Effect of Plastic Curtain in Greenhouse Upon Temperature Distribution and Heat Loss

Abstract

This paper described the effect of an internal curtain on temperature distribution and heat flow rate from the roof in a greenhouse covered with plastic film. The greenhouse used was shown in Fig. 1, and the internal curtain material provided was a transparent polyethylene plastic film with 0.05mm in thickness.
On this heating system hot air (approximately 50°C) blew off through the outlets of polyethylene duct to heat up the air in the greenhouse. The air temperature became higher with height from the soil surface.
The air temperature of the upper portion (2.8-4.1m above the soil surface), which appeared not to be related with the plant growth, was usually higher than that of the lower portion (0.2-1.0m above the soil surface), which appeared to be related with the plant growth. It became 2.3-4.1°C higher when the temperature difference (Δt) between the inside and the outside was 9.7-13.1°C. When the internal curtain was installed, the air temperature of the upper portion became lower than that of the lower portion. It became 6.9°C lower when Δt was 15.6°C. It was shown that the internal curtain in the greenhouse reduced the unnecessary air heating.
In regard to the uniformity of air temperature, the maximum values of temperature difference and standard deviation in the case of internal curtain installation were larger than in the case of no internal curtain installation for heating volume in the greenhouse was decreased. But mean temperature distribution became more uniform because the heat insulation was improved resulting in the decreased heater operation.
The internal curtain installation resulted in a lower heat flow rate from the roof by means of the lower air temperature under the roof. The values of heat flow rate were equivalent to 55-60% in the case of no internal curtain installation. In addition fuel consumption was equivalent to 36% (Δt=10°C) and 50% (Δt=15°C) in the case of no internal curtain installation.