Survey of Energy Resources 2007
Geothermal - Introduction
Although geothermal energy is categorised in international energy tables amongst the 'new renewables', it is not a new energy source at all. People in many parts of the world have used hot springs for bathing and washing of clothes since the dawn of civilisation. An excellent book has been published with historical records and stories of geothermal utilisation from all over the world (Cataldi, et al., 1999).
Electricity has been generated by geothermal steam commercially since 1913, and geothermal energy has been used on the scale of hundreds of megawatts for five decades, both for electricity generation and direct use. The utilisation has increased rapidly during the last three decades. Geothermal resources have been identified in some 90 countries and there are quantified records of geothermal utilisation in more than 70 countries. Electricity is produced by geothermal in some 25 countries. Five of these countries obtain 15-22% of their national electricity production from geothermal. In 2004, the worldwide use of geothermal energy was about 55 TWh of electricity (Bertani, 2005), and 76 TWh for direct use (Lund, et al., 2005). Fig. 11-1 shows the installed capacity and the energy produced by geothermal by continent.
Electricity production increased by 16% from 1999 to 2004 (annual growth rate of 3%), and direct use by 43% (annual growth rate of 7.5%). Only a small fraction of the geothermal potential has been developed so far, and there is ample space for an accelerated use of geothermal energy, both for direct applications and electricity production.
Of the total electricity production from renewables of 2 968 TWh in 2001, 91% came from hydropower, 5.7% from biomass, 1.8% from geothermal and 1.4% from wind. Solar electricity contributed 0.06% and tidal 0.02%. A comparison of the renewable energy sources shows the electrical energy cost to be US$ 0.02-0.10/kWh for geothermal and hydro, US$ 0.04-0.08/kWh for wind, US$ 0.03-0.12/kWh for biomass, US$ 0.25-1.60/kWh for solar photovoltaic and US$ 0.12-0.34/kWh for solar thermal electricity. Heat from renewables is also commercially competitive with conventional energy sources. The current cost of direct heat from biomass is US$ 0.01-0.06/kWh, geothermal US$ 0.005-0.05/kWh, and solar heating US$ 0.02-0.25/kWh (WEA, 2004).
Geothermal energy is independent of weather conditions, contrary to solar, wind, or hydro applications. It has an inherent storage capability and can be used both for base-load and peak power plants. However, in most cases, it is more economical to run the geothermal plants as base-load suppliers. Fig. 11-2 clearly reflects the variable capacity factors of power stations using the four renewable sources. At end-2001, wind energy was in the leading position with 70.1% of installed capacity, followed by geothermal with 24.4%. Geothermal was, however, the leading electricity producer with 53.8% of total production, followed by wind energy with 43.7%. Geothermal's relatively high share of electricity production reflects the reliability of its plants, which can be operated at capacity factors in excess of 90%.
Geothermal energy has until recently had a considerable economic potential only in areas where thermal water or steam is found concentrated at depths of less than 3 km in restricted volumes, analogous to oil in commercial oil reservoirs. This has changed in the last two decades with developments in the application of ground-source heat pumps using the earth as a heat source for heating or as a heat sink for cooling, depending on the season. This has made it possible for all countries to use the heat of the earth for heating and/or cooling, as appropriate. It should be stressed that heat pumps can be used basically everywhere.
Geothermal energy is independent of weather conditions, contrary to solar, wind, or hydro applications.