Geothermal - Ground Source Heat Pumps

Geothermal Energy – Ground Source Heat Pumps

Ground-source heat pumps (GSHP) are an excellent way to improve energy performance over conventional systems. Originally used for residential applications, it is now being widely used for commercial buildings. In large buildings, GSHPs save not just energy, but water as well when they are used to replace cooling towers. The following article will explain coupled and direct-exchange systems, vertical boring and horizontal boring. Site characterizations will also be mentioned.

Coupled vs Direct-Exchange Systems

Ground-coupled heat pumps (GCHP) are a subset of a GSHPs and are often referred to as a closed-loop heat pump (see Figure 1 below). The most common ground-coupled system consists of a refrigerant-to-water heat exchanger so that the ground loop liquid is not directly used for hydronic heating or cooling. In the coupled system, the ground loop liquid is used in combination with compressors, reversing valves and other equipment to exchange its heat with a second, ‘coupled’ fluid that is used for the hydronic cooling/heating. The second type of system used in GSHPs is called a direct-exchange configuration heat pump (DXGCHP) where the ground loop liquid is directly used for hydronic cooling/heating.

Figure 1: Ground-coupled heat pump system

Vertical and Horizontal GSHPs

Ground-source heat pumps are further divided into how the ground loops are oriented. Both types can be used for different applications and have their pros and cons.

Vertical Ground Loops

A vertical system typically consists of two high-density polyethylene (HDPE) tubes. These are placed in vertical boreholes filled with a solid medium. The tube diameters range from 0.75 to 1.5 inches and extend 50 to 400 feet deep, depending on soil conditions and available equipment. Bores can go as deep as 600 feet or more if certain procedures are followed. The individual boreholes range from 4 to 6 inches in diameter.

Able to be laid out in a grid pattern or a single row, vertical ground loop arrangements can be adjusted to meet the plot contour and shape. In a grid, bores are recommended to be 20 feet apart to mitigate thermal interference between bores. This distance can be reduced to approximately 10 feet when a single row of bores are used. Distances can also be reduced with the following:

Annual ground load is balanced (energy released in ground equals energy extracted)
Water movement or evaporation mitigates effect of heat build-up

The following are advantages of a vertical ground loop system:

Requires relatively small plot of land
Contacts soil that varies little in temperature and thermal properties
Requires smallest amount of piping and pumping requirements
Most efficient system performance

Below are disadvantages of a vertical ground loop system:

Higher cost due to expensive installation equipment
Limited availability of contractors to install

Figure 2: Vertical ground loop system

Horizontal Ground Loop

Horizontal ground loops are made up of single-pipe, multiple-pipe, spiral and horizontally bored layouts. Single pipe configurations consist of narrow trenches at least 4 feet in depth. The drawback of the single pipes is that it requires the greatest amount of ground area.

Multiple pipes configurations consist of two, four or six pipes placed in a single trench. The length of these multiple pipe trenches can be less than a single pipe layout, but the total pipe length must be increased. This is due to the need to overcome thermal interference from adjacent pipes.

When horizontal bored loops are grouted and placed in deep earth, design lengths are near those for vertical systems. This is because annual temperature and moisture content remains relatively constant throughout the year.

Figure 3: Horizontal ground loop system diagram

Figure 4: Horizontal ground loop trench configuration

The following are advantages of a horizontal ground loop system:

Less expensive than vertical GCHPs
Trained equipment operators more widely available

The following are disadvantages of horizontal ground loop system:

Larger ground area required
Ground temperatures and thermal properties fluctuate creating variations in performance
Higher pumping-energy required
Lower system efficiencies

Site Evaluation

The site characteristics heavily influence the type of GSHP system that shall be used. Important issues are:

Presence or absence of ground water
Soil/rock temperature
Depth of rock
Rock type
Topsoil composition

The types of soil and rock allow a preliminary evaluation of the range of thermal conduct/diffusivity that might be expected. The thickness and nature of the topsoil (gravel, sand, clay, etc.) materials overlying the rock affect whether casing is required in the upper portion of boreholes and play a factor that can increase drilling costs.

After the initial site characterizations have been studied and a GSHP system has been chosen, specific details of the subsurface material are needed. These details include rock/soil thermal conductivity and diffusivity, pumping levels, rock depth, rock density and more. This information can come from geologic and hydrologic maps, state geology and water regulatory agencies and geotechnical studies at the site. A particularly good reference would be completion reports for nearby water wells. These reports are filed by the driller upon completion of the water well, providing a great deal of information.

For vertical bore systems, what is often referred to as a test well can be drilled if none of the information mentioned above is available. This test well is just a bore dug at the anticipated design depth. A report is produced afterward determining the soil depth, rock depth, rock density, rock thermal conductivity and more. If the results from a test well are not particularly promising, a new system type may be considered or a GSHP system may need to be reconsidered.

Expected Life Cycle

Ground-source heat pumps are a long-term investment. Indoor components (pumps, heat exchangers, etc.) usually last 25 years. The exterior components (bore piping, coil loops, etc.) can last nearly 50 years. However, an issue to consider is the thermal conductivity and temperature of the subsurface material throughout the life of the system. Eventually, the ground loses its efficient heat exchanging capabilities because it is constantly being used as a heat sink. The temperature of the subsurface material begins to increase permanently over time.

Conclusion

Ground-source heat pumps are a great way to save energy, water and ultimately money. Initial installation costs may be greater than a normal system, but the return on investment is something that needs to be taken into consideration. There are many types and subsets of GSHPs, more than what is mentioned in this article, so one considering the installation is not locked into a particular design. No matter what system is installed. the energy efficiency of your building will be immediately noticeable. Something every building owner desires.