There is a compelling elegance in using the earth’s relatively constant temperatures as a source and sink for heat. Indeed, ground-source heat pumps can be a highly efficient space conditioning option and, although their overall market share is very low, they are increasingly popular in the many dozens of model green homes and light commercial buildings cropping up around the country.
The U.S. Department of Energy and many electric utilities tout them as the best technology for green buildings. And schools, whether or not they consider themselves “green,” are adopting them in droves. Clearly, ground-source heat pumps (GSHPs) have some great environmental advantages over other heating and cooling systems, but do they make sense everywhere?
According to the U.S. Department of Energy’s Energy Information Administration (EIA), over 38,000 GSHPs were shipped in the U.S. in 1998, up nearly 50% from 26,600 in 1994. Due to the diffuse nature of the industry and discrepancies in how ground-source heat pumps are tallied, these numbers are rough at best, but they compare well with data from other sources. Other than the significant growth they reflect, these numbers are notable for how small a share of the overall market for heating and cooling equipment they represent. Compared with the 1.1 million air-source heat pumps and 4.2 million central air conditioners sold in 1997, GSHPs represent a mere 0.5% of the market for unitary cooling equipment. Four companies control about 80% of the U.S. GSHP market: WaterFurnace International, ClimateMaster, FHP Manufacturing, and The Trane Company.
This article seeks to explain the benefits of ground-source heat pumps, while also outlining some drawbacks that haven’t been widely explored.
Ultimately, we hope to provide a context that can help you decide if a GSHP is right for your next project.
Heat pumps in general are categorized according to the heat source.
Air-source heat pumps rely on outside air as the heat source during the heating season and the heat sink during the cooling season. Currently, approximately 90% of all heat pumps sold fall into this category. Ground-source heat pumps rely on the ground as the heat source/sink. A few meters underground, the earth maintains a very uniform temperature year-round. This ground temperature is warmer than the outside air during the winter and cooler than the outside air during the summer—thus improving heat pump performance (efficiency) year-round. Ground-source heat pumps deliver more heat (or cooling) per unit of electricity consumed than air-source heat pumps.
Since nearly all ground-source heat pumps rely on water or a water-antifreeze mixture to carry heat to or from the ground, the heat pumps themselves are actually a type of water-source heat pump—that is, they are designed to transfer heat to or from water to cool or heat the air. Water-source heat pumps are commonly used in commercial buildings to extract heat from, or dump heat into, circulating water that has been heated or chilled. GSHPs are typically a type of water-source heat pump that has been modified to work with cooler incoming water temperatures.
GSHPs are divided into two main types:
closed loop and
open loop. Closed-loop heat pumps rely on a ground loop of plastic or copper piping, through which water, a water-antifreeze mixture, or (in the case of
direct-exchange, DX, heat pumps) the refrigerant fluid, is circulated directly. Depending on local conditions, the ground loop may be installed either horizontally or vertically. Horizontal systems are less expensive to install, but they require more land and thus are usually confined to smaller installations. A variation on the horizontal loop is a coiled (“slinky”) loop, which allows a long length of pipe to fit in a relatively short, large trench. Instead of relying on the earth as the heat source/sink, a body of surface water (such as a lake) can be used in
surface-water heat pump systems.
With open-loop systems, the water is pumped out of the ground or water body, passed through the heat exchanger, then either returned to the source or discharged elsewhere. Using groundwater directly in this way can be more efficient than a closed-loop system, but it also introduces potential problems if the heat exchanger becomes clogged with sediment from the water.
Ground-source heat pumps are often called
geothermal heat pumps, but only deep-well, vertical-loop systems really draw on geothermal heat. With horizontal-loop heat pumps the ultimate energy source is solar, not geothermal, since it is solar energy that maintains the ground temperature within the upper few meters of the surface. The term “geothermal” can be confusing because it also refers to systems that extract high-temperature heat from the ground directly, without the use of pumps, as occurs naturally in geysers and hot springs.
Environmental Advantages of Ground-Source Heat Pumps
There are three primary environmental advantages to ground-source heat pumps: low energy usage; potential for renewable electricity use; and absence of on-site combustion. Each of these is discussed below.
A less significant but still notable advantage is the absence of an outdoor compressor, which reduces noise pollution and improves aesthetics—although having the compressor indoors can be a noise issue for occupants if it isn’t managed carefully.
Low energy use
A good GSHP in the right application is often the most energy-efficient technology for meeting a building’s cooling and heating loads. If one looks only at energy use on-site, GSHPs win hands down, because a heat pump can use a kilowatt-hour of electricity to create several times more heat than is contained in that energy, and an equivalent amount of cooling. Electricity used on-site, however, has usually been generated some distance away using other fuels, so it makes more sense to compare the
source energy for various systems, including energy used to generate and transmit the electricity. In most of the U.S., roughly one-third of the source energy ends up as electricity available at the site, but even after factoring this loss of two-thirds of the energy, GSHPs come away as the most efficient solution.
Looking at simple electric resistance heat from this perspective, even though nearly 100% efficient in terms of site energy, it usually results in significantly more greenhouse gas emissions per unit of delivered heat than direct combustion of natural gas or oil on-site. With air-source heat pumps, the situation is a little better, but still generally worse, than direct combustion of natural gas. Ground-source heat pumps provide the only way of using fossil-fuel-generated electricity to exceed the efficiency of direct natural gas combustion and reduce greenhouse gas emissions per unit of delivered heat.
As with other heating and cooling systems, the overall efficiency of the system depends not only on the heating or cooling source, but also on how the conditioned air is delivered. Ducts that leak, or poorly insulated ducts running through unconditioned spaces, can waste a large fraction of the energy, so good duct design and careful installation are essential to retain efficiency.
It is often claimed that the heat that GSHPs draw from the earth (or the earth’s ability to receive unwanted heat) is a form of renewable energy. In a sense, this is true, as heat deposited in the ground eventually dissipates out to the surface, and parts of the ground that have been cooled by a heat pump are eventually reheated by solar energy (near the surface) and heat from the earth’s core (below about 50 feet—15 m—from the surface). One could argue that heat from the earth’s core is not, strictly speaking, renewable, but we’ll leave that debate to others.
Even if the earth as a heat source or sink is a form of renewable energy, electricity is still needed to power the compressors—and, to a lesser extent, the pumps and fans—of a ground-source heat pump. Drawn from a typical electric utility, this electricity is likely to be generated primarily from fossil fuels, nuclear power, or, in the Pacific Northwest, large-scale hydropower. It is possible, however, to power a heat pump with electricity generated from renewable sources. Due to the large amount of storage needed, an off-the-grid system would be expensive, but a photovoltaic or wind-power system using the grid as storage/backup is more feasible. In some states it is also possible to purchase green power directly from the grid, with a contract that ensures that new renewable-energy electricity generation is added to the grid in proportion to the electricity purchased through such contracts. Running on either on-site or utility-produced green power, ground-source heat pumps are truly a low-impact heating and cooling system.
Elimination of combustion
A third environmental benefit of heat pumps, especially in residences, is that they can heat a home efficiently without any type of combustion. Gases released from burning fuel in furnaces or boilers can be a significant health hazard in buildings. The primary concern is for carbon monoxide poisoning, which is blamed for the death of some 300 Americans each year. Combustion heating equipment can be used safely, but only if combustion gases are isolated from the indoor air, or if pressure relationships in the building are carefully managed to prevent flue-gas spillage indoors.
In terms of global impact, the main drawback to GSHPs is the refrigerant. GSHPs currently use R-22, a hydrochlorofluorocarbon or HCFC, as their refrigerant. HCFCs are considered transitional refrigerants because they do damage the stratospheric ozone level, albeit much less than CFCs. The likely replacements to HCFCs in the U.S. are hydrofluorocarbons or HFCs. Lacking a chlorine atom, these do not affect the ozone layer. They are, however, significant greenhouse gases. The refrigerants only represent a problem if they escape into the atmosphere, so proper maintenance of the heat pumps is critical. To their credit, GSHPs use much less refrigerant than air-source heat pumps—a residential units uses about 3 pounds (1.4 kg) as opposed to 6 or 7 pounds (2.7 to 3.2 kg) for an air-source unit of the same capacity.
Perhaps the more significant environmental concerns are very local, however. Ground-source heat pumps require a connection to the earth (or groundwater), and in making this connection the immediate environment can be compromised. The environmental issues vary with the different types of ground loop—the key issues are summarized here.
Site and water issues
For horizontal loops the main issue is disturbance of the soil and the plant and animal systems it supports. This disturbance may involve a series of narrow trenches spread out over a relatively large area, or the excavation of a smaller area in which coiled tubes are installed. In either case, the surface can be replanted afterwards (though not with trees or other deep-rooted plants).
The disruption is similar to other aspects of preparing a site for construction, or the installation of a leach-field for a septic system. As long as adequate measures are taken to prevent erosion during and after the installation, the impacts of this work are very localized. Few, if any, regulations address issues specific to the installation of horizontal-loop systems.
Vertical bores used in large-scale commercial installations also involve disruption of the surface by the well-drilling rigs. The surface area affected is much smaller in relation to the size of the heat pump system, however, than with horizontal loops.
A more significant issue with vertical bores is the potential for groundwater contamination, either from the fluid in the pipes (in case of a leak), or from contaminants at the surface or in other bodies of groundwater. In most regions of the U.S., the fluid in a closed-loop ground-source system is a mixture of water and antifreeze. Some types of antifreeze are more hazardous than others, and many jurisdictions now require the use of those that will not pose a serious environmental threat in the case of an underground leak.
The concern regarding transfer of pollutants from the surface or from another body of groundwater is that pollutants may use the well holes as conduits into previously sealed underground aquifers, including some that supply drinking water. Typically, the drilled hole might be 4” to 6” (100 to 150 mm) in diameter, in which two 1” (25 mm) plastic pipes are installed to carry the heat-transfer fluid. Pollutants might originate from the surface in the form of lawn chemicals, pesticides, or agricultural runoff, or they might travel from a polluted (perched) aquifer into another—previously uncontaminated—one.
States regulate both water well construction and vertical borehole installation to prevent groundwater contamination. The National Ground Water Association has prepared guidelines for state regulators. The key for vertical boreholes is using a very-low-permeability material to seal the borehole completely. Depending on local conditions, this may require sealing the entire borehole (to prevent interaquifer contamination) or just the top 10 to 20 feet (3 to 6 m) to prevent downwash of contaminants. Two classes of seal materials are commonly employed. The first is based on bentonite, an engineered material made from a natural clay. The clay may have additives such as silica sand to improve its thermal conductivity. The other class is based on Portland cement, which may be augmented with silica sand. These rules are still evolving—in California it was just recently determined that horizontal loops are exempt from well-drilling regulations only if they go no deeper than 20 feet (6 m), according to Carl Hauge of the California Department of Water Resources. This ruling did not please installers of coiled loop (“slinky”) systems, because they were digging holes 30 feet (10 m) deep and 3 feet (1 m) wide for coil placement.
Open-loop GSHPs introduce another set of water-related environmental issues. These systems pump water out of the ground, use it as a heat source or sink for the heat pump, and then either return it to the ground or release it at the surface. Maximum system efficiency is achieved with water supplies of 1 to 3 gallons per minute (0.06 to 0.19 l/sec), depending on the depth of the water in the well, heat pump efficiency, and other system variables. With either approach, it is important to design and install a system that prevents any contact between the groundwater and air. Such contact can lead to chemical reactions or bacterial growth, which can clog heat exchangers and, in the case of reinjection systems, possibly contaminate groundwater. The same Underground Injection Control program of the EPA covering many industrial wells that inject toxics into the ground also covers open-loop systems, even though there is no contact between the groundwater and the system, only a change in its temperature.
Even more problematic are GSHPs that do not return the water to the aquifer. Known in the industry as “pump and dump” systems, they are illegal in some states but still encouraged in others due to concerns about reinjecting the water. Bruce O’Conner is principal geologist for the Georgia Geological Survey in the Environmental Protection Division of the Department of Natural Resources. “Historically we have done everything we can to discourage reinjection,” O’Conner reports, adding, “We try to give people alternative suggestions, including to discharge to the surface or use a closed loop.”
Under current Georgia law, only large industrial or agricultural users, who withdraw over 100,000 gallons of water per day (375 m
3/day) are required to get a permit, so the state doesn’t have any record of how many surface-discharge open-loop heat pumps exist. Given the State’s current drought conditions, this policy seems especially short-sighted, a situation that O’Conner acknowledges: “It’s a perverse logic, of course, because we do have a depleted groundwater situation, but it will take somebody with the right information and connections to get it changed.” Not only do these pump-and-dump systems draw-down valuable underground aquifers, but they can also cause problems at the surface where the water is discharged. In one unfortunate instance in Wisconsin, a residential-scale system dumping water into a field killed several mature oak trees. Distributing this excess water via irrigation systems can also cause buildup of excess salt and other minerals in the soil.
Given the fact that ground-source heat pumps change soil and groundwater temperatures, it is remarkable how little attention the industry and regulators are paying to this issue. Horizontal-loop systems routinely cause the soil immediately around the pipes to freeze in winter in climates and at depths where such freezing would not happen otherwise. They also make the soil warmer in summer, which has the additional effect of driving moisture out. Arrays of wells used for commercial-scale heat-pump systems often raise the temperatures of soil and groundwater deep below the surface because they operate primarily in cooling mode.
System designers and installers are sensitive to these temperature changes to the extent that they affect the performance and efficiency of the heat pump system. “A change of up to 10°F (5.5°C) is generally considered okay in terms of system efficiency,” reports Dr. Lynn Stiles, Dean of Natural Sciences and Mathematics at Stockton College in Pomona, New Jersey, site of the world’s largest heat pump well field. This change refers to average background temperatures in the soil. “Over the course of a few hours or a day, temperature fluctuations will be larger,” Stiles notes. If cooling and heating loads on a large system are roughly in balance, such temperature changes will balance out on an annual basis, and they may even increase system efficiency by using the ground for seasonal thermal storage.
As thermal pollution goes, ground-source heat pumps affect ground temperatures less radically than other sources of heat into the soil or groundwater, including industrial sources and cooling water from power plants. Experiments with large-scale, seasonal thermal storage using vertical wells have had serious consequences, according to Stiles. One such project at the University of Minnesota tried storing waste heat from a generator underground, only to find that the wells were fouled by blooms of microbes stimulated by the high temperatures.
Ground-source heat pumps have the potential to become much more widespread than these higher-temperature applications, however, so it is surprising that there has been little effort to study effects of smaller temperature changes. Stiles directed the only large-scale effort of this type in the U.S., a study of temperature changes and the associated ecological effects in and around a huge field of boreholes for heat pumps that cool and heat much of the Stockton College campus. This closed-loop borehole field includes 400 borehole heat exchangers, each 425 feet (130 m) deep, spread over 3.5 acres (1.4 ha). The wells penetrate three separate aquifers.
Stiles and his team closely monitored temperatures over several years both in and near the field, and analyzed microbial populations in the groundwater both upstream and downstream of the field. Because the heat pumps are primarily used for cooling, the average temperature in the field initially rose about 1°F (0.6°C) each year, eventually stabilizing at about 5°F to 7°F (-15°C to -14°C) above normal. Test wells positioned in and downstream of the field revealed a notable change in the bacteria populations that corresponded with the change in temperature. However, when the water temperatures returned to normal, the bacteria populations recovered as well, so the results of this research are generally regarded in the industry as inconclusive.
EBN was unable to uncover any research on ecological effects of soil temperature changes due to horizontal-loop systems. Responding to this issue, Stiles notes that, based on temperature measurements he has done, the effect is certainly less severe than that of an asphalt parking lot. According to Stiles, a blacktop surface collecting solar radiation during the summer adds much more heat to the soil than a cooling loop adds. Stiles even measured significant temperature rise 30 to 40 feet (9 to 12 m)away from a parking lot—a result, he believes, of rainwater runoff heated by the asphalt surface. From these measurements, Stiles concludes that “if you are willing to build a parking lot 30 feet (10 m) from a sensitive stream, then you should also be willing to build a well-field, because the well-field will have less impact.” While this information says little about the actual impact that temperature changes from GSHP might have on the soil, it does put the concern into perspective.
Due to the cost of installing ground loops, residential ground-source heat pumps almost always have a higher first-cost than more conventional heating and cooling systems. In many areas there have been, at least until recently, rebates available from electric utility companies to help pay this premium. The advent of electricity deregulation is affecting the availability of such rebates in some places, though it is possible that alternative sources of subsidies will be found.
The payback on this investment depends both on the amount of the premium and on the operating cost savings that result. Calculating these savings can be tricky because the GSHP must be compared to options that provide both heating and cooling, and the methods for describing the efficiency of these systems are not consistent. In houses, in particular, spending such a premium on energy upgrades to the building may make it possible to downsize both the heating and cooling requirements to the point where it makes little difference how the remaining loads are met. Done well, such an investment in up-front conservation is often a better use of resources than a similar investment in efficient equipment, such as GSHPs.
The extent of the first-cost premium varies greatly from region to region, due to market conditions and geologic conditions. The cost of ground loops tends to be lowest in areas with an established market for GSHPs and a number of experienced installers who can accurately predict their costs. Soil conditions are a factor for two reasons. First, soil characteristics determine how effectively heat will be conducted to or from the heat pump loops, and therefore how large those loops must be. Moist, sandy soils, for example, conduct heat relatively well and therefore require smaller coolant loops or well-fields than for dry or clay soils. Second, soil conditions determine how difficult and expensive it is to dig trenches or drill wells to install the loops. This latter factor is not as important as the experience of the installers, however. Steve Kavanaugh, Professor of Mechanical Engineering at the University of Alabama, notes that in and around Austin, Texas the drilling conditions are poor, but wells are routinely installed for under $5 per foot ($16/m), while in his region of southern Alabama conditions are much better, yet drillers charge as much as $12 per foot ($40/m). “In contrast with residential systems, in regions with experienced designers and competitive drillers, the first-cost of a well-designed commercial-scale ground-source system can be less than that of a competing system with the same amenities,” according to Harvey Sachs, former Technical Director of the Geothermal Heat Pump Consortium.
The operating cost savings depend on many factors, including the cost of electricity and alternative heating fuels, and how conducive the climate and underground conditions are to optimal performance of the GSHP. Some GSHPs also include
desuperheater coils, which basically provide free hot water when the heat pump is operating in cooling mode. Advocates often note that maintenance costs for GSHPs are lower than for air conditioners or chillers because the outdoor coils or cooling towers needed for the latter systems are replaced by the maintenance-free ground loops.
Except in the Deep South, heating loads tend to overwhelm cooling loads in single-family homes, so the potential savings from a GSHP are closely tied to the cost of heating fuel. In areas with inexpensive natural gas, that makes GSHPs a tough sell, according to Sachs: “Geothermal heat pumps are unlikely to be strongly cost-competitive with natural gas in the single-family market,” he says. Mark Kelley of Building Science Engineering in Harvard, Massachusetts has designed GSHP systems for multifamily and small office buildings. His conclusion is that GSHPs only make sense for such heating-load dominated building if the cost per kWh of electricity is less than one-tenth the cost per therm of natural gas—for example, if natural gas costs 80¢ per therm, electricity would have to cost less than 8¢ per kWh.
Larger buildings tend to have more significant cooling loads due to the internal heat gains from lighting and equipment, along with their low surface-to-volume ratio. In a northern climate, small- to medium-size commercial buildings might have a better balance of heating and cooling loads, resulting in a more cost-effective GSHP installation.
Although GSHPs are relatively trouble-free once installed, there are many variables that, if not considered properly, can result in an inefficient system. They demand a lot of pumping, for example, so if the pumps are not efficient or well controlled, a lot of energy may be wasted. Also, in residential systems, the high cost of ground-coupling and a concern about getting enough dehumidification in cooling mode sometimes lead to an undersized ground loop. As with air-source heat pumps, in residential systems the additional heating capacity is supplied with electric resistance coils. This backup is even less efficient than simple electric baseboard heat because it heats air that is delivered via ducts rather than heating the space directly. As a result, a system that is significantly undersized for its heating load can be a major liability.
There are many different types of GSHPs and even more variations among all the possible installations, so it is hard to draw any blanket conclusions about their suitability. Their increased efficiency over standard air conditioning is a huge asset, especially in applications with roughly matched cooling and heating loads. The environmental concerns with refrigerants and groundwater contamination suggest that careful, experienced installers are essential. And the unknown effects of changing ground temperatures, while not a reason to avoid GSHPs, certainly cry out for good research. To be considered against these unknowns is the fact that for many buildings the right system, carefully selected, designed, and installed, will provide the benefits of reduced energy consumption.
As with any heating or cooling technology, the best approach is to reduce the demand as much as possible. The relatively high first-cost of GSHP systems provides a good incentive to do this, even in buildings where it is not possible to reduce demand so much that a central mechanical system is no longer needed. The remaining heating and cooling loads should be met with technologies that are the most cost-effective and lowest in environmental impacts. Figuring out which approaches perform best on these scales is not always easy and will invariably include some intangible “fudge-factors,” such as the impact of GSHPs on the site as opposed to the health risks from combustion equipment. But it is an exercise worth doing for the benefit of your clients—and the planet.