Science - December 1, 2005

New water-efficient greenhouse

Wageningen researchers have developed a new water-efficient greenhouse. The ‘Watergy-greenhouse’ is intended for arid areas. It is kept closed, so that water given off by plants does not escape. The first measurements indicate that about seventy percent of the irrigation water is recouped.

A prototype of the Watergy greenhouse has been set up in the province of Almeria in southern Spain. The greenhouse was developed by the Systems and Control Group together with A&F Greenhouse Technology, test station Cajamar in Spain and the Technische Universität of Berlin.

The greenhouse has a rounded roof with a tower rising out of it. This resembles a chimney, but has no opening at the top. Air rises into the tower and circulates back into the greenhouse. Bas Speetjens of the Systems and Control Group: ‘Of course the temperature gets too high in a closed system if no cooling takes place. That’s why we built the tower with a heat exchanger in it. The air circulates through natural convection. Hot air rises, passes through the heat exchanger and then the cool air sinks.’

The heat exchanger also keeps the greenhouse cool during the day, and the heat stored is then used to keep the greenhouse warm at night. Water is also recycled in the greenhouse. During the daytime the moist air moves towards the cooling tower where the water condenses on the cold surface of the heat exchanger. This water can be used again to water the plants in the greenhouse.

Speetjens: ‘The extra advantages of a closed greenhouse are that there are no CO2 losses and that insects can be kept out. That way you save on insecticides.’ In addition salt water evaporates on the inside of the lower roof, the moisture then condenses in the tower, making it possible to win back fresh water.

Growing plants in the greenhouse is a success. This spring the greenhouse was full of beans, and in the summer okra was grown. ‘The bean yields were similar to those obtained by commercial greenhouses in Spain. We are still studying the okra, but as we win back seventy percent of the irrigation water, much less water is used than in a conventional greenhouse.’

In Berlin researchers are testing the Watergy greenhouse in an urban setting. Recycled bath and rinsing water from buildings near the greenhouse is used, and the greenhouse is run in a seasonal rhythm rather than a day-and-night rhythm, where heat from the summer is stored to heat the greenhouse in the winter. / HB