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Introduction to Ground Loop application in Directional Drilling...


Heat pumps provide wintertime heating by extracting heat from a source and transferring it to the building. In theory, heat can be extracted from any source, no matter how cold, but a warmer source allows higher efficiency. A ground source heat pump uses the shallow ground as a source of heat, thus taking advantage of its seasonally moderate temperatures.

Ground Loop Configurations

Earth-coupled heat exchangers for GSHP systems can be grouped into three categories: closed-loop, open-loop and direct-exchange (DX). Closed-loop and DX systems can be arranged in either series or parallel piping configurations, as described below. The ground loop arrangement section that follows is an overview of the three basic ground loop categories.

Series Loop

Arrangement in Series or Parallel

Multiple closed ground loops can be arranged in series, parallel, or a combination of both. In series systems (Figure 1), the working fluid can take only one path through the loop, whereas in parallel systems (Figure 2) 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 GSHP systems can use either series or parallel arrangements, but large commercial and institutional buildings such as schools usually employ parallel loops.

Parallel-Loop

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.

Closed-loop systems

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. The loop is placed below the frost line where the temperature is more stable or submerged in a body of water if available. Systems in wet ground or in water are generally more efficient than drier ground loops since it is less work to move heat in and out of water than solids in sand or soil.
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 five different closed-loop configurations described in this section: horizontal directional drilling, traditional trenching, spiral (“slinky”) trenching, 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.

Horizontal Directional Drilling HDD

Horizontal directional drilling uses a little less land surface area per ton than the traditional trenching method (Figure 3) due to deeper ‘stacking’. HDD has a few specific advantages over the other ground loop methods which are discussed in more detail here:

Traditional Trenching of horizontal closed loops

Horizontal-loop

Traditional trenching to lay straight horizontal loops requires the greatest amount of land surface area per system ton. Pipe loops are laid in trenches, typically at a depth of 1 to 2 m (Figure 3). 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 30 to 100 meters per ton, depending on soil texture and moisture content, and the number of loops per trench. Trenches typically are spaced 2 to 4 meters apart.

Overall, the land area requirement for horizontal ground loops typically ranges from 750 to 1,000 square meters per system ton, depending on soil properties and mean earth temperature.

Horizontal trenched 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.

Slinky closed loops

Slinky loop
A variation on the horizontal loop is the spiral loop, commonly referred to as the “slinky.” As shown in Figure 4, 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 4). In the vertical slinky layout, coils stand upright in narrow trenches (Figure 4) that are deep enough to accommodate the coil diameter and a sufficient overburden so that the tops of the coils do not experience large seasonal temperature swings
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.

 

Advantages: Slinky loops requires less land area and less trenching than traditionally trenched horizontal-loop systems, and installation costs may be significantly less than vertical boreholes.

Disadvantages: Higher running costs due to pipe overlap and 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.

Summary of horizontal ground loop methods

Advantages: Directional Drilling and 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 (Mini HDD, digger, or trencher) and number of pipe loops per surface area.

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.

Vertical closed loops

Vertical loops are only considered when the available land area is limited. Wells are bored to depths that typically range from 70 to 100 meters deep. The closed-loop pipes are inserted into the vertical boreholes (Figure 5). Piping requirements range from 125 to 200 linear meters per system ton, depending on soil and temperature conditions. Vertical Loop

Vertical loops typically require one to two boreholes per ton of system load, the exact number depending on soil thermal conductivity. To avoid long-term degradation of the thermal resource, boreholes should be spaced minimum 3 meters apart, depending on climate and soil conditions.

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.

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.

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 much higher than cost of horizontal drilling or trenching, and vertical-loop designs tend to be the most costly GSHP 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 100 linear meters of piping per system ton. Depending on the pond depth and degree of water column stratification (persistence of thermocline), ponds can support GSHP systems ranging from 15 to 85 tons per acre of pond surface area. This range corresponds to a unit area requirement of 150 to 1,000 square meters per system ton. The minimum acceptable pond depth for submerged ground loops is 3 meters.

Pond-loop-fullPond loop

Concrete anchors are used to secure the piping coils, preventing their movement and holding the coils 25 to 50 cm 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 2 to 4 meters below the pond surface (Figure 6), preferably deeper, in order to maintain adequate thermal mass in times of extended drought or other low-water conditions.
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.

Open-loop


In open-loop GSHP 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. Open-loop

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.

Surface water systems use a large water body such as an ocean bay or inland lake for water supply, as well as discharge.

Evaluating open-loop feasibility

Water Quality: The primary heat exchanger is exposed to a continuous stream of dissolved ions and suspended solids and micro organisms carried into the system from the supply well, making it prone to scaling and the build-up 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 re-circulated, 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.

Direct Exchange/Expansion (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.

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.

Direct- Expansion-Loop

DX ground loops can be installed in a horizontal trenched configuration or a vertical U-tube configuration. Horizontal-loop DX systems require about 100 meters of copper tubing per system ton, as opposed to 150 to 200 meters per ton for polyethylene ground loops. Similarly, vertical DX systems require only a 3-inch diameter bores to a depth of 40 meters per ton, as opposed to 4- to 6-inch diameter bores to a depth of 70 to 100 meters per ton for polyethylene U-tubes in conventional vertical closed loops.

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 possibility 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.

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