Energy performance comparison of two small scale combined geothermal heating plants for greenhouse heating

The focus of this paper is on the energy performance and thermo-economic assessment of small scale (50 kWth) heating plants to match a greenhouse (468 m2) energy demand. The annual energy demand of an air inflated-double layer polyethylene film greenhouse located in Apulia region (South of Italy) is considered. Two different system configurations are designed to produce hot-water by using low enthalpy geothermal source and a natural gas engine. The systems analyzed are: i) a grid-connected and electricity-driven ground source heat pump, ii) a gas engine-driven ground source heat pump feed by natural gas. The heat pump Model NRW 127 HA (brand AERMEC), which uses R407c as a refrigerant fluid circulating inside its circuit, is the unit used in the i) system. Instead, the Model AWGP450E1 16HP manufactured by Aisin (TOYOTA) is the gas heat pump unit used in the ii). According to the technical data provided by the manufacturers, the GSHP and GSGHP output is 48 kW thermal power and the input is 36 kW. The GSHP and the GSGHP are equipped with ten geothermal closed-loop vertical boreholes 100 m deep and modelled assuming data from existing commercial plants. The global thermal resistance values of the covering material of the greenhouse were 0.13 m2· °C·W-1. The investment profitability is assessed in light of the Italian regulations. The coefficient of performance (COP) of the heat pumps is 4 for configuration i), while the gas utilization efficiency of the ii) systems is 1.6 and the heating consumption of methane is 2.2 kg·h-1. The heating system increased the greenhouse air temperatures by 10 °C respect to the external air temperatures and climate conditions. Average hot water outlet temperatures between 35 °C and 45 °C are obtained over the considered range of the external operating parameters and this met the temperature demand of the greenhouse.