Performance of a Covered Closed Loop Shallow Geothermal Greenhouse Heating System

A great variety of heating applications are being used nowadays in order to cover the greenhouse thermal needs. During the last decade a significant increase in installation of Shallow Geothermal Heat (SGH) systems assisted by closed loop Geothermal Heat Pumps (GHP) has been observed. The SGH systems are highly efficient, renewable energy systems for greenhouse heating and cooling. The main advantage of these systems consists on the simplicity of the application, the low cost of structure and the fact that no drilling is required. The SGH systems exploit the underground stored heat, even in shallow depths. The external air temperature variations interact directly only with the surface soil temperature, due to the soil heat capacity. As a result, the effect of the external air temperature variations is being reduced at deeper ground layers. The underground temperature value remains almost constant at a depth greater than 5.0 m, while in 2.0 m depth the underground temperature value has been proved that do not change significantly if the ground is under cover. Therefore, a fluid of a more or less constant temperature can be supplied to the primary GHP circuit by installing a ground heat exchanger. GHPs, operating with such a system, consume less energy than air-to-air heat pumps which are widely used. The present study presents a technical analysis concerning the installation and performance of a covered closed loop GHP used in a SGH system for greenhouse, without an auxiliary conventional heating system. The innovation of the examined system relies on the fact that the ground surface, above which the piping installation is under a polyethylene film cover or it is located inside a greenhouse. This system was proved experimentally to improve the heating performance of the GHPs.