Effect of fixed heat preservation quilt on indoor light heat environment and yield in solar greenhouses

Solar greenhouse is one of the most important horticultural facilities in China. A great contribution has been made to produce fruits and vegetables in winter in northern China. Among them, the material of the south roof is usually plastic films with high light transmittance. The heat preservation quilt can also be used with better thermal insulation performance on the south roof, in order to reduce the heat loss in the low temperature outdoor environment at night. However, artificial management cannot fully meet large-scale production, due to the time-consuming and laborious in the large area of heat preservation quilt. Therefore, the heat preservation quilt can be accomplished using rolling machine at present. A fixed heat preservation quilt is usually laid between the rolling machine and the covering plastic film, in order to prevent the rolling machine from damaging the covering plastic film of the south roof during actual work. Specifically, the fixed heat preservation quilt is located in the center of the east-west length of the solar greenhouse. The sunlight can be blocked into the inside solar greenhouse in the daytime. The resulting shadow can move in the solar greenhouse over time, inevitably leading to the indoor light, temperature distribution, and crop yield. In this study, a calculation model was proposed for the shadow width of the fixed heat preservation quilt, according to the theory of direct solar radiation. A series of tests were also performed on the indoor light and temperature under the fixed heat preservation quilt. The experimental solar greenhouse was selected in Hongke Farm (39.6°N, 116.0°E), Fangshan District, Beijing. The results show that the average photosynthetic photon flux density (PPFD) was 198 μmol/(m²·s) under the fixed heat preservation quilt. PPFD gradually increased with the center of the fixed heat preservation quilt moving on both sides. The average PPFD was greater than 400 μmol/(m²·s) at a position greater than 3.6 m from the center of the fixed heat preservation quilt. Meanwhile, the light cumulant at the distance of 4.8 m increased by more than 130%, compared with the directly underneath fixed heat preservation quilt. In the daytime, the indoor air temperature, wall temperature and soil temperature directly below the fixed heat preservation quilt were 2.2, 5.8 and 2.3 ℃ lower than those on both sides of the quilt. Solar energy was collected and stored in the daytime, and then released heat at night, because the walls and the soil were the heat storage media. As a result, the higher temperature at night was achieved in the walls and the soil that received strong sunlight in the daytime. At night, the maximum wall temperature and the soil temperature directly below the fixed heat preservation quilt were 1.2 and 1.3 ℃ lower than those on both sides of the quilt. There was 14.3 kg average yield reduction of tomato in each furrow under the fixed heat preservation quilt, which was 36.2% than before. The average reduction of plant stem thickness was 2.0-4.0 mm. The shadow width was mainly affected by the illumination time, the geographical location, the orientation and the structure of the solar greenhouse. The shadow width decreased first and then increased over time in the daytime. The shadow width was the smallest at noon. The average shadow width of the solar greenhouse in the western region was larger than that in the eastern. Once the orientation of the solar greenhouse gradually changed from south-west to south-east, the shadow width decreased in the morning and increased in the afternoon. When the greenhouse azimuth was in range of -9° to 9°, the maximum difference of the average shadow width was less than 10.0%. Light and temperature are the important influencing factors on the normal growth of crops. Therefore, it is necessary to rationalize the management of the fixed heat preservation quilt.