Every day buildings use heating-and-cooling machines, or HVAC systems, to provide a comfortable environment to support living and working activities. Most conventional heatingand- cooling machines use fossil fuels or biofuels and electricity to provide heating and cooling for space conditioning and industrial processes. These machines can be inefficient, expensive and polluting.
To increase efficiency, a geoexchange machine, also known as a ground-source or geothermal heat pump, can be used in place of a polluting carbon-fired boiler. Geoexchange machines have been in use in the U.S. since the late 1940s, but it took more than 50 years for the technology to be recognized for its environmental benefits. A 1993 Washington, D.C.-based U.S. Environmental Protection Agency report shows that geoexchange systems are the most energy-efficient and environmentally friendly heating-and-cooling machines available for buildings and have the lowest life-cycle cost when compared to conventional machines used in most buildings. Although projects are seeing similar results today, they will vary by location and the utility power supply that applies to the specific project being considered.
THE BASICS / Conventional heating-and-cooling machines burn carbon-based fossil fuels or biofuels to add heat to interior spaces. Burning these fuels sends pollutants into the atmosphere, not to mention the cleanup required for the residues of combustion. When cooling is required, these heating-and-cooling machines typically use electric air conditioners or chillers and cooling towers to reject unwanted heat into the atmosphere.
Regardless of how efficient conventional heating-and-cooling machines are, a steady stream of pollutants will be produced whenever the heating system is on. Heat-pump machines, however, remove the burning of carbon fuels from the heating-and-cooling equation. They operate by transferring heat rather than creating it. Generally, it takes less heat to move energy than it does to make it. Like refrigerators and air conditioners, heat pumps rely on the vapor compression of a refrigerant to heat and cool depending on the direction of refrigerant flow. There are two basic types of heat-pump machines, air source and water source. Air-source heat pumps use the energy stored in the air to heat and cool. When cooling is required, the heat pump captures heat from the air inside a facility and transfers it to the outdoor air through a condensing unit. When heating is required, the process is reversed and heat is captured from outdoor air, compressed and released to the air inside the facility. However, in most climates, the ability of the heat pump to heat is limited when outdoor temperatures drop below 40 F (4 C). When this happens, inefficient electric resistance heating is used. This condition can reduce the energy efficiency of the heat pump substantially. The outdoor equipment used by these systems is exposed to weather, which further reduces their efficiency and increases the need for maintenance.
Water-source heat pumps generally are located in the space to be conditioned and are served by a building water loop that acts as a heat sink, or storage, when heat is removed from the space and a heat source when heat is supplied to the space. The temperature of this loop typically is maintained between 60 and 90 F (16 and 32 C). Any excess heat is removed from the water loop by a cooling tower through evaporation, and heat is added with a central carbon-fired boiler. Ventilation air is ducted directly to the heat pumps with outside air through the building envelope or from a central air handler. These systems generally are more energy efficient than air-source systems, but they still have the problem of requiring a carbon-fired boiler and consuming significant power and water by the chiller/cooling tower (2 gallons [8 L] per hour per ton of cooling). Geoexchange systems provide significant improvements over traditional water- and air-source heat-pump systems. Geoexchange machines use extended-range water-source heat-pump equipment (30 to 100 F [1.1 to 38 C]), and they eliminate the chiller/cooling tower and carbon-fired boiler, replacing them with the ground under and surrounding a facility. Planet Earth acts as a giant heat absorber and supplier, absorbing heat when cooling is necessary and supplying heatwhen it’s required. The geoexchange machine also can be designed to provide domestic or process hot water, supply heat to radiant floors and swimming pools, and make and melt ice. Visual and noise pollution also are reduced because these machines are virtually invisible and silent.
THE PROOF / An Oak Ridge, Tenn.-based Oak Ridge National Laboratory study found that a geoexchange machine is very good at reducing peak power usage for heating and cooling. A comprehensive analysis of monitored data at the U.S. Army’s Fort Polk, La.-based Joint Readiness Training Center, which features 4,003 housing units retrofitted with a 1 1/2 to 2 ton (1.4 to 1.8 metric ton) geoexchange system, demonstrated a 34 percent reduction in kilowatt hours and Btu energy use; 43 percent reduction in kilowatt peak demand; and complete elimination of heating with carbon-based fuels, which provided a reduction of 22,400 tons (20321 metric tons) of carbon emissions annually. The geoexchange system also achieved a 10 percent improvement in the load factor of the local electric utility. Housing is one thing, but does geoexchange work in nonresidential building types? The answer is “yes” to all building types that require heating and cooling wherever they are located, from Alaska to Florida, and from the highest mountains to the lowest valleys.
One of the largest geoexchange projects is the Galt House Hotel in Louisville, Ky. The hotel’s newer east tower is a mirror image of the original 600-room west tower. The west tower used a conventional boiler/chiller/cooling tower heating-and-cooling system. A comparison of the two towers reveals that the geoexchange machine provided annual savings of more than 5.5 million kWh, 1.1 million kW and $270,000 at 1984 utility rates.
THE FUTURE OF GEOEXCHANGE / Geoexchange use has been growing, but its growth is hovering around 1 percent of the U.S. HVAC market. Given the increased interest in energy efficiency, peak power reduction, green-building design and reduced carbon footprints, these clean, green geoexchange machines are positioned for take-off in the HVAC marketplace.
Assisting with geoexchange systems’ growth in the marketplace are a number of green-rating programs that encourage more holistic and environmentally friendlybuilding-design and construction practices. Most employ a scoring system and progressive ratings to reward strategies that improve indoor and outdoor environments. The Washington-based U.S. Green Building Council’s LEED rating system is being applied to growing numbers of building projects and in some cases is required as part of community project planning and building-code approval. A review of the LEED for New Construction program suggests that a geoexchange machine can be considered for credits in five categories with the potential of contributing to as many as 16 points.
Most of the environmental benefits of Earth-coupled geoexchange systems relate directly to the reduction of carbon-fuel burning to generate electricity for space, hot water and process heating. Jack DiEnna, executive director of the Washington-based Geothermal Heat Pump National & International Initiative, states: “Geoexchange systems represent less than 1 percent of all heating and cooling units in the U.S., but that small number of units is conserving more than 21 million barrels of crude oil and eliminating 6 million tons [5.4 million metric tons] of CO2 annually. Just think what we could do if we had 30 percent of the market. Try this on for size: 477 million barrels of crude, 36,564 megawatts of electric demand; 175 billion kWh of energy; 35 million tons [32 million metric tons] of carbon; and more than 130 million metric tons [143 million tons] of CO2 per year.”
In 1993, EPA indicated geoexchange systems have the lowest total CO2 emissions of all the major heating-and-cooling technologies for all regions of the U.S. How can you have a smaller footprint than that?
>> CRAIG W. HOELLWARTH is a principal of GREEN INQ, Elk Grove, Calif. He provides green design, project management and technology development services for the building industry. He can be reached at email@example.com or (916) 683-5151.
GEOTHERMAL HEAT PUMP CONSORTIUM, Washington, D.C., www.geoexchange.org GEOTHERMAL HEAT PUMP NATIONAL & INTERNATIONAL INITIATIVE, Washington, www.geo-nii.org INTERNATIONAL GROUND SOURCE HEAT PUMP ASSOCIATION, Stillwater, Okla., www.igshpa.okstate.edu U.S. DEPARTMENT OF ENERGY, Washington, www1.eere.energy.gov/geothermal U.S. ENERGY INFORMATION ADMINISTRATION, Washington, www.eia.doe.gov U.S. ENVIRONMENTAL PROTECTION AGENCY, Washington, www.epa.gov
WHY IS GEOEXCHANGE NOT SPECIFIED MORE OFTEN?
Currently, there are no code restrictions in the U.S. that ban the use of geoexchange systems. In fact, geoexchange often is encouraged. However, there can be local requirements for drilling the ground loops, especially where the drilling technology is not well understood.
Primary reasons most owners, architects, engineers and contractors do not consider geoexchange systems are that they are not familiar with them; their perception is that the systems tend to have a higher first cost, which is true for single-family residential and rooftop systems; they don’t consider cost savings from the systems’ low maintenance requirements; and geoexchange is a significant departure from the conventional air- and water-source systems that are well understood and supported by the heating and cooling industry.
Geoexchange systems also suffer from the electric utility and mechanical engineer’s focus on energy benefits instead of the additional holistic and qualitative benefits that go beyond just energy efficiency, such as eliminating use of fossil fuels.