While high-temperature geothermal resources are suitable for power generation and lower-temperature resources are often exploited for district heating, it is possible to combine the two. This is typical, for example, of geothermal power plants in Iceland where there is high demand for both power and heat.

Iceland’s geothermal reservoirs generally provide a hot brine and energy is extracted from this with a flash plant as described previously. However, after the brine exits the flash plant it still contains a significant amount of heat energy that can be exploited to provide heat for a district heating system. For this, the brine is passed through a series of heat exchangers through the second side of which fresh water is passed and heated. The hot water is then stored in a large tank from which it can be taken to supply the district heating system. The brine, upon exiting these heat exchangers, is reinjected into the reservoir.


The exploitation of geothermal power is often considered to be more risky than development of other renewable resources because of the uncertainty and high costs associated with finding suitable geothermal fields. Initial surveys of geo- thermal resources are often carried out by national institutions, but for developers, these then need to be backed up with test wells to determine the exact nature of the resource available. Data from Pacific Rim volcanic regions suggests the presence of a single hot spring will provide a 50% change of an exploitable geothermal field. A boiling spring or fumarole increases the prob- ability to 70%.

Having identified a suitable surface site, prefeasibility studies are likely to cost around $1 million, with a 30% change of failure. Test drilling, usually three wells at up to $2 million per well, has a similar prospect of failure. This risk can be reduced by careful surface study followed by prioritization of the avail- able sites. Such an approach has led to success rates for well drilling in excess of 83% in countries such as Indonesia, Kenya, and New Zealand that high-temperature resources. However, success rates can be much lower where low-temperature resources are concerned.

Once a usable underground reservoir has been located, its size must be deter- mined. This involves fluid withdrawal over a long period; indeed, it may not be until several years after production has started that a good picture of the resource can be obtained. Careful sizing of the geothermal plant to match the reservoir size will prolong the lifetime of a reservoir. However, this may not be possible if the plant has to be constructed before full data is available and, as noted, the data may not be available anyway until production has started. Oversized plants such as that installed at the Geysers in the United States have led to a premature fall in output, which will have an impact on overall economics.

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