Inground Heat Extraction
While it may be difficult to explain the technology of extracting heat from air, water or ground, the concept is easier to comprehend once one understands the principles of heat extraction and heat exchange. Consider the following:
- All matter contains heat. Zero degrees Kelvin/Rankine (minus 273 degrees Celsius/minus 460 degrees Farenheit) is absolute zero. This is a hypothetical, but fairly well substantiated, theory. There is nowhere in the universe where absolute zero exists. Temperatures in outer space have been found to be approximately three degrees Kelvin, which supports the theories developed by scientists.
- Cold is the absence of heat. Cold exists only in relative terms, and plays no part in scientific theory. While we all verbalize such expressions as "It is cold out", to be technically correct we should say "the heat level outside is ten degrees farenheit" (which, admittedly, is pretty cold).
- Heat always flows from higher temperature matter to lower temperature matter by conduction (from molecule to molecule), by convection (air currents) and by radiation (electro-magnetic waves).
- Heat can be moved or "extracted" from one source and delivered to another by various means such as "heat exchangers".
What is a Heat Pump?
A heat pump, as the name suggests, is a device that "pumps" heat from one location to another. The most popular heat pump is the air-source type (air-to-air), which operates in two basic modes:
- As an air-conditioner, a heat pump's indoor coil (heat exchanger) extracts heat from the interior of a structure and pumps it to the coil in the unit outside where it is discharged to the air outside (hence the term air-to-air heat pump) and
- As a heating device the heat pump's out door coil (heat exchanger) extracts heat from the air outside and pumps it indoors where it is discharged to the air inside.
The problem in comprehending such technology is that it is difficult to understand how heat extracted from ten degree air (or water) can heat anything. This is where the unit's compressor and the "phase-change" physical properties of the refrigerant come into play: the compressor boosts the extracted heat to a much higher temperature gas which gives up its heat as it condenses to a liquid in the condensing coil and is distributed to the structure by the fan or blower in the air-handler.
Differences between air-source and inground geothermal heat pumps
As with air-to-air heat extraction technology, geothermal (ground water/ground source) technology utilizes a type of inground heat pump known as a geothermal heat pump. This type of inground geothermal heat pump device extracts its heat from the ground or water rather than from air (your typical a/c unit). While the principles are fundamentally similar, the methodology varies in that water or air is pumped through a special type of heat exchanger and is either "chilled" by the evaporating refrigerant (in the heating mode) or heated by the condensing refrigerant (in the cooling mode).
Geothermal Heat Pumps
Why geothermal heat pumps are BETTER.
A geothermal heat pump or ground source heat pump (GSHP) is a central heating and/or cooling system that pumps heat to or from the ground. It uses the earth as a heat source (in the winter) or a heat sink (in the summer). This design takes advantage of the moderate temperatures in the ground to boost efficiency and reduce the operational costs of heating and cooling systems, and may be combined with solar heating to form a geosolar system with even greater efficiency. Inground geothermal heat pumps are also known by a variety of other names, including inground geoexchange, earth-coupled, earth energy or water-source heat pumps. The engineering and scientific communities prefer the terms "geoexchange" or "ground source heat pumps" to avoid confusion with traditional geothermal power, which uses a high temperature heat source to generate electricity. Ground source heat pumps harvest a combination of geothermal power and heat from the sun when heating, but work against these heat sources when used for air conditioning.
Water stores tremendous quantities of heat. In nature, few substances have a higher specific heat (one BTU per pound) than does water, making it an ideal heat storage medium for both natural and man-made phenomena.
Depending on latitude, the upper 3 metres (9.8 ft) of Earth's surface maintains a nearly constant temperature between 10 and 16°C (50 and 60°F). Like a refrigerator or air conditioner, these systems use a heat pump to force the transfer of heat from there. Inground geothermal heat pump systems can transfer heat from a cool space to a warm space, 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 at heating than pure electric heaters, even when extracting heat from cold winter air. But unlike an air-source heat pump, which transfers heat to or from the outside air, a ground source heat pump exchanges heat with the ground. This is much more energy-efficient because underground temperatures are more stable than air temperatures through the year. Seasonal variations drop off with depth and disappear below seven meters 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 ground heat in the winter (for heating) and transfers heat back into the ground in the summer (for cooling). Some systems are designed to operate in one mode only, heating or cooling, depending on climate.
The geothermal pump systems reach fairly high efficiencies (300%-600%) on the coldest of winter nights, compared to 175%-250% for air-source heat pumps on cool days. Ground source heat pumps (GSHPs) are among the most energy efficient technologies for providing HVAC and water heating.
The setup costs for geothermal systems are higher than its more prevelant cousin; conventional air conditions systems, but the difference is usually returned in energy savings in 3 to 10 years. Geothermal system life is estimated at 25 years for inside components and 50+ years for the ground loop.
In an open loop system (also called a groundwater heat pump), the secondary loop pumps natural water from a well or body of water into a heat exchanger inside the heat pump. Water stores tremendous quantities of heat. In nature, few substances have a higher specific heat (one BTU per pound) than does water, making it an ideal heat storage medium for both natural and man-made phenomena. Air, on the other hand has a very low specific heat (.018 BTU per cubic foot). There is 3472 times more heat stored in a cubic foot of water (62.5 BTU per degree F) as in a cubic foot of air . In other words it would be necessary to move 3472 cubic feet of air through a heat exchanger in an air-to-air heat pump in order to expose that heat exchanger to the same quantity of heat stored in a cubic foot of water (7 1/2 gallons) that is moved thru a geothermal heat pump.
While these differences are significant, there is more: the heat transfer characteristics of water make it superior to air. Conduction is more rapid, more complete, and more efficient a heat transfer phenomenon than convection. A ground-water heat pump extracting heat from water at freezing is approximately equal in performance to that of an air-source heat pump extracting heat from 60 degree air.
ASHRAE calls open loop systems groundwater heat pumps or surface water heat pumps, depending on the source. Heat is either extracted or added by the primary refrigerant loop, and the water is returned to a separate injection well, irrigation trench, tile field or body of water. The supply and return lines must be placed far enough apart to ensure thermal recharge of the source. Since the water chemistry is not controlled, the appliance may need to be protected from corrosion by using different metals in the heat exchanger and pump. Limescale may foul the system over time and require periodic acid cleaning. Also, as fouling decreases the flow of natural water, it becomes difficult for the heat pump to exchange building heat with the groundwater. If the water contains high levels of salt, minerals, iron bacteria or hydrogen sulfide, a closed loop system is usually preferable.
Deep lake water cooling uses a similar process with an open loop for air conditioning and cooling. Open loop systems using ground water are usually more efficient than closed systems because they are better coupled with ground temperatures. Closed loop systems, in comparison, have to transfer heat across extra layers of pipe wall and dirt.
A closed loop is one in which both ends of the loop's piping are closed. The water or other fluid is recirculated over and over and no new water is introduced to the loop. The heat is transferred thru the walls of the piping to or from the source, which could be ground, ground water, or surface water. As heat is extracted from the water in the loop the temperature of the loop falls and the heat from the source flows toward the loop. In closed loop operation water quality is not an issue because corrosives become rapidly "spent" or used up and corrosion caused by poor water quality is quickly curtailed. The wire-to-water efficiencies of circulators used in closed loop operation are very high and the costs of pumping the water are lower as compared to open loops. System efficiencies are somewhat lower in closed loop operation, but given the lower pumping costs associated with this method, economics favor this approach.
Closed loop tubing can be installed horizontally as a loop field in trenches or vertically as a series of long U-shapes in wells (see below). The size of the loop field depends on the soil type and moisture content, the average ground temperature and the heat loss and or gain characteristics of the building being conditioned. A rough approximation of the initial soil temperature is the average daily temperature for the region.
Closed Pond Loop Configuration
A closed pond loop is not common because it depends on proximity to a body of water, where an open loop system is usually preferable. A pond loop may be advantageous where poor water quality precludes an open loop, or where the system heat load is small. A pond loop consists of coils of pipe similar to a slinky loop attached to a frame and located at the bottom of an appropriately sized pond or water source.
Advantages: Can require the least total pipe length and can be the least expensive of all closed-loop systems if a suitable water body is available.
Vertical Loop Configuration
In vertical loop installation, deep holes are bored into the ground and pipes with U-bends are inserted into the holes, the holes are grouted, the piping loops are manifolded together, brought into the structure and closed. The argument for this type of ground-loop heat exchanger is that because the piping is in the deeper ground - unaffected by surface temperatures - performance will be higher. Generally, installed costs are higher than with a horizontal loop.
Vertical loop fields are typically used when there is a limited area of land available. Bore holes are spaced at least 5–6 m apart and the depth depends on ground and building characteristics. For illustration, a detached house needing 10 kW (3 ton) of heating capacity might need three boreholes 80 to 110 m (260 to 360 ft) deep. (A ton of heat is 12,000 British thermal units per hour (BTU/h) or 3.5 kilowatts.) During the cooling season, the local temperature rise in the bore field is influenced most by the moisture travel in the soil. Reliable heat transfer models have been developed through sample bore holes as well as other tests.
Advantages: Requires less total pipe length than most other closed-loop systems; requires the least amount of land area; seasonal soil temperature swings are not a concern.
Disadvantages: Cost of geothermal drilling is usually higher than cost of horizontal trenching, and vertical-loop designs tend to be the most costly geothermal heat pump systems; potential for long-term soil temperature changes if boreholes not spaced far enough apart.
Horizontal Loop Configuration
In horizontal loop installation, trenches are dug, usually by a backhoe or other trenching device, in some form of horizontal configuration. Various configurations of piping are installed in the trenches. A larger number of horizontal loop designs have been tried and utilized successfully by the industry. While installed costs have been lower, horizontial loops have been thought to be less efficient than vertical loops because of the effect of air temperatures near the surface of the ground.
Resistance to heat transfer two significant factors need to be considered when designing and sizing a ground-loop: 1) Resistance of the heat source to heat transfer eg. ground, pond, lake, etc. and 2) Resistance of the pipe to heat tansfer. Of the two factors, pipe resistance is the dominant one. But, while little control can be exercised over source resistance, a great deal of influence can be exercised by the designer over the pipe resistance. Plastic pipes are generally poor conductors as compared with metal. Increasing the ratio of pipe surface area to trench length yields significant gains in loop performance.
Advantages: Trenching costs for horizontal loops usually are much lower than well-drilling costs for vertical closed-loops, and there are more contractors with the appropriate equipment; flexible installation options depending on type of digging equipment (bulldozer, backhoe, or trencher) and number of pipe loops per trench.
Disadvantages: Largest land area requirement; performance more affected by season, rainfall, and burial depth; drought potential (low groundwater levels) must be considered in estimating required pipe length, especially in sandy soils and elevated areas; ground-loop piping can be damaged during trench backfill; longer pipe lengths per ton than for vertical closed loops; antifreeze solution more likely to be needed to handle winter soil temperatures.
Slinky Loop Configuration (Spiral Loop)
A variation on the horizontal loop is the spiral loop, commonly referred to as the "slinky."
The slinky ground loop, developed by the International Ground Source Heat Pump Association (IGSHPA) represents a good compromise between performance and installed costs. The slinky can be laid out in two ways, depending on the width of the trench that holds the pipe coils. The horizontal slinky layout consists of piping unrolled in overlapping circular loops that are laid flat in trenches of approximately the same width as the coil diameters, typically 3 to 6 feet wide.
In the vertical slinky layout, coils stand upright in narrow trenches that are deep enough to accomodate the coil diameter and a sufficient overburden so that the tops of the coils do not experience large seasonal temperature swings.
Slinky systems typically require 700 to 900 feet of piping per system ton, depending on soil properties and depth of burial. Depending on the coil pitch (overlap betweeen adjacent spirals), slinky installations can accommodate 80 to 120 feet of piping for every 10 feet of trench length. Slinky trenches typically are spaced about 12 feet apart. Overall, slinky systems require three to five times less land area than straight horizontal-loop systems, in the range of 500 to 800 square feet per ton.
Slinky coils are more prone to damage by backfill, and there also is a concern that careless backfilling could result in large voids around the slinky, particularly if the backfill material has large rocks or clods in it. Because air is a poor heat conductor, voids greatly reduce the loop's ability to exchange heat with the surrounding soil. To address these concerns, a flowable backfill has been developed, that can be dispensed directly into the trench by a mixer truck in the field, and this is described in the section below on ground loop installation procedures.
Advantages: Slinky loops equires less land area and less trenching than other horizontal-loop systems, and installation costs may be significantly less.
Disadvantages: Greater pumping energy needed than for straight horizontal-loops; backfilling the trench while ensuring that there are no voids around the pipe coils is difficult with certain types of soil, and even more so with upright coils in narrow trenches than with coils laid flat in wide trenches.
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 moves 3 to 5 times more heat energy than the electric energy it consumes, the total energy output is much greater than the input. This results in net thermal efficiencies greater than 100% for most electricity sources. Traditional combustion furnaces and electric heaters can never exceed 100% efficiency, but inground geothermal heat pumps provide extra energy by extracting it from the ground.
Geothermal heat pumps can reduce energy consumption— and corresponding air pollution emissions—upto 44% compared to air source heat pumps and up to 72% compared to electric resistance heating with standard air-conditioning equipment.
The dependence of net thermal efficiency on the electricity infrastructure tends to be an unnecessary complication for consumers and is not applicable to hydroelectric power, so performance of heat pumps is usually expressed as the ratio of heating output or heat removal to electricity input. 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. Performance is influenced by all components of the installed system, including the soil conditions, the ground-coupled heat exchanger, the heat pump appliance, and the building distribution, but is largely determined by the "lift" between the input temperature and the output temperature.
For the sake of comparing heat pump appliances to each other, independently from other system components, a few standard test conditions have been established by the American Refrigerant Institute (ARI) and more recently by the International Organization for Standardization. Standard ARI 330 ratings were 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 were 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.ASHRAE transitioned to ISO 13256-1 in 2001, which replaces ARI 320, 325 and 330. The new ISO standard produces slightly higher ratings because it no longer budgets any electricity for water pumps.
Efficient compressors, variable speed compressors and larger heat exchangers all contribute to heat pump efficiency. Inground geothermal heating and cooling systems in the market today have standard COPs ranging from 2.4 to 5.0 and EERs ranging from 10.6 to 30. 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.
Actual installation conditions may produce better or worse efficiency than the standard test conditions. COP improves with a lower temperature difference between the input and output of the heat pump, so the stability of ground temperatures is important. If the loop field or water pump is undersized, the addition or removal of heat may push the ground temperature beyond standard test conditions, and performance will be degraded. Similarly, an undersized blower may allow the plenum coil to overheat and degrade performance.
Soil without artificial heat addition or subtraction and at depths of several meters or more remains at a relatively constant temperature year round. This temperature equates roughly to the average annual air-temperature of the chosen location, usually 7–12 °C (45–54 °F) at a depth of six meters in the northern USA. Because this temperature remains more constant than the air temperature throughout the seasons, geothermal heat pumps perform with far greater efficiency during extreme air temperatures than air conditioners and air-source heat pumps.
Standards ARI 210 and 240 define Seasonal Energy Efficiency Ratio (SEER) and Heating Seasonal Performance Factors (HSPF) to account for the impact of seasonal variations on air source heat pumps. 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 HSPFs for ground-source heat pumps in Canada. The NRC HSPFs 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.
Geothermal DIY 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 therefore depends on the characteristics of the electricity supply. The GHG emissions savings from a heat pump over a conventional furnace can be calculated based on the following formula:
|GHG savings relative to|
|natural gas||heating oil||electric heating|
|Canada||223 ton/GWh||2.7 ton/yr||5.3 ton/yr||3.4 ton/yr|
|Russia||351 ton/GWh||1.8 ton/yr||4.4 ton/yr||5.4 ton/yr|
|USA||676 ton/GWh||-0.5 ton/yr||2.2 ton/yr||10.3 ton/yr|
|China||839 ton/GWh||-1.6 ton/yr||1.0 ton/yr||12.8 ton/yr|
- HL = seasonal heat load ≈ 80 GJ/yr for a modern detached house in the northern USA
- FI = emissions intensity of fuel = 50 kg(CO2)/GJ for natural gas, 73 for heating oil
- AFUE = furnace efficiency ≈ 95% for a modern condensing furnace
- COP = heat pump coefficient of performance ≈ 3.2 seasonally adjusted for northern USA heat pump
- EI = emissions intensity of electricity ≈ 200-800 ton(CO2)/GWh, depending on region
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 as 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 was, until recently,chlorodifluoromethane, which is an ozone depleting substance. Although harmless while contained, leaks and improper end-of-life disposal contribute to enlarging the ozone hole. This refrigerant is being phased out in favor of ozone-friendly R410A for new construction.
Open loop systems that draw water from a well and drain to the surface may contribute to aquifer depletion, water shortages, groundwater contamination, and subsidence of the soil. An inground heating systems project in Staufen im Breisgau, Germany, is suspected to have caused considerable damage to buildings in the city center. The ground has subsided by up to eight millimeters under the city hall while other areas have been uplifted by a few millimeters.
Ground source heat pumps are characterized 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. However, many family size installations are reported to use much more electricity then their owners had expected from advertisements. This is often partly due to bad design or installation: Heat exchange capacity with groundwater is often too small, heating pipes in house floors are often too thin and too few, or heated floors are covered with wooden panels or carpets.
Capital 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 10 kW (3 ton) thermal capacity for a detached rural residence in the USA averaged $8000–$9000 in 1995 US dollars. More recent studies found an average cost of $14,000 in 2008 US dollars for the same size system. The US Department of Energy estimates a price of $7500 on its website, last updated in 2008. Prices over $20,000 are quoted in Canada, with one source placing them in the range of $30,000-$34,000 Canadian dollars. 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.
|Country||Payback period for replacing|
|natural gas||heating oil||electric heating|
|Canada||13 years||3 years||6 years|
|USA||12 years||5 years||4 years|
|Germany||net loss||8 years||2 years|
Capital costs may be offset by substantial subsidies from many governments, for example totaling over $7000 in Ontario for residential heat pump systems installed in the 2009 fiscal year. Some electric companies offer special rates to customers who install a geothermal 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 lifespan of the system is longer than conventional heating 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 have warranties 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, propane or electric systems may be returned in energy savings in 2–10 years for residential systems in the USA. If compared to natural gas systems, the payback period can be much longer or non-existent. 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.
Governments that promote renewable energy will likely offer, renewable energy incentives for the consumer (residential), or industrial markets. For example, in the United States, incentives are offered both on the state and federal levels of government.
Because of the technical knowledge and equipment needed to properly design and size the system (and install the piping if heat fusion is required), a residential geothermal installation requires a professional's services. TERRASource Geothermal Systems offers qualified geothermal installation services in the USA and Canada.