This marks my 40th year working in the renewable energy space that is solar energy.
Geothermal energy is, literally, the heat of the earth. The heat itself derives from radioactive decay beneath the earth's surface and, in certain locations, it is concentrated enough and is close enough to surface waters to be brought to the surface for a variety of purposes. When it is above 150 degrees C (302 F), it is usually considered hot enough to be used to generate electricity as it is in Italy, El Salvador, Mexico, Japan, Iceland, and Indonesia, among other countries. No such operations exist in Arizona, but several power plants are currently in operation just west of Yuma, Arizona in the Imperial Valley of southeastern California. Although some high temperature geothermal resources exist southeast of Phoenix near the now-retired Williams Air Force Base, they have never been deemed economically feasible.
Resources less than 150 degrees C, have wide non-electric applicability. Indeed, the worldwide potential of such temperatures is many times larger than that used to generate electricity. Such temperatures are used in greenhouses, hot baths, onion dehydration, laundries, and even hotel space heating. The capital of Iceland is almost entirely heated with geothermal water. Several heating districts exist in the US, although none are as large as those in Iceland. These include projects in Reno, Klamath Falls, Boise, Susanville, and other locations. The best source of information in the US on such non-electric applications is the Oregon Institute of Technology Geo-Heat Center.
In Arizona, the opportunity to use geothermal water is limited, in part by population distribution, yet at least three locations are well known. These are Buckhorn Baths in Apache Junction, Castle Hot Springs in the Bradshaw Mountains, and Childs on the Verde River. Additionally, the two highest temperature springs in the state are Clifton and Gillard, both in the Clifton-Morenci area of southeastern Arizona. The water temperature at these springs ranges from 158-180 degrees Fahrenheit. Even though temperatures may exceed 284 degrees Fahrenheit at depth, these two sites are only suitable for low grad steam.
The only types of geothermal energy to be commercially developed are those called "hydrothermal". These include steam, as developed at The Geysers (north of San Francisco), and liquid, as developed in southeastern California. Geothermal energy is also available in several other forms. One of these forms, known as hot-dry rock has attracted some attention in the volcanic areas of the White Mountains, east of Phoenix. In such resource areas, heat is available, but there is insufficient water to conduct the heat to the surface. In some of these cooler climes, geothermal heat pumps might be a sensible application. The Geothermal Heat Pump Consortium maintains a web site with more information.
In summary, major geothermal resources exist near but not in Arizona. The resource that exists in the state has been recognized and, to some degree, explored, but no sites are considered economically commercial at this time. For more information on geothermal power, visit: http://www.geothermal.org/links.html