Air temperature prediction model to control solar energy heating in a germination greenhouse at night

Abstract

Summary. The outdoor air temperature is not constant especially in spring in Okayama city. The average night temperature ranges from –2 to 20°C which is too low for the germination of most seeds. A good knowledge of the future outdoor air temperature is necessary to decide if greenhouse heating is needed for the next day. Using measured temperatures from the preceding days and considering the minimum and maximum temperatures given by the weather forecast, it was possible to accurately compute the temperature for the next day. Pumpkin, eggplant and tomato seeds were used in this study. A solar system was used to heat the air inside a greenhouse at night using an air–water–air heat exchanger and make a comparison with an unheated greenhouse. The performance of the solar collector and methods of heat exchange were tested. It was shown that the solar energy collected was sufficient for warming a nursery greenhouse overnight. The system operated with a hot water flow rate of 0.647 L/min and an air flow rate of 9.21 m3/min and could maintain the greenhouse temperature between 16 and 20°C. The quantity of heat collected and delivered by the solar system from incident solar radiation was about 50% in a day. Heating the air inside the greenhouse at night produced 100% germination for all seedlings. In comparison, in the unheated greenhouse the germination ratio was 100, 93 and 27% for pumpkin, eggplant and tomato respectively. The germination ratio outside the greenhouses was 100% for pumpkin, 67% for eggplant and zero for tomato.