ほろ型プラスチックハウスにおける日射量解析

抄録

Direct and diffuse components of solar radiation were measured inside and outside a single cylindrical vaulted greenhouse. The greenhouse oriented quasi-north-south, had been constructed with thin steel pipes (1.9cm dia.). The inner surface of the polyvinyl chloride film (0.1mm thick) which is the covering material of the greenhouse had been wet with dew, under the same conditions as normal cultivation greenhouses.
The direct solar radiation transmitting the cylindrical vaulted greenhouse was calculated theoretically, and it was compared with actual measurements.
The differences in inside direct radiation between the greenhouse with the new film and that with the used film, were investigated.
1. Results of observation.
The components of direct and diffuse radiation inside and outside the greenhouse were expressed as follows:
τ(Q+q)=(τ_Q+τ′_Q)Q+τ_qq=τ_QQ+τ′_QQ+τ_qq
where, Q+q: outside total radiation (direct solar radiation plus diffuse sky radiation), τ(Q+q): inside total radiation, τ: inside/outside ratio of total radiation, τ_QQ: inside direct radiation, τ_Q: inside/outside ratio of direct radiation, τ′_QQ+τ_qq: inside diffuse radiation (effective scattered component of direct solar radiation plus transmitted diffuse sky radiation), τ_q: inside/outside ratio of diffuse sky radiation, τ_Q: effective scattering coefficient of direct solar radiation. The numerical values in the formula were obtained on a clear day. Assuming that the inside/outside ratio of diffuse sky radiation is equal to the inside/outside ratio of total radiation on an overcast day, we have τ_q=75% as an average (Fig. 1). Inside direct radiation is lower than outside (τ_QQ<Q), conversely, inside diffuse radiation is higher than outside (τ_Q′Q+τ_qq>q). The intensity of τ_Q′Q is equally matched for τ_qq, and it cannot be disregarded even in the transparent plastic greenhouse. The value of τ increased from 56.0% in January to 60.2% in February. The increase of τ depends on the increase of τ_Q from 30.8% to 37.9%, despite a little decrease of τ′_Q from 20.3% to 18.6% (Fig. 2). It appears that the seasonal variation of inside/outside ratio of total radiation is mostly affected by inside/outside ratio of direct radiation.
Generally, the transmissivities of direct solar radiation in the polyvinyl chloride films have a tendency to decrease with increasing angle of incidence (Fig. 4). The tendency of dirty film, contaminated by dust in the city, is steeper than that of new one. Moreover, this tendency is the most remarkable of dewy film now in use.
2. Consideration of inside/outside ratio of direct radiation.
An equation was deduced for calculating the solar incident angle on a curved surface of cylindrical vaulted greenhouse. The angle of incidence I is written as follows:
I=cos^-1|sin h⋅cosecω⋅sinρ|
ρ=tan^-1{√sec²ω-k²/k}
ω=tan^-1(cosecθ⋅tan h)
excepting that the angle I=90°-h for θ=0, and ρ=90° for k=0. Where, h: solar altitude, θ=α-α′: angle between solar azimth α and greenhouse orientation α′, k=l/r: ratio of ground level above the center of cylinder l to radius of greenhouse section r (Appendix Fig. 1).
Diurnal variations of direct radiation inside the greenhouse under observation were calculated by using I in the above equation with the values of k=0.4 and α′=