IMPROVEMENT OF A HEAT PUMP COEFFICIENT OF PERFORMANCE USED IN GREENHOUSES

A convenient and economic method for greenhouse heating and cooling is the installation of Ground Source Heat Pumps (GSHP) to exploit shallow geothermal energy. GSHPs are attractive alternatives to conventional heating and cooling systems due to their higher energy utilisation efficiency. The technique relies on the fact that GSHP utilises the earth as a heat source in heating mode and as heat sink in cooling mode operation. In heating mode, GSHP absorbs heat from the ground and uses it to heat the greenhouse. In cooling operation mode, heat is absorbed from the internal greenhouse air and is transferred to the ground through its in ground heat exchanger. The heating performance of a GSHP is expressed by a dimensionless unit called Coefficient of Performance (COP). COP value depends on many factors, most important of which are: (a) the temperature difference (DT) between the generated heat pump thermal fluid and the heat source, and (b) the temperature stability of the heat source particularly between 0 and 30°C. The main objective of this research is to study the temperature variations among different depths and soil covering materials in order to maintain a higher COP value. Different soil covering materials, such as bare ground, ground covered by insulation material (i.e., extruded polystyrene) and ground covered by polyethylene film (greenhouse simulation) affects the underground temperature, even at the very shallow depth of 2.0 m. This can lead to an improved COP value due to a reduction of the temperature difference between the heat pump thermal fluid and the ground, as well as due to the fact that at 2.0 m depth the temperature remains rather constant.