MEASUREMENTS OF HEAT LOSS FROM GLASSHOUSES WITH DIFFERENT DOUBLE SHELTERS

Q = Q_T + Q_L = k' x A_Gl x (t_i - t_o)

The heat consumption coefficient k' consists of two components, the coefficient for heat transmission k and the coefficient for ventilation heat transfer. It is

k' = k + k_L

The k_L value is depending on the leakages of the glasshouse, the windspeed, the temperature and humidity of the air inside and outside the glasshouse. k_L is a linear funktion of the air change coefficient z (1/h). For a single span glasshouse (10. 8 x 21. 5 m) with single shelter of glass we have measured and calculated the value of z. We found z being a function of the windspeed and the orientation of the glasshouse to the winddirection. (See paper of H. Chr. Gudehus)

The coefficient for heat transmission k is depending on the heating system, the kind of shelter and the amount of long wave radiation from the roof. Therefore energy saving is possible by the choice of the heating system, by using double shelters and by reducing the long wave radiation to the roof. The area of the roof is depending on single- or multispan construction.

We have measured the heat consumption of glasshouses under the following conditions:

1. Single span glasshouse, 11 m width, covered with single glass and fiberglass plates ( GUP ). The GUP plates are covering the glazing bars and reduce the heat loss by transmission through the iron construction. ( See figure 1 )
2. Venlo type glasshouse ( multispan ) with single and double glass. ( Pipe heating )
3. Single span glasshouse, 11 m width, with an internal blind of aluminised plastic film. The layer of aluminium is between two layers of polyethylene and polyester film. The inside layer is the polyester film.

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