抄録
Using a newly developed phase change material (PCM) which has a melting point of 20-23°C, a freezing point of 17-20°C and a heat of fusion of 56cal/cm3 (40cal/g) (Fig. 1), an experiment of solar heating was carried out in a Venlo type glasshouse in Kanagawa Horticultural Experiment Station from January through March 1983. The floor and surface areas of the greenhouse were 352m2 and 560m2 respectively, and two layers of thermal screens were furnished for heat insulation in the nighttime.
Batches of PCM weighing 6.2kg each were encapsulated in bags made of aluminum laminated polyethylene film. The thickness, width and length of a bag were 1.8cm, 30cm and 90cm, respectively. Two bags were put side by side, and twenty-five pairs of bags vertically separated by 1.7cm spaces for the passage of air composed a heat storage box. Four boxes were lined up in the direction of the air flow, composing the total heat storage unit (Fig. 2). The height, width and length of the heat storage unit were 0.9m, 0.6m and 4.7m, respectively. A ventilating fan provided 72m3/min of air. The inside air was sucked into the heat storage unit, exchanged heat with the PCM and was returned into the greenhouse through a polyethylene film duct. Two identical heat storage units were installed in the greenhouse. The total amount of the PCM was 2, 500kg, and the potential value of heat to be stored or released was 1125×102kcal.
In the daytime, the two ventilating fans of the heat storage units were operated to store heat when the inside air temperature was above 22°C. The roof ventilators were opened when the inside air temperature was above 28°C. In the nighttime, the ventilating fans were operated to release heat when the inside air temperature was below the set points: 12°C from 17:00 to 21:00, 11°C from 21:00 to 23:00 and 8°C from 23:00 to 8:00, according to a blueprint for fine days. No auxiliary heating system was needed during the experimental period, and tomatoes were grown in the greenhouse.
As the results, the inside air temperature was kept above the set point during almost the entire experimental period (Fig. 3 and Table 1). In addition to this, the following results were made clear. In the present paper, a day is defined from 8:00 to 8:00 the next day.
(1) Daily stored heat into the PCM and released heat from the PCM on fine days ranged on the whole between 600×102kcal and 900×102kcal (Table 1). The efficiency of utilization of the PCM was on the average 70%. The efficiency of utilization of the PCM near the air inlet was above 80%, while that near the air outlet was below 60% (Fig. 5).
(2) The amount of heat due to latent heat transfer from the air flowing through the heat storage unit to the surfaces of the PCM bags was about the same that due to sensible heat transfer (Table 3). It is considered that between 55 and 80 liters of water condensed in the daytime and evaporated in the nighttime at the surfaces of the PCM bags.
(3) The thermal effectiveness of the heat storage unit was about 0.75-0.80 (Fig. 4).
(4) The ratios of the released heat from the PCM and the soil to the total released heat in the case of two layers of thermal screens were 62-69% and 30-36% respectively, while those in the case of one layer of thermal screen were 54-59% and 40-45% respectively (Table 2).
(5) The efficiency of heat storage, defined as the ratio of the stored heat into the PCM to the integrated global solar radiation outside the greenhouse, was 6-9% on fine days.
(6) The relative humidity of the inside air was on the whole 80-90% in the daytime and 95-100% in the nighttime.
