In order to compare the effect of several material curtains on reducing the heat requirement of a greenhouse, the overall heat transfer coefficient, k, was investigated with two small model green-houses (Fig. 1); One was covered with glass and the other with PVC film. Five material curtains (PVC, PE, Nonwoven polyester fabric, net and aluminum PE) were used to compare the ability of reduction in heat requirement.
Experiments were carried out on several winter nights under natural weather conditions, and k was calculated from equation (1). Results obtained were as follows:
(1) The values of k for the single glazing greenhouses indicated a correlation with the windspeed, w (Fig. 5).
Glasshouse k=4.41+0.40w
PVC house k=4.87+0.54w
(2) The PVC and PE curtains with which no condensation occurred were examined for the effect on k in the glass and PVC houses (Fig. 7, 8).
Glass+PVC k=2.93+0.10w
+PE k=3.00+0.16w
PVC+PVC k=2.91+0.14w
+PE k=3.15+0.17w
The PE curtain which was the most transparent to thermal radiation indicated a larger k than the PVC curtain which had less transparency to thermal radiation.
(3) When evaporation occurred, the value of k for the greenhouse with nonwoven fabric curtain which had vapor permeability was larger compared with no evaporation occurring (Fig. 10).
(4) Aluminum PE curtain showed a high reduction of k. This result suggests that curtains which are reflective to thermal radiation are more effective in reducing heat losses in greenhouses.
(5) PVC, PE, nonwoven fabric, net, and aluminum PE curtains in glasshouses showed a reduction in k at 1m/sec windspeed of 37%, 33%, 27%, 8%, and 58% respectively. PVC and PE curtains in the PVC house showed a heat loss reduction rate of 43% and 39%, respectively.