Underground geo heat exchanger types (GHEX)
Underground heat exchangers for geothermal heat pumps (GHPs) are not placed until the heating loads of the buildings they’ll serve are known. To start drilling or digging and just “throw some pipe about” will not result in an effective installation. That would be more like covering the front your car’s radiator with cardboard at Death Valley in July. We need proper rates of heat exchange for our heat pumps to work effectively (or to save your car’s engine). When you want to use the earth as your thermal battery, that battery must have adequate capacity to export the heat you want to grab (HEAT SOURCE) or to import the heat you wish to dispose of (HEAT SINK). And like all batteries, the cabled connection needs to be of adequate size to transport this energy at the rate that you need it.
GHP ground loops are not rocket science (particularly in smaller buildings) but they do require planning and careful installation. There are many types we will describe and illustrate here, but they are all designed for the import or export of thermal energy in a GHP’s connection to the earth. While boreholes and the loops in trenches are where the heat exchange “action” takes place, the headers that supply and return these are critical features requiring some design work. Simple is often cost-efficient as well as elegant. There are installed systems that use too much loop pumping power, wasting electric energy that could have been saved with a smarter design. Proper testing and commissioning of systems is important so that the design proves itself at start-up.
UNDERGROUND HEAT EXCHANGER TYPES
There are two basic styles of GHEX delivery systems, closed loops or open loops. Closed loops are by far the most prevalent, and circulate the same fluid between the heat pump’s main heat exchanger and the underground regime—over and over. Open loops pump water from a well, pond, or lake in a once-through pass through the heat pump’s main heat exchanger. Where this return product ends up of can be a bit of a challenge (and the purity of this water is important in keeping the heat exchanger free of debris or biological contamination)—but it often represents a gain in efficiency (COP, Coefficient of Performance) over closed loop designs. Nearly all loops use HDPE (high density polyethylene) pipe and some use PEX (cross-linked polyethylene). There is an even rarer arrangement called DX (Direct Exchange), where the refrigerant loop itself makes contact with underground strata, but we will not cover that method here.
Grouted Geothermal Boreholes
A vertically-drilled bore of 4.5-to-8″ diameter is punched into the ground to a specified depth (based on access, the geology, conductivity, or choice). A U-bend loop of HDPE pipe is lowered to the bottom of the bore and a temporary line called a “Tremie” is sent to the bottom where it pumps a mixture of bentonite clay, sand, and water as it is retracted toward the surface. This mixture actually swells 5-to-8% against the borehole wall, helping to seal against aquifer cross-contamination or surface water penetration along the grouted
Multiple bores in the same bore field are usually separated 15-to-20 feet laterally to provide access to more of the formation and to minimize the chances of drilling drift into neighboring bores. Depending on access, the formation’s geologic characteristics, and the type of project, such bores can be as little as 150′ and as great as 600′ in depth. In the latter, dual U-bend loops are sometimes deployed.
Since the temperature underground does not change below 25 feet, once a grouted geothermal borehole is in place, the temperatures fed to the GHP’s ground loop heat exchanger will be nearly constant. For this reason, extreme climate locations such as North Dakota in winter and Las Vegas in summer will benefit greatly by such stable entering water temperatures.
Horizontal drilling of a GHEX is also possible, usually in smaller diameters, with or without grout depending upon soil conditions. Drilling rigs are lesser cost and have great ability to descend to advantageous depths under existing buildings and infrastructure (assuming all of that is known in advance). Horizontal has great potential to be deployed in GHP retrofits of existing buildings.
Trenched Installations (Straight Pipe, Racetrack, and Slinky® Configurations)
A shallower but still effective GHEX installation is the trench installation. The methods of digging can be a backhoe, a regular excavator, a mini excavator, or a chained trencher that digs a deep but narrow gash in the earth. In each case the make-up of the soil is more critical than vertical boring because of the variety of soils and their widely varying conductivity. If there is lots of room and trenching is not expensive, the straight pipe deployment method is common. When multiple pipes travel in parallel in the same plane, we call this racetrack configuration. When less GHEX territory is available, a more concentrated exchange method is possible. It’s called a Slinky® and its trademark derives not from a toy but from a unique method of arranging conducting pipe.
In one of the gallery photos (below), a jig is used to form Slinky® loops at a specific diameter and spacing. Diameter is easy to understand. Pitch may be something less comprehensible. It refers to the distance (in inches) between the 9 o’clock position of the loop you just secured to the 9 o’clock position of the next one you are placing. For example, a 36″ diameter loop at 36″ pitch would represent two loops touching at their 9 o’clock (loop on the right) and 3 o’clock (loop on the left) positions. The Slinky® loop illustrated in the jig below deploys over 9 feet of conducting pipe for every linear foot of trench. The six pipe method illustrated in the first slide deploys 6 feet of conducting pipe for every linear foot, and the upper three pipe runs are in a less effective, shallower location.
Slinky® fields of loops are also possible, but unless they are widely spaced between each, they are less effective than single, widely spaced runs (that are correspondingly more expensive to install).
Click any image to enlarge and advance
Going Liquid (and exchanging heat underwater)
In this case using the term “ground loop” is a bit of a misnomer, but heat exchange between two submerged liquids separated by conducting pipe is even better than under ground. That’s because the density of media increases when you go from air, to dirt, to liquid, and lastly to solid rock. It’s all about increased density (heat capacity) to improve conductive heat transfer. But in the case of liquids there is an additional advantage. Water is a good conductive media but it is also a convective one. That means th
at when you draw heat away from or back into
water it mixes itself convectively to involve the water that hasn’t yet been in touch with your loop ipe or heat exchange plate. The greater the difference in temperatures, the greater the rate of heat exchange between the loop pipe’s contents and the water body. Whole coils of loop pipe with pre-inserted spacers to improve contact can be placed on rafts in bundles. Also, loop pipes can be configured as Slinkies® and strapped to raft structures. In both cases they need positioning while neutrally buoyant and will sink to the pond or lake bottom when charged with water.
Spacing of the sunken raft off the pond bottom will keep mud and debris away from the loops, allowing them to interact better with the water medium. The earth at the bottom of the pond helps stabilize the pond’s water temperature. In the upper midwest, this is an important consideration, particularly as the surface of smaller ponds or lakes become covered with a helpful layer of insulating ice during very cold weather.
Slim Jim® Lake Plates
The newest member of the underwater heat conduction loop family is the solid, metallic plate exchanger, whose honeycomb is fed water across the inside between two metallic plates to transfer heat. These plates have been used in lakes, large decorative ponds for multi-residential GHPs as well as in busy lakes and harbors. When the lake is not deep enough or snagging of headers with anchor lines, fishing gear, or other tethers might be an issue, plates are suspended directly under dock walkways where they are safer. For salt water harbors, the normally stainless steel plates are replaced by Titanium because its metallic structure is not affected by either corrosion or sea life. When bodies of water are smaller and heat transfer capacity might be less certain, such ponds may incorporate central fountains which can cool the water body significantly.