Articles
ENERGY SAVING AND CLIMATE IMPROVEMENT WITH THERMAL SCREENS
The use of these screens is quite wide spread in modern glasshouses in North-western Europe, both with ornamental and with vegetable crops.
The big advantage of screens when compared with other insulating methods is the fact that the screen can be retracted during day time, thus limiting the light loss for a big deal.
Another advantage is the positive side effect of the screen on the greenhouse climate.
In order to realise a high energy saving, the screen material needs to reduce heat losses from the greenhouse as much as possible.
Heat loss occurs in three forms:
- convection/conduction;
- radiation;
- transpiration/condensation.
The first process, where hot air is rising until it touches the cold roof, where it is transmitted to the outside by means of conduction through the covering material, can be limited by using a type of screen that blocks the rising air stream as much as possible, so a screen that is as air tight as possible.
There is however a limit: a too closed structure inevitably results in condensation on the bottom of the screen, which has to be considered as undesired.
The second process can be stopped by using a material which is not transparent for long wave IR radiation.
Polyester is quite an attractive material in this view, as it absorbs up till 80% of this radiation.
Covering this polyester with a thin layer of aluminium, further limits the radiation loss with nearly 100% due to the very low emission value of aluminium.
Heat loss through transpiration/condensation can hardly be reduced, as a certain amount of transpiration (that costs energy) is needed in order to maintain a good functioning crop.
This means that too vapour tight screens are not so recommended, as they increase humidity too much, through which transpiration is limited.
Climate improvement with the aid of screens can be distinguished in different direction.
- Sun shading/cooling during the hot season.
The aluminised screens appear very effective in controlling the temperature, as the sun radiation is reflected, not absorbed.
Each crop has its own shade demand; so there need to be available a whole range of screens with different shade values. - Humidity control during cold season.
This is especially important for a starting vegetable crop, where evaporation and transpiration is still very low.
With special vapour tight screens, this can be solved, so that humidity will be more optimal for crop development.