Six steps for designing ground heat exchangers

Designing ground-coupled heat pump (GCHP) HVAC systems requires careful consideration of system configuration and the ability to clearly navigate the design paths within the owner’s budget. A GCHP system can be either an economical system or a source of continual heartburn for the engineer and owner. Understanding GCHP design considerations and operational requirements is necessary to design an economical and efficient project.

09/07/2017


Learning objectives:

  • Understand the basic concepts and equipment of a ground-source heat pump system.
  • Review key elements of ground-coupled heat pump (GCHP) design.
  • Understand maintenance and operations requirements for GCHP and incorporate them in the design.

Figure 1: This is an example of a ground-coupled heat pump unit in a vertical configuration. Courtesy: Stanley ConsultantsThe term ground-source heat pump (GSHP) refers to the family of heat pump systems that use the ground, groundwater, or surface water as a heat sink and source to condition buildings. This article focuses on ground-coupled heat pump (GCHP) systems, which fall under the GSHP family. A GCHP system is characterized as a reversible vapor-compression-cycle heat pump connected to a closed-loop ground heat exchanger. Ground-coupled systems typically use water-to-air heat pumps or water-to-water heat pumps. Conventional hydronic pumps are used to circulate a water or water-antifreeze mixture between the building piping system and the ground heat exchanger.

GCHPs are typically connected to either a vertical ground heat exchanger (GHEX) or a horizontal GHEX. Vertical GHEX systems are constructed of two small-bore (0.75- to 1.5-in. diameter) high-density polyethylene (HDPE) pipes inserted in a 4- to 6-in.-diameter vertical borehole filled with thermally enhanced grout. Borehole depths usually range from 100 to 400 ft deep, depending on soil conditions and drilling equipment. Vertical GHEX systems achieve high system performance by taking advantage of the relatively constant temperature and soil thermal properties of the deep earth. Ground temperatures are nearly equal to the average annual air temperature at depths of 12 ft or more, providing a constant-temperature heat source and sink.

Horizontal GHEX systems have either a shallow trench, typically 4 to 8 ft deep, or are horizontally bored. Trenched systems can contain several different piping arrangements, such as single-pipe, multiple-pipe (2-, 4-, or 6-pipe arrangements), or spiral-pipe. Horizontally bored systems are drilled at depths greater than 12 ft and are grouted similar to vertical GHEXs. Due to their shallow depths, horizontal GHEX systems are subjected to seasonal ground-temperature swings and varying soil thermal properties, which result in lower system performance. Table 1 provides a summary of the advantages and disadvantage to consider when selecting between a vertical or horizontal GHEX.

The decision between a vertical or horizontal GHEX system is a multifaceted process. There are many factors to consider:

  • Land availability.
  • Required system capacity.
  • Excavation costs.
  • Drilling costs.
  • Labor costs.

Figure 2: This picture shows the dedicated outdoor-air system (DOAS) units used to precondition outdoor air before delivering fresh air to the individual heat pump units. Courtesy: Stanley ConsultantsVertically bored systems typically require less land area than a horizontal system. Available land area at the project site will allow the designer to make an initial determination between the two ground heat-exchanger systems. For most projects, the decision between vertical or horizontal GHEXs is based on economics. Based on the combination and evaluation of the above parameters, borefield economics typically show that horizontal systems are cost-effective for systems with capacity requirements of less than 15 tons. Horizontal systems larger than 15 tons are often costly due to excavation and backfill costs. Vertical systems are typically used for systems larger than 15 tons.

Designing GCHP systems can take several iterations and design paths based on owner input, ground thermal conductivity, codes, budgetary constraints, and which energy-rating program (U.S. Green Building Council LEED or Energy Star) used. Commercially available ground heat exchanger design software programs, such as GLHEPRO, GSHPCalc, and LoopLink PRO, have been developed to aid with the design of the ground heat exchanger and to allow designers to evaluate alternative designs. These programs are specifically programmed to analyze the effects of borehole spacing, unbalanced design loads, and soil properties to determine the required ground heat exchanger loop lengths. Use these six design strategies when designing ground heat exchangers.

1. Determine ground properties

Soil composition, undisturbed soil temperatures, thermal conductivity, and allowable drilling depths are completely out of the designer’s control. These properties are estimated using one or more of the three available methods. The first method is to estimate the information based on “local information” and “best guess” of the soil composition. For obvious reasons, this is the least accurate method and is not often used. The second available method is to obtain soil-formation information from local drilling logs, discussions with local drilling contractors, or geological reports; the engineer then estimates soil properties from soil thermal property tables using a weighted-average process. This method provides a more reasonable estimation of soil thermal properties and is typically used for horizontal GHEXs.

The best and most accurate method to estimate soil thermal properties is to perform a formation thermal conductivity (FTC) test. Vertical GHEXs require a more accurate estimation of the site ground thermal properties due to installation costs and the system effects of oversizing or undersizing the GHEX. International Ground Source Heat Pump Association 2016 Standards and ASHRAE 2015 Handbook—HVAC Applications provide recommended FTC test procedures and specifications.

In short, a test borehole is drilled to an estimated design depth and a drill log is used to record the soil and rock types encountered as the drill progresses. A vertical U-bend loop is installed and grouted in place. The test loop is connected to equipment designed to introduce a controlled amount of heat to the loop, and then temperatures are recorded over a 36- to 48-hour period. The data gathered is used to determine the undisturbed soil temperature, average thermal conductivity, and average thermal diffusivity. The test loop is capped and later incorporated into the GHEX. The soil information is input into a ground heat exchanger design software package to determine the required loop lengths.

A bonus feature to conducting a FTC test is the production of drill log reports. The FTC report and drill logs are often incorporated within the construction documents to provide contractors with important information about the efforts that will be required to install the GHEX formation. This information may alleviate bid uncertainties and reduce the vertical GHEX bid price.

2. Grout thermal conductivity effect

Figure 3: This ground heat-exchange borehole detail depicts the installation requirements of the vertical borehole loop. Courtesy: Stanley Consultants

Grout conductivity is the major controlling factor when determining the required vertical borehole length. Soil and the U-bend pipe thermal conductivity are predefined parameters and beyond the designer’s control. However, the designer has direct control over the specification of the grout thermal conductivity. Grout is the pathway or roadblock to heat transfer between the pipe and the ground. Therefore, it makes sense to increase the thermal conductivity (reduce the thermal resistance) as much as economically reasonable.

As the grout thermal conductivity is increased, the overall thermal resistance of the ground heat exchanger is reduced. As a result, the GHEX can deliver or remove the same amount of heat with shorter boreholes or fewer boreholes. Nevertheless, the trade-off between increased grout conductivity and reduced borehole design lengths follow the law of diminishing returns. Eventually, the design borehole lengths or number of boreholes will be dictated by the soil thermal conductivity, and increasing the grout thermal conductivity will not result in a decrease in the borehole lengths or the number of boreholes.

GHEX designers will need to perform an economic analysis to determine which grout thermal conductivity will result in the optimal performance for the best value, given the local conditions. Typically, grout thermal-conductivity analyses start with conductivities of 0.88 to 1.00 Btu/hrxftx°F. Many thermally enhanced grout manufacturers have developed grout-volume calculators to assist designers with their economic calculations.

3. Outdoor air strategies

Figure 4: A mud rotary drilling rig is used to bore the 175-ft-deep vertical boreholes for this ground heat exchanger. Once the correct depth is reached, coiled premanufactured vertical loops will be fed into the hole and grouted in place. This procedure wOften, the GCHP system is designed for the building’s total peak HVAC load. Careful consideration should be given to this approach if available land is limited or budget is a concern. Outdoor-air (OA) peak loads can account for a significant portion of a building’s conditioning loads. OA loads are calculated at peak occupancy, a load that the building rarely experiences. These high incidental loads can result in an oversized and inefficient heat pump system.

Oversizing the heat pumps could cause short cycling, which in turn can cause early equipment failure, humidity-control issues, and temperature-control problems. Also, installing a ground heat exchanger sized for the total peak load may be a higher project cost than treating the OA with a separate system. Generally, high-capacity requirements and low run time will result in less-than-desirable payback for GCHP systems.

Systems with high OA conditioning loads are usually designed with dedicated outdoor-air systems (DOAS), energy-recovery ventilation (ERV) systems, or heat-recovery ventilation (HRV) systems, dependent on building size and OA requirements. Typically, these OA-treatment systems are completely decoupled from the ground heat exchanger, which has its own heating and cooling systems, so the OA loads do not affect the ground heat exchanger sizing and performance. However, when the OA load is relatively small and constant, the fresh-air load may be added to the ground-source heat pump system. Several heat pump manufacturers offer heat pumps with integrated ERV or HRV options.

An alternative option to installing a separate heating and cooling system for the DOAS would be to install a water-to-water heat pump (WWHP) to serve the DOAS. This approach will require analysis to determine if it will provide an acceptable performance and economic benefit when compared with air-cooled chillers and hot-water boilers that serve the DOAS. The WWHP alternative should still incorporate an energy-recovery process to precondition the OA to reduce the load on the GCHP system.


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