By Jim Ashley and George Harvey
From the point of view of general economy, there is no more efficient way to heat and cool most buildings than with heat pumps. The most efficient heat pumps are geothermal, which move the heat from or to the Earth’s crust, partly because the temperature underground is constant at about 50° F. The efficiency of such systems is heavily dependent on the ground loop, the heat exchanger in the ground.
Nearly all ground loops in the Northeast use water as the heat transfer fluid. The fluid absorbs underground heat, delivers it to the geothermal equipment so the heat can be extracted from it, and then is returned to the earth to get more heat for the next round trip. A few of the systems use refrigerant directly in copper pipes, and are called DX systems.
The most effective ground loops use vertical drilled holes. An example is at Champlain College in Burlington, VT, which started with two high yield wells. Water was drawn from one well to supply its heat to the geothermal equipment in Roger H. Perry Hall, and then it was returned to the aquifer in the other well. This is referred to as an “open loop” system. The heating of the supply water is done by the aquifer.
A second effective type of open ground loop is called a “standing column well” (SCW). In this open loop system the water is returned to the well for rewarming by conductive heat exchange. As the well gets chilled with heavy usage it can be refreshed by bleeding off about 10% of the return water causing new, ground-temperature water to be supplied by the aquifer. The SCW can be the same water well that supplies water to the building, as the quality of the water is not changed. More than 50,000 potential wells that can be used for SCWs have been identified in the Vermont Energy Atlas.
A single SCW can often supply an ordinary home or business with all the heating and cooling it needs. Some wells, however, need to be deepened to provide sufficient additional heat exchange for the size of the geothermal equipment.
In some cases, multiple deep, large diameter SCWs are needed for a project. The 235,000-square-foot Merrimack County Nursing Home, in Boscawen, New Hampshire has over a dozen. The SCWs supply a total of 326 geothermal heating and cooling units, keeping the temperatures in the nursing home just right!
Geothermal designers also use what are termed “deep no-yield wells.” These wells are most common in towns like Charlotte, Vermont, where the shale-like rock produces little or no water, and homeowners often resort to drilling multiple wells to find it. When this happens, one of the abandoned wells can then be filled with water and used as an SCW without removing water for other purposes. As they cannot be refreshed, they must be deeper for the same annual heating load than a regular SCW.
A third common type of earth coupling is a “closed loop system.” The technology for these has advanced considerably in the last few years. Multiple vertical holes are drilled, and a loop of plastic pipe is put in each hole. The holes are then sealed with special conductive thermal grout. Conductive heat transfer is through the grout and plastic pipe, through which the fluid is pumped. Major disadvantages are the thermal resistance of the plastic pipe, the loop field cannot be refreshed, and larger equipment must be used in comparison to that needed in a similar SCW installation. Large projects, however, can put fields of closed loop wells to effective use, with examples at St. Michael’s College in Burlington and Stowe Resort.
The designers at Green Mountain Geothermal saw that if the rock fractures in a standard SCW could be sealed with cement or a sleeve, the well would be isolated from any aquifer problems. Instead of installing a closed-loop earth coupling, a more efficient, lower cost open loop system that does not require water to be bled off could be installed. With a hope of broadening availability of geothermal options, Green Mountain Geothermal applied for and has received a 2016 Green Mountain Power CEED grant to install and test ten sealed no-bleed SCWs in the former CVPS service area, which GMP purchased a few years ago. Please see the solicitation ad on this page. Inquiries are appreciated.
Jim Ashley has a degree in hydrogeology and worked for most of his 26 years with the VT Agency of Natural Resource as the well drilling specialist in charge of Vermont’s Well Driller Licensing program. In 2003, after retirement, Jim formed Green Mountain Geothermal, LLC, a geothermal consulting business. In July of 2015 Jim also assumed the presidency of the New England Geothermal Professional Association (NEGPA).