Sustainable development
The concept of sustainable development originates in the report "Our Common Future", which was written by a working group on behalf of the United Nations in 1987. The concept describes development where our needs at the present moment are met without compromising the ability of later generations to meet theirs. Sustainability thus describes the use of a resource, such as geothermal heat.
Sustainable use of geothermal energy
The definition of the concept of sustainable use of geothermal energy entails that for each geothermal area there exist limits called the maximum sustainable production. If geothermal production is below them, then it can be maintained for a very long time, but production above the limits cannot be maintained in the long run. It must be borne in mind that the limits depend on the production technology used and can therefore change over time. This entails that the properties of the geothermal system and the technology for extracting the energy determine the maximum sustainable production.
Example: Deep pumps, which began to be used in boreholes in Reykjavík about half a century ago, raised the sustainable limits there greatly. The same is expected of deep drilling in high-temperature areas in the future. It is therefore not possible to give a definite number for a particular geothermal system that indicates its sustainable production capacity.
In order to determine whether the use of a resource is sustainable, time limits must be set on how far into the future we make such demands. In the case of geothermal use it has been considered reasonable to reckon with 1-3 centuries. In the first months or years of extensive geothermal production there is, without exception, a pressure drop in the geothermal system concerned. For production to be sustainable, the pressure and temperature in the geothermal system must reach equilibrium, or decline very slowly over time. If production from a geothermal system is above the sustainable limits, it is said to be aggressive and cannot be maintained over the long term. But experience and research have shown that geothermal systems can recover their former state if they are rested after periods of aggressive production. A production arrangement where periods of aggressive production and production pauses alternate can therefore count as sustainable.
How is sustainable production capacity assessed?
In order to find how much energy may be extracted from a geothermal system sustainably, we need information about its size and temperature, plus data from boreholes that have been thoroughly tested. It only becomes clear, however, after some years of production what the maximum sustainable production of the system concerned is, given the production technology used. This is due, among other things, to the fact that the final pressure responses of geothermal systems are very slow to appear and temperature changes in them are exceedingly slow. Production from geothermal systems is often aggressive at the start. This is, on the one hand, because the sustainable limits are not known at the beginning of use and, on the other hand, because of conditions in the energy market. Temporary aggressive production is not harmful; the production simply has to be reduced or the production technology changed over the longer term.
Example: No geothermal system has been used with present production technology for centuries. There are many examples of geothermal systems that have reached equilibrium under production for several decades, plus examples of aggressive production that has not been possible to maintain over the long term. From all these examples much can be learned about how production should be arranged so that it is sustainable.
The Laugarnes area in Reykjavík is a good example of a geothermal system that has reached reasonable equilibrium under substantial production. There production was increased greatly in the 1960s. With that the pressure in the system fell considerably, but reached a new equilibrium again. The production there is now considered sustainable.
The Geysers area in California is an example of production that is unsustainable. There the installed electrical capacity of steam power stations had reached 2000 MW by about 1985. That led to such aggressive production that it was impossible to maintain it over the long term. Production there has now been greatly reduced, and injection is also being used to an increasing degree. It is, however, still not clear whether the production there is beginning to approach the sustainable limits.