Geothermal Technologies Compared
Geothermal power production dates back to the early 1900s, although global commercialization did not begin in earnest until approximately 1960. However, while there have been significant advances in surface power generation equipment, little has changed in the subsurface design. All current designs still require communication between open-ended injection and production wells that extend vertically into open, hot groundwater reservoirs.
There are two types of conventional hydrothermal projects. Dry steam, the most rare type, collects steam from various wells and conducts it by pipeline to a central generating plant. Other hydrothermal systems lift water to the surface where it is flashed into steam, or, in the case of binary systems, run through a heat exchanger to transfer the heat into another fluid that actually drives the turbines.
Where there is insufficient subsurface permeability to allow for water flow, a newer technology, known as “Enhanced Geothermal Systems” or “EGS”, attempts to create flow by opening fractures in hot, dry rock. Approximately US$1 billion has been spent in this R&D effort over the past 30 years with only partial success. Other than the creation of fractures, water-based EGS is very similar to conventional hydrothermal; water needs to be pumped into and through the formation and then back to the surface, which constitutes a significant and costly parasitic energy loss. This process is made more difficult because water is relatively viscous and does not flow easily through small cracks.
In both conventional hydrothermal and hydro EGS geothermal designs, the injection and production wells extend into an “open” underground reservoir. This means that the geothermally heated water or brine is directly exposed to the subsurface rock formation and often brings unwanted materials to the surface, such as impurities, dangerous gases, and other toxic chemicals. These impurities also foul the aboveground facilities, adding to operating costs and often resulting in waste streams. Furthermore, geothermal power production can consume significant amounts of water, not only because water can be lost into the formation but also because water-based cooling towers are used to cool the water before reinjection.
GreenFire Energy’s ECO2G™ changes all of this by using supercritical carbon dioxide (sCO2) circulating in a closed loop that includes the injection and production wells. Creating a closed-loop system requires the use of the same advanced drilling and completion technologies that revolutionized the oil and gas industry. A closed-loop system has the twin benefits of preventing the loss of sCO2, and of keeping the sCO2 stream from interacting with water and minerals that result in scaling and corrosion. Only a closed loop system of this type can move fluid through the impermeable ductile, or "plastic," zone.