Heating of greenhouse crops with microwave energy

The greenhouse vegetable industry in Northern Europe is an extensive, high value and energy-intensive horticultural sector that is vital for food supply and security in Europe. Despite technical and legislative innovation over a period of thirty years, little improvement has been made in the reduction of absolute energy consumption for heating greenhouse growing space. A novel free-space microwave system for heating greenhouse crops volumetrically is proposed. and demonstrated. It is estimated that such a system can use one-third of the energy of a conventional heating system. This would allow for significant reductions in energy consumption, reduce environmental damage
and provide financial savings of several billion Euros. Experiments on real plants
demonstrate microwave heating utilising ISM band frequencies as a viable method
for heating growing plants from seedling to fruition and reveal plant and fruit
quality that is comparable to that attained by conventional heating.
Non-uniformity of electric-field distribution within the plants, both between and
within plant components, is identified as the main limiting factor during microwave heating and can result in localised burning. This can cause catastrophic
failure when occurring on stems. The necessary reduction in power and the
resultant decrease in average plant temperature delays development relative to
conventionally-heated plants. Poor leaf heating and development is the main
source of delay in fruit formation. A novel practical technique for improving inter-
and intra-object heating uniformity is thus also presented and demonstrated. This
method utilises circularly polarised incident waves to allow more regular heating
of plant components and greater consistency of heating between different plant
components. Furthermore, this technique can be applied generally to other dielectric heating scenarios where heating non-uniformity is a problem, specifically
in industrial processes.
The concept of plant sectors and sections is also defined to allow for valid comparison of the energy consumption of conventional and microwave heating systems,
1Il a commercial setting.