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Energies, Volume 5, Issue 2 (February 2012), Pages 181-530

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Research

Jump to: Review

Open AccessArticle Experimental Research on the Mechanical Properties of Methane Hydrate-Ice Mixtures
Energies 2012, 5(2), 181-192; doi:10.3390/en5020181
Received: 8 December 2011 / Revised: 11 January 2012 / Accepted: 17 January 2012 / Published: 30 January 2012
Cited by 13 | PDF Full-text (531 KB) | HTML Full-text | XML Full-text
Abstract
The mechanical properties of methane hydrate are important to the stability of borehole and methane extraction from a methane hydrate reservoir. In this study, a series of triaxial compression tests were carried out on laboratory-formed methane hydrate-ice mixtures with various methane hydrate [...] Read more.
The mechanical properties of methane hydrate are important to the stability of borehole and methane extraction from a methane hydrate reservoir. In this study, a series of triaxial compression tests were carried out on laboratory-formed methane hydrate-ice mixtures with various methane hydrate contents. Axial loading was conducted at an axial strain rate of 1.33%/min and a constant temperature of −10 °C. The results indicate that: (1) the deformation behavior is strongly affected by confining pressure and methane hydrate content; (2) the failure strength significantly increases with confining pressure when confining pressure is less than 10 MPa, and decreases with methane hydrate content; (3) the cohesion decreases with methane hydrate content, while the internal friction angle increases with methane hydrate content; (4) the strength of ice specimens are higher than that of methane hydrate-ice mixture specimens; Based on the experimental data, the relationship among failure strength, confining pressure and methane hydrate content was obtained, and a modified Mohr-Coulomb criterion considering the influence of methane hydrate content on shear strength was proposed. Full article
(This article belongs to the Special Issue Natural Gas Hydrate 2011)
Open AccessArticle Ground Thermal Inertia for Energy Efficient Building Design: A Case Study on Food Industry
Energies 2012, 5(2), 227-242; doi:10.3390/en5020227
Received: 9 November 2011 / Revised: 13 January 2012 / Accepted: 20 January 2012 / Published: 2 February 2012
Cited by 8 | PDF Full-text (5882 KB) | HTML Full-text | XML Full-text
Abstract
The search for energy efficient construction solutions is still pending in the agro-food industry, in which a large amount of energy is often consumed unnecessarily when storing products. The main objective of this research is to promote high energy efficiency built environments, [...] Read more.
The search for energy efficient construction solutions is still pending in the agro-food industry, in which a large amount of energy is often consumed unnecessarily when storing products. The main objective of this research is to promote high energy efficiency built environments, which aim to reduce energy consumption in this sector. We analyze the suitability of using the thermal inertia of the ground to provide an adequate environment for the storage and conservation of agro-food products. This research compares different construction solutions based on the use of ground thermal properties, analyzing their effectiveness to decrease annual outdoor variations and provide adequate indoor conditions. The analysis undertaken is based on over five million pieces of data, obtained from an uninterrupted four year monitoring process of various constructions with different levels of thermal mass, ranging from high volume constructions to others lacking this resource. It has been proven that constructive solutions based on the use of ground thermal inertia are more effective than other solutions when reducing the effects of outdoor conditions, even when these have air conditioning systems. It is possible to reach optimal conditions to preserve agro-food products such as wine, with a good design and an adequate amount of terrain, without having to use air conditioning systems. The results of this investigation could be of great use to the agro-food industry, becoming a reference when it comes to the design of energy efficient constructions. Full article
(This article belongs to the Special Issue Energy Efficient Building Design)
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Open AccessArticle Influence of n-Hexane on in Situ Transesterification of Marine Macroalgae
Energies 2012, 5(2), 243-257; doi:10.3390/en5020243
Received: 20 December 2011 / Revised: 13 January 2012 / Accepted: 2 February 2012 / Published: 6 February 2012
Cited by 11 | PDF Full-text (515 KB) | HTML Full-text | XML Full-text
Abstract
The purpose of this work is to investigate the influence of n-hexane addition on in situ transesterification of a solid raw material for biodiesel production. Extraction and reaction of macroalgae oil has been performed simultaneously in a batch reactor adding n- [...] Read more.
The purpose of this work is to investigate the influence of n-hexane addition on in situ transesterification of a solid raw material for biodiesel production. Extraction and reaction of macroalgae oil has been performed simultaneously in a batch reactor adding n-hexane with the reactants. In order to analyze the influence of n-hexane on the transesterification, the reaction was also carried out with sunflower oil. The results show that the presence of n-hexane does not have an important effect on the transesterification. It was also observed that this method requires large quantities of methanol to carry out the reaction. The best reaction conditions for in situ transesterification of marine macroalgae were 300:1 methanol-to-oil molar ratio, 1% catalyst concentration, 60 °C reaction temperature and 11 h reaction time, resulting in a methyl esters yield of 17.1%. Thus, biodiesel production from macroalgae by transesterification in situ could be feasible, using hexane for the extraction and eliminating the previous extraction. This integrated method is thus effective and technically attractive. Full article
(This article belongs to the Special Issue Algae Fuel)
Open AccessArticle Field Surveys of Non-Residential Solar Water Heating Systems in Taiwan
Energies 2012, 5(2), 258-269; doi:10.3390/en5020258
Received: 21 December 2011 / Revised: 31 January 2012 / Accepted: 6 February 2012 / Published: 7 February 2012
Cited by 14 | PDF Full-text (908 KB) | HTML Full-text | XML Full-text
Abstract
To develop indigenous alternative and renewable energy resources, long-term subsidy programs (1986–1991 and 2000–present) for solar water heaters have been enforced in Taiwan. By the end of 2010, the total installed area of solar collectors had exceeded 2 million square meters. However, [...] Read more.
To develop indigenous alternative and renewable energy resources, long-term subsidy programs (1986–1991 and 2000–present) for solar water heaters have been enforced in Taiwan. By the end of 2010, the total installed area of solar collectors had exceeded 2 million square meters. However, over 98% of solar water heaters were used in residential systems for hot water production, with the areas of installed solar collector being less than 10 square meters. There were only 98 systems with area of solar collectors installed exceeding 100 square meters put into operation from 2001 to 2010. These systems were mainly installed for water heating in dormitories, swimming pools, restaurants, and manufacturing plants. In the present study, a comprehensive survey of these large-scale solar water heaters was conducted. The objectives of the survey were to assess the system performance and to collect feedback from individual users. It is found that lack of experience in system design and maintenance are the key factors affecting reliable operation of a system. Hourly, daily and long-term field measurements of a dormitory system were also examined to evaluate its thermal efficiencies. Results indicated that thermal efficiency of the system is associated with the daily solar radiation. Hot water use pattern and operation of auxiliary heater should be taken into account in system design. Full article
Open AccessArticle Experimental Investigation on NOx Reduction by Primary Measures in Biomass Combustion: Straw, Peat, Sewage Sludge, Forest Residues and Wood Pellets
Energies 2012, 5(2), 270-290; doi:10.3390/en5020270
Received: 6 December 2011 / Revised: 29 January 2012 / Accepted: 1 February 2012 / Published: 8 February 2012
Cited by 19 | PDF Full-text (8445 KB) | HTML Full-text | XML Full-text
Abstract
An experimental investigation was carried out to study the NOx formation and reduction by primary measures for five types of biomass (straw, peat, sewage sludge, forest residues/Grot, and wood pellets) and their mixtures. To minimize the NOx level in biomass-fired [...] Read more.
An experimental investigation was carried out to study the NOx formation and reduction by primary measures for five types of biomass (straw, peat, sewage sludge, forest residues/Grot, and wood pellets) and their mixtures. To minimize the NOx level in biomass-fired boilers, combustion experiments were performed in a laboratory scale multifuel fixed grate reactor using staged air combustion. Flue gas was extracted to measure final levels of CO, CO2, CxHy, O2, NO, NO2, N2O, and other species. The fuel gas compositions between the first and second stage were also monitored. The experiments showed good combustion quality with very low concentrations of unburnt species in the flue gas. Under optimum conditions, a NOx reduction of 50–80% was achieved, where the highest reduction represents the case with the highest fuel-N content. The NOx emission levels were very sensitive to the primary excess air ratio and an optimum value for primary excess air ratio was seen at about 0.9. Conversion of fuel nitrogen to NOx showed great dependency on the initial fuel-N content, where the blend with the highest nitrogen content had lowest conversion rate. Between 1–25% of the fuel-N content is converted to NOx depending on the fuel blend and excess air ratio. Sewage sludge is suggested as a favorable fuel to be blended with straw. It resulted in a higher NOx reduction and low fuel-N conversion to NOx. Tops and branches did not show desirable NOx reduction and made the combustion also more unstable. N2O emissions were very low, typically below 5 ppm at 11% O2 in the dry flue gas, except for mixtures with high nitrogen content, where values up to 20 ppm were observed. The presented results are part of a larger study on problematic fuels, also considering ash content and corrosive compounds which have been discussed elsewhere. Full article
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Open AccessArticle Low-Carbon Development Patterns: Observations of Typical Chinese Cities
Energies 2012, 5(2), 291-304; doi:10.3390/en5020291
Received: 7 November 2011 / Revised: 30 January 2012 / Accepted: 30 January 2012 / Published: 13 February 2012
Cited by 18 | PDF Full-text (977 KB) | HTML Full-text | XML Full-text
Abstract
Threatened by the huge pressure caused by climate change, low-carbon cities have become an inevitable part of urban evolution. It is essential to evaluate urban low-carbon development levels to smoothly promote the construction of low-carbon cities. This paper proposes an evaluation index [...] Read more.
Threatened by the huge pressure caused by climate change, low-carbon cities have become an inevitable part of urban evolution. It is essential to evaluate urban low-carbon development levels to smoothly promote the construction of low-carbon cities. This paper proposes an evaluation index system for urban low-carbon development from the points of view of economic development and social progress, energy structure and usage efficiency, living consumption, and development surroundings. A weighted sum model was also established. Selecting 12 typical Chinese cities as cases studies, an integrated evaluation was conducted based on the index system and the assessment model. The development speed and limiting factors of different cities were also analyzed. The 12 cities were ultimately classified into three groups in terms of their low-carbon development patterns by integrating all of the analysis results. Furthermore, suitable regulation and management for different patterns were suggested. This study both aids in assessing the executive effect of low-carbon city construction and helps to determine existing problems and suggest effective solutions. Full article
(This article belongs to the Special Issue Low Carbon Transitions Worldwide)
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Open AccessArticle Assessing Climate Change Impacts on Global Hydropower
Energies 2012, 5(2), 305-322; doi:10.3390/en5020305
Received: 15 December 2011 / Revised: 13 January 2012 / Accepted: 6 February 2012 / Published: 14 February 2012
Cited by 31 | PDF Full-text (4976 KB) | HTML Full-text | XML Full-text
Abstract
Currently, hydropower accounts for close to 16% of the world’s total power supply and is the world’s most dominant (86%) source of renewable electrical energy. The key resource for hydropower generation is runoff, which is dependent on precipitation. The future global climate [...] Read more.
Currently, hydropower accounts for close to 16% of the world’s total power supply and is the world’s most dominant (86%) source of renewable electrical energy. The key resource for hydropower generation is runoff, which is dependent on precipitation. The future global climate is uncertain and thus poses some risk for the hydropower generation sector. The crucial question and challenge then is what will be the impact of climate change on global hydropower generation and what are the resulting regional variations in hydropower generation potential? This paper is a study that aims to evaluate the changes in global hydropower generation resulting from predicted changes in climate. The study uses an ensemble of simulations of regional patterns of changes in runoff, computed from global circulation models (GCM) simulations with 12 different models. Based on these runoff changes, hydropower generation is estimated by relating the runoff changes to hydropower generation potential through geographical information system (GIS), based on 2005 hydropower generation. Hydropower data obtained from EIA (energy generation), national sites, FAO (water resources) and UNEP were used in the analysis. The countries/states were used as computational units to reduce the complexities of the analysis. The results indicate that there are large variations of changes (increases/decreases) in hydropower generation across regions and even within regions. Globally, hydropower generation is predicted to change very little by the year 2050 for the hydropower system in operation today. This change amounts to an increase of less than 1% of the current (2005) generation level although it is necessary to carry out basin level detailed assessment for local impacts which may differ from the country based values. There are many regions where runoff and hydropower generation will increase due to increasing precipitation, but also many regions where there will be a decrease. Based on this evaluation, it has been concluded that even if individual countries and regions may experience significant impacts, climate change will not lead to significant changes in the global hydropower generation, at least for the existing hydropower system. Full article
(This article belongs to the Special Issue Energy Policy on Climate Change)
Open AccessArticle A Novel Multiscale Ensemble Carbon Price Prediction Model Integrating Empirical Mode Decomposition, Genetic Algorithm and Artificial Neural Network
Energies 2012, 5(2), 355-370; doi:10.3390/en5020355
Received: 8 December 2011 / Revised: 8 February 2012 / Accepted: 8 February 2012 / Published: 17 February 2012
Cited by 9 | PDF Full-text (594 KB) | HTML Full-text | XML Full-text
Abstract
Due to the movement and complexity of the carbon market, traditional monoscale forecasting approaches often fail to capture its nonstationary and nonlinear properties and accurately describe its moving tendencies. In this study, a multiscale ensemble forecasting model integrating empirical mode decomposition (EMD), [...] Read more.
Due to the movement and complexity of the carbon market, traditional monoscale forecasting approaches often fail to capture its nonstationary and nonlinear properties and accurately describe its moving tendencies. In this study, a multiscale ensemble forecasting model integrating empirical mode decomposition (EMD), genetic algorithm (GA) and artificial neural network (ANN) is proposed to forecast carbon price. Firstly, the proposed model uses EMD to decompose carbon price data into several intrinsic mode functions (IMFs) and one residue. Then, the IMFs and residue are composed into a high frequency component, a low frequency component and a trend component which have similar frequency characteristics, simple components and strong regularity using the fine-to-coarse reconstruction algorithm. Finally, those three components are predicted using an ANN trained by GA, i.e., a GAANN model, and the final forecasting results can be obtained by the sum of these three forecasting results. For verification and testing, two main carbon future prices with different maturity in the European Climate Exchange (ECX) are used to test the effectiveness of the proposed multiscale ensemble forecasting model. Empirical results obtained demonstrate that the proposed multiscale ensemble forecasting model can outperform the single random walk (RW), ARIMA, ANN and GAANN models without EMD preprocessing and the ensemble ARIMA model with EMD preprocessing. Full article
(This article belongs to the Special Issue Low Carbon Transitions Worldwide)
Open AccessArticle Development and Evaluation of an Economic-Driving Assistance Program for Transit Vehicles
Energies 2012, 5(2), 371-385; doi:10.3390/en5020371
Received: 30 January 2012 / Revised: 9 February 2012 / Accepted: 12 February 2012 / Published: 21 February 2012
Cited by 2 | PDF Full-text (556 KB) | HTML Full-text | XML Full-text
Abstract
This paper focuses on development and evaluation of an economic-driving assistance program for transit vehicles (EDTV) which can minimize energy consumption, air pollution emission of buses, and improve the level of service of transit system as well. Taking advantage of the latest [...] Read more.
This paper focuses on development and evaluation of an economic-driving assistance program for transit vehicles (EDTV) which can minimize energy consumption, air pollution emission of buses, and improve the level of service of transit system as well. Taking advantage of the latest advances in information and communication technologies, the EDTV system can provide bus drivers with optimal recommended bus holding times at near-side bus stops and dynamic bus speed to adapt to the real-time traffic control plan at downstream intersections. In order to address the impacts of the stochastic variation of bus dwell time, the total link between adjacent intersections is divided into three parts: upstream of bus stop part; bus stop part; and downstream of bus stop part. The methods for calculating recommended parameters, including bus holding time and bus speed in each of the three parts are proposed based on real-time bus status and signal status at downstream intersections. A VISSIM-based simulation platform was designed and used for simulating and evaluating the proposed EDTV system. Extensive experimental analyses have shown that the proposed EDTV system can improve the performance of a transit system in terms of reducing fuel consumption, air pollution emissions and level of service of the transit system. Full article
Open AccessArticle Decomposition Analysis of the Mechanism Behind the Spatial and Temporal Patterns of Changes in Carbon Bio-Sequestration in China
Energies 2012, 5(2), 386-398; doi:10.3390/en5020386
Received: 28 December 2011 / Revised: 13 February 2012 / Accepted: 14 February 2012 / Published: 22 February 2012
Cited by 7 | PDF Full-text (1256 KB) | HTML Full-text | XML Full-text
Abstract
Great attention has been paid to carbon bio-sequestration due to increasing concerns over global warming. Understanding the relationship between carbon bio-sequestration and its influencing factors is of great significance for formulating appropriate management measures for global warming mitigation. Since change in carbon [...] Read more.
Great attention has been paid to carbon bio-sequestration due to increasing concerns over global warming. Understanding the relationship between carbon bio-sequestration and its influencing factors is of great significance for formulating appropriate management measures for global warming mitigation. Since change in carbon bio-sequestration is a complex process, it is difficult to take into account all of its influencing factors, while the panel data model may provide an effective way to measure their subtle effects. In this paper, decomposition analysis is applied to further analyze these influencing factors. The results indicate that climatic, demographic and geographical variables play important roles in explaining the spatial heterogeneity of carbon bio-sequestration in China, which is consistent with previous researches. Meanwhile, the irrigation rate is found to be the most critical factor influencing carbon bio-sequestration changes, followed by climatic and economic factors. These results may provide decision makers in China with important scientific reference information for formulating regional carbon bio-sequestration management policies, which are of great significance to alleviating and adapting to global warming. Full article
(This article belongs to the Special Issue Low Carbon Transitions Worldwide)
Open AccessArticle The Driving Forces of Guest Substitution in Gas Hydrates—A Laser Raman Study on CH4-CO2 Exchange in the Presence of Impurities
Energies 2012, 5(2), 420-437; doi:10.3390/en5020420
Received: 15 December 2011 / Revised: 2 February 2012 / Accepted: 14 February 2012 / Published: 22 February 2012
Cited by 11 | PDF Full-text (2139 KB) | HTML Full-text | XML Full-text
Abstract
The recovery of CH4 gas from natural hydrate formations by injection of industrially emitted CO2 is considered to be a promising solution to simultaneously access an unconventional fossil fuel reserve and counteract atmospheric CO2 increase. CO2 obtained from [...] Read more.
The recovery of CH4 gas from natural hydrate formations by injection of industrially emitted CO2 is considered to be a promising solution to simultaneously access an unconventional fossil fuel reserve and counteract atmospheric CO2 increase. CO2 obtained from industrial processes may contain traces of impurities such as SO2 or NOx and natural gas hydrates may contain higher hydrocarbons such as C2H6 and C3H8. These additions have an influence on the properties of the resulting hydrate phase and the conversion process of CH4-rich hydrates to CO2-rich hydrates. Here we show results of a microscopic and laser Raman in situ study investigating the effects of SO2-polluted CO2 and mixed CH4-C2H6 hydrate on the exchange process. Our study shows that the key driving force of the exchange processes is the establishment of the chemical equilibrium between hydrate phase and the surrounding phases. The exchange rate is also influenced by the guest-to-cavity ratio as well as the thermodynamic stability in terms of p-T conditions of the original and resulting hydrate phase. The most effective molecule exchange is related to structural changes (sI-sII) which indicates that hydrate decomposition and reformation processes are the occurring processes. Full article
(This article belongs to the Special Issue Natural Gas Hydrate 2011)
Open AccessArticle Numerical Simulation of Methane Production from Hydrates Induced by Different Depressurizing Approaches
Energies 2012, 5(2), 438-458; doi:10.3390/en5020438
Received: 23 December 2011 / Revised: 13 February 2012 / Accepted: 17 February 2012 / Published: 22 February 2012
Cited by 12 | PDF Full-text (1730 KB) | HTML Full-text | XML Full-text
Abstract
Several studies have demonstrated that methane production from hydrate-bearing porous media by means of depressurization-induced dissociation can be a promising technique. In this study, a 2D axisymmetric model for simulating the gas production from hydrates by depressurization is developed to investigate the [...] Read more.
Several studies have demonstrated that methane production from hydrate-bearing porous media by means of depressurization-induced dissociation can be a promising technique. In this study, a 2D axisymmetric model for simulating the gas production from hydrates by depressurization is developed to investigate the gas production behavior with different depressurizing approaches. The simulation results showed that the depressurization process with depressurizing range has significant influence on the final gas production. On the contrary, the depressurizing rate only affects the production lifetime. More amount of cumulative gas can be produced with a larger depressurization range or lowering the depressurizing rate for a certain depressurizing range. Through the comparison of the combined depressurization modes, the Class 2 (all the hydrate dissociation simulations are performed by reducing the initial system pressure with the same depressurizing range initially, then to continue the depressurization process conducted by different depressurizing rates and complete when the system pressure decreases to the atmospheric pressure) is much superior to the Class 1 (different depressurizing ranges are adopted in the initial period of the gas production process, when the pressure is reduced to the corresponding value of depressurization process at the different depressurizing range, the simulations are conducted at a certain depressurizing rate until the pressure reaches the atmospheric pressure) for a long and stable gas production process. The parameter analysis indicated that the gas production performance decreases and the period of stable production increases with the initial pressure for the case of depressurizing range. Additionally, for the case of depressurizing range, the better gas production performance is associated with higher ambient temperature for production process, and the effect of thermal conductivity on gas production performance can be negligible. However, for the case of depressurizing rate, the ambient temperature or thermal conductivity is dominant in different period of gas production process. Full article
(This article belongs to the Special Issue Natural Gas Hydrate 2011)
Open AccessArticle Thermodynamic Stability of Structure H Hydrates Based on the Molecular Properties of Large Guest Molecules
Energies 2012, 5(2), 459-465; doi:10.3390/en5020459
Received: 31 December 2011 / Revised: 11 February 2012 / Accepted: 14 February 2012 / Published: 22 February 2012
Cited by 8 | PDF Full-text (2289 KB) | HTML Full-text | XML Full-text
Abstract
This paper report analyses of thermodynamic stability of structure-H clathrate hydrates formed with methane and large guest molecules in terms of their gas phase molecular sizes and molar masses for the selection of a large guest molecule providing better hydrate stability. We [...] Read more.
This paper report analyses of thermodynamic stability of structure-H clathrate hydrates formed with methane and large guest molecules in terms of their gas phase molecular sizes and molar masses for the selection of a large guest molecule providing better hydrate stability. We investigated the correlation among the gas phase molecular sizes, the molar masses of large molecule guest substances, and the equilibrium pressures. The results suggest that there exists a molecular-size value for the best stability. Also, at a given molecule size, better stability may be available when the large molecule guest substance has a larger molar mass. Full article
(This article belongs to the Special Issue Natural Gas Hydrate 2011)
Open AccessArticle Climate and Energy Policy in Hungary
Energies 2012, 5(2), 494-517; doi:10.3390/en5020494
Received: 15 December 2011 / Revised: 3 February 2012 / Accepted: 16 February 2012 / Published: 22 February 2012
Cited by 6 | PDF Full-text (822 KB) | HTML Full-text | XML Full-text
Abstract
The energy problem has been redefined as one of the most important elements of sustainable development by climate change, adaptation and mitigation. Meeting energy needs is always a current issue in Hungary, irrespective of climate change because of the country’s high dependency [...] Read more.
The energy problem has been redefined as one of the most important elements of sustainable development by climate change, adaptation and mitigation. Meeting energy needs is always a current issue in Hungary, irrespective of climate change because of the country’s high dependency on oil and gas imports, limited opportunities to replace them with domestic production, and the pollution associated with using fossil energy sources. Increasing effectiveness and saving energy can provide relatively short-term solutions with bearable costs and a relatively quick return on investment. The aim of the present paper is to give an overview about the climate and energy policy in Hungary with a special focus on the new energy strategy. Energy policy has a pivotal role in the economic recovery plan of the Hungarian government. The National Energy Strategy 2030 taking shape in Hungary takes climate policy into account with respect to adaptation and mitigation and lists renewable energy sources as the second most important tool for achieving strategic goals. As in most countries, it is also possible in Hungary to introduce climate strategy measures with zero social costs. The expedient management of climate change requires the combination of prevention, adaptation and dissemination initiatives. Strategies must meet a dual requirement: they must face the economic risks associated with premature measures, while also considering the adverse effects of delay. Full article
(This article belongs to the Special Issue Energy Policy on Climate Change)
Open AccessArticle Experimental Study on Methane Hydrate Dissociation by Depressurization in Porous Sediments
Energies 2012, 5(2), 518-530; doi:10.3390/en5020518
Received: 23 December 2011 / Revised: 6 February 2012 / Accepted: 7 February 2012 / Published: 22 February 2012
Cited by 15 | PDF Full-text (495 KB) | HTML Full-text | XML Full-text
Abstract
Based on currently available data from site measurements in the Shenhu Area of the South China Sea, methane hydrate dissociation behavior by depressurization is studied in a one-dimensional experimental apparatus. According to time variation of temperature, resistance and gas production, the hydrate [...] Read more.
Based on currently available data from site measurements in the Shenhu Area of the South China Sea, methane hydrate dissociation behavior by depressurization is studied in a one-dimensional experimental apparatus. According to time variation of temperature, resistance and gas production, the hydrate dissociation process is divided into three stages: free gas release, rapid dissociation and gradual dissociation. The experimental results show that as the hydrate saturation increases the proportion of hydrate decomposed decreases in the rapid dissociation stage. The hydrate dissociation rate and the dissociation heat increase as the dissociation pressure decreases. Furthermore, the decrease of the dissociation pressure works against the secondary formation of the hydrate. Full article
(This article belongs to the Special Issue Natural Gas Hydrate 2011)

Review

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Open AccessReview The SSG Wave Energy Converter: Performance, Status and Recent Developments
Energies 2012, 5(2), 193-226; doi:10.3390/en5020193
Received: 24 October 2011 / Revised: 6 January 2012 / Accepted: 17 January 2012 / Published: 31 January 2012
Cited by 23 | PDF Full-text (2072 KB) | HTML Full-text | XML Full-text
Abstract
The Sea-wave Slot-cone Generator (SSG) is a Wave Energy Converter based on the wave overtopping principle; it employs several reservoirs placed on top of each other, in which the energy of incoming waves is stored as potential energy. Then, the captured water [...] Read more.
The Sea-wave Slot-cone Generator (SSG) is a Wave Energy Converter based on the wave overtopping principle; it employs several reservoirs placed on top of each other, in which the energy of incoming waves is stored as potential energy. Then, the captured water runs through turbines for electricity production. The system works under a wide spectrum of different wave conditions, giving a high overall efficiency. It can be suitable for shoreline and breakwater applications and presents particular advantages, such as sharing structure costs, availability of grid connection and recirculation of water inside the harbor, as the outlet of the turbines is on the rear part of the system. Recently, plans for the SSG pilot installations are in progress at the Svaaheia site (Norway), the port of Hanstholm (Denmark) and the port of Garibaldi (Oregon, USA). In the last-mentioned two projects, the Sea-wave Slot-cone Generator technology is integrated into the outer harbor breakwater and jetty reconstruction projects. In the last years extensive studies have been performed on the hydraulic and the structural response of this converter, with the aim of optimizing the design process. The investigations have been conducted by physical model tests and numerical simulations and many results have been published on both conference proceedings and journals. The main scope of this paper is reviewing the most significant findings, to provide the reader with an organic overview on the present status of knowledge. Full article
(This article belongs to the Special Issue Progress in Ocean Energy Conversion)
Open AccessReview Carbon Lock-Out: Advancing Renewable Energy Policy in Europe
Energies 2012, 5(2), 323-354; doi:10.3390/en5020323
Received: 1 November 2011 / Revised: 16 January 2012 / Accepted: 8 February 2012 / Published: 15 February 2012
Cited by 25 | PDF Full-text (447 KB) | HTML Full-text | XML Full-text
Abstract
As part of its climate strategy, the EU aims at increasing the share of electricity from renewable energy sources (RES-E) in overall electricity generation. Attaining this target poses a considerable challenge as the electricity sector is “locked” into a carbon-intensive system, which [...] Read more.
As part of its climate strategy, the EU aims at increasing the share of electricity from renewable energy sources (RES-E) in overall electricity generation. Attaining this target poses a considerable challenge as the electricity sector is “locked” into a carbon-intensive system, which hampers the adoption of RES-E technologies. Electricity generation, transmission and distribution grids as well as storage and demand response are subject to important path dependences, which put existing, non-renewable energy sources at an advantage. This paper examines how an EU framework for RES-E support policies should be designed to facilitate a carbon lock-out. For this purpose, we specify the major technological, economic and institutional barriers to RES-E. For each of the barriers, a policy review is carried out which assesses the performance of existing policy instruments and identifies needs for reform. The review reveals several shortcomings: while policies targeting generation are widely in place, measures to address barriers associated with electricity grids, storage and demand are still in their infancy and have to be extended. Moreover, the implementation of policies has been fragmented across EU Member States. In this respect, national policies should be embedded into an integrated EU-wide planning of the RES-E system with overarching energy scenarios and partially harmonized policy rules. Full article
(This article belongs to the Special Issue Energy Policy on Climate Change)
Open AccessReview A Review on Research on Replacement of CH4 in Natural Gas Hydrates by Use of CO2
Energies 2012, 5(2), 399-419; doi:10.3390/en5020399
Received: 28 December 2011 / Revised: 6 February 2012 / Accepted: 8 February 2012 / Published: 22 February 2012
Cited by 30 | PDF Full-text (862 KB) | HTML Full-text | XML Full-text
Abstract
This paper introduces the research advances on replacement of CH4 in Natural Gas Hydrates (NGHs) by use of CO2 and discusses the advantages and disadvantages of the method on the natural gas production from such hydrates. Firstly, the feasibility of [...] Read more.
This paper introduces the research advances on replacement of CH4 in Natural Gas Hydrates (NGHs) by use of CO2 and discusses the advantages and disadvantages of the method on the natural gas production from such hydrates. Firstly, the feasibility of replacement is proven from the points of view of kinetics and thermodynamics, and confirmed by experiments. Then, the latest progress in CH4 replacement experiments with gaseous CO2, liquid CO2 and CO2 emulsion are presented Moreover, the superiority of CO2 emulsion for replacement of CH4 is emphasized. The latest experiment progress on preparation of CO2 emulsions are introduced. Finally, the advancements in simulation research on replacement is introduced, and the deficiencies of the simulations are pointed. The factors influencing on the replacement with different forms of CO2 are analyzed and the optimum conditions for the replacement of CH4 in hydrated with different forms of CO2 is suggested. Full article
(This article belongs to the Special Issue Natural Gas Hydrate 2011)
Open AccessReview Experimental Simulation of the Exploitation of Natural Gas Hydrate
Energies 2012, 5(2), 466-493; doi:10.3390/en5020466
Received: 20 January 2012 / Revised: 7 February 2012 / Accepted: 8 February 2012 / Published: 22 February 2012
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Abstract
Natural gas hydrates are cage-like crystalline compounds in which a large amount of methane is trapped within a crystal structure of water, forming solids at low temperature and high pressure. Natural gas hydrates are widely distributed in permafrost regions and offshore. It [...] Read more.
Natural gas hydrates are cage-like crystalline compounds in which a large amount of methane is trapped within a crystal structure of water, forming solids at low temperature and high pressure. Natural gas hydrates are widely distributed in permafrost regions and offshore. It is estimated that the worldwide amounts of methane bound in gas hydrates are total twice the amount of carbon to be found in all known fossil fuels on earth. A proper understanding of the relevant exploitation technologies is then important for natural gas production applications. In this paper, the recent advances on the experimental simulation of natural gas hydrate exploitation using the major hydrate production technologies are summarized. In addition, the current situation of the industrial exploitation of natural gas hydrate is introduced, which are expected to be useful for establishing more safe and efficient gas production technologies. Full article
(This article belongs to the Special Issue Natural Gas Hydrate 2011)

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