Selecting the Optimal Use of the Geothermal Energy Produced with a Deep Borehole Heat Exchanger: Exergy Performance

Geothermal sector has a strength point respect to other renewable energy sources: the availability of a wide range of both thermal and power applications depending on source temperature. Several researches have been focused on the possibility to produce geothermal energy without brine extraction, by means of a deep borehole heat exchanger. This solution may be the key to increase the social acceptance, to reduce environmental impact of geothermal projects, and to exploit the unconventional geothermal systems, where the extraction of brines in technically complex. In this work, exergy efficiency has been used to investigate the best utilization strategy downstream the deep borehole heat exchanger. Five configurations have been analyzed: a district heating plant, an absorption cooling plant, an Organic Rankine Cycle, a cascade system composed by district heat and absorption chiller, a cascade system composed by the Organic Rankine plant and the district heating plant. District heating results a promising and robust solution: it ensures high energy capacities per well depth and high exergy efficiency. Power production shows performances in line with typical geothermal binary plants, but the system capacity per well depth is low and the complexity increases both irreversibilities and sensibility to operative and source conditions.