Research On Photoelectric&Thermal Performance Of Photovoltaic Direct-expansion Heat Pumps With Various Novel Combined Structures

In recent years,sustainable energy has been vigorously developed with the"carbon peaking and carbon neutrality" goals.The exploration&ultilization of solar energy is one of the most crucial components.Photovoltaic direct-expansion solar-assisted heat pumps(PV-SAHP)have received wide attention due to its superior PV cooling and higher evaporating temperature,showing excellent comprehensive system performance.But the present research mostly focuses on optimizing PV operating temperature.There is no combination with thermoelectric generator(TEG)to realize secondary electrical output.Besides,most of them are flat PV evaporators with low surface irradiance,unit power output,and large PV area which limits the selection of PV materials.Using expensive PV cells with high efficiency is difficult.Concentrated photovoltaic(CPV)can solve the problems,but the high temperature caused by the thermal conversion of the high-density and low-energy photon is unneglectable.To cool PV and improve solar energy utilization,electricity&heat cogeneration structure or further combination with TEG that can directly convert solar heat into electricity is proposed.Effective cooling could largely improve the output of mentioned systems,but mostly,cheap air and water are used as cooling fluid with unbreakable lower temperature limitation of water temperature in cogeneration systems.However,the evaporator could provide better cooling:Ⅰ.Provide much lower temperature than the water temperature,which greatly reduces the temperature of CPV&TEG and improve the reliability&stability;2.Supply almost unchanged cooling temperature while absorbing a lot of heat due to the phase change process,providing more uniform cooling;3.Generate water with higher temperature while the evaporating temperature is changeless,achieving more durable and stable cooling.Effectively avoid the above contradictions.In application,PV evaporators and air-source evaporators are always combined to improve the stability,which is called dual-source heat pump system.But in existing papers,different evaporators are always independent,resulting in low utilization of space and energy.Considering the status&limitations of the present PV-SAHP and CPV/TEG systems,this thesis firstly proposes a comprehensive system combining CPV,TEG and DX-SAHP,and a composite heat exchange structure with three-dimensional design of CPV and air-source heat exchanger.The photoelectric&thermal performance of the photovoltaic direct-expansion heat pumps with various novel combined structures is analyzed experimentally and theoreticaly.The specific points are listed as:1.It is the first time to combine the evaporator with the Fresnel concentrated photovoltaic and TEG(FPV/TEG-SAHP)to achieve more durable and stable cooling,enabling broaden selection of PV materials and secondary electricial conversion in PVSAHP system.The novel structure is introduced.The models of each component in the system are established using lumped parameter method and the distributed parameter method.The refrigerant state,electrical/thermal/exergy efficiencies and performance coefficient(COP)of the system under different ambient conditions are analyzed in detail.The influence of TEG parameters on the system performance is discussed based on the quality factor characteristics of TEG materials.2.Based on the coupling characteristics between CPV and TEG,the benefit of secondary electrical output and the harm of hindering PV cooling are compared.The advantages&disadvantages of the novel structure are discussed in contrast to traditional flat PV-SAHP system.The models of two-dimensional composite parabolic concentrating photovoltaic evaporator(CPC/PV)and cross composite parabolic concentrating photovoltaic evaporator(CCPC/PV)were established,and the performance characteristics of heat pumps with different CPV evaporators are compared and analyzed under different boundary conditions.3.A novel double-condensing PV-SAHP system combining micro-channel heat pipes(MCHP)and TEG is firstly constructed.The TEG is designed to convert collected condensing heat into electricity,avoiding hindering PV cooling and simultaneously promoting electrical output.Based on the novel composite condenser and traditional water coil condenser,the mathematical model of the double-condensing system is constructed,and the test rig is built to test system performance in outdoors.Mathmatical prediction and test results are concluded and analyzed.4.A multi-function dual-source direct-expansion heat pump system combining CPC/PV and fins is firstly proposed.The novel 3D comprehensive design of dualsource evaporator realizes the layer and partial utilization of limited area and space energy,improving the total output of evaporator and system stability.The operating principle is introduced,the mathematical model is constructed,the test rig is built,and the performance in outdoors is tested and analyzed.The mathematical model is verified.The refrigerant state&operating performance of each part of the system under different environmental conditions are analyzed.5.Combining experiments and simulation predictions,the photoelectric&thermal performance of concentrating and non-concentrating dualsource heat pumps are compared,proving the superiority of the proposed novel structure.Further improvements is realized by optimizing the throttling process and matching more appropriate refrigerant flow rate.The novel system is compared with traditional heat pump under different two-dimensional environmental coordinate systems.The comprehensive impacts of multiple ambient parameters on the systems are discussed,and the superior operating environmental areas of the novel system are concluded.6.The concentrator can not only help increasing high-quality energy output by converging irradiation on PV,but also help shielding the condensing fins in cooling mode,which reduces condensing temperature and increases cooling COP.Based on experiments and modeling,the improvement of condenser shielding is discussed under conditions of fixed evaporating state or fixed throttling process by controlling variables Internal relationship and specific performance of refrigerant parameters at different parts are all explored in depth,and so as the optimizing mechanism.The impacts of wind speed,water temperature and ambient temperature on the cooling improvement are all analyzed.