Energetic and exergetic performance evaluations of a geothermal power plant based integrated system for hydrogen production

In this paper, we propose an integrated system aiming for hydrogen production with by-products using geothermal power as a renewable energy source. In analyzing the system, an extensive thermodynamic model of the proposed system is developed and presented accordingly. In addition, the energetic and exergetic efficiencies and exergy destruction rates for the whole system and its parts are defined. Due to the significance of some parameters, the impacts of varying working conditions are also investigated. The results of the energetic and exergetic analyses of the integrated system show that the energy and exergy efficiencies are 39.46% and 44.27%, respectively. Furthermore, the system performance increases with the increasing geothermal source temperature and reference temperature while it decreases with the increasing pinch point temperature and turbine inlet pressure.     

Energetic and exergetic parametric study of a SOFC-GT hybrid power plant

A parametric study is conducted on a hybrid SOFC-GT cycle as part of a national program aiming to improve the efficiency of the actual gas turbine power plants and to better undertake the future investigations. The proposed power plant is mainly constituted by a Gas Turbine cycle, a SOFC system, and an ammonia water absorption refrigerating system. An external pre-reformer is installed before the SOFC. Heat recovery systems are adopted to valorize the waste heat at the SOFC and GT exhausts. The gas from the SOFC exhaust is also used as additional supply for the combustion chamber. An extraction is performed on the gas Turbine in order to feed the SOFC cycle by thermal heat flux at medium pressure.The equations governing the electrochemical processes, the energy and the exergy balances of the power plant components are established. Numerical simulation using EES software is performed. The influences of key operating parameters, such as humidity, pre-reforming fraction, extraction fraction from the Gas Turbine and fuel utilization on the performances of the SOFC-GT hybrid system are analyzed. Obtained results show that the integration of the SOFC enhances significantly the hybrid overall cycle efficiency. The increase of the ambient temperature and humidity reduces the system efficiencies. The utilization factor has a negative effect on the SOFC temperature and voltage. That leads to a decrease in the power plant performances. While the pre-reforming fraction, has a positive effect on the indicated parameters.     

Effects of evaporative inlet and aftercooling on the recuperated gas turbine cycle

Inlet air cooling and cooling of the compressor discharge using water injection boost both efficiency and power of gas turbine cycles. Four different layouts of the recuperated gas turbine cycle are presented. Those layouts include the effect of evaporative inlet and aftercooling (evaporative cooling of the compressor discharge). A parametric study of the effect of turbine inlet temperature (TIT), ambient temperature, and relative humidity on the performance of all four layouts is investigated. The results indicate that as TIT increases the optimum pressure ratio increases by 0.45 per 100 K for the regular recuperated cycle and by 1.4 per 100 K for the recuperated cycle with evaporative aftercooling. The cycles with evaporative aftercooling have distinctive pattern of performance curves and higher values of optimum pressure ratios. The results also showed that evaporative cooling of the inlet air could boost the efficiency by up to 3.2% and that evaporative aftercooling could increase the power by up to about 110% and cycle efficiency by up to 16%.     

Exergetic comparison of efficiency indicators for combined heat and power (CHP)

Legislative regulations in favor of combined heat and power (CHP) production have been implemented in many countries. Although these regulations put different emphasis on power production vs. process heat production, they are based on energy quantities and not on exergy. In order to analyze and compare the exergetic consequences of the various legislations, a relative avoided irreversibility (RAI) is defined. This can be regarded as the exergy loss that is avoided when reference plants with separate production are replaced by an actual CHP plant. Some series of industrial and district heating CHP plants, under varying operational conditions, are used as test cases. It is seen that some, but not all, CHP cases are exergetically beneficial to separate generation. Comparison with the RAI allows a quantitative assessment of the various performance indicators. It is seen that exergetic improvements were only captured to a limited degree by the various energy-based efficiency indicators. Some legislatively defined indicators even appear to discourage thermodynamic improvements.     

Energetic and exergetic analysis of a biomass-fueled CCHP system integrated with proton exchange membrane fuel cell

As a high-efficiency and eco-friendly way of energy conversion, fuel cell has received much attention in recent years. A novel residential combined cooling, heating and power (CCHP) system, consisting of a biomass gasifier, a proton exchange membrane fuel cell (PEMFC) stack, an absorption chiller and auxiliary equipment, is proposed. Based on the established thermodynamic models, the effects of operating parameters, biomass materials type and moisture content on the system performance are closely investigated. Overall system performance is then compared under four different operating modes. From the viewpoints of energy utilization and CO₂ emissions, the CCHP mode has the best performance with corresponding energy efficiency of 57.41% and CO₂ emission index of 0.516 ton/MWh. Exergy analysis results suggest that the optimization and transformation on the gasifier and PEMFC stack should be encouraged. Energy and exergy assessments in this research provide pragmatic guidance to the performance improvement of the integrated CCHP systems with PEMFC. This research also achieves a reasonable combination of efficient cogeneration, green hydrogen production and full recovery of low grade waste heat.     

Comparative exergoeconomic analyses of the integration of biomass gasification and a gas turbine power plant with and without fogging inlet cooling

Inlet cooling is effective for mitigating the decrease in gas turbine performance during hot and humid summer periods when electrical power demands peak, and steam injection, using steam raised from the turbine exhaust gases in a heat recovery steam generator, is an effective technique for utilizing the hot turbine exhaust gases. Biomass gasification can be integrated with a gas turbine cycle to provide efficient, clean power generation. In the present paper, a gas turbine cycle with fog cooling and steam injection, and integrated with biomass gasification, is proposed and analyzed with energy, exergy and exergoeconomic analyses. The thermodynamic analyses show that increasing the compressor pressure ratio and the gas turbine inlet temperature raises the energy and exergy efficiencies. On the component level, the gas turbine is determined to have the highest exergy efficiency and the combustor the lowest. The exergoeconomic analysis reveals that the proposed cycle has a lower total unit product cost than a similar plant fired by natural gas. However, the relative cost difference and exergoeconomic factor is higher for the proposed cycle than the natural gas fired plant, indicating that the proposed cycle is more costly for producing electricity despite its lower product cost and environmental impact.     

Exergy analysis of a solar assisted absorption cooling system on an hourly basis in villa applications

A solar assisted absorption refrigeration system (SAARS) was designed for acclimatizing of villas in Mardin which is located in Turkey and the performance of the system under different temperatures was analyzed by using MATLAB. Hourly cooling load calculation of the villas was done between 15th of May and 15th of September by considering the season for the cooling. Cooling capacity of the system (SAARS) was calculated as 106 kW. During the cooling period, the temperature of the environment shows the alteration between 40.3 °C and 13.2 °C. In the study, hourly exergy loss values are calculated with the software developed in matlab program and for the entire components of SAARS. The effect of the temperature alterations of the dead state on the exergy results is determined by taking dead state temperature as 25 °C and with more realistic approach, by taking it as the environment temperature. It was observed that the most of the exergy losses in the system have taken place in the solar collectors and then in the generator. Exergy loss in the collector changes between 10% and 70% while exergy loss in the generator changes between 5% and 8%. The effects of environmental temperature and solar insolation were stated for optimization.     

Thermal performance of a building coupled to an evaporative cooling tower

The thermal performance of a building fitted with an evaporative cooling tower has been evaluated in terms of discomfort index for two climates, namely, composite and hot-dry, typified by New Delhi and Jodhpur, respectively. The effects of various evaporative cooling parameters (height and cross-sectional area of the tower, packing factor, area of the pads, resistance offered to the air flow and local wind conditions) on the performance of the building have been analysed. It was found that, for given parameters of the tower and wind conditions, there is an optimum height of the tower for which the thermal discomfort condition in the building is minimum. The optimum values of the tower height for comfort conditions in the building for various other tower parameters have been obtained for each climate.     

Energetic and exergetic assessments of a novel solar power tower based multigeneration system with hydrogen production and liquefaction

In this article, a thermodynamic investigation of solar power tower assisted multigeneration system with hydrogen production and liquefaction is presented for more environmentally-benign multigenerational outputs. The proposed multigeneration system is consisted of mainly eight sub-systems, such as a solar power tower, a high temperature solid oxide steam electrolyzer, a steam Rankine cycle with two turbines, a hydrogen generation and liquefaction cycle, a quadruple effect absorption cooling process, a drying process, a membrane distillation unit and a domestic hot water tank to supply hydrogen, electrical power, heating, cooling, dry products, fresh and hot water generation for a community. The energetic and exergetic efficiencies for the performance of the present multigeneration system are found as 65.17% and 62.35%, respectively. Also, numerous operating conditions and parameters of the systems and their effects on the respective energy and exergy efficiencies are investigated, evaluated and discussed in this study. A parametric study is carried out to analyze the impact of various system design indicators on the sub-systems, exergy destruction rates and exergetic efficiencies and COPs. In addition, the impacts of varying the ambient temperature and solar radiation intensity on the irreversibility and exergetic performance for the present multigeneration system and its components are investigated and evaluated comparatively. According to the modeling results, the solar irradiation intensity is found to be the most influential parameter among other conditions and factors on system performance.     

Performance assessment of an integrated PV/T and triple effect cooling system for hydrogen and cooling production

In this paper we study an integrated PV/T absorption system for cooling and hydrogen production based on U.A.E weather data. Effect of average solar radiation for different months, operating time of the electrolyzer, air inlet temperature and area of the PV module on power and rate of heat production, energy and exergy efficiencies, hydrogen production and energetic and exergetic COPs are studied. It is found that the overall energy and exergy efficiency varies greatly from month to month because of the variation of solar radiation and the time for which it is available. The highest energy and exergy efficiencies are obtained for the month of March and their value is 15.6% and 7.9%, respectively. However, the hydrogen production is maximum for the month of August and its value is 9.7 kg because in august, the solar radiation is high and is available for almost 13 h daily. The maximum energetic and exergetic COPs are calculated to be 2.28 and 2.145, respectively and they are obtained in the month of June when solar radiation is high for the specified cooling load of 15 kW.     

Thermodynamic performance assessment of an indirect intercooled reheat regenerative gas turbine cycle with inlet air cooling and evaporative aftercooling of the compressor discharge

This study provides a computational analysis to investigate the effects of cycle pressure ratio, turbine inlet temperature (TIT), and ambient relative humidity (φ) on the thermodynamic performance of an indirect intercooled reheat regenerative gas turbine cycle with indirect evaporative cooling of the inlet air and evaporative aftercooling of the compressor discharge. Combined first and second‐law analysis indicates that the exergy destruction in various components of gas turbine cycles is significantly affected by compressor pressure ratio and turbine inlet temperature, and is not at all affected by ambient relative humidity. It also indicates that the maximum exergy is destroyed in the combustion chamber; which represents over 60% of the total exergy destruction in the overall system. The net work output, first‐law efficiency, and the second‐law efficiency of the cycle significantly varies with the change in the pressure ratio, turbine inlet temperature and ambient relative humidity. Results clearly shows that performance evaluation based on first‐law analysis alone is not adequate, and hence more meaningful evaluation must include second‐law analysis. Decision makers should find the methodology contained in this paper useful in the comparison and selection of gas turbine systems. Copyright © 2006 John Wiley & Sons, Ltd.     

Energetic and exergetic performance evaluation of the quadruple-effect evaporator unit in tomato paste production

Quadruple-effect evaporator units are commonly used in food focus area in sector is evaporative unit. It consumes about 60% of total energy input. The present study evaluates the performance of quadruple-effect evaporator unit (QEEU) by using exergy analysis based on actual operational data. A tomato paste factory is chosen for the analysis. The highest exergy destruction/loss occurs in the first effect with 158.2 kW, 52.7% of exergy input in first effect. Steam temperature should be decreased in order to decrease exergy destruction in first effect. Also, third effect achieves the highest exergy efficiency with 93.3%. Exergetic improvement potential of each effect varies between 0.3 kW and 83.6 kW. The highest and lowest exergetic improvement potential occurs in first and third effect of QEEU system, respectively. Exergetic improvement potential is equals to 52.80%, 11.10%, 6.73% and 69.8% of exergy loss/destruction from the first effect to the last effect, respectively. Total exergetic improvement potential is achieved as 128 kW (55% of total exergy loss/destruction) in QEEU system. It is expected that analyses result provide important information for designer and/or resources of multiple effect evaporator unit.     

Exergoenvironmental analysis and optimization of a cogeneration plant system using Multimodal Genetic Algorithm (MGA)

In the present work, a combined heat and power plant for cogeneration purposes that produces 50 MW of electricity and 33.3 kg/s of saturated steam at 13 bar is optimized using genetic algorithm. The design parameters of the plant considered are compressor pressure ratio (r_AC), compressor isentropic efficiency (η_comp), gas turbine isentropic efficiency (η_GT), combustion chamber inlet temperature (T₃), and turbine inlet temperature (TIT). In addition, to optimally find the optimum design parameters, an exergoeconomic approach is employed. A new objective function, representing total cost rate of the system product including cost rate of each equipment (sum of the operating cost, related to the fuel consumption) and cost rate of environmental impact (NO_x and CO) is considered. Finally, the optimal values of decision variables are obtained by minimizing the objective function using evolutionary genetic algorithm. Moreover, the influence of changes in the demanded power on various design parameters are parametrically studied for 50, 60, 70 MW of net power output. The results show that for a specific unit cost of fuel, the values of design parameters increase, as the required, with net power output increases. Also, the variations of the optimal decision variables versus unit cost of fuel reveal that by increasing the fuel cost, the pressure ratio, r_AC, compressor isentropic efficiency, η_AC, turbine isentropic efficiency, η_GT, and turbine inlet temperature (TIT) increase.     

Energetic and exergetic analyses of a combined system consisting of a high-temperature polymer electrolyte membrane fuel cell and a thermoelectric generator with Thomson effect

A combined system model consisting of a high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC), a regenerator and a thermoelectric generator (TEG) is proposed, where the TEG is applied to harness the generated waste heat in the HT-PEMFC for extra electricity production. The TEG considers not only the Seebeck effect and Peltier effect but also the Thomson effect. The mathematical expressions of power output, energy efficiency, exergy destruction rate and exergy efficiency for the proposed system are derived. The energetic and exergetic performance characteristics for the whole system are revealed. The optimum operating ranges for some key performance parameters of the combined system are determined using the maximum power density as the objective function. The combined system maximum power density and its corresponding energy efficiency and exergy efficiency allow 19.1%, 12.4% and 12.6% higher than that of a stand-alone HT-PEMFC, while the exergy destruction rate density is only increased by 8.6%. The system performances are compared between the TEG with and without the Thomson effect. Moreover, the impacts of comprehensive parameters on the system performance characteristics are discussed. The obtained results are helpful in developing and designing such an actual combined system for efficient and clean power production.     

Lithium bromide high-temperature absorption heat pump: Coefficient of performance and exergetic efficiency

A theoretical study on the employment of a lithium bromide absorption heat pump in Spain, used as machine type I and aimed to produce heat at 120°C via waste heat sources at 60°C, is given in the paper. Real performance conditions are stated for each component of the machine, namely the absorber, the heat recoverer, the generator, the condenser, the solution pump, the expansion valve and the evaporator. By means of thermodynamic diagrams (p, t, x) and (h, x), the required data are obtained for calculation of the heat recovered in the evaporator Q_e, and the heat delivered to the absorber Q_a and to the condenser Q_c, as well as the heat supplied to the generator Q_g. In addition, the heat delivered by the hot solution to the cold solution in the heat recovered Q_r, and the work W_p done by the solution pump are calculated. The probable COP is calculated, and values are obtained close to 1.4. The working temperature in the generator is determined; it ranges from 178 to 200°C. The heat produced by the lithium bromide absorption heat pump is 22% cheaper than the heat obtained from a cogeneration system comprising natural gas internal combustion engine and a high temperature heat pump with mechanical compression. Compared with a high temperature heat pump with mechanical compression, the heat produced by the absorption heat pump is 31% cheaper. From (h, x) and (s, x) diagrams, exergy losses for each component can be determined, and, from these results, an exergetic efficiency of 75% is obtained, which provides the quality index of absorption cycle.     

Energetic and exergetic aspects of cotton stalk production in establishing energy policies

Exergy analysis is important for energy resource utilization, because exergy, which is a way to a sustainable future, is a part of the energy analysis. Exergy analysis starts to play a role in several countries in developing energy policy. This paper deals with the exergetic assessment of the cotton stalk (CS) production. In this regard, Turkey, which is one of the eight countries producing 85% of the world's cotton, is given as an application country first. Energy and exergy relations used in the analysis are then presented. Finally, the Turkish CS production in 2003 is evaluated using energy and exergy analyses method, while the results obtained are discussed. The values for the net energy and exergy gained are obtained to be about 49,146 and 59,395 MJ/ha, respectively. Turkey's total energy and exergy are estimated to be 75.45 and 81.87 PJ. It may be concluded that this amount of energy is equal to 7.77% and 2.38% of Turkey's primary energy production and consumption in the same year, respectively. The overall mean energy and exergy efficiencies of the cotton production in the year studied are found to be 33.06% and 33.12%, respectively. It is also expected that the results of this study will be helpful in developing highly applicable and productive planning for energy policies.     

Thermally driven cooling coupled with municipal solid waste-fired power plant: Application of combined heat,cooling and power in tropical urban areas

Energy recovery from flue gases in thermal treatment plants is an integral part of municipal solid waste (MSW) management for many industrialized nations. Often cogeneration can be employed for both enhancing the plant profitability and increasing the overall energy yield. However, it is normally difficult to justify traditional cogeneration in tropical locations since there is little need for the heat produced. The main objective of this article is to investigate the opportunities and potentials for various types of absorption technologies driven by MSW power plants for providing both electricity and cooling. Results show that cogeneration coupling with thermally driven cooling is sustainably and economically attractive for both electricity and cooling production. The thermally driven cooling provides significant potential to replace electrically driven cooling: such systems are capable of providing cooling output and simultaneously increasing electricity yield (41%). The systems are also capable of reducing the fuel consumption per unit of cooling in comparison with conventional cooling technology: a reduction of more than 1 MW_fuel/MW_cooling can be met in a small unit. MSW power plant coupled with thermally driven cooling can further reduce CO₂ emissions per unit of cooling of around 60% as compared to conventional compression chiller and has short payback period (less than 5 years).     

Comparative study of the performance of the M-cycle counter-flow and cross-flow heat exchangers for indirect evaporative cooling – Paving the path toward sustainable cooling of buildings

This paper provides a comparative study of the performance of cross-flow and counter-flow M-cycle heat exchangers for dew point cooling. It is recognised that evaporative cooling systems offer a low energy alternative to conventional air conditioning units. Recently emerged dew point cooling, as the renovated evaporative cooling configuration, is claimed to have much higher cooling output over the conventional evaporative modes owing to use of the M-cycle heat exchangers. Cross-flow and counter-flow heat exchangers, as the available structures for M-cycle dew point cooling processing, were theoretically and experimentally investigated to identify the difference in cooling effectiveness of both under the parallel structural/operational conditions, optimise the geometrical sizes of the exchangers and suggest their favourite operational conditions. Through development of a dedicated computer model and case-by-case experimental testing and validation, a parametric study of the cooling performance of the counter-flow and cross-flow heat exchangers was carried out. The results showed the counter-flow exchanger offered greater (around 20% higher) cooling capacity, as well as greater (15%–23% higher) dew-point and wet-bulb effectiveness when equal in physical size and under the same operating conditions. The cross-flow system, however, had a greater (10% higher) Energy Efficiency (COP). As the increased cooling effectiveness will lead to reduced air volume flow rate, smaller system size and lower cost, whilst the size and cost are the inherent barriers for use of dew point cooling as the alternation of the conventional cooling systems, the counter-flow system is considered to offer practical advantages over the cross-flow system that would aid the uptake of this low energy cooling alternative. In line with increased global demand for energy in cooling of building, largely by economic booming of emerging developing nations and recognised global warming, the research results will be of significant importance in terms of promoting deployment of the low energy dew point cooling system, helping reduction of energy use in cooling of buildings and cut of the associated carbon emission.     

Thermodynamic analysis of a novel integrated geothermal based power generation-quadruple effect absorption cooling-hydrogen liquefaction system

In this paper, we propose a novel integrated geothermal absorption system for hydrogen liquefaction, power and cooling productions. The effect of geothermal, ambient temperature and concentration of ammonia-water vapor on the system outputs and efficiencies are studied through energy and exergy analyses. It is found that both energetic and exergetic coefficient of performances (COPs), and amounts of hydrogen gas pre-cooled and liquefied decrease with increase in the mass flow rate of geothermal water. Moreover, increasing the temperature of geothermal source degrades the performance of the quadruple effect absorption system (QEAS), but at the same time it affects the liquefaction production rate of hydrogen gas in a positive way. However, an increase in ambient temperature has a negative effect on the liquefaction rate of hydrogen gas produced as it decreases from 0.2 kg/s to 0.05 kg/s. Moreover, an increase in the concentration of the ammonia-water vapor results in an increase in the amount of hydrogen gas liquefied from 0.07 kg/s to 0.11 kg/s.