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 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.

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).

Open-Loop System Designs

An open-loop system uses surface water or well water as its heat exchange liquid, which circulates directly through the system. After one pass, the water is returned to the ground via a well, surface discharge, or recharge well. This option is clearly feasible only where an ample supply of reasonably clean water is available. All local regulations and codes pertaining to groundwater discharge must also be followed (Energy.Gov, n.d.).

Open-loop GHPs can employ several techniques to dispose of used water. One method is surface drainage, in which water is deposited into a river, pond, or other low-lying area. A second method is re-injection, in which water is pumped back to its source via a separate discharge well. When returning water to the ground, care must be taken not to generate pollution. The only difference between the water entering and leaving the GHP should be a slight temperature variation (Lund, 2001; Energy.Gov, n.d.). Before installing an open-loop system, it is essential to confirm that the water source contains sufficient water to power the GHP. While wells generally do contain adequate water, they may deplete a neighboring well's source. Local contractors should be consulted to verify that sufficient water exists for open-loop GHP installation.

This system has the simplest configuration of all. Successfully used for many decades, the system operates as follows: groundwater drawn from aquifers passes through the heat exchanger of the pump and is subsequently discharged. After the water exits the building, it may be disposed of by one of the following methods (Ohio Water Resources Council, 2012). Note that local regulations and codes may restrict available discharge techniques.

An open-loop system typically includes at least one supply well and at least one discharge, diffusion, return, injection, or recharge well. In these systems, the supply well draws groundwater from an aquifer and pumps it to a heat pump, where it serves as a heat sink or heat source during the cooling or heating process. After passing through the pump, the groundwater is returned to the aquifer via the injection well. The only difference between the original and returned water is a temperature change (Kavanaugh & Rafferty, 1997). Generally, a flow rate of two to three gallons per minute per ton is required for efficient heat exchange. Because groundwater temperature remains nearly constant throughout the year, open-loop systems remain a commonly adopted option in areas where their use is permitted.

These systems are not used as frequently as closed-loop GHPs; however, where ample groundwater is available, they can be deployed cost-effectively. Local environmental authorities should be consulted before installation, as in some areas such systems may be partly or entirely governed by local regulations, agreements, or licensing requirements. Poor water quality can cause serious problems in an open-loop system (Idaho Geothermal, n.d.). Iron content, hardness, and acidity should all be tested before heat pump installation. Poor water quality can lead to mineral deposit accumulation inside the pump's heat exchanger, which may require periodic cleaning.

An open-loop GHP causes no harm to the environment; the only difference between supplied and re-injected water is a mild temperature change. One key design consideration is the distance between the production well and the injection well. Complete prevention of water flow between the two wells is not essential; however, flow between them must be low enough that discharged water reaches the supply well at nearly the same temperature as the aquifer's ambient temperature (Idaho Geothermal, n.d.; Rafferty, 1995). Typically, wells are spaced 200–600 feet apart. This spacing depends on the natural aquifer flow rate and thickness, the maximum system heating or cooling load, and the standard maximum load duration. Failure to address this critical design factor can cause undesirable temperature rise in the aquifer, promoting the growth of organisms that increase incrustation and bio-fouling.

Conclusion

An analysis of both systems reveals that open-loop systems require closer monitoring in the weeks immediately following installation. Regularly checking new systems is good practice. Flow meters should be inspected to verify correct water flow rates, particularly during spring when water sprinklers are also in use. Installation costs are another important consideration. Having access to a well does not automatically make an open-loop system the most economical option. Cost savings also depend on finding an effective means of discharging water — for example, digging a large hole with a backhoe and backfilling it with washed rock. While this approach works well in sandy soils, it can still cost approximately $1,500 in labor and equipment time. Geothermal heat pump systems, whether open or closed loop, ultimately deliver efficiencies that exceed all other currently available heating and cooling alternatives, making them a sound long-term investment for both residential and commercial applications.

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.

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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

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