Geothermal (ground-source) heat pumps (GHP) are one of the fastest growing applications of renewable energy in the world, with annual increases of 10% in about 30 countries over the past 10 years. Its main advantage is that it uses normal ground or groundwater temperatures (between about 5 and 30 C), which are available in all countries of the world. Most of this growth has occurred in the United States and Europe, though interest is developing in other countries such as Japan and Turkey. The present worldwide installed capacity is estimated at almost 12,000 MWt (thermal) and the annual energy use is about 72,000 TJ (20,000 GWh). The actual number of installed units is around 1,100,000, but the data are incomplete