Abstract
Because in summer the outside air temperature in many Asian countries is too high to grow certain flowers and vegetables, these crops are often grown in a greenhouse which is cooled by cooling equipment. For example, the development and growth-rate of carnation, roses and melons can be improved by using both daytime and nighttime cooling.
In this paper cooling load of a single span greenhouse in summer nighttime is investigated. The experiments were made under the following three different thermal screen conditions:
1) two thermal screens open
2) one thermal screen closed
3) two thermal screens closed.
The greenhouse was cooled by a heat pump that was in operation continuously during the night, and no crop was present in the greenhouse. The results can be summarized as follows.
(1) Under the condition of a constant inside air temperature the cooling load of the greenhouse is due to
1) air infiltration Qv 42%
2) soil heat flux Qs 38%
3) overall heat transmission Qt 20%.
(2) An empirical relation for estimating air infiltration rate of the cooled greenhouse Vg was proposed. In the relation, Vg is a function of the outside wind speed and the air temperature difference between inside and outside the greenhouse. With wind speed being 1m/s and the temperature difference ranging from 3 to 15K, the value of Vg ranged from 1.5 to 3.0m3/(m2hr).
(3) From measurements it appeared that 70% of the whole heat transfer due to air infiltration Qv was caused by the transfer of latent heat. Because of this result Qv was proposed to be described as being proportional to the enthalpy difference between inside and outside air instead of temperature difference. Qv decreased to 63% with one thermal screen closed and 58% with two thermal screens closed.
(4) The effects of thermal screen on the heat transmission ht [kcal/(m2hr K)] were:
1) thermal screen open: ht=1.62
2) upper thermal screen closed: ht=1.20
3) lower thermal screen closed: ht=0.51
4) two thermal screens closed: ht=0.50
The upper thermal screen was an aluminium powder mixed polyester film with a thickness of 0.05mm. The lower thermal screen was a polyethylene film of 0.05mm thick.
(5) The process of greenhouse cooling is considered to be a first order process of which the time-constant is estimated by parameters of the greenhouse and the cooling system.
Based on this assumption it is proposed that two definitions of the maximum cooling load of a greenhouse are possible, the one is static and the other is dynamic. The static conception is used in the case the heat pump or any cooling equipment needs continuous operation for keeping a setpoint air temperature. In this case the cooling load of greenhouse is equal to the capacity of the cooling equipment. The dynamic definition is used in the case the capacity of the cooling equipment is so high that no continuous operation is needed to maintain a setpoint air temperature.
