Hot Dry Rock (HDR): Plentiful But Deep
Conventional geothermal technology entails the production of useful energy from natural sources of steam or, much more commonly, hot water. These hydrothermal resources are found in a number of locations around the world, but they are the exception rather than the rule. In most places, the earth grows hotter with increasing depth, but mobile water is absent. The vast majority of the world’s accessible geothermal energy is found in rock that is hot but essentially dry -- the so-called hot dry rock (HDR) resource.
The total amount of heat contained in HDR at accessible depths has been estimated to be on the order of 10 billion quads (a quad is the energy equivalent of about 180 million barrels of oil and 90 quads represents the total US energy consumption in 2001). This is about 800 times greater than the estimated energy content of all hydrothermal resources and 300 times greater than the fossil fuel resource base that includes all petroleum, natural gas, and coal. (Tester, et al. 1989).
Deep Direct Use Geothermal (DDUG)
Deep Direct Use Geothermal is the use of deep reservoirs of heated water that are of sufficient temperature to be used directly for building heat and domestic hot water (DHW). Typically wells are drilled in pairs; one supply and another for return once heat has been extracted (called “doublets”). Direct Use Geothermal is less expensive than HDR, and is practical in many regions where the heated aquifer resources are available. However, there is an even greater abundance of lower temperature geothermal than can be tapped using Enhanced Geothermal Systems (EGS).
Enhanced Geothermal Systems (EGS) and Organic Rankine Cycle (ORC)
EGS are used when the temperature of the geothermal resource is not quite hot enough to make steam to drive a turbine. One of the methods or processes is the Organic Rankine Cycle (ORC). The Organic Rankine Cycle is a thermodynamic process where heat is transferred to a fluid at a constant pressure. The fluid is vaporized and then expanded in a vapor turbine that drives a generator, producing electricity. ORC’s are also used to generate electricity from waste heat such as exhaust gas from generators or other heating processes. This is called co-generation.
While HDR geothermal produces electricity efficiently and effectively, it also has the costliest capital investment, and is limited to specific regions where hot dry rock resources are accessible.
Deep Direct Use Geothermal and GSHP have the lowest capital cost, and can be effectively applied nearly everywhere.
For more Information on geothermal power generation, visit www.geo-energy.org.