Articles
GREENHOUSE HEATING BY MEANS OF LATENT HEAT STORAGE UNITS
These systems have an advantage of low cost but lose the effectiveness of heat storage due to the temperature change.
In order to use solar energy more effectively, solar greenhouse with latent heat storage systems have been studies (e.g.
Takakura and Nishina, 1981). The preliminary results of our previous studies show the basic feasibilities of the systems provided that an inorganic phase change material (PCM) with a melting point around 20°C is available.
By adding some chemical materials to sodium sulfate decahydrate (Na2SO4·10H2O), a new PCM which has a melting point around 20°C and a heat of fusion of 56cal/cm3 has been developed in recent years.
Therefore, during the last winter, the experiment of solar greenhouse was conducted using this newly developed PCM.
The experiment was carried out in a Venlo type glasshouse in Kanagawa Horticultural Experiment Station from December 1982 through March 1983. The floor and surface areas of the greenhouse were 352m2 and 560m2, respectively.
Two layers of thermal screens, PVC film and non-woven fabric, were furnished for heat insulation in the nighttime.
Twelve and a half kilogrammes of PCM was encapsulated in a bag made of aluminium laminated polyethylene film.
The thickness, width and length of a bag were 1.8cm, 60cm and 90cm, respectively.
A heat storage box consisted of 25 bags separated by 1.7cm spaces for the passage of air.
Four boxes were lined up in the direction of the air flow, which made the total heat storage unit (Fig. 1). The total number of bags in the heat storage unit was 100 and the height, width and length of the unit were 0.9m, 0.6m and 4.7m, respectively.
An electric fan attached to the unit consumed 490W and provided 72m3/min of air.
The inside air was sucked into the 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 (Fig. 2). The total amount of the PCM was 2.5 tonnes and the potential value of latent heat was 1×105kcal.
The inside air temperature was controlled by a microcomputer.
In the daytime, the two 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 fans were operated to deliver heat when the inside air temperature was below the set point.
The set point of the inside air from 17:00 to 21:00 was 12°C, from 21:00 to 23:00 11°C and from 23:00 to 8:00 8°C, according to a blue print for fine days.
The thermal screens were normally opened at a time between 7:00 and 8:00 and closed between 16:00 and 17:00 ; these times were dependent upon weather.
No auxiliary heating system was needed during the experimental period, and tomatoes grew well.