Choosing the Right Heat Sink for a Geothermal Heat Exchanger

According to the US Department of Energy the most effective type of heat pump is the geothermal heat pump, GHP, more accurately called ground-source heat pumps. These systems have been proven capable of producing large reductions in energy use in buildings. A study by the U.S. Environmental Protection Agency (EPA) comparing the major HVAC options for residential applications determined that GHPs were the most energy efficient and environmentally benign option. The overall performance of these systems will be to a large extent determined by selecting the ideal heat sink for your site and sizing your heat sink and system correctly. Only about 60,000 GHP units are installed each year in the combined US new built and retrofit market. The number is so small because the market is fragmented, the total systems are difficult to understand, the installation costs appear to be about twice the costs of less efficient system, and the distribution and installation network is inefficient. Better understanding of all elements of these systems is the first step in choosing a GHP.

In winter GHPs collects the Earth's natural heat from a heat sink, typically that is through a series of pipes, called a loop, installed below the surface of the ground or submersed in a pond or lake. Fluid circulates through the loop via an electric pump and carries the heat to the house. There, an electrically driven compressor and a heat exchanger concentrate the Earth's energy and release it inside the home at a higher temperature. Ductwork distributes the heat to different rooms. In summer, the process is reversed. The underground loop draws excess heat from the house and allows it to be absorbed by the Earth. The system cools your home in the same way that a refrigerator keeps your food cool by drawing heat from the interior, not by blowing in cold air.

As you may have experienced in a cave, the temperature at least six feet beneath ground surface is cooler in summer and warmer in winter than the ambient temperature. Using this temperature as its source the GHP can operate within its most efficient range for heat transport. The coefficient of performance or COP, of a GHP (or any heating and/or air conditioning unit) is essentially a measurement of the amount of work (either electricity or other source of power) necessary to change the temperature in the desired direction. A theoretical maximum achievable COP would be 7.8 on the usual design parameters. Test results of the best systems are above 5.0. However, when looking at cost savings on an installed unit this savings would be reduced by the energy needed to operate the pumps for the water and/or antifreeze through the piping systems.

The typical GHP has a water loop heat sink installed either horizontally or vertically. Horizontal ground loops are usually the most cost effective when trenches are easy to dig, the size of the yard is adequate and there is adequate soil. Workers use trenchers or backhoes to dig the trenches six feet below the ground in which they lay a series of parallel plastic pipes or loops. They then backfill the trench. Fluid runs through the pipe in a closed system. A typical horizontal loop will be 400 to 600 feet long for each ton of heating and cooling.

The vertical loop is used where there is little yard space, when surface rocks make digging impractical or the heat exchange properties of the rocky soil are unacceptable, or when you want to disrupt the landscape as little as possible. Vertical holes are typically 150 to 450 feet deep and contain a single loop of pipe with a U-bend at the bottom. Each vertical pipe is then connected to a horizontal underground pipe that carries fluid in a closed system to and from the indoor exchange unit. Vertical loops are generally more expensive to install than horizontal loops, but require less piping than horizontal loops because the Earth's temperature is more stable farther below the surface. The performance of these heat sink systems can also be enhanced by the presence of groundwater to improve the thermal transport properties of the water loop.

These heat sink systems whether vertical or horizontal have two loops: the primary refrigerant loop is contained in the GHP cabinet where it exchanges heat with a secondary water loop that is buried underground. They are thus called water loop systems. The secondary loop is generally HD polyethylene pipe and contains a mixture of water and an anti-freeze. After leaving the GHP, the water flows through the secondary loop outside the building to exchange heat with the ground before being returned to the building. In the extremes of weather, they efficiency of the GHP system can be negatively impacted by the water in the secondary loop’s ability to loose or gain enough heat.

The secondary loop is placed deep enough below the surface (and frost line) that the temperature remains relatively stable. Siltstone, balls bluff and other rocky soil are a poor medium for temperature transport and HD polyethylene is not an efficient heat transport medium. Loop systems installed in wet ground or in water are significantly more efficient than drier ground loops since it easier to move heat in water than air, solids, sand or soil because there is better surface contact.

A variation on the ground loop duel system is the direct loop system. The refrigerant leaves the GHP cabinet, and is pumped directly through a loop of copper tubes buried underground, and exchanges heat with the ground before returning to the pump. The name "direct exchange" refers to heat transfer between the refrigerant and the ground without the use of an intermediate fluid. Heat transfer takes place through the piping wall. It is best to use an environmentally friendly anti-freeze like denatured alcohol or methane because all these systems will someday fail and a simple and easy remediation should be planned in advance. Direct exchange heat pumps are not to be confused with “water loop heat pumps" since there is no water in the ground loop. Ground loop heat pumps have a higher efficiency than the water loop systems.

A groundwater or open loop system is the most efficient in terms of heat transport. These systems were originally designed to pump natural water from a well or body of water into a heat exchanger inside the heat pump. Heat would be either extracted or added depending on the season, and the water is returned to a separate injection well, or body of water. The supply and return lines must be placed far enough apart to ensure thermal recharge of the source. These systems were subject to limescale clogging or fouling if the water contains high levels of salt, calcium carbonate, iron and or magnesium, iron bacteria or hydrogen sulfide. In addition, these open-loop systems may drain aquifers or contaminate wells.

A standing column well system is a specialized type of open loop system. According to Dr. Zheng Dong O’Neill who has done extensive research on the topic, though there were only about 1,000 standing column systems installed in the United States in 2005 these systems utilize a shorter borehole depth and have more stable water temperature so they are cheaper to install than a vertical loop system and are more efficient than the water loop systems. In standing columns water is drawn from the bottom of a deep well, passed through a heat pump, and returned to the top of the well. Heat is lost or gained from the water as it travels back down the well. This system because it utilizes direct water contact for the heat sink (which is really the bedrock) is more efficient. In addition, because the water is returned to the well there is limited impact on the groundwater system. A high density of these groundwater systems could potentially impact a groundwater basin.

The standing column system is ideal where the bedrock is near the surface and the soils are rocky so that the well casing required is less than 60 feet. The standing column well method is popular in residential and small commercial applications in the New England states and should be ideal for the geology in my backyard. I have a strong shallow aquifer that runs from Bull Run Mountain to the river behind my land. There are acres of woods between the house and the yard (that I own) to allow the aquifer to regain its natural temperature profile. My land is down gradient of all my neighbors with the river serving as a hydraulic barrier to the basin. Any additional wells I install down gradient of my own drinking water well will not impact any other homes. Access to the well location for the well drilling equipment is an important factor. Well drillers tend to want to locate the well where it is easiest to drill. Make sure that the location will not impact drinking water wells, and that the trenching for the connection to the unit in the house is at least six feet deep. If that trench is too long and too shallow you defeat any benefit of the geothermal system. This is also true for the vertical loop wells.

If flow and sizing are properly done, this type of groundwater system can potentially have some heat storage benefits. The idea is that heat is rejected from the building will raise the temperature of the well by a few degrees during the Summer cooling months can then be harvested for heating in the Winter months, thereby increasing the efficiency of the heat pump system. Of course if the temperature of the well is raised too much in the summer the efficiency of the cooling system just falls. As with closed loop systems, sizing of the standing column system is critical. The heat exchange in this system is actually with the bedrock, using groundwater as the transfer medium. A large volume of water flow is not required for a standing column system to work and the water is returned to the well so the net effect on the groundwater table should be negligible. However, if there is adequate water production and the groundwater is well recharged, then the thermal performance of the well system can be increased by discharging a small percentage of the system water flow during the peak summer and winter months.

Since this is essentially a water pumping system, standing column well design is the most difficult to obtain peak operating efficiency. The well designer and driller needs to be fully trained and certified in GHP systems to ensure that the system is properly designed and includes all essential design elements including system shut-off valves to install a system that operates at maximum efficiency. Since the number of national installation has been so limited, experienced well drillers are difficult to find. The American Society of Heating, Refrigerating and Air Conditioning Engineers, ASHRAE, has funded Dr. O’Neill’s research to model standing column wells in GHP systems. Finding a well driller who has been certified in GHP by ASHRAE is a place to begin.