Some proponents of geothermal energy have described this energy source as renewable, but many geothermal fields show declines in output as exploitation proceeds. On the other hand, those who would call geothermal a depletable energy source have to explain how some mature fields are able to produce with negligible declines and no apparent limit to the amount of recoverable energy. The confusion arises out of the attempt to describe geothermal resources using inappropriate conceptual models of how resources behave. This paper presents a conceptual model that is better suited to describe the expected performance of a geothermal field over its entire life cycle. The paper also describes a set of terms that can be used to quantify geothermal resources and to guide plans for development.

For the purposes of this paper, sustainability is defined as the ability to economically maintain the installed capacity, over the amortized life of a power plant, by taking practical steps (such as, make-up well drilling) to compensate for resource degradation (pressure drawdown and/or cooling). Renewability is defined here as the ability to maintain the installed power capacity indefinitely without encountering any resource degradation; renewable capacity is, however, often too small for commercial development. This paper also considers an additional level of commercial capacity (above the sustainable level) that is not planned to be maintained fully over the entire plant life as mitigation of resource degradation would become uneconomic or otherwise impractical at some point. This declining capacity above the sustainable level is considered commercial only if the levelized power cost is lower than that from alternative renewable, or environmentally benign, energy sources. Even if power cost at this unsustained commercial generation level proves higher than that from fossil fuels, this additional capacity can reduce fossil fuel usage if power from renewable or environmentally benign energy resources is given adequate tax breaks or price support. Displacement of fossil fuel usage is a social imperative that would reduce environmental pollution today and preserve these fuels as raw material for organic chemicals, and for potentially cleaner power generation in the future.