A data-driven approach to low-enthalpy shallow geothermal energy extraction: A case study on indoor heating for precision agriculture applications

Shallow geothermal energy systems utilize the upper layers of the ground to provide space heating and cooling, with a relatively high efficiency. The research presented aims to assess the potential harnessing of this source and applying it for indoor conditioning of a precision agriculture facility. An experimental phase was conducted to measure the temperature of the ground at 0.75 m, 1.5 m, 2.25 m, and 3.0 m. Field testing was performed employing two probes; one uncovered and a second insulated with a polyethylene sheet, to evaluate the effect soil water content variations exert over the soil temperature. It was found that, given the high permeability and good drainage of the soil material, water content is not statistically significant, ruling out the presence of microconvective phenomena within the interstices of the undisturbed earth. Based on the temperature profile obtained, achievable heat extraction capacity was computed by modelling a buried pipe system. A parametrization was conducted showing that, of all the involved factors, pipe depth, pitch and their interaction, are the ones that contribute the most to the heat output variability. The computed available heat flow was used to simulate the performance of a geothermal heat pumping system for greenhouse heating based on real operation data, provided by the owners. The results show that, shallower layers of the ground, can provide adequate solutions for immediate thermal needs; a 5-pipe, 100m-length ground loop at 0.75 m, could provide 90% of the total thermal demand, whilst an 11-pipe, 200m-length loop, at 1.5 m depth, reaches 84% of the same demand, with a variability reduction in availability of 10% to 20% depending on the severity of surface conditions. However, for deeper layers, i.e., 2.25 m and 3.0 m, respective maximum heat output amounts to 43% and 49% of the total thermal demand. Nonetheless, these configurations yield the most stable and constant heat flow of the lot, making them particularly suitable for preheating/subcooling processes or even long-term thermal storage. Despite not fulfilling the total thermal demand of the greenhouse, these setups are more than capable of providing support to on-site conditioning devices; requiring a lower upkeep and yielding a higher efficiency, which would reduce overall production costs. Furthermore, from the conducted data analysis, it was found that controlling the agronomic values, associated to indoor agriculture, facilitates the physical aspect of greenhouse energy management, paving the way to more optimized thermal solutions, tailored to different sets of operation conditions.