Geothermal Sustainability or Heat Mining?
Heat mining” is, in fact a complete deceptive misnomer. When a mineral deposit (e.g. copper) is mined and the ore has been taken out, it will be gone forever. Not so with geothermal resources: The heat and the fluid are coming back! Namely, the heat and fluid extraction create heat sinks and hydraulic minima; around these, strong temperature and pressure gradients develop. Along the gradients, natural inflow of heat and fluid arises to replenish the deficits. The inflow from the surroundings can be strong: around borehole heat exchangers, heat flow densities of several W/m2 result, whereas terrestrial heat flow amounts only to about 50 – 100 mW/m2.
The regeneration of geothermal resources after production, in other words, extraction of fluid and/or heat) is a process that runs over different timescales, depending on the kind and size of the utilization system, the production rate, and the resource characteristics. The resource renewal depends directly on the heat/fluid backflow rate. Heat, respectively fluid production from geothermal resources can be accomplished with different withdrawal rates. Although forced production is more attractive financially (with quick payback), it can nevertheless degrade the resource permanently. The longevity of the resource (and thus the sustainability of production) can be ensured by moderate production rates. The sustainable geothermal production level depends on the utilization technology as well as on the local geologic conditions. The stipulation of the sustainable production level requires specific clarifications, especially by numerical modelling, based on long-term production strategies. In general, resource regeneration proceeds asymptotically: strong at the beginning and slowing down subsequently, reaching the original conditions only after infinite time. However, regeneration to 95 % can be achieved much earlier, e.g. within the lifetime of the extraction/production system. In other words, geothermal resources may under certain circumstances may be considered as having potential regrowth, like biomass. Concerning the requirements for such sustainable production, it is convenient to consider four resource types and utilization schemes. These may be treated by numerical model simulations that consider heat extraction by geothermal heat pumps, hydrothermal aquifer, used by a doublet system for space heating, high enthalpy two-phase reservoir, tapped to generate electricity, and enhanced Geothermal Systems (EGS).
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Open Access statement
The International Journal of Terrestrial Heat Flow and Applied Geothermics (IJTHFA) provides immediate open access to its content on the principle that making research freely available to the public supports a greater global exchange of knowledge. Full-text access to scientific articles of the journal is presented on the official website in the Archives section.
The IJTHFA is an open access journal, which means all its content is freely available without charge to the user or his/her institution. Users are allowed to read, download, copy, distribute, print, search, or link to the full texts of the articles, or use them for any other lawful purpose, without asking prior permission from the publisher or the author as long as they cite the source. The journal is licensed by Creative Commons Attribution International BY-NC-SA 4.0.
Journal metrics and indexing:
International Scientific Indexing (ISI)
Impact Factor: 1.078