SOLAR HEAT SUPPLY FOR GREENHOUSES WITH STRATIFIED SOIL HEAT STORAGE: DATA COLLECTION AND FEASIBILITY STUDY
These investigations were intensified lately with an objective of utilization of waste heat of power plants, geothermal heat and of solar energy. However, the available results reveal several shortcomings with regard to warmer climates.
While heat may be continuously applied at relatively shallow depths (30- 0 cm.) in colder climates, if applied by day under local winter conditions it may cause overheating of the root zone with attendant water losses and increase in salinity. Application of heat at night only would require costly storage capacity, heat transfer and control system.
We use the greenhouse soil as a heat storage medium and a self control mechanism for the supply of heat to the root zone of flower beds and subsequently to the greenhouse air. Heat is stored initially at different depths in the soil to allow for the delay in delivery by conduction. Low cost, pool heating solar collectors are used to supply warm water which is circulated through plastic irrigation pipes buried in the ground. The depth of storage depends on local soil thermal properties, on the irrigation method and the season, with 2-3 layers of heat input pipes placed at the optimal depths. This creates a stratified storage of heat which is utilized according to seasonal and weather conditions.
Model calculations are used to find the optimal depth for each stratum. Measurements of heat distribution are used to calibrate the model for the local values of thermal parameters.
The effect on root zone heating will be evaluated in terms of flower yield and quality. Air temperature is enhanced with the aid of thermal curtains of IR absorbing polyethylene. This study is expected to provide input data for technological and economic evaluation of an autonomous solar greenhouse.
While this experiment is performed with the aid of a solar heat supply, the general design is open ended and can be modified to accept warm water originating from geothermal and waste heat.