Component exergy analysis of solar powered transcritical CO<Subscript>2</Subscript> rankine cycle system

In this paper,exergy analysis method is developed to assess a Rankine cycle system,by using supercritical CO2 as working fluid and powered by solar energy.The proposed system consists of evacuated solar collectors,throttling valve,high-temperature heat exchanger,low-temperature heat exchanger,and feed pump.The system is designed for utilize evacuated solar collectors to convert solar energy into mechanical energy and hence electricity.In order to investigate and estimate exergy performance of this system,the energy,entropy,exergy balances are developed for the components.The exergy destructions and exergy efficiency values of the system components are also determined.The results indicate that solar collector and high temperature heat exchanger which have low exergy efficiencies contribute the largest share to system irreversibility and should be the optimization design focus to improve system exergy effectiveness.Further,exergy analysis is a useful tool in this regard as it permits the performance of each process to be assessed and losses to be quantified.Exergy analysis results can be used in design,optimization,and improvement efforts.     

Energy- and exergy-based comparison of coal-fired and nuclear steam power plants

The results are reported of energy- and exergy-based comparisons of coal-fired and nuclear electrical generating stations. A version of a process-simulation computer code, previously enhanced by the author for exergy analysis, is used. Overall energy and exergy efficiencies, respectively, are 37% and 36% for the coal-fired process, and 30% and 30% for the nuclear process. The losses in both plants exhibit many common characteristics. Energy losses associated with emissions (mainly with spent cooling water) account for all of the energy losses, while emission-related exergy losses account for approximately 10% of the exergy losses. The remaining exergy losses are associated with internal consumptions, mainly in components which generate heat by combustion or nuclear reactions, and in components which transfer heat across large temperature differences. It is anticipated that the results will prove useful to those involved in the improvement of existing and design of future electrical generating stations.     

Energy and exergy analyses of a production process for methanol from natural gas

Results are reported of energy and exergy analyses of the Imperial Chemical Industries low-pressure process for methanol from natural gas. The process involves generation of a synthesis gas by steam-methane reforming, compression of the synthesis gas, methanol synthesis, and distillation of the crude methanol. The analyses are carried out using a computer code capable of performing process-simulation and energy and exergy analyses. The energy and exergy efficiencies for the overall process are found to be 39 and 41%, respectively. The majority of energy losses is found to be associated with emissions of cooling water and stack gas. The majority of exergy losses is found to be due to internal consumptions, particularly within the combustion, compression and methanol synthesis systems. The energy losses associated with emissions of cooling water and stack gas, because of their low quality, are shown to be relatively insignificant on an exergy basis. The results may prove valuable to those involved in the design, optimization and modification of production plants for methanol and related fuels.     

Exergetic analysis of hydrogen utilisation pathways – Part 1: Energy supply in buildings

This article analyses exergy losses along hydrogen utilisation pathways recently discussed in Germany and other countries. As a renewable fuel hydrogen will be an important part of sustainable future economies. Hydrogen can be used in all sectors, especially in buildings, for mobility and in industry, e.g. in steel production or ammonia synthesis. However, hydrogen has to be produced in a sustainable way. The most promising production is via water electrolysis using renewable electricity. In the first part of this work, exergy analysis is made for the complete hydrogen pathways from production until final utilisation for energy supply in buildings. The second part will focus on pathways for mobility. In the third part, the results are compared with available alternatives to hydrogen such as direct use of electricity in building supply or mobility. The results for building energy supply show that firstly transportation in pipelines (mixture with natural gas and pure hydrogen) is very efficient. Secondly, major exergy losses are caused by the electrolyser. Thirdly, combustion of renewable hydrogen for room heating in common boilers cause the highest exergy losses, but the use of combined heat and power (CHP) units or fuel cells can improve the exergy efficiency substantially.     

Thermodynamic assessment of photovoltaic systems

In this paper, an attempt is made to investigate the performance characteristics of a photovoltaic (PV) and photovoltaic-thermal (PV/T) system based on energy and exergy efficiencies, respectively. The PV system converts solar energy into DC electrical energy where as, the PV/T system also utilizes the thermal energy of the solar radiation along with electrical energy generation. Exergy efficiency for PV and PV/T systems is developed that is useful in studying the PV and PV/T performance and possible improvements. Exergy analysis is applied to a PV system and its components, in order to evaluate the exergy flow, losses and various efficiencies namely energy, exergy and power conversion efficiency. Energy efficiency of the system is calculated based on the first law of thermodynamics and the exergy efficiency, which incorporates the second law of thermodynamics and solar irradiation exergy values, is also calculated and found that the latter is lower for the electricity generation using the considered PV system. The values of “fill factor” are also determined for the system and the effect of the fill factor on the efficiencies is also evaluated. The experimental data for a typical day of March (27th March 2006) for New Delhi are used for the calculation of the energy and exergy efficiencies of the PV and PV/T systems. It is found that the energy efficiency varies from a minimum of 33% to a maximum of 45% respectively, the corresponding exergy efficiency (PV/T) varies from a minimum of 11.3% to a maximum of 16% and exergy efficiency (PV) varies from a minimum of 7.8% to a maximum of 13.8%, respectively.     

Thermodynamic investigation and comparison of selected production processes for hydrogen and hydrogen-derived fuels

The results are reported of comparisons based on energy and exergy analyses of a wide range of production processes for hydrogen and hydrogen-derived fuels (HDFs). A commercial process-simulation computer code, previously enhanced by the author for exergy analysis, is used in the analyses. Depending on the process and the efficiency definition used, overall efficiencies are determined to range widely, from 21 to 92% for energy efficiencies and from 19 to 83% for exergy efficiencies. The losses for all processes are found to exhibit many common factors. Energy losses associated with emissions account for 100% of the total energy losses, while exergy losses associated with emissions account for 4 to 11% of the total exergy losses. The remaining exergy losses are associated with internal consumptions. It is anticipated that the results will prove useful to those involved in the improvement of existing and design of future production processes for hydrogen and HDFs.     

Assessment of the energy and exergy utilization efficiencies in the Turkish agricultural sector

This study deals with evaluating the energy and exergy utilization efficiencies in the Turkish agricultural sector over a 12‐year period from 1990 to 2001. In the energy and exergy analyses, two main energy sources, namely fuels and electricity, are taken into consideration, while the sectoral energy and exergy efficiencies are compared for this period. These main energy sources include diesel for tractors and other vehicles, and electricity for pumps. Overall energy utilization efficiencies are obtained to vary between 29.1 and 41.1%, while overall exergy utilization efficiencies are found to range from 27.9 to 37.4% in the analysed years, respectively. It may be concluded that the present technique proposed here may be used as a useful tool in analysing and evaluating the energy and exergy utilization efficiencies, identifying energy efficiency and/or energy conservation opportunities and dictating the energy strategies of countries. Copyright © 2005 John Wiley & Sons, Ltd.     

Exergy analysis of the solar cylindrical-parabolic cooker

For the first time the simple solar parabolic cooker (SPC), of the cylindrical trough shape, is analysed from the exergy viewpoint. The paper presents the methodology of detailed exergy analysis of the SPC, the distribution of the exergy losses, and, on the example of the cooker surfaces, explains the general problem of how the exergy loss on any radiating surface, should be determined, if the surface absorbs many radiation fluxes of different temperatures. An imagined surface was used in the considerations to close the system of the cooker surfaces. It was shown that optimization is needed, to increase the energy and exergy efficiencies of the cooker.Equations for heat transfer between the three surfaces: cooking pot, reflector and imagined surface making up the system, were derived. The model allowed for theoretical estimation of the energy and exergy losses: unabsorbed insolation, convective and radiative heat transfer to the ambient, and additionally, for the exergy losses: the radiative irreversibilities on the surfaces, and the irreversibility of the useful heat transferred to the water.The exergy efficiency of the SPC, was found to be relatively very low (1%), and to be about 10 times smaller than the respective energy efficiency which is in agreement with experimental data from the literature. The influence of the input parameters (geometrical configuration, emissivities of the surfaces, heat transfer coefficients and temperatures of water and ambience) was determined on the output parameters, the distribution of the energy and exergy losses and the respective efficiencies.     

On energetic,exergetic and environmental aspects of drying systems

In this study, a comprehensive discussion of energetic, exergetic and environmental aspects of drying systems is presented. Some theoretical and practical limitations on increased energy and exergy efficiencies and discussions of the relations between energy and exergy, and the environment, along with two illustrative examples are presented. A number of issues relating to energy, exergy and the environment are examined from the drying industry perspectives. It is pointed out that exergy is a suitable technique for furthering the goal of more efficient energy‐resource use and it is a key tool to determine the locations, types, and true magnitudes of wastes and losses in the drying systems. It is believed that this paper will provide some guidance to drying industry people in attaining optimum system design and operation. Copyright © 2002 John Wiley & Sons, Ltd.     

A review and assessment of the energy utilization efficiency in the Turkish industrial sector using energy and exergy analysis method

Exergy has been seen a key component for a sustainable society, and in the recent years exergy analysis has been widely used in the design, simulation and performance evaluation of thermal and thermo chemical systems. A particular thermo dynamical system is the society of a country, while the energy utilization of a country can be assessed using exergy analysis to gain insights into its efficiency and potential for improvements.Energy and exergy utilization efficiencies in the Turkish industrial sector (TIS) over the period from 1990 to 2003 are reviewed and evaluated in this study. Energy and exergy analyses are performed for eight industrial modes, namely iron–steel, chemical–petrochemical, petrochemical–feedstock, cement, fertilizer, sugar, non-metal industry, other industry, while in the analysis the actual data are used. Sectoral energy and exergy analyses are conducted to study the variations of energy and exergy efficiencies for each subsector throughout the years studied, and these heating and overall energy and exergy efficiencies are compared for the eight subsectors. The chemical and petrochemical subsector, and the iron and steel subsector appear to be the most energy and exergy efficient sectors, respectively. The energy utilization efficiencies for the Turkish overall industrial sector range from 63.45% to 70.11%, while the exergy utilization efficiencies vary from 29.72% to 33.23% in the analyzed years. Exergetic improvement potential for this sector is also determined to be 681 PJ in 2003, with an average increase rate of 9.5% annually for the analyzed years. It may be concluded that the methodology used in this study is practical and useful for analyzing sectoral and subsectoral energy and exergy utilization to determine how efficient energy and exergy are used in the sector studied. It is also expected that this study will be helpful in developing highly applicable and productive planning for energy policies.     

Assessment of the Turkish utility sector through energy and exergy analyses

The present study deals with evaluating the utility sector in terms of energetic and exergetic aspects. In this regard, energy and exergy utilization efficiencies in the Turkish utility sector over a wide range of period from 1990 to 2004 are assessed in this study. Energy and exergy analyses are performed for eight power plant modes, while they are based on the actual data over the period studied. Sectoral energy and exergy analyses are conducted to study the variations of energy and exergy efficiencies for each power plants throughout the years, and overall energy and exergy efficiencies are compared for these power plants. The energy utilization efficiencies for the overall Turkish utility sector range from 32.64% to 45.69%, while the exergy utilization efficiencies vary from 32.20% to 46.81% in the analyzed years. Exergetic improvement potential for this sector are also determined to be 332 PJ in 2004. It may be concluded that the methodology used in this study is practical and useful for analyzing sectoral and subsectoral energy and exergy utilization to determine how efficient energy and exergy are used in the sector studied. It is also expected that the results of this study will be helpful in developing highly applicable and productive planning for energy policies.     

An innovative approach for geothermal-wind hybrid comprehensive energy system and hydrogen production modeling/process analysis

In this paper, the energy, exergy, economic, environmental, steady-state, and process performance modeling/analysis of hybrid renewable energy (RE) based multigeneration system is presented. Beyond the design/performance analysis of an innovative hybrid RE system, this study is novel as it proposes a new methodology for determining the overall process energy and exergy efficiency of multigeneration systems. This novel method integrates EnergPLAN simulation program with EES and Matlab. It considers both the steady-state and the process performance of the modeled system on hourly timesteps in order to determine the overall efficiencies. Based on the proposed new method, it is observed that the overall process thermodynamic efficiencies of a hybrid renewable energy-based multigeneration system are different from its steady-state efficiencies. The overall energy and exergy efficiencies reduce from 81.01% and 52.52% (in steady-state condition) to 58.6% and 39.33% (when considering a one-year process performance). The integration of the hot water production with the multigeneration system enhanced the overall thermodynamic efficiencies in steady-state conditions. The Kalina system produces a total work output of 1171 kW with a thermal and exergy efficiency of 12.23% and 52% respectively while the wind turbine system produces 1297 kW of electricity in steady-state condition and it has the same thermal/exergy efficiency (72%). The economic analysis showed that the Levelized cost of electricity (LCOE) of the geothermal energy-based Kalina system is 0.0103 $/kWh. The greenhouse gas emission reduction analysis showed that the proposed system will save between 1,411,480 kg/yr and 3,518,760 kg/yr of greenhouse gases from being emitted into the atmosphere yearly. The multigeneration system designed in this study will produce electricity, hydrogen, hot water, cooling effect, and freshwater. Also, battery electric vehicle charging is integrated with process performance analysis of the multigeneration system.     

Dynamic multi-objective optimization applied to a solar-geothermal multi-generation system for hydrogen production,desalination, and energy storage

The design of optimal energy systems is vital to achieving global environmental and economic targets. In the design of solar-geothermal multi-generation systems, most previous investigations have relied on the static multi-objective optimization approach (SMOA), which may leave considerable room for improvement under certain conditions. In this numerical study, the optimal condition at which to operate a solar-geothermal multi-generation system – which can simultaneously produce hydrogen, fresh water, electricity, and heat, along with storing energy ? is determined via a dynamic multi-objective optimization approach (DMOA). Optimization is performed using a combination of NSGA-II and TOPSIS, and the results are benchmarked against those of SMOA. The decision variables include the solar area, geothermal water extraction mass flow, and hydrogen storage pressure. The objective functions include the production of electricity, heat, hydrogen, and fresh water, along with the exergy and energy efficiencies and the payback period. It is found that when compared with SMOA, DMOA can significantly improve all the objective functions. The annual production of electricity, heat, hydrogen, and fresh water increases by 14.4, 16.1, 13.5, and 14.3%, respectively, while the average annual exergy and energy efficiencies increase by 5.2 and 3.0%, respectively. The use of DMOA also reduces the payback period from 5.56 to 4.43 years, with a 4.4% reduction in hydrogen storage pressure. This shows that compared with a static approach such as SMOA, DMOA can improve the exergy and energy efficiencies, economic viability, and safety of a solar-geothermal multi-generation system.     

The role of equilibrium thermodynamics in process synthesis

The role of classical thermodynamics in generating design options in a systematic way is emphasised. Two thermodynamic approaches to solving this problem, the pinch and exergy methods, are compared. It is demonstrated that both approaches are based on analysis of exergy losses within a designed system. The difference is that in pinch analysis knowledge of exergy losses precedes design, while the latter allows us to establish energy targets prior to design. In conventional exergy analysis, losses may be evaluated only after the configuration of the process has been chosen. As a result, the conventional exergy approach remains trial-and-error. In order to overcome this limitation, this paper proposes a new exergy-based approach to generating design alternatives. A special graphic presentation of exergy balance around the ideal processes, where exergy losses are nil, allows us to generate alternatives with predetermined targets in a systematic way. In the next design step, real processes with the same structure as the selected ideal ones are identified. The final solution is carried out through optimization of a superstructure embedding the limited number of real process alternatives.     

Design and analysis of an integrated concentrated solar and wind energy system with storage

This study analyzes a renewable energy‐driven innovative multigeneration system, in which wind and solar energy sources are utilized in an efficient way to generate several useful commodities such as hydrogen, oxygen, desalted water, space cooling, and space heating along with electricity. A 1‐km² heliostat field is considered to concentrate the solar light onto a spectrum splitter, where the light spectrum is separated into two portions as reflected and transmitted to be used as the energy source in the concentrated solar power (CSP) and concentrated photovoltaics (CPV) receivers, respectively. As such, CSP and CPV systems are integrated. Wind energy is proposed for generating electricity (146 MW) or thermal energy (138 MW) to compensate the energy need of the multigeneration system when there is insufficient solar energy. In addition, multiple commodities, 46 MW of electricity, 12 m³/h of desalted water, and 69 MW of cooling, are generated using the Rankine cycle and the rejected heat from its condenser. Further, the heat generated on CPV cells is recovered for efficient photovoltaic conversion and utilized in the space heating (34 MW) and proton exchange membrane (PEM) electrolyzer (239 kg/h) for hydrogen production. The energy and exergy efficiencies of the overall system are calculated as 61.3% and 47.8%, respectively. The exergy destruction rates of the main components are presented to identify the potential improvements of the system. Finally, parametric studies are performed to analyze the effect of changing parameters on the exergy destruction rates, production rates, and efficiencies.     

Parametric Study of the Effect of Reference State on Energy and Exergy Efficiencies of Geothermal District Heating Systems (GDHSs): An Application of the Salihli GDHS in Turkey

A parametric study of the effect of reference state on the energy and exergy efficiencies of geothermal district heating systems is presented. In this regard, the work consists of two parts: a modeling study covering energy and exergy analysis and a case study covering the actual system data taken from the Salihli Geothermal District Heating System (SGDHS) in Manisa, Turkey. General energy and exergy analysis of the geothermal district heating systems is introduced along with some thermodynamic performance evaluation parameters. This analysis is then applied to the SGDHS using actual thermodynamic data for its performance evaluation in terms of energy and exergy efficiencies. In addition, a parametric study on the effect of varying dead state properties on the energy and exergy efficiencies of the system that has been conducted to find optimum performance and operating conditions is explained. Two parametric expressions of energy and exergy efficiencies were developed as a function of the reference temperature. Both energy and exergy flow diagrams illustrate and compare results under different conditions. It has been observed that the exergy destructions in the system particularly take place as the exergy of the fluid lost in the heat exchanger, the natural direct discharge of the system (pipeline losses), and the pumps, which account for 31.17%, 8.98%, and 4.27% of the total exergy input to the SGDHS, respectively. For the actual system that is presented, the system energy and exergy efficiencies vary between 0.53 and 0.73 and 0.58 and 0.59, respectively.     

Energy savings by co-production: A methanol/electricity case study

The overall exergy losses of co-production systems were decomposed into five sub-systems: chemical reaction processes, heat exchange processes, external exergy losses, turbine/mechanical exergy losses and others. By defining new parameters called energy-saving factors, we quantitatively describe the contribution of these processes to the overall energy savings relative to separate production systems. A methanol/electricity co-production system is taken as case study, results show that heat exchange processes are the main contribution to the energy savings.     

An application of energy and exergy analysis in agricultural sector of Malaysia

Thermodynamic losses usually take place in machineries used for agricultural activities. Therefore, it is important to identify and quantify the losses in order to devise strategies or policies to reduce them. An exergy analysis is a tool that can identify the losses occurred in any sector. In this study, an analysis has been carried out to estimate energy and exergy consumption of the agricultural sector in Malaysia. Energy and exergy efficiencies have been determined for the devices used in the agricultural sector of Malaysia, where petrol, diesel and fuel oil are used to run the machineries. Energy and exergy flow diagrams for the overall efficiencies of Malaysian agricultural sector are presented as well. The average overall energy and exergy efficiencies of this sector were found to be 22% and 20.728%, respectively, within the period from 1991 to 2009. These figures were found to be lower than those of Norway but higher than Turkey.     

Effect of reference state on exergy efficiencies of one- and two-stage crude oil distillation plants

In this paper we deal with the effects of varying reference temperature on the exergy efficiencies of one- and two-stage crude oil distillation units. Such units essentially consist of the crude oil heating furnace, the distillation column and a network of heat exchangers. Since the exergy efficiency is a key parameter to see how well the system is performing, we undertake a study on the influence of the reference temperature on such efficiencies. In this regard, a commercial software package, SimSci/PRO II program is used for the calculations. The results show that increasing reference temperature decreases the exergy efficiency in both one- and two-stage crude oil distillation systems and also increases the difference between the exergy efficiencies of both systems. Therefore, the exergy losses increase in the crude oil distillation systems.     

Entropy production and exergy destruction: Part I—hierarchy of Earth''s major constituencies

Earth does not consume energy. Neither do we nor does our energy system. Rather, we consume exergy. So using an exergy optic should be preferred when we plan future hydrogen-electricity systems. In this article, we provide a perspective on the exergy optic from the viewpoint of Earth's major energy-transaction constituencies. We first consider the Earth as a whole and then consider the following constituencies: the biosphere, people and civilization's energy system. Our objectives are to understand better the nature and sources of thermodynamic losses and to provide a background against which future hydrogen opportunities may be evaluated. As a codicil, we hope our results can improve our understanding of how people, civilizations and nature operate, and are connected. In a companion article, representative components within the energy system hierarchy (e.g., Niagara Falls, home heating systems and fuel cells) are considered.