Study On Heat Loss Of Flat Plate Solar Collector At Different Altitudes

Due to the advantages of large lighting area and stable and reliable operation,flat plate solar collector has become one of the widely used collector types.Accurately obtaining the heat loss coefficient of flat plate collector in different regions is the key to the design and calculation of solar collector system.However,the heat loss coefficient of flat plate collector is mainly based on the experimental results of the standard laboratory under steady state conditions.On the one hand,this method ignores the actual outdoor environment in different regions,and it is difficult to reflect the heat loss characteristics of flat plate collectors under outdoor dynamic conditions.On the other hand,for high altitude areas,special environmental conditions such as low pressure and thin air make the convective and radiative heat loss of flat plate collector change significantly.Therefore,in order to accurately calculate the heat loss of flat plate solar collector in high altitude area and provide calculation basis for the design of solar collector system,it is necessary to study the thermal process and heat loss of flat plate solar collector in different altitude areas.Firstly,the heat loss calculation model of flat plate collector is established by theoretical analysis.On this basis,through numerical calculation,the convective and radiative heat loss characteristics of flat plate collectors at different altitudes under the comprehensive influence of environmental factors are analyzed.Finally,considering convective and radiative heat loss,the variation law of total heat loss of flat plate collector at different altitudes is obtained,and the correction calculation method of total heat loss at different altitudes is proposed.The research process and main conclusions are as follows:(1)The dynamic heat transfer process of the flat plate collector is analyzed,and the thermal network model of the heat transfer process is established.The mathematical model of heat transfer is established according to the heat balance relationship,and then the mathematical model of convective,radiative and total heat loss of flat plate collector is proposed.The simulation calculation model of heat loss of flat plate collector is established by Simulink,and the accuracy of the mathematical model of heat loss is verified by experimental test.(2)The variation of convective heat loss of flat plate collector with air pressure,wind speed,ambient temperature and solar irradiance is analyzed,and convective heat loss at different altitudes is obtained.The results show that convective heat loss process is mainly affected by convective heat transfer coefficient and heat transfer temperature difference.The air pressure or wind speed decreases,the air density or flow rate decreases,so does the convective heat transfer coefficient,and the convective heat loss and heat loss coefficient decrease.When the ambient temperature increases or the solar irradiance decreases,the temperature difference and convective heat transfer rate between the flat plate collector and the environment decrease,and the convective heat loss and heat loss coefficient decrease.For different altitude areas,affected by the above environmental factors,with the increase of altitude,the convective heat loss and heat loss coefficient of flat plate collector decrease.From Xi’an(about 400 meters above sea level)to Lhasa(about 3600 meters above sea level),the convective heat loss coefficient decreases by 1.7 W/(m~2·K)and the convective heat loss decreases by 29.5W/m~2.(3)The effects of air pressure,water vapor pressure,sunshine percentage,ambient temperature,surface temperature and solar irradiance on the radiative heat loss of flat plate collector are analyzed,and the radiative heat loss at different altitudes is obtained.The results show that the radiative heat loss process is mainly affected by atmospheric inverse radiation,ground and collector long-wave radiation;The decrease of atmospheric and vapor pressure,or the increase of sunshine percentage,the decrease of atmospheric inverse radiation,the increase of radiative heat loss and heat loss coefficient.With the increase of ambient temperature,the atmospheric inverse radiation is enhanced.At the same time,the long wave radiation of the collector is also increased,resulting in the increase of radiative heat loss and heat loss coefficient.Rise of surface temperature,increase of ground long wave radiation,decrease of radiative heat loss and heat loss coefficient.Solar irradiance increases,long-wave radiation of collector increases,so do radiative heat loss and heat loss coefficient.For different altitude areas,affected by the above factors,with the increase of altitude,the radiative heat loss and heat loss coefficient of flat plate collector increase.From Xi’an(about 400 meters above sea level)to Lhasa(about 3600 meters above sea level),the radiative heat loss increased by 52.7 W/m~2,and the radiative heat loss coefficient increased by 3.6 times.(4)Based on the variation of convective and radiative heat loss of flat plate collectors at different altitudes,the total heat loss at different altitudes is obtained.The results show that with the increase of altitude,the convective heat loss coefficient of the flat plate collector decreases,and the radiative heat loss coefficient increases.However,there are differences in the variation amplitudes of the two,making the maximum variation amplitude of the total heat loss coefficient reach about 10%.Combined with the laboratory standard working conditions and the total heat loss coefficient of the flat plate collector under the actual outdoor conditions in different altitude areas,the correction calculation method of the heat loss coefficient in different altitude areas is proposed.In this study,aiming at the problem that the heat loss coefficient deviates from the heat loss coefficient under the standard working condition due to not considering the actual outdoor environmental conditions,a correction calculation method for the heat loss of flat plate collector is proposed.This provides theoretical guidance for the selection and design of solar collector system in high altitude area.