At What Time Scale Are Geothermal Resources Renewable?

Abstract

The geothermal resource base (=heat content of the accessible Earthís crust) is immense and ubiquitous. Geothermal resources are utilised by withdrawing the fluid and/or extracting the heat content of a reservoir, which causes depletion. After production stops the re-establishment of the original resource/reservoir conditions begins, by processes driven by natural forces like pressure and temperature gradients. The time-scale of recovery has been addressed by numerical simulations. The recovery times are, for the resource/utilization types considered: 1) high enthalpy, twophase reservoir, produced to generate electricity: several 100 years, 2) hydrothermal aquifer, used by doublet system for space heating, 100 ñ 200 years, 3) conductive heat extraction by shallow ground-source heat pumps: roughly the time of production (e.g. practical recovery in 30 years after a 30 years production period). Thus geothermal resources can be considered renewable on time-scales of technological/societal systems and do not need geological times for regeneration as fossil fuel reserves do (coal, oil, gas). Sustainable production can be achieved for types 2) and 3).