Page 133 - Geothermal Energy Renewable Energy and The Environment

7 Resource Assessments

Assessing a resource is a fundamental activity that is common to all enterprises in which the amount
of a commodity must be established for economic, management, or scientific purposes. With respect
to geothermal resources, the scale at which this effort is carried out can be quite local, such as
assessing the long-term energy availability from a single hot spring, or it can be extremely broad,
such as a national resource assessment. Despite these vastly different scales, the methodologies
employed are surprisingly similar since, in all cases, what is sought is a measure of the amount of
heat that is accessible, the extent to which it can be economically extracted using available technol-
ogy, and what its lifetime is. Beyond this similarity, however, important differences exist in how
the necessary information is collected and how it is processed. This chapter will first consider how
a local resource is evaluated, and then consider the ensemble of information that is necessary to
conduct national and international assessments and how those data are processed.

assessInG a GeoThermal resoUrce

We will consider here the issues associated with establishing the available heat for applications that
will either directly use the heat, so called direct use applications (discussed in detail in Chapter 11)
or will use the heat to generate power (Chapter 9). We will not consider resource assessments for
ground source heat pump applications (Chapter 10) since the heat available for their deployment
exists everywhere: The most significant challenge for their economic viability is the local geother-
mal gradient, drilling costs, and competition with other energy sources and is not directly a function
of the availability of heat.
Geothermal resource assessments have been an integral part of developing a geothermal resource
ever since the industry began producing power. Such an effort is the first step in deciding whether
investment in a facility is an economically viable undertaking. However, to better understand the
magnitude of the geothermal resource base and the ability of this energy source to contribute to
local, regional, or national energy markets, it is necessary to develop a more systematic analysis
than is usually pursued when considering the development of a single facility.
In 1979 the United States Geological Survey published its most recent systematic assessment
of the geothermal resource in the United States (Muffler 1979), which followed the first national
assessment carried out in 1975 (White and Williams 1975). A new assessment is currently underway
and should be available by the end of 2010 (Williams et al. 2008b).
Since publication of the 1979 report, several other assessments have been carried out on a vari-
ety of scales, most of them regional or state-wide (Gawell 2006; Lovekin 2004; Petty et al. 1992;
Western Governor’s Association 2006). As summarized by Gawell (2006), different approaches
and underlying methodologies have given rise to a broad range of results that are not directly
comparable. This diversity of results emphasizes the important fact that resource assessments
provide results that are sensitive to the methodology and assumptions employed in the analysis.
As a result, it is entirely possible that different studies will produce widely different estimates of a
resource, and yet they each may be correct. In the approach we will take here, we will follow the
methodology that was developed by Muffler and Cataldi (1978), Nathenson (1975), and, White and
Williams (1975), employed by Muffler in his 1979 assessment, and modified by Williams, Reed,
and Mariner (2008a).

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