Geothermal is one of many tools in the renewable sector that can help us move towards a cleaner planet and brighter future. Geothermal is generally less understood and less invested in than other renewables with around 1% of the world’s energy being derived by it. The sector is still emerging and key technologies in cost-effective drilling and capture are developing. As companies like Arctic Green prove geothermal’s benefits and scalability, the industry will continue to expand.

Geothermal has two key differentiators from other renewables that add to its appeal:

• It is the only renewable baseload energy. It is not reliant on the weather, sun, wind, or water flow to produce consistent generation. Geothermal’s constant availability at consistent rates is incredibly useful to reliably power and heat our cities.
• Geothermal energy has cascading usage. This means that the heat produced can be re-used for more than one application. While the highest heat can be used to produce electricity and green hydrogen, excess heat can be used for other applications with lower ideal temperatures, like direct-use heating and cooling, agriculture, aquaculture, etc. The cascading usage of geothermal makes it one of the most efficient renewable energies.

Geothermal heat pumps

Heat pump technology relies on the relatively constant temperature all year round of the shallow earth right below the surface. The temperature is warmer than the air during the winter and cooler during the summer, acting as a source or sink of heat depending on the season. Ground source heat pumps use some electricity to pump liquid through underground loop systems and then extract the heat energy for heating and cooling applications. The electricity required to operate heat pumps can come from the local electrical grid or green sources. Today’s advanced heat pumps are highly efficient with an output: input COP ratio of 6:1. In recent years, great progress has been made in industrial-size heat pumps with drilling fields with multiple shallow wells, often hundreds drilled in a grid, up to 200 meters deep.

Hydrothermal geothermal

Hydrothermal drilling technology makes use of natural underground hot water reservoirs trapped in permeable rock. Heated by the earth’s core, water rises through rock fissures and fractures and becomes trapped under solid caprocks. This forms natural reservoirs of pressurized hot water. These reservoirs often reveal themselves through hot springs or fumaroles. Once a reservoir is located, developers drill exploratory wells to map the field and then drill production wells to extract the hot water. The water’s heat is captured using heat exchangers or turbines, and the remaining fluids are returned to the field through re-injection wells to maintain reservoir pressure. These wells are typically 1-4km in depth and produce temperatures of 55 °C to over 150 °C depending on the quality of the site. As a rule of thumb, the Earth’s heat gradient rises 30°C for every km of increased depth.

Enhanced Geothermal Systems (EGS)

Developers can make enhanced geothermal systems by effectively creating underground hot water reservoirs from solid rock. Developers drill into solid rock and inject water and geothermal brine at high pressure. This fractures the rock and allows fluid to permeate and become heated. The hot fluid is then extracted through the production well, and the heat is captured. Remaining fluid is re-injected, restarting the process again. The effectiveness of an EGS is based upon the site’s reservoir permeability, temperature, well permeability, and depth. Enhanced drilling technology can increase the productivity of a site by branching out from “in field” into “near field” resources. This horizontal drilling increases the amount of permeable rock, allowing more fluid to be heated and extracted.

Hot Rock Geothermal (DEEP EGS)

Hot rock geothermal relies on the principles of EGS but explores heat sources at much greater depths and temperatures. Hot rock geothermal drills at least 6 miles beneath the surface where water temperatures exceed the “supercritical” 373ºC threshold. These wells create significantly more energy than traditional EGS wells. A hot rock geothermal project at 400°C can produce ~50MW capacity, compared to roughly 5MW capacity of an EGS project at 200°C — 42 percent hotter, 10 times the power. While these energy enhancements are appealing, hot rock technology with such deep drilling is still developing. Engineering and profitability concerns need to be resolved before this technology is scalable.

Advanced Geothermal Systems (AGS)

This new, emerging technology relies on the concept of an engineered renewable closed loop. Instead of water being extracted from and injected into the earth, fluid moves through man-made underground pipes in the shape of a loop. The fluid assumes the earth’s heat and returns to the surface where the heat is captured using turbines. AGS could potentially supply both electricity and heating and cooling applications. Advocates maintain that AGS may one day surpass EGS because it eliminates any fracking concerns associated with the latter. However promising, AGS has not been proven yet. Advances in drilling and depth exploration are being made every year to make these systems possible.