What is Geothermal Energy?
The Heat Beneath Our Feet
Heat generated from the Earth is in rocks deep underground. When these rocks have naturally occurring water (i.e. something akin to sea water or a brine), this water can be pumped to surface for useful work. For conventional geothermal energy, this heat extraction involves drilling to depths of 2,000 to 4,000 metres to reach the heat resource.
Types of Geothermal Sources
Moderate-High Temperatures
P O W E R G E N
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High temperatures (over 150°C) are suitable for power generation
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Includes plate subduction zones (“Ring of Fire”), rift zones (East African Rift), and extensional environments such as mid-oceanic ridges (Iceland)
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Mature technology - over 100 years old
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Considered conventional geothermal
Low-Moderate Temperatures
D I R E C T U S E
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Moderate temperatures range from 90°C to 150°C
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Can be shallow or deep
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Can be used for direct-use applications in a cascading system or stand-alone
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Can produce power through Organic Rankine Cycle (ORC) technology if resource is over 100°C
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Considered conventional geothermal
Low Temperatures
G E O E X C H A N G E
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Low temperatures can be used for space conditioning (heating and cooling) and hot water
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Shallow, low-temperature source (the ground, lakes, etc.)
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Geoexchange systems relies on seasonal temperature differentials, treating the Earth as a thermal battery
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Should not be confused with conventional geothermal
How Geothermal Energy Works
To produce geothermal energy, a developer first conducts a desktop investigation to identify a heat resource that is more than 100°C. These temperatures in Alberta can be found as shallow as 2,000 metres, but in other areas are deeper. They then drill a well to confirm the geothermal resource. If the brine is hot enough for the intended purpose and the flow is high enough, an additional large diameter well is drilled to complete the doublet. Upon successful testing, the remainder of the geothermal wellfield is drilled out and a power facility is established.
There are three types of geothermal power plants:
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Dry Steam
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Flash Steam
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Binary
Alberta No. 1 is a binary power plant utilizing a geothermal resource of 118°C.
To generate power with a binary power plant:
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Hot brine is pumped to the surface
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The brine's thermal energy energy is transferred to another "working fluid" through a heat exchange process. This working fluid is an organic fluid with a low boiling point, which, when heated, produces a vapour.
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The vapour drives an Organic Rankine Cycle (ORC) turbine to generate clean electricity.
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After the heat to power conversion, the residual thermal energy from the brine can be put through another heat exchanger to be transferred to a district heat network.
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After useful work is complete, the geothermal brine is reintroduced underground through an injection well, where it will heat up again over time, creating a sustainable system that supplies green heat and power for decades.
Conventional Geothermal vs Geo-Exchange
Deep or conventional geothermal should not be confused with "geo-exchange", which is commonly used to heat one's home or an urban area (e.g. Blatchford's District Energy Sharing System). Geo-exchange, or ground-source heat pumps (GHPs), is a renewable energy system that uses the seasonal temperature differences and the space it is heating or cooling to drive the heat transfer. This technology does not generate power nor produce hot brines for geothermal heating, but instead uses the shallow subsurface like a battery. These geo-exchange systems store heat (cooling the air) in the summer and release it in the winter.
