Measurements of the heavy oil consumption for heating, heating degree hour, soil heat flux at the floor surface, solar radiation outside etc, were made on a four-span glasshouse (see Table 1). Based on the analyses of the measured data, a set of equations for estimating the daily heating load was proposed. The results can be summarized as follows:
(1) Nighttime heating load was not proportional to nighttime heating degreehour (Fig. 2). During the period from December in 1980 to February in 1981, the nighttime heating load accounted for only 80% of the nighttime heat loss from walls, on the average. The heat released from floor surface (soil heat flux) during night accounted for the rest of the nighttime heat loss (Fig. 1).
In March, the nighttime air temperature inside the glasshouse without heating was maintained about 5 degrees centigrade higher than that outside, due to the heat released from the floor.
(2) The daytime air temperature inside the glasshouse without heating was maintained about 10 degrees centigrade higher than that outside, under rainy or overcast conditions when the solar radiation outside was about 100kcal/m2/hr (Fig. 3). This temperature difference between inside and outside was considered to be maintained by the solar radiation input into the glasshouse, because another possible heat input, the soil heat flux at the floor surface, was almost zero.
(3) On the other hand, a hot air heater was often turned on within one hour after sunrise even on a sunny day, when solar radiation outside was less than 100kcal/m2 /hr and the air temperature difference between inside and outside was kept more than about 14°C. Daytime heating load, almost of which was consummed within one hour after sunrise, was about 5% of the daily heating load, on the average.
(4) Daily average of the heating load, qg [kcal/m2 (floor area)/day], was considered to be estimated by using the following equations.
qg=qd+qn (1)
qd=hd⋅DHd/β-qsolar (2)
qn=hn⋅DHn/β-qsoil (3)
qsolar=∫t2t1q(t)dt (4)
q(t)=Min(qs·τ·α, hd·Δθ/β) (5)
where subscripts d and n denote, respectively, daytime and nighttime; h and DH being, respectively, overall heat transmission coefficient (kcal/m2/hr/°C) of the glasshouse walls and heating degree hour; Δθ and β being respectively, temperature difference between inside and outside at time t and wall area to floor area ratio; qsolar being a portion of the transmitted solar radiation which contributes to the reduction of daytime heating load (see also Fig. 4); qsoil being the nighttime soil heat flux at the floor surface. t1 and t2 denote, respectively, the times of sunrise and sunset.