Screens

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Unlike cover materials, which have fixed optical properties, energy screens installed in the greenhouse can be deployed and 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. In this way, the screens reduce radiative heat losses towards the cold roof and outside the greenhouse. 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.

Screen management is typically done by defining radiation and temperature levels above/below which the screen should close:

• Radiation is not as important a criterion for energy screens as for blackout screens - the screens are typically kept open during hours with sunlight to minimize light loss, and closed during night time. During warmer months, screens may be partially closed (e.g. 80%) at night to facilitate air exchange for cooling and humidity control.

• The temperature is an important criterion for maximum heat retention under cooler night-time or winter-time conditions. Especially for temperate winter conditions, energy screens may also be needed during daytime, and a ‘close below’ temperature criterion is defined where the screen is kept closed at lower temperatures despite some potential solar radiation loss.

N.b.:

• Also when screens are completely stowed there can be significant air exchange 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.

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

More intensive usage of screens - such as by closing screens for longer hours, especially during night time, and by using double screens - significantly improves insulation, and thus energy-saving. Some model calculations have shown that even adapting the criteria to close or open a normal screen can save up to 30 MJ/m² energy per year. Adding a screen with a high insulating value can further decrease the energy demand by 115 MJ/m² per year.

In general development and usage of screens that retain maximum PAR transparency, but are highly insulating and airtight, will retain energy in the greenhouse without compromising production. The desirable properties for screens are: high PAR transmission, low thermal transmittance (U-value) - which is a combination of low thermal conductivity, low air permeability, low emissivity and low thermal infrared transmissivity.