A preliminary modeling study published in the Journal of Engineering Research suggests that underutilized geothermal reservoirs could support compressed air and CO2 energy storage. Geothermal heating improved round-trip efficiency by approximately five to six percentage points, although more realistic CO2 modeling revealed significant performance limitations that require further investigation.
Study: Preliminary Feasibility of Geothermal-Assisted Compressed Air and CO2 Energy Storage in Underutilized Geothermal Reservoirs. Image Credit: Zebra-Studio/Shutterstock.com
Can Underground Storage Solve Renewable Energy's Supply Challenge?
Managing surplus electricity from fluctuations in wind and solar generation, and releasing it when demand surges, remains a key challenge.
Thermal power plants can also generate surplus electricity when demand remains low. As a result, long-term energy storage has become crucial for balancing supply and demand in power systems.
Subsurface energy storage is often discussed as a candidate for long-term storage because geological formations can offer far greater capacity than traditional surface systems. Studies have reported that underground energy storage is a feasible alternative for large-scale integration of renewable energy.
Why Reservoir Integrity Matters for Energy Storage
Subsurface alternatives are particularly attractive in nations with significant geothermal resources. For instance, in the United States, approximately three million abandoned wells have great potential
for reuse.
However, the presence of old wells alone does not confirm that a reservoir is suitable for subsurface storage. Based on compressed air energy storage (CAES), storage deliverability and integrity constantly play a crucial role within porous media.
Studies have reported that seal quality, fluid recirculation, and gas bubble stability within the reservoir significantly affect utility-scale CAES in aquifers. Further developments emphasized the use of the reservoir as a heat source and a pressure reservoir.
The selection of working fluids is another major problem. Studies have shown that, for compressed CO2 energy storage (CCES), the thermodynamic behavior of the storage fluid plays a critical role in the system's output within a saltwater aquifer.
Against this backdrop, the magnitude of the realizable geothermal uplift, the adequacy of storage integrity, the acceptability of the pressure margin, and the justification of the added operational complexity by the advantage of CO2 require further investigation.
Comparing Air and CO2 for Geothermal Energy Storage
In this work, researchers assessed the preliminary feasibility of CO2 and air storage in inactive geothermal reservoirs using a low-order surface-well reservoir model. The model, developed using Python programming, allowed CO2 and air to be compared under equivalent pressure boundary, cycle scheduling, and geometric conditions.
Using the fixed-property, ideal-gas-style screening model, the researchers compared CO2 and air across electrical performance (average net discharge power and round-trip efficiency), pressure margin, plume size (dispersion radius), mass retention, geothermal reheating, and storage integrity.
Additionally, the authors combined field-based evaluation, geothermal reheating, well-reservoir response, and electrical performance approaches into one reduced-order model of the coupled well, surface, and reservoir system.
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Modeling 18 Underground Energy Storage Scenarios
The researchers conducted a preliminary modeling assessment informed by published geothermal field conditions. They considered inactive reservoir sectors with high temperatures and potentially reusable wells, while using assumed values for several reservoir properties that had not been verified through site-specific measurements.
Based on three steps, including discharge, storage, and charging, the investigated system was developed. The primary architecture was dependent on one reversible access well for production and injection according to the conceptual model.
Additionally, the location of the injection-production pathway, gas plume, and overburden described the energy sequence. Initially, geothermal reservoir sectors were screened depending on well integrity, pressure margin, caprock condition, and temperature.
After defining the initial candidates, two working-fluid alternatives, along with three storage and temperature-integrity classes, were matched to each sector, yielding 18 scenarios under shared reservoir pressure and geometry limits. For every combination, the coupled surface–well–reservoir model was operated.
Geothermal Heat Boosts Efficiency, but CO2 Faces Limitations
The results demonstrated that high temperatures offered dual advantages: gas could be stored as pressure energy, while geothermal heat provided thermal uplift during storage and discharge.
In the high-temperature geothermal case (553.15 K), air produced a geothermal temperature increase, average net discharge power, round-trip efficiency, energy density, and thermal assist fraction of 58.9 K, 6.20 MW, 27.18%, 5.634 kWh/m3, and 13.91%, respectively.
Under matching conditions, CO2 produced 49.7 K, 6.90 MW, 29.46%, 7.615 kWh/m3, and 11.55%, respectively, with a 31.4 m plume radius, less than 34.5 m for air.
Geothermal reheating increased round-trip efficiency by 6.08 and 5.48 percentage points and net power by 1.42 and 1.25 MW for CO2 and air, respectively, relative to non-geothermal conditions.
However, the model indicated that only approximately 56% of the injected gas mass was recovered during the simulated cycles for both air and CO2. This relatively low recovery rate was a major contributor to the system's limited round-trip efficiency, as a substantial proportion of the energy used to compress the injected gas could not be recovered.
The findings, therefore, highlight the importance of improving gas recovery when assessing the feasibility of underground compressed-gas energy storage.
A separate phase-guarded real-gas CO2 assessment yielded 3.40 MW net discharge power and 19.77% round-trip efficiency, below the corresponding air screening values of 6.20 MW and 27.18%. However, because this assessment also used a different exhaust-pressure boundary, the researchers cautioned that a definitive comparison between the two fluids requires further investigation.
In conclusion, the findings of this study indicated that underutilized geothermal reservoirs could be used to support storage with thermal assistance, conditional on slightly lower energy density and narrower pressure margins. However, these results remain preliminary, and further investigation is required to assess reservoir integrity, operating conditions, and CO2 under realistic thermodynamic conditions.
Journal Reference
Pambudi, N. A., & Aziz, M. (2026). Preliminary Feasibility of Geothermal-Assisted Compressed Air and CO2 Energy Storage in Underutilized Geothermal Reservoirs. Journal of Engineering Research. DOI: 10.1016/j.jer.2026.09.060, https://www.sciencedirect.com/science/article/pii/S2307187726004955.
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