The many options for utilizing geothermal energy—from shallow heat exchange for space heating and cooling, to direct utilization for district heating and industrial heat, to geothermal power production, to closed-loop systems and to enhanced geothermal systems (EGS)—provide multiple value chains that have one thing in common: reducing the CO2 footprint of our energy systems. Additional synergies can be realized through combining geothermal with other technologies in hybrid applications. There are many such applications, starting with hybrids between different types of geothermal resources; for example, combined heat and power projects bring not only clean, baseload electricity but also a benefit to local communities through direct utilization. This paper explores the growing interest in four other hybrid solutions: geothermal + oil & gas in deep sedimentary basins; geothermal + solar; geothermal + wind; and geothermal + green hydrogen production. Each of these provides opportunities for innovative business models and synergistic relationships.
Oil & gas production from deep sedimentary basins has been ongoing for more than 100 years, resulting in a wealth of knowledge and technology associated with extracting hydrocarbons. Most producing oil & gas wells also produce water—depending on depth, some of this water is hot. This provides an obvious geothermal opportunity that has been previously recognized, but largely ignored until now. Considering the fundamental differences between 1) how oil & gas wells and geothermal wells are completed, 2) the difference in energy content between hydrocarbons and hot water, requiring comparatively high flow rates of hot water to be economic, and 3) the fact that most sedimentary basins are in areas with normal temperature gradients, geothermal production of hot waters in sedimentary basins is not without its challenges. However, oil & gas technologies have long been leveraged for geothermal drilling and production, and operators entering the geothermal space are taking advantage of this history, working with geothermal experts to maximize energy production from geothermal resources. Taking advantage of the knowledge and the significant amount of data available from these basins, the geothermal + oil & gas hybrid is an obvious opportunity for increasing the amount of geothermal energy produced today.
Several geothermal operators in the United States have adopted the geothermal + solar hybrid model, particularly in arid areas with high summer temperatures, which decreases the conversion efficiency of air-cooled binary power plants. The addition of solar PV to this type of geothermal project enables geothermal operators to mitigate the decrease in geothermal output during periods of hot weather. Particularly where water cooling is not an option, the addition of solar PV helps maintain power generation at times when energy pricing may be at its peak. Concentrated Solar Power (CSP) also offers highly useful options for increasing geothermal output, using approaches such as boosting the temperature of the geothermal fluid to increase conversion efficiency and output, and using steam topping turbines to make an even bigger impact on the output of geothermal facilities, regardless of the temperature of the geothermal fluid. Thus, an intermittent energy source like solar creates a useful complement to baseload geothermal.
Geothermal + wind is similar to solar; however, in this hybrid, geothermal supports the continued provision of power from wind resources that vary daily and seasonally. Although no such hybrids exist today, there is potential for synergistic relationships between the two sources in areas where the wind power density is reasonably high. The highest power density is found offshore, but there are land areas around the world where the wind density is reasonably good, though more variable. One example is the wind belt in the central United States, which extends from Texas to the Canadian border. In this region, there are many deep basins that are developed for oil & gas production, many of which contain fluids at attractive temperatures for power generation. The installation of a modest increment of power produced from these fluids using binary power technology could add an increment of baseload power that improves the overall performance of wind projects.
The final hybrid discussed herein is using geothermal for the production of hydrogen, which is fast becoming the fuel of the future owing to its zero-emission characteristic and the fact that it is readily available through electrolysis of water. In addition, hydrogen is an important component for the production of ammonia, urea, methanol and melamine. Gray hydrogen (commonly using natural gas for electrolysis) becomes blue hydrogen when carbon capture, utilization and storage (CCUS) techniques are applied. Green hydrogen is produced by using clean energy sources for electrolysis. The current discussion around the production of green hydrogen is centered around solar power, but using geothermal power for electrolysis is a topic of increased interest, and would improve the efficiency of hydrogen production by using a clean baseload power source with a constant output. Although there are many more, this paper presents two scenarios that provide immediate and meaningful opportunities for the production of green hydrogen: 1) in island settings where demand is less than supply; and 2) where geothermal assets lack access to markets (because of remoteness, lack of transmission or both).
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