Reproduction of radiative cooling in a greenhouse using a hot box with a thermal plant model

This study aimed to develop a scheme to simulate the effect of radiative cooling on plant leaf temperature under transparent covering materials used in greenhouses.
In greenhouses, radiative cooling affects plants at nighttime during winter by occasionally decreasing the leaf temperatures below the air temperature.
Since integrated temperature influences plant growth, understanding the actual leaf temperature is important for predicting growth and yield.
We developed a hot box with a heating and cooling room to estimate the temperature difference between leaves and air in the greenhouse.
Heating and cooling rooms simulated the inside and outside environments of greenhouses on winter nights.
Moreover, a cooling plate was installed in the cooling room to induce radiative cooling.
The heating and cooling rooms were set at 15 and 0°C, assuming temperatures inside and outside the greenhouse in winter, respectively.
The cooling plate surface temperature was determined assuming winter nighttime downward longwave radiation, and was set at -22°C. Covering materials made of glass or polyethylene (PE) film were installed between the rooms.
A thermal plant model of a strawberry (emissivity: 0.94) was created, which evaluates the thermal condition of a plant; it had an emissivity similar to that of real plants and was installed in the heating room.
The hot box was sealed and the temperature difference between the leaf and air was measured using copper-constantan thermocouples.
Average leaf temperatures under glass and PE film were 1.1 and 2.3°C lower than that of air, respectively.
This experimental method could estimate leaf temperature under different covering materials in a greenhouse considering the radiative cooling effect.
Temperature differences between leaves and air under PE film were greater than that under glass due to differences in the longwave radiation transmittance of the materials, which caused greater radiative cooling under PE film than glass.

IV Asian Horticultural Congress – AHC2023

Y. Ohashi, R. Tsuchiya, M. Ishii, M. Hayashi

covering material, leaf temperature, longwave radiation, physical measurement, radiation heat transfer, thermal environment

https://doi.org/10.17660/ActaHortic.2024.1404.170