Agenda - Planning Commission - 06/03/2010

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The lifetime of a gepthefmal power plant is somewhat uncertain, and depends in part on the quality and extent of the geothermal resource and whether the water vapor is re- injected. Because ofthe expense ofresearch and development, geothermal energy devel- opersare careful to develop resources where scientific data indicates that the resource is likely to be strong and stable over a[ least 30 years. However, there is always uncertainty about what may occur deep below the earth's crustthat could shorten or extend the pre- dictedlifetime orrequire retooling the plant or drilling newwells. CHARACTERISTICS AND IMPACTS OF GEOTHERft1AL DEVELOPMENT Footprint According to the U.S. Department of Energy, geothermal electricitygenerationrequires one to eight acresper megawatt (MW) versus five to io acres per MW for nuclear opera- tionsand igacres per MW forcoal power plants.Atypicalgeothermalpowerplant often sits on five to io acres awned by the energy company. - Awellfield for a geothermal power plant can easily span z,ooo acres. The wellfield subsurface rights are typically leased by the energycompany, while land ownership remains in private or public hands. Above- groundstructures inthe wellfield associated .with the ppwer plant are generally quite lim- ited, consisting primarily of insulatedpipes that conduct [he heat to the power plant. Generally, wellfields that tap h igh-temper- ature, utility-grade geothermal resources are compatible with agricultural and other., low-intensity uses. However, uses such as residential develbpment; which would Concentrate people nearthe pipes, create concern about possible tampering or un- likelyaccidents such as blow-outs that could potentially create hazards.Forthisreason, - rural areas developed with high-temperature wellfields generally should noYbe converted to more urbanized uses until or unless the wellfield is no longer productive. A geoth ermal direct use typically relies on one or more wells located on the same property as the surface use. These uses have footprints that are comparable to simi- laruses that use other heat sources. Emissions and Effluent While there is a range oftechnologies in use in geothermal power plants (including flash, dry steam, and binary), all geothermal power plants emit some amount ofwater vapor and many emit trace-amounts of other gases. In some cases, the vaporemitted by the plant may have a sulfurous "rotten egg" or other noticeable odor caused by gases such As hydrogen sulfide. Mitigation ofsuch odors and the substances that cause them may fall under state or federal air quality standards or may need to be regulated at ttie total level. As the steam is used far electricity generation or a di rect use, it cools into liquid water. Many geothermal.plants reinject such effluent, which can help pro- longthe life of the geothermal resource. Otherwise, the discharged water, which is warmer than surface waters and may contain higher concentrations of dissolved mi ner- als,may have environmental and habitat impacts on surface waters into which it may be discharged. Geothermal direct uses also use steam that cools into liquid water and must be reinjected ordischarged. Reinjection typi- cally prolongsthe life ofthe resource and is a hedge against subsidence (See the discussion below.) However, because injec- tionwells, like production wells, may cast as much as a million dollars to drill, geothermal direct uses may not be financially feasible if reinjection is required. Noise.. During development, drilling noise in the geothermal wellfield area is considerable and can last for months to years, depending ZONINGPRACTICE 5.ao eC AMERIGW PIANNINGASSOCIAiIONlpoge® C