A preliminary comparison of subsurface energy applications from the exergy perspective as a tool for sustainable use of subsurface resources assessment

The subsurface (geological space beneath  Earth’s surface) is increasingly treated as a multifunctional resource that must serve multiple purposes in the era of the low-carbon economy. The most important current and near-future energy-related uses may include:
•    conventional and enhanced geothermal systems (EGS) 
•    low- and medium-temperature aquifer thermal energy storage (ATES) 
•    borehole thermal energy storage (BTES) 
•    underground hydrogen storage (porous reservoirs or salt caverns)
•    etc.
Exergy analysis offers a rational way to compare different applications while answering the question: how much useful energy could theoretically be obtained from each cubic meter of subsurface space used in a given way?
Purely volumetric approaches („how many m³ do we have?") can be very misleading – exergy density is usually a much better indicator of real resource value. In this view, priority should be given to high-exergy applications in the most valuable parts of the subsurface.  In the case of energy storage technologies, exergy loss is proportional to the entropy change due to heat dissipation to the environment. This effect will be greater the higher the temperature of the stored heat. The article considers temperature ranges typical for heat storage technologies.