Geothermal power plants can be divided into two main groups, steam cycles and binary cycles. Typically the steam cycles are used at higher well enthalpies, and binary cycles for lower enthalpies. The steam cycles allow the fluid to boil, and then the steam is separated from the brine and expanded in a turbine. Usually the brine is rejected to the environment (re-injected), or it is flashed again at a lower pressure. Here the Single Flash (SF) and Double Flash (DF) cycles will be presented.

A binary cycle uses a secondary working fluid in a closed power generation cycle. A heat exchanger is used to transfer heat from the geothermal fluid to the working fluid, and the cooled brine is then rejected to the environment or re-injected. The Organic Rankine Cycle (ORC) and Kalina cycle will be presented.

A geothermal reservoir is a volume of rocks in the subsurface which exploitaton in terms of heat can be economically profitable. It should be noted that for producing the heat from the subsurface is necessary the presence of a transport fluid (usually water), and that drilling to an enough depth to reach the optimum operation temperatures is also necessary. These factors and the technical and other concerns entail costs which increase with depth. The temperature of the fluid and the possible applications are important to sort reservoirs.

Geothermal reservoirs of low-to moderate-temperature water — 68°F to 302°F (20°C to 150°C) — provide direct heat for residential, industrial, and commercial uses. This resource is widespread in the United States, and is used to heat homes and offices, commercial greenhouses, fish farms, food processing facilities, gold mining operations, and a variety of other applications. In addition, spent fluids from geothermal electric plants can be subsequently used for direct use applications in so-called "cascaded" operation.

Direct use of geothermal energy in homes and commercial operations is much less expensive than using traditional fuels. Savings can be as much as 80% over fossil fuels. Direct use is also very clean, producing only a small percentage (and in many cases none) of the air pollutants emitted by burning fossil fuels.

ADVANTAGES

1) It is a renewable source of energy.

2) By far, it is non-polluting and environment friendly.

3) There is no wastage or generation of by-products.

4) Geothermal energy can be used directly. In ancient times, people used this source of energy for heating homes, cooking, etc.

5) Maintenance cost of geothermal power plants is very less.

6) Geothermal power plants don't occupy too much space and thus help in protecting natural environment.

7) Unlike solar energy, it is not dependent on the weather conditions.

DISADVANTAGES

1) Only few sites have the potential of Geothermal Energy.

2) Most of the sites, where geothermal energy is produced, are far from markets or cities, where it needs to be consumed.

3) Total generation potential of this source is too small.

4) There is always a danger of eruption of volcano.

5) Installation cost of steam power plant is very high.

6) There is no guarantee that the amount of energy which is produced will justify the capital expenditure and operations costs.

7) It may release some harmful, poisonous gases that can escape through the holes drilled during construction.