At Lans Zeeland, the first tomato cultivation in practice was monitored during a few winter months under ± 215 μmol / (m2.s) full-LED in terms of light distribution, greenhouse climate and heat demand. The LED lamps are supplied by Q-Westland. The lamps distinguish themselves from the most common LED lamps by a broad light spectrum (white light) and a fan that blows the lamp heat downwards, Marcel Raaphorst and Kees Weerheim of Wageningen University & Research Business Unit Greenhouse Horticulture write on Greenhouse as Energy Source.
Light and temperature distribution
Monitoring has shown that the used generation of lamps is not yet ready for use in practice because 10-20% of the lamps failed, probably due to moisture. At 1.92 μmol / J, the efficiency of the lamps under practical conditions is 10% lower than under lab conditions (according to the supplier's statement). The light distribution turned out to be much less even at top height (180 cm below the lamps) than with SON-T lamps. However, at a height of 230 cm below the lamps, the light distribution was uniform and by crop growth and production the PAR difference at top height could not be recognized.
Not only for the amount of light, but also for the temperature and the humidity at top height, it makes a difference whether measurements are taken directly under the LED lamps or between them. The fan in the LED fixture blows warm air downwards and smoke tests have shown that this air is blown more down than sideways and reaches just as far as the top of the crop. The smoke did not penetrate deeper into the crop than at the top.
Higher greenhouse temperature
Until January 30, when the measuring box was moved at crop top height from underneath the lamps to between the lamps, a higher greenhouse temperature and lower RH were measured than afterwards. This is also one of the reasons that hardly any heat was used in the lower net during the first few months. The temperature and the RH of the measuring box indicated a high temperature and a low RH. Because the crop had grown as desired despite this deviating measurement, the strategy with a small, lower pipe rail was not changed and the heat demand remained ± 35% lower than in the SON-T departments. Crop intake (≈ evaporation) also remained lower than in the SON-T departments with a comparable lighting level. However, this does not yet explain the lower heat consumption.
The temperature at top height was found to be slightly higher in the LED department during lighting hours even after moving the measuring boxes to the level of the ripening fruit, while no difference was measured in the SON-T departments. This is in contrast to previous tests with LEDs above the crop, and can partly be explained by the lower temperature of the low net and by the effect of the fan in the LED lamp.
The crop growth and production under LED lamps went according to the satisfaction of the grower. To be able to give quantitative conclusions, a reference greenhouse with SON-T lamps and the same variety is needed, the researchers conclude.