New report explores geothermal cooling potential in Hawai‘i
Considering a potential option for reducing greenhouse gas emissions on O‘ahu, in a new report, University of Hawai‘i (UH) at Mānoa researchers explored the feasibility of using geothermal heat pumps for cooling buildings. A team from the UH Mānoa School of Ocean and Earth Science and Technology (SOEST) and the Lawrence Berkeley National Laboratory collaborated to assess the feasibility of developing shallow ground heat exchangers near Stan Sheriff Center on the UH Mānoa campus and across Oʻahu.
“Reducing electrical load through use of Ground Source Heat Exchanger technology is referred to as shallow geothermal,” said Nicole Lautze, report co-author and director of the Hawai‘i Groundwater and Geothermal Resources Center (HGGRC) in SOEST’s Hawai‘i Institute of Geophysics and Planetology. “This research aimed to assess the applicability of this technology for specific buildings on UH campuses, but has the potential to expand across the state.”
While geothermal energy is often thought to be using heat from deep in the earth to generate electricity, there are other uses of the heat gradient that have different applications, including heating and cooling buildings. Just beneath Earth’s surface, around 10 to 30 feet, the ground maintains a fairly constant temperature of 50°F to 59°F. This area is close to the ground surface; therefore, the approach does not involve drilling thousands of feet underground or extracting hot water.
Geothermal heat pumps use geothermal heat exchangers to exchange heat with the earth. Through circulating groundwater or a heat-transfer solution through looping pipes in the shallow ground, the exchangers bring the cool liquid to a warmer place, the same process used in a refrigerator or air conditioner. Although heat pumps transport heat from one place to another using electricity, air conditioning that relies on geothermal energy lessens the overall load on the electric grid.
When exploring using geothermal heat pumps on O‘ahu, researchers considered these options:
- Open-loop geothermal system–In an area with sufficient groundwater flow, wells extract and inject groundwater, which transports heat to and from the ground. These exchangers use cooler groundwater from outside the system for cooling and expel the warmer water afterward.
- Closed-loop geothermal system–Circulating through pipes, a heat-transfer solution moves heat to and from the ground via thermal conduction. This system can operate in areas without ready access to groundwater.
The researchers developed favorability maps for closed-loop and open-loop geothermal heat exchangers systems. They used geographic information system layers with 11 attributes—including elevation, geology, and soil permeability. Restrictions on water supply limited the available area for geothermal heat exchangers system deployment across the island, while many coastal areas were highly favorable. Overlays showed potential customers and restricted areas.
Possibilities at Stan Sheriff Center
On the UH Mānoa campus, the researchers considered the Stan Sheriff Center for the geothermal heat exchanger technology. The sports arena, which can host 10,300 people, holds a high cooling load and stands in an area with lots of surrounding open spaces and athletic fields. By using a hydrogeologic model, researchers analyzed a potential closed-loop system at the site. They modeled groundwater and heat flow, analyzed subsurface heat flow, and completed a techno-economic analysis. Cold seawater may also be an option, the analysis concluded, and the Natural Energy Laboratory of Hawaiʻi operates such a system. The report authors encouraged further study.
According to the UH Office of Sustainability, HVAC systems (heating, ventilation, and air conditioning) is one of the largest consumers of energy on the campuses. They account for 58% of total energy consumption across the UH system. Nationwide, universities have been testing and adopting geothermal heat pump systems for campus heating and cooling.
“The transition to geothermal systems can boost energy efficiency, cut electricity costs, and help the ten-campus UH system reach its net-zero emission goals by 2035,” said Lautze.
Report authors and funding
HGGRC and the Lawrence Berkeley National Laboratory produced this report through the U.S. Department of Energy’s Energy Technology Innovation Partnership Project. Christine Doughty from Berkeley Lab is the lead author. Co-authors from Berkeley Lab include Jianjun Hu, Craig Ulrich, Sean Murphy, Patrick Dobson, and Mike Campton. Co-authors from HGGRC include Lautze and Donald Thomas.
A statewide Geothermal Energy Grant Program sponsored this research, and the U.S. Department of Energy’s Energy Technology Innovation Partnership Project (ETIPP) provided technical support. Managed by the National Renewable Energy Laboratory, ETIPP helps remote, coastal, and island communities build more reliable and affordable energy systems. Communities apply for up to 24 months of technical assistance, and those communities drive the scopes and focuses of their energy projects.