Study On Comprehensive Energy-saving Of Shading And Photovoltaics Of Roof Added Pv Module

As a kind of important renewable energy,solar energy photo-thermal and photovoltaics utilization plays a significant role in reducing building energy consumption.Roof added PV module has the potential to become a major way of solar energy utilization not only because of electricity production to reduce extra energy requirement,which is obviously the largest beneficiary aspect,but also for the shading effect on thermal performance of roof.The radiation heat flux on the thermal boundary condition of conventional roof is mainly composed of the solar radiation and the long wave radiation.After adding PV module,due to its own characteristics of photovoltaics and thermal response,those two radiation heat flux will be reduced to a certain extent and the radiation heat transfer between the rear surface of PV panel and roof will be introduced.In spite that roof is one of the main sources of load,they added PV because their shading and power generation reduce the building energy consumption greatly,and even become the building’s gain components.In this paper,integrated energy-saving of shading and photovoltaics of roof added PV module was studied.Mechanisms of the two were analyzed and combined into the boundary condition of roof.Then the influences of shading on incident solar radiation,roof longwave radiation and the heat transfer between PV backsheet and roof were discussed.Furthermore,a model was set up with TRNSYS to simulate the electricity generating performance.Combined with the roof heat transfer,taking sol-air temperature as thermal boundary of roof,the heat gain,heating and cooling load were studied by using radiation time series method.Besides,according to synthetical benefit,integrated energy efficiency of roof added PV module was put forward.In addition,a comparative experiment was carried out in Shaanxi province to study the thermal performance of roof added PV module and the results showed that the proposed thermal model of PV roofs can be considered reliable.Eventually,based on the solar energy resources levels of regional division and thermotechnical design division standard,the typical city was selected to analyze the seasonal and regional differences of PV roofs,and the adaptability of roof added PV module was finally obtained.The main conclusion were listed as follow:(1)The seasonal and regional differences in thermal response of PV roofs are exist.More concretely,in some areas where solar energy resources level above IV,temperature of roof surfaces underneath the PV panels is significantly lower than that of the exposed roof in the daytime.At night,the roof surfaces under the solar panels remained warmer.However,in some zones where the solar energy resources level below IV,the temperature of exterior surface of PV roof is higher than conventional roof’s day and night.(2)The seasonal and regional differences in heat gain of PV roofs are also exist.In summer,in some areas where there are large difference in temperature between day and night and strong solar radiation,overhead added PV module can play a larger role in reducing roof heat gain and cooling load,while in areas where the temperature difference between day and night is small and the average temperature is high,added PV module conversely has a negative impact which is undesirable for energy consumption.In winter,total heat loss and peak heating load of roofs under overhead PV panels are both attenuated significantly.(3)In summer,shading benefit of tilted overhead PV roof is highest among the three while integrated energy efficiency of flat overhead PV roof is supreme,but just the opposite in winter.Therefore,flat overhead photovoltaics roof is more suitable for summer and tilted overhead photovoltaics roof is more favorable in winter.The study on synthetical benefits of shading and photovoltaics of roof added PV module can play a guiding role in estimating energy consumption of building integrated PV,designing,operation and maintenance air conditioning system,which also can provide a theoretical basis for designing PV roof in different regions.