Unlike cover materials, which have fixed optical properties, energy screens installed in the greenhouse can be deployed on stowed on demand, enabling better control of these properties with changing seasons and light availabilities.
When the screens are closed, part of the thermal infrared (heat) radiation from inside the greenhouse is transmitted, reflected, or absorbed and emitted by the screen material - thus the radiative heat losses towards the cold roof and outside the greenhouse are reduced due to the screens. The air permeability of the screen also allows or restricts transfer of heat through the screen by air-convection - the lower the air permeability, the lower is the loss of greenhouse heat when the screen is closed.
The highest possible energy saving would be attained by screens with:
low air permeability
low absorption and emissivity for thermal radiation, coupled with
low transmissivity for thermal radiation (all radiative heat reflected by the screen)
Energy screens can be aluminized, or transparent. Transparent screens can even be used during daytime in winters for retaining heat but at the same time minimizing the loss in production. However, usage of an energy screen always lowers the total PAR transmission, hence screen management is crucial to balance energy-saving with light-loss and potential yield loss.
N.b.:
Also when screens are completely stowed there can be significant air axchange due to leakage.
Low permeability (air-tight) could require more frequent use of a screen gap, in order to control excess heat or humidity.
More information on screens can be found on the KasKieswijzer.
Contribution to energy balance and resource use of greenhouse:
The more hours an energy screen is used, the more energy can be saved. In practice this means that the screen should be deployed longer in the morning and earlier in the evening.
Screen management by defining radiation criteria ‘Close above’
In cold periods when screens are needed in daytime, define a temperature criterion ‘close below’
The air permeability of the screen also impacts the permeability of water vapour and CO2 through the screen - thus affecting the humidity and CO2 concentration in the greenhouse. Due to increased humidity levels under airtight screens, additional dehumidification systems might be required.
Possible steps towards sustainable, energy-efficient greenhouses
Better insulation = more energy saving. (add some numbers)
More transparent + insulating screens = less compromise with production while retaining insulating properties. Moving towards the goal of : impermeable + low emissivity + low thermal infrared transmissivity