Geothermal power plants tap into underground reservoirs of hot water or steam found deep below the Earth’s surface to generate electricity. These reservoirs are accessed through wells drilled into geothermal fields located in areas with high underground temperatures. There are two main types of geothermal fields – hydrothermal and hot dry rock.
Hydrothermal reservoirs contain naturally occurring hot water or steam trapped in porous rock or underground water reservoirs. To access this, geothermal plants drill production and injection wells into known hydrothermal fields. Production wells are drilled to depths ranging from 1-3 km and bring the hot water or steam to the surface. Injection wells are also drilled and are used to return cooled geothermal fluid back underground after it has passed through the power plant.
The location of these hydrothermal reservoirs is identified through extensive geological, geophysical, and geochemical exploration of areas with recent volcanic activity and/or nearby magma chambers. Areas like the Ring of Fire in the Pacific Ocean or volcanic zones in Iceland and Africa have many of the highest temperature hydrothermal fields accessible for geothermal power production. Once promising locations are identified, test wells are drilled to establish temperature gradients and find productive zones of permeability and fluid saturation in the bedrock.
After exploration identifies commercial quantities of recoverable geothermal resources, power plant development begins. Production wells capable of handling high temperatures are carefully drilled using drilling mud to prevent damage from heat. Well casings made of stainless steel, Inconel, or other corrosion resistant alloys are installed to line the wellbore and prevent collapse while withstanding high pressures and temperatures. Downhole instrumentation is also installed to monitor reservoir conditions and performance over the life of the plant.
Once drilling is complete, a pipeline network transports the geothermal fluid from the production wells to the power plant for utilization. Typical geothermal fluid reservoir temperatures can range from 150-350°C. Lower temperature hydrothermal resources between 90-150°C can also be used with binary cycle power plants utilizing an additional heat exchange process. Upon arrival at the plant, geothermal fluid is first passed through separators which separate steam, liquid, and other gases. The steam is then used to drive turbines which spin generators to produce electricity, just like in conventional steam plants.
After passing through the turbines, the lower pressure steam is condensed back into liquid form using cooling towers. The geothermal fluid now at a lower temperature is piped back underground through the injection wells to be reheated by the hot reservoir rock. Careful reservoir management is needed as injection returns some of the fluid but also cools the reservoir if not balanced by natural reinjection. Sustaining sufficient reservoir pressures and temperatures over the 25-30 year lifetime of the plant is important for continuous power generation.
With hot dry rock resources, the naturally fractured basement rock itself is the target reservoir without naturally occurring fluids. Special techniques are required to access this type of resource. Long injection and production wells extending 2-5 km deep are drilled parallel to each other into the hot basement rock. Then a procedure called hydraulic stimulation is used to fracture open the rock and connect the two wells by pumping water or other fluids down one well under high pressure. This creates an artificial reservoir where once established, water can be circulated and heated between the wells to temperatures of 150-300°C suitable for power production. These engineered reservoirs are still experimental and require further research to prove commercial viability compared to hydrothermal resources.
Geothermal power plants access vast subsurface heat reservoirs through carefully engineered well systems. Hydrothermal reservoirs containing naturally occurring hot fluids are the most developed resource and provide base load renewable power by tapping into underground zones of permeable rock saturated with hot water or steam. Future potential also lies in creating engineered reservoirs within hot basement rocks if techniques for artificially enhancing permeability and conductively heating injected fluids can be proven on a utility scale. Geothermal energy harnesses the Earth’s natural internal heat for power generation utilizing sustainable reservoirs that can last for decades.