Studies on the temperature control of glasshouses. I

On the temperature and the heat balance in an empty unventilated glasshouse

Abstract

Summer glasshouse temperatures in Japan are too high for plant growth, so it is emphasized that glasshouse temperature should be controlled. As a fundamental investigation into temperature control the temperature distribution and heat balance in an empty, unventilated glasshouse was measured in July 1959.
One of the questions arising is that the temperature difference between the inside and the outside is very small at night, whereas in daytime it is considerably large. The daytime difference between the inside and the outside of the glasshouse has often been attributed to the optical properties of the glass that transmit a high proportion of solar radiation but prevent the direct exchange of longwave radiation between the internal surface and the atmosphere. This, so called “greenhouse effect” does not explain the temperature differences observed at night.
Table 1 shows that the maximum internal temperature was about 60°C, 28°C higher than the external temperature; but the minimum air temperature (26.5°C) was only 2.9°C above the external temperature. The temperature distribution revealed an up-draft in the centre of the house with a downdraft along the walls; the direction of the circulation was reversed at night.
The “greenhouse effect” was unimportant because the loss of solar radiation by reflection and absorption exceeded the gain in long-wave back radiation from the glass.
The difference in the amount of heat that is advected to the internal and external surfaces of the glass appeared to be considerably more important than it was previously supposed and it has a great influence on the net radiation of the soil surface. Excess heating of the internal air should be attributed to decreased turbulent transfer of sensible and latent heat.
At night the net radiation measurements showed a marked “greenhouse effect, ’ but heat was lost rapidly in two important ways, by convection to the walls, which are cooled by radiation from their outer surfaces, and by horizontal conduction in the soil. (Fig. 4, 5). This explains why glasshouses cool rapidly after sunset and why the temperature difference is very small throughout the night.