A geothermal heat pump system is a central heating and/or air conditioning system that actively pumps heat to or from the shallow ground. It uses the earth as either a source of heat in the winter, or as a coolant in the summer. This design takes advantage of moderate temperatures in the shallow ground to boost efficiency and reduce operational costs. It may be combined with solar heating to form a geosolar system with even greater efficiency.
Geothermal heat pumps are also known by a variety of other names, including geoexchange, earth-coupled, earth energy, ground-source or water-source heat pump. The engineering and scientific community tend to prefer the terms “geoexchange” or “ground-source heat pumps” because very little of the heat originates from true geological sources. Instead, these pumps draw energy from shallow ground heated by the sun in the summer. Genuine geothermal energy from the core of Earth is available only in places where volcanic activity comes close to the surface, and can usually be extracted without the help of a heat pump.
Like a refrigerator or air conditioner, these systems use a heat pump to force the transfer of heat. Heat pumps can capture heat from a cool area and transfer it to a warm area, against the natural direction of flow, or they can enhance the natural flow of heat from a warm area to a cool one.
The core of the heat pump is a loop of refrigerant pumped through a vapor-compression refrigeration cycle that moves heat. Heat pumps are always more efficient than pure electric heating, even when extracting heat from air.
But unlike an air-source heat pump, which extracts or exhausts heat to or from the outside air, a ground-source heat pump exchanges heat with the ground. This is much more efficient because underground temperatures are relatively stable through the year. Seasonal variations drop off with depth and disappear below 10 m due to thermal inertia. Like a cave, the shallow ground temperature is warmer than the air above during the winter and cooler than the air in the summer.
A ground-source heat pump extracts that ground heat in the winter (heating) and exhausts heat back into the ground in the summer (cooling).
The system cost are much higher than conventional systems, but the difference is usually returned in energy savings in 3–10 years. System life is estimated at 25 years for the inside components and 50+ years for the ground loop. As of 2004, there are over a million units installed worldwide providing 12 GW of thermal capacity, with an annual growth rate of 10%. If deployed on a large scale, this technology may help alleviate energy costs and global warming.
Seasonal thermal storage
The efficiency of ground-source heat pumps can be improved by using seasonal thermal storage. If heat loss from the ground source is sufficiently low, the heat pumped out of the building in the summer can be retrieved in the winter. Heat storage efficiency increases with scale, so this advantage usually only applies to commercial or district heating systems. Geosolar combisystems further augment this efficiency by collecting extra solar energy during the summer (more than is needed for air conditioning) and concentrating it in the store.
Such a system has been used to heat and cool a greenhouse using an aquifer for thermal storage. In summer, the greenhouse is cooled with cold ground water. This heats the water in the aquifer which can become a warm source for heating in winter. The combination of cold and heat storage with heat pumps can be combined with water/humidity regulation. These principles are used to provide renewable heat and renewable cooling to all kinds of buildings.
The heat pump was invented by Lord Kelvin in 1852. After experimenting with a freezer, Robert C. Webber built the first direct exchange ground-source heat pump in the late 1940s. The first successful commercial project was installed in the Commonwealth Building (Portland, Oregon) in 1946, and has been designated a National Historic Mechanical Engineering Landmark by ASME. The technology became popular in Sweden in the 1970’s, and has been growing slowly in worldwide acceptance since then. Open loop systems dominated the market until the development of polybutylene pipe in 1979 made closed loop systems economically viable. As of 2004, there are over a million units installed worldwide providing 12 GW of thermal capacity. Each year, about 80,000 units are installed in the USA and 27,000 in Sweden.
The net thermal efficiency of a heat pump should take into account the efficiency of electricity generation and transmission, typically about 40%. Since a heat pump takes heat from the ground, the total heat energy output to the building is greater than the electricity input. This results in thermal efficiencies greater than 100%, up to around 150%.
Net thermal efficiency tends to be confusing to consumers, so heat pump performance is generally expressed as the ratio of heat output to electricity input. An allowance is included for electricity used by the fluid pumps. Cooling performance is typically expressed in units of BTU/hr/Watt as the Energy Efficiency Ratio, (EER) while heating performance is typically reduced to dimensionless units as the Coefficient of Performance. (COP) The conversion factor is 3.41 BTU/hr/Watt. Both of these measures will vary depending on the temperature difference between the ground source and the building, which can vary greatly between installations and over the course of the year.
For the sake of comparing ground source heat pump appliances to each other, independent of installation variations, a few standard test conditions have been established by the American Refrigerant Institute (ARI) and the International Standards Organization. Standard ARI 330 ratings are intended for closed loop ground-source heat pumps, and assumes secondary loop water temperatures of 77°F for air conditioning and 32°F for heating. These temperatures are typical of installations in the northern USA. Standard ARI 325 ratings are intended for open loop ground-source heat pumps, and include two sets of ratings for groundwater temperatures of 50°F and 70°F. ARI 325 budgets more electricity for water pumping than ARI 330. Neither of these standards attempt to account for seasonal variations. Standard ARI 870 ratings are intended for direct exchange ground-source heat pumps. ISO 13256-1 ratings are intended for open loop and closed loop systems.
Residential ground source heat pumps on the market today have COP’s ranging from 2.4 to 3.8 and EER’s ranging from 10.6 to 17.5. To qualify for an Energy Star label, heat pumps must meet certain minimum COP and EER ratings which depend on the ground heat exchanger type. For closed loop systems, the ISO 13256-1 heating COP must be 3.3 or greater and the cooling EER must be 14.1 or greater.
Undisturbed earth below the frost line remains at a relatively constant temperature year round. This temperature equates roughly to the average annual air-temperature of the chosen location. It is usually 7-12°C (45-54°F) at a depth of six meters in locations where heating is needed in winter.
Ground-source heat pumps rely on this near constant temperature as a base temperature that is raised or lowered minimally to create a desirable indoor temperature. Because this temperature remains more constant than the air temperature throughout the seasons, geothermal heat pumps perform with far greater efficiency and are stressed less during extreme air temperatures than fueled or electric conventional air conditioners and furnaces. A particular advantage is that they can use electricity to heat spaces and water much more efficiently than an electric heater.
Seasonal variations are much more important for air-source heat pumps, and ARI 210 and 240 define Seasonal Energy Efficiency Ratios (SEER) and Heating Seasonal Performance Factors (HSPF) to take into seasonal variations into account for these units. These numbers are normally not applicable and should not be compared to ground-source heat pump ratings. However, Natural Resources Canada has adapted this approach to calculate typical seasonally adjusted HSPF’s for ground-source heat pumps in Canada. The NRC HSPF’s ranged from 8.7 to 12.8 BTU/hr/Watt (2.6 to 3.8 in nondimensional factors, or 255% to 375% seasonal average electricity utilization efficiency) for the most populated regions of Canada. When combined with the thermal efficiency of electricity, this corresponds to net average thermal efficiencies of 100% to 150%.
The U.S. Environmental Protection Agency (EPA) has called ground-source heat pumps the most energy-efficient, environmentally clean, and cost-effective space conditioning systems available. Heat pumps offer significant emission reductions potential, particularly where they are used for both heating and cooling and where the electricity is produced from renewable resources.
Ground-source heat pumps have unsurpassed thermal efficiencies and produce zero emissions locally, but their electricity supply almost always includes components with high greenhouse gas emissions. Their environmental impact therfore depends on the characteristics of the electricity supply.
Ground-source heat pumps always produce less greenhouse gases than air conditioners, oil furnaces, and electric heating, but natural gas furnaces may be competitive depending on the greenhouse gas intensity of the local electricity supply. In countries like Canada and Russia with low emitting electricity infrastructure, a residential heat pump may save 5 tons of carbon dioxide per year relative to an oil furnace, or about as much taking an average passenger car off the road. But in countries like China or USA that are highly reliant on coal for electricity production, a heat pump may result in 1 or 2 tons more carbon dioxide emissions than a natural gas furnace.
The fluids used in closed loops may be designed to be biodegradable and non-toxic, but the refrigerant used in the heat pump cabinet and in direct exchange loops is almost universally chlorodifluoromethane, which is an ozone depleting substance. Although harmless while contained, leaks and improper end-of-life disposal contribute to enlarging the ozone hole.
Open loop systems that draw water from a well and drain to the surface may contribute to aquifer depletion, water shortages, and subsidence of the soil. There are also some concerns about groundwater contamination.
Ground-source heat pump technology, like building orientation, is a natural building technique (bioclimatic building).
Ground-source heat pumps are characterised by high capital costs and low operational costs compared to other HVAC systems. Their overall economic benefit depends primarily on the relative costs of electricity and fuels, which are highly variable over time and across the world. Based on recent prices, ground-source heat pumps currently have lower operational costs than any other conventional heating source almost everywhere in the world.
Natural gas is the only fuel with competitive operational costs, and only in a handful of countries where it is exceptionally cheap, or where electricity is exceptionally expensive. In general, a homeowner may save anywhere from 20% to 60% annually on utilities by switching from an ordinary system to a ground-source system.
Captical costs and system lifespan have received much less study, and the return on investment is highly variable. One study found the total installed cost for a system with 10kW (3 ton) thermal capacity for a detached rural residence in the USA averaged $8000–$9000 in 1995 US dollars. A more recent study found an average cost of $14,000 in 2008 US dollars for the same size system in Indiana. Prices over $20,000 are quoted in Canada.
The rapid escalation in system price has been accompanied by rapid improvements in efficiency and reliability. Capital costs are known to benefit from economies of scale, particularly for open loop systems, so they are more cost-effective for larger commercial buildings and harsher climates. The initial cost can be two to five times that of a conventional heating system in most residential applications, new construction or existing. In retrofits, the cost of installation is affected by the size of living area, the home’s age, insulation characteristics, the geology of the area, and location of the home/property. Proper duct system design and mechanical air exchange should be considered in the initial system cost.
Capital costs may be offset by substantial subsidies from many governments, for example totalling over $7000 in Ontario for residential systems installed in the 2009 fiscal year. Some electric companies will offer special rates to customers who install a ground-source heat pump for heating/cooling their building.
This is due to the fact that electrical plants have the largest loads during summer months and much of their capacity sits idle during winter months. This allows the electric company to use more of their facility during the winter months and sell more electricity. It also allows them to reduce peak usage during the summer (due to the increased efficiency of heat pumps), thereby avoiding costly construction of new power plants.
For the same reasons, other utility companies have started to pay for the installation of ground-source heat pumps at customer residences. They lease the systems to their customers for a monthly fee, at a net overall savings to the customer.
The life span of the system is longer than conventional heating and cooling systems. Good data on system lifespan is not yet available because the technology is too recent, but many early systems are still operational today after 25–30 years with routine maintenance. Most loop fields are warrantied for 25 to 50 years and are expected to last at least 50 to 200 years.
Ground-source heat pumps use electricity for heating the house. The higher investment above conventional oil or electric systems may be returned in energy savings in 2–10 years for residential systems in the USA.
If compared to compared to natural gas systems, the payback period can be much longer. The payback period for larger commercial systems in the USA is 1-5 years, even when compared to natural gas.
Ground-source heat pumps are recognized as one of the most efficient heating and cooling systems on the market. They are often the second-most cost effective solution in extreme climates, (after co-generation,) despite reductions in thermal efficiency due to ground temperature. (The ground source is warmer in climates that need strong air conditioning, and cooler in climates that need strong heating.)
Commercial systems maintenance costs in the USA have historically been between $0.11 to $0.22 per m2 per year in 1996 dollars, much less than the average $0.54 per m2 per year for conventional HVAC systems.
Because of the technical knowledge and equipment needed to properly install the piping, a GHP system installation is not a do-it-yourself project. To find a qualified installer, call your local utility company, the Geothermal Heat Pump Consortium or the Canadian GeoExchange Coalition for their listing of qualified installers in your area. Installers should be certified and experienced. The U.S. Department of Energy has a useful reference page.