| With the continuous development of new energy sources,solar air flat plate collectors are used more and more widely in the heating field.They have simple structure and strong durability.However,they still have the problems of low heat collection efficiency and low heat collection when used in cold areas in winter.In order to further improve this problem,this paper develops an active solar collector for greenhouses,which mainly includes the following contents:1.Make solar flat panel heat collectors using air as the heat transfer medium to directly convert solar radiant energy into heat energy and transfer it to the air medium.It can be used directly,without conversion,clean energy,and convenient maintenance.Compared with other collectors,there is no need to consider the freezing problem caused by low temperature in winter.Choose the three factors that have the greatest impact on the heat collection efficiency,namely the type of cover plate,the shape of the heat-absorbing plate structure,and the material of the insulation layer.Choose two levels for the type of cover plate and the shape of the heat-absorbing plate structure,and choose the material of the cover plate 4.At each level,a mixed orthogonal experimental design was carried out,and the relationship between the selected factors and the experimental indicators was analyzed.The test results show that the average heat collection efficiency of the collector with double-layer hollow glass and reflector as the cover plate reaches 50.70%,and the average heat collection efficiency of the collector with the V-shaped heat-absorbing plate is higher than that of the parabolic heat-absorbing plate collector 6.25%,the average heat collection efficiency of the heat collector with polyethylene board as the insulation layer reaches 51.97%.After analysis,it is concluded that the structure of the heat-absorbing plate is the most important factor that affects the heat collection efficiency of the collector.Through further analysis,it is obtained that when a double-layer hollow glass cover plate and a reflector are used,the structure of the V-shaped heat-absorbing plate and the heat preservation material are used for the collector.When the vinyl board is used,the heat collection performance of the heat collector is the best,and the average heat collection efficiency in winter in Changchun City can reach between 40% and 62%.2.By comparing the instantaneous efficiency curve,it can be seen that when the cover material is the same,the heat collection efficiency of the collector using the V-shaped plate is superior,and the heat loss of the collector using the parabolic type is greater.By comparing the same heat collector using a parabolic plate,the heat collector using a double-layer hollow glass cover with a reflector has better heat collection efficiency.Through calculation,it is known that the error between the experimental and theoretical values of the total heat loss coefficient of the collector is within 5%,and the measured heat loss coefficients are all 2.5 W/(㎡· ℃)-4.5W/(㎡· ℃),are less than 6.0 W/(㎡· ℃),in line with national standards.3.With reference to the optimized design results of the collector,a reasonable design condition was determined,and the flat-plate collector with double-layer hollow glass and EVA film as the cover plate was re-made,calculated according to the latitude of Changchun City and the declination of the winter solstice.The optimal installation angle of the collector is between 22°15’-67°15’,and it is arranged on the side facing the greenhouse from north to south,facing due south.The experiment has studied the instantaneous efficiency of the two collectors and the improvement effect of the collector temperature under different ambient temperatures and solar irradiance near the winter solstice.Comprehensive analysis of the thermal efficiency diagrams of the two types of collectors,the following conclusions are drawn: the average heat collection efficiency of the double-layer hollow glass cover collector can be increased by 10%-21% compared with the EVA film collector,and the outlet temperature of the collector can be Increase 21-24 ℃.4.The double-layer hollow glass cover collectors was arranged in a 10 m extension meter,a 8m span heating greenhouse and a comparison greenhouse for 24 h tests.The room temperature measurement point was set 50 cm above the ground in the middle of the greenhouse,and 10,20,30,40 cm underground in the middle of the greenhouse Set the ground temperature measurement points respectively.The results showed that the temperature in the heating greenhouse increased by 3.5℃ on average,and the ground temperature increased by 2.5℃ on average. |
