Investigation Of Optical Properties Of Nanofluids And Their Utilization In The Photovoltaic/Thermal Hybrid System

The efficient harvesting of solar energy is one of the most promising ways to overcome the excessive usage of conventional fossil fuels.Concentration photovoltaic/thermal(CPVT)system,which using liquid as both optical filter and heat transfer fluid(HTF),is an effective technology to achieve better utilization of solar radiation.The system is producing both electrical and thermal energy and recovering thermal energy at high temperature.However,the main drawback of such system is the structural design,since it plays an important role in controlling the operating temperature of the PV cells and the temperature difference across the thermal unit,which has a crucial effect on the overall system performance.Despite this challenge,there are still some obstacles to overcome for the application of liquid absorption filtering technology in CPVT systems,including the lack of non-degradable liquids with suitable absorption spectra and the difficulty to obtain a stable nanofluid in a high-temperature environment.In addition,research on optical characteristics of the nanofluids is still in the initial stage,especially for nanofluids working at elevated temperatures.Hence,it is of great significance to tackle such problems.In this study,the performance of CPVT system based on liquid absorption spectral beam filter with different structure configurations is theoretically investigated.Main efforts are put to reveal the effects of vacuum enclosure surrounding the optical filter and the heat transfer medium between the thermal receiver and the solar cells on the system performance at different operating conditions using Si-based PV cell.The effect of long-term operation under high-temperature exposure is also analyzed.The simulation results show both the vacuum enclosure around the thermal unit and the heat transfer medium between the thermal receiver and solar cells can influence the transmitted energy arriving the solar cells and the temperature of the whole module,which are very sensitive to the electrical and thermal performance.The use of vacuum enclosure decreases the electrical efficiency by about 4%,while it leads to a much higher thermal efficiency.With large coolant flow rates,the configuration without the vacuum enclosure shows both higher overall energy and exergetic efficiencies.It is also suggested to use an air gap between the receiver and PV cells instead of a vacuum layer or a solid layer.In order to obtain a more efficient CPVT configuration,another comparative analysis is conducted,in terms of overall system performance,between an optimized CPVT configuration investigated from the different CPVT configurations and the same optimized CPVT configuration with separate channels.As thus,the optical window,which mostly depends on the spectral response of PV cells,and heat capacity of fluids,which determines the fluid temperature and the capacity of HTF as heat carriers,are comprehensively studied.In this instance,the results indicate that increasing the specific heat capacity of the liquid filter can improve the thermal efficiencies for the D-CPVT and S-CPVT by 3.52%and 2.19%,and overall efficiencies of the system by 5.8%and 4.5%,respectively.In addition,it is noted that the suitable type of the absorptive liquid filter considerably depends on the desired purpose,where the use of liquid filter with both higher ideality factor and specific heat capacity is recommended to enhance the electrical performance of CPVT system.Furthermore,since the NFs are more effective as both HTF and absorptive liquid filter,in this thesis,the SiO2-H2O NFs under different conditions and different nanoparticle volume fractions are synthesized.The NF dispersion stability and optical properties at elevated temperatures are investigated.The effects of several factors including the sonication temperature,the exposure temperature,the nanoparticle volume fraction and the exposure time are analyzed.The experimental results show that reducing the maximum sonicating temperature during the NF fabrication process is beneficial to prolonging the NF stability,and the prepared NF stability is rather good with the maximum sonicating temperature of 30?.The absorptance of NFs decreases when the exposure temperature increases from 25 to 90?,and agglomeration more likely occurs for higher exposure temperatures.It is also observed that the adding of nanoparticles helps to increase the spectrum absorption ability of NF.As a result,the overall energy efficiencies of the photovoltaic/thermal device using the NFs as the optical filter can be increased compared with that using the water filter.