Thermal Power Engineering
摘要：Solar energy has emerged as one of the most rapidly growing renewable sources of power generation.It has a minimum time of replenishment and maximum capacity among all available energy resources.Furthermore,it is an attractive option for coupling with low-medium temperature organic Rankine cycle(ORC)system.The system is beneficial due to good thermodynamic performance in the utilization of low-grade heat,small unit size,low technical demand in heat storage,decentralized application and suitability in regions with less direct solar radiation resource.
The present study consists of four major parts.In the first part,a novel solar ORC system with DVG is proposed.A phase change material heat storage unit is embedded in the ORC to guarantee the stability of power generation.Compared with conventional solar ORCs,the proposed system avoids the secondary heat transfer intermediate and shows good reaction to the fluctuation of solar radiation.The technical feasibility of the system is discussed.Performance is analyzed by using17dry and isentropic working fluids.Fluid effects on the efficiencies of ORC,collectors and the whole system are studied.The results indicate that the collector efficiency generally decreases while the ORC and system efficiencies increase with the increment in fluid critical temperature.
In the second part,an indicator,namely equivalent hot side temperature(TEHST)is proposed for the organic Rankine cycle.TEHST is derived from the ideal thermodynamic process but can denote the efficiency of irreversible ORC.In this work,the basic organic Rankine cycle is considered.Study on27fluids shows that given the operating conditions,the fluid of higher TEHST generally offers higher ORC efficiency.This relationship is stronger and more universal than those established with respect to the critical temperature,boiling point temperature,Jacobs's number and Figure of Merit.An ORC model by the method of error transfer and compensation is further built,in which the efficiency is quantitatively correlated with TEHST.
In the third part,a thermodynamic comparison between a novel direct solar ORC system(DSOS)and indirect solar ORC system(ISOS)is carried out.A phase change material heat storage unit is integrated with both systems to ensure the stability of power generation.Water and R245fa are selected as a heat transfer fluids(HTFs)for ISOS and DSOS respectively.However,R245fa is used as working fluid for both systems.Weekly,monthly and annual dynamic simulations are carried out to compare the performance of both systems using hourly weather data of Islamabad,Pakistan.ISOS has shown1.71％system efficiency and able to provide34.02kW/day power while DSOS has shown4.5times higher system efficiency and2.8times higher power on annual basis.
In fourth and final part,the thermal performance of the heat pipe evacuated tube solar collectors(HPETCs)with and without heat shield are investigated theoretically and experimentally.The two types of solar collectors with wickless heat pipes are tested in parallel on an outdoor test rig.A considerable agreement between the experimental and simulated results is obtained.The efficiency of the new type is up by11.8％compared to the original one in test results at the normalized temperature difference of0.166m2K/W.In instantaneous efficiency curves,the first and second order heat loss coefficients of the new type of solar collector result in28.4％and29.9％decrease compared to the original one,respectively.Furthermore,the efficiency of the two types of collectors is simulated at different cooling water flow rates,solar radiations,and ambient temperatures.It is observed that cooling water temperature is the most affecting parameter that affects the collector performance.Moreover,numerical simulations are carried out for the HPETCs based ORC systems.Results of collector and overall solar ORC system are demonstrated and compared.