Most large-scale geothermal developments in New Zealand extract water and heat at a greater rate than the natural recharge to the system , and are therefore “heat mining”. However, where pressures have been reduced significantly by exploitation, as at Wairakei, in some cases the rate of replenishment from depth has increased several-fold to match the discharge rate. This, combined with the ability to recycle fluids through reinjection, means that it can be difficult to define what is meant by renewability and sustainable management of a geothermal resource.
Although some geothermal resources have been over-exploited and had to reduce their generating capacity, as at Ohaaki, no geothermal field has ever been run to exhaustion. In practice, all resource consents to take and reinject are framed in terms of fluid mass withdrawal, and in some cases there are limits on how much energy can be extracted. The fact that Wairakei has been operating for 50 years, resource consents for a further 25 years at the same generation capacity have been given, and reservoir modelling indicates that the resource will remain viable for at least another 25 years beyond that all demonstrate the longevity of geothermal resources.
- Centre for Advanced Engineering 2003. New Zealand’s energy future: a sustainable energy future after Maui. CAE Comments 03.