There are three classifications of geothermal power
Geothermal Energy Schematic Diagram [Source: DOE Geothermal Energy Technical Site (INEL)].
In general, geothermal can create air pollution from radon gas, hydrogen sulfide (H2S), CO2, methane, and ammonia emissions. Geothermal also has sizable thermal pollution due to its low thermal efficiency (approximately 20%) because of the relatively low steam temperatures.
Hydrothermal is the traditional commercial geothermal source. Hydrothermal uses hot water or steam directed to a steam turbine either on an indirect or direct basis, respectively. Most hydrothermal is based on hot water although steam can be found near geysers. The typical steam conditions are 400°F and 100 psi (significantly lower than fossil and nuclear generation schemes). Other hydrothermal drawbacks include its problem of depositing minerals on the components, and new wells must be drilled after a few years of use. Most hydrothermal in the U.S. is found in the western states (e.g., California).
Schematic of a Binary Cycle Power Plant [Source: DOE Geothermal Energy Technical Site (INEL)].
Schematic of a Flash Steam Power Plant [Source: DOE Geothermal Energy Technical Site (INEL)].
Geopressurized water is hot water around 350°F and 16,000 psi. This geothermal source is located primarily along Texas Gulf Coast, and thus, the water is unfortunately saline.
Electric Solar Geothermal Biomass