Feasibility of an electrical radiative heating system in greenhouses

Electrical heating elements were tested for their possible use as a radiative heating system in greenhouses and to find out whether these elements - despite the high cost of electrical energy - could economically be used for the prevention of frost damage in those greenhouses where conventional heating systems cannot be considered because of their high capital cost. The performance of the heating elements was studied in, an "unheated" plastic tunnel and in a single glazed glasshouse heated to 5 degrees C by means of a conventional pipe heating system. Heating elements were spaced 1, 1.5, and 2 m apart and the effect on vegetation temperature was followed. To simulate the maximal performance of the radiative heating elements, a simple steady state model describing vegetation and element temperatures as a function of inside climate, temperature of the surroundings and power input into the elements was built. From the experiments it was found that the radiative heating system could not be used successfully in the tunnel: the vegetation temperature almost equalled the outside air temperature. In the glasshouse the use of the radiative elements resulted only in a slight increase of the vegetation temperature. Comparison between experimental and simulated results showed that only 40% of the maximally achievable vegetation temperature increase was really obtained. Since even the maximum vegetation temperature increase at rather high electrical energy inputs (100 W.m(-2) greenhouse floor area) was too low, it was concluded that the system was not beneficial from both a technical and economic point of view