Heat Loss Mechanism And Characteristics Of The Heat Dissipation Side In Solar Energy Photovoltaic-Thermal System

Due to the increasingly serious energy crisis and environmental problem,photovoltaic technology which realizes the solar energy conversion is one of the most important ways of renewable energy utilization.Among them,photovoltaic-thermal system(PV-T)combining electricity generation and heat collection together takes the unutilized heat away in time.Then the working temperature of PV cells is controlled and the comprehensive efficiency of solar energy is obviously enhanced.In the field of solar energy utilization technologies,PV-T has become one of the research focuses.To improve thermal performance of the system,laying a glass cover upon the base PV panel is mostly adopted,forming an enclosure in the heat dissipation side of PV-T system.Because of the existence of external environment,there are various modes of heat dissipation losses,which directly influence the comprehensive efficiency of the whole system.Therefore,research related with the heat dissipation side is the key and difficult point to improve the system performance.Additionally,if converting the collected heat in PV-T system into electricity again with the help of thermoelectric generator,the photovoltaic-thermoelectric system(PV-TE)is developed.Recently,PV-TE receives ever-increasing attention,thanks to the efficient development of thermoelectric materials.Aiming to the heat dissipation side of PV-T system,a systematic study concerned with the heat loss mechanism and characteristic was carried out in this dissertation.Firstly,for the unglazed PV-T system,the impacts of tilt angle,environmental wind and different heat boundary conditions on the convection heat transfer of PV panel were investigated numerically.Accordingly,the convective Nusselt number correlation coupling multiple parameters was obtained.Meanwhile,the relevant experiment was set up to reveal heat dissipation mechanism under the natural convective and windy conditions.Electric heating method was used to simulate the heat loss of PV panels.Then,taking the glazed PV-T system as the object,two-and three-dimensional numerical simulations were separately performed to study the heat transfer process in the shallow enclosure constituted by the glass cover and PV panel.In two-dimensional simulation,the effects of PV cells arrangement and optical constants on the flow and heat transfer were discussed.While in three-dimensional simulation,the influences of initial condition and thermal boundary condition on flow instability were explored.Besides,the flow pattern transition and features from steady state to chaotic state were analyzed gradually.Based on above works,two-dimensional numerical model which contains both inside and outside the enclosure was established to get insight into the coupled heat transfer effect on some parameters,such as Nusselt numbers inside and outside the enclosure,total heat loss coefficient and hot spot temperature of PV cells.In addition,partition was introduced and the heat transfer performance between partitioned and non-partitioned enclosures was compared.Finally,the adoptions of the glass cover and nanofluid cooling were proposed in the PV-TE system.With the help of numerical results,the mathematic model was established to compare electrical performance of the system with and without a glaze cover,also,to discuss the coupling effect of several important parameters on efficiency.This dissertation,on the one hand,could enrich and develop the theory of heat dissipation loss in PV-T system,on the other hand,it provides scientific basis for the structural design and performance improvement of PV-T and PV-TE systems.The main research results are as follows:(1)For heat dissipation in the unglazed PV-T system,three-dimensional numerical results indicate that,compared with the constant heat flux,boundary condition considering the photovoltaic conversion has small effect on convection Nusselt number(Nuc),while a completely opposite distribution is shown on the released heat flux contours.With tilt angle varying,Nuc decreases firstly and then rises when exceeding a critical point,and the critical point shifts to a larger angle at higher wind velocity.Owing to the combination of wind incident angle and wind yaw angle,the effect of wind direction changes with tilt angle.At 30° and 45°,wind direction mainly has a positive impact on Nuc,while Nuc decreases firstly and then rises at other tilt angles.As for heat flux,the obvious effect on convection heat transfer appears only at smaller wind velocity,but it notably enhances radiation heat exchange whatever the wind velocity.Moreover,experimental results find that,under the natural convective condition,Nuc supplied with the uniform heat flux is always larger at smaller tilt angles,while the difference among various angles has shrunk with the increase of heat flux gradient parameter.As for the windy enviroment,Nuc is higher on the windward side.And on leeward side,the decreasing tendency of Nuc with tilt angle has been observed.(2)For the shallow enclosure in the glazed PV-T system,two-dimensional simulation shows that,air flow maintains steady state in most cases.Radiation heat transfer occupies the major proportion,which could be reduced by increasing refractive index or decreasing absorption coefficient.However,three-dimensional simulation points out that,the results attaining unsteady state are in the majority.Depending on the initial condition,hysteresis of the flow state transition may occur near the critical stable angles.With the decrease of tilt angle,the flow pattern gradually undergoes the transformation from periodic flow to multimodal,and finally the chaotic flow.(3)For the coupled heat transfer inside and outside the enclosure,two-dimensional numerical results reveal that,the hot spot temperature distribution is affected by the tilt angle and wind velocity simultaneously.Different from the sudden change characteristic of Nuc inside the enclosure,total heat loss coefficient achieves smooth variation within most of the tilt angle range.The partition effectively reduces the maximum temperature of PV cells and improves the temperature uniformity,also the hot spot temperature difference among PV cells has decreased at higher tilt angle.Nevertheless,the influence of partition length on heat transfer differs.When the longer partitions are employed,Nuc inside the enclosure has enhanced within most of the tilt angle range,while the total heat transfer ability is relatively suppressed.(4)For the theoretical analysis about PV-TE system,comparison between glazed and unglazed systems indicates that,as concentration ratio increases,the system electrical efficiency could increase,decrease or remain unchanged,determined by the temperature coefficient of PV cells and the figure of merit of thermoelectric material.When the figure of merit is larger,the higher efficiency for glazed system is achieved after a critical concentration ratio.Further,nanofluid has effectively increased the electrical efficiency as comparing with water.Nanofluid flow rate always has a promotion effect on the electrical efficiency,while the behavior of wind velocity is complex.When the temperature coefficient is larger but the figure of merit is smaller,the increase of wind velocity improves system efficiency;as the temperature coefficient is decreasing,it is always presented as the negative relationship between them.