The low energy greenhouse ‒ heat transfer by long wave radiation
In the ZINEG-Project of Hanover the Low Energy Greenhouse is covered with double glazing in the roof and equipped with two thermal screens and one black out screen. Compared with a conventional reference greenhouse (single glass and one thermal screen) the reduction of energy consumption is in the range of 70% (Tantau, 2013). A heat resistance model was used to calculate the heat transfer through the covering material. This model generates good results for the heat flux from inside to the outer roof surface. The convective heat flux outside (e.g. by wind) was modelled using a resistor according to the wind speed. But the long wave heat transfer outside depends on the difference between the roof surface and a virtual sky temperature. In nights with clear sky the long wave radiation heat flux will be larger than the conduction heat flux through the cover material resulting in cover temperatures below outside air temperatures. In this case the greenhouse extracts heat from the outside air. During spring and autumn when the heat requirement is low, the air temperature between the screens and the roof may be decreased below outside air temperature. Thus the heat flux by long wave radiation is more important for low energy greenhouses than for conventional greenhouses. But looking at the heat transfer through the screens and the double glazing it is apparent that the largest resistors for heat transfer are the screens and the double glazing. The resistor for heat transfer from the cover to the environment can be neglected; therefore the U-value is not significantly influenced either by long wave radiation or by wind speed. A radiative heat flux of up to 90 W m-2 decreases the roof temperature 6 degrees below the outside air temperature.
Tantau, H.-J. and Akyazi, G. 2017. The low energy greenhouse ‒ heat transfer by long wave radiation. Acta Hort. (ISHS) 1170:847-854
greenhouse, energy saving, heat transfer model, energy consumption