Ground Loop Configuration and Installation
Earth-coupled heat exchangers for GHP systems can be grouped into three categories: open-loop, closed-loop, and direct-exchange (DX). Closed-loop and DX systems can be arranged in either series or parallel piping configurations, as described in the ground loop arrangement section that follows our overview of the three basic ground loop categories. Finally this page ends with a section on ground loop installation.
In open-loop GHP systems, a groundwater or surface water supply is used as a direct heat transfer medium, such that the water flows “one-way” through the building heat pump units and is then discharged.
Note, however, that wells designed for domestic water supply or grounds irrigation may not be large enough to meet the needs of a groundwater heat pump (GWHP) system, and additional production wells may have to be drilled. For example, residential domestic water supply wells are normally designed to produce 300 to 400 gallons per day, whereas a GWHP system for the same household might require thousands of gallons per day.
An important variation of the single-well open loop is the standing-column well, where all or most of the discharge water is re-injected back into the source well. This minimizes the amount of surface discharge, which may have to be limited for environmental or regulatory reasons.
Standing-column wells are feasible in areas with fractured bedrock aquifers near the surface, which is the case throughout much of Virginia's Piedmont region, where depth to bedrock typically ranges from 2 to 10 feet, and where the underlying rock formations are deeply fractured. Standing column wells are installed by drilling uncased boreholes, typically 6 inches in diameter, to depths in the range of 1,000 to 1,500 feet. The uncased borehole is in direct contact with the surrounding aquifer, creating a standing column of water from the top of the groundwater table down to the bottom of the well.
As shown in Figure 2, water for the GHP system is drawn from the bottom part of the well and returned at the top, with no net withdrawal of groundwater. During periods of peak heating and cooling demand, however, the system can "bleed" or discharge a portion of the return water at the surface (to a river, lake, pond, or drainage field) rather than returning it all to the standing column. Such surface bleeding causes net groundwater inflow to the column from the surrounding rock formation. This chills the standing column during periods of peak heat rejection (when building demand for cooling is greatest) and/or warms it during peak heat extraction (when heating demand is greatest), thus reducing the required bore depth.
Properly sited and designed, standing-column wells can be significantly less expensive to install than closed-loop GHP systems. They also require the least amount of ground area of any GHP system and should be considered for urban settings where land availability is limited, provided that proper geologic conditions exist.
The distance between the production and injection wells is an important design consideration. It is not necessary to completely prevent flow from the injection well to the production well, but simply to make sure that any flow between the wells is sufficiently low that discharged water arrives at the production well at a temperature at nearly the same temperature as the aquifer. Well spacing typically will be in the range of 200 to 600 feet, depending on the maximum system cooling or heating load, the typical duration of the maximum load, and the thickness and natural flow rate of the aquifer.
Surface water systems use a large water body such as an ocean bay or inland lake for water supply, as well as discharge. Thermal stratification in the ocean or deep lakes results in cold water below the thermocline remaining undisturbed throughout the year. In some cases this water is cold enough to provide direct space cooling simply by being circulated directly through water/air heat exchangers in the building, which eliminates the need for a heat pump or refrigerant to cool the interior spaces. In such cases, however, building loop water temperatures must be kept below 55 °F to dehumidify the air effectively. Data from lakes in Alabama suggest that significant thermal stratification occurs in lakes deeper than 30 feet, with bottom water temperatures between 45 and 55 °F throughout the year, even when summer surface water temperatures reach 80 to 90 °F.
A leading example of a surface water open-loop system is the Lake Source Cooling (LSC)project at Cornell University. As shown in Figure 4, deep water from Cayuga Lake is pumped to a water/water heat exchanger at the shore, where it absorbs heat from the chill-water loop used to cool Cornell University and Ithaca High School. The lake-water open loop and campus-water closed-loop are completely isolated from each other. Because heat flows naturally from hot to cold, no external energy is required for refrigeration compressors.
The LSC system has a power demand of 0.1 kilowatts per ton of cooling, which is only 13% of that required by the original campus cooling system. In addition, the Ithaca City School District will save more than $100,000 in capital expense for a new chiller it would have had to build at the high school, and $750,000 in cooling energy costs over the next twenty years.
It is important to note that the Cornell LSC project does not use heat pumps. The campus chill-water loop circulates through water/air heat exchangers in hydronic fan-coil units, where room air is blown over water-cooled heat exchanger fins similar to those on a car radiator.
Isolation heat exchangers
Direct use of groundwater or surface water in the building loop is an extension of residential open-loop GHP design practice and recommended for only the smallest institutional or commercial building applications where an isolation heat exchanger (such as used in the Cornell LSC project) would not be practical or economical, and where supply water quality is excellent.
Indirect open-loop systems employ an isolation heat exchanger between the building loop and the water supply. This eliminates exposure of building water loop or heat pump components to poor-quality supply water, making more sites potentially attractive for open-loop systems. The isolation heat exchanger also allows the building loop and supply water loops to be operated at different flow rates and pressures for optimal thermal and hydraulic performance.
Evaluating open-loop feasibility
Water Quality: The primary heat exchanger is exposed to a continuous stream of dissolved ions and suspended solids and microorganisms carried into the system from the supply well, making it prone to scaling and the buildup of bio-fouling or corrosion films. These increase the thermal resistance to heat transfer, thereby reducing system performance. They also increase the hydraulic resistance to flow through the heat exchanger, increasing pump energy consumption. Because large quantities of water are used, water treatment is not usually economical and may not be permitted. If water quality testing indicates that treatment is required, then an isolation heat exchanger should be considered.
Water Availability: The required groundwater flow rate through open-loop earth-coupled heat exchanger is typically between 2 and 3 gallons per minute per system ton. For a school or other institutional or commercial building, this may exceed permissible withdrawals allowed by local groundwater regulations. In this case, a standing column well might be a feasible alternative, since the water in the standing column is continuously recirculated, and there is no net withdrawal of groundwater, or minimal withdrawal during times of peak system load.
Discharge Water Permitting: The groundwater must either be re-injected into the ground or discharged to a storm-water drainage system, or into a surface water body such as a river or lake. Local codes and regulations may restrict such discharges.
In an open loop, water is usually discharged at a significantly higher elevation than the intake point, which represents a static pressure head that must be overcome by the main system circulating pump. This requires more electric power than a closed loop, where the pump only has to overcome pipe friction and pressure drop through any valves or heat exchangers. Because of their simpler design, however, open-loop systems can be much less costly to install, possibly yielding the most economical GHP solution.
Advantages: Simpler design; lower drilling costs than for vertical closed-loop systems; more efficient performance by avoiding thermal degradation associated with heat transfer across pipe wall from ground or water body to antifreeze solution in closed-loop; lower installation cost if a supply well already exists for domestic water or grounds irrigation, with sufficient surplus production capacity to supply heat pump system.
Disadvantages: Subject to local, state, and Federal groundwater and surface water withdrawal and discharge permitting; large water flow requirements may exceed local water availability; supply-side of heat exchangers subject to corrosive and abrasive agents, chemical scaling, and microbial fouling; main circulating pumps typically require more power in open loops than in closed loops; water discharge regulations may preclude single-well systems or constrain the design of standing-column systems; higher installation cost if a separate injection well is required for loop water discharge.
Closed-loop systems use an underground network of sealed, high-strength plastic piping, which acts as the earth-coupled heat exchanger. The most commonly used closed-loop piping material is high-density polyethylene (HDPE). The ground loop piping is filled with a working fluid that is continuously re-circulated without ever directly contacting the soil or water in which the loop is buried or immersed. Once filled with fluid and purged of air, nothing enters or leaves the closed loop. This eliminates the potential shortcomings of water quality and availability associated with open-loop systems.
There are four different closed-loop configurations described in this section: horizontal, spiral (“slinky”), vertical, and submerged (pond or lake loop). Each type is described briefly below, together with “rule of thumb” estimates on the land or water area required per system ton. The advantages and disadvantages of each type also are noted. This section continues with an overview of three hybrid closed-loop systems, where a supplemental heat sink or heat source is added to the loop, such as a heat-rejection pond, cooling tower, or solar thermal collector.
In Virginia, particularly in the southern and southeastern regions of the state, the energy demand for school space conditioning is dominated by cooling loads. Even where heating and cooling loads are approximately equal, a ground loop that is properly sized to meet the heating demand will be only about half the size needed to meet the cooling demand. This is because schools have high internal heat gains (lighting, computer and audio-visual equipment, student bodies), which reduce the demand for space heating but increase the demand for space cooling. Also, when a GHP operates in heating mode, about 30% of the heat input to the building comes from the heat given off by the compressors and fan motors in the heat pump units. So per ton (12,000 BTU/hr)of heating demand, only about 8,400 BTU/hr needs to be supplied by the ground loop; the rest comes from the compressors and fan motors. In the cooling season, however, the ground loop must supply not only 12,000 BTU/hr per ton of school- and weather-based cooling demand, but an additional 3,600 BTU/hr to remove the heat from the compressors and fan motors. Thus the ground loop has to be 85% larger (15,600 divided by 8,400) to meet the total cooling load, even when seasonal heating and cooling demands are balanced.
Some buildings may not have enough land area to accomodate a ground loop for meeting the full cooling load. Even where sufficient land area is available for a fully sized ground loop, it may be more economical to install a supplemental heat sink, such as a heat-rejection pond or a cooling tower, which can cost significantly less per ton of installed capacity, especially in areas with high drilling costs (difficult sub-surface geology). Therefore, hybrid systems may be the most economical application of GHP technology for schools in Virginia, and our Web site visitors are particularly encouraged to read the sections on supplemental cooling ponds and cooling towers.
The working fluid used in closed ground loops, including hybrid systems, is water or water mixed with an antifreeze additive. Antifreeze protection is required if loop temperatures are expected to fall below about 40 ºF, which is possible with buried horizontal and slinky ground loops, as well as submerged loops in lakes or ponds. Different antifreeze additives have different densities, viscosities, and thermal properties, which in turn will affect the total length of ground loop piping (installation cost) and the required pumping power (operating cost). Furthermore, health, safety, and environmental concerns will limit the selection and use of certain antifreeze chemicals. This section concludes with a description and comparison of antifreeze solutions.
Horizontal closed loops
Horizontal loops require the greatest amount of land surface area per system ton. Pipe loops are laid in trenches, typically at a depth of 4 to 10 feet (Figure 5). From one to six loops can be installed in each trench. Although such multiple-loop systems conserve land area and require less trenching, they use more linear feet of pipe per system ton. Nevertheless, multiple loops frequently cost less to install than single loops. Trench lengths can range from 100 to 400 feet per ton, depending on soil texture and moisture content, and the number of loops per trench. Trenches typically are spaced 6 to 12 feet apart.
Horizontal loops are most attractive where there is ample land area for trenching, and where a high water table ensures good heat transfer even in relatively shallow trenches. In school applications, such large areas can exist beneath athletic fields, playgrounds, or parking lots.
Horizontal loops can be particularly attractive in a new school construction project. After the site has been cleared, a large area beneath a future athletic field or parking lot can be excavated with a bulldozer, and a "mat" of horizontal loops laid to cover the area before it is backfilled (Figure 6).
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 closed loops
A variation on the horizontal loop is the spiral loop, commonly referred to as the “slinky.” As shown in Figure 7, 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 (Figure 8). In the vertical slinky layout, coils stand upright in narrow trenches (Figure 9) 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.
Vertical closed loops
Vertical loops are generally considered when the available land area is limited. Wells are bored to depths that typically range from 200 to 300 feet deep. The closed-loop pipes are inserted into the vertical boreholes (Figure 10). Typical piping requirements range from 400 to 600 linear feet per system ton, depending on soil and temperature conditions.
Vertical loops typically require one to two boreholes per ton of system load, the exact number depending on soil thermal conductivity (see Figure 4 on Soil Thermal Properties page). To avoid long-term degradation of the thermal resource, boreholes should be spaced 15 to 20 feet apart, depending on climate and soil conditions.
Vertical ground loops typically require 150 to 300 square feet of land area per system ton.
The most common configuration for the vertical loop piping element in the drilled bore is a U-tube, where a 180-bend fitting has been factory fused to join two lengths of HDPE pipe, and this inserted into the borehole (Figure 11). More details and photographs are provided in the ground loop installation section on this page.
Where the local water table is known to be reliably near the surface, the borehole can be backfilled with pea gravel, which allows groundwater circulation around the U-tube elements. Where the soil is dry or where there are large seasonal fluctuations in the groundwater level, or where local regulations require permanent sealing of the borehole, a thermally enhanced grout should be used to backfill around the U-tube. Thermal performance also can be enhanced by the use of spacer clips at 5-foot intervals along the length of the piping element, which force the legs of the U-tube against the borehole wall. See grouting section at the end of this page for details.
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 drilling is usually higher than cost of horizontal trenching, and vertical-loop designs tend to be the most costly GHP systems; potential for long-term soil temperature changes if boreholes not spaced far enough apart.
Submerged closed loops
If a large river or moderately sized pond or lake is available, the closed-loop piping system can be submerged. Some commercial and institutional buildings have artificial ponds for aesthetic reasons, and these may have adequate surface area and depth for fully immersing a closed-loop heat exchanger.
Submerged-loop systems typically require about 300 linear feet of piping per system ton. Depending on the pond depth and degree of water column stratification (persistence of thermocline), ponds can support GHP systems ranging from 15 to 85 tons per acre of pond surface area. This range corresponds to a unit area requirement of 500 to 3,000 square feet per system ton. The minimum acceptable pond depth for submerged ground loops is 10 feet.
Concrete anchors are used to secure the piping coils, preventing their movement and holding the coils 9 to 18 inches above the pond floor, to allow good convective circulation of water around the piping. It also is recommended that the coils be submerged at least 6 to 8 feet below the pond surface (Figure 13), preferably deeper, in order to maintain adequate thermal mass in times of extended drought or other low-water conditions.
Although there are no schools in Virginia that have pond-based GHP systems, there is an impressive example in Fond du Lac, Wisconsin, of a large (400,000 square feet) high school with a 720-ton GHP system connected to a pond loop that consists of 41 miles of 3/4-inch, HDPE piping. Thirty rafts, each containing 24 piping coils, were submerged in the school's two retention ponds. These ponds are about 20 feet deep and have a total surface area of 12 acres. Links to Web pages and reports describing this project can be found on the Other Resources page of this Web site.
The Fond du Lac High School GHP project originated with two teachers, ultimately evolving into a core group that facilitated this project, bringing in outside professional expertise as needed. Because of its success, this project is one from which we have drawn heavily in developing our own participation program, as described on the Get Involved pages of this Web site.
Rivers typically are not as attractive as lakes or ponds for closed-loop immersion, since they are more affected by drought and flooding conditions. Moreover, river-bottom installations may be subject to moving boulders or logs, which can damage the submerged coils. Finally, anchoring requirements will be greater in rivers than in lakes or ponds, since the anchors must hold the coils against the force of flowing water.
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.
Disadvantages: Submerged loops are likely to require more regulatory permitting than buried closed-loop systems; unless properly marked, can be damaged by boat anchoring.
Hybrid loop with cooling pond
As explained in the introduction to this section, GHP systems for school buildings in Virginia are expected to reject significantly more heat to the ground loop than they extract from it during the course of a year. This seasonal imbalance can be accommodated either by a full-size buried ground loop, more than half of which would be not needed during the winter heating season, or a supplemental means of heat rejection such as a pond or cooling tower.
Research at Oklahoma State University suggests that it is less costly to build artificial ponds than to install cooling towers, particularly if the pond is built on school property and additional land does not have to be purchased for pond construction. As shown in Figure 14, the only mechanical equipment associated with a supplemental cooling pond is the pond loop pump. Thus in addition to having a lower first cost, a supplemental cooling pond is expected to have a lower operating and maintenance cost than a cooling tower.
The land area required to build an artificial cooling pond is considerably less than the land area taken up by the ground loop piping that it replaces, even for the most compact vertical closed-loop systems. For example, in the moderate climate of Tulsa, Oklahoma, a supplemental cooling pond having a surface area of 240 square feet would have the same heat rejection capability as a vertical closed-loop occupying 720 square feet of land area (with boreholes compactly spaced at just 12 feet apart).
Hybrid loop with cooling tower
A cooling tower is an alternative to consider in cases where an artificial pond cannot be built for supplemental heat rejection. This would be particularly attractive in a retrofit project where an existing school already has a cooling tower for its conventional HVAC system. In this type of hybrid GHP configuration, the cooling tower can be connected directly to the ground loop, or it may require an isolation heat exchanger, depending on the type of cooling tower.
There are two types of cooling towers: open and closed. In an open tower, the water to be chilled is open to the atmosphere and is cooled by evaporation as it cascades down through a structured packing material (called "fill") designed to increase the surface area of falling water films, thereby increasing the evaporation rate. Induced-draft towers use a suction fan to pull air up through the fill ("counterflow") or across the fill ("crossflow"), as shown in Figure 15.
Counterflow towers tend to be the most compact. This is because the upward flowing air stream is in contact with the entire cross section of water from top to bottom as it falls through the fill. Less space is needed because of their increased thermal efficiency and lack of the warm water distribution plenum needed by crossflow towers. Their disadvantage, however, is the increased fan power resulting from air flowing directly against, rather than across, the falling water. Although bulkier, crossflow towers consume less power and usually have a lower capital cost as well.
Forced-draft cooling towers use a bottom-mounted centrifugal blower to push air through the fill, as shown in Figure 16. Centrifugal blowers are inherently quieter than the propellor-type fans of induced-flow towers, and by locating the blower beneath the unit, fan and motor noise is baffled, further reducing the noise of operation. Maintenance access is easier because all moving parts are located at the tower base. Corrosion is reduced because the blower handles dry, ambient air unlike the fans of induced-flow towers, which must handle moisture-laden air. The major disadvantage of forced-draft towers is that they consume about twice the power of induced-flow towers for the same cooling capacity.
When used in a hybrid GHP system, open cooling towers must be isolated from the ground loop with a plate heat exchanger, as shown in Figure 17, in order to prevent contamination by debris in the cooling tower air stream, which otherwise would lead to corrosion and clogging of the building heat pump units. If a school already has an open cooling tower as part of its existing HVAC system, and the tower is in good condition, then a hybrid GHP retrofit using this tower might be economical. If a new cooling tower must be installed, then a closed-circuit tower would be preferred.
In a closed-circuit tower (also called a "fluid cooler"), the water to be chilled is contained within a coil and isolated from the water that is evaporating, as shown in Figure 18. Closed towers typically are larger, cost more, and consume more power than open towers of the same cooling capacity. On the other hand, closed towers are less expensive to maintain, particularly if not used continuously throughout the cooling season, as would be the case in schools without summer programming. Unlike open towers, closed-circuit fluid coolers do not have to be placed at the highest point in the water loop (typically on the roof), but can be ground-mounted in a suitably designed enclosure.
Closed towers are recommended for any hybrid GHP system where there is no pre-existing cooling tower already in place. Because of its closed-circuit design, a fluid cooler can be connected directly to the ground loop, as shown in Figure 19.
There are no hybrid GHP systems at schools in Virginia, but there are two examples in the Mid-Atlantic region. The first is a school near Erie, Pennsylvania and the second is an elementary school in West Atlantic City, New Jersey.
The Wattsburg School located near Erie has a total area of 180,000 square feet, with a cooling load of 330 tons and a heating load of 250 tons. A full-size ground loop would have required 133 bores 300 feet deep. By installing a 125-ton closed cooling tower, the ground loop field was reduced to 90 bores. The capital cost saving was $154,800, while the fluid cooler cost approximately $28,000, yielding a net saving of $126,800 compared with the fully sized ground loop.
The elementary school in West Atlantic City has a total area of 85,000 square feet. A ground loop fully sized to meet this school's cooling load would have required 90 bores 400 feet deep. By installing a 117-ton cooling tower, the ground loop field was reduced to 66 bores, at a capital cost saving of $93,120. Although the cooling tower cost is unknown, it probably is similar to the cost of the Wattsburg School cooling tower, again yielding a significant net saving.
Hybrid loop with solar collector
For schools without summer programming and which are located in the cooler, elevated regions of western Virginia, heating load might be the driving ground loop design factor. In such cases, supplementing a GHP system with solar thermal collectors will reduce the required size of the ground loop and increase heat pump efficiency by providing significantly higher building loop temperatures than could be attained by the ground heat exchanger alone.
In most cases, the solar thermal collector can be connected directly to the ground loop, as shown in Figure 20. A liquid/liquid isolation heat exchanger would be required, however, if the solar recirculating loop needs a different level of antifreeze protection than the ground loop or uses a different antifreeze additive. Solar thermal collectors almost always use propylene glycol for both antifreeze and anti-boiling protection, whereas methanol is the preferred antifreeze additive for closed ground loops, where environmental and health regulations permit its use, as described in the antifreeze section below.
The supplemental solar collector loop shown in the above diagram includes a photovoltaic (PV) panel for powering the recirculating pump. When the evacuated-tubes cannot collect enough solar heat to add to the ground loop, the PV panel will not have enough power for the recirculating pump. This provides automatic recirculation control without the need for a separate electronic controller. By being suitably sized to match the pump motor and solar thermal array, the PV panel IS the controller.
As mentioned at the beginning of this section, closed-loop antifreeze protection is required if the water temperature entering the building (i.e., leaving the ground loop) is expected to fall below 40 ºF. Since a 10 ºF bulk fluid temperature difference typically exists across the tubing walls of the water-to-refrigerant heat exchanger in the building heat pump units, an interior loop temperature of 40 ºF potentially creates a coil surface temperature below the freezing point of water. If a layer of ice forms on the water side of the heat exchanger, heat transfer efficiency will be reduced (which is why refrigerators must be periodically defrosted) and water flow will be restricted or blocked, raising loop pressure and possibly causing pump shutdown. Since solid water is less dense than liquid water, extreme ice formation could burst the heat exchanger tubing, entailing a major expense for heat pump repair or replacement.
Vertical closed loops in Virginia usually will not require antifreeze protection, since they are subject to a year-round constant temperature of 52 to 62 ºF in most parts of the state, except the extreme northwest (see Figure 2 on the "Earth Temperature and Site Geology" page).
Depending on burial depth and type of soil, horizontal and "slinky" ground loops may be subject to seasonal temperature swings of ±10 ºF, and in such cases the late-winter, natural ground temperature would drop below 50 ºF. The heat extraction process can lower the soil temperature in the immediate vicinity of the ground loop by a further 10 ºF, such that the water leaving the ground loop would approach or even fall below 40 ºF. For submerged closed loops, ponds or lakes that freeze at the surface during the winter can have bottom water temperatures below 40 ºF, also creating a condition that would require antifreeze protection.
At least seven different antifreeze additives have been proposed or used in closed ground loop solutions, falling into three general categories: alcohols (methanol, ethanol), glycols (ethylene glycol, propylene glycol), and salts (sodium chloride, calcium chloride, and potassium acetate). The antifreeze additives in each of these categories are reviewed in the next three paragraphs. This topic concludes with a recommendation of methanol or propylene glycol as the preferred antifreeze options and a comparison of these two additives in terms of their impact on loop length and required pumping power, as well as the cost of the chemical itself.
Solutions of alcohols in water - methanol and ethanol - have low viscosity (which translates to lower pumping power) and relatively high heat transfer capability (which translates to shorter ground loops). On the negative side, however, they are highly volatile, flammable in concentrated or pure form, and toxic. Once mixed with water in typical antifreeze concentrations, the resulting solution is not flammable. Unlike methanol, pure ethanol is not toxic, but only denatured ethanol can be purchased for commercial use as an antifreeze. Denaturants render the ethanol toxic and some denaturants also will chemically attack polyethylene piping. Methanol is preferred because it eliminates the possiblity of such damage to ground loop piping, even though it is more toxic than denatured ethanol.
Ethylene glycol is the most commonly used antifreeze for engine cooling in automobiles. Animals and children can be attracted to its sweet taste, however, and may ingest it if they encounter an open or used container, a spill, or a leak. Countless cats and dogs are poisoned every year by discarded or leaking automotive antifreeze, and nearly nine out of ten animal encounters with this toxin will end in fatality. Its toxicity has prevented widespread use of ethylene glycol in the GHP industry, despite its fairly common use in conventional water-source heat pump (chiller/boiler) applications. Although more expensive, propylene glycol is recommended as a less toxic alternative and has been successfully used in many GHP systems. Additional aspects of propylene glycol usage are discussed in the last paragraph under this topic.
Salt solutions of calcium chloride or sodium chloride are commonly used for antifreeze protection, typically in industrial applications. Due to their high corrosiveness, the use of an antifreeze salt solution in GHP systems would require that all air traps or vents be designed out of the loop and it would eliminate the use of certain metals or combinations of metals. Salt solutions also require additives to inhibit corrosion and maintain pH, and the levels of these additives must be continually monitored and maintained. Potassium acetate is a more environmentally acceptable member of the salt family and a corrosion-inhibited 50% solution is marketed specifically for GHP applications under the trade name "Vanguard GS4." Due to its low surface tension, GS4 has been known to leak through mechanical fittings and certain pipe-thread sealing compounds, exposing the solution to air and enabling corrosion. For this reason, GS4 is not recommended.
Methanol is the most preferred antifreeze additive for closed-loop working fluids and has delivered outstanding performance for more than a decade in GHP applications. To increase safety, a premixed solution should be used on the job site. Although methanol enjoys widespread consumer use as a windshield washer fluid, local health or environmental regulations may prohibit or restrict its use in some ground loop applications.
In situations where local regulations prohibit methanol use, then propylene glycol is the recommended alternative. Only food-grade propylene glycol should be used, since other grades often contain corrosion inhibitors, which react with local water and precipitate to form slime coatings inside heat exchangers, hindering heat transfer.
Propylene glycol should be used only in those instances where methanol is prohibited, because it has the following disadvantages relative to methanol:
- Compared with pure water at 40 ºF, the poorer heat transfer capability of propylene glycol can lead to a 10-15% increase in loop length, depending on the level of antifreeze protection, whereas methanol has less than a 5% impact for the same heating or cooling capacity.
- Again compared with pure water at 40 ºF, the higher viscosity of propylene glycol, in combination with the longer loop required for a given system capacity, can increase pumping power requirements by 35-40%, depending on the level of antifreeze protection, whereas methanol entails only a 15-25% pumping power increase.
- Finally, for a given level of antifreeze protection, the cost of propylene glycol is ten times greater than that of methanol, typically in the range of tens of dollars per ton of system heating or cooling capacity, compared with dollars per system ton for methanol solutions.
Considering all three of the above aspects, use of propylene glycol for antifreeze protection can have a significant negative impact on GHP system economics. Therefore if methanol use is prohibited, it might be worth exploring whether more costly ground loop designs that would eliminate the need for antifreeze, such as deeper burial of horizontal and slinky loops or the use of a vertical loop configuration, might yield a lower total system cost over its operating lifetime.
Direct Exchange (DX) Loops
The closed ground-loops described above use water or a water-antifreeze solution as an intermediate working fluid to move heat energy between the ground (or water body) and the building, with a liquid/refrigerant heat exchanger in each heat pump unit. Direct-exchange (DX) systems do not use an intermediate working fluid or heat exchanger. Intead, DX systems employ closed loops of soft copper tubing to directly transfer heat between the ground and the refrigerant -- the heat pump's refrigerant loop is buried in the ground (Figure 15).
By eliminating the intermediate heat exchanger, the refrigerant's temperature is closer to the ground's temperature, which lowers the heat pump's required compression ratio, reducing its size and energy consumption. Also a shorter ground loop can be used, because copper tubing is more efficient at transferring heat than the polyethylene pipe used in conventional closed loops; the thermal conductivity of copper is about 19 Btu/sq.ft-hr-°F per inch of wall thickness, whereas that of HDPE pipe is only 2.7 Btu/sq.ft-hr-°F per inch.
DX ground loops can be installed in a horizontal trenched configuration (Figure 16) or a vertical U-tube configuration (Figure 17). Horizontal-loop DX systems require about 350 feet of copper tubing per system ton, as opposed to 450 to 500 feet per ton for polyethylene ground loops. Similarly, vertical DX systems require only a 3-inch diameter bores to a depth of 120 feet per ton, as opposed to 4- to 6-inch diameter bores to a depth of 200 to 300 feet per ton for polyethylene U-tubes in conventional vertical closed loops.
DX geothermal heat pumps are offered by only two manufacturers in North America and are commonly available in the 2- to 5-ton size range. To date DX GHPs have been installed only in residential and small commercial applications, where a blower forces air through a refrigerant/air heat exchanger, and a duct sytem distributes the warmed or chilled air throughout the building. In larger building applications a refrigerant/water heat exchanger is used to transfer the heat to a pipe system that can distribute warmed or chilled water to hydronic terminal systems such as radiant floor slabs or fan-coil units. A 25-ton geothermal DX-water heat pump system for a commercial building is shown in Figure 18.
Because of their shorter length, horizontal DX ground loops need only about 500 square feet of land area per system ton, considerably less than the 1,500 to 3,000 square feet needed for conventional horizontal closed-loops. Vertical DX loops, on the other hand, need at least the same land area as their conventional counterparts, or even somewhat more. Vertical DX boreholes should be spaced at least 20 feet apart to minimize the possiblity of ground freezing and buckling in the heating mode or excessive warming and drying of the soil in the cooling mode.
Heat from DX ground loops can bake fine-grained soils, reducing their thermal conductivity and thus the performance of the system. DX ground loops perform best in moist sandy soils or sand bed installations. Because DX ground loops are copper, they are subject to corrosion in acidic soils and should be installed in soils with a pH between 5.5 and 10, which are common in Virginia.
Advantages: Higher thermal efficiency; no liquid/liquid heat exchangers required; less land area needed for horizontal configuration.
Disadvantages: Soil in contact with ground loop subject to freezing; copper tubing should not be buried near large trees where growing root system could damage the coil; ground-loop leaks can lead to catastrophic loss of refrigerant; smaller supporting infrastructure in GHP industry, with greater care and higher skill needed to install and consequently higher installation costs.
Arrangement in Series or Parallel
Multiple closed ground loops can be arranged in series, parallel, or a combination of both. In series systems (Figure 19), the working fluid can take only one path through the loop, whereas in parallel systems (Figure 20) the fluid can take two or more paths through the circuit. Note that parallel arrangements use a reverse rather than direct return to the building so that all parallel flow paths are of equal length, helping to ensure a balanced flow distribution. Residential and small commercial GHP systems can use either series or parallel arrangements, but large commerical and institutional buildings such as schools usually employ parallel loops.
The type of arrangement will affect pipe diameter, pump power requirements, and installation cost. The relative advantages and disadvantages of series and parallel arrangements are summarized below.
Series Advantages: Single pipe diameter entails simpler pipe fusion joints, enabling quicker installation; single flow path enables easier purging to remove air from the loop when filling with water or antifreeze solution.
Series Disadvantages: Longer flow path requires larger-diameter pipe to minimize pressure drop and maintain pump power at reasonable levels; larger diameter also entails greater antifreeze volumes; system capacity limited by total pressure drop from end to end, so not suitable for large building applications.
Parallel Advantages: Shorter flow paths enable smaller pipe diameter to be used, which lowers unit piping cost and requires less antifreeze; reduced pressure drop along shorter flow paths results in smaller pump power requirements.
Parallel Disadvantages: Header lines must be larger diameter than individual loops and so require more complex pipe joining operations than series installation; special care needed to ensure complete air removal from all flow paths when purging system at start-up.
Ground Loop Installation
High-density polyethylene (HDPE) is the preferred material for ground loop piping. Closed loops use relatively small pipe sizes for the portion of the loop that exchanges heat with the ground, typically ranging from 3/4 to 2 inches in diameter. Header and supply/return lines are larger, typically ranging from 3 to 10 inches in diameter. Field procedures for thermally fusing HDPE pipe in butt splices (pipe end to pipe end, same diameter) and sidewall splices (heat-exchanging pipe element to larger-diameter header pipe) have been proven in the natural gas distribution industry, where tests have shown the joints to be stronger than the piping itself. CAUTION: During the fusion process, equipment and piping can reach temperatures in excess of 450°F, and pipe elements should not be removed from fusion machine clamps until cooled. WARNING: Barbed, clamped, or other mechanical connections should never be used for joining ground loop piping eliments; such connections are prone to leakage, requiring costly repairs and or ground-loop replacement.
Horizontal and slinky closed loop installation
As shown in Figure 21, three different types of equipment can be used to excavate and backfill horizontal or slinky ground loops. Bulldozers are appropriate if the entire loop field can be excavated, as might be possible at the start of a new construction project. Site-wide excavations enable rapid installation the ground loops in a "mat" covering the entire floor of the excavated area (see Figure 6 for horizontal loop example, and Figure 22 for slinky loop example). This may be the only feasible installation option for horizontal or slinky loops in areas where the soil is so sandy that trench walls collapse before loop piping can be laid down.
Trenches made with a wide backhoe bucket are typically used for four-loop and six-loop arrangements of horizontal closed loops that are stacked two deep in the soil. When loops are placed at two depths, the lower loops are laid on the bottom of the trench, which is then partially backfilled with a soil layer at least 2 feet thick. The upper loops are laid on top of this backfill layer, and then the trench is backfilled all the way to the surface. Multiple slinky loops also may be installed in a single trench dug with a backhoe, laid side by side in a single layer (Figure 23).
Where large rocks and boulders are not present, a chain trencher may be more economical than a backhoe. A 65-horsepower chain trencher can excavate a trench 6 inches wide and 7 feet deep, which would be suitable for burial of one or two horizontal straight loops or a single upright slinky loop (Figure 24).
Cohesive clay soils capable of supporting deep, narrow trenches also tend to form clods when excavated. If trench cuttings are used to backfill around an upright slinky loop, then large quantities of water should be used to eliminate voids around the overlapping spirals. Alternatively, a special flowable backfill can be used (Figures 25 and 26), but this will add to the installation cost and require off-site disposal of trench cuttings.
Vertical closed loop installation
Vertical borehole drilling equipment is more costly and less widely available than the excavation equipment used for horizontal and slinky loops. The most commonly used drilling equipment for vertical borehole excavation is a wet rotary rig, where a mud slurry (often with bentonite and other additives) is pumped down the rotating drill stem to lubricate and cool the cutting head and wash drill cuttings to the ground surface (Figure 27). Drilling rates for ground loop installation are higher than for water well drilling, since logging and completion procedures are not required.
Photographs showing the major steps during installation of a vertical closed loop GHP system are shown in Figures 28 through 32 for Y E Smith Elementary School in Durham, North Carolina, a new school construction project where the ground loop was installed beneath a future recess area immediately behind the school.
Note in Figure 29 that the two U-tube risers have been taped together (which keeps them from springing back against the borehole wall) and strapped to a piece of scrap metal rebar (adding weight and stiffness). This makes it easier to feed the U-tube element into the borehole, particularly for deep bores. On the other hand, taping the U-tube risers together can significantly reduce the thermal performance of the element if the borehole has to be grouted. The thermal conductivity of typical grouting materials is low compared with the thermal conductivity of most soil formations. When the U-tube elements are taped together, it pulls them inward, away from contact with the borehole wall, thereby increasing the insulating thickness of the grout and hindering heat transfer.
This would not be as much of a problem where pea gravel or naturally sandy soil can be used to backfill the borehole and the groundwater level is reliably high enough to circulate around most of the U-tube length. Note, however, that state or local environmental regulations may require the permanent sealing of the annular space with a grouting material that has low permeability to water. Such a regulatory requirement eliminates the possibility of potential contaminants at the ground surface from flowing down into an aquifer, and it prevents movement of groundwater from one aquifer to another.
Where regulations require the grouting of boreholes, the following measures should be taken to minimize grout impact on thermal performance:
- Reduce borehole diameter as much as possible to minimize the amount of grout needed
- Use a thermally-enhanced grout formulation with a higher ratio of sand to bentonite in the grout mixture (typically 100 to 300 pounds of sand per 50 pounds of bentonite)
- Use Geo-Clips to push the U-tube elements apart, holding them against the borehole walls
Field measurements suggest that the use of a thermally enhanced grout in combination with Geo-Clips spaced at 5-foot intervals can yield a 30% decrease in ground loop length relative to standard installations with a 27% bentonite grout. There is a limit to increasing grout thermal conductivity, however, beyond which the added cost of materials, including transportation to the job site, and added labor to handle and install the heavier grouts offsets the savings in ground loop length. Specifying grout thermal conductivities higher than 0.85 or 1.0 BTU/hr-ft-°F may not be economical when considering these added material and labor costs.