Evaluating Light And Thermal Environment And Insulation Performance Of A Multi-span Greenhouse With External Insulation

Large-scale greenhouse is the development trend of the greenhouse horticulture industry.However,in northern China,multi-span greenhouses consume a large amount of energy for summer cooling energy consumption is very large,and the operating cost is high,resulting in poor profitability and sustainability.For reducing greenhouse roof heat loss purposes,this study designed a multi-span greenhouse with large roofs and external insulation,innovatively applying the external insulation system to the multi-span greenhouse.Such a greenhouse design was expected to improve thermal insulation performance and reduce heating energy consumption.The light and thermal environment and thermal insulation performance of the multi-span greenhouse with external insulation were tested in Shouguang,Shandong Province,taking Venlo-type multi-span greenhouse in the same area as a reference.The experimental data from continuous 40 winter days were analyzed.The main research contents and results are as follows:(1)During the day(10:00-16:00),the average solar radiation above the crop canopy inside the tested greenhouse was 152 W/m2,and the total light transmittance was 40%,which was 7%higher than that of Venlo type multi-span greenhouse.Influenced by the gutter,the daylighting in the middle of the greenhouse span was the best.And the solar radiation intensity at the east and west of the greenhouse span and under the gutter was reduced by 17%,29%,and 46%respectively compared to that in the middle.(2)After the sun rose,the external thermal blankets covering the east and west greenhouse roofs were folded in turn.From 09:30 to 12:00,the indoor air temperature rose at 1.9℃/h,which was 0.3℃/h slower than that of the Venlo-type multi-span greenhouse.However,the sudden drop in air temperature of the multi-span greenhouse with external insulation was reduced after folding insulation devices.The tested greenhouse was heated by internal air circulation,with air coming out from the ground and then returning to the equipment room through its inner side windows.During the heating period(20:00-07:00),the average temperature difference of indoor air in the horizontal direction did not exceed 1.2℃,and that in the vertical direction did not exceed 1.0℃.The temperature decreased with the increase of height from 1 m to 6 m,and rose slightly at 7 m.During the day when the greenhouse is closed and the insulation is fully collected,the average temperature difference in the horizontal direction of the indoor air in the outdoor insulation greenhouse does not exceed 0.4 ℃,and the maximum average temperature difference in the vertical direction is 3.8℃.The temperature increases with the increase of height from 1-6 m,and falls back and gradually decreases at 7 m.The horizontal temperature distribution of the multi-span greenhouse with external insulation was uniform,and its vertical temperature difference was smaller than that of the Venlo-type multi-span greenhouse.(3)The average air temperature at nighttime inside the multi-span greenhouse with external insulation ranged from 13.1 to 16.1℃,and the average temperature difference between indoor and outdoor air was 12.8 to 21.0℃.The average heat flux of the glass roof covered with the external thermal blanket was 50.0-97.7 W/m2,and that of the single-layer glass roof was 217.6 to 367.9 W/m2.Covering with the external thermal blanket reduced the heat loss of the glass greenhouse roof by 75%.At the same time,the average heat flux of the Venlo-type multi-span greenhouse with double-layer indoor thermal screens in use was 141.1232.2 W/m2.Compared to the Venlo-type multi-span greenhouse,the roof heat loss of the multi-span greenhouse with external insulation was reduced by 36%,Since the average temperature difference between the two greenhouses does not exceed 0.6℃,the external insulation greenhouse in this study is compared with the traditional greenhouse.indicating a better insulation performance.During the heating period,the mean heat energy input of the multi-span greenhouse with external insulation was measured to be 74.5 W/m2,maintaining an average temperature difference between indoor and outdoor air of 17.4℃.Thus,the energy consumption for heating the multi-span greenhouse with external insulation was low.Addition,the influence of indoor and outdoor air temperature differences on the heat fluxes of greenhouse roofs was fitted.(4)After the external thermal insulation of external insulated greenhouse is put up in fine weather,indoor solar radiation will increase,indoor air temperature also rises but the relative air humidity drops.After it is covered with outer insulation quilts,the indoor solar radiation drops to 0 and the air temperature drops,but the relative air humidity rises,which then started to heat the greenhouse at night.After that,the indoor air temperature rises and the relative humidity drops gradually.After the outer insulation quilt is put up in snowy days,the solar radiation gradually increases,but the indoor air temperature does not drop suddenly.However,the temperature rises slowly.Around the noon,the solar radiation of the greenhouse reached the maximum 87.9 W/m2.At that time,the indoor air temperature reached 16.8℃.When the greenhouse is covered with an outer insulation quilt,the temperature can drop to the lowest of 12.9℃ and the relative air humidity in the room can reach the highest of 93%.This study provides a new type of greenhouse structure for low-carbon and energy-saving production of multi-span greenhouses and offers a data basis for the optimal design and engineering application of the multi-span greenhouse with external insulation.