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.     

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.     

Effect of irreversibilities on performance of an absorption heat transformer used to increase solar pond’s temperature

Absorption thermal systems are attractive for using waste heat energy from industrial processes and renewable energy such as geothermal energy, solar energy, etc. The Absorption Heat Transformer (AHT) is a promising system for recovering low-level waste heat. The thermal processes in the absorption system release a large amount of heat to the environment. This heat is evolved considerably at temperature, the ambient temperature results in a major irreversible loss in the absorption system components. Exergy analysis emphasises that both losses and irreversibility have an impact on system performance. Therefore, evaluating of the AHT in exergy basis is a much more suitable approach. In this study, a mathematical model of AHTs operating with the aqua/ammonia was developed to simulate the performance of these systems coupled to a solar pond in order to increase the temperature of the useful heat produced by solar ponds. A heat source at temperatures not higher than 100 °C was used to simulate the heat input to an AHT from a solar pond. In this paper, exergy analysis of the AHT were performed and effects of exergy losses of the system components on performance of the AHT used to increase solar pond’s temperature were investigated. The maximum upgrading of solar pond’s temperature by the AHT, is obtained at 51.5 °C and gross temperature lift at 93.5 °C with coefficients of performance of about 0.4. The maximum temperature of the useful heat produced by the AHT was ˜150 °C. As a result, determining of exergy losses for the system components show that the absorber and the generator need to be improved thermally. If the exergy losses are reduced, use of the AHT to increase the temperature of the heat used from solar ponds will be more feasable.     

Energy and exergy analysis of an indirect solar cabinet dryer based on mathematical modeling results

In the present study, using a previously developed dynamic mathematical model for performance analysis of an indirect cabinet solar dryer [1], a microscopic energy and exergy analysis for an indirect solar cabinet dryer is carried out. To this end, appropriate energy and exergy models are developed and using the predicted values for temperature and enthalpy of gas stream and the temperature, enthalpy and moisture content of the drying solid, the energy and exergy efficiencies are estimated. The validity of the model for predicting variations in gas and solid characteristics along the time and the length of the solar collector and/or dryer length was examined against some existing experimental data. The results show that in spite of high energy efficiency, the indirect solar cabinet dryer has relatively low exergy efficiency. Results show that the maximum exergy losses are in midday. Also the minimums of total exergy efficiency are 32.3% and 47.2% on the first and second days, respectively. Furthermore, the effect of some operating parameters, including length of the collector, its surface, and air flow rate was investigated on the exergy destruction and efficiency.     

Thermodynamic performance comparison of SOFC-MGT-CCHP systems coupled with two different solar methane steam reforming processes

Multi-energy complementary distributed energy system integrated with renewable energy is at the forefront of energy sustainable development and is an important way to achieve energy conservation and emission reduction. A comparative analysis of solid oxide fuel cell (SOFC)-micro gas turbine (MGT)-combined cooling, heating and power (CCHP) systems coupled with two solar methane steam reforming processes is presented in terms of energy, exergy, environmental and economic performances in this paper. The first is to couple with the traditional solar methane steam reforming process. Then the produced hydrogen-rich syngas is directly sent into the SOFC anode to produce electricity. The second is to couple with the medium-temperature solar methane membrane separation and reforming process. The produced pure hydrogen enters the SOFC anode to generate electricity, and the remaining small amount of fuel gas enters the afterburner to increase the exhaust gas enthalpy. Both systems transfer the low-grade solar energy to high-grade hydrogen, and then orderly release energy in the systems. The research results show that the solar thermochemical efficiency, energy efficiency and exergy efficiency of the second system reach 52.20%, 77.97% and 57.29%, respectively, 19.05%, 7.51% and 3.63% higher than those of the first system, respectively. Exergy analysis results indicate that both the solar heat collection process and the SOFC electrochemical process have larger exergy destruction. The levelized cost of products of the first system is about 0.0735$/h that is lower than that of the second system. And these two new systems have less environmental impact, with specific CO₂ emissions of 236.98 g/kWh and 249.89 g/kWh, respectively.     

Analysis of terrestrial solar radiation exergy

Based on Candau’s definition of radiative exergy, the exergy of the extraterrestrial and the terrestrial solar radiation are computed and compared by using the solar spectral radiation databank developed by Gueymard. The results show that within the spectrum region from 0.28 to 4.0 μm, the total energy quality factor (i.e., the exergy-to-energy ratio) of extraterrestrial solar radiation is about 0.9292, and that of the global terrestrial solar radiation is about 0.9171 under US standard atmosphere condition and zero solar zenith angle. The terrestrial solar spectral radiation exergy flux is large in the near ultraviolet and the visible light region. The reference radiation exergy spectra are obtained under atmospheric conditions consistent with ASTM standard G173-03. The effect of tilt angle on the terrestrial solar radiative exergy for inclined surface, and the effect of air mass on total energy quality factor of the terrestrial solar radiation for horizontal surface are analyzed. With the increase of tilt angle, the terrestrial solar spectral radiation exergy flux initially increases and then decreases, the total energy quality factor of the diffuse part decreases monotonically, while that of the direct part is invariant. The total energy quality factor of the direct, the diffuse and the global terrestrial solar radiation all decrease with the increase of air mass.     

Parametric analysis of a solar energy based multigeneration plant with SOFC for hydrogen generation

Renewable energy-based hydrogen production plants can offer potential solutions to both ensuring sustainability in energy generation systems and designing environmentally friendly systems. In this combined work, a novel solar energy supported plant is proposed that can generate hydrogen, electricity, heating, cooling and hot water. With the suggested integrated plant, the potential of solar energy usage is increased for energy generation systems. The modeled integrated system generally consists of the solar power cycle, solid oxide fuel cell plant, gas turbine process, supercritical power plant, organic Rankine cycle, cooling cycle, hydrogen production and liquefaction plant, and hot water production sub-system. To conduct a comprehensive thermodynamic performance analysis of the suggested plant, the combined plant is modeled according to thermodynamic equilibrium equations. A performance assessment is also conducted to evaluate the impact of several plant indicators on performance characteristics of integrated system and its sub-parts. Hydrogen production rate in the suggested plant according to the performance analysis performed is realized as 0.0642 kg/s. While maximum exergy destruction rate is seen in the solar power plant with 8279 kW, the cooling plant has the lowest exergy destruction rate as 1098 kW. Also, the highest power generation is obtained from gas turbine cycle with 7053 kW. In addition, energetic and exergetic efficiencies of solar power based combined cycle are found as 56.48% and 54.06%, respectively.     

SECOND LAW ANALYSIS OF A SOLAR THERMAL POWER SYSTEM

This communication presents second law analysis based on exergy concept for a solar thermal power system. Basic energy and exergy analysis for the system components (viz. parabolic trough collector/receiver and Rankine heat engine etc.) are carried out for evaluating the energy and exergy losses as well as exergetic efficiency for typical solar thermal power system under given operating conditions. Relevant energy flow and exergy flow diagrams are drawn to show the various thermodynamic and thermal losses. It is found that the main energy loss takes place at the condenser of the heat engine part whereas the exergy analysis shows that the collector-receiver assembly is the part where the losses are maximum. The analysis and results can be used for evaluating the component irreversibilities which can also explain the deviation between the actual efficiency and ideal efficiency of solar thermal power system.     

EXERGOECONOMIC EVALUATION OF A SOLAR THERMAL POWER PLANT

The second law of thermodynamics combined with economics represents a very powerful tool for the systematic study and optimization of energy systems. This combination forms the basis of the relatively new field of thermoeconomics (exergoeconomics). This approach allows the monetary evaluation of costs caused by irreversibilities (exergy losses) as well as comparisons between these costs and the investment and operating costs for each component of a power plant. A 35MW solar thermal power plant has been analyzed with the aid of the exergoeconomics. It is found that in the reheater, heat exchanger and pumps it will be profitable to reduce exergy losses even by increasing the capital costs. Whereas it will be profitable to reduce capital cost in feed water heaters (no. 1, 2 and 3) by compromising with exergy losses. Most crucial component is the solar collector field where exergy losses are severe and it requires efficient collector design. Unfortunately not much could be done at the moment for the collectors. Its fate will depend on the advancements in collector technology.

The analysis permits identification and evaluation of inefficiencies in the plant and opportunities for improvement in the plant components.     

Estimation of monthly solar radiation distribution for solar energy system analysis

The concept of probability density frequency, which is successfully used for analyses of wind speed and outdoor temperature distributions, is now modified and proposed for estimating solar radiation distributions for design and analysis of solar energy systems. In this study, global solar radiation distribution is comprehensively analyzed for photovoltaic (PV) panel and thermal collector systems. In this regard, a case study is conducted with actual global solar irradiation data of the last 15 years recorded by the Turkish State Meteorological Service. It is found that intensity of global solar irradiance greatly affects energy and exergy efficiencies and hence the performance of collectors.     

A comprehensive comparative energy and exergy analysis in solar based hydrogen production systems

In the presented paper, energy and exergy analysis is performed for thermochemical hydrogen (H₂) production facility based on solar power. Thermal power used in thermochemical cycles and electricity production is obtained from concentrated solar power systems. In order to investigate the effect of thermochemical cycles on hydrogen production, three different cycles which are low temperature Mg–Cl, H₂SO₄ and UT-3 cycles are compared. Reheat-regenerative Rankine and recompression S–CO₂ Brayton power cycles are considered to supply electricity needed in the Mg–Cl and H₂SO₄ thermochemical cycles. Furthermore, the effects of instant solar radiation and concentration ratio on the system performance are investigated. The integration of S–CO₂ Brayton power cycle instead of reheat-regenerative Rankine enhances the system performance. The maximum exergy efficiency which is obtained in the system with Mg–Cl thermochemical and recompression S–CO₂ Brayton power cycles is 27%. Although the energy and exergy efficiencies decrease with the increase of the solar radiation, they increase with the increase of the concentration ratio. The highest exergy destruction occurred in the solar energy unit.     

Residential solar air conditioning: Energy and exergy analyses of an ammonia–water absorption cooling system

Large scale heat-driven absorption cooling systems are available in the marketplace for industrial applications but the concept of a solar driven absorption chiller for air-conditioning applications is relatively new. Absorption chillers have a lower efficiency than compression refrigeration systems, when used for small scale applications and this restrains the absorption cooling system from air conditioning applications in residential buildings. The potential of a solar driven ammonia–water absorption chiller for residential air conditioning application is discussed and analyzed in this paper. A thermodynamic model has been developed based on a 10 kW air cooled ammonia–water absorption chiller driven by solar thermal energy. Both energy and exergy analyses have been conducted to evaluate the performance of this residential scale cooling system. The analyses uncovered that the absorber is where the most exergy loss occurs (63%) followed by the generator (13%) and the condenser (11%). Furthermore, the exergy loss of the condenser and absorber greatly increase with temperature, the generator less so, and the exergy loss in the evaporator is the least sensitive to increasing temperature.     

Performance assessment of a solar hydrogen and electricity production plant using high temperature PEM electrolyzer and energy storage

This paper investigates the performance of a high temperature Polymer Electrolyte Membrane (PEM) electrolyzer integrated with concentrating solar power (CSP) plant and thermal energy storage (TES) to produce hydrogen and electricity, concurrently. A finite-time-thermodynamic analysis is conducted to evaluate the performance of a PEM system integrated with a Rankine cycle based on the concept of exergy. The effects of solar intensity, electrolyzer current density and working temperature on the performance of the overall system are identified. A TES subsystem is utilized to facilitate continuous generation of hydrogen and electricity. The hydrogen and electricity generation efficiency and the exergy efficiency of the integrated system are 20.1% and 41.25%, respectively. When TES system supplies the required energy, the overall energy and exergy efficiencies decrease to 23.1% and 45%, respectively. The integration of PEM electrolyzer enhances the exergy efficiency of the Rankine cycle, considerably. However, it causes almost 5% exergy destruction in the integrated system due to conversion of electrical energy to hydrogen energy. Also, it is concluded that increase of working pressure and membrane thickness leads to higher cell voltage and lower electrolyzer efficiency. The results indicate that the integrated system is a promising technology to enhance the performance of concentrating solar power plants.     

Integration of wind turbine with heliostat based CSP/CPVT system for hydrogen production and polygeneration: A thermodynamic comparison

In this study, two wind-solar-based polygeneration systems namely CES-1 and CES-2 are developed, modeled, and analyzed thermodynamically. CES-1 hybridizes a heliostat based CSP system with wind turbines while CES-2 integrates heliostat-based CPVT with wind turbines. This study aims to compare the production and thermodynamics performance of two heliostat based concentrated solar power technologies when hybridized with wind turbines. The systems have been modeled to produce, freshwater, hot water, electricity, hydrogen, and cooling with different cycles/subsystems. While the overall objective of the study is to model two polygeneration systems with improved energy and exergy performances, the performances of two solar technologies are compared. The wind turbine system integrated with the comprehensive energy systems will produce 1.14 MW of electricity and it has 72.2% energy and exergy efficiency. Also, based on the same solar energy input, the performance of the heliostat integrated CPVT system (CES-2) is found to be better than that of the CSP based system (CES-1). The polygeneration thermal and exergy efficiencies for the two systems respectively are 48.08% and 31.67% for CES-1; 59.7% and 43.91% for CES-2. Also, the electric power produced by CES-2 is 280 kW higher in comparison to CES-1.     

Performance analysis of humid air turbine cycle with solar energy for methanol decomposition

According to the physical and chemical energy cascade utilization and concept of synthesis integration of variety cycle systems, a new humid air turbine (HAT) cycle with solar energy for methanol decomposition has been proposed in this paper. The solar energy is utilized for methanol decomposing as a heat source in the HAT cycle. The low energy level of solar energy is supposed to convert the high energy level of chemical energy through methanol absorption, realizing the combination of clean energy and normal chemical fuels as compared to the normal chemical recuperative cycle. As a result, the performance of normal chemical fuel thermal cycle can be improved to some extent. Though the energy level of decomposed syngas from methanol is decreased, the cascade utilization of methanol is upgraded. The energy level and exergy losses in the system are graphically displayed with the energy utilization diagrams (EUD). The results show that the cycle’s exergy efficiency is higher than that of the conventional HAT cycle by at least 5 percentage points under the same operating conditions. In addition, the cycle’s thermal efficiency, exergy efficiency and solar thermal efficiency respond to an optimal methanol conversion.     

Exergy analysis of a passive solar still

This paper presents a steady-state and transient theoretical exergy analysis of a solar still, focused on the exergy destruction in the components of the still: collector plate, brine and glass cover. The analytical approach states an energy balance for each component resulting in three coupled equations where three parameters—solar irradiance, ambient temperature and insulation thickness—are studied. The energy balances are solved to find temperatures of each component; these temperatures are used to compute energy and exergy flows. Results in the steady-state regime show that the irreversibilities produced in the collector account for the largest exergy destruction, up to 615 W/m² for a 935 W/m² solar exergy input, whereas irreversibility rates in the brine and in the glass cover can be neglected. For the same exergy input a collector, brine and solar still exergy efficiency of 12.9%, 6% and 5% are obtained, respectively. The most influential parameter is solar irradiance. During the transient regime, irreversibility rates and still temperatures find a maximum 6 h after dawn when solar irradiance has a maximum value. However, maximum exergy brine efficiency, close to 93%, is found once T_col<T_w (dusk) and the heat capacity of the brine plays an important role in the modeling of collector–brine interaction. Nocturnal distillation is characterized by very low irreversibility rates due to reduced temperature difference between collector and an increase in exergy efficiency towards dawn due to ambient temperature decrease.     

Comparative performance analysis of solar powered supercritical-transcritical CO2 based systems for hydrogen production and multigeneration

CO₂ based power and refrigeration cycles have been developed and analyzed in different existing studies. However, the development of a CO₂ based comprehensive energy system and its performance analysis have not been considered. In this study, the integration of a CO₂ based solar parabolic trough collector system, a supercritical CO₂ power cycle, a transcritical CO₂ power cycle, and a CO₂ based cascade refrigeration system for hydrogen production and multigeneration purpose is analyzed thermodynamically. This study aims to analyze and compare the difference in the thermodynamic performance of comprehensive energy systems when CO₂ is used as the working fluid in all the cycles with a system that uses other working fluids. Therefore, two comprehensive energy systems with the same number of subsystems are designed to justify the comparison. The second comprehensive energy system uses liquid potassium instead of CO₂ as a working fluid in the solar parabolic trough collector and a steam cycle is used to replace the transcritical CO₂ power cycle. Results of the energy and exergy performance analysis of two comprehensive energy systems showed that the two systems can be used for the multigeneration purpose. However, the use of a steam cycle and potassium-based solar parabolic trough collector increases the comprehensive energy systems’ overall energy and exergy efficiency by 41.9% and 26.7% respectively. Also, the use of liquid potassium as working fluid in the parabolic trough collectors increases the absorbed solar energy input by 419 kW and 2100 kW thereby resulting in a 23% and 90.7% increase in energetic and exergetic efficiency respectively. The carbon emission reduction potential of the two comprehensive energy systems modelled in this study is also analyzed.     

Energy and exergy performance assessment of a novel solar-based integrated system with hydrogen production

In this study, a new solar power assisted multigeneration system designed and thermodynamically analyzed. In this system, it is designed to perform heating, cooling, drying, hydrogen and power generation with a single energy input. The proposed study consists of seven sub-parts which are namely parabolic dish solar collector, Rankine cycle, organic Rankine cycle, PEM-electrolyzer, double effect absorption cooling, dryer and heat pump. The effects of varying reference temperature, solar irradiation, input and output pressure of high-pressure turbine and pinch point temperature heat recovery steam generator are investigated on the energetic and exergetic performance of integration system. Thermodynamic analysis result outputs show that the energy and exergy performance of overall study are computed as 48.19% and 43.57%, respectively. Moreover, the highest rate of irreversibility has the parabolic dish collector with 24,750 kW, while the lowest rate of irreversibility is calculated as 5745 kW in dryer. In addition, the main contribution of this study is that the solar-assisted multi-generation systems have good potential in terms of energy and exergy efficiency.     

Thermodynamic study of a simplified model of the solar chimney power plant

A simplified model of solar chimney power plant (SCPP) consists of a heating air collector, turbine and chimney. Thermodynamic interpretation of processes occurring in these SCPP components is based on the derived energy and exergy balances. The examples of the energy and exergy flow diagrams show how the SCPP input of 36.81 MW energy of solar radiation, corresponding to 32.41 MW input of radiation exergy, is distributed between the SCPP components. Responsive trends to the varying input parameters are studied. Additionally, the concept of mechanical exergy (ezergy) of air is applied and it allowed for quantitative determination of the effect attributed to the terrestrial gravity field on the component processes of the SCPP.     

Simulated energy and exergy analyses of the charging of an oil-pebble bed thermal energy storage system for a solar cooker

Energy balance equations are used to model the solar energy capture (SEC) system and the thermal energy storage (TES) system of a proposed indirect solar cooker. An oil-pebble bed is used as the TES material. Energy and exergy analyses are carried out using two different charging methods to predict the performance of the TES system. The first method charges the TES system at a constant flowrate. In the second method, the flowrate is made variable to maintain a constant charging temperature. A Simulink block model is developed to solve the energy balance equations and to perform energy and exergy analyses. Simulation results using the two methods indicate a greater degree of thermal stratification and energy stored when using constant-temperature charging than when using constant-flowrate charging. There are greater initial energy and exergy rates for the constant-flowrate method when the solar radiation is low. Energy efficiencies using both methods are comparable whilst the constant-temperature method results in greater exergy efficiency at higher levels of the solar radiation. Parametric results showing the effect of each charging method on the energy and exergy efficiencies are also presented.