The word geothermal comes from the Greek geo, meaning earth, and therme, meaning heat. Geothermal technologies take advantage of the heat resources found in the earth’s crust. This can be at either high temperatures for electricity production, and at lower temperatures for use in heating and cooling. Geothermal energy can be captured for human uses just about anywhere on earth, and it is a common misconception that it cannot. But for practical purposes it is not cost-effective to drill down too deep to reach temperatures hot enough for electrical production, and these technologies are better sourced where hotter tempuratures are found closer to the Earth’s surface (volcanic zones and hot spots). Lower grade heat resources, however, can be utilized nearly anywhere.

The Geothermal Resource

The earth’s crust generally maintains a constant temperature at depths and temperatures that vary according to location. The technology used for space heating is called ground source heat pumps or geoexchange systems. In ground source technology, pumps are used to transfer heat from within a few metres of the earth’s surface to a building. During winter months, heat pumps can store this cooler air from buildings in the earth, and extract it during the summer for cooling.  Bodies of water can serve the same purpose, and the City of Toronto’s Deep Lake Water Cooling System is a prime application.

The Technology

To begin with, geothermal energy for space and water heating can be accessed in a number ways:

• With the use of water or another fluid in a closed loop system, to exchange the geothermal heat picked up by the water to a building or district. This can also include a loop system that utilizes the heat energy in a pond.
• Or, in an open loop system, that directly heats the air, water or another liquid by the earth.

Geothermal electrical generation requires much higher temperatures. The most common form of geothermal power plants is the flash-steam geothermal plants or hydrothermal plants. These plants utilize the water in hot aquifers (upwards of 100°C) by tapping into this heated water, and bringing it to the surface. This water is then used to flash heat hydrothermal liquids into steam to run a turbine generator. With the flash-steam technology, water is transferred back underground to keep aquifer water levels even. The hot water from aquifers in these hot geological stratas or deeper surfaces, can be used for in power generation or for direct use.

There is also a hot dry rock process, which is similar to the flash steam process, though in this process the heat (upwards of 150°C) stored in rocks with little to no water content in the earth’s crust, by fracturing them. Water or liquid is then forced down, converted to steam, and then the steam is used to run a turbine. This can be quite a disruptive process.