Analyzing light and thermal environment and insulation performance of a multi-span greenhouse with external insulation

Abstract: Large-scale greenhouse can be expected to serve as the future direction in the horticulture industry. However, the multi-span greenhouses can consume a large amount of energy for heating in winter in northern China, resulting in low profitability and sustainability. In this study, a multi-span greenhouse was designed with large roofs and external insulation, in order to reduce the heat loss of the greenhouse roof. The external insulation system was innovatively applied to the multi-span greenhouse. The greenhouse design was expected to improve thermal insulation performance and reduce heating energy consumption. A field test was carried out in Shouguang, Shandong Province, China. Taking the Venlo-type multi-span greenhouse in the same area as a reference, a systematic investigation was made on the light and thermal environment, thermal insulation performance of the multi-span greenhouse with external insulation. The experimental data were analyzed from continuous 40 winter days. The results show that: 1) The average solar radiation was 152 W/m2 above the crop canopy inside the tested greenhouse during the day (10: 00-16: 00), and the total light transmittance was 40%, which was 7% higher than that of Venlo type multi-span greenhouse. The best daylighting was found in the middle of the greenhouse span, due to the influence of the gutter. 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 with the middle. 2) There was the folded in turn for the external thermal blankets covering the east and west greenhouse roofs after the sun rose. Specifically, the indoor air temperature rose at 1.9 ℃/h from 09:30 to 12:00, which was 0.3 ℃/h slower than that of the Venlo-type one. However, the sudden drop in the air temperature of the multi-span greenhouse with the external insulation was reduced by 0.3℃ within 10 min after folding insulation devices. The tested greenhouse was heated by the internal air circulation, with the air coming out from the ground and then returning to the equipment room through the 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 ℃, without exceeding 1.0 ℃ in the vertical direction. The uniform distribution was observed in the horizontal temperature of the multi-span greenhouse with the external insulation. The vertical temperature difference was smaller than that of the Venlo-type one. 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 12.8-21.0℃. The average heat flux of the glass roof that was covered with the external thermal blanket was 50.0-97.7 W/m2, while the single-layer glass roof was 217.6-367.9 W/m2. The greenhouse covering with the external thermal blanket was reduced by 75% in the heat loss of the glass greenhouse roof. At the same time, the average heat flux was 141.1-232.2 W/m2 in the Venlo-type one with double-layer indoor thermal screens in use. The roof heat loss of the multi-span greenhouse with the external insulation was reduced by 36%, indicating a better insulation performance. The mean heat energy input of the multi-span greenhouse with external insulation was measured to be 74.5 W/m2 during the heating period, maintaining an average temperature difference between indoor and outdoor air of 17.4 ℃. Thus, the energy consumption of heating the multi-span greenhouse with the external insulation was low. Finally, the fitted influence of indoor and outdoor air temperature differences on the heat fluxes of greenhouse roofs was presented, and the tested greenhouse showed better goodness of fitting. This finding can provide a new type of greenhouse structure for the low-carbon and energy-saving production of multi-span greenhouses. A data basis can also be offered for the optimal design and engineering application of the multi-span greenhouse with external insulation.