Studies on Solar Greenhouses with Latent Heat Storage Systems

Abstract

As the first step of a study on solar greenhouses with latent heat storage systems, thermal properties of phase change materials (PCMs) were tested and solar heating experiments applying these PCMs to small greenhouses were performed.Several PCMs which have melting points between 10-30°C were selected from the literatures (e.g. Hale et al., 1971), and measurements of melting points and heats of fusion were made. Considering these test results (Table 1 and Fig. 1), and because of the extremely high price of paraffin, we chose polyethylene glycol (PEG) and calcium chloride hexahydrate as the most suitable PCMs for solar heating greenhouses at the present time.Three types of solar greenhouse systems were constructed, and solar heating experiments were performed in both 1979 and 1980. We used two small identical glasshouses (floor area is 7.2m2, surface area 25.2m2) with the north wall (4.8m2) insulated.In the Type I solar greenhouse (without thermal screen), polyethylene tubes (3.0cm in diameter) were filled with 600kg of PEG (# 600 and # 400) and hung by steel bars in the insulated heat storage unit. Air was circulated by a fan between the greenhouse and the heat storage unit, and solar heat was collected from the air inside the greenhouse (Fig. 2). Typical patterns of diurnal changes in temperatures in the Type I solar greenhouse on a clear day are shown in Fig. 3. Solar heat stored in the daytime was 9, 900kcal and heat released in the nighttime was 11, 060kcal. The average efficiency of heat storage on clear days was 20%, based on the outside solar radiation.In the Type II solar greenhouse (with one layer thermal screen), three 1.6m2 air-type solar collectors were attached. PCMs, 300kg of CaCl2⋅6H2O and 200kg of PEG (# 600), were encapsulated in double-layered polypropylene panels (1.2cm in thickness) and were installed in the heat storage unit. In the daytime, air was sucked from the greenhouse to the collectors, and heated air was then sent to the heat storage unit and was returned to the greenhouse. In the nighttime, the path to the collectors was closed by a damper, and air was circulated between the greenhouse and the heat storage unit, although the direction of the air flow through the heat storage unit was opposite to that in the daytime (Figs. 4 and 5). The heat collected in the collectors was 12, 060kcal, and the heat stored in the heat storage unit was 8, 380kcal (7, 520kcal to CaCl2⋅6H2O, 860kcal to PEG) on Feb. 3, 1980. The temperatures of CaCl2⋅6H2O and PEG were kept almost at the melting point of each, which indicated that the storage capacity of latent heat was not yet filled. The inside air temperature was kept at 8.0°C in the early morning on Feb. 4, when the outside air temperature was -0.6°C. The average efficiency of heat storage on clear days was 17%, taking into account the receiving area of both the collectors and the greenhouse.In the Type III solar greenhouse (with one layer thermal screen), double-layered polypropylene panels (1.5cm in thickness) which contained 56kg of CaCl2⋅6H2O were installed in front of the inside surface of the north wall. They could be called a heat storage panel. In addition to this, 200kg of PEG (#600) was encapsulated in PVC pipes (3.2cm in diameter) and was installed in the small heat storage unit. The heat storage panel can store heat from direct solar radiation. In the heat storage unit, heat was collected from the inside air by circulating air between the greenhouse and the heat storage unit. Typical patterns of diurnal changes in temperatures in the Type III solar greenhouse on a clear day are shown in Fig. 6. The heat stored in the heat storage panel and the heat storage unit was 2, 860kcal and 7, 560kcal, respectively.