ISHS Acta Horticulturae 534: International Conference and British-Israeli Workshop on Greenhouse Techniques towards the 3rd Millennium
COLLECTING EVAPORATED WATER BY AN EARTH-TO-AIR HEAT EXCHANGER INSTALLED IN A GREENHOUSE |
| Authors: | Y. Nara, H. Horino, T. Mitsuno |
| Keywords: | Heat exchanger, water budget, greenhouse, condensation, vapor collection, dehumidification |
| Abstract:
An earth-to-air heat exchange system that utilizes the temperature difference between the internal air of the greenhouse and the underground soil for cooling or heating was tested. It consists of, in principle, buried pipes inside which the internal air is circulated. This system can be used as a water vapor collector if the underground temperature is lower than the dew point of the circulating air. The condensed and collected water can partially compensate the water needs of the greenhouse. To evaluate efficiency of such use, experiments were conducted using a small-scale experimental greenhouse with a heat exchanger sunk in a water bath instead of a buried earth pipe. The apparatus was kept airtight, and the air inside was continously circulated in the greenhouse and the heat exchanger. In the greenhouse, an evaporation vat filled with water was left preliminarily instead of plants. Water condensed in the heat exchanger was led to an outside collection tank. During the experimental period (40 days), total evaporation in the greenhouse was 31 kg, and 83% of this was collected in the heat exchanger. This result implies that the earth-to-air heat exchanger can collect water evaporated in the greenhouse at high rate for reuse. Changes in pressure of the water vapor passing through the heat exchanger and the pipes, by which the heat exchanger and the greenhouse were connected, were calculated by a simple procedure. For a cooling process of the air flowing through a certain section, one of the three equations representing the vapor pressure at the outlet of the section is chosen according to the relation between the cooling temperature and the dew point of the air at the inlet of the section. The calculated vapor pressure at the outlet of the pipe (or inlet of the greenhouse) showed good agreement with the measurements. Using these calculation results of the water vapor pressure at the inlet and the outlet of the heat exchanger and the measurement of the flow rate of the circulating air, then, the condensation rate within the heat exchanger was calculated. The results obtained showed good agreement with the measurements for various averaging time intervals as far as the cooling temperature was sufficiently low. | |
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