Introduction to Water Source Heat Pump Systems Part 5: Geothermal Systems

By Chad Edmondson

Commercial water source heat pump systems don’t necessarily have to rely on boilers and cooling towers to add or remove heat from the building loop. They can actually use the earth or a nearby water source (e.g. a pond) instead. In such cases, the WSHP system transfers heat directly to or from the earth or water source. In the HVAC world, these systems are typically referred to as “geothermal” or “ground source” heat pumps. If the building location allows this for this approach, a geothermal system can really amp up your efficiency game and offer some other advantages as well.

Efficient and Environmentally Friendly

Geothermal systems typically have the lowest operating cost of any WSHP system. That’s because the earth maintains fairly stable ground temperatures (approximately 62°F year-round here in our part of the south), whether the outdoor air temperature is 5°F or 100°F. Geothermal heat pumps circulate water through a sealed underground piping loop where it is naturally warmed (or cooled) by the earth. Obviously, it is more efficient to extract heat from 62°F soil than 5°F air or to reject heat into 62°F ground than 100°F air.

Geothermal systems can reduce or even eliminate the need for a cooling tower and/or a boiler depending on the climate. As such, the entire system (not just the heat pump itself) can be entirely carbon-free, another reason why geothermal systems are considered in green building projects.

Geothermal systems are mostly protected from the elements, so they are also extremely long-lasting and low maintenance. Ground loops can, and often do, last a lifetime. Most or all of the equipment is totally hidden from view and operationally very quiet.

Loop Field Options

Figure 1 - Vertical Ground Loop

The only caveats to geothermal heat pumps systems are that (1) not every building lot has suitable acreage for a loop field and (2) installing the loop field can be expensive.

Vertical loop fields can be especially expensive since they require drilling deeply into the earth to allow for sufficient heat transfer surface between the earth and the hydronic loop (Figure 1). The deeper the well fields, the more costly and potentially problematic the installation can be, depending on the geological make-up of the site.

Figure 2 - Horizontal Coiled Loop

Horizontal loops are usually just a few feet from the surface and be installed in trenches without engaging the services of well drillers and are thus less expensive to install. But not every building has sufficient land for horizontal well fields. A coiled or “slinky” loop field configuration can help minimize the lot space required for a horizontal installation.

If there is a nearby pond or other water source, an owner may be able to save installation costs by floating and sinking the piping into water instead of the earth and avoid drilling or trenching altogether (Figure 2).

Hybrid Can Be a Better Approach

Figure 3 - Hybrid Loop

In many cases, a hybrid approach, one that utilizes a ground loop and closed loop cooling tower is the best solution (Figure 3). Hybrid systems work well in buildings with load profiles that are distinctly cooling dominated. In the south, we typically size the well field for the heating load only and use a smaller closed-loop cooling tower to help offset any supplemental cooling needed during the summer. This lowers the installed cost of the system by reducing the field size.

Every building is unique, and most sites have their limitations. It is up to the engineer to help the owner decide which if any of the above solutions will work best.