Successful production of electricity from geothermal heat was first achieved in Larderello in Italy, in the early 1900s. Since then, the production of geothermal electricity has steadily increased. Unlike other weather dependant renewables, geothermal is available 24 hours a day, every day of the year, to match the consumer demand of providing grid stability.
Geothermal energy comes from wells drilled into the earth to reach a reservoir. Steam or hot water are piped to the surface to power a turbine that generates electricity. A power plant has a capacity ranging from 1 to 40 Megawatts electric to produce baseload electricity. These plants require high temperatures in deep reservoirs to produce electricity. But temperature and depth can vary widely according to the different regions and the characteristics of the site. Moreover, with the most recent technological developments, like Enhanced Geothermal Systems, make it now possible to produce geothermal electricity anywhere, and not only in areas with rich geothermal reservoirs.
Geothermal electricity is key to stabilise the grid and reduce the overall system costs of future electricity systems. Providing a local source of electricity for the base load will also allow the total energy bill to decrease and make energy more sustainable and affordable.
The systems for geothermal electricity production can be subdivided in three main categories, which are also linked to the temperature ranges:
- 80°C<T<180°C (Medium Enthalpy resources): this range of temperature is appropriate for use with binary plants (Organic Rankine or Kalina cycle), with typical power in the range 0.1-10 MWe. These systems are also suitable for heat & power co-generation, typically for single edifice to small towns heating.
- 180°C-390°C (High Enthalpy resources): temperatures in this range can be exploited with dry steam, flash and hybrid plants, with typical power in the range 10-100 MWe. These systems, characterised by high efficiency up to more than 40%, also allow heat co-generation for large towns’ district heating. Above 200°C, these resources are generally limited to volcanic areas.
- 390°C-600°C (Supercritical unconventional resources): temperatures in this range, limited to volcanic areas, generally involve superheated dry steam plants, with power per unit volume of fluid up to one order of magnitude larger than conventional resources.
Besides the temperature range, the methods of exploitation of geothermal energy can be further subdivided in two main categories:
- Conventional hydrological systems, which use natural aquifers
- EGS (Enhanced Geothermal Systems), which use the high temperature of rocks with artificial water injection and, generally, with enhancement of permeability of the hot reservoir. An EGS is an underground reservoir that has been created or improved artificially.
In electricity applications, the commercialization and use of EGS can play a central role in establishing the size of the contribution of geothermal energy to long term GHG emissions reductions. The estimated technical potential for geothermal electricity and geothermal heat excludes advanced geothermal technologies that could exploit hot rock or offshore hydrothermal, magma and geopressured resources.
The future looks strong for increasing geothermal electricity production, and with the right tools and continued efforts to break down barriers, the process can be accelerated to make the best use of this dynamic resource.
The GEOELEC project, funded by the EU from 2011-2013, actively worked to increase geothermal electricity generation in Europe, with the objective to double installed geothermal power capacity in Europe from 11 TWh per year in 2010 generated mainly in Italy and Iceland, to 22 TWh per year in the short/medium term and initiate new projects in every EU member state leading to 55 TWh per year by 2020.
The project also looked at concrete actions to reach these objectives, for example conditions for financial feasibility, regulatory frameworks, and public acceptance.