Solar Organic Rankine Cycle System Integrated With Phase Change Materiel Storage

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.A temperature of 100℃ or slightly higher is enough to run a small scale solar 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 small-scale solar ORC system is a promising technology.However,there are few problems associated with the system such as fluctuating and less efficient power generation.In past few years,the author has dedicated to reduce the thermodynamic irreversibility and to optimize the solar ORC system.In current work,different innovative solutions are proposed,including direct vapor generation solar ORC system and a solar ORC with novel heat pipe evacuated tube collectors.The present work includes effect of working fluids on the direct vapor generation(DVG)solar ORC system,modelling of a novel indicator for ORC efficiency,thermodynamic comparison between phase change material(PCM)storage based direct and indirect solar ORC systems,and thermodynamic comparison between novel and conventional heat pipe evacuated tube collectors based solar ORC systems.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 using 17 dry 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.The heat collection is strongly correlated with the latent and sensible heat of the working fluid.Although R123 has environmental constraints such as high level of Ozone depletion potential(ODP)and global warming potential(GWP),however,it exhibits the highest overall performance among the selected fluids.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 on 27 fluids 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.Unlike the conventional ORC efficiency model,this one consists of thermodynamic parameters on the liquid/vapor curve and is independent of fluid properties at superheated state,and hence is more convenient.It has high accuracy especially for basic ORC and the relative deviation of the estimated efficiency from that calculated by the conventional model is from-0.7%to 3.4%.The novel model is applied for the thermodynamic performance prediction of a recently developed fluid of HF01336mzzZ based on the phase equilibrium data.The results indicate HF01336mzzZ is more efficient than R245fa on the conditions of high evaporation temperature and low pump efficiency.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 shown 1.71%system efficiency and able to provide 34.02 kW/day power while DSOS has shown 4.5 times higher system efficiency and 2.8 times higher power on annual basis.A numerical model for the PCM storage is developed and validated with the previous experimental data.Average annual amount of energy stored by PCM during charging phase for ISOS is 4.24 MW/day higher than DSOS.However,in comparison with ISOS,DSOS has delivered 33.80 kW/day more power to HTF during discharging phase of the PCM on annual basis.Maximum benefits of PCM storage are observed during the summer season compared to the winter season at selected operating conditions.Furthermore,the average annual increment in capacity factor by using PCM storage is found to be 21.71%and 17%for DSOS and ISOS respectively.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 heat shield improve the collector efficiency at any cooling water temperature and have a better performance at higher temperatures.The efficiency of the new type is up by 11.8%compared to the original one in test results at the normalized temperature difference of 0.166 m2K/W.In instantaneous efficiency curves,the first and second order heat loss coefficients of the new type of solar collector result in 28.4%and 29.9%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.