Sustainable geothermal utilization – Case histories; definitions; research issues and modelling

Sustainable development by definition meets the needs of the present without compromising the ability of future generations to meet their own needs. The Earth's enormous geothermal resources have the potential to contribute significantly to sustainable energy use worldwide as well as to help mitigate climate change. Experience from the use of numerous geothermal systems worldwide lasting several decades demonstrates that by maintaining production below a certain limit the systems reach a balance between net energy discharge and recharge that may be maintained for a long time (100–300 years). Modelling studies indicate that the effect of heavy utilization is often reversible on a time-scale comparable to the period of utilization. Thus, geothermal resources can be used in a sustainable manner either through (1) constant production below the sustainable limit, (2) step-wise increase in production, (3) intermittent excessive production with breaks, and (4) reduced production after a shorter period of heavy production. The long production histories that are available for low-temperature as well as high-temperature geothermal systems distributed throughout the world, provide the most valuable data available for studying sustainable management of geothermal resources, and reservoir modelling is the most powerful tool available for this purpose. The paper presents sustainability modelling studies for the Hamar and Nesjavellir geothermal systems in Iceland, the Beijing Urban system in China and the Olkaria system in Kenya as examples. Several relevant research issues have also been identified, such as the relevance of system boundary conditions during long-term utilization, how far reaching interference from utilization is, how effectively geothermal systems recover after heavy utilization and the reliability of long-term (more than 100 years) model predictions.