From the Ground Up

Geothermal-heat exchange is a great concept, but inefficiency can quickly turn a heat sink into a money pit.
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From GreenSource
Tudor Van Hampton

Heat pumps in their simplest form are found in every typical American kitchen: the fridge. Refrigerators use a mechanical compressor and two coils that pump heat from the icebox and reject it to the outside air. That is why the back or bottom of the fridge is warm to the touch, while the contents inside remain nicely chilled.

 

Illustration by Alan Kikuchi

 

Home air conditioners are another example. Typical, "split" air conditioners feature an evaporator coil that rests inside the building while the condenser coil sits outside. With GHPs, you can imagine the "condenser" as a continuous loop of high-density polyethylene (HDPE) pipe, from 3/4 inch to 2 inches in diameter, buried underground or submerged in a body of water. A solution of water, methanol, or glycol provides heat transfer between the ground source and the heat pump. Unlike a home air conditioner, the flow can be reversed in winter to heat the building.

WATER SOURCES

A limnologist is a person who studies the properties of lakes and other inland bodies of water. This specialized field includes the movement of heat between the water's surface-exposed to solar heating during the day and evaporative cooling as wind passes over it at night-and the very depths, which are denser and cooler. As WSHPs have matured, designers have increasingly employed limnologists to create more efficient systems.

WSHPs comprise both open- and closed-loop systems. In an open-loop system, water is pumped from a lake, pond, aquifer, or other nearby source of water, exchanges heat inside the building via a heat pump, and is returned back to its source along with rejected heat. In a closed-loop system, HDPE tubing is submerged in the water and relies on conduction through the tubing to transfer heat.

Examples of open-loop systems are scattered across the campus of Harvard University, where installed WSHPs on seven buildings use "standing-column wells," about 6 inches in diameter and drilled as far as 1,500 feet. These wells-an average of three per building-use a pump that extracts water from the bottom of the well where the water is cooler, exchanges it with heat pumps in the building, and returns it to the top of the well, where the water is naturally warmer.

Sherman Health's runoff-retention pond doubles as a 15-acre water-source loop. The 650,000-square-foot facility expects to save $1 million a year in heating and cooling costs.

Photo © Tudor Van Hampton

 

Naturally, as these wells heat up, COP drops. Poorly designed and constructed wells inhibit heat dissipation, so owners may need to dump or "bleed" water to offset the heat spike. Wells that are not drilled down deep enough, or "short-drilled," also may lack the capacity to allow heat to escape quickly enough, warns Nathan Gauthier, assistant director of Harvard's sustainability office and a member of the U.S. Green Building Council's Energy and Atmosphere Technical Advisory Group.

Another environmental problem is the cross-linking of aquifers. Since drilling exposes soil strata, underground water that may not naturally come into contact with other sources is suddenly introduced. "If there is an aquifer at 100 feet and another one at 300 feet, a gas station may have contaminated the one at 100 feet," Gauthier explains. While geotechnical consultants can help identify the risks, the construction industry as a whole has not yet solved this potential problem.

 

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Originally published in GreenSource
Originally published in September 2009

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