ABSTRACT
A variety of methods or criteria have been applied to analyze geothermal systems, reecting their complex and multidisciplinary nature. Some of the main schemes employed and discussed below include the following:
• How heat is transferred (conductive systems vs. convective systems) • The types of heat sources (presence or absence of underlying molten rock
or magma) • The geologic or tectonic settings (location along or near plate boundaries or
within the interior portions of continents) • Environments of low, moderate, and high enthalpy or heat content • The type of uid medium present in the geothermal reservoir (liquid-or
vapor-dominated) • The uses of geothermal systems (e.g., power production, direct use of geo-
thermal uids, or geoexchange, also known as ground-source heat pumps or geothermal heat pumps)
In the Earth, heat is transferred mainly by either conduction or convection. Conduction is the transfer of heat by contact, and convection involves the transfer of heat via motion, mainly of a uid, such as a liquid or gas. Convection is initiated
from a thermal gradient that causes changes in the density of a uid. In the presence of a gravitational eld, the changes in density cause hot material to rise (become more buoyant) and cold material to sink (become less buoyant). Convection, therefore, requires good permeability and porosity* in order for uid to ow and transfer heat. Where porosity and permeability are low, on the other hand, heat is more slowly transferred by conduction. Conductive and convective heat ow processes are discussed in further detail in Chapter 3. The difference in rates of heat transport between conduction and convection has implications for the sustainability of developed geothermal systems, as explored in Chapter 12.
