Geothermal energy is most often used for power generation and over 20 countries have geothermal power plants. All geothermal power projects share a common feature: a geothermally-heated fluid must be transported to the surface where a turbine is used to convert the heat to electric power.
Today, all conventional geothermal projects operate using water as the heat transfer fluid. These are hydrothermal systems where water is injected into the ground, allowed to percolate through moderately hot and permeable rock formations, and then pumped to the surface where the heat is extracted, and finally power generated. The success of hydrothermal projects depends on sufficient heat, water, and subsurface permeability.
But many hot geothermal regions lack water, permeability or both. The limitations of conventional geothermal heat extraction methods spurred research into two new approaches Enhanced Geothermal Systems (EGS) and closed-loop systems.
Enhanced Geothermal Systems
EGS is designed to create permeability by fracturing hot, dry rock where there is insufficient permeability or fluid saturation. An EGS fracture system is created by injecting water at a high pressure to create a fracture system within the heat reservoir. Water is injected into the fracture system by an injection well and a second well is drilled to intersect the fracture system and conduct the water to the surface. This system makes it possible to circulate water and extract heat from the original hot, dry rock. Although EGS reservoirs can create permeability, the system requires large volumes of water for fracturing and ongoing operations.
Although research and Investment into EGS has been ongoing for more than three decades, the technology has not yet reached commercial viability. Hydraulic fracturing is complex, risky, and expensive and the resulting system requires enormous water resources. EGS projects require substantial volumes of water not only for the hydraulic fracturing process, but on a continuous basis, for heat transport through the system and to make up for significant fluid loss into the surrounding rock formations.
Closed-Loop Geothermal Systems
Closed-loop systems can be used in two different ways. First, closed-loop projects can access high temperature geothermal resources in hot, dry rock regions where conventional hydrothermal technology cannot operate. Second, closed-loop systems can be inserted in existing but unproductive wells to produce power. This is particularly cost effective because no new drilling is required. A closed-loop assembly is inserted into the well and a working fluid is circulated through the sealed tube to collect heat. The sealed tube makes heat transport highly efficient and prevents leakage of the working fluid into the earth. Different working fluids can be used (water, sCO2, other), depending on the project.
GreenFire Energy’s closed-loop geothermal technology, called GreenLoop, overcomes the limitations associated with low subsurface permeability and water availability to enable a vast expansion of geothermal power generation. Hot, dry rock formations contain more than 90% of geothermal heat but lack the permeability required for hydrothermal projects. Closed-loop systems ensure enough heat flow from the geothermal resource to the surface.
Closed-loop systems are also capable of removing more of the heat from a given geothermal site. This capability is important because, according to the U.S. Geological Survey (USGS), hydrothermal projects generally remove only about 10% of the available heat from a given resource. This occurs because heat can be removed only from those portions of the resource that have natural fractures to allow fluid flow; the rest of the heat is stranded. In contrast, GreenFire Energy’s research shows that closed-loop wells can be spaced at close intervals for intensive heat extraction. By accessing a higher percentage of available heat, closed-loop systems reduce the levelized cost of energy (LCOE) and provide higher returns on project development expenditures, fixed assets, and transmission.