Research On Key Processes And Simulation And Construction Of Multi-Generation System Thermally Driven By Solar Energy

With the rapid development of economic construction of China’s society in recent years,the coal-based primary energy consumption is growing severely.Meanwhile,the air pollution caused by combusting foil fuels has intensified which threatens the living environment of people seriously.This situation urges us to find a type of sustainable and clean energy.The distributed multi-generation system thermally driven by solar energy has gradually become the focus of researches and applications due to its characteristic of inexhaustible.Solar energy collection technology using parabolic trough is very suitable for distributed multi-generation system because of its high maturity,flexible scale and relatively low investment cost.The regularities of collector’s efficiency with working fluid’s temperature,incidence angle and intensity of solar radiation are the basic inputs for designing multi-generation system.The annual effective incident radiation and available heat gain from per collector’s area are prerequisites for evaluating the system’s economy.Based on the actual configuration of 1000 m~2 parabolic trough solar collector,the performances of two tracking methods of east-west axis and north-south axis are studied by combining the location information of Tianjin city,data of typical meteorological year and outdoor experimental data of large-scale parabolic collector.Under these two tracking methods,the annual effective incident direct radiations on per collector’s area are 3662.51 MJ/(m~2·y)and 4621.10 MJ/(m~2·y)respectively,resulting in the capture ratios of normal direct radiation of 72.47%and 91.44%.Under the condition of that working fluid’s average temperature is 200℃and the reflector’s cleanliness factor is 0.95,the annual average conversion efficiencies of radiation to heat are 47.93%and 54.93%,respectively.When the working fluid’s average temperature increases to 300℃,the annual average conversion efficiencies are 30.60%and 41.20%,respectively.Organic Rankine cycle is the most suitable power cycle for distributed solar thermal system because of its technology characteristic of multi-generation,while selection of working fluid is the first question needs to be solved to construct an organic Rankine cycle.There are already many researches concerning the selection of working fluid,but there is no specific one or several types of working fluid that can be widely recommended or recognized,which mainly results from that the candidates,minimum temperature difference and performance indicators are different.Based on this situation,all pure fluids in the authoritative database Refprop are studied with the objective of thermal efficiency.When the heat source temperature is below 200℃,the top four types of fluid with highest efficiency are R141b,R123,diethyl ether and pentane.After considering the ODP and flammability of fluids,R1233zd is the first recommended and the performance of R245fa is medium.When the heat source temperature is above200℃,after considering the ODP,flammability and toxicity,there is no proper working fluids except for the siloxanes.However,the condensation temperatures of them under standard atmosphere pressure are too high,and about 200℃.The nearly isentropic fluids have limited prospect in subcritical organic Rankine cycle due to their low critical temperatures.Currently,the existing studies concerning the selection of working fluid is a lack of verifications about the key property parameters influencing the cycle’s thermal efficiency,neither take cognizance of the existence of less-ideal cycle’s thermal efficiency between organic Rankine cycle and Carnot cycle.In a subcritical organic Rankine cycle without recuperator,superheat and the key parameters influencing the thermal efficiency are analyzed.The results prove that superheat is not necessary,and smaller value of the ratio of sensible heat to latent heat in evaporation process can lead to higher thermal efficiency.Finally,the thermal efficiency of a less-ideal cycle between the Carnot efficiency and actual efficiency is proposed by applying the Watson equation.This conclusion has cut down the difference between actual cycle and ideal cycle,and has clarified the improvement potential of thermal efficiency of high-performance organic working fluid.The compression process is the only power-consuming thermodynamic process in organic Rankine cycle,which may consume the power output of expansion process accounting for 10~30%in actual applications.However,there is a lack of study focusing the correlation between the working fluid’s properties and the isentropic efficiency of compression process.Four kinds of working fluid R245fa,R123,R152a and R600a,are selected to study the variation of isentropic efficiency in compression process.A diaphragm pump is installed on the test rig.The experimental results indicate that the isentropic efficiency increases with the volume flow rate and the pressure difference between inlet and outlet.The analysis of the transmission mechanism of diaphragm pump proves that the hitting velocity of diaphragm to the liquid at the moment of starting compression is the main reason to the production of entropy in the compression process.A new compound parameterα_V/ρc_p is proposed to define the effect of fluid’s properties on isentropic efficiency in compression process.Smaller value ofα_V/ρc_p can lead to higher isentropic efficiency under the same volume flow rate and pressure difference.Upon the research results of solar energy collection process and the thermodynamic processes of organic Rankine cycle,a new design method for the multi-generation system driven by solar energy is proposed.The new method has pointed out the’least configuration’of multi-generation system,and has improved the feasibility of design by dividing the design scheme into two stages.The optimized minimum temperature difference in heat exchanger is 39.1℃for system A which only fulfills the heating(cooling)load.System B is the combination of system A and an organic Rankine cycle.The optimized minimum temperature differences in the evaporator and condenser of the organic Rankine cycle is 10.69℃and 8.44℃respectively,and the corresponding system’s yield rate is 122.03 W/m~2.The Dymola platform is used to test the dynamic response of system B.The results indicate,without active control strategy,that the heat-transfer oil’s temperature exceeds the design value with 37.79℃and the superheat of working fluid at evaporator outlet reaches 86.46℃.