Electricity saving in greenhouse production with adjustable LED fixtures

Chlorofylfluorscense can maybe be used for on-line control of the lamps in the darkest time of the year when the maximum natural PAR < 500 μmol m

-2 s-1. In the spring – autumn season it is more doubtful due to the high natural light levels that cause extreme light fluctuations and thereby unstable photosynthesis, which is directly, reflected in the fluorescence measurements.

Fluctuations under 5 minutes (even big ones) does not trigger deactivation of the plant metabolism or serious stomatal closure. It is therefore not necessary to "fill holes" of short duration in the light by turning on the light for only few minutes. If this pays off or not, will be determined more by how big part of the total daily light integral that comes from the supplemental light.

The Greenhouse LED, SON-T, and terrass heater experiment showed that SON-T and LED can be used for growth, but SON-T provides higher leaf temperature. This will be an important issue for all species that require growth temperature above ca 20°C. A change to LED light will require a different climate control strategy via the heating system, to keep the required leaf temperature. To produce a certain amount of plant biomass will always require a certain amount of resources in terms of light, water/nutrients, CO

2 and temperature, regardless of the source. The trick is to balance the different resources, so no one is in excess that cannot be fully utilized.

In our greenhouse experiment it is obvious "a photon is a photon", i.e. the actual color of the supplemental light has no direct influence on the growth of the roses. However, this is because of the natural background daylight in the greenhouse. It is well known from the literature that different monochromatic light in growth rooms can cause severe problems with plant development and morphology, like leaf shape and branching. With increasing use of energy saving non-transparent, white insulation screens that block out the natural daylight in commercial greenhouses, this is an issue that has to be addressed further.

The energy saving model can probably save electricity in spring and autumn but the target for final plant size must be chosen with care and the higher target the smaller energy saving. However, to create a dynamic light control that saves energy while still ensuring an appropriate plant production one will have to include energy prices and weather forecasts, as well as photosynthesis models, as has been done in the Dynalight project that has run as collaboration between AU and SDU.

Energy saving in the greenhouse industry will today depend on energy saving models and hopefully in the near future on more effective LED’s.