Geothermal energy is the renewable source of energy because Earth's heat is being constantly replenished. The energy is environmentally friendlier than fossil fuels because it is connected with far less greenhouse gas emissions. Geothermal energy can improve our energy security and energy independence because by using the more geothermal energy we would need to import less foreign oil and other expensive fuels. Geothermal energy is heat energy generated and stored in the Earth. The geothermal energy of the Earth's crust originates from the original formation of the planet and from radioactive decay of materials, but possibly roughly equal. The geothermal gradient, which is the difference in temperature between the core of the planet and its surface, drives a continuous conduction of thermal energy in the form of heat from the core to the surface. The use of geothermal energy for power generation is attracting favorable attention because, unlike other types of renewable energy, it is independent of weather and seasons and can achieve high-load-factor power generation all year round. CO2 emissions are as low as those for nuclear power. There are two types of geothermal power generation systems. One type involves the use of high-temperature steam to drive a turbine. The steam is obtained from a wellbore into the hot water zone. The other type is binary power generation in which the remaining thermal energy of hot water is separated from steam. This thermal energy is transferred to another low-boiling-point medium to generate power.