as environmental considerations. A risk analysis for the transport at sea has been carried out in this early stage of development, and the safety level of the new concept was compared to the safety level of other ship types with similar scopes, i.e., LNG carriers and crude oil tankers. Based on the results of the technological part of this study, and with best knowledge available on the alternative technologies, i.e., pipeline, LNG and CNG transportation, an evaluation of the economic competitiveness of the methane hydrate transport technology has been performed. The analysis considers capital investment as well as operational costs and comprises a wide set of scenarios with production rates from 20 to 800 103 Nm3·h−1 and transport distances from 200 to 10,000 km. In contrast to previous studies, the model calculations in this study reveal no economic benefit of methane hydrate transportation versus competing technologies. Full article
(This article belongs to the Special Issue Natural Gas Hydrate 2011)
Open AccessArticle Dynamic Modeling of Anode Function in Enzyme-Based Biofuel Cells Using High Mediator Concentration
Energies 2012, 5(7), 2524-2544; doi:10.3390/en5072524
Received: 21 March 2012 / Revised: 5 June 2012 / Accepted: 27 June 2012 / Published: 17 July 2012
Cited by 2 | PDF Full-text (1647 KB) | HTML Full-text | XML Full-text
Abstract
The working principle of enzyme-based biofuel cells (EBFCs) is the same as that of conventional fuel cells. In an EBFC system, the electricity-production process is very intricate. Analysis requires a mathematical model that can adequately describe the EBFC and predict its performance. This
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The working principle of enzyme-based biofuel cells (EBFCs) is the same as that of conventional fuel cells. In an EBFC system, the electricity-production process is very intricate. Analysis requires a mathematical model that can adequately describe the EBFC and predict its performance. This paper develops a dynamic model simulating the discharge performance of the anode for which supported glucose oxidase and mediator immobilize in the EBFC. The dynamic transport behavior of substrate, redox state (ROS) of enzyme, enzyme-substrate complex, and the mediator creates different potential changes inside the anode. The potential-step method illustrates the dynamic phenomena of substrate diffusion, ROS of enzyme, production of enzyme-substrate complex, and reduction of the mediator with different potential changes. Full article
(This article belongs to the Special Issue Hydrogen Energy and Fuel Cells)
Open AccessArticle Analysis of Transient Phenomena Due to a Direct Lightning Strike on a Wind Energy System
Energies 2012, 5(7), 2545-2558; doi:10.3390/en5072545
Received: 29 May 2012 / Revised: 27 June 2012 / Accepted: 9 July 2012 / Published: 17 July 2012
Cited by 5 | PDF Full-text (312 KB) | HTML Full-text | XML Full-text
Abstract
This paper is concerned with the protection of wind energy systems against the direct effects of lightning. As wind power generation undergoes rapid growth, lightning damages involving wind turbines have come to be regarded as a serious problem. Nevertheless, very few studies exist
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This paper is concerned with the protection of wind energy systems against the direct effects of lightning. As wind power generation undergoes rapid growth, lightning damages involving wind turbines have come to be regarded as a serious problem. Nevertheless, very few studies exist yet in Portugal regarding lightning protection of wind energy systems using numerical codes. A new case study is presented in this paper, based on a wind turbine with an interconnecting transformer, for the analysis of transient phenomena due to a direct lightning strike to the blade. Comprehensive simulation results are provided by using models of the Restructured Version of the Electro-Magnetic Transients Program (EMTP), and conclusions are duly drawn. Full article
Open AccessArticle The Impact of Power Switching Devices on the Thermal Performance of a 10 MW Wind Power NPC Converter
Energies 2012, 5(7), 2559-2577; doi:10.3390/en5072559
Received: 23 May 2012 / Revised: 25 June 2012 / Accepted: 9 July 2012 / Published: 17 July 2012
Cited by 14 | PDF Full-text (1775 KB) | HTML Full-text | XML Full-text
Abstract
Power semiconductor switching devices play an important role in the performance of high power wind energy generation systems. The state-of-the-art device choices in the wind power application as reported in the industry include IGBT modules, IGBT press-pack and IGCT press-pack. Because of significant
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Power semiconductor switching devices play an important role in the performance of high power wind energy generation systems. The state-of-the-art device choices in the wind power application as reported in the industry include IGBT modules, IGBT press-pack and IGCT press-pack. Because of significant deviation in the packaging structure, electrical characteristics, as well as thermal impedance, these available power switching devices may have various thermal cycling behaviors, which will lead to converter solutions with very different cost, size and reliability performance. As a result, this paper aimed to investigate the thermal related characteristics of some important power switching devices. Their impact on the thermal cycling of a 10 MW three-level Neutral-Point-Clamped wind power converter is then evaluated under various operating conditions; the main focus will be on the grid connected inverter. It is concluded that the thermal performances of the 3L-NPC wind power converter can be significantly changed by the power device technology as well as their parallel configurations. Full article
(This article belongs to the Special Issue Electrical Power and Energy Systems)
Figures

Open AccessArticle Nonintrusive Energy Monitoring for Microgrids Using Hybrid Self-Organizing Feature-Mapping Networks
Energies 2012, 5(7), 2578-2593; doi:10.3390/en5072578
Received: 3 May 2012 / Revised: 11 July 2012 / Accepted: 12 July 2012 / Published: 18 July 2012
Cited by 4 | PDF Full-text (945 KB) | HTML Full-text | XML Full-text
Abstract
Microgrids can increase power penetration from distributed generation (DG) in the power system. The interface (i.e., the point of common coupling, PCC) between the microgrid and the power utility must satisfy certain standards, such as IEEE Sd. 1547. Energy monitoring of
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Microgrids can increase power penetration from distributed generation (DG) in the power system. The interface (i.e., the point of common coupling, PCC) between the microgrid and the power utility must satisfy certain standards, such as IEEE Sd. 1547. Energy monitoring of the microgrid at the PCC by the power utility is crucial if the utility cannot install advanced meters at different locations in the microgrid (e.g., a factory). This paper presents a new nonintrusive energy monitoring method using a hybrid self-organizing feature-mapping neural network (SOFMNN). The components of the FFT spectra for voltage, current, kW and kVAR, measured at the PCC, serve as the signatures for the hybrid SOFMNN inputs. The nonintrusive energy monitoring at the PCC identifies different load levels for individual linear/nonlinear loads and output levels for wind power generators in the microgrid. Using this energy monitoring result, the power utility can establish an energy management policy. The simulation results from a microgrid, consisting of a diesel generator, a wind-turbine-generator, a rectifier and a cyclo-converter, show the practicability of the proposed method. Full article
(This article belongs to the Special Issue Smart Grid and the Future Electrical Network)
Open AccessArticle Insulator Contamination Forecasting Based on Fractal Analysis of Leakage Current
Energies 2012, 5(7), 2594-2607; doi:10.3390/en5072594
Received: 25 April 2012 / Revised: 25 June 2012 / Accepted: 12 July 2012 / Published: 20 July 2012
Cited by 6 | PDF Full-text (374 KB) | HTML Full-text | XML Full-text
Abstract
In this paper, an artificial pollution test is carried out to study the leakage current of porcelain insulators. Fractal theory is adopted to extract the characteristics hidden in leakage current waveforms. Fractal dimensions of the leakage current for the security, forecast and danger
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In this paper, an artificial pollution test is carried out to study the leakage current of porcelain insulators. Fractal theory is adopted to extract the characteristics hidden in leakage current waveforms. Fractal dimensions of the leakage current for the security, forecast and danger zones are analyzed under four types of degrees of contamination. The mean value and the standard deviation of the fractal dimension in the forecast zone are calculated to characterize the differences. The analysis reveals large differences in the fractal dimension of leakage current under different contamination discharge stages and degrees. The experimental and calculation results suggest that the fractal dimension of a leakage current waveform can be used as a new indicator of the discharge process and contamination degree of insulators. The results provide new methods and valid indicators for forecasting contamination flashovers. Full article
Open AccessArticle Use of Anion Exchange Resins for One-Step Processing of Algae from Harvest to Biofuel
Energies 2012, 5(7), 2608-2625; doi:10.3390/en5072608
Received: 15 March 2012 / Revised: 5 June 2012 / Accepted: 13 July 2012 / Published: 24 July 2012
Cited by 3 | PDF Full-text (528 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Some microalgae are particularly attractive as a renewable feedstock for biodiesel production due to their rapid growth, high content of triacylglycerols, and ability to be grown on non-arable land. Unfortunately, obtaining oil from algae is currently cost prohibitive in part due to the
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Some microalgae are particularly attractive as a renewable feedstock for biodiesel production due to their rapid growth, high content of triacylglycerols, and ability to be grown on non-arable land. Unfortunately, obtaining oil from algae is currently cost prohibitive in part due to the need to pump and process large volumes of dilute algal suspensions. In an effort to circumvent this problem, we have explored the use of anion exchange resins for simplifying the processing of algae to biofuel. Anion exchange resins can bind and accumulate the algal cells out of suspension to form a dewatered concentrate. Treatment of the resin-bound algae with sulfuric acid/methanol elutes the algae and regenerates the resin while converting algal lipids to biodiesel. Hydrophobic polymers can remove biodiesel from the sulfuric acid/methanol, allowing the transesterification reagent to be reused. We show that in situ transesterification of algal lipids can efficiently convert algal lipids to fatty acid methyl esters while allowing the resin and transesterification reagent to be recycled numerous times without loss of effectiveness. Full article
(This article belongs to the Special Issue Algae Fuel)

Review

Jump to: Research

Open AccessReview Alternative Technologies for Biofuels Production in Kraft Pulp Mills—Potential and Prospects
Energies 2012, 5(7), 2288-2309; doi:10.3390/en5072288
Received: 22 May 2012 / Revised: 21 June 2012 / Accepted: 2 July 2012 / Published: 6 July 2012
Cited by 15 | PDF Full-text (536 KB) | HTML Full-text | XML Full-text
Abstract
The current global conditions provide the pulp mill new opportunities beyond the traditional production of cellulose. Due to stricter environmental regulations, volatility of oil price, energy policies and also the global competitiveness, the challenges for the pulp industry are many. They range from
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The current global conditions provide the pulp mill new opportunities beyond the traditional production of cellulose. Due to stricter environmental regulations, volatility of oil price, energy policies and also the global competitiveness, the challenges for the pulp industry are many. They range from replacing fossil fuels with renewable energy sources to the export of biofuels, chemicals and biomaterials through the implementation of biorefineries. In spite of the enhanced maturity of various bio and thermo-chemical conversion processes, the economic viability becomes an impediment when considering the effective implementation on an industrial scale. In the case of kraft pulp mills, favorable conditions for biofuels production can be created due to the availability of wood residues and generation of black liquor. The objective of this article is to give an overview of the technologies related to the production of alternative biofuels in the kraft pulp mills and discuss their potential and prospects in the present and future scenario. Full article
(This article belongs to the Special Issue Wood to Energy)
Open AccessReview The Global Inventory of Methane Hydrate in Marine Sediments: A Theoretical Approach
Energies 2012, 5(7), 2449-2498; doi:10.3390/en5072449
Received: 1 February 2012 / Revised: 29 June 2012 / Accepted: 2 July 2012 / Published: 16 July 2012
Cited by 54 | PDF Full-text (11226 KB) | XML Full-text
Abstract
The accumulation of methane hydrate in marine sediments is controlled by a number of physical and biogeochemical parameters including the thickness of the gas hydrate stability zone (GHSZ), the solubility of methane in pore fluids, the accumulation of particulate organic carbon at the
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The accumulation of methane hydrate in marine sediments is controlled by a number of physical and biogeochemical parameters including the thickness of the gas hydrate stability zone (GHSZ), the solubility of methane in pore fluids, the accumulation of particulate organic carbon at the seafloor, the kinetics of microbial organic matter degradation and methane generation in marine sediments, sediment compaction and the ascent of deep-seated pore fluids and methane gas into the GHSZ. Our present knowledge on these controlling factors is discussed and new estimates of global sediment and methane fluxes are provided applying a transport-reaction model at global scale. The modeling and the data evaluation yield improved and better constrained estimates of the global pore volume within the modern GHSZ ( ≥ 44 × 1015 m3), the Holocene POC accumulation rate at the seabed (~1.4 × 1014 g yr−1), the global rate of microbial methane production in the deep biosphere (4−25 × 1012 g C yr−1) and the inventory of methane hydrates in marine sediments ( ≥ 455 Gt of methane-bound carbon). Full article
(This article belongs to the Special Issue Natural Gas Hydrate 2011)

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