Renewability Versus Sustainability. A Reservoir Management Approach.

Abstract

Within the renewable energy spectrum, geothermal heat deserves a special statute. Contrary to solar, wind and tidal, geothermal energy is not an inexhaustible energy source. Quoting Henry J. Ramey Jr. (1987), commenting, in a historic overview, on the fact that geothermal fluids were of meteoric origin and geothermal systems subject to transfer and recharge ?A popular theory was that production of a geothermal system at the natural recharge rate would lead to an inexhaustible source of energy. This idea appeared often and since unchallenged, was eventually accepted as a fact?. Actually, terrestrial heat is renewable as it flows to surface at a 0.06 Wm-2 (continental average) rate. Practically, it is exhaustible, given that its resupply is structurally diffuse (conductive) and its exploitation necessarily concentrated (convective) to meet, via a heat carrier fluid, the end users demand. Therefore, reclamation of geothermal resources complies with an exploration/production mining rationale (search of anomalies, i.e. occurrence of geothermal reservoirs and higher than normal heat flows in selected geodynamic environments). There is factual evidence, from pressure depletion and reservoir cooling, supporting the foregoing. The ground source (coupled) heat pump (GSHP) concept somehow mitigates this ?fatal? mining issue. Not only has it significantly widened the scope of the geothermal exploitable resource base by adding, irrespective of any reservoir/aquifer occurrence whatsoever, i.e. almost anywhere provided there is a local user, a considerable reserve stored in the uppermost 400 m of soil, it allows for extracting terrestrial heat at near to equilibrium conditions. The foregoing are illustrated and discussed through selected case studies addressing high enthalpy (dry steam, liquid dominated) and low enthalpy reservoirs and power generation and geothermal district heating (GDH) uses respectively. Emphasis is placed on design of optimum management strategies aimed at extending reservoir life via water injection, evolutive (production/injection) well arrays and absorption cooling, thus securing the sustainability objective. Last but not least, the renewability issue is challenged by modelling typical GSHP configurations.