Environmental Engineering Reference
In-Depth Information
10.4 Economics
Drilling is by far the biggest cost factor with deep geothermal energy. Yet it is not
only the cost of the drilling itself that is the problem. The risks associated with it,
especially on commercial projects, are something that should not be underestimated.
Even the best geologists can never accurately predict what conditions will be like
underground. Drilling costs shoot up immediately if drills unexpectedly hit crystal-
line hard rock instead of soft sedimentary rock. If, in addition, the temperatures
underground are well below what was projected, a geothermal project can fail at an
early stage.
Underground conditions can often spring other surprises. For example, during work
on a geothermal project in Speyer in Germany, drillers not only found the thermal
water they were hoping for but also discovered an oilfi eld 2000 m below the surface.
So in addition to accessing hot thermal water of 160 °C, further drilling will also
exploit this oilfi eld.
Even when drillings are successful, geothermal power plants have to spend up to
half of all investment on the drilling activities themselves. Therefore, the cost of
geothermal power is often considerably higher than that of electricity from wind
and hydropower systems.
The renewable energy law in Germany also promotes geothermal power generation.
In 2007 the feed-in-tariff for geothermal power plants with a power capacity of up
to 5 megawatts was 15 cents per kilowatt hour. As this compensation was not high
enough to motivate the building of large numbers of commercial plants, a planned
revision to the renewable energy law is to raise this compensation to 20.5 cents per
kilowatt hour. This compensation includes a bonus for simultaneous power-heat
coupling and for petrothermal systems; that is, HDR (hot dry rock) power plants.
With new plants this compensation will drop by 1% per year.
The costs in countries with good geothermal conditions are considerably lower than
those in Germany. At drilling depths of a few hundred metres the drilling costs in
those areas end up being very low. If, in addition, high temperatures exist close to
the surface, some countries such as Iceland can even use geothermal heat to keep
their pavements free of ice in winter (Figure 10.10).
10.5 Ecology
Geothermal cogeneration plants have little ecological impact. Most of the plant is
located underground and is not visible, and therefore does not have a direct negative
impact on people or landscape. Only the power plant complex is above ground. Like
other heat plants, these also require cooling water for the plant processes. However,
water is readily available at most geothermal sites.
What can cause problems are some of the working materials used in the generation
of power. For instance, the material PF5050 used in ORC processes has a very high
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