What is Geothermal Energy

Geothermal energy is the energy harnessed from steam and hot water enclosed in the Earth’s crust. The word geothermal is a hybrid word that comes from “geo”, a Greek word meaning “Earth,” and “therme” meaning “heat.” The term geothermal energy denotes the combined thermal energy below the considerably cooler and thinner surface of the Earth, constituting about 260 billion cubic miles (1012 cubic kilometers) of rock at melting temperatures.

Geothermal energy is generated from the formation of the planet, decay of long-lived radioactive isotopes (radiogenic heat), volcanic activity and solar energy absorbed at the Earth’s surface. Most of geothermal energy is generated by radiogenic heat— when radioactive isotopes of Thorium (232Th), Uranium (235U, 238U) and Potassium (40K) decay to a more stable state.

The geothermal energy of the Earth is enormous. For each mile of depth below the Earth’s surface, the temperature increases around 80 °F (26.6 °C). The difference in temperature between the core of the Earth and the surface is known as geothermal gradient. The geothermal gradient forces a permanent transfer of thermal energy in the form of heat from the Earth’s core to the surface.

The Earth is made up of 3 circular layers: the core (inner and outer), the mantle, and the crust (Picture 1). At the center of Earth is a solid inner core with an estimated temperature of about 8,000 °F (4426.6 °C). The heat emitted in the Earth’s core is strong enough to melt rock into a molten fluid of extremely high-temperature also known as magma. The molten fluid or magma forms the liquid outer core of the Earth. The core, consisting largely of iron and smaller quantities of nickel, takes up 16{d7514f11c7b5caf74388f511feb4632d0e65596f28f25c91e5fa75b3dade25af} of the Earth’s volume. The layer that surrounds the core is called the mantle (Picture 1) and comprises 83{d7514f11c7b5caf74388f511feb4632d0e65596f28f25c91e5fa75b3dade25af} of the Earth’s volume.

Magma is fluidlike and less dense than the nearby rock from which it was derived. Thus, it tends to rise and moves slowly toward the Earth’s crust. The temperature of magma is between 600°F (315.6°C) and 2,400 °F (1,316 °C). The tendency of a warmer, less dense material to rise and cooler, more dense material to sink is referred to as convection and results in the transfer of heat. Geothermal energy is being perpetually generated in the Earth’s mantle as magma travels upward while surface groundwater escapes down to the Earth’s interior. The hot magma heats the water and forces it back up through Earth’s cracks. In its liquid form, geothermal energy is also referred to as hydrothermal energy.

In most of the world, the geothermal gradient through the crust is 124–138 °F per mile (25–30°C per kilometer) of depth. The conductive heat flux (the rate of heat energy transfer) is about 0.1 MW/km2 on average. Near tectonic plate boundaries, where the Earth’s crust is thinner, these values are much higher.

Geothermal energy powers natural hot springs, volcanoes and geysers. Volcanoes, in particular, are perhaps the most eminent geothermal occurrences. Volcanism most frequently occurs at spreading ridges where tectonic plates diverge or alongside subduction zones where tectonic plates converge. It is not surprising that volcanic areas have the best potential for geothermal energy given the function of magma in geothermal energy generation. The hot magma typically remains well below the Earth’s surface to heat rocks as well as water originating from rainfall and snowmelt that is sinking deep into the Earth. When a volcano erupts, however, hot magma from below ground shoots through the Earth’s crust, flowing onto the surface. When magma erupts onto the Earth’s surface it is called lava.

The heat for most hot springs and geysers originates from the underground water that is heated by warm, volcanic rock. A hot spring is a geothermal manifestation in which the water temperature is higher than the average human body temperature. The word “geysir” is of Icelandic origin meaning to “rush forth”. Geyser is a hot spring that irregularly emits hot water and steam with great force. Well-known geysers are found in Iceland, New Zealand and Wyoming. Due to their highly mineralized content, the waters of many hot springs are believed to have therapeutic properties.

Most of the hot geothermal water remains trapped deep underground in cracks and porous rocks. This natural underground water is called a geothermal reservoir.Geothermal resources are classified into three main categories depending on phases and forms: Convective hydrothermal resource (vapor or hot-water-dominated), other hydrothermal resources (sedimentary, geopressed and radiogenic) and hot rock resources (magma and solidified).