Thermal performance analysis of assembled active heat storage wall in Chinese solar greenhouse

Abstract: Solar greenhouse is efficient energy-saving greenhouse which plays an important role in the development of protected horticulture in China. The biggest differences between solar greenhouse and other types of greenhouse are the walls, which act as load-bearing, heat storage and heat preservation. At present, a lot of research has been done on the material and structure of solar greenhouse. In terms of material research, it is generally believed that the performance of composite heterogeneous wall is better than that of single material wall. In terms of structure, many scholars have studied the reasonable thickness of the wall and developed an efficient active heat storage wall structure. Active heat storage wall solar greenhouse has a good heat storage capacity, which is the development trend of solar greenhouse. However, there are some disadvantages such as slow construction speed and high cost. In order to solve these problems, in our research, we simplified the construction techniques of active heat storage wall., The Chinese solar greenhouses with traditional active heat storage wall (G1), backfill-assembled active heat storage wall (G2) and modular assembled active heat storage wall(G3) were constructed, the indoor environments were tested and the thermal performances were analysed. The results showed that under continuous sunny conditions (from 2017-12-30 09:00 to 2018-01-02 09:00), the average temperatures in G1, G2 and G3 were 19.9, 20.1, 20.8 ℃; the night average temperature were 15.2, 16.0, 17.3 ℃ respectively; the night average temperature in G2 and G3 were 0.8 and 2.1 ℃ higher than that in G1; the night minimum average temperatures in G1, G2, G3 were 13.4, 14.7, 15.6 ℃; night minimum average temperature in G2 and G3 were 1.3 and 2.2 ℃ higher than that in G1. Under continuous cloudy conditions (from 2018-01-13 09:00 to 2018-01-16 09:00), the average temperature in G1, G2 and G3 were 12.9, 14.4, 14.3 ℃; the night average temperatures in G1, G2, G3 were 11.3, 12.9, 13.0 ℃, the night minimum average temperatures in G1, G2, G3 were 9.8, 11.5, 11.7 ℃. Therefore, under continuous cloudy conditions, the temperature in G2 and G3 were almost at the same level but were better than that in G1. The temperature analysis results of the coldest month (from 2017-12-22 to 2018-01-21) showed that the average temperature and night minimum temperature in G2 and G3 were higher than that in G1; and the lowest temperature not higher than 8 ℃ were 2 and 1 days in G2 and G3. Therefore, G2 and G3 had stronger ability to resist continuous low temperature than G1 which could meet the heat demand of greenhouse vegetables without extra heating. The overall performance of G3 was better than that of G2, and the overall performance of G2 better than that of G1. Under typical sunny conditions (from 2017-12-31 09:00 to 2018-1-1 09:00), the heat storage thickness of the wall of G1, G2, G3 were 700-800, 800-900 and 700-800 mm; under typical cloudy conditions (from 2018-01-14 09:00 to 2018-01-15 09:00), the heat storage thickness of G1, G2, G3 were 300-400, 500-600, 500-600 mm. The cost of the wall of G1 was 461.1 yuan/m2, however, the costs of the walls of G2, G3 were significantly reduced to 389.9 and 299.0 yuan/m2 respectively. In summary, the thermal properties of G2 and G3 were not significantly different, but were both better than that of G1, which could meet the production needs of thermophilic crops in greenhouse. The assembled active heat storage wall was feasible, and the cost was reduced, and had a certain popularization value in some area suitable for constructing greenhouse.