What it is and How it Works

Credit for the above text as it comes, and is adapted from, the Union of Concerned Scientists.

Heat from the earth can be used as an energy source in many ways, from large and complex power stations to small and relatively simple pumping systems.

Many regions of the world are already tapping geothermal energy as an affordable and sustainable solution to reducing dependence on carbon-based fuels.

The Geothermal Resource

Below Earth's crust, there is a layer of hot and molten rock, called magma. Heat is continually produced in this layer, mostly from the decay of naturally radioactive materials such as uranium and potassium. The amount of heat within 10,000 meters (about 33,000 feet) of Earth's surface contains 50,000 times more energy than all the oil and natural gas resources in the world.

The areas with the highest underground temperatures are in regions with active or geologically young volcanoes. These typically occur at tectonic plate boundaries or at places where the crust is thin enough to let the heat through. The Pacific Rim, often called the Ring of Fire for its many volcanoes, has many “hot spots”.

Tectonic plate boundaries and where the earth’s crust is very thin are not the only places where geothermal energy can be found. There is a vast amount of heat energy available from dry rock formations very deep (4–10 km) below the surface . Using modified, established technology we can harness this heat for electricity production on a much larger scale than before. While still early stage, the first projects have provided electricity to grids in the United States and Australia.

If the full economic potential of geothermal resources can be realized, they would represent an enormous source of electricity production capacity.

As technologies improve and become competitive, more of the largely untapped geothermal resource could be developed.

Not only do geothermal resources in Australia offer great potential, they can also provide continuous baseload electricity. Studies show that the capacity factors of geothermal plants - a measure of the ratio of the actual electricity generated over time compared to what would be produced if the plant was running nonstop for that period - are comparable with those of coal and nuclear power. With the combination of both the size of the resource base and its consistency, geothermal can play an indispensable role in a cleaner, more sustainable power system.

How geothermal energy is captured

Geothermal Power Plants

Geothermal power plants draw heat from underground by pumping fluids through rock unites deep underground and then back to the surface to produce fluid heated to high temperatures. This fluid (steam or hot liquid) then passes through a heat exchanger to drive turbines that generate electricity before the heat depleted fluid is reinjected back underground.

There are three main types of geothermal power plant technologies: dry steam, flash steam, and binary cycle. The type of heat transfer at the surface determines the power plant design and generally depends on the state of the fluid pumped underground and its temperature.   

Dry Steam Power Plant

Dry steam plants use hydrothermal fluids that are already mostly steam, which is a relatively rare natural occurrence. The steam is drawn directly to a turbine, which drives a generator that produces electricity. After the steam condenses, it is frequently reinjected into the reservoir as water.

Dry steam power plant systems are the oldest type of geothermal power plants, first used in Lardarello, Italy, in 1904. Steam technology is still relevant today and is currently in use in northern California at The Geysers, the world's largest single source of geothermal power.

Flash Steam Power Plant

Flash steam plants are a common type of geothermal power plant in operation today. Fluids at temperatures greater than 182°C (360°F), pumped deep underground and back to the surface, travel under high pressures to a low-pressure tank at the earth’s surface. The change in pressure causes some of the fluid to rapidly transform, or “flash,” into vapor. The vapor then drives a turbine, which drives a generator. If any liquid remains in the low-pressure tank, it can be “flashed” again in a second tank to extract even more energy.

Binary-Cycle Power Plant

Binary-cycle geothermal power plants can use lower temperature geothermal resources, making them an important technology for deploying geothermal electricity production in more locations. Binary-cycle geothermal power plants differ from dry steam and flash steam systems in that the geothermal reservoir fluids never come into contact with the power plant’s turbine units. Low-temperature (below 182°C/360°F) geothermal fluids pass through a heat exchanger with a secondary, or "binary," fluid. This binary fluid has a much lower boiling point than water, and the modest heat from the geothermal fluid causes it to flash to vapor, which then drives the turbines, spins the generators, and creates electricity.

Conventional Geothermal

(Open System)

Enhanced Geothermal

(Closed Loop System - ECLS)

These schematic diagrams are adapted from GreenFire Energy.