|Authors: ||K.H. Strauch, Chr. von Zabeltitz|
Water shortage is a serious limiting factor for agricultural and horticultural production in arid regions.
In order to develop remote areas which are provided with little or no external services, an adequate greenhouse system must consist of several characteristics.
Water purification by solar desalination seems to be an appropriate way of utilizing reservoirs of brackish water or even seawater for small communities because its application reduces the dependence on external energy and technology supply.
The most effective use of purified water is to balance water supply (solar still) and water demand (greenhouse crop). The most important requirements for such greenhouse systems are the following:
- water saving by reduced evaporative water losses (closed atmosphere), water collection by extraction of moisture from the enclosed air by condensation on parts of the greenhouse cover, reduced water demand of the crop in a closed atmosphere (high air humidity) compared with open air conditions.
- simple, low technology structure which is based on the use of ordinary construction profiles used in common greenhouse design.
- low energy demand which includes the application of solar destillation and the avoidance of active cooling devices.
Some closed system greenhouse constructions have been developed in recent years.
Figure 1 shows a closed system greenhouse with salt water surface cooling (system Daunicht, West Germany). The outside surface of the roof is irrigated continuously by salt water which evaporates and cools the cover material.
At the inside surface of the roof the inside air is cooled by convection at the inner surface.
There is a sufficient cooling effect because the greenhouse surface area is twice that of the greenhouse floor.
The water produced by evaporation from soil and plants condenses at the cooler inner roof surface and is collected by the inner gutters.
The collected water is again used for irrigating the plants.
This system saves water and has a low energy demand for pumping the salt water.
The fluid roof closed system greenhouse (Fig. 2) with separate water desalination has been developed in south France and in Israel.
The covering material consists of rigid double plastic material (acrylic or polycarbonate) through which a special fluid flows.
This fluid absorbs a high proportion of the near infrared radiation (NIR) in the solar spectrum.
The fluid thus cools the greenhouse during the day and heats it at night, if heating is required.
Beside the greenhouse a
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