Geothermal Heat Pump Open Loop vs. Closed Loop Systems

Introduction to Geothermal Heat Pumps

Geothermal heat pumps (GHPs), sometimes called GeoExchanges, are earth-coupled, water-source, or ground-source pumps that have been in use since the latter part of the 1940s. They employ the earth's relatively constant temperature — rather than external air temperature — as their medium for heat exchange. This enables GHP systems to achieve quite high efficiencies, ranging between 300% and 600%, on extremely cold winter nights, compared with air-source heat pumps, which achieve only 175–250% efficiency during winter (Energy.Gov, n.d.). While many regions experience extreme seasonal temperature swings — from sweltering summer heat to below-freezing winter temperatures — just a few feet beneath the earth's surface, ground temperature remains fairly stable year-round. Depending on latitude, ground temperature ranges between 7°C (45°F) and 21°C (75°F). As in caves, ground temperature is higher than the air above it during winter months, and cooler than the air during summer. GHPs exploit this phenomenon by exchanging heat energy with the ground through ground heat exchangers.

Despite the high installation costs associated with geothermal systems compared to air-source systems of identical cooling and heating capacity, those additional costs are typically returned to the buyer through energy savings over the subsequent 5–10 years. The system's estimated lifespan exceeds half a century for ground loops and roughly half that for internal components. A ground loop system can be either open loop or closed loop. The former uses one or more wells, while the latter comes in three configurations: pond/lake, horizontal, and vertical. Which proves most effective depends on soil conditions, climate, local installation expenses, and available land. Both approaches may be applied in residential and commercial buildings (Energy.Gov, n.d.).

Closed-Loop System Designs

The majority of closed-loop GHPs circulate an antifreeze solution through a plastic closed loop that is buried underground or immersed in water. Heat exchangers transfer heat between the antifreeze solution in the closed loop and the refrigerant in the heat pump. The loop may be configured in one of three ways: pond/lake, horizontal, or vertical (Goldscheider & Bechtel, 2009). A variant of this approach — known as direct exchange — pumps coolant directly through copper tubing buried underground in a vertical or horizontal configuration, bypassing heat exchangers. Such systems require a larger compressor and perform best in moist soil conditions, sometimes requiring additional irrigation to maintain soil moisture. However, direct exchange systems should not be installed in soils that can corrode copper tubing. Because they circulate coolant through the ground, some local environmental regulatory authorities may prohibit their use.

Horizontal Closed Loop

Horizontal installation is often the most economical option for residential complexes, particularly new constructions where ample land is available. It requires digging trenches approximately four feet deep. The most common layout uses two pipes, one buried six feet deep and the other four feet deep. Another common layout places two pipes side by side at a depth of five feet within a two-foot-wide trench. The Slinky pipe-looping method allows more pipe length to be incorporated into a narrower trench (Energy.Gov, n.d.), reducing installation expenses and making horizontal installation feasible in locations where traditional horizontal approaches would not work.

Vertical Closed Loop

Schools and large commercial buildings typically use vertical loop systems because of the prohibitive land area that horizontal loop installation would require. Vertical loop systems are also used in locations where the ground is too shallow for trenching, and they minimize disturbance to existing landscaping. For vertical installations, holes approximately four inches in diameter are drilled to depths of 100–400 feet, spaced roughly 20 feet apart. Two pipes are lowered into each hole and connected at the bottom with a U-bend, forming a loop. Vertical loops are then connected using horizontal manifold piping set in trenches and attached to the building's heat pump (Energy.Gov, n.d.; Kavanaugh & Rafferty, 1997).

References

Energy.Gov. (n.d.). Geothermal heat pumps. U.S. Department of Energy.

Goldscheider, N., & Bechtel, T. D. (2009). Editors' message: The housing crisis from underground — damage to a historic town by geothermal drillings through anhydrite, Staufen, Germany. Hydrogeology Journal, 17(3), 491–493.

Idaho Geothermal. (n.d.). Open loop vs. closed loop.

Kavanaugh, S. P., & Rafferty, K. (1997). Ground-source heat pumps. ASHRAE.

Lund, J. W. (2001). Ground-source (geothermal) heat pumps. European Summer School on Geothermal Energy Applications, University of Oradea, Romania.

Ohio Water Resources Council. (2012). State of Ohio: Recommendations for geothermal heating and cooling systems. http://wwwapp.epa.ohio.gov/ddagw/SCCGW/Documents/GHCS.pdf

Orio, C. D., Johnson, C. N., Rees, S. J., Chiasson, A., Deng, Z., & Spitler, J. D. (2004). A survey of standing column well installations in North America. ASHRAE Transactions, 110(4), 637–655.

Rafferty, K. (1995). A capital cost comparison of commercial ground-source heat pump systems. Geo-Heat Center Quarterly Bulletin, 16(2).

Rafferty, K. (2004). Water chemistry issues in groundwater heat pump systems. ASHRAE Transactions, 110(1), 550–555.

Rafferty, K. (2008). Design issues in commercial open loop heat pump systems. ASHRAE Transactions, 114(2), 316–327.

Rezaei, B., Amir, K., Dargush, G. F., & Weber, A. S. (2012). Ground source heat pump pipe performance with tire-derived aggregate. International Journal of Heat and Mass Transfer, 55(11–12), 2844–2853.

Rybach, L., & Sanner, B. (1999). Ground-source heat pump systems — the European experience. Proceedings of the International Summer School, GeoHeat Center, Klamath Falls, OR.