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Ground Source Heat Pumps Dig In

Home Energy Magazine Online September/October 1999

Ground Source Heat Pumps Dig In

by Sandy Cataldo

Sandy Cataldo is warming up to ground source heat pumps as a freelance writer in Lakeville, Massachusetts.

The United States government is pushing ground source heat pump technology in a big way. Even the military has caught on. Within the last year, more than 5,000 air force and army residential housing units have been retrofitted with ground source heat pumps for their heating, cooling, and hot-water uses.

About 680 miles of 1-in plastic tubing was used to make the systems at 4,003 homes at the Fort Polk army base.

When burying the plastic tubing (black), the contractors are careful to avoid other buried pipes, such as sewer lines and water supply pipes. This short section of tubing runs horizontally between the borefield and the heat pump at the house.

Where the tubing comes out of the ground, cement is used to seal the borehole and prevent surface water runoff and its contaminants from flowing down into the water table.

Ground source heat pumps (sometimes called geothermal heat pumps, Geo-Exchange Systems, or GHPs) most often exchange heat with the ground by means of a ground heat exchanger. The heat exchanger consists mainly of long pipes, either drilled vertically into the ground or buried in trenches, that use the tempering effect of the earth to heat cold water or cool warm water. When the system operates, a pump circulates the water through the heat exchanger and the heat pump, and the heat pump moves energy between the conditioned space and the water. Because it relies on the earth, not on outdoor air, as the heating or cooling source, it is substantially cheaper to run than a conventional heating and air conditioning system. For example, based on results to date, the Department of Energy (DOE) estimates a savings of as much as 20%-40% of the energy consumption at each site that is retrofitted.

Military Hits the Dirt

The family housing facilities at the Fort Polk, Louisiana, army base shelter more than 12,000 military personnel and their dependents. Here, all 4,003 housing units were retrofitted with ground source heat pumps.

Eighty percent of these homes previously had air source heat pumps, while the remainder had gas-fired furnaces with central air conditioning systems. Because Louisiana has a cooling-dominated climate, most of the energy requirement of the homes is for cooling and hot water, rather than for heating. Each housing unit required a 1.5- to 2.5-ton heat pump. Even though heating is required for only two to three months out of the year, residents appreciate comfort during that period.

In heating mode, the ground source heat pumps deliver air at about 105ºF. This is about 10ºF to 15ºF warmer than air delivered by air source heat pumps, and warm enough to eliminate complaints about the system blowing cold air. Most of the newly installed ground source heat pumps included desuperheaters that use recovered heat drawn from the refrigerant circuit--originating from the ground (in heating mode) or from the conditioned space (in cooling mode)--to make hot water. Separately, some lighting upgrades were also included in the retrofit.

DOE officials say the apparent energy savings for the overall program in a typical year were determined through examining occupant bills to be an impressive 33% of the preretrofit electric consumption, or 26 million kWh per year. A simulation model calibrated to measurements showed that 66% of the savings could be attributed to the new heat pumps, 29% to the lighting retrofit, and 5% to the installation of low-flow showerheads. Because they replaced old, failing units, the heat pumps alone accounted for about 17.2 million kWh per year of electricity savings (and, of course, 100% of the gas savings, amounting to 260,000 therms per year). Also impressive, the summer peak demand of the family housing was reduced by 43%, dropping 7.5 megawatts (MW) and saving on high demand charges.

The Lowdown on Ground Source

Ground source heat pumps are superior to conventional heating and cooling systems because, with ground source units, air needs to be moved on only one side of the unit. On the other side, it moves water--and it takes less electricity to move 2.5 gpm/ton of water (or anti freeze, in northern climates) than it would take to move the 900 CFM/ton of air required in air source heat pumps across the outdoor unit.

Unlike air source heat pumps, ground source systems do not need to defrost. Whereas air source units need to engage backup electric-resistance heat at low outdoor air temperatures in all locations, ground source heat pumps require backup only in extreme heating-dominated climates. Because there is no outdoor unit, there are no defrost controls to maintain and no performance deterioration from corrosion, vandalism, or clogging with debris. Furthermore, since ground temperatures remain relatively constant, ground source heat pumps don't have to contend with the capacity-limiting and efficiency-zapping operating conditions caused by extreme outdoor temperatures.

They also generally require less babysitting and fewer repairs. The heat pump is a packaged water-to-air unit that is factory charged with refrigerant, avoiding the problems associated with field-charged split systems. Also, the underground piping is high-density polyethylene, which is usually guaranteed for 50 years. ASHRAE gives the median service life of a water source heat pump as 4 years longer than that of an air source pump. Such longevity factors tend to lower the charges for maintenance contracts for the equipment by as much as 25%, as was the case at Fort Polk.

The Deep and Dirty Details

Patrick Hughes, of DOE's Oak Ridge National Laboratory, led DOE's evaluation of the Fort Polk project. He explained how the drilling was done at Fort Polk. More than 8,000 boreholes were drilled to a depth of about 200 ft, and about 680 miles of 1-inch polyethylene pipe was used. According to Hughes, major movement of earth is not required with vertical borehole ground heat exchanger systems such as those installed at Fort Polk. The drilling operation is relatively nonintrusive. Noise can be an issue during the brief period when the drillers are in the neighborhood, but it was managed in this case by scheduling the work for a time when most occupants had left for their daily routines. The drillers worked through the neighborhoods drilling bores, inserting 1-inch U tubes in the bores, and pumping in the bore backfill bottom to top. Generally there were two bores per apartment, each with two pipes that were stubbed to the surface when the drillers were done.

Separate crews came through to perform the indoor heat pump work and connect the loops to the indoor heat pump. Connecting the loops involves the use of chain trenchers to slice trenches about 6 inches wide for the header pipes, from where they connect to the vertical U tubes to where they enter the buildings. Since the bores are spaced about 20 ft apart and are also about 20 ft from the foundation, modest trench lengths are involved, and trenches are 3 to 4 ft deep. After the pipes are installed, cuttings from the trench are pushed back in and sprayed with high pressure water hoses, in order to prevent voids and reduce pipe settling.

The heat pump indoor work and loop tie-ins were completed in one day, and no temporary occupant relocation was required, although, for security reasons, it was necessary for an occupant to be present during the indoor work. Surveying and design prior to construction required apartment access for several hours for a sample of each apartment type.

Digging for Dollars

During the life of the 20-year project, the folks at Fort Polk expect to keep about $742,500 in savings on energy and maintenance costs, net of payments to the energy service company (ESCO) that did the work. Because the work was done under a performance contract with the ESCO, the army paid nothing up front, got all new equipment, delegated maintenance to the ESCO, and improved the comfort of their residents. The costs (about $4,700 per housing unit) will be paid as a percentage of the savings over the life of the contract. "The beauty of it all is that the onus to save Btu is on the contractor," says Jim Kelley, manager of engineering and planning at the Fort Polk Directorate of Public Works. "I'm a happy camper, knowing that I have a single entity that I am going to deal with over the next 20 years, an entity with a profit motivation for saving energy and maintenance dollars."

The Department of Defense found similarly high savings at another project, a family housing retrofit of ground source heat pumps at the Little Rock, Arkansas, air force base. Here, in the first month of operation, energy savings for the housing measured 26%-27%.

Federal Mandate for Savings

DOE's Federal Energy Management Program (FEMP) has made ground source heat pumps a part of its Super Energy Savings Performance Contract (Super ESPC) program, which is under the administrative jurisdiction of the Office of Energy Efficiency and Renewable Energy. This gives the technology an important role in meeting President Clinton's June 3, 1999, Executive Order 13123 to reduce energy use and to significantly increase the use of renewable energy in federal facilities.

All totaled, under the Super ESPC program, DOE sees a potential for $10 billion in energy savings over the life of all the projects, once they are completed. The technology-specific, performance-based GHP Super ESPC program represents an important slice of that pie, because it enables federal agencies to privately finance $500 million worth of projects without waiting for capital appropriations.

Contact info: U.S. Department of Energy
Office of Geothermal Technology
Mailstop EE12
1000 Independence Ave. SW
Washington, DC 20585-0121
Tel:(202)586-5340
Web site: www.eren.doe.gov/geothermal

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