Warming and strawberry cultivation effect of building heat storage walls in assembled solar greenhouse

To solve the problem of low temperature inside the solar greenhouses during the winter nights in Northwest desert area of China, a case study was conducted on the assembled solar greenhouse, which is widely used in Hetian area, Xinjiang Province. Three types of walls with different building materials, i.e., the brick wall, block wall, and the sand-filled block wall, were built in the north sides of the solar greenhouses with the purpose to improve its heat storage properties and consequently to increase the indoor temperatures during the winter nights. Temperatures of the north walls and indoor environments of the solar greenhouses were continuously monitored with the TC100 sensors. The effect of different heat storage walls on strawberry cultivation inside the greenhouses was evaluated. The mass weight and solid soluble content of the strawberry were measured by ACS-30 balance and LH-B55 digital refractometer, respectively. Results showed that the heat storage performances of the three types of north walls with identical volumes and dimensions were as followings: Sand-filled block wall > block wall > brick wall. The internal temperatures of the three heat storage walls were 9.7, 10.4, and 11.3 ℃, respectively. The brick wall, block wall and sand-filled block wall averagely increased the indoor temperatures of the assembled solar greenhouses by 2.2, 2.9, and 3.8 ℃ at 07:00 in the morning, respectively. The indoor temperatures of four greenhouses are 7.4, 9.6, 10.3, and 11.2 ℃ respectively. The same planting management technology was adopted for the three tested solar greenhouses. The flowering dates of the cultivated strawberries using the three heat storage walls were 7 days, 11 days, and 14 days earlier than those of the original solar greenhouses without any modifications, and the maturities of strawberries were 14 days, 17 days, and 20 days earlier, respectively. The strawberry yields were 24.2%, 30.1%, and 33.4% higher, and meanwhile, the soluble solid contents of the strawberry in the treated groups were 1.4 percentage points, 2.1 percentage points, and 2.6 percentage points greater than those in the original fabricated greenhouses. The average soluble solids content of strawberry cultivated in three greenhouses reached 11.5%, 12.2%, and 12.7% respectively. The results again verify the importance of thermal properties of the north wall on the microenvironment of the solar greenhouses. Among the modifications, the mortar block wall is more effective than the brick wall, providing a better environment for strawberry growth in the greenhouse. The heat storage capacity of the new tested walls was consistent with the calculated heat load of the assembled greenhouses, indicating that the newly built walls in the greenhouse without supplemental heating systems were the driven source for the indoor environment improvement during the nighttime. The thermal load of fabricated solar greenhouse envelope was 70.3 MJ/h. The calculated value of heat emitted from the heat storage wall into the greenhouse was 91.1和119.3 MJ, respectively. This paper provides a reference for the heat storage material selection and structural design, as well as a basis for the application and research of mortar blocks in assembled solar greenhouses in the desert area of Northwest China.