Analysis of heat transfer properties of three-layer wall with phase-change heat storage in solar greenhouse

Abstract: At present, the wall structures of a Chinese solar greenhouse puts more emphasis upon wall thermal insulation than on heat storage of the wall, such as, brick wall / air layer / brick wall (indoor), brick wall / polystyrene board / brick wall (indoor), rammed earth wall / brick wall (indoor), and so on. In order to solve the problems which existed in the thermal performance design method of the wall structure in a solar greenhouse, the construction method of a three-layer wall with phase-change thermal storage, that is, the inner wall built with the phase change material (PCM) wallboard, the outer insulating layer built with polystyrene board and the middle layer built with block bricks, was proposed in this paper. To quantitatively evaluate the heat transfer performances and heat storage/release characteristics of the three-layer wall, an experimental device used for the heat performance of the three-layer wall was constructed at a vegetable planting base located in Beijing. Combined with the experimental results, the analysis method of heat transfer performances of solar greenhouse wall and its evaluation indexes were put forward. The analysis results showed that: 1) The three-layer wall had better heat storage/release performance than the traditional brick wall of the reference greenhouse. The PCM wallboard can significantly improve the utilization rate of solar energy and increase the indoor air temperature. The effective heat storage capacity of the three-layer wall was increased by 26.6% more than the north wall of the reference greenhouse in which the daily accumulation of solar radiation was 9.32 MJ/m2. At night, the cumulative heating capacity of the three-layer wall was increased by 16.2% over the capacity of the brick wall of the reference greenhouse during the time when the heat preservation quilt was closed. Moreover, the effective heat storage capacity of the per unit volume PCM wallboard was 80.0 MJ/m3 and it was about 10 times that of the block brick in the middle layer of the three-layer wall. 2) The depth of the solar greenhouse north wall that the temperature variation caused by the solar transmitted radiation through the front sloping roof surface could affect was limited, and the depth accounted for about 33.3% of the 0.90m-thick three-layer wall. Furthermore, there was the temperature stable zone in the three-layer wall and its thickness accounted for about 61.1% of the 0.90m-thick three-layer wall. Obviously, the method that only increased the three-layer wall thickness to improve the sensible heat storage efficiency was very limited. These results can provide references for the construction method of the solar greenhouse wall, the application of phase-change thermal storage technology, and the analysis of the phase change heat transfer problem in a solar greenhouse.