Page 196 - Low Temperature Energy Systems with Applications of Renewable Energy

Heating with geothermal systems 183

other on the surface, say 10 m apart, but kilometers apart where they intercept the deep
sedimentary formation. This minimizes interference between the cold injectate and the
hot producing fluid.

5.2.3 Development

Once a resource has been identified and characterized, the next step is to match it to the
requirements of particular applications. Naturally, certain applications will be in mind
when the project is first conceptualized, but only when the resource becomes known
and characterized can specific, appropriate applications be brought into consideration.
For example, in Europe where low-to-moderate temperature geothermal fluids can
be accessed at depths of 2500 m and deeper, applications such as district heating, hor-
ticulture, aquaculture, recreation, and balneology are typical appropriate applications.
The return on investment for various facilities will be weighed against the cost of
development, construction, installation, operation, and maintenance to determine
feasibility of any project, even before the resource is verified. This involves risks
for investors and requires some form of incentive or insurance to mitigate the risk
of failure to find a commercial resource. Governments and municipalities often provide
such guarantees for a limited time period in order to stimulate sustainable and environ-
mentally friendly projects. The drilling phase is the most risky, especially when depths
of 4000e5000 m are needed to reach adequate temperatures, because sufficient perme-
ability must be achieved in addition to good temperatures. The costs and risks escalate
when the reservoir must be stimulated (e.g., hydrofractured) to improve the perme-
ability and thereby enhance the productivity and injectivity of the wells.

5.3 Geothermal district heating systems

Many examples exist of geothermal district heating systems. Only two will be pre-
sented in this section; interested readers can find many others in the literature. The first
one is a very old system in Europe that dates back about 700 years and the other is a
fairly modern one in a city in the western United States.

5.3.1 Chaudes-Aigues, France
One of the earliest district heating systems in historic time occurred in a small French
town, Chaudes-Aigues. Set in the valley of the Remontalou River in south-central
France, the community of fewer than 900 residents hosts about 30 natural hot springs

with waters ranging in temperature from 45 C to as hot as 82 C; see Fig. 5.3. The

Remontalou flows through the elongated town from generally south to north; the
elevation varies from about 800 masl in the south to about 755 m in the north.
Certainly the springs were known to the early inhabitants of the area dating back to
at least 450 AD. Excavations in the area have unearthed artifacts and thermal baths in
caves that date back to the Romans. By 1332 a district heating system had been