Showing papers in "International Journal of Energy Research in 2007"

Size effects on the hydrogen storage properties of nanostructured metal hydrides: A review

Abstract: Hydrogen is considered a good energy carrier candidate for future automotive applications that could be part of a carbon-free cycle. Metal hydrides are often preferred over pressurized gas and other hydrogen storage methods because of their gravimetric and volumetric storage capacities and safe operating pressures. In addition to the hydrogen storage capacity, other properties that have often been disregarded must now be addressed before hydrogen storage in metal hydrides becomes feasible. The slow hydriding/dehydriding kinetics, high release temperature, low storage efficiency due to the high enthalpy of formation, and thermal management during the hydriding reaction remain important difficulties in meeting the objectives set by the Department of Energy (DOE) for hydrogen storage systems. Nanotechnology offers new ways of addressing those issues by taking advantage of the distinctive chemical and physical properties observed in nanostructures. Nanostructured materials significantly improve the reaction kinetics, reduce the enthalpy of formation, and lower the hydrogen absorption and release temperatures through destabilization of the metal hydride and multiple catalytic effects in the system. But nanostructures can also lead to poor cyclability, degradation of the sorption properties, and a significant reduction of the thermal conductivity that could make metal hydrides impractical for hydrogen storage. This review summarizes the effects that nanotechnology can have on the main properties of metal hydrides and highlights the main competing behaviours between the system requirements, the necessary trade-offs, and the research priorities necessary to obtain hydride storage materials for practical automotive applications. Copyright © 2007 John Wiley & Sons, Ltd.

Environmental and sustainability aspects of hydrogen and fuel cell systems

Abstract: Discussed in this paper are current environmental problems, potential solutions to these problems, possible future hydrogen energy-utilization patterns for better environment and sustainable development through life cycle assessment (LCA), and how the principles of thermodynamics via exergy can be beneficially used to evaluate hydrogen and fuel cell systems and their role in sustainable development. Throughout the paper current and future perspectives of hydrogen and fuel cell systems based on exergetic, LCA and sustainability aspects development are considered. The results will likely be useful to scientists, researchers and engineers as well as policy and decision makers. Two case studies on the LCA aspects of hydrogen and fuel cell systems are presented to highlight the importance of the hydrogen and fuel cell systems and show that these can help achieve better environment and sustainability. Copyright © 2006 John Wiley & Sons, Ltd.

Microscale radiation in thermophotovoltaic devices- : A review

Abstract: Fast depleting reserves of conventional energy sources has resulted in an urgent need for increasing energy conversion efficiencies and recycling of waste heat. One of the potential candidates for fulfilling these requirements is thermophotovoltaic (TPV) devices. TPV devices generate electricity from either the complete combustion of different fuels or the waste heat of other energy sources, thereby saving energy. The thermal radiation from the emitter is incident on a TPV cell, which generates electrical currents. Applications of such devices range from hybrid electric vehicles to power sources for microelectronic systems. Absence of any moving parts and versatile fuel usage has made TPV devices very appealing for military and space applications. However, the presently available TPV systems suffer from low conversion efficiency. A viable solution to increase their efficiency is to apply microscale radiation principles in the design of different components to utilize the characteristics of thermal radiation at small distances and in microstructures. In order to have a clear understanding of microscale radiation and its role in TPV devices, several critical issues are reviewed in the present work. Emphasis is given to the development of wavelength-selective emitters and filters and the aspects of microscale heat transfer as applied to TPV systems. Recent progress, along with challenges and opportunities for future development of TPV systems are also outlined. Copyright © 2006 John Wiley & Sons, Ltd.

Experimental investigation of an adsorptive thermal energy storage

Abstract: A zeolite-water adsorption module, which has been originally constructed for an adsorption heat pump, has been experimentally investigated as an adsorptive thermal energy storage unit. The adsorber/desorber heat exchanger contains 13.2 kg of zeolite 13X and is connected to an evaporator/condenser heat exchanger via a butterfly valve. The flow rate of the heat transfer fluid in the adsorber/desorber unit has been changed between 0.5 and 2.0 l min−1, the inlet temperature to the evaporator between 10 and 40°C. It turned out that the higher the flow rate inside the adsorber/desorber unit the faster and more effective is the discharge of heat. However, at lower flow rates higher discharge temperatures are obtained. Storage capacities of 2.7 and 3.1 kWh have been measured at the evaporator inlet temperatures of 10 and 40°C, respectively, corresponding to thermal energy storage densities of 80 and 92 kWh m−3 based on the volume of the adsorber unit. The measured maximum power density increases from 144 to 165 kWh m−3 as the flow rate in the adsorber increases from 0.5 to 2 l min−1. An internal insulation in form of a radiation shield around the adsorber heat exchanger is recommended to reduce the thermal losses of the adsorptive storage. Copyright © 2006 John Wiley & Sons, Ltd.

Electrochemical model for performance analysis of a tubular SOFC

Abstract: Tubular solid oxide fuel cells (SOFC) are promising candidates for future energy conversion systems and expected to be applied widely for small-scale distributed generation to large-scale central station power plants because of their high electrical efficiency and high temperature exhaust gas utilization. This study presents an electrochemical model to determine the performance characteristics of tubular solid oxide fuel cell. Activation, ohmic and concentration polarizations are regarded as the major sources of irreversibility. The Butler-Volmer equation, Fick's law and Ohm's law are used to determine the polarization terms. Performance curves are simulated for single cell voltage and power under variable current density and validated with published experimental data for given operating conditions. All the variations of tubular SOFC's operational conditions such as operating pressure and temperature in the electrochemical processes is taken into consideration. The contribution of each polarization term to voltage losses is analysed with local characteristics such as pore size, electrolyte thickness and activation energy for evaluating the relative changes. Cell performance represented by cell voltage, power, efficiency and heat generation are analysed at its complete operating range, aiming at finding the set of optimal operating conditions maximizing the overall cell performance. As a conclusion from this study, the developed model is a simple and effective tool to analyse tubular SOFC in obtaining insight information about cell performance characteristics under different conditions.

Micro-fuel cell power sources

Abstract: This paper presents a review and discussion of micro-fuel cell technologies, providing insight into the innovations that have been made to date. Discussion of concepts and results leading towards increased levels of integration and performance for micro-fuel cell systems will elucidate the potential of thin film and microfabrication methods in meeting the challenges and requirements necessary for consumer applications. While the amount of literature in this area is substantial, a representative sampling of key developments will be presented in this paper, in order to gain a sense of the design methodologies being implemented for micro-fuel cell power sources. Copyright © 2007 John Wiley & Sons, Ltd.

Selected nanotechnologies for renewable energy applications

Abstract: As the world faces serious energy challenges, the development and implementation of renewable energy technologies become increasingly important. In this article, we offer a glimpse of the role nanotechnology, in particular, innovations of nanostructures and nanomaterials, is playing in the development of selected renewable energy technologies. These technologies, based on the authors' research interests, include (1) converting the energy of sunlight directly into electricity using solar cells; (2) converting solar energy into hydrogen fuel by splitting water into its constituents; (3) storing hydrogen in solid-state forms; and (4) utilizing hydrogen to generate electricity through the use of fuel cells. It is clear that nanotechnology-enabled renewable energy technologies are starting to scale up dramatically. As they become mature and cost effective in the decades to come, renewable energy could eventually replace the traditional, environmentally unfriendly, fossil fuels. Published in 2007 by John Wiley & Sons, Ltd.

Exergy analysis of a coal-based 210 MW thermal power plant

Abstract: In the present work, exergy analysis of a coal-based thermal power plant is done using the design data from a 210 MW thermal power plant under operation in India. The entire plant cycle is split up into three zones for the analysis: (1) only the turbo-generator with its inlets and outlets, (2) turbo-generator, condenser, feed pumps and the regenerative heaters, (3) the entire cycle with boiler, turbo-generator, condenser, feed pumps, regenerative heaters and the plant auxiliaries. It helps to find out the contributions of different parts of the plant towards exergy destruction. The exergy efficiency is calculated using the operating data from the plant at different conditions, viz. at different loads, different condenser pressures, with and without regenerative heaters and with different settings of the turbine governing. The load variation is studied with the data at 100, 75, 60 and 40% of full load. Effects of two different condenser pressures, i.e. 76 and 89 mmHg (abs.), are studied. Effect of regeneration on exergy efficiency is studied by successively removing the high pressure regenerative heaters out of operation. The turbine governing system has been kept at constant pressure and sliding pressure modes to study their effects. It is observed that the major source of irreversibility in the power cycle is the boiler, which contributes to an exergy destruction of the order of 60%. Part load operation increases the irreversibilities in the cycle and the effect is more pronounced with the reduction of the load. Increase in the condenser back pressure decreases the exergy efficiency. Successive withdrawal of the high pressure heaters show a gradual increment in the exergy efficiency for the control volume excluding the boiler, while a decrease in exergy efficiency when the whole plant including the boiler is considered. Keeping the main steam pressure before the turbine control valves in sliding mode improves the exergy efficiencies in case of part load operation. Copyright © 2006 John Wiley & Sons, Ltd.

Minimizing shell-and-tube heat exchanger cost with genetic algorithms and considering maintenance

Abstract: This paper presents a procedure for minimizing the cost of a shell-and-tube heat exchanger based on genetic algorithms (GA). The global cost includes the operating cost (pumping power) and the initial cost expressed in terms of annuities. Eleven design variables associated with shell-and-tube heat exchanger geometries are considered: tube pitch, tube layout patterns, number of tube passes, baffle spacing at the centre, baffle spacing at the inlet and outlet, baffle cut, tube-to-baffle diametrical clearance, shell-to-baffle diametrical clearance, tube bundle outer diameter, shell diameter, and tube outer diameter. Evaluations of the heat exchangers performances are based on an adapted version of the Bell–Delaware method. Pressure drops constraints are included in the procedure. Reliability and maintenance due to fouling are taken into account by restraining the coefficient of increase of surface into a given interval. Two case studies are presented. Results show that the procedure can properly and rapidly identify the optimal design for a specified heat transfer process. Copyright © 2006 John Wiley & Sons, Ltd.

A review and comparative study of the investigations on micro heat pipes

Abstract: A review of recent investigations on micro heat pipes of various geometrical designs, fabrication methods, operational environments and applications is presented, along with a comparative analysis of the performance of various designs, based on quantitative design parameters and performance indicators. A comprehensive report of the salient features of these investigations has been presented in a tabular form, in order to provide a quick reference of the state-of-the-art of micro heat pipe research. Both theoretical and experimental investigations have been included in this review, with special attention given to some of the new concepts and proposed designs for micro heat pipes, currently under development. Copyright © 2007 John Wiley & Sons, Ltd.

Energy and exergy analyses of a gasoline engine

Abstract: SUMMARY This study presents comparative energy and exergy analyses of a four-cylinder, four-stroke spark-ignition engine using gasoline fuels of three different research octane numbers (RONs), namely 91, 93 and 95.3. Each fuel test was performed by varying the engine speed between 1200 and 2400 rpm while keeping the engine torque at 20 and 40 Nm. Then, using the steady-state data along with energy and exergy rate balance equations, various performance parameters of the engine were evaluated for each fuel case. It was found that the gasoline of 91-RON, the design octane rating of the test engine, yielded better energetic and exergetic performance, while the exergetic performance parameters were slightly lower than the corresponding energetic ones. Furthermore, this study revealed that the combustion was the most important contributor to the system inefficiency, and almost all performance parameters increased with increasing engine speed. Copyright # 2006 John Wiley & Sons, Ltd.

An energy benchmarking model based on artificial neural network method utilizing US Commercial Buildings Energy Consumption Survey (CBECS) database

Abstract: This study focuses on development of an energy benchmarking model utilizing U.S. Commercial Buildings Energy Consumption Survey (CBECS) Database. An artificial neural networks (ANN) method based approach was used in the study. Office type buildings in the CBECS database were used in the benchmarking model development and weighted energy use intensity (EUI) was selected as the benchmarking index. The benchmarking model included input variables describing building's physical properties, occupancy and climate. Yearly electricity consumption per square meter, or EUI, was estimated by the ANN model. The correlation coefficient for each census division benchmarking model varied between 0.45 and 0.73, and mean squared error (MSE) varied between 9.60 and 15.25. It was observed that when the data set for a census division was grouped by different climate zones, ANN benchmarking model provided more accurate predictions. It was also observed that ANN model provides more accurate estimations when compared with predictions obtained with multi-linear regression models. For comparison, the MSE values varied between 10.24 and 40.43. Overall, the ANN model proved itself a better prediction model for energy benchmarking. Copyright © 2006 John Wiley & Sons, Ltd.

Performance analysis of the vapour compression cycle using ejector as an expander

Abstract: The purpose of incorporating an ejector into vapour compression cycle is to improve the COP by reducing the throttling loss associated with the expansion device. A computer simulation of the improved cycle is carried out using a one-dimensional model based on mass, momentum and energy balances. Refrigerant characteristics were evaluated using NIST subroutines for equations of state solutions. According to the results of simulation of the improved cycle, it has been shown that the geometric parameters of the ejector design have considerable effects on the system's performance. The maximum COP is obtained for Φopt whose value is around 10. Several refrigerants are considered; it has been observed, at Φopt and for given operating conditions, that the best performances are obtained with R141b. Compared with the standard cycle the COP of the improved cycle shows an increase of about 22%. Copyright © 2006 John Wiley & Sons, Ltd.

Exergetic performance evaluation of a combined heat and power (CHP) system in Turkey

Abstract: This study deals with the exergetic performance assessment of a combined heat and power (CHP) system installed in Eskisehir city of Turkey. Quantitative exergy balance for each component and the whole CHP system was considered, while exergy consumptions in the system were determined. The performance characteristics of this CHP system were evaluated using exergy analysis method. The exergetic efficiency of the CHP system was accounted for 38.16% with 49 880 kW as electrical products. The exergy consumption occurred in this system amounted to 80 833.67 kW. The ways of improving the exergy efficiency of this system were also analysed. As a result of these, a simple way of increasing the exergy efficiency of the available CHP system was suggested that the valves-I–III and the MPSC could be replaced by a 3500 kW-intermediate pressure steam turbine (IPST). If the IPST is installed to the CHP system (called the modified CHP (MCHP) system), the exergetic efficiency of the MCHP system is calculated to be 40.75% with 53 269.53 kW as electrical products. The exergy consumption is found to be 77 444.14 kW in the MCHP system. Copyright © 2006 John Wiley & Sons, Ltd.

Performance analysis on industrial refrigeration system integrated with encapsulated PCM-based cool thermal energy storage system

Abstract: Cool thermal energy storage (CTES) is an advanced energy technology that has recently attracted increasing interest for industrial refrigeration applications such as process cooling, food preservation and building air conditioning systems. An experimental investigation on the performance of an industrial refrigeration system integrated with encapsulated phase change material (PCM)-based CTES system is carried out in the present work. In the experimental set-up a vertical storage tank is integrated with the evaporator of the vapour compression refrigeration system. Effect of the inlet temperature of heat transfer fluid (HTF) on the temperature variation of the PCM and the HTF in the storage tank and the performance parameters namely average rate of charging, energy stored, specific energy consumption (SEC) of the chiller with and without storage system are studied in detail. The effect of porosity variation in the storage tank is also studied. A 1°C decrease in evaporator temperature results in about 3–4% increase in SEC and 1°C decrease in condensing temperature leads to 2.25–3.25% decrease in SEC. The range of HTF inlet temperature and porosity values for optimum performance is reported. Copyright © 2007 John Wiley & Sons, Ltd.

Rapid and slow pyrolysis of pistachio shell: effect of pyrolysis conditions on the product yields and characterization of the liquid product

Abstract: This study reports the experimental results for the pyrolysis of pistachio shell under different conditions in a tubular reactor under a nitrogen flow. For the different conditions of pyrolysis temperature, nitrogen flow rate and heating rate, pyrolysis temperature of 773 K gave the highest bio-oil yield with a value of 27.7% when the heating rate and carrier gas flow rate were chosen as 300 K min−1 and 100 cm3 min−1, respectively. Column chromatography was applied to this bio-oil and its subfractions were characterized by elemental analysis, FT-IR and 1H-NMR. Aliphatic subfraction was conducted to gas chromatography–mass spectroscopy for further characterization. The results for the characterization show that using pistachio shell as a renewable source to produce valuable liquid products is applicable via pyrolysis. Copyright © 2006 John Wiley & Sons, Ltd.

Comparative thermal performance evaluation of an active solar distillation system

Abstract: In this paper, thermal models of all types of solar collector-integrated active solar stills are developed based on basic energy balance equations in terms of inner and outer glass temperatures. In this paper, hourly yield, hourly exergy efficiency, and hourly overall thermal efficiency of active solar stills are evaluated for 0.05 m water depth. All numerical computations had been performed for a typical day in the month of 07 December 2005 for the climatic conditions of New Delhi (28°35′N, 77°12′E, 216 m above MSL). The thermal model of flat-plate collector integrated with active solar still was validated using the experimental test set-up results. Total daily yield from active solar still integrated with evacuated tube collector with heat pipe is 4.24 kg m−2 day−1, maximum among all other types of active solar stills. Copyright © 2007 John Wiley & Sons, Ltd.

Energy saving in building with PCM cold storage

Abstract: This article presents an experimental and numerical analysis of cooling buildings using night-time cold accumulation in phase change material (PCM), otherwise known as the ‘free-cooling principle’. Experimental and numerical studies of the ceiling and floor free-cooling principle, as well as passive cooling, are presented. The free-cooling principle is explained and some of the types of PCMs suitable for summer cooling are listed. An experiment was conducted using paraffin with a melting point of 22°C as the PCM to store cold during the night-time and to cool hot air during the daytime in summer. Air temperatures and heat fluxes as a function of time and dimensionless cold discharging values are presented for different air velocities. Experimental analysis of one week of measuring under real conditions is presented in order to show how cold storage functions. Copyright © 2007 John Wiley & Sons, Ltd.

Recent advances in microdevices for electrochemical energy conversion and storage

Abstract: The application of silicon microfabrication technologies to electrochemical devices allows reduction of overall device package to potentially increase volumetric power densities. This review first focuses on some exciting developments in microfuel cells, in particular, solid oxide fuel cells (SOFCs) and proton exchange membrane fuel cells (PEMFCs). The emphasis is given to innovative 2D processing methods, novel 2D architectures of microfuel cells, and demonstrated performance in terms of area power densities. Emerging 3D fabrication techniques that are potentially promising to produce 3D electrochemical devices such as 3D cell and stack architectures on the micrometer scale will then be discussed. Lastly this paper highlights some new opportunities in electrode kinetics studies enabled by microfabricated devices—investigation of scaling relationship between microelectrodes and electrochemical responses, which has led to improved fundamental understanding of electrode reactions and rate-limiting steps. Copyright © 2007 John Wiley & Sons, Ltd.

Exergetic performance assessment of a ground-source heat pump drying system

Abstract: In evaluating the efficiency of heat pump (HP) systems, the most commonly used measure is the energy (or first law) efficiency, which is modified to a coefficient of performance (COP) for HP systems. However, for indicating the possibilities for thermodynamic improvement, energy analysis is inadequate and exergy analysis is needed. This study presents an exergetic assessment of a ground-source (or geothermal) HP (GSHP) drying system. This system was designed, constructed and tested in the Solar Energy Institute of Ege University, Izmir, Turkey. The exergy destructions in each of the components of the overall system are determined for average values of experimentally measured parameters. Exergy efficiencies of the system components are determined to assess their performances and to elucidate potentials for improvement. COP values for the GSHP unit and overall GSHP drying system are found to range between 1.63–2.88 and 1.45–2.65, respectively, while corresponding exergy efficiency values on a product/fuel basis are found to be 21.1 and 15.5% at a dead state temperature of 27°C, respectively. Specific moisture extraction rate (SMER) on the system basis is obtained to be 0.122 kg kW−1 h−1. For drying systems, the so-called specific moisture exergetic rate (SMExR), which is defined as the ratio of the moisture removed in kg to the exergy input in kW h, is also proposed by the authors. The SMExR of the whole GSHP drying system is found to be 5.11 kg kW−1 h−1. Copyright © 2006 John Wiley & Sons, Ltd.

Exergetic analysis of an aircraft turbofan engine

Abstract: The main objective of the present study is to perform an exergy analysis of a turbofan kerosene-fired engine with afterburner (AB) at sea level and an altitude of 11 000 m. The main components of this engine include a fan, a compressor, a combustion chamber, a turbine, an AB and an exhaust. Exergy destructions in each of the engine components are determined, while exergy efficiency values for both altitudes are calculated. The AB unit is found to have the highest exergy destruction with 48.1% of the whole engine at the sea level, followed by the exhaust, the combustion chamber and the turbine amounting to 29.7, 17.2 and 2.5%, respectively. The corresponding exergy efficiency values for the four components on the product/fuel basis are obtained to be 59.9, 65.6, 66.7 and 88.5%, while those for the whole engine at the sea level and an altitude of 11 000 m are calculated to be 66.1 and 54.2%. Copyright © 2007 John Wiley & Sons, Ltd.

Influencing user behaviour with energy information display systems for intelligent homes

Abstract: This paper considers the design of energy consumption displays from a theoretical standpoint in relation to user–appliance interactions in the home. It presents the main options for display position and discusses what detailed information to display in order to promote energy saving. To guide the designer in making decisions regarding the merits of displaying appliance-specific information (ASI) and grouped appliance information (GAI) around the home, classifications are presented of appliance-associated micro-behaviours and appliance control to end-use relationships. A hybrid display approach is suggested as a most effective solution. Copyright © 2006 John Wiley & Sons, Ltd.

Waste pyrolysis and generation of storable char

Abstract: Sustainable cities require the generation of energy from waste that cannot be economically reused or recycled. This study focuses on slow pyrolysis that can generate a high yield of char along with liquid and gas products from waste. Char is high in energy content, storable and transportable with low cost so that it can be used as an intermediate medium for high efficiency energy conversion. Pre-processed municipal waste pellets, wood and grass were pyrolysed in a batch type reactor for a final temperature ranging from 350 to 700°C, and the char products were characterized. The mass yields of char ranged from 55 to 20% for the tested temperature range, recovering 70–30% of energy and 62–30% of carbon in the raw material. The gross calorific value of char was 30–35 MJ kg−1 on a dry ash free basis. The ash content of raw materials was a key parameter for the quality of char, since its proportion increased by 2–4 times in char depending on the mass yield. A significant amount of volatile metals such as Hg, As and Pb in the waste sample was evaporated at 500°C. Therefore, evaporation of volatile metals was another important parameter in determining the pyrolysis temperature and fuel residence time. The char did not show significant morphological change in the tested range of temperatures. It was concluded that slow pyrolysis of waste for char production should be performed below 500°C in order to increase the energy yield and also to reduce the evaporation of heavy metals. Copyright © 2006 John Wiley & Sons, Ltd.

Design and techno-economical optimization for stand-alone hybrid power systems with multi-objective evolutionary algorithms

Abstract: The optimal design of the hybrid energy system can significantly improve the economical and technical performance of power supply. However, the problem is formidable because of the uncertain renewable energy supplies, the uncertain load demand, the nonlinear characteristics of some components, and the conflicting techno-economical objectives. In this work, the optimal design of the hybrid energy system has been formulated as a multi-objective optimization problem. We optimize the techno-economical performance of the hybrid energy system and analyse the trade-offs between the multi-objectives using multi-objective genetic algorithms. The proposed method is tested on the widely researched hybrid PV-wind power system design problem. The optimization seeks the compromise system configurations with reference to three incommensurable techno-economical criteria, and uses an hourly time-step simulation procedure to determine the design criteria with the weather resources and the load demand for one reference year. The well-known efficient multi-objective genetic algorithm, called NGAS-II (the fast elitist non-dominated sorting genetic algorithm), is applied on this problem. A hybrid PV-wind power system has been designed with this method and several methods in the literature. The numerical results demonstrate that the proposed method is superior to the other methods. It can handle the optimal design of the hybrid energy system effectively and facilitate the designer with a range of the design solutions and the trade-off information. For this particular application, the hybrid PV-wind power system using more solar panels achieves better technical performance while the one using more wind power is more economical. Copyright © 2006 John Wiley & Sons, Ltd.

Thermodynamic analysis of the CO2-based Rankine cycle powered by solar energy

Abstract: Using carbon dioxide as working fluid receives increasing interest since the Kyoto Protocol. In this paper, thermodynamic analysis was conducted for proposed CO2-based Rankine cycle powered by solar energy. It can be used to provide power output, refrigeration and hot water. Carbon dioxide is used as working fluid with supercritical state in solar collector. Theoretical analysis was carried out to investigate performances of the CO2-based Rankine cycle. The interest was focused on comparison of the performance with that of solar cell and those when using other fluids as working fluids. In addition, the performance and characteristics of the thermodynamic cycle are studied for different seasons. The obtained results show that using CO2 as working fluid in the Rankine cycle owns maximal thermal efficiency when the working temperature is lower than 250.0°C. The power generation efficiency is about 8%, which is comparable with that of solar cells. But in addition to power generation, the CO2-based solar utilization system can also supply thermal energy. Copyright © 2007 John Wiley & Sons, Ltd.

Heat exchanger: from micro- to multi-scale design optimization

Abstract: This paper presents a consideration of micro-heat exchangers design optimization for the aim of process intensification Two examples are discussed to illustrate different ways of heat transfer intensification in micro-heat exchangers To solve the key issue of the link between the micro-scale and the macro-scale, a multi-scale design optimization method using fractal and constructal approaches is introduced The concept of a novel constructal heat exchanger is also proposed

Exergetic evaluation of drying of laurel leaves in a vertical ground-source heat pump drying cabinet

Abstract: This paper is concerned with the exergy analysis of the single layer drying process of laurel leaves in a ground-source heat pump drying cabinet, which was designed and constructed in the Solar Energy Institute, Ege University, Izmir, Turkey. The effects of drying air temperature on exergy losses, exergy efficiencies and exergetic improvement potential of the drying process are investigated. The results have indicated that exergy efficiencies of the dryer increase with rising the drying air temperature. Moreover, the laurel leaves are sufficiently dried at the temperatures ranging from 40 to 50°C with relative humidities varying from 16 to 19% and a drying air velocity of 0.5 m s−1 during the drying period of 9 h. The exergy efficiency values are obtained to range from 81.35 to 87.48% based on the inflow, outflow and loss of exergy, and 9.11 to 15.48% based on the product/fuel basis between the same drying air temperatures with a drying air mass flow rate of 0.12 kg s−1. Copyright © 2006 John Wiley & Sons, Ltd.

Thermodynamic and related analysis of natural gas combined cycle power plants with and without carbon sequestration

Abstract: Thermodynamic and related 'exergoeconomic' performance criteria have been used to evaluate natural gas combined cycle (NGCC) power generation systems, with and without carbon dioxide (CO 2 ) removal technologies. These plants were previously studied by the US National Energy Technology Laboratory employing conventional energy and mass balance results, and have now been evaluated using detailed energy, exergy and exergoeconomic analyses. The plant consisted of a gas turbine together with a steam cycle having three pressure levels. Such NGCC plants show the least exergetic improvement potential amongst competing fossil fuel generators going forward, because they are already enhanced by use of a thermodynamic 'topping' cycle. Carbon capture was simulated on the basis of CO 2 recovery from the flue gas stream that leaves the heat recovery steam generator via a commercial amine process. Ninety per cent of the CO 2 was captured in this way, and then compressed into a high-pressure liquid. This was achieved with significant power penalty (some 21%) and increase in generating cost per MWh (44%). Combustion and heat transfer processes are the main sources of exergy degradation within power cycles. Fuel combustion accounted for some 32% of exergy destruction. Even with CO 2 sequestration, the NGCC system is still a thermodynamically attractive option compared with modern fossil fuel alternatives. Overall, the exergoeconomic results indicate that significant improvements can be achieved by considering the power generation systems as a whole, rather than concentrating on the improvement in performance of individual components (which is a common practice in exergoeconomic optimization studies).

Thermodynamic analysis of spark‐ignition engine using a gas mixture model for the working fluid

Abstract: This paper presents thermodynamic analysis of spark-ignition engine. A theoretical model of Otto cycle, with a working fluid consisting of various gas mixtures, has been implemented. It is compared to those which use air as the working fluid with variable temperature specific heats. A wide range of engine parameters were studied, such as equivalence ratio, engine speed, maximum and outlet temperatures, brake mean effective pressure, gas pressure, and cycle thermal efficiency. For example, for the air model, the maximum temperature, brake mean effective pressure (BMEP), and efficiency were about 3000 K, 15 bar, and 32%, respectively, at 5000 rpm and 1.2 equivalence ratio. On the other hand, by using the gas mixture model under the same conditions, the maximum temperature, BMEP, and efficiency were about 2500 K, 13.7 bar, and 29%. However, for the air model, at lower engine speeds of 2000 rpm and equivalence ratio of 0.8, the maximum temperature, BMEP, and efficiency were about 2000 K, 8.7 bar, and 28%, respectively. Also, by using the gas mixture model under these conditions, the maximum temperature, BMEP, and efficiency were about 1900 K, 8.4 bar, and 27%, i.e. with insignificant differences. Therefore, it is more realistic to use gas mixture in cycle analysis instead of merely assuming air to be the working fluid, especially at high engine speeds. Copyright © 2007 John Wiley & Sons, Ltd.

Thermodynamic analysis of an LNG fuelled combined cycle power plant with waste heat recovery and utilization system

Abstract: This paper has proposed an improved liquefied natural gas (LNG) fuelled combined cycle power plant with a waste heat recovery and utilization system. The proposed combined cycle, which provides power outputs and thermal energy, consists of the gas/steam combined cycle, the subsystem utilizing the latent heat of spent steam from the steam turbine to vaporize LNG, the subsystem that recovers both the sensible heat and the latent heat of water vapour in the exhaust gas from the heat recovery steam generator (HRSG) by installing a condensing heat exchanger, and the HRSG waste heat utilization subsystem. The conventional combined cycle and the proposed combined cycle are modelled, considering mass, energy and exergy balances for every component and both energy and exergy analyses are conducted. Parametric analyses are performed for the proposed combined cycle to evaluate the effects of several factors, such as the gas turbine inlet temperature (TIT), the condenser pressure, the pinch point temperature difference of the condensing heat exchanger and the fuel gas heating temperature on the performance of the proposed combined cycle through simulation calculations. The results show that the net electrical efficiency and the exergy efficiency of the proposed combined cycle can be increased by 1.6 and 2.84% than those of the conventional combined cycle, respectively. The heat recovery per kg of flue gas is equal to 86.27 kJ s−1. One MW of electric power for operating sea water pumps can be saved. The net electrical efficiency and the heat recovery ratio increase as the condenser pressure decreases. The higher heat recovery from the HRSG exit flue gas is achieved at higher gas TIT and at lower pinch point temperature of the condensing heat exchanger. Copyright © 2006 John Wiley & Sons, Ltd.