1997 Volume 53 Issue 3 Pages 175-183
Light transmittance, temperature and humidity of greenhouses covered with three PolyOlefin films of PO1 (Mikado Chemical M.F.G. Co., Super solar 0.15mm in thickness), PO2 (Sumitomo Chemical Co., Cleantate-LF, 0.15mm in thickness), and PO3 (Mikado Chemical M.F.G., Co., U-lack, 0.1mm in thickness), and a ploy vinyl chloride film PVC (Mitsubishi Chemical MKV Co., Clean Ace-CPM, 0.13mm in thickness) were examined to elucidate the differences of microclimate inside the greenhouses. All the films were covered in October 1993 except PO3 which was covered in October 1994. Several kinds of vegetables such as spinach, daikon-radish, Chinese-cabbage and red-radish were cultivated under usual management in each autumn-winter-spring season. Air temperature, humidity, solar rediation, and soil heat flux inside and outside (control) each greenhouse were measured during every winter-spring season. Dewdrop and/or condensation water on the covering film and the water content in the air were measured in order to check the humidity conditions in each greenhouse. The relationship between light transmittance and the dewdrop on films of the greenhouses was analyzed using the data obtained mainly in 1995-1996 season and 1997. The main results are as follows.
1. Transmittance of solar radiation of PO1, PO2, PO3-greenhouses was 76.7%, 79.7%, and 63.5%, respectively, while that of the PVC-greenhouse was 71.6% at early March in 1996.
2. Ventilation rate of PO1 and PO2-greenhouses was high but that of PVC was very low. The differences in ventilation rate were caused by different draft condition of each greenhouse.
3. As compared to other three greenhouses, air temperature difference between the inside and outside of the PO3-greenhouse was lower by 2-4°C during daytime, resulting from the heavy dewing on the covering film.
4. Average dewdrop on the films of the PVC and PO3-greenhouses was from 140-250g/m2 on cloudy day, while that of the PO1 and PO2-greenhouses was less than 40g/m2. Dewing on the greenhouse films reduced the transmittance of the solar rariation at a rate of exp (-0.9d), where d is equivalent water film thickness of dewdrop and/or condensation.
5. An empirical equation was obtained using the relationship between light transmittance and the dewdrop thickness in order to estimate the air temperature difference between the inside and outside of greenhouse. Estimated values agreed well with measured values, suggesting that the effect of the dewdrop on the heat budget of the greenhouse can be quantified accurately.
6. High level of dewdrop was caused by a lack of the drip control performance (runoff function) of films which decreased light transmittance, thereby increasing humidity and decreasing air temperature inside greenhouses.
