# Techno-economic analysis of geothermal combined with direct and biomass-based carbon dioxide removal for high-temperature hydrothermal systems - **Authors:** K.A. Titus (ORCID 0000-0002-3885-1682); David Dempsey; Rebecca A.M. Peer (ORCID 0000-0002-9951-2625); Rosalind Archer - **Journal:** Geothermics, vol. 125, pp. 103159 (2025) - **Publisher:** Elsevier BV - **DOI (version of record):** [https://doi.org/10.1016/j.geothermics.2024.103159](https://doi.org/10.1016/j.geothermics.2024.103159) - **This page:** https://ddempsey.github.io/papers/2025-titus-techno-economic-analysis-geothermal-combined/ - **Open PDF:** [https://ddempsey.github.io/papers/2025-titus-techno-economic-analysis-geothermal-combined/titus2025-techno-economic-analysis.pdf](https://ddempsey.github.io/papers/2025-titus-techno-economic-analysis-geothermal-combined/titus2025-techno-economic-analysis.pdf) (Published version) - **Full text (Markdown):** [https://ddempsey.github.io/papers/2025-titus-techno-economic-analysis-geothermal-combined/fulltext.md](https://ddempsey.github.io/papers/2025-titus-techno-economic-analysis-geothermal-combined/fulltext.md) - **Notice:** Published version, reproduced under the CC BY 4.0 licence. © The Author(s). Version of record: https://doi.org/10.1016/j.geothermics.2024.103159 - **Explainer:** all explainers on this site are AI-generated. - **BibTeX:** https://ddempsey.github.io/papers/2025-titus-techno-economic-analysis-geothermal-combined/citation.bib - **Topics:** Geothermal systems; CO2 & hydrogen storage - **Keywords:** Geothermal gradient; Hydrothermal circulation; Carbon dioxide; Biomass (ecology); Environmental science ## In brief We modelled geothermal power plants that also remove CO2 from the atmosphere, by burning forestry residues or by capturing CO2 from air, and store it dissolved in reinjected water. At USD 100 per tonne of CO2, the biomass design had the lowest median costs of electricity and of carbon abatement. - In our model, 100 kg/s of geothermal fluid gave 16.4 MWe with biomass superheating, 13.7 MWe in a conventional plant and 8.3 MWe with direct air capture. - Direct air capture removed more CO2, a net 36.5 kt per year against 25.6 kt per year for the biomass design. - {'At a CO2 price of USD 100 per tonne, the biomass design had the lowest median costs of the three designs': 'USD 88/MWh for electricity and USD 122 per tonne of CO2 abated.'} - An initial estimate for retrofitting biomass and CO2 capture to an existing geothermal plant gave an abatement cost of under USD 50 per tonne of CO2. - In a New Zealand case, removing 1 Mt of CO2 a year would need 62 wells and about 790 kt of residues with biomass, or 49 wells with air capture. ## The problem Limiting global warming to 1.5 to 2 °C will require net-zero CO2 emissions by 2050 (IPCC, 2023). Carbon dioxide removal (CDR), taking CO2 out of the atmosphere, will likely be needed to offset hard-to-eliminate emissions, from agriculture, aviation, shipping and industry (IPCC, 2023). Geothermal power stations return their cooled water to the reservoir. In 2012, the Carbfix project at Hellisheidi, Iceland, began dissolving CO2 in this water to store it underground (Sigfusson et al., 2015). If that CO2 came from the atmosphere, the plant would remove carbon. Direct air carbon capture and storage (DACCS) removes CO2 from air using heat and electricity that a geothermal plant can supply (Fasihi et al., 2019; Sabatino et al., 2021). Bioenergy with carbon capture and storage (BECCS) burns biomass and captures the CO2 it releases. We compare the costs of both options with a conventional geothermal plant. ## What we did We modelled the energy and carbon balances and the costs of a generic plant fed by 100 kg of geothermal fluid per second from a reservoir at 250 to 300 °C. The benchmark plant generates power from separated steam, and from the hot brine in a binary turbine. It vents reservoir CO2 at the worldwide average of 122 grams of CO2 per kilowatt-hour (gCO2/kWh) (Bertani and Thain, 2002). The geothermal-BECCS plant adds a boiler that burns biomass in pure oxygen (oxy-combustion) to superheat the steam. Its exhaust (flue) gas is more than 95% CO2 and is compressed and dissolved in the reinjected brine. The geothermal-DACCS plant uses the brine heat and part of the steam turbine output to capture CO2 from air, and dissolves it in the same way. We dissolved CO2 at 50 bar, below the supercritical pressure of CO2 (73.8 bar). For this plant's brine flow, that limits storage to about 50 thousand tonnes (kt) of CO2 per year. Costs were discounted at 8% over a 30-year life. We report levelised costs of electricity (LCOE, net of CO2 revenue), of sequestration (LCOS, per tonne of CO2 removed, net of electricity revenue) and of carbon abatement (LCCA, per tonne of CO2 avoided when the plant replaces fossil-fuel generation). Reference prices were USD 100 per tonne of CO2, USD 88 per tonne of biomass and USD 60 per megawatt-hour (MWh). We drew 1000 random sets of uncertain inputs and report medians. We then modelled a field in New Zealand's Taupō Volcanic Zone (TVZ). ## What we found In our model, geothermal-BECCS produced 16.4 megawatts of electricity (MWe), against 13.7 MWe for the benchmark and 8.3 MWe for geothermal-DACCS. Geothermal-DACCS removed more CO2, a net 36.5 kt per year against 25.6 kt. Neither removed as much CO2 per unit of electricity as standalone BECCS plants (Khallaghi et al., 2021; Yang et al., 2021). This is because the solubility of CO2 in brine limits storage. At the reference prices, geothermal-BECCS had a median LCOE of USD 88/MWh, close to the benchmark (USD 89/MWh) and, by our arithmetic, about half that of geothermal-DACCS (USD 181/MWh). Its median LCCA was also lowest, USD 122 per tonne of CO2, against 157 for the benchmark and 187 for geothermal-DACCS. Removal costs were closer: a median LCOS of USD 241 per tonne for geothermal-BECCS and 311 for geothermal-DACCS, with uncertainty ranges that could overlap. We conclude that the extra electricity makes geothermal-BECCS the more cost-effective of the two CO2-removal designs. For geothermal-BECCS, the CO2 yield of the feedstock (the CO2 released per tonne burned) and the bioenergy capital cost were the largest contributors to the uncertainty in its costs. A low yield leaves brine storage capacity unused. The advantage of geothermal-BECCS over the benchmark also depends on the CO2 price. On median values, its LCOE is lower than the benchmark's at CO2 prices above about USD 100 per tonne. With model uncertainty, the two LCOE ranges overlap for CO2 prices between about USD 30 and 170 per tonne. Retrofitting an existing plant could cost less, because its wells, steam field and plant equipment are already paid for. As an initial estimate, we find a retrofit LCCA under USD 50 per tonne, by our arithmetic less than half the new-build value. However, a practical retrofit would need a design specific to the original plant. For a TVZ reservoir at 275 °C, removing 1 million tonnes (Mt) per year with geothermal-BECCS would need 62 wells. The plants would generate about 510 MWe and burn about 790 kt of forestry residues per year. For comparison, Wairakei Power Station (175 MWe) had 59 active production wells in 2008 (Bixley et al., 2009). New Zealand produces 3 Mt of unused forestry residues per year (MPI, 2020). Geothermal-DACCS would need 49 wells and no fuel, but would give about 190 MWe. Capital costs were similar, USD 2.2 and 2.3 billion.
The calculator scales the TVZ case in proportion to the target, as the paper does. For 1000 MWe of geothermal-BECCS, we estimate about 121 wells, 1.5 Mt of residues and 2 Mt of CO2 removal per year. ## Limitations Our plant designs are generic, and we did not include emissions from construction or land-use change. With our median inputs, net removal held only where the reservoir emits less than about 400 gCO2/kWh (BECCS) or 450 gCO2/kWh (DACCS). High-gas fields, such as those in Türkiye (Aksoy, 2014), may be unsuitable. A global inventory of plants suitable for retrofit could be compiled in future work. However, existing turbines may limit the extra power from superheated steam. ## Glossary A **bar** is a unit of pressure, about equal to atmospheric pressure at sea level. A **binary turbine** is driven by a separate working fluid, here isopentane, heated by geothermal brine. **Brine** is the hot liquid left after steam is separated from the geothermal fluid. **Carbon dioxide removal (CDR)** takes CO2 out of the atmosphere and stores it. **Discounting** counts future costs and income as worth less than present ones, here by 8% for each year in the future. **Kilotonne (kt)** is a thousand tonnes, and a **megatonne (Mt)** is a million tonnes. The **levelised cost** of a product is the discounted lifetime cost of a plant divided by its discounted lifetime output of that product. A **megawatt of electricity (MWe)** is a million watts of electrical output, and a **megawatt-hour (MWh)** is the energy delivered by one megawatt for one hour. **Oxy-combustion** burns fuel in pure oxygen. In general its flue gas is 90 to 99% CO2 (Zhou et al., 2016), and in our design it is more than 95%. A **supercritical** fluid is above the temperature and pressure at which liquid and gas can no longer be told apart. **Superheating** heats steam above its boiling temperature to raise the work a turbine extracts from it. ## Abstract Limiting global temperature rise to between 1.5 and 2 °C will likely require widespread deployment of carbon dioxide removal (CDR) to offset sectors with hard-to-abate emissions. As financial resources for decarbonization are finite, strategic deployment of CDR technologies is essential for maximizing atmospheric CO2 reductions. Carbon capture and sequestration (CCS), using either direct air capture (DACCS) or bioenergy (BECCS) technologies has a particular synergy with geothermal electricity generation. This is because expensive geothermal infrastructure can be leveraged to transport dissolved CO2 for storage in subsurface reservoirs. Here, we present a techno-economic comparison of renewable electricity generation coupled with either BECCS or DACCS at high-temperature, low-gas hydrothermal systems. We use a systems model that quantifies energy, carbon and financial flows through a generic hybrid power plant. At a CO2 market price of $100/tonne, the geothermal-BECCS system has a lower median cost of electricity generation ($88/MWh) than geothermal-DACCS ($181/MWh) and conventional geothermal ($89/MWh). Geothermal-BECCS also had the lowest costs of overall emissions abatement, $122/tCO2 , accounting for carbon removal and assuming displacement of fossil-fuel generation. Abatement costs are even lower, $45/tCO2 , for BECCS retrofit of existing geothermal plants, owing to discounted costs of pre-existing injection wells, steam fields, and plant equipment. For a case study based on a geothermal field in New Zealand's Taupō Volcanic Zone (TVZ), we determined that achieving CDR rates of 1 MtCO2 /year via new geothermal-BECCS builds would require 62 standard geothermal wells and 790 kt/year of feedstock and result in 511 MWe in installed capacity. In contrast, geothermal-DACCS would need 49 wells and no external fuel source to achieve 1 MtCO2 /year scale but result in only 190 MWe in installed capacity. Both pathways are calculated to require similar upfront investment costs at $2.2 billion and $2.3 billion for geothermal-BECCS and geothermal-DACCS respectively. Although geothermal-DACCS removes CO2 at high rates, its high parasitic load increases the overall decarbonization cost ($187/tCO2 ). In contrast, when biomass hybridization is considered, geothermal-BECCS has a lower cost of emissions abatement and produces 20 % more electricity than the benchmark geothermal plant. We conclude that this increase in electricity production makes geothermal-BECCS the more cost-effective geothermal-based CDR configuration. Finally, we argue that revenues from net-negative CO2 emissions and increased power production make geothermal-CDR a cost-competitive decarbonization technology. ## Cite Titus, K.A., Dempsey, David, Peer, Rebecca A.M., Archer, Rosalind (2025). Techno-economic analysis of geothermal combined with direct and biomass-based carbon dioxide removal for high-temperature hydrothermal systems. Geothermics, 125, 103159. https://doi.org/10.1016/j.geothermics.2024.103159 ```bibtex @article{titus2025techno, title = {Techno-economic analysis of geothermal combined with direct and biomass-based carbon dioxide removal for high-temperature hydrothermal systems}, author = {Titus, K.A. and Dempsey, David and Peer, Rebecca A.M. and Archer, Rosalind}, journal = {Geothermics}, year = {2025}, volume = {125}, pages = {103159}, publisher = {Elsevier BV}, doi = {10.1016/j.geothermics.2024.103159}, url = {https://ddempsey.github.io/papers/2025-titus-techno-economic-analysis-geothermal-combined/} } ```