|Authors: ||U. Schmidt, C. Huber, D. Dannehl, T. Rocksch, H.-J. Tantau, J. Meyer|
|Keywords: ||DescFog, leaf temperature, phytomonitoring, solar energy, thermal radiation, tomato, vapour pressure difference|
Closed greenhouses are operating for simultaneous solar energy collecting and plant production.
For this, a compromise has to be made between climate control in order to withdraw a maximum of heat energy and optimal growth condition.
The coefficient of performance of a solar heat system increases with low temperature differences between heat source and heat sink.
This calls for low temperature heating systems for heat transmission to the plant itself.
On the other hand, higher temperatures and higher relative humidity should be accepted to increase the energy output from the closed greenhouse.
In a three year experiment these kinds of special microclimate conditions were tested with tomatoes.
In a comparison experiment the impact of higher temperatures in combination with high relative humidity and higher CO2 content on vegetative growth, fruit yield and fruit ingredient was examined.
In the result a 15 up to 40% higher yield with a significant increase of lycopene content in the fruits was estimated.
On the other hand, there was a dramatical decrease of transpiration on day time induced fungi diseases if the vapour pressure deficit was too low.
Consequences of heat transmission by thermal radiation on transpiration and latent heat losses were obtained.
At night time thermal radiation transmission leads to higher leaf temperatures.
Leaf temperature differs depending on the distance to the radiation source.
The maximum difference was measured with a 7 K higher leaf temperature at a distance of 700 mm from a glass body radiator.
Higher vapour pressure differences induced higher transpiration and increased latent heat losses.
Thus, a combined control of convective heat and heat radiation is an essential prerequisite for the optimization of heat transfer to the plant.
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