Effect NIR-filter in greenhouses climate disappointing

Half of the sun energy reaching the earth cannot be seen and cannot be used for photosynthesis, but does add to the warming and evaporation. This energy is near infrared radiation (NIR). By keeping this radiation away from the greenhouse it is possible to control the temperature inside the greenhouse better. This is an advantage both with semi-closed greenhouses and with tunnel greenhouses in the Mediterranean areas.

The chemical industry have developed new materials containing particles to absorb or reflect the NIR-radiation. Reflection can also be achieved by specially manufactured foil (with the so-called multi layer technique). It is expected, that the addition of these nano particles will prevent the NIR-radiation from entering the greenhouses and that as a result the temperature in the greenhouse will not increase so quickly.

Absorber and Reflector

Calculations on models by researchers of Wageningen UR Glastuinbouw have, however, shown that the interaction between the roof of the greenhouse and the plant will destroy this effect. In the greenhouse various processes play a role in the warming and the reflection of light. The relatively high NIR-reflection of the plants cause ‘multiple reflection’ between NIR-reflecting greenhouse roof and the plants and in this way the NIR-radiation passing through the roof is ‘caught’ in the greenhouse. The nett effect of the roof is very small as a result. The desired effect can, according to calculations, be achieved when the NIR-reflectors become much better than the present generation.

A different situation arises by a NIR-absorber. Then the temperature in the glasshouse increases. Model calculations show that a large part of the energy, which was removed, can enter the greenhouse in other forms. Finally it becomes clear, that during the winter months the effect of a NIR-absorbing filter causes a decrease in temperature, but not during the summer. During the summer months the NIR-absorbers even show a negative effect, according to the calculations the temperature increases.

There are few possibilities to manage the climate in tunnel greenhouses. Especially in so-called passive greenhouses these new materials appear to have little effect. The calculations, however, were done on a model not specifically developed for this kind of greenhouse. The researchers will validate the model and the calculations with practical research in a multi-tunnel greenhouse in Almeria, Spain.