**5.6 Geothermal energy**

Geothermal energy is derived from hot water or steam that, in both cases, emanates from hot rocks or soil and exists below the earth's surface. Some of the high-temperature rocks are discovered around volcanic activities as well as low-temperature rocks and soil that we can find everywhere, including in the ice field [18]. Geothermal energy is used to supply heat directly or at high temperature it is able to supply electricity. Lower temperature (0C–120C) is employed to warm buildings, and hot-temperature warmth (120C–400C) is normally used to generate electrical power. For electricity production, there are three types of geothermal plants: flash steam, dry steam, and binary geothermal plant [18]. Energy storage for geothermal energy works by technology that transfers heat energy from underground water to electricity, and after that, extra energy is stored into the underground water. Geothermal storage also has disadvantages such as fluctuation in the binary geothermal plant, mostly used in the US as a heating agent, that can cause more damage than we imagined. If it is used for other purposes, environmental impact is pretty huge, especially when geothermal fluid is not stored and recycled in a pipe, which can then absorb toxic compounds such as arsenic, boron, and fluoride. These poisonous compounds may be taken to the surface and leak as the water evaporates [32].

In the framework of the effectiveness of geothermal energy, we are mainly talking about the fact that geothermal systems can provide any combination of forced-air heating, radiant in-floor heating, domestic hot water, and air conditioning all from the same unit.

The heating efficiency of a geothermal heat pump is assessed by the coefficient of performance (COP), while the cooling efficiency is reflected by the energy efficiency ratio (EER). These metrics relate the number of units of heat given or withdrawn to the number of units of power consumed to complete the task. Ground source heat pumps generally provide 4 units of heat energy for every unit of electricity consumed.

So, we are talking about 400% effectiveness. In comparison, the most efficient conventional systems on the market today are just 98% efficient (**Figures 1** and **2**).
