We know the Earth has a hot core about 6,350 km below the surface. The temperature decreases from >6700 °C to an average of 15 °C. With such a gradient and the large size of the Earth, the thermal energy stored below the Earth’s surface is huge. We call this form of thermal energy as geothermal energy, which can be categorized into several forms, hydrothermal, magma, hot dry rock, geopressured and earth energy, etc. The hydrothermal energy is the most observed and utilized geothermal energy, such as steam or hot springs in many areas. People have used hot springs and earth steams for heating greenhouses and homes for long time. In recent decades, many power plants have been built for generating electricity from hydrothermal sources. As technology advances, we have grown interest in utilizing other forms of geothermal energy, such as hot dry rocks (HDR).
Hot Dry Rock which is also known as enhanced geothermal systems (EGS) that has two wells drilled in the granitic rock, one called injection well for pumping cold water down 3-6 km to the hot dry rock heated by the magma and another well called production well for carrying the hot steam produced at 175 °C to 180 °C up to the turbine for generating electricity. It is reported that a geothermal power plant based on the super hot rock geothermal energy technology and a single geothermal well just 8 inches in diameter operating at above 400 °C can generate the same amount of electricity as that by a 320 acre of solar PV.
Geothermal Engineering Ltd. (GEL) has announced to build four more geothermal power plants in Cornwall at its United Downs site. Each of the four new hot rock geothermal plant is designed to supply 5MW to the National Grid as well as 20MW of heat supply for local use. The four new power plants will be built in 18 months and fully operational in 2026.