Expanding geothermal energy use
Why is this gap important?
So far, utilisation of geothermal energy has been concentrated in areas with naturally occurring water or steam, and relatively permeable rock. However, the vast majority of geothermal energy within drilling reach – up to 5 km with current technologies and economics – is in relatively dry, low-permeability rock. Heat stored in low-porosity and/or low-permeability rock is commonly referred to as a hot rock resource, and in contrast with most hydrothermal resources in use today for power generation, hot rock resources are available worldwide.
Hot rock resources are characterised by limited pore space and/or minor fractures, and therefore contain insufficient water and permeability for natural exploitation. Hot rock resources can be found anywhere in the world, although they are closer to the surface in regions with an increased presence of naturally occurring radioactive isotopes (e.g. South Australia) or where tectonics have resulted in a favourable state of stress (e.g. in the western United States). In stable, old continental tectonic provinces that have low temperature gradients (7°C/km to 15°C/km) and permeability, but a less favourable state of stress, depths will be significantly greater and developing an enhanced geothermal system (EGS) resource will be less economic.
Technologies that allow energy to be tapped from hot rock resources are still in the demonstration stage and require innovation and experience to become commercially viable; the best-known such technology is EGS. Other approaches to engineering hot rock resources, which are still at the conceptual phase, experiment with methods other than fracturing the hot rock. They aim instead to create water inlet and outlet connectivity, for example by drilling a subsurface heat exchanger made of tubes underground, or by drilling a 7‑km to 10‑km vertical well of large diameter that contains water inlets and outlets at different depths.
EGS research, testing and demonstration is also under way in the United States and Australia. The United States has included large EGS RD&D components in its recent clean energy initiatives as part of a revived national geothermal programme. A global map of hot rock resources is not yet available, but some countries, including the United States, have started mapping EGS resources.
Colored bars represent the Technology Readiness Level (TRL) of each technology. Learn more about TRLs