Question

Draw a neat labelled diagram and explain the process of generating electricity from geothermal energy.

Answer 1

Geothermal energy is thermal energy generated and stored in the Earth. The geothermal energy of the Earth's crust originates from the original formation of the planet (20%) and from radioactive decay of minerals (80%).

This thermal energy is contained in the rocks and fluids beneath Earth’s crust. It can be found from shallow ground to several miles below the surface, and even farther down to the extremely hot molten rock called magma.

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.

Geothermal energy contributes less than 1% of the world’s electricity generation. There were 315 MW of new geothermal power capacity installed in 2015, raising the total capacity to 13.2 GW.

Principle:
Geothermal power plants use steam produced from reservoirs of hot water found a few miles or more below the Earth's surface to produce electricity. The steam rotates a turbine that activates a generator, which produces electricity.

There are three types of geothermal power plants:
1. Dry steam
2. Flash steam
3. Binary steam

Dry Steam:
• Dry steam power plants take advantage of underground resources of steam.
• The steam is piped directly from the production well, drilled into the geothermal reservoir.
• The high pressure dry steam passes up the production well and through a series of mesh filters which catch any rocks, stones or other debris, which would otherwise damage the turbine blades.
• The steam then passes through a steam turbine that drives an electrical generator, which produces electricity for the grid.
• The steam exits the low pressure stage of the turbine and into the condenser.
• The condensate is then re-injected down the injection well into the geothermal reservoir.

Flash Steam:
Flash steam power plants are the most common and use geothermal reservoirs of water with temperatures greater than 360°F (182°C). This very hot water flows up through wells in the ground under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbine/generator. Any leftover water and condensed steam are injected back into the reservoir, making this a sustainable resource.

Binary Steam: Binary cycle power plants operate on water at lower temperatures of about 225°– 360°F (107°–182°C). Binary cycle plants use the heat from the hot water to boil a working fluid, usually an organic compound (isobutane) with a low boiling point. The working fluid is vaporized in a heat exchanger and used to turn a turbine. The water is then injected back into the ground to be reheated. The water and the working fluid are kept separated during the whole process, so there are little or no air emissions.