GEOTHERMAL
What is Geothermal Energy?
Geothermal energy is the Earth’s heat, which can be harnessed for electricity generation or direct uses like heating and cooling. In Hawaiʻi, the main application of geothermal energy is electricity generation, which is possible where subsurface heat, permeability, and transport fluid create an “optimum geothermal play.” As a firm, dispatchable, low-carbon, local resource, geothermal energy is accessed through wells that tap steam, hot water, or geothermal fluid, which is used to turn turbines and drive generators, creating electricity.
Understanding Geothermal as a Public Trust Resource
A “public trust resource” refers to natural resources such as land, water, minerals, and wildlife held in trust by the government for all citizens. In Hawaiʻi, the Public Trust Doctrine is enshrined in the constitution and law, extending beyond water to land, air, minerals, natural beauty, and Native Hawaiian cultural practices. Article XI, Section 1 of the Hawaiʻi Constitution states: “For the benefit of present and future generations, the State and its political subdivisions shall conserve and protect Hawai’i’s natural beauty and natural resources, including land, water, air, minerals and energy sources, and shall promote the development and utilization of these resources in a manner consistent with their conservation and in furtherance of the self-sufficiency of the State. All public natural resources are held in trust by the State for the benefit of the people.”
The doctrine mandates the state to protect public natural resources while considering the traditional and customary rights of Native Hawaiians. It recognizes both the public’s right to use trust resources and the private, traditional, and customary rights in those resources, with a presumption in favor of public use, access, and enjoyment. Ultimately, it requires balancing protection with maximum beneficial use, recognizing that these obligations are not mutually exclusive.
History of Geothermal in Hawaiʻi
For decades, geothermal energy has been one of the most environmentally responsible methods for generating large-scale power. It can play a crucial role in phased transition plans toward cleaner fuel and electricity generation, potentially with energy storage such as hydrogen. It offers greater long-term reliability than other renewable sources, providing baseload capacity. Most importantly, it is a public trust resource and must be approached with care and reverence if Hawai‘i is to expand its use.
HSEO has played a pivotal role in advancing geothermal energy for over a decade. Through strategic partnerships with the University of Hawaiʻi’s Groundwater and Geothermal Resource Center (HGGRC), the U.S. Department of Energy (USDOE), and the National Renewable Energy Laboratory (NREL), HSEO has led efforts to understand the regulatory landscape for responsible development. Some of this research is outlined in our State Energy Resource Library.
Geothermal Energy Today
The Kīlauea East Rift Zone is home to the 38-megawatt Puna Geothermal Venture (PGV) plant and is the only region in Hawaiʻi with geothermal development. PGV is in the process of upgrading its technology, from 12 generators to 3, increasing its capacity to 54 MW. Other areas show suitable attributes, and surface research suggests potential outside Kīlauea, but limited deep-well studies leave their resources largely unknown. Generally, geothermal probability decreases north along the older islands, but areas such as Oʻahu, with high demand, may hold plausible resources pending further subsurface study.
A Path Forward
Why should the resource be developed?
The state’s energy transition to 100% renewable energy by 2045 will require more than the intermittent sources of energy that solar and wind provide. Firm and dispatchable resources, such as geothermal, are paramount to our energy reliability and grid stability. Geothermal energy, as a public trust resource, has the potential to provide the reliability that Hawaiʻi’s energy ecosystem requires.
What work is HSEO doing?
Research
In 2024, Governor Green allocated $5 million from the Coronavirus State Fiscal Recovery Fund for SEO to initiate a slim-hole resource characterization program to improve our understanding of Hawaiʻi’s geothermal potential. Slim-hole characterization is required to identify geothermal resources, and a good overview of possible resources can be developed using geophysical surveys, subsurface stress analysis, and groundwater chemistry. The University of Hawaiʻi is conducting research using slim hole drilling on Maui to gather, more information on the resources available and possible locations for geothermal energy development.
Community Outreach
The Hawaiʻi State Energy Office has committed to fostering a healthy community conversation about geothermal energy by introducing education and engagement programs. By providing improved access to information on geothermal energy, the state energy office intends to improve communications and foster community input. The energy office has also convened a Geothermal Working Group to evaluate the regulatory and policy landscape surrounding geothermal energy in Hawaiʻi.
Our approach involves the engagement of Paʻakai Communications in an extension of the HSEO Clean Energy Wayfinders program to ensure community engagement occurs within the larger context of achieving state RPS and decarbonization goals. This outreach involves sensitive conversations with the intent to improve access to information and community empowerment in determining Hawaiʻi’s energy future, including consideration of geothermal energy.
Community Input
The community plays a key role in the decision-making process and the success of all renewable energy projects in Hawaiʻi. HSEO recognizes the importance of prioritizing community engagement, education, and collaboration throughout the research and planning processes of a project to ensure the project meets the community’s needs and priorities.
Geothermal Working Group
House Concurrent Resolution (H.C.R.) 58 House Draft (H.D.) 1 was passed by the Thirty-third Hawaiʻi State Legislature during the 2025 Regular Session, and requests that HSEO convene a Geothermal Energy Working Group to evaluate the regulatory and policy landscape surrounding geothermal energy in Hawaiʻi. The Working Group is to be made up of designated representatives from relevant State and local government offices, representatives from relevant industry stakeholders, and other invited members. In addition, the H.C.R. requests that HSEO submit a report of its findings and recommendations, including any proposed legislation, to the Legislature no later than 20 days prior to the convening of the 2027 Regular Session.
Requirements
H.C.R 58 H.D. 1 states that the working group will be convened with the overarching premise of having a community-inclusive process to evaluate the role of geothermal development in Hawai’i. This group is tasked with identifying a clear development pathway that aligns with state policy, environmental stewardship and cultural sensitivity.
The working group will identify any regulatory, permitting, and policy challenges affecting the state; review best practices from other jurisdictions, such as New Zealand; assess geothermal energy’s potential expansion and its role in supporting energy resilience and affordability; and provide recommendations to the legislature on policy and regulatory reforms necessary to establish a clear and efficient pathway for geothermal energy in Hawaiʻi.
Geothermal Energy Resource Studies
Used with permission. Image sources: Lautze, Ito, G., Thomas, D., Frazer, N., Martel, S., Hinz, N., Tachera, D., Hill, G., Pierce, H., Wannamaker, P., Martin, T., (2020). Geothermics, 86, Article 101798. Play Fairway analysis of geothermal resources across the State of Hawai‘i: 4. Updates with new groundwater chemistry, subsurface stress analysis, and focused geophysical surveys Results of the initial geothermal play fairway probability analysis for the State of Hawai‘i. Probabilities of a geothermal resource are colored. Areas with restricted land access are shown in stippled and crosshatch patterns (e.g., National Park lands, protective conservation districts, and urban areas). Red boxes outline areas proposed for subsequent study. Red triangles indicate the calderas of the main shield volcanoes. White stars mark the locations of the Saddle Road well and Puna Geothermal Venture (PGV).
Hawai’i Play Fairway Reports
The Hawai’i Playfairway reports were commissioned by the US Department of Energy in conjunction with the US Office of Science and the University of Hawai’i. These reports were the first if their kind conducted in the state of Hawai’i since 1985. Its purpose was to conduct assessments to find hydrothermal resources which sit below the surface and characterize sources in rift-zone settings. These studies concluded with recommendations of areas for further exploration, a roadmap for additional exploration activities, and an updated resource assessment.
Play fairway analysis of geothermal resources across the state of Hawai‘i: 3. Geothermal 101: Understanding the Technology
Geothermal energy can be categorized into two areas: extraction type and plant type. “Extraction type” refers to the process that occurs underground, while “plant type” refers to the process that occurs above ground. Conventional geothermal plant types are hydrothermal systems, which include binary cycle, flash steam, and dry steam power plants. These systems differ slightly in the way they process the resource.
Geothermal energy can provide a massive amount of power to the energy grid, but it also has direct use applications such as heating for buildings.
Geothermal Plant Types: Conventional Hydrothermal
Binary cycle plants use a working fluid that is separate from the reservoir fluid; this fluid is ideally an alcohol with a lower boiling point. Binary cycle plants utilize a heat exchanger to turn the working fluid to steam. Hot reservoir fluid is brought up in a pipe and is used to heat working fluid in a neighboring pipe, turning the working fluid into steam without the two ever mixing. The steam is used to drive a turbine, which creates electricity. The reservoir fluid is then pumped back underground via injection well.
Hawaiʻi’s only geothermal power plant, Puna Geothermal Venture, is an advanced binary cycle system.
Dry steam plants draw steam directly from the underground reservoir resource. The steam rises, and is pushed through a turbine, creating mechanical energy. The turbine is connected to a generator, which turns the mechanical energy into electrical energy. Once used, the steam is moved through a condenser to turn it back into fluid and re–injected into the reservoir to preserve the resource.
Currently, the Geysers in northern California and Yellowstone National Park in Wyoming are the only known underground resources of steam in the United States.
Flash Steam Plants draw high-pressure reservoir fluid from deep underground and pipe it into a low-pressure tank. The difference in pressure causes the reservoir fluid to vaporize, or “flash,” into steam. The steam is driven through a turbine, which is connected to a generator, creating electricity. The steam is then moved through a condenser, turned back into water, and re–injected into the reservoir to be used again.
Geothermal Extraction: Types of Systems
Hydrothermal geothermal systems are the most frequently and conventionally used geothermal system. Hydrothermal systems tap into existing reservoirs and require three characteristics: heat, permeability, and water. The steam or water is extracted and used to create energy.
Open-loop systems are a geothermal system that pumps water from a nearby aquifer or lake, to a geothermal heat pump; the water is used once and then expelled. Because the water does not return to its original source, it has been referred to as a “pump and dump system.” Open-loop systems can degrade over time if water quality issues are present. Environmental concerns are also present, as the system can stir up sediment and silt, which can affect the aquifers.
Closed loop systems are a geothermal system that circulates a fluid through the ground and back to the surface through a system of sealed pipes, where the resource can be recycled and used continuously. Closed-loop systems are more durable and last longer than open-loop systems. Closed-loop systems do not put anything into the Earth, nor do they take anything out. Making them much more deployable in a variety of locations.
Enhanced Geothermal Systems (EGS) facilitate the development of low-permeability geothermal resources. EGS uses oil and gas techniques to fracture the rock, such as pumping liquid or carbon dioxide and horizontal drilling, to artificially stimulate preexisting reservoirs or create new ones. The reservoir can then be utilized as a geothermal resource. This technology has enormous potential to be a high-producing clean energy resource.
U.S. DOE Public Domain