Closed loop systems are the second most common type of geothermal source. Closed loops utilize polyethylene piping buried or drilled into the ground, filled with a water/anti-freeze solution. The loop fluid circulating in this closed piping system absorbs heat or rejects heat into the surrounding earth. Closed loops are virtually maintenance free, and since they re-circulate the same fluid, have no need for a well or a place to discharge the water.
Horizontal earth loops are used where the space allowed for the loop is not extremely limited. There are various designs of horizontal loops. There is not one type of horizontal loop that is best for every application. The selection of which type to use should be based on system size, space available, soil conditions and the type of excavating equipment used. Regardless of the type selected, operating costs will not vary substantially.
Here are a few pictures of a horizontal loop installation we did in the Adirondack Park.
Horizontal directional drilling (HDD) is becoming a much more common method for the placement of geothermal ground heat exchangers. Horizontally-drilled systems mimic vertically-bored applications in almost every aspect (including the application of grout) except that they are horizontal. The method is often simply described as “a vertical system set on its side”…which also means that it will generally require a much bigger lot size than a vertically-bored–or even horizontally-trenched–system, since everything must fit horizontally beneath the property.
Minimum length required is approximately 225 feet of U-bend loop per nominal ton of heat pump capacity at minimum depth and spacing of 15 feet…although on a smaller lot it is possible to drill two or three shorter holes and combine them as one–or even stack them vertically (e.g., at horizontal depths of 15, 30, and 45 feet). On larger properties where much longer HDD boreholes can be drilled, it is possible to utilize a smaller number of holes to achieve sufficient geothermal capacity.
One benefit of horizontally-drilled systems over other methods is that they can be installed under structures, lawn and garden obstacles, play fields, etc., without disturbing those existing structures. This often provides access to ground heat exchange areas that would not otherwise be available. Horizontally-bored systems can also be installed at lower cost in areas where depth to rock is shallow and the economics of drilling into the rock or drilling shallow boreholes to stay above the rock make a vertically-bored system more cost prohibitive.
Here are a few pictures of an installation we did in the Adirondack Park in near Whiteface Mountain.
Vertical loops are used where space is limited or where soil conditions are not conductive to horizontal loops. Installing vertical loops requires the use of a drilling rig. Multiple holes are bored. A double pipe connected with a U-bend is inserted into each hole. The hole is then filled with grout to provide good contact around the pipe and to seal the hole. Vertical pipes are then connected to a header system horizontally a few feet below the system.
Vertically-bored ground heat exchange systems require the least amount of surface area for buried closed-loop GHEX applications. They are typically the most expensive of all of the closed-loop options, but are sometimes the only one feasible depending on available space, site geology, and system design requirements. While vertical GHEX designs may vary widely, a general rule of thumb is to utilize one borehole per nominal ton of GHP capacity drilled 150 to 250 feet deep, with 15 to 25 foot spacing between boreholes. Longer drilling depths are possible to reduce the number (or spacing) of boreholes…and shorter ones in greater numbers may be used if shallower drilling conditions require it.
Most commonly, a single loop of pipe with a U-bend at the end is placed down the length of each borehole, which is then back-filled bottom-up with a special grout to enhance conductivity and seal it against aquifer erosion. Multiple U-bend pipes per hole are also possible if some additional thermal capacity is needed due to certain site limitations. Each vertical pipe is then connected to a horizontal header piping system, which is buried 6 to 8 feet underground with supply-return pipes to and from the GHP.