Page 213 - Geothermal Energy Renewable Energy and The Environment

11 Direct Use of Geothermal

Resources

Unlike geothermal power generation, in which heat energy is converted to electricity, direct use
applications use heat energy directly to accomplish a broad range of purposes. The tempera-
ture range of these applications is between ~10°C to ~150°C. Given the ubiquity of this temperature
range in the shallow subsurface, these types of applications of geothermal energy have the potential
to be installed almost anywhere that has sufficient fluid available. In this chapter the basic principles
that underlay these applications will be detailed and examples of the applications will be discussed.
We will begin the discussion by examining estimates of the magnitude of the resource.

assessInG The maGnITUde oF The dIrecT Use reserVoIr
Approximately 5.4 × 10 J (Dickson and Fanelli 2006) of thermal energy are available in the con-
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tinents of the Earth, of which nearly a quarter is available at depths shallower than 10 km (Lund
2007). To be useful directly, the heat must be significantly above ambient surface temperatures and
capable of being transferred efficiently. Such conditions have traditionally been satisfied in places
where warm or hot springs emerge at the surface, or in locations where high thermal gradients allow
shallow drilling to access heated waters. Such sites are relatively restricted in their distribution,
being concentrated in regions where volcanic activity has occurred recently or where rifting of con-
tinents has happened (Chapter 2). For these reasons, a relatively small fraction of the large amount
of heat contained within the continents can be economically employed for geothermal direct use
applications.
The proportion of the heat that is readily available is not well known since thorough assessment
efforts to quantitatively map the distribution of such resources have thus far been limited. If it is
assumed that active plate margins and collision zones are the most likely sites where elevated, near-
surface temperatures are sufficient to drive the circulation of warm water, approximately 1–10% of
the area of continents have the potential to support direct use applications using available technol-
ogy for accessing subsurface fluids. As drilling technology improves and fluid circulation to support
heat harvesting at depth improve, the proportion of the continental thermal resource that can be
accessed will significantly expand.
As of 2004, approximately 76 TWh/yr of thermal energy were used for direct use purposes
worldwide, which was derived from an installed capacity of 28,268 MW (Lund, Freeston, and Boyd
2005). For comparison, global consumption of electricity in 2006 was 16,378 TWh/yr (Energy
Information Agency 2009). The installed capacity of direct use applications nearly doubled between
2000 and 2005, growing at a rate of about 13.3% per year (Lund et al. 2005).
The growth in installed capacity of direct use applications reflects a rapid growth in interna-
tional development of this type of system. In 1985 11 countries reported using more than 100 MW
of direct use geothermal energy. In 2005 that number had increased to 33. Shown in Table 11.1 is a
summary of installed capacity of direct use geothermal energy systems in the 72 countries that have
such systems installed. The global distribution of these systems reflects the diversity of applications
to which they can be engineered.
Figure 11.1 summarizes some of the types of applications that have been installed that utilize
warm geothermal fluids. The applications have been grouped according to broad categories that

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