| Abstract |
International organisations have classified geothermal energy as a renewable energy source, but it is sometimes stated that this is not the case. The meaning of the concepts ìrenewable energy sourcesî and ìsustainable energy productionî is discussed in this paper. Comparison is made with the resources of hydropower and fish stocks in order to clarify the meaning of the two concepts. It is found, that the rate of energy recharge to geothermal systems is the most critical aspect for the classification of geothermal energy as a renewable energy source. In the exploitation of natural geothermal systems, the recharge of energy takes place by advection of thermal water at the same time scale as the production from the resource. This justifies the classification of geothermal energy as a renewable energy source. In the case of hot dry rock, and eventually some of the hot water aquifers in sedimentary basins, the energy recharge is only by thermal conduction and, due to the slowness of this process, hot dry rock and some sedimentary reservoirs should be considered as finite energy sources. |