Experimental investigation of solar-assisted transcritical CO2 Rankine cycle for summer and winter conditions from exergetic point of view

This study deals with energy and exergy analysis of the experimental solar-assisted Rankine cycle working with an environmentally friendly working fluid transcritical CO₂. The experimental system consists of evacuated solar collectors, a heat recovery system, condenser, a pump, and an expansion valve to simulate the realistic turbine operation. The system was designed for electricity production and the heat supply for various applications. The experiments were made funder typical winter and summer days to evaluate seasonal system performance in Kyoto, Japan. According to the obtained results, the turbine capacity was calculated as 0.118 kW and 0.177 kW for winter and summer seasons. From the exergetic point of view, solar collectors were found to be the major contributor to the total exergy destruction with 96.32% for summer and 93.58% for the winter season. Therefore, the efforts should be focused on the collectors. Thus, any attempt for improving the system performance should be focused on solar collectors first. Furthermore, the exergetic efficiency of the overall system was calculated as 7.63% for the winter season and 4.08% for the summer season. As a result, the utilization of CO₂ in the energy conversion cycle can be sustainably developed and extended by providing a glimpse into the carbon-free clean energy future.     

Performance optimization and analysis of solar combi-system with carbon dioxide heat pump

In this paper, a solar combi-system which consists of solar collectors and a carbon dioxide heat pump is proposed and investigated through simulation and optimization. Performance analysis and comparison are primarily conducted to show the feasibility and reasonability of using a CO₂ heat pump as an auxiliary heater under local weather conditions. Then, a system model with a test building in TRNSYS is developed for performance optimization. The most influential variables are identified using influence and sensitivity analyzes of single parameters. Subsequently, a multi-parameter optimization using the high-weight parameters is carried out to obtain a final design result. The simulated results of the optimized case show that the average coefficient of performance of the CO₂ heat pump is 2.38, and the solar fraction of the system is 69.0% for the entire heating season. The time when a comfortable temperature level can be achieved in the indoor environment accounts for 81.6% of the entire heating season. Furthermore, the performance characteristics of the proposed system are evaluated in terms of the thermal balance, fraction of the thermal energy saving, feasibility of net zero energy, economic factor, and CO₂ emissions reduction.     

Assessment of electricity and hydrogen production performance of evacuated tube solar collectors

In this work, a unified renewable energy system has designed to assess the electricity and hydrogen production. This system consists of the evacuated tube solar collectors (ETSCs) which have the total surface area of 300 m², a salt gradient solar pond (SGSP) which has the surface area of 217 m², an Organic Rankine Cycle (ORC) and an electrolysis system. The stored heat in the heat storage zone (HSZ) transferred to the input water of the ETSCs by means of an exchanger and thereby ETSCs increase the temperature of preheated water to higher level as much as possible that primarily affects the performance of the ORC. The balance equations of the designed system were written and analyzed by utilizing the Engineering Equations Solver (EES) software. Hence, the energy and exergy efficiencies of the overall system were calculated as to be 5.92% and 18.21%, respectively. It was also found that hydrogen generation of the system can reach up to ratio 3204 g/day.     

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.     

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.     

The potential applications and advantages of powering solar air-conditioning systems using concentrator augmented solar collectors

Non-concentrated evacuated tube heat pipe solar collectors have been reported to show higher fluid temperatures with improved thermal performance in the low to medium temperature range (?60 °C) due to low heat losses but suffer higher heat losses at the medium to higher temperature range (?80 °C) which reduces their efficiency compared to concentrated evacuated tube heat pipe solar collectors. To operate as stand-alone systems capable of attaining temperatures in the range of 70-120 °C, an innovative concentrator augmented solar collector can be an attractive option. The performance of a combined low-concentrator augmented solar collector in an array of evacuated tube heat pipe solar collectors defined as concentrator augmented evacuated tube heat pipe array (CAETHPA) and an array of evacuated tube heat pipe collectors (ETHPC) were tested and compared and results presented in this paper. The analysis of the experimental data allows concluding that the use of a CAETHPA is a more efficient alternative for integrating renewable energy into buildings with higher fluid temperature response, energy collection and lower heat loss coefficient compared to the use of evacuated tube heat pipe collector array (ETHPA).     

Modeling and analysis of energetic and exergetic efficiencies of a LiBr/H<Subscript>2</Subscript>0 absorption heat storage system for solar space heating in buildings

The development of efficient long-term heat storage systems could significantly increase the use of solar thermal energy for building heating. Among the different heat storage technologies, the absorption heat storage system seems promising for this application. To analyze the potential of this technology, a numerical model based on mass, species, energy, and exergy balances has been developed. The evolution over time of the storage imposes a transient approach. Simulations were performed considering temperature conditions close to those of a storage system used for space heating coupled to solar thermal collectors (as the heat source), with ground source heat exchangers (as the cold source). The transient behavior of the system was analyzed in both the charging and discharging phases. This analysis highlights the lowering of energetic and exergetic performance during both phases, and these phenomena are discussed. The thermal efficiency and the energy storage density of the system were determined, equal to 48.4 % and 263 MJ/m³, respectively. The exergetic efficiency is equal to 15.0 %, and the exergy destruction rate is 85.8 %. The key elements in terms of exergy destruction are the solution storage tank, the generator, and the absorber. The impact of using a solution heat exchanger (SHX) was studied. The risk of the solution crystallizing in the SHX was taken into account. With a SHX, the thermal efficiency of the system can reach 75 %, its storage density was 331 MJ/m³, and its exergetic efficiency and exergy destruction rate was 23.2 and 77.3 %, respectively.     

Design of solar thermal systems utilizing pressurized hot water storage for industrial applications

A large number of industrial processes demand thermal energy in the temperature range of 80–240 °C. In this temperature range, solar thermal systems have a great scope of application. However, the challenge lies in the integration of a periodic, dilute and variable solar input into a wide variety of industrial processes. Issues in the integration are selection of collectors, working fluid and sizing of components. Application specific configurations are required to be adopted and designed. Analysis presented in this paper lays an emphasis on the component sizing. The same is done by developing a design procedure for a specific configuration. The specific configuration consists of concentrating collectors, pressurized hot water storage and a load heat exchanger. The design procedure follows a methodology called design space approach. In the design space approach a mathematical model is built for generation of the design space. In the generation of the design space, design variables of concern are collector area, storage volume, solar fraction, storage mass flow rate and heat exchanger size. Design space comprises of constant solar fraction curves traced on a collector area versus storage volume diagram. Results of the design variables study demonstrate that a higher maximum storage mass flow rates and a larger heat exchanger size are desired while limiting storage temperature should be as low as possible. An economic optimization is carried out to design the overall system. In economic optimization, total annualized cost of the overall system has been minimized. The proposed methodology is demonstrated through an illustrative example. It has been shown that 23% reduction in the total system cost may be achieved as compared to the existing design. The proposed design tool offers flexibility to the designer in choosing a system configuration on the basis of desired performance and economy.     

S-CO2布雷顿循环太阳能电力淡水系统火用分析

设计一种使用S-CO₂布雷顿循环的太阳能电力淡水系统,对系统的工作原理和结构组成进行介绍,并对系统开展运行性能和火用分析。结果表明,设计工况下系统的输出电功率为233.8 MW,布雷顿循环效率为37.5%,淡水日产量为3981.6 t。增大太阳辐照度有利于提高系统的电力输出和总的能量效率。定工况下的火用分析结果表明,太阳塔集热器中的火用损最大,为303.99 MW,对应的火用效率为64.45%。海水淡化换热器的火用效率最低,且其火用损值也较大。随着太阳辐照度的增加,太阳塔集热器、海水淡化系统换热器和回热器内的火用损均有不同幅度的增加。因此,对于该S-CO₂布雷顿循环太阳能电力淡水系统的后续优化而言,应重点考虑改进这些部件的性能。     

Modeling, simulation and optimization of a solar collector driven water heating and absorption cooling plant

A cogeneration system consisting of a solar collector, a gas burner, a thermal storage reservoir, a hot water heat exchanger, and an absorption refrigerator is devised to simultaneously produce heating (hot water heat exchanger) and cooling (absorption refrigerator system). A simplified mathematical model, which combines fundamental and empirical correlations, and principles of classical thermodynamics, mass and heat transfer, is developed. The proposed model is then utilized to simulate numerically the system transient and steady state response under different operating and design conditions. A system global optimization for maximum performance (or minimum exergy destruction) in the search for minimum pull-down and pull-up times, and maximum system second law efficiency is performed with low computational time. Appropriate dimensionless groups are identified and the results presented in normalized charts for general application. The numerical results show that the three way maximized system second law efficiency, η_{II,max,max,max}, occurs when three system characteristic mass flow rates are optimally selected in general terms as dimensionless heat capacity rates, i.e., (ψ_sp,s,ψ_wx,wx,ψ_H,s)_opt≅(1.43,0.23,0.14). The minimum pull-down and pull-up times, and maximum second law efficiencies found with respect to the optimized operating parameters are sharp and, therefore important to be considered in actual design. As a result, the model is expected to be a useful tool for simulation, design, and optimization of solar collector based energy systems.     

Exergetic optimization of flat plate solar collectors

In this paper, an exergetic optimization of flat plate solar collectors is developed to determine the optimal performance and design parameters of these solar to thermal energy conversion systems. A detailed energy and exergy analysis is carried out for evaluating the thermal and optical performance, exergy flows and losses as well as exergetic efficiency for a typical flat plate solar collector under given operating conditions. In this analysis, the following geometric and operating parameters are considered as variables: the absorber plate area, dimensions of solar collector, pipes' diameter, mass flow rate, fluid inlet, outlet temperature, the overall loss coefficient, etc. A simulation program is developed for the thermal and exergetic calculations. The results of this computational program are in good agreement with the experimental measurements noted in the previous literature. Finally, the exergetic optimization has been carried out under given design and operating conditions and the optimum values of the mass flow rate, the absorber plate area and the maximum exergy efficiency have been found. Thus, more accurate results and beneficial applications of the exergy method in the design of solar collectors have been obtained.     

Experimental performance analysis and optimisation of medium temperature solar thermal collectors with silicon oil as a heat transfer fluid

Heat transfer fluids (HTFs) play an essential role in solar water heating systems by transferring collected energy from the collector, perhaps via a heat exchanger to the store. If the store is at a much higher temperature than the fluid, the store acts as a heat source, whereas the fluid acts as a coolant, thus reversing the collection process. This action must be avoided through good controls. Experimental performance analysis and comparison of three different types of solar collectors; a non‐concentrating evacuated tube heat pipe and two concentrating single‐sided and double‐sided coated evacuated tube heat pipes collectors are installed and tested using Dow‐corning 550® silicon oil as an HTF under the same operating in‐door control conditions, and results are presented in this paper. The performance of these solar collectors was determined from the overall increase in inlet and outlet fluid temperatures, overall fluid temperature differential, energy collection rate, optical efficiencies, and thermal performances. Temperature differential, energy, and collection efficiency diagrams plotted against time were used to represent and compare the solar collectors. Finally, a comparative analysis of these solar collectors using either pressurised water or Dow‐corning 550 silicon oil as HTF is presented. Copyright © 2011 John Wiley & Sons, Ltd.     

小型太阳能热泵地板供暖系统的优化研究

建立了太阳能热泵地板供暖系统的能量分析、可用能分析数学模型,模拟了上海供暖期的气候条件,给出了系统各部件的可用能损失情况。着重从太阳能集热器并联的组数出发对系统进行了优化研究,并给出了系统供暖性能系数和可用能效率,为该系统的设计及应用提供参考。     

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.     

Experimental investigation on thermal performance of thermosyphon flat-plate solar water heater with a mantle heat exchanger

The thermal performance of thermosyphon flat-plate solar water heater with a mantle heat exchanger was investigated to show its applicability in China. The effect on the performance of the collector of using a heat exchanger between the collector and the tank was analyzed. A “heat exchanger penalty factor” for the system was determined and energy balance equation in the system was presented. Outdoor tests of thermal performance of the thermosyphon flat-plate solar water heater with a mantle heat exchanger were taken in Kunming, China. Experimental results show that mean daily efficiency of the thermosyphon flat plate solar water heater with a mantle heat exchanger with 10 mm gap can reach up to 50%, which is lower than that of a thermosyphon flat-plate solar water heater without heat exchanger, but higher than that of a all-glass evacuated tubular solar water heater.     

Solar energy powered Rankine cycle using supercritical CO2

A solar energy powered Rankine cycle using supercritical CO₂ for combined production of electricity and thermal energy is proposed. The proposed system consists of evacuated solar collectors, power generating turbine, high-temperature heat recovery system, low-temperature heat recovery system, and feed pump. The system utilizes evacuated solar collectors to convert CO₂ into high-temperature supercritical state, used to drive a turbine and thereby produce mechanical energy and hence electricity. The system also recovers heat (high-temperature heat and low-temperature heat), which could be used for refrigeration, air conditioning, hot water supply, etc. in domestic or commercial buildings. An experimental prototype has been designed and constructed. The prototype system has been tested under typical summer conditions in Kyoto, Japan; It was found that CO₂ is efficiently converted into high-temperature supercritical state, of while electricity and hot water can be generated. The experimental results show that the solar energy powered Rankine cycle using CO₂ works stably in a trans-critical region. The estimated power generation efficiency is 0.25 and heat recovery efficiency is 0.65. This study shows the potential of the application of the solar-powered Rankine cycle using supercritical CO₂.     

Exergy modeling of a new solar driven trigeneration system

In this paper, exergy modeling is used to assess the exergetic performance of a novel trigeneration system using parabolic trough solar collectors (PTSC) and an organic Rankine cycle (ORC). Four cases are considered: electrical-power, cooling-cogeneration, heating-cogeneration, and trigeneration. In this trigeneration system a single-effect absorption chiller is utilized to provide the necessary cooling energy and a heat exchanger is utilized to provide the necessary heating energy. The trigeneration system considered is examined using three modes of operation. They are: solar mode during the low-solar radiation time of the day, solar and storage mode during the high-solar radiation time of the day, and storage mode during night time. The storage mode is operated through the heat collected in a thermal storage tank during the solar and storage mode. The exergy efficiencies and exergy destruction rates are examined under the variation of the ORC evaporator pinch point temperature, ORC pump inlet temperature, and turbine inlet pressure. This study reveals that the maximum electrical-exergy efficiency for the solar mode is 7%, for the solar and storage mode is 3.5%, and for the storage mode is 3%. Alternatively, when trigeneration is used, the exergy efficiency increases noticeably. The maximum trigeneration-exergy efficiency for the solar mode is 20%, for solar and storage mode is 8%, and for the storage mode is 7%. Moreover, this study shows that the main sources of exergy destruction rate are the solar collectors and ORC evaporators. Therefore, careful selection and design of these two components are essential to reduce the exergy destructed by them and, thus, increase the exergy efficiencies of the system.     

Performance analysis on a novel micro‐channel heat pipe evacuated tube solar collector‐incorporated thermoelectric generation

In China, because of the emergence of a large number of high‐rise buildings, the solar hot water heater system often uses the balcony wall‐mounted method for installation. The thermoelectric energy converter is proposed as one of the possible technologies to incorporate solar water heater to produce electricity for building application. In this paper, the conceptual development and theoretical analysis of a novel micro‐channel heat pipe evacuated tube solar collector‐incorporated thermoelectric generation are all proposed. The new system takes into account many advantages, including the high heat transfer, low convective heat loss, and low contact thermal resistance. The exergy analysis method based on the second law of thermodynamics is also introduced to evaluate the performance of this system. The results show that a novel micro‐channel heat pipe evacuated tube solar collector‐incorporated thermoelectric generation has a high thermal performance with addition of electricity production. Copyright © 2016 John Wiley & Sons, Ltd.     

The need for exergy analysis and thermodynamic optimization in aircraft development

This paper outlines a newly emerging body of work that relies on exergy analysis and thermodynamic optimization in the design of energy systems for modern aircraft. Exergy analysis establishes the theoretical performance limit. The minimization of exergy destruction brings the design as closely as permissible to the theoretical limit. The system architecture springs out of this constrained optimization principle. A key problem is the extraction of maximum exergy from a hot gaseous stream that is gradually cooled and eventually discharged into the ambient. The optimal configuration consists of a heat transfer surface with a temperature that decays exponentially in the flow direction. This configuration can be achieved in a counterflow heat exchanger with an optimal imbalance of flow capacity rates. The same optimal configuration emerges when the surface is minimized subject to specified exergy extraction rate. Similar opportunities for optimally matching components and streams exist in considerably more complex systems for power and refrigeration. They deserve to be pursued, and can be approached first at the conceptual level, based on exergy analysis and thermodynamic optimization. The application of such principles in aircraft energy system design also sheds light on the “constructal” design principle that generates all the systems that use powered flight, engineered and natural, cf. constructal theory.     

Design of a solar‐driven cogeneration system using flat plate collectors and evacuated tube collectors

The objective of this work is the investigation of a novel solar‐driven cogeneration system, which combines both flat plate collectors (FPCs) and evacuated tube collectors (ETCs). This system includes an organic Rankine cycle in order to produce electricity and heat exchangers in order to produce useful heating at 50°C, which is a usual temperature level for domestic applications. The combination of FPCs and ETCs aims to reduce the investment cost and so to design a cost‐effective solar‐driven cogeneration system. The FPCs are located before the ETCs in order to work at lower temperature levels and the ETC at higher temperature levels, a design which provides optimum compatibility between temperature levels and solar thermal efficiency values. The system is examined energetically, exergetically, and financially. The power production is selected at 5 kW, while the heating production is studied from 5 kW up to 35 kW. According to the final results, it is found that in the typical case of 20‐kW heating production, the simple payback period of the system is around 11 years, while the energy and exergy efficiency at 16% and 4%, respectively. The analysis is conducted with a developed model in Engineering Equation Solver under steady‐state conditions.