温室の空気調和に関する設計資料

1. 夜間の放射冷却の算定

抄録

Thermal radiation cooling during the night is one of the most significant phenomena for the greenhouse and the long wave net radiation exchange between the cold sky and the greenhouse surface has been analysed under the assumption that there is no significant temperature difference betwen the greenhouse surface and the surrounding ground surface.
In order tlo calculate the air-conditioning load, especially the heating load of the greenhouse, it is indispensable to analyse the radiative heat transfer mechanism of the whole system which includes the green house and to calculate the amount of the heat flow of each section involved. The diagrammatic representation each radiation exhange path is given in Fig. 3.
In the present study, the theoretical analysis is expanded to the thermal radiation cooling of the heated greenhouse as well as experimental verification. The view factor to the sky of the finite glass plane whose inclination angle is α is expressed as (1+cosα)/2 under the assumption that the long wave radiation from the sky is uniform and this factor is introduced as one of the important factor of the heat loss from the unheated greenhouse (Takakura 1967 a). Using this relationship, the ratio of the net radiation between the greenhouse surface and the sky to that between the sky and the horizontal surface whose area is the same as the greenhouse floor is expressed as (1+β)/2, where β is the ratio of the greenhouse floor area to the greenhouse surface area (Tachibana 1971).
The net radiation between the greenhouse surface and the sky is a function of the water vapor pressure in the air and the angular height of the net radiation intensity. Therfore, the numerical integration of Kondrat'yev's equation has been conducted and one of the results, which is the curve for 6mmHg is given in Fig. 2. The deviation from this curve due to the variation of the water vapor pressure is given as vertical deviation bands at three inclination angles in the same figure instead of giving the family of curves. This curve is expressed by the function (1+cosα-0.23 sin²α)/2 under 2% error.
Using this function, the ratio of the net radiation loss from the greenhouse surface to the net radiation of the horizontal surface is expressed as (0.77+β+0.23β²)+γ(1-β)/2, where γ is the ratio of the net radiation between the greenhouse wall and the surrounding ground surface to the net radiation between the greenhouse wall and the cold sky. Suppose that there is no temperature difference between the greenhouse wall and the surrounding ground surface, γ is equal to zero. On the other hand, if there is a significant temperature difference between them, e. g., 10 deg C, γ is approximately 0.5 and the value is increased 60% more than the value at γ is zero in the smaller range of β. The results are given in Fig. 4. Therefore, it is concluded that the thermal radiation cooling of the heated greenhouse should be calculated by Eqs. 13 and 14.
The calculation results are verified experimentally.