Performance comparison between fuel cell coupled with geothermal source heat pump and geothermal source gas engine heat pump system for greenhouse heating: A mathematical study

LPG, diesel and natural gas are generally used for greenhouse conditioning. Alternative technologies should be developed to increase the productivity of the protected environments. Innovative solutions are represented by photovoltaic, geothermal, wind and solar thermal integrated in a stand-alone system in agriculture land. The present paper compares the performances of two renewable energy systems for greenhouse heating based on geothermal and hydrogen technologies. The first integrated system is composed by a photovoltaic array, an electrolyzer, a hydrogen storage tank, a fuel cell and a ground source heat pump connected to a geothermal borehole. The second system, instead, is composed by a photovoltaic array, an electrolyzer, a hydrogen storage tank and a gas engine heat pump connected to a geothermal borehole. In order to compare the two systems, both heat pumps produced the same greenhouse heating power input. The results show a difference between the internal and external greenhouse air temperature from 7 to 15 ºC in winter, considering a deep insulating greenhouse cover material. As regarding the first system, the following energy efficiency has been calculated, photovoltaic arrays 13 %, electrolyzer 50 %, fuel cell 40 % and the ground source heat pump coefficient of performance 400 %. Than the total energy efficiency of the first system is 10.4 %. Instead, the overall efficiency of the second system is 11.9 % considering the same performance of the photovoltaic arrays and the electrolyzer of the first system and the ground source gas engine heat pump's primary energy ratio of 181 %. The primary energy ratio of the ground source gas engine heat pump seems to be low and not competitive respect to the coefficient of performance of a ground source heat pump, but considering the overall efficiencies of the both systems the performances are reversed. Furthermore, the first system is more complex than the second one.