| Abstract |
Geothermal resources are of a double nature, a combination of an energy current (through heat convection and conduction) and of stored energy. The renewability of these two aspects is quite different as the energy current is steady (fully renewable) while the stored energy is renewed relatively slowly, in particular the part renewed by heat conduction. Production from geothermal resources is, therefore, not a mining process. Unlike in ore mining or hydrocarbon extraction, the heat/fluid removed from the resources is continuously replaced at the site of production. Experience from the use of geothermal systems worldwide, lasting several decades (in some cases half a century or more), demonstrates that the systems often appear to attain a sort of semi-equilibrium in physical conditions during long-term production. In some cases, this equilibrium is found by gradually increasing production rate and in other cases equilibrium is found after a limited period of higher extraction rates. In yet other cases, physical changes in geothermal systems are so slow that their output is not affected for decades. Therefore, a sustainability time-scale of 100 to 300 years has been proposed. Studies furthermore indicate that the effect of heavy utilization is often reversible on a time-scale comparable to the period of utilization. Sustainable geothermal management basically involves setting up and maintaining a specific long-term production scheme. The most practical way to utilize a geothermal resource in a sustainable manner is through a step-wise increase in production, even though other sustainable utilization schemes can be envisioned (like moderate circulation flow-rates in EGS or variable production to follow load coming from wind and solar utilization). Distinction needs to be made between sustainable production from a particular geothermal resource and the more general sustainable geothermal utilization, which involves integrated resource, economic, social and environmental development. Yet, the key element of sustainable geothermal utilization is sustainable production from a given resource. A sustainability policy (protocol) should be based on general sustainability goals and include specific sustainability indicators to measure the degree of sustainability of a given geothermal operation. |