Where Does Geothermal Energy Come From?Geothermal energy is energy that originates from the Earth’s subsurface. Geothermal energy is an important natural energy resource. It has the potential to significantly contribute to satisfying human energy needs. Unlike other natural resources, such as hydrocarbons and minerals, geothermal energy is a renewable energy resource as it naturally replenishes with time. Other benefits of geothermal energy are significantly lower emissions of greenhouse gases and pollutants during production and a smaller energy footprint (i.e., land disturbed by production activities). It is possible for us to mine this underground heat energy and turn the heat into other forms of energy like electrical energy.
About Geothermal Energy
Wherever we are in the world, there is heat to some degree in the subsurface below our feet. Sources of heat contributing to the Earth’s internal heat flow include heat generated
- during the early formation of the Earth, which the Earth has not yet lost,
- continuously by the decay of long-lived radioactive isotopes of elements (e.g., uranium, thorium, rubidium, and potassium) in minerals in the Earth’s mantle and crust,
- from solar absorption in the shallow subsurface, and
- from friction between rock units during mantle convection and plate tectonics.
The temperature in the ground increases with depth indicating that heat is moving from the Earth’s interior towards the surface. This increase in ground temperature with depth is known as the geothermal gradient. The temperature a few metres below ground surface generally reflects the average annual air temperature at a given location, whereas the estimated temperature is about 900°C at the base of the Earth’s crust, about 4000°C at the boundary between the mantle and the core, and likely more than 5500°C within the Earth’s core. We can see a surface manifestation of the heated subsurface at thermal springs. In thermal springs, geothermal energy heats groundwater that travels to the surface through natural pathways in the rock called fractures. The Miette Hot Springs near Jasper and the Cave and Basin Hot Springs near Banff (western Alberta) are excellent examples of thermal springs in Alberta.
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Three main mechanisms move heat from its deep sources towards the Earth’s surface:
- conduction, where heat transfers between materials of different temperature through direct physical contact, but without physically moving them
- convection, where heat transfers by a combination of physical flow of material and conduction between the warmer and colder materials
- advection, where heat transfers by the flow of a fluid, such as magma rising below active volcanoes
Large-scale convection of material in the Earth’s mantle transports heat from the Earth’s outer core to the crust. The major form of heat transfer in the crust is conduction, but some heat transfers by convection in areas of circulating crustal fluids and by the rise of magma below active volcanoes. Areas of high convective heat flow were traditionally the primary targets for geothermal exploration and development because high volumes of fluids at high temperatures were accessed at shallow depths, compared to deeper purely conductive settings. With advancements in technology, it is now possible to extract heat from deeper conductive settings. We can inject fluids that fracture low-permeability rocks (i.e., hydrofracturing) creating a reservoir where injected fluids absorb the Earth’s heat and are then piped back to the surface (enhanced geothermal systems).
To produce geothermal energy, you want to bring the heat stored in rocks and fluids from the subsurface to the surface. To accomplish this, we use different techniques, depending on the geographic location, geological conditions, and temperature of the fluids. The heat associated with volcanoes and high-level tectonic activity is high-temperature geothermal energy (>150°C) and can generate electricity. On the other hand, the low-temperature geothermal energy (<40°C) stored everywhere in the ground is a result of the natural geothermal gradient. As the temperature increases with depth, influences from surface weather conditions diminish. At certain depths the subsurface temperature is higher than the air temperature during the winter and lower than the air temperature during the summer in Alberta. By tapping into this natural phenomenon, we can use low-temperature geothermal energy for heating in winter and cooling in summer.
Although geothermal research and development is in its early stages, Albertans are currently using this renewable energy source for residential heating and cooling applications. For electricity generation and industrial and commercial heating and cooling, companies in Alberta are developing advanced geothermal technologies to access and use geothermal energy. Our province's first conventional geothermal heat and power facility is expected to be operational in 2025. The future of this green energy source in our province is positive.