EIGHT YEARS PRACTICAL EXPERIENCE WITH THE ACRYLIC DOUBLE SKIN SHEET IN GREENHOUSES

The first application of the double skin sheet was greenhouse glazing. Therefore, most of our experience with the double skin sheet derives from this field of application:

1. Saving of heating costs

With a heat transfer index of k = 3.2 W/m² °C the theoretical heat loss of the double skin sheet is about one half that of single glazing. The heat actually saved by double skin sheet greenhouses compared to conventionally glazed houses was measured in different places.

1.1. In an experimental double skin sheet greenhouse at the agricultural college of Alnarp an about 50 % reduction of heating costs was found at a greenhouse temperature of 20 °C and wind velocities between 2 and 8 m/s (Gustavsson, 1974).

1.2. In another experimental double skin sheet greenhouse at the Versuchsanstalt Straelen, West-Germany, the measured saving was 44 % at 20 °C. This value increased to 58 % with strong winds (Gartenbauliche Versuchsberichte, 1976).

1.3. In winter the fuel saving in the double skin sheet greenhouse Kirchardt, West-Germany, was over 50 % at a greenhouse temperature of about 10 °C.

1.4. Model experiments by the Institute for Technology and Horticulture of the Technical University of Hannover revealed that double skin sheet greenhouses save up to 45 % in energy (Kohlmeier, 1975).

A characteristic feature of the double skin sheet is the high surface temperature on the inside of the greenhouse, as a result of which the loss of heat by radiation from the ground or from the plants is reduced. This was found by measurements at the agricultural college of Alnarp (Gustavsson, 1974). The ground temperature is higher and the plants grow equally well at a greenhouse temperature which is 1 or 2 °C lower.

Generally speaking, for the calculation of the

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