Thermodynamic investigation of solar energy-based triple combined power cycle

The present work deals with the thermodynamic analysis of a solar-powered triple combined power cycle to generate emission-free power. The triple combined cycle comprises one topping cycle as Brayton cycle and two bottoming cycles, namely, steam Rankine cycle (SRC) and organic Rankine cycle (ORC). The Brayton cycle employs double-stage compression with intercooling. During intercooling, heat energy rejected by the compressed air was further utilized in the ORC. The energy carried away after the turbine exit was used in the SRC. The proposed cycle performance is investigated for three working fluids to use with the bottoming ORC. Results showed that the maximum overall thermal efficiency and work output of solar energy-based triple combined cycle are found 21.89% and 218.98 kJ/kg air, respectively, for organic fluid R245fa at the topping cycle pressure ratio of 31.     

Thermodynamic analysis and optimization of a solar-driven regenerative organic Rankine cycle (ORC) based on flat-plate solar collectors

This paper presents a regenerative organic Rankine cycle (ORC) to utilize the solar energy over a low temperature range. Flat-plate solar collectors are used to collect the solar radiation for their low costs. A thermal storage system is employed to store the collected solar energy and provide continuous power output when solar radiation is insufficient. A daily average efficiency is defined to evaluate the system performance exactly instead of instantaneous efficiency. By establishing mathematical models to simulate the system under steady-state conditions, parametric analysis is conducted to examine the effects of some thermodynamic parameters on the system performance using different working fluids. The system is also optimized with the daily average efficiency as its objective function by means of genetic algorithm under the given conditions. The results indicate that under the actual constraints, increasing turbine inlet pressure and temperature or lowering the turbine back pressure could improve the system performance. The parametric optimization also implies that a higher turbine inlet temperature with saturated vapor state could obtain the better system performance. Compared with other working fluids, R245fa and R123 are the most suitable working fluids for the system due to their high system performance and low operation pressure.     

Analysis of low temperature solar thermal electric generation using regenerative Organic Rankine Cycle

The innovative configuration of low temperature solar thermal electric generation with regenerative Organic Rankine Cycle (ORC) is designed, mainly consisting of small concentration ratio compound parabolic concentrators (CPC) and the regenerative ORC. Advantages of the innovative configuration such as effectively reducing heat transfer irreversibility and permitting the use of thermal storage with phase change materials (PCMs) are outlined. The numerical simulation of the heat transfer and power conversion processes are carried out based on distributed parameters. The effects of regenerative cycle on the collector, ORC, and overall electricity efficiency are then analyzed. The results indicate that the regenerative cycle has positive effects on the ORC efficiency but negative ones on the collector efficiency due to increment of the average working temperature of the first-stage collectors. Thus, it is necessary to evaluate the overall electricity efficiency when regenerative cycle is adopted. Further investigation shows that there are maximum efficiencies for both the ORC and the system electric generation on conditions of constant irradiance, evaporation temperature, and environment temperature. And the regenerative temperature at which the system electricity efficiency reaches its maximum is smaller than that at which the ORC efficiency reaches its maximum by 12–21 °C. Thus, the regenerative cycle optimization of the solar thermal electric generation differs from that of a solo ORC. The system electricity efficiency with regenerative ORC is about 8.6% for irradiance 750 W/m² and is relatively higher than that without the regenerative cycle by 4.9%.     

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.     

Hydrogen as a battery for a rooftop household solar power generation unit

Decentralization of electrical power generation using rooftop solar units is projected to develop to not only mitigate power losses along transmission and distribution lines, but to control greenhouse gases emissions. Due to intermittency of solar energy, traditional batteries are used to store energy. However, batteries have several drawbacks such as limited lifespan, low storage capacity, uncontrolled discharge when not connected to a load and limited number of charge/discharge cycles. In this paper, the feasibility of using hydrogen as a battery is analyzed where hydrogen is produced by the extra diurnal generated electricity by a rooftop household solar power generation unit and utilized in a fuel cell system to generate the required electrical power at night. In the proposed design, two rooftop concentrated photovoltaic thermal (CPVT) systems coupled with an organic Rankine cycle (ORC) are used to generate electricity during 9.5 h per day and the extra power is utilized in an electrolyzer to produce hydrogen. Various working fluids (Isobutane, R134a, R245fa and R123) are used in the ORC system to analyze the maximum feasible power generation by this section. Under the operating conditions, the generated power by ORC as well as its efficiency are evaluated for various working fluids and the most efficient working fluid is selected. The required power for the compressor in the hydrogen storage process is calculated and the number of electrolyzer cells required for the hydrogen production system is determined. The results indicate that the hybrid CPVT-ORC system produces 2.378 kW of electricity at 160 suns. Supplying 65% of the produced electricity to an electrolyzer, 0.2606 kg of hydrogen is produced and stored for nightly use in a fuel cell system. This amount of hydrogen can generate the required electrical power at night while the efficiency of electrolyzer is more than 70%.     

不同太阳能利用区有机朗肯循环工质的选择

为太阳能有机朗肯循环系统(organic Rankine cycle,ORC)合理选择工质需要综合考虑太阳能辐射强度和环境温度的影响。基于建筑热工设计分区和太阳能资源区划标准,将我国分为了11个太阳能利用区。利用太阳能ORC热力学能量流通数学模型,以太阳能ORC系统典型月平均热发电效率最高为原则,为太阳能ORC系统绘制出推荐的工质地图。     

不同工质对太阳能有机朗肯循环系统性能的影响

循环工质的特性是影响有机朗肯循环系统性能的重要因素之一,在不同的蒸发温度条件下,选取R600、R600a、R245fa、R236fa、R236ea、R601、R601a、RC318及R227ea共9种有机工质,基于热力学第一定律和第二定律对其热力循环特性进行了计算分析,并对各有机工质的蒸发压力、热效率、功比和不可逆损失等进行了比较.结果表明：R245fa作为太阳能低温热发电朗肯循环系统的循环工质具有较高的热效率和效率,并且产生的系统总不可逆损失较小,是一种较理想的有机工质;其次,R236fa和R236ea作为系统循环工质也具有较为良好的性能.     

Performance and design optimization of a low-cost solar organic Rankine cycle for remote power generation

Recent interest in small-scale solar thermal combined heat and power (CHP) power systems has coincided with demand growth for distributed electricity supplies in areas poorly served by centralized power stations. One potential technical approach to meeting this demand is the parabolic trough solar thermal collector coupled with an organic Rankine cycle (ORC) heat engine.The paper describes the design of a solar organic Rankine cycle being installed in Lesotho for rural electrification purpose. The system consists of parabolic though collectors, a storages tank, and a small-scale ORC engine using scroll expanders.A model of each component is developed taking into account the main physical and mechanical phenomena occurring in the cycle and based on experimental data for the main key components.The model allows sizing the different components of the cycle and evaluates the performance of the system. Different working fluids are compared, and two different expansion machine configurations are simulated (single and double stage).     

A study of organic working fluids on system efficiency of an ORC using low-grade energy sources

Rankine cycles using organic fluids (as categorized into three groups: wet, dry, and isentropic fluids) as working fluids in converting low-grade energy are investigated in this study. The main purpose is to identify suitable working fluids which may yield high system efficiencies in an organic Rankine cycle (ORC) system. Efficiencies of ORC systems are calculated based on an assumption that the inlet condition of the working fluid entering turbine is in saturated vapor phase. Parameters under investigation are turbine inlet temperature, turbine inlet pressure, condenser exit temperature, turbine exit quality, overall irrversibility, and system efficiency. The low-grade energy source can be obtained from a solar pond or/and an ocean thermal energy conversion (OTEC) system. Results indicate that wet fluids with very steep saturated vapor curves in T-s diagram have a better overall performance in energy conversion efficiencies than that of dry fluids. It can also be shown that all the working fluids have a similar behavior of the efficiency-condenser exit temperature relationship. Furthermore, an appropriate combination of solar energy and an ORC system with a higher turbine inlet temperature and a lower condenser temperature (as operated deeply under sea level) would provide an economically feasible and environment-friendly renewable energy conversion system.     

Performance analysis of cogeneration systems based on micro gas turbine (MGT), organic Rankine cycle and ejector refrigeration cycle

In this paper, the operation performance of three novel kinds of cogeneration systems under design and off-design condition was investigated. The systems are MGT (micro gas turbine) + ORC (organic Rankine cycle) for electricity demand, MGT+ ERC (ejector refrigeration cycle) for electricity and cooling demand, and MGT+ ORC+ ERC for electricity and cooling demand. The effect of 5 different working fluids on cogeneration systems was studied. The results show that under the design condition, when using R600 in the bottoming cycle, the MGT+ ORC system has the lowest total output of 117.1 kW with a thermal efficiency of 0.334, and the MGT+ ERC system has the largest total output of 142.6 kW with a thermal efficiency of 0.408. For the MGT+ ORC+ ERC system, the total output is between the other two systems, which is 129.3 kW with a thermal efficiency of 0.370. For the effect of different working fluids, R123 is the most suitable working fluid for MGT+ ORC with the maximum electricity output power and R600 is the most suitable working fluid for MGT+ ERC with the maximum cooling capacity, while both R600 and R123 can make MGT+ ORC+ ERC achieve a good comprehensive performance of refrigeration and electricity. The thermal efficiency of three cogeneration systems can be effectively improved under off-design condition because the bottoming cycle can compensate for the power decrease of MGT. The results obtained in this paper can provide a reference for the design and operation of the cogeneration system for distributed energy systems (DES).     

Design recommendations for solar organic Rankine cycle (ORC)-powered reverse osmosis (RO) desalination

This paper deals with the design recommendations for solar reverse osmosis (RO) desalination based on solar organic Rankine cycles (SORC). This technology can be the most energy-efficient technology for seawater and brackish water desalination within the small to medium power output range (up to 500 kW) of the power cycle if the system is properly designed. However, theoretical studies, design proposals and experimental works are very scarce and only very few solar reverse osmosis systems driven by ORC has been either implemented or analysed in the past. In this paper, those systems are outlined and general design recommendations from previous detailed analysis already publish are given for future RO desalination system to be designed based on SORC. Useful information is given about the selection of the working fluid and boundary conditions of the ORC, operation temperature and configuration of the solar field, suited solar collector and thermal energy storage technology, etc. Recommendations are exemplified with well selected numerical cases based on recommended working fluids and solar cycle configuration with proper values of design point parameters. Recommendations given in this paper could be helpful in future initiatives regarding the research and development of this promising solar desalination technology.     

A procedure to select working fluids for Solar Organic Rankine Cycles (ORCs)

The selection of working fluid and working conditions of the Organic Rankine Cycle (ORC) has a great effect on the system operation, and its energy efficiency and impact on the environment. The main purpose of this study is to develop a procedure to compare capabilities of working fluids when they are employed in solar Rankine cycles with similar working conditions. The Refprop 8.0 database with 117 organic fluids has been considered as the reference in this study. A procedure to compare ORC working fluids based on their molecular components, temperature–entropy diagram and fluid effects on the thermal efficiency, net power generated, vapor expansion ratio, and exergy efficiency of the Rankine cycle has been proposed. Fluids with the best cycle performance have been recognized in two different temperature levels within two different categories of fluids: refrigerants and non-refrigerants. Based on categories of solar collectors, 11 fluids have been suggested to be employed in solar ORCs that use low or medium temperature solar collectors. Collector efficiency improvement and use of the regenerative ORC instead of the basic cycle reduce irreversibility of a solar ORC. Calculation results show that for selected fluids, the theoretical limits for irreversibility reduction and exergy efficiency enhancement through collector efficiency improvement are 35% and 5% respectively, when the collector efficiency increases from 70% to 100%. The effect of regeneration on the exergy efficiency of the cycle is fluid dependent while the effect of collector efficiency improvement on the exergy efficiency of the cycle is nearly independent of fluid type. At the two temperature levels studied, higher molecular complexity results in more effective regenerative cycles except for Cyclohydrocarbons.     

Material Compatibility of Hexamethyldisiloxane as Organic Rankine Cycle Working Fluids at High Temperatures

The organic Rankine cycle(ORC)is a promising technology for industrial waste heat recovery and renewable energy utilization.High temperature ORCs have attracted particular interest because of their high thermal efficiencies and outputs.The material compatibility of working fluid is a significant limitation for the working fluid selection and system design for high temperature ORCs.This work presents a method for studying the material compatibility of ORC working fluids based on the calculated conditions of the ORCs and matching of components,temperatures,and materials.Hexamethyldisiloxane(MM)was chosen as the test fluid.The experimental results show that 304 stainless steel has better compatibility with MM than copper as the material of evaporators.Fluoric rubber is not a suitable sealing material for high temperature ORCs with MM as the working fluids because of the bad compatibility.Mineral oil has better compatibility with MM than polyol ester(POE)lubricant as the lubricant for the fluid pump.     

Review of the Working Fluid Thermal Stability for Organic Rankine Cycles

The organic Rankine cycle(ORC) is an efficient power generation technology and has been widely used for renewable energy utilization and industrial waste heat recovery. Thermal stability is a significant property of ORC working fluids and is the primary limitation for working fluid selection and system design. This paper presents a review of the working fluid thermal stability for ORCs, including an analysis of the main theoretical method for thermal stability, a summary of the main experimental method for thermal stability, a summary of the decomposition experimental results for working fluids, and a discussion of the decomposition influence on ORC systems. Further research trends of thermal stability are also discussed in this paper.     

太阳能驱动有机朗肯循环的工质比较

针对太阳能热水和其它低品位热能的动力利用,研究了工作在100℃供热温度和30℃冷凝温度之间的有机朗肯循环工质的优化选择,以满足较高的循环效率、较大的机械能输出、较小的排气量需求等要求。工质模型采用RKS状态方程,针对R22在-30～95℃温度区间内,计算结果与ASHRAE20-2005数据比较,除液相密度外,其它的热力学参数计算绝对误差小于5%,满足工程模拟要求。利用RKS模型,文中分析了19种有机工质的动力循环参数,发现工质R11的热力学性能系数最高。结合GWP和ODP环境指标,发现R142b、Rc318与R600适合于低温朗肯循环。     

Selection and Evaluation of Dry and Isentropic Organic Working Fluids Used in Organic Rankine Cycle Based on the Turning Point on Their Saturated Vapor Curves

The organic Rankine cycle(ORC) is a popular technique used in the utilization of low-grade thermal energy. Among wet, dry, and isentropic organic working fluids, the latter two types are more appropriate for ORC systems. In this paper, the definition of turning point on saturated vapor curve of dry fluid and isentropic fluid was given according to the shape of the saturated curve of working fluids in a T-s diagram. On this basis, the model of near-critical region triangle was established. Using this model, the thermodynamic performance of 57 kinds of dry and isentropic organic working fluids in ORC was evaluated. The performance includes the relation between turning point temperature and cycle thermal efficiency, the relation between near-critical region triangle area and cycle thermal efficiency, the relation between near-critical region triangle area and exergy at turning point temperature, the relation between near-critical region triangle area and reciprocal value of slope of saturated vapor curve. Moreover, working fluid selection was also conducted in terms of heat source type. It was found through theoretical analysis results that the popular R123 is an acceptable choice especially for the utilization of closed type heat source. Considering it will be phased out in near future, then cis-butene, butane, trans-butene, and isobutene are worth studying as its successor. Dodecane is worthy of attention and further research and it can be a good choice for utilization of open type heat source.     

Research and Development of Supercritical Carbon Dioxide Coal-Fired Power Systems

Using supercritical carbon dioxide(S-CO2)Brayton cycle instead of the traditional steam Rankine cycle is a promising technique to enhance the coal-fired power generation efficiency.Researchers from all over the world are actively designing and exploring efficient S-CO2 coal-fired power plants in recent years with great efforts made to overcome the significant technical challenges in the cycle layouts of S-CO2 and its specific thermal integration with coal-fired heat resources.This paper provides a detailed review of the research progress on the coal-fired power generation using S-CO2 Brayton cycles.The basic knowledge of S-CO2 properties,the promising S-CO2 power cycles and the conceptual designs for S-CO2 coal-fired power plants are comprehensively summarized,with some key issues in the constructing process and the corresponding engineering solutions being emphatically discussed.Based on the current achievements,the overall technical and economic evaluations on the S-CO2 coal-fired power system are figured out.Furthermore,the specific integration applications of S-CO2 cycles with different coal firing devices and modes including pulverized coal combustion,circulating fluidized bed combustion,oxy-coal combustion,pressurized fluidized bed combustion,chemical looping combustion are discussed.Finally,the main challenges requiring further studies are highlighted.     

基于动态透平效率的有机朗肯循环性能分析及优化

常规有机朗肯循环(ORC)中透平效率多假设为定值,而实际上透平效率因工质种类和运行参数的不同而有较大差异。因此,采用向心透平效率计算模型,将动态透平效率与ORC系统耦合,分析透平效率随蒸发温度与冷凝温度的变化规律,比较固定透平效率与动态透平效率ORC系统热效率的差异。综合考虑热力性与经济性,采用多目标优化算法,对固定透平效率与动态透平效率ORC系统进行工质筛选及参数优化,并对优化结果进行分析比较。结果表明:透平效率随蒸发温度的下降或者冷凝温度升高而增大;不同工质及不同蒸发冷凝温度条件下,透平效率差异较大,最大达0.148。固定透平效率ORC系统与动态透平效率ORC系统的热效率随蒸发温度的变化规律有较大差异,尤其在高蒸发温度区间更为明显。对于固定透平效率ORC系统,R245ca和R236ea为最佳工质;而对于动态透平效率ORC系统,R114为最佳工质。在引入动态透平效率前后,各工质的最佳蒸发温度与最佳冷凝温度也有较大变化。     

跨临界-亚临界复叠式有机朗肯循环性能分析

热源温度高于473.15 K时,复叠式有机朗肯循环(organic Rankine cycle,ORC)可避免高温下工质热分解、膨胀比过大等缺点,相对单级ORC更具优势。跨临界循环相较常规亚临界具有更高的吸热压力及更好的热源匹配性,其与复叠式ORC耦合有望获得更优的热力性能。因此,构建了跨临界-亚临界复叠式ORC(TSORC),以473.15~573.15 K工业烟气为热源,针对5组工质,探究了吸热压力、冷凝压力对系统热力性能的影响,优化系统参数以获得最大净输出功;并与常规亚临界-亚临界复叠式ORC(SSORC)进行对比。结果表明:TSORC相对SSORC可有效增大系统净输出功,最多可提高23.9%;但当热源温度低于"等值热源温度"时,SSORC的输出功反而更大;以R1233zd-R1234ze(E)为工质的TSORC具有最大净输出功。     

Design and Analysis of S-CO_2 Cycle and Radial Turbine for SOFC Vehicle Waste-Heat Recovery

Solid oxide fuel cell(SOFC) vehicles are considered to have broad prospects for development, and the high operating temperature of SOFC results in great potential for waste-heat recovery. Many concepts for utilizing waste heat of SOFC have been suggested and studied, and most of them directly couple an SOFC to a gas turbine, which require the SOFC to operate at an elevated pressure and make the system less flexible and thus harder to operate. In recent years, with the development of turbine and heat exchanger technology, the supercritical carbon dioxide(S-CO_2) power cycle has raised widespread attractions for the waste recovery. This study explores the potential of S-CO_2 Brayton cycle to realize waste-heat recovery for an SOFC vehicle. The SOFC can operate at atmospheric pressure, and the hybrid system is easier to operate than the directly coupled systems. In this paper, a simple recuperated S-CO_2 Brayton cycle is proposed and the key component, radial inflow turbine is designed and focused. The flow state of the designed turbine is analyzed in detail based on computational fluid dynamics(CFD) numerical simulation. Five cases with different impeller tip clearances are numerically simulated to study its influence on the turbine performance. In addition, off-design performance analysis of the radial inflow turbine is conducted considering the temperature fluctuation of SOFC in practical applications.