Session 3D: Energy Conversion/Utilization

The world is facing a climate crisis as a result of greenhouse gas (GHG)-driven warming. Many geothermal operators and researchers are actively implementing and finding new solutions to the challenge of abating GHG emissions from geothermal plants to ensure a sustainable future.

In geothermal power operations, non-condensable gases (NCG) are brought to the surface dissolved in / with the geothermal fluids. The most abundant geothermal gases are carbon dioxide (CO2 - the most dominant NCG), hydrogen sulfide (H2S) and methane (CH4), with other gases considered trace gases. Their inability to condense (i.e. be turned into a liquid) by the cooling which occurs in geothermal power stations at the surface means NCGs are most often released to atmosphere during the power generation process. Decarbonization drivers on industry means reinjection of NCGs back into reservoirs is increasingly becoming standard practice at many locations globally, to close the emissions loop.

There is a need, and an opportunity, to find different uses and ways to abate the NCG emissions from geothermal plants that can be beneficial to operations, lower costs and potentially add value. In this paper, we examine several avenues of abatement through gas capture and re-use, into applications including horticulture, green fuels and chemicals production, and industrial uses, as an added benefit of geothermal energy production. Several pathways for utilization are illustrated that may have been overlooked.

Monetizing Low-Moderate Enthalpy Reservoirs: Lessons Learned

Dr. Russell Roundtree, Dr. Ben Burke, and Lia Sedillos

Gradient Geothermal, Inc. Denver, Colorado

Low and medium temperature hydrothermal reservoirs are differentiated from high-temperature geothermal reservoirs in several important ways: the predominant phase is liquid water, the reservoir temperature is usually less than 150 degrees Centigrade, the lithology is typically sedimentary rocks with conventional pore space and permeability, and the pore space can be filled with hydrocarbons. Most sedimentary systems are dominantly conduction driven heat flow instead of convective.

In May of 2022, Transitional Energy installed a pilot energy plant at the Blackburn Field in northeast Nevada to pilot the integration of an Organic Rankine Cycle Engine with existing oil and gas facilities to generate electricity at the still-operating mature oil field. Conventional subsurface assessments of total enthalpy then multiplied by an optimistic 10-15% ORC efficiency number to predict total electricity generation potential underestimate the complexities of extracting real work from the produced fluid mass flow.

Considerable enthalpy potential at the reservoir is lost by the time the fluids enter the ORC at oil field flow rates. Estimation of reservoir temperatures utilizing abundant well log header temperature readings is prone to error for several well known reasons. Corrections to those errors is problematic at best. An intermediate heat conveyance loop isolating the oil field fluids from the ORC creates additional engineering complexity and inefficiency. Tubulars limit the maximum flow possible from individual wells and create an upper limit on power generation from individual wells. Daily and seasonal variations in ambient temperature significantly effect net electricity generation due to heat rejection strategies in air cooled ORCs. The use of shallow groundwater or surface water to cool the condensing side of the ORC can help mitigate this effect and stabilize ORC output.

This study evaluates the techno-economic feasibility of two different deep geothermal system designs for direct-use heating to meet the Calgary Airport Authority’s energy demands and support net-zero carbon targets. Geological targets of interest include Upper Devonian carbonates and pre-Cambrian basement rocks, assessed for their geothermal potential and reservoir characteristics. A closed-loop geothermal system in u-loop configuration and an enhanced geothermal system were simulated to compare environmental and economic performance using the GEOPHIRES-X software. Results show that systems deployed at 4 km depth with water as the working fluid have the capacity to reduce 59% of natural gas consumption and abate approximately 396,000 tCO2-equivalent emissions for YYC over the study period. This research demonstrates the feasibility of deep geothermal systems in low-enthalpy basins within urban centres and highlights important areas for future work to support geothermal development in these basins.

This study builds on the existing techno economic and environmental life cycle assessments performed for a geothermal district heating and cooling system implementation in Tuttle Oklahoma with four existing oil and gas wells. Its resilience in meeting the peak annual heating and cooling loads of the district assuming a temporary disconnection from the electrical grid is assessed both qualitatively and quantitatively. Attributes of resilience and qualitative criteria established by Kolker et al. (2022) for geothermal district heating systems are applied to the proposed case to highlight its vulnerabilities. Results indicate that increasing the redundancy and diversity in the physical configuration of the distributional piping can considerably increase the system’s resilience. A quantitative assessment executed via the REopt tool predicts that the ancillary electricity required to power the geothermal system’s heating and cooling can be met by an onsite emergency diesel generator during times of grid outages for over 28 days.

Geothermal power generation typically (but not always) results in greenhouse gas emissions as a result of these gases being naturally present in geothermal fluids. Globally, there is an increasing focus on reducing naturally occurring emissions from geothermal electricity generation to contribute to wider decarbonization goals and efforts. Several countries have already implemented carbon emissions pricing, generally as a carbon-dioxide equivalent (CO2e) value, or are in the process of considering doing so. Three major factors will increase the focus of operators in reducing emissions: (1) increased direct costs due to carbon pricing, (2) societal pressure from the general public, and (3) investment funds or funding institutions with ESG targets that influence the pricing and availability of funding for both new and existing projects.

This paper attempts to catalog details of global geothermal power plants that have meaningful greenhouse gas emissions reduction technologies in place, with details on the methods employed. The authors intend to republish this paper with an updated catalog regularly, and we encourage feedback from readers to ensure this catalog is updated accurately.

Keywords

Geothermal ORC Power Plant, Green Hydrogen, Hydrogen Liquefaction, Electrolyser, Proton Exchange Membrane

ABSTRACT

Investment in and development of geothermal power projects have been going through ‘feast and famine’ cycles during the last few decades. Capital expenditure, private or institutional, are rushed into geothermal power plant development as soon as governments enact some form of incentives for renewable energy and there are no activities as soon as the incentive terms expires.

The worldwide campaign to reduce significantly or eliminate environmental pollution and using clean hydrogen as a preferred energy source is well underway and being sponsored by almost all countries. Hydrogen production in general and green hydrogen in particular is rapidly increasing. Total investment in net zero emission projects that was projected at $3,000 billion for 2021-2025 will exceed $4,300 billion by 2030.

Green hydrogen is defined as that which is produced by splitting water through electrolysis. Electrolysers powered by electricity from a geothermal power plant produce green hydrogen. Utilizing a renewable source energy that generates no polluting emissions into the atmosphere produces the most sustainable and cleanest hydrogen.

The fast growth of demand for green hydrogen is an excellent opportunity for investment and development of geothermal power plants. In this paper, the authors present a brief description of green hydrogen production and liquefaction facilities. There is also a short reference to oxygen liquefaction as a byproduct of an electrolyser. The paper continues with a simulated evaluation of green hydrogen production and liquefaction from a 15 MW geothermal ORC power plant. The simulation is configured to demonstrate green hydrogen production as an energy storage option for the period of low demand for electrical power. Geothermal energy can be used for green hydrogen production, a direct power source or both, as in the case with our simulation.