Exergetic utilization of solar energy for feed water preheating in a conventional thermal power plant

This communication is based on exergy concept for the utilization of solar thermal energy in a Rankine cycle‐based fuel‐fired thermal power plant (FFTPP). It has been shown that solar thermal energy as an aided source for feed water preheating helps to reduce the exergy loss in feed water heater (FWH) of Rankine cycle and develops more work than that could have been produced in a solar thermal power plant (STPP). It has been found that this enhancement in work increases for low‐pressure FWHs. For further illustration, a case study has been carried out of a typical 50 kW STPP and a 220 MW FFTPP. The effect of utilizing the same input solar thermal energy of typical STPP, if used as an aided source in a 220 MW FFTPP for feed water preheating is investigated. The work output of STPP is 59.312 kW, while the extra work output of FFTPP by using solar thermal energy of STPP is 90.27 kW. It has been found that the efficiency of work conversion of aided solar thermal energy in FFTPP is higher than the efficiency of work conversion in STPP. Copyright © 2009 John Wiley & Sons, Ltd.     

Exergy analysis and investigation for various feed water heaters of direct steam generation solar–thermal power plant

The energy and exergy analysis has been carried out for the different components of a proposed conceptual direct steam generation (DSG) solar–thermal power plant (STPP). It has been found that the maximum energy loss is in the condenser followed by solar collector field. The maximum exergy loss is in the solar collector field while in other plant components it is small. The possibilities to further improve the plant efficiency are identified and exploited. For minimum exergy loss in receiver the inlet temperature of water to the receiver, which is governed by the number of feed water heaters (FWHs), bleed pressure and mass fraction of bleed steam, must be optimum. The only one FWH has been proposed in conceptual DSG STPP. In order to evaluate the optimum bleed pressure and mass fraction of bleed steam to maximize the STPP efficiency, the investigations are carried out for various bleed pressure and mass fractions of bleed steam of proposed conceptual DSG STPP having one FWH. The investigations for bleed pressure and mass fraction of bleed steam are also carried out by incorporating two and three FWHs. It has been found that there will be significant improvement in efficiency by using three FWHs and further gain in efficiency is possible by making provision for more FWHs.     

Solar multiple optimization for a solar-only thermal power plant, using oil as heat transfer fluid in the parabolic trough collectors

Usual size of parabolic trough solar thermal plants being built at present is approximately 50 MW_e. Most of these plants do not have a thermal storage system for maintaining the power block performance at nominal conditions during long non-insolation periods. Because of that, a proper solar field size, with respect to the electric nominal power, is a fundamental choice. A too large field will be partially useless under high solar irradiance values whereas a small field will mainly make the power block to work at part-load conditions.This paper presents an economic optimization of the solar multiple for a solar-only parabolic trough plant, using neither hybridization nor thermal storage. Five parabolic trough plants have been considered, with the same parameters in the power block but different solar field sizes. Thermal performance for each solar power plant has been featured, both at nominal and part-load conditions. This characterization has been applied to perform a simulation in order to calculate the annual electricity produced by each of these plants. Once annual electric energy generation is known, levelized cost of energy (LCOE) for each plant is calculated, yielding a minimum LCOE value for a certain solar multiple value within the range considered.     

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.     

Exergetic analysis and economic evaluation of central tower receiver solar thermal power plant

This article presents the analytical evaluation of a central tower receiver solar thermal power plant with air‐cooled volumetric receiver using exergy analysis. The energetic and exergetic losses as well as the efficiencies of a typical central tower receiver‐based solar thermal power plant have been carried out under the specific operating conditions. The enhancement in efficiency of the plant from the variation in power input to constant power input achieved by thermal storage backup condition has been determined. It is found that the year round average energetic efficiency can be increased from 24.15% to 25.08% and year round average exergetic efficiency can be increased, from 26.10% to 27.10% for the selected location Jodhpur. The unit cost of electric energy generation (kWh_e) is found to be INR 10.09 considering 30‐year lifespan of the solar plant along with a 10% interest rate. The present study provides a base for the development of future solar thermal power plants in India. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

Energy and exergy analyses of a solar driven Mg–Cl hybrid thermochemical cycle for co-production of power and hydrogen

Analysis and performance assessment of a solar driven hydrogen production plant running on an Mg–Cl cycle, are conducted through energy and exergy methods. The proposed system consists of (a) a concentrating solar power cycle with thermal energy storage, (b) a steam power plant with reheating and regeneration, and (c) a hybrid thermochemical Mg–Cl hydrogen production cycle. The results show that higher steam to magnesium molar ratios are required for full yield of reactants at the hydrolysis step. This ratio even increases at low temperatures, although lowering the highest temperatures appears to be more favorable for linking such a cycle to lower temperature energy sources. Reducing the maximum cycle temperature decreases the plant energy and exergy efficiencies and may cause some undesirable reactions and effects. The overall system energy and exergy efficiencies are found to be 18.8% and 19.9%, respectively, by considering a solar heat input. These efficiencies are improved to 26.9% and 40.7% when the heat absorbed by the molten salt is considered and used as a main energy input to the system. The highest exergy destruction rate occurs in the solar field which accounts for 79% of total exergy destruction of the integrated system.     

Analysis of an innovative direct steam generation‐based parabolic trough collector plant hybridized with a biomass boiler

Direct steam generation (DSG) is the process by which steam is directly produced in parabolic trough fields and supplied to a power block. This process simplifies parabolic trough plants and improves cost effectiveness by increasing the permissible temperature of the working fluid. Similar to all solar‐based technologies, thermal energy storage is needed to overcome the intermittent nature of solar. In the present work, an innovative DSG‐based parabolic trough collector (PTC) plant hybridized with a biomass boiler is proposed and analyzed in detail. Two additional configurations comprising indirect steam generation PTC plants were also analyzed to compare their energy and exergy performance. To consider a wide range of operation, the share of biomass input to the hybridized system is varied. Energy and exergy analyses of DSG are conducted and compared with an existing indirect steam generation PTC power plants such as Andasol. The analyses are conducted on a 50 MW regenerative reheat Rankine cycle. The results obtained indicate that the proposed DSG‐based PTC plant is able to increase the overall system efficiency by 3% in comparison with indirect steam generation when linked to a biomass boiler that supplies 50% of the energy.     

Thermal energy storage technologies and systems for concentrating solar power plants

This paper presents a review of thermal energy storage system design methodologies and the factors to be considered at different hierarchical levels for concentrating solar power (CSP) plants. Thermal energy storage forms a key component of a power plant for improvement of its dispatchability. Though there have been many reviews of storage media, there are not many that focus on storage system design along with its integration into the power plant. This paper discusses the thermal energy storage system designs presented in the literature along with thermal and exergy efficiency analyses of various thermal energy storage systems integrated into the power plant. Economic aspects of these systems and the relevant publications in literature are also summarized in this effort.     

Energy management in solar thermal power plants with double thermal storage system and subdivided solar field

In the paper, two systems for solar thermal power plants (STPPs) are devised for improving the overall performance of the plant. Each one attempts to reduce losses coming from two respective sources. The systems are simulated and compared to a reference STPP.     

Review on storage materials and thermal performance enhancement techniques for high temperature phase change thermal storage systems

Designing a cost-effective phase change thermal storage system involves two challenging aspects: one is to select a suitable storage material and the other is to increase the heat transfer between the storage material and the heat transfer fluid as the performance of the system is limited by the poor thermal conductivity of the latent heat storage material. When used for storing energy in concentrated solar thermal power plants, the solar field operation temperature will determine the PCM melting temperature selection. This paper reviews concentrated solar thermal power plants that are currently operating and under construction. It also reviews phase change materials with melting temperatures above 300 °C, which potentially can be used as energy storage media in these plants. In addition, various techniques employed to enhance the thermal performance of high temperature phase change thermal storage systems have been reviewed and discussed. This review aims to provide the necessary information for further research in the development of cost-effective high temperature phase change thermal storage systems.     

Exergy analysis of an integrated solar combined cycle system

Exergetic analysis has become an integral part of thermodynamic assessment of any power generation system. Energy and exergy studies for power plants optimum design and for combined chemical industries received much attention recently. An Integrated Solar Combined Cycle System (ISCCS) is proposed as a means of integrating a parabolic trough solar thermal plant with modern combined cycle power plants. In this study attempt will be made to analyze the Integrated Solar Combined Cycle in Yazd, Iran using design plant data. Energy and exergy analysis for the solar field and combined cycle is carried out to assess the plant performance and pinpoint sites of primary exergy destruction. Exergy destruction throughout the plant is quantified and illustrated using an exergy flow diagram, and compared to the energy flow diagram. The causes of exergy destruction in the plant include: losses in combustor, collector, heat exchangers, and pump & turbines which accounts for 29.62, 8.69, 9.11 and 8% of the total exergy input to the plant, respectively. Exergetic efficiencies of the major plant components are determined in an attempt to assess their individual performances.     

Effect of stratification on energy and exergy capacities in thermal storage systems

The increase in exergy storage capacity that is attained in thermal storages through stratification is assessed. A design‐oriented temperature‐distribution model for vertically stratified thermal storages that facilitates the evaluation of storage energy and exergy contents is utilized. The paper is directed towards demonstrating the thermodynamic benefits achieved through stratification, and increasing the utilization of exergy‐based performance measures for stratified thermal storages. A wide range of realistic storage‐fluid temperature profiles is considered, and for each the relative increase in exergy content of the stratified storage compared to the same storage when it is fully mixed is evaluated. The results indicate that, for all temperature profiles considered, the exergy storage capacity of a thermal storage increases when it is stratified, and increases as the degree of stratification, as represented through greater and sharper spatial temperature variations, increases. Furthermore, the percentage increase in exergy capacity is greatest for storages at temperatures near to the environment temperature, and decreases as the mean storage temperature diverges from the environment temperature (to either higher or lower temperatures). It is concluded that (i) the use of stratification in thermal storage designs should be considered as it increases the exergy storage capacity of a thermal storage and (ii) exergy analysis should be applied in the analysis and comparison of stratified thermal storage systems. Copyright © 2004 John Wiley & Sons, Ltd.     

Energy and exergy analyses of a solar based hydrogen production and compression system

In this study, a solar thermal based integrated system with a supercritical-CO₂ (sCO₂) gas turbine (GT) cycle, a four-step Mg–Cl cycle and a five-stage hydrogen compression plant is developed, proposed for applications and analyzed thermodynamically. The solar data for the considered solar plant are taken for Greater Toronto Area (GTA) by considering both daily and yearly data. A molten salt storage is considered for the system in order to work without interruption when the sun is out. The power and heat from the solar and sCO₂-GT subsystems are introduced to the Mg–Cl cycle to produce hydrogen at four consecutive steps. After the internal heat recovery is accomplished, the heating process at required temperature level is supplied by the heat exchanger of the solar plant. The hydrogen produced from the Mg–Cl cycle is compressed up to 700 bar by using a five-stage compression with intercooling and required compression power is compensated by the sCO₂-GT cycle. The total energy and exergy inputs to the integrated system are found to be 1535 MW and 1454 MW, respectively, for a 1 kmol/s hydrogen producing plant. Both energy and exergy efficiencies of the overall system are calculated as 16.31% and 17.6%, respectively. When the energy and exergy loads of the receiver are taken into account as the main inputs, energy and exergy efficiencies become 25.1%, and 39.8%, respectively. The total exergy destruction within the system is found to be 1265 MW where the solar field contains almost 64% of the total irreversibility with a value of ～811 MW.     

Numerical analysis of latent heat thermal energy storage using encapsulated phase change material for solar thermal power plant

Thermal energy storage improves the load stability and efficiency of solar thermal power plants by reducing fluctuations and intermittency inherent to solar radiation. This paper presents a numerical study on the transient response of packed bed latent heat thermal energy storage system in removing fluctuations in the heat transfer fluid (HTF) temperature during the charging and discharging period. The packed bed consisting of spherical shaped encapsulated phase change materials (PCMs) is integrated in an organic Rankine cycle-based solar thermal power plant for electricity generation. A comprehensive numerical model is developed using flow equations for HTF and two-temperature non-equilibrium energy equation for heat transfer, coupled with enthalpy method to account for phase change in PCM. Systematic parametric studies are performed to understand the effect of mass flow rate, inlet charging system, storage system dimension and encapsulation of the shell diameter on the dynamic behaviour of the storage system. The overall effectiveness and transient temperature difference in HTF temperature in a cycle are computed for different geometrical and operational parameters to evaluate the system performance. It is found that the ability of the latent heat thermal energy storage system to store and release energy is significantly improved by increasing mass flow rate and inlet charging temperature. The transient variation in the HTF temperature can be effectively reduced by decreasing porosity.     

Energy and exergy analysis at the utility and commercial sectors of Malaysia

In this paper, sectoral energy and exergy analysis model is applied to the utility and commercial sectors of Malaysia by considering the energy and exergy flows from 1990 to 2003. The energy and exergy efficiencies are determined for the sub-sectors and devices used in these two sectors. It has been found the hydroelectric power plant sub-sector is more energy and exergy efficient compared to the thermal power plant sub-sector. The energy and exergy efficiencies of utility and commercial sectors of Malaysia are compared with a few other countries around the world as well. The utility and commercial sectors of Malaysia are found to be more efficient than that of Thailand, Brunei, China, and Vietnam in 1999.     

Hybrid solar parabolic dish power plant and high‐temperature phase change material energy storage system

In this study, a conventional steam power plant with two regenerative boilers is considered, and one of its boilers is replaced with parabolic solar dish collectors and storing the produced thermal energy by the phase change material (PCM) in a storage tank. The results show the necessity of the existence of an auxiliary fired‐gas boiler to provide constant load during the whole 24 hours. The performance of the proposed hybridized system is evaluated through energy and exergy analyses. It was demonstrated that substituting solar collectors with one of the boilers marginally lowers the energy efficiency but increases the exergy efficiency of the whole power plant up to 41.76%. Moreover, it is found out that this hybridization decreases the total irreversibility of the power plant in comparison with the base case, from 51.1 to 47.2 MW. The parametric analysis states that raising the mass flow rate of the heat transfer fluid in the solar collectors not only enhances the system performance but also increases the volume of the PCM tank.     

Performance assessment of a direct steam solar power plant with hydrogen energy storage: An exergoeconomic study

Power generation and its storage using solar energy and hydrogen energy systems is a promising approach to overcome serious challenges associated with fossil fuel-based power plants. In this study, an exergoeconomic model is developed to analyze a direct steam solar tower-hydrogen gas turbine power plant under different operating conditions. An on-grid solar power plant integrated with a hydrogen storage system composed of an electrolyser, hydrogen gas turbine and fuel cell is considered. When solar energy is not available, electrical power is generated by the gas turbine and the fuel cell utilizing the hydrogen produced by the electrolyser. The effects of different working parameters on the cycle performance during charging and discharging processes are investigated using thermodynamic analysis. The results indicate that increasing the solar irradiation by 36%, leads to 13% increase in the exergy efficiency of the cycle. Moreover, the mass flow rate of the heat transfer fluid in solar system has a considerable effect on the exergy cost of output power. Solar tower has the highest exergy destruction and capital investment cost. The highest exergoeconomic factor for the integrated cycle is 60.94%. The steam turbine and PEM electrolyser have the highest share of exergoeconomic factor i.e., 80.4% and 50%, respectively.     

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.     

Control scheme for direct steam generation in parabolic troughs under recirculation operation mode

Electricity production using solar thermal energy is one of the main research areas at present in the field of renewable energies, these systems being characterised by the need of reliable control systems aimed at maintaining desired operating conditions in the face of changes in solar radiation, which is the main source of energy. A new prototype of solar system with parabolic trough collectors was implemented at the Plataforma Solar de Almería (PSA, South-East Spain) to investigate the direct steam generation process under real solar conditions in the parabolic solar collector field of a thermal power plant prototype. This paper presents details and some results of the application of a control scheme designed and tested for the recirculation operation mode, for which the main objective is to obtain steam at constant temperature and pressure at the outlet of the solar field, so that changes produced in the inlet water conditions and/or solar radiation will only affect the amount of steam produced by the solar field. The steam quality and consequently the nominal efficiency of the plant are thus maintained.