Amid the current push to “go green” by building high-efficiency, environmentally sound, sustainable HVAC systems, heat pumps — particularly those that use the earth's geothermal energy to serve as a heat sink and source — are generating a lot of buzz. Ground-source heat-pump (GSHP) systems operate like every other HVAC system: They move energy from one place to another. Although that sounds simple, HVAC engineers and designers owe their livelihoods to the complexities.

In most cases, GSHP systems are a suitable green solution, providing sustainability and energy efficiency. They reduce the amount of heat rejected into the atmosphere during cooling periods and decrease the amount of heating energy required from local utility companies — a win-win situation. However, popularity sometimes can lead to improper applications. This article will discuss various elements important to sound design and installation.

Because this article can only scratch the surface of the myriad conditions engineers need to consider before installing a GSHP system, references are included for further study. Additionally, for the purposes of this article, the term “GSHP” refers to a variety of heat-pump systems that use geothermal energy but utilize a variety of technologies. For brevity, the article will focus on ground-coupled systems.

BACKGROUND

For the last five decades, designers have used water-source heat pumps in commercial buildings successfully. However, geothermal pumps are a more recent application. During the oil embargo of the 1970s, builders became more concerned with alternative HVAC solutions, such as air-to-air heat pumps. While the heat pumps provided excellent efficiency at temperatures above 15°F to 20°F, defrost cycles resulted in ice formations during subfreezing temperatures. Below 15°F, their efficiency was equivalent to or less than that of resistance-heat equipment.

Alternately, geothermal energy proved to be an efficient and popular solution because ground temperatures remain constant, making it easier to extract heat during winter and cool water in summer. Additionally, GSHPs are equipped for small temperature-control zones and varying weather conditions. They require minimal chemical treatment and little service once installed.

GSHPs were developed primarily for use in the residential and light-commercial marketplace, but engineers dealing with energy-efficiency concerns and “green” objectives soon adapted them for larger buildings. GSHPs began moving into suburban markets and currently are in the high-density environs of major cities. Installations have moved from regional locations to national levels. Now, not only are GSHPs in larger buildings, but in applications across more climatic zones.

Designers have been quick to accept GSHPs, but as applications expand, so do design concerns. To that end, designers are experiencing a new challenge: geology. With GSHPs, HVAC engineers have had to move past the traditional 5-ft-from-the-building-line mandate into a world of drilling and bore-hole heat exchangers. These have become key elements of the geothermal-heating concept.

Currently, pure GSHP systems are being used in larger buildings successfully. However, there are limitations, meaning hybrid solutions sometimes are required. A pure GSHP system is bound by the amount of land available for the ground loop — the portion of the system that connects with the ground for heat transfer — and the viability of creating a sufficient bore field. However, hybrid systems also can be used in larger buildings successfully. Closed-circuit fluid coolers (cooling towers) can meet the heat-rejection needs of larger cooling-load applications, while, in a similar fashion, a heat-driven application may supplement capacity with a boiler or solar array.

GSHP systems typically are not installed in zones with design temperatures well below 0°F and zones that experience more than 7,000 degree-days. Cooling requirements can be met by lowering ground temperatures with a hybrid GSHP system that includes booster water heaters to optimize heat-pump efficiencies and give startup capacity after night setbacks.