In a world first, scientists propose geothermal power plants that also work as valuable clean energy reservoirs
A team of researchers from Princeton University claims that enhanced geothermal systems (EGS) could enable up to five terawatts of power generation in the U.S. alone. This is huge because, currently, the total amount of electricity produced annually in the country from all the sources stands at around one terawatt only.
Geothermal energy is the heat that is naturally produced in the interior parts of the planet including the Earth’s crust. It is considered among the most stable, reliable, and sustainable forms of energy. However, it is not as popular as solar and wind power because setting up a conventional geothermal power plant requires a high initial investment, has a long-payback time; and is limited by various factors including location, and resource availability.
The researchers have proposed an EGS that has the potential to overcome various such limitations. They claim the geothermal system can not only work as an efficient power plant for producing clean energy but also as a cheap solution for effective energy storage and dispatch.
One of the authors of the study and Ph.D. candidate at Princeton University, Wilson Ricks told IE, “Geothermal energy is currently limited by the need for naturally-occurring hydrothermal reservoirs, which are extremely rare. These are sites that host both significant geothermal heat and an aquifer that can be pumped to extract that heat. Enhanced geothermal systems (EGS) are an emerging technology that does away with this second requirement, allowing geothermal development in locations with hot rock but no existing aquifers. These conditions are much more common and could enable up to 5 terawatts of geothermal development in the US alone.”
The untapped potential of an enhanced geothermal system
The researchers suggest the development of an artificial geothermal system that makes use of advanced technologies similar to those that are employed in the oil and gas industries. Such an EGS is driven by an artificial fracture network that can demonstrate powerful drilling and hydraulic functions. Whereas conventional geothermal reservoirs can naturally occur in permeable zones, the proposed EGS is required to be established only within hot and impermeable rock formations.
Since the rock surrounding the reservoir is effectively impermeable, the extra water left after power generation can be pumped into the reservoir and stored there. Later, when required, this excess water can be pumped to the surface, enabling greater electricity production than would otherwise be impossible.
The researchers point out that extra water storage does not require any modifications in the existing structure of the geothermal system because these are inherent capabilities of any enhanced geothermal reservoir found within impermeable rocks. The storage, therefore, comes effectively free with the enhanced geothermal power plant.
However, it is important to note that this storage approach does not work with conventional geothermal reservoirs — naturally-occurring hot aquifers — as most of the excess water pumped into the reservoir will simply leak off into the surrounding permeable rock.
Another aspect of this technique is that it does not store electricity generated by other sources. Instead, it accomplishes the same task effectively by storing the geothermal power plant's own energy for use at specific times.
EGS is a win-win situation for everyone
Geothermal power is a much more stable and sustainable clean energy alternative when compared to solar and wind energy which are highly fluctuating in nature and depend upon the external environment conditions. The interior parts of the Earth are not much affected by external atmospheric factors. Therefore, the proposed EGS can function as a better and more reliable power source and energy storage solution than any wind or solar farm.
When asked about the limitation of the technology, Ricks explained, “our study is based on simulations of geothermal reservoirs, and the performance of EGS in-reservoir energy storage can only be verified through true at-scale field testing. We also only assess the value of storage for individual geothermal plants and do not explore the implications for the electricity system as a whole.”
The research shows that EGS can provide significantly more value than previously assumed, and may prove to be a motivation for greater investment in the technology. Ricks believes that if the in-reservoir energy storage technique is successfully demonstrated in the field, the additional value gained could even help EGS power plants to compete in the electricity market.
The storage ability of plants itself could help reduce the cost of decarbonized electricity systems as a whole, allowing geothermal plants to store up their energy during times when wind and solar generation is plentiful and release it when it is most needed.
The findings from this initial research have also motivated a follow-on project aiming to demonstrate in-reservoir energy storage in the field, which has received $4.5 million in funding from the US Department of Energy. The researchers are currently working on the follow-up study and will release the same shortly.
The study is published in the journal Applied Energy.
Geothermal systems making use of advanced drilling and well stimulation techniques have the potential to provide tens to hundreds of gigawatts of clean electricity generation in the United States by 2050. With near-zero variable costs, geothermal plants have traditionally been envisioned as providing “baseload” power, generating at their maximum rated output at all times. However, as variable renewable energy sources (VREs) see greater deployment in energy markets, baseload power is becoming increasingly less competitive relative to flexible, dispatchable generation and energy storage. Herein we conduct an analysis of the potential for future geothermal plants to provide both of these services, taking advantage of the natural properties of confined, engineered geothermal reservoirs to store energy in the form of accumulated, pressurized geofluid and provide flexible load-following generation. We develop a linear optimization model based on multi-physics reservoir simulations that captures the transient pressure and flow behaviors within a confined, engineered geothermal reservoir. We then optimize the investment decisions and hourly operations of a power plant exploiting such a reservoir against a set of historical and modeled future electricity price series. We find that operational flexibility and in-reservoir energy storage can significantly enhance the value of geothermal plants in markets with high VRE penetration, with energy value improvements of up to 60% relative to conventional baseload plants operating under identical conditions. Across a range of realistic subsurface and operational conditions, our modeling demonstrates that confined, engineered geothermal reservoirs can provide large and effectively free energy storage capacity, with round-trip storage efficiencies comparable to those of leading grid-scale energy storage technologies. Optimized operational strategies indicate that flexible geothermal plants can provide both short- and long-duration energy storage, prioritizing output during periods of high electricity prices. Sensitivity analysis assesses the variation in outcomes across a range of subsurface conditions and cost scenarios.
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