Comprehensive Energy Saving Performance Of Building Fa?ade Integrated Photovoltaics

The building energy consumption accounts for 30%of the total energy consumption in China,building energy sustainability has become a major component of achieving sustainable development.Building energy demands has greatly reduced through passive design and the application of high efficient HVAC equipment,on this basis,renewable energy is required to supply energy for building.Photovoltaic(PV)as an important utilization form of renewable energy in building can realize the organic combination with buildings.At present,most of PV modules are installed on the roof,however,for the urban construction,the building facade the area that can be used to install photovoltaic modules is larger than the roof,and the energy saving potential is huge.When PV modules are installed on the building facade,PV will work at very high temperatures due to the limitation of installation conditions,which will affect the power generation of PV modules,at the same time,the thermal resistance of the wall will increases when PV modules are installed,and PV block the direct sunlight on the wall,which affects the cooling and heating load of the building.Based on the above analysis,this paper studies the comprehensive energy-saving performance of photovoltaic modules installed on building facades(BIPV)in different air gap depth,which include the power production of PV and cooling and heating load of the building,the main research includes the followings:Firstly,the power generation performance of BIPV and it’s influence mechanism on the building heating and cooling load are theoretically analyzed,and a numerical model is developed to predict the thermal performance of PV modules,which include the heat transfer process of PV modules coupled with the wall and the convective heat transfer in the air gap,and was verified by experimental.A preliminary analysis was made on the comprehensive energy saving of BIPV.Secondly,the power generation,heat production and the influence of heat transfer on BIPV by installing monocrystalline silicon(m-Si)double glass photovoltaic modules on the south fa?ade in Beijing is investigated through experiment.The mathematical model is verified by photovoltaic power generation and heat gain of the air gap,the results shows highly agreement between simulated and experimental values.Experiment results show that:in a sunny day in winter,the daily power production and heat gains of per square meter m-Si photovoltaic module is 0.58 k Wh/(m~2?d)and 0.96k Wh/(m~2?d).In addition,compared with conventional wall without PV fa?ade,PV fa?ade system can decrease outside surface temperature of the wall by 27.9℃in the daytime,while increased by 4.6℃at night,the heat transfer through the per unit area increased by 2.55 Wh/m~2.According to the experimental results,when sunny day,the photovoltaic modules installed on the building facade increase the thermal load of the building,and the heat production of the modules can be used for building heating or preheating the fresh air.Then,a numerical model based on Transient System Simulation Program(TRNSYS)package was developed to assess the thermal and energy performance of PV fa?ade system in different air gap in Beijing,which has been verified using experiment result.The results show that:(1)The power generation of PV modules increases firstly and then decreases slightly with the increase of the air gap depth;(2)The influence of photovoltaic modules on heating and cooling load of buildings are different,when the air gap is small,the heating load of the building has been reduced,with the increase of the air gap depth,the heating load of the building increases first and then decreases slightly,while the cooling load changes in reverse,and the heating load changed more than the cooling load.And according to the PV influence of the heat transfer on the wall during the day and night,it is found that PV modules are suitable for application of residential buildings dominated by heating load and public buildings dominated by cooling load;(3)In winter,PV module can heating the air between the module and the wall at a temperature 5～10℃higher than the ambient temperature,which can be used to heating the building or preheat the fresh air.According to the above analysis,PV fa?ade system should be installed with an appropriate air gap according to the type of PV modules and make full use of the heat gain from air gap to achieve the optimal comprehensive energy saving performance.Finally,the annual comprehensive energy saving performance of PV modules on the south facade of buildings in different climate resource regions in China was studied through TRNSYS simulation.The results indicate that:(1)Compared with the ambient temperature,solar irradiation has a greater impact on the power generation of PV modules.In addition,the optimal depth of air gap for power generating is different in different regions.(2)For the building consumption dominated by heating,the small air gap can reduce the annual energy consumption of the building,for the building consumption dominated by air conditioning,the appropriate increase of air gap can reduce the annual consumption of the building.(3)For the buildings which is demand for heating,using PV thermal performance to heating the building or preheat the new wind can reduce the heating load about 7%～15%.Through the above research,we can find that PV fa?ade system in different areas have different air gap to get the largest comprehensive energy-saving performance,different air gap should be used for different project.And the mathematical models in this paper can be integrated into the performance-based design research tools,subject to local climate conditions and the specific project,to solve the appropriate air gap,which has reference value and guiding significance in the practical engineering.