Development of a microwave system for greenhouse heating

Greenhouse heating in cold and mild climates is usually by traditional heating methods such as hot-water and hot-air systems. In both methods a certain amount of energy is wasted on heating the greenhouse air and construction, which results in an increase in heat losses to the surroundings. An alternative heating method, which is of interest because of its potential to rapidly heat the plants with less heat losses to the surrounding is microwave heating (MWH). This was tested in the present study, in a small scale greenhouse. A 500 Watt (maximum power) microwave generator at a frequency of 2.45 GHz was used to heat mature tomato and pepper plants. For reference and comparison, an identical nearby greenhouse was heated with hot air. The experimental results showed that it is possible to heat the plants with microwaves without visible damage, and no increase in susceptibility to gray mold (Botrytis cinerea). During the heating period, the leaves were always warmer than the greenhouse air. Hence, it can be expected that the risk of occurrence of humidity promoted diseases (e.g. those that are caused by B. cinerea, Fulvia fulva and Sclerotinia sclerotiorum) will be reduced. In addition, the greenhouse air did not warm up and its temperature was nearly constant. It appears that the non-uniformity in temperature distribution with MWH was not much worse than with the hot-air heating. The energy required for MWH was about 0.55 of that required by hot-air heating.