“Geothermal energy” has been in the statewide news recently with an endorsement by Governor Jared Polis of a “Heat Beneath our Feet” (HBOF) initiative with the Western Governors’ Association.
As currently used, “geothermal energy” encompasses a broad variety of technologies utilizing the heat of the earth. Locally, we are familiar with natural “hydrothermal” hot water from the ground along the Crystal River and around Glenwood Springs. Geothermal power is the production of electricity from the earth’s hot water or steam.
Because the geologic availability of natural hydrothermal water is limited, the HBOF initiative focuses mostly on “Enhanced Geothermal Systems” (EGS) for power generation, and a bit on ground source heat pumps (GSHP). “Geothermal” heat pumps utilize the more constant temperature of the shallow subsurface, from 10 feet to 1,000 feet, to reduce the electricity consumption of heat pumps warming buildings. More specifically, these are geo-exchange loops circulating tempered water to GSHP systems.
In Carbondale and nearby, large GSHP systems are currently proposed and already at work. A proposal to evaluate a networked district of GSHPs to displace gas heating at the Third Street Center and nearby buildings has been accepted by the Department of Energy. Federal funds should soon be available for design and engineering and CLEER will provide more details when the grant is finalized.
A “district,” “network” or “grid” of ground loops refers to multiple buildings sharing the geo-exhange water loop. This was pioneered at Colorado Mesa University and Ball State University around 2010. The idea of using this for heating conversions emerged from the Home Energy Efficiency Team in Massachusetts. There, the local gas company is being utilized for construction and financing since they’re experienced in laying pipes in the street.
The largest nearby GSHP system was built for the Powers Art Center (13110 Highway 82) in 2010. The geo-exchange tempering utilizes “slinky tubes” which are buried under a meadow to the east of the building. According to the staff, the HVAC performs perfectly, and inexpensively for a museum.
These ground loop systems have a reputation for low maintenance and long lifetimes. The buried heat exchangers are not subject to extreme temperatures, can’t be clogged with cottonwood seeds, hit by falling ice or wayward bike parking, etc. The other components are usually inside the building. Typically, they contain smaller quantities of refrigerant, which is less likely to leak because the units are factory-sealed. These use less power for heating during extreme cold compared to air source heat pumps. The drawback is higher installation costs and sometimes challenging ground loop installation.
Power generation from geothermal heat requires hotter temperatures. These are found over a mile deep, and are much lower than fuel-burning power plant temperatures, which contributes to high costs. Unless water is naturally flowing, the heat is also difficult to extract. The “enhanced” part of EGS is the use of horizontal drilling and then fracturing for a permeable path between two deep wells. A closed loop of water is pumped down an injection well, across the path, and up a production well where the heat is exchanged with a “binary” power plant. Instead of steam, these usually use another sealed system with a turbogenerator, and dry cooling to operate without water-consuming cooling towers.
The HBOF initiative identifies oil and gas regions as candidates for EGS. Currently, no proposals have been announced for EGS power production in the nearby Piceance Basin, the area west of the Grand Hogback where local oil and gas production is located. However, some have been announced and tested in the Denver-Julesburg Basin in Weld County. Preliminary information shows lower temperatures and efficiencies and presumably higher costs of power production compared to recently announced EGS production in Utah.
EGS power plants are distinguished from wind turbines and solar panels by their ability to generate 24/7 and provide electrical “inertia” to the grid. They can modulate or even stop and start, but near continuous operation is probably needed to recoup construction costs, which are projected to remain many times those of wind and solar.
Coincidentally, a contrasting approach to “firming” the output from low-cost but variable renewable power is under construction in Delta, Utah. There, a hydrogen production, storage and electricity generation system is under construction. The power plant is comparatively inexpensive, but the hydrogen fuel created from wind and solar power is expensive, so the generator will be dispatched intermittently for backup. When the plant is not generating power, grid inertia will be provided by a component called a synchronous condenser.
Hopefully we end up with plenty of both, as soon as possible.