| Hybrid photovoltaic/thermal(PV/T)collectors are the integration of solar collectors and photovoltaic modules and exhibit higher exergy and overall efficiency in solar energy utilization.Initially,the main purpose of PV/T systems is to reduce the operating temperature and collects the excess heat released by solar cells.Thermal energy is considered as a secondary benefit of PV/T systems,which are mainly used in low-temperature applications such as domestic hot water and space heating.Recently,medium temperature PV/T systems have attracted more and more attention.It has a wide range of potential applications in the temperature of 70～100℃,including PV/T-absorption cooling,PV/T-thermoelectric generation,and hybrid PV/T and organic Rankine cycle power generationAt present,the mainstream photovoltaic/thermal(PV/T)systems employ crystalline silicon(c-Si)cells for power generation.However,some characteristics of c-Si cells make the PV/T system only suitable for low-temperature applications.First,common c-Si cells have high power temperature coefficients,which are about-0.4%/℃ to-0.50%/℃.The electrical efficiency of the solar cells decreases significantly as the operating temperature increases.Second,PV/T using c-Si cells may suffer from low energy efficiency in the season when heat is unfavorable.In summer,the demand for domestic hot water is significantly reduced compared with that in winter,though the solar radiation is more attainable.Meanwhile,the temperature inside the PV/T collector is higher than the ambient temperature,which will adversely affect the performance of the solar cells.Third,since fluctuation in the temperature and its gradient and the coefficient of thermal expansion of c-Si cells is far less than that of aluminum,the c-Si PV/T may be easily broken in the long-term operation.However,the aforementioned problems associated with the PV/T using c-Si cells can be solved or eased by using amorphous silicon(a-Si)cells.First,the power temperature coefficient of a-Si cells is about-0.1%/℃,to-0.2%/℃,which is significantly lower than that of c-Si cells,making it suitable for operating at medium and high temperature without significant power loss.Second,the electrical properties of a-Si cells have seasonal variation:higher electrical efficiency in summer months and lower electrical efficiency in winter months,which is the opposite of c-Si cells.Besides,the thin-film characteristic of a-Si cells presents lower thermal resistance than c-Si cells.Moreover,a-Si cells can avoid huge thermal stress and overcome the problem of interruptions at fluctuating temperatures in PV/T collectors.One compelling characteristic that limited the development of a-Si cell is the Staebler-Wronski effect,which is related to the light-induced degradation of electrical performance and the creation of defect states.However,a-Si cells can benefit from thermal annealing at the operating temperature higher than 150℃ and have the reversible ability to reduce defect states.The electrical performance of the a-Si cells even can be restored to the original state.Therefore,a-Si cells are promising solar cells and ideal choices for PV/T collectors in medium and high temperature fields.Based on the above advantages of a-Si cells in PV/T systems,it is the first time that a-Si cells deposited on stainless steel have been used in a practical PV/T system in this paper.Experimental and simulation study of the a-Si PV/T system is deeply investigated,and the specific research content is as follows:1.a-Si cells with stainless steel as the substrate is applied to the actual PV/T system.Two sets of a-Si PV/T collectors are designed and processed,and the experimental system is established to conduct preliminary tests on the reliability of a-Si PV/T system.After more than half a year of operation,the performances of the PV/T do not show a significant drop,and there is no technical failure.The feasibility of the a-Si PV/T is preliminarily demonstrated.2.Long-term outdoor tests from December 2017 to June 2019 have been conducted on the a-Si PV/T system.To investigate the long-term temperature-dependent performance,two a-Si PV/T systems are operated at a low temperature of 30℃ and a medium temperature of 60℃.In the initial stage,the difference in the electrical efficiency between the two systems at low temperature and medium temperature is 0.47%,and after the long-term operation,it narrows to only 0.13%.Therefore,a higher temperature can reduce the Staebler-Wronski effect of a-Si cells,and this characteristic proves that a-Si cells are suitable for PV/T applications.The two a-Si PV/T systems operate stably during the long-term operation.Moreover,neither of the two a-Si PV/T systems show detectable technical failures and observable deformations.3.The effects of various parameters on the PV/T system using a-Si cells have been studied theoretically and experimentally.A distributed parameter model is established,and its reliability is verified through experiments.Through simulation research,the effects of thermodynamic parameters(solar irradiance,operating temperature,mass flow rate)and structural parameters(number of copper tubes,cover factor)on the performance are analyzed.In addition,the influence of frame shadow on a-Si PV/T system is analyzed through experiments.4.Two novel PV/T structures suitable for medium and high temperature applications are proposed(micro-channel loop heat pipe PV/T collector and evacuated flat PV/T collector).Distributed parameter models have been established to investigate the medium temperature performance and optimal design.When the inlet water temperature is 90℃,the thermal efficiency of the two novel PV/T collectors can still reach 17.46%and 20.95%,which is significantly higher than that of the traditional flat plate PV/T system(4.76%).5.Based on long-term experimental results,the electrical efficiency function suitable for the light-induced degradation process of a-Si cells is fitted for the first time,and this function is applied to the annual simulation.The annual performance analysis of a-Si PV/T system and evacuated flat plate PV/T system in different regions has been carried out.The annual simulation further confirmed the advantages of the evacuated flat plate PV/T system using a-Si cells in medium temperature applications,and this also breaks the limitation that PV/T can only be used in low-temperature applications due to the use of c-Si cells. |
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