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Geothermal Heat Pump Systems Are Becoming More Popular, So What’s the Problem? By Todd Giddings, Ph.D., P.G., IGSHPA Accredited Trainer Reprinted from the Fourth Quarter 2007 Pennsylvania Driller Newsletter of the Pennsylvania Ground Water Association As prices for heating oil, natural gas, and propane continue to rise, geothermal heat pump systems are becoming more popular because of their very high efficiency. When you can buy one unit of energy (a kilowatt-hour of electricity) and have your geothermal heat pump system deliver four kilowatt-hours of heat into your home or office building, that’s 400% efficiency! Closed loop (also called earth-coupled) geothermal heat pump systems often use vertical loops of plastic pipe, installed in several bore holes, to take heat out of and to put heat into the earth. So what is the problem? The problem is that some closed loop installers are not using any grout material to seal the boreholes to prevent surface and shallow soil water from directly entering the bedrock aquifer. I have observed closed-loop geothermal heat pump installers backfilling around the two loop pipes in the 6-inch diameter bore holes with only sand material and drill cuttings. Bentonite grout has a lower thermal conductivity than the sand material and it costs more to install. Thermally enhanced bentonite grout has silica sand added to the grout slurry, and this thermally enhanced grout also costs more than just dumping sand down the borehole around the loop pipes. Drill cuttings and sand have a good thermal conductivity and cost less than grout. Using bentonite grout in closed loop boreholes is analogous to having an air bag in your truck or car. Not every driver has had needed his or her air bag and not every un-grouted borehole has carried contamination into a deep aquifer. But when you do need your air bag, it will probably save your life. When surface water or soil water contamination is present (such as from a spill), the grouted boreholes will protect the quality of the ground water in the aquifer. Pennsylvania’s ground-water resources are too valuable to have hundreds of geothermal systems installed using un-grouted boreholes. Closed-loop boreholes should always be grouted, just as residential wells should have grout around their casings.
Crushed limestone sand from the pile in the foreground was the only material used to backfill the 36 boreholes on this commercial geothermal site. The coils of pipe on the ground are the ends of the loops in each borehole.
Reprinted from the Fourth Quarter 2007 Pennsylvania Driller Newsletter of the Pennsylvania Ground Water Association
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