Development And Performance Investigation Of Novel Photovoltaic/Thermal Systems

Photovoltaic/Thermal Technologies(PV/T)combines photovoltaic technology with photothermal technology to generate electricity and thermal simultaneously,with the advantages of low material cost,simple manufacturing process,high space using rate and effective solar energy utilization.However,the contradiction between the temperature requirements of electricity gain and heat gain is a long-standing problem in the PV/T field.Current studies mainly focus on the overall performance optimization rather than the temperature requirements contrary and energy trade-off.This thesis develops 4 novel PV/T systems and experimentally&theoretically studied their electrical and thermal performance.1.For the hot area in South China,this thesis proposes a G-PV/T system which laminates the PV cells to the back of the glass cover.It aims to improve the electrical performance in hot ambient conditions at the cost of a proper decrease in heat gain.And it also helps to overcome the ununiform temperature distribution,thermal stress,and frame shadow in traditional A-PV/T(i.e.,PV/T that laminates PV cells to the absorber).(1)The G-PV/T and A-PV/T are manufactured and experimentally tested and compared.Results show that the all-day electrical efficiency of the G-PV/T system can be 19.7%relatively higher than A-PV/T system,the temperature peak is reduced by 15.6℃,and the final water tank temperature can meet the hot water requirement of residents.(2)The dynamic heat transfer models and the thermodynamic models for the two PV/T systems are established.The thermal stress distributions are simulated and compared.The effect of non-uniform temperature distribution and temperature gradients are explored.The results confirm that the G-PV/T system can effectively cool down PV cells,improve the uniformity of temperature distribution,reduce the thermal stress on PV cells,and improve the reliability of the system.(3)The existing optimization methods of the A-PV/T cannot be directly applied to G-PV/T due to the large changes in the core structure,so this thesis conducts a parametric analysis on several key parameters and finds out that some key parameters have different degrees or even opposite effects on the two kinds of PV/T.(4)Current studies on PV/T are based on univariate discussion,in which case there are limitations in parameter correlation.A comprehensive theoretical prediction on the all-day performance of G-PV/T under a matrix consisting of two key structural parameters(i.e.PV packing factor and air gap thickness)is carried out.The first/second laws of thermodynamics are used to evaluate the system thermal/electrical performance.The coupling relationship between the two parameters for performance optimization is discussed,and the fluctuation tendency and turning point(0.57,28.5%)are founded.Recommendations are also given on how to optimize these two key structural parameters,discussing how and to what extent the external conditions(wind speed,solar irradiation,tank volume)affect the performance of G-PV/T and the movement of coupling relationships.Suggestions are given for energy trade-off and overall performance improvement of the system.2.The crystalline silicon cells commonly used in PV/T systems have a high power temperature coefficient(-0.4%/℃～-0.5%/℃),which shows significant performance attenuation at high temperatures.This thesis proposes to use temperature-insensitive solar cells-Cadmium telluride thin-film PV modules(CdTe)to replace the silicon cells for the first time.(1)CdTe-PV/T systems with different transmittances are designed,assembled,experimentally tested and compared with Poly-Si-PV/T.(2)Then the mathematical model of the G-PV/T proposed above is adjusted according to its new structure and then the parametric discussions of the CdTe-PV/T system for performance optimization are carried out and find out that the choice of heat-absorbing plate coating material is also important.In contrast to traditional A-PV/T systems,increasing the emissivity of the absorbing plate can significantly optimize the performance of this system.When the surface of the heat-absorbing plate is changed from selective coating to black TPT,its primary energy-saving efficiency is increased by 7.4%.3.For the cold area in North China,this thesis proposes a PV/T collector with vacuum double glass as the cover.It can reduce the top heat loss and enhance thermal performance by daylight,and putting off the water freezing and branch pipes cracking on cold nights.Current studies on PV/T pay little attention to the top heat loss and frizzing process in water pipes.(1)The proposed system is experimentally tested during the daytime and then tested in an Enthalpy Difference Laboratory,compared to single-glass PV/T.The vacuum double glass reduces the top heat loss of the PV/T system by about 35%,putting off the icing start time by 1 hour and the complete freezing time by 0.78 hours.(2)The corresponding optical model,heat transfer model and enthalpy model are established and verified.(3)A 24-hour simulation of the two PV/T systems is carried out for three typical cities and weather conditions in Hefei,Beijing and Urumqi,and their all-day electrical/thermal performance and freezing process are evaluated.It is found that vacuum double glass PV/T can effectively prevent PV/T from freezing in the cold winter by adjusting the structural parameters in Hefei and Beijing areas.(4)Finally,the impact of several structural parameters(water pipe diameter,glass extinction coefficient,branch pipe spacing,PV packing factor)on the daytime performance and nighttime antifreeze performance of the two systems are predicted,and optimization suggestions are given.4.For cold areas,changing the cooling channel and working fluid is another effective way against freezing besides changing the glass cover.This thesis proposes a novel LT-PV/T(Loop thermosiphon PV/T)with a new concentric copper tube heat exchanger as the condenser.It can avoid the challenges of airtightness risk and uniform return of working fluid in the large-scale combination of PV/T application of the existing LT-PV/T systems which use immersion coil heat exchangers.(1)The prototypes of the new LT-PV/T systems are designed and produced,and their performance with three filling ratios(26.5%,34.8%,43.2%)and three kinds of working fluids(water,ethanol and R134A)are explored.(2)Based on the experimental results,semi-empirical system efficiency models are established to further evaluate the performance of the system.The typical thermal efficiency and typical primary energysaving efficiency of LT-PV/T system with 40%filling R134a are 58.0%and 73.0%,respectively,which performs well and has obvious advantages compared with the published existing LT/LHP-PV/T or collector.(3)The semi-empirical system efficiency models are then used to predict the annual operating performance of LT-PV/T systems in South China,including a single LT-PV/T collector and the combination utilization of 4 LT-PV/T collectors.This research aims to alleviate the temperature requirement and trade off the energy production.It develops four novel PV/T system and studies their electrical/thermal performance and optimization strategies,filling the gap in this aspect of research,providing reference ideas for further research and development and largescale application in the field of PV/T,and playing a positive role in promoting sustainable energy development and achieving carbon neutrality.