| Abstract |
The fastest growing segment of the geothermal industry is Geothermal Heat Pumps. In 2008, approximately 250 U.S. drill rigs will complete over 25,000,000 feet of vertical bores at market value near $300 million. Current U.S. investment in geothermal heating and cooling systems exceeds $800 million, annually. Canada installs about 10% more. In the United States alone, 1.8 million vertical loops of waterfilled, High Density Polyethylene (“HDPE”) pipe exchange 2,500 net megawatts-thermal of heat with the earth, annually, at 100% capacity factor (estimated). Total global capacity is about 50% higher. This form of geothermal drilling, looping and grouting procedures has evolved to a state of high efficiency, low risk and affordable cost. (8,000 installed MWth of geothermal heat pumps operate in the U.S. near 30% load). Commercial and institutional ground-coupled heat exchangers are closed, vertical loops of 0.75" to 1.25" HDPE installed to depths of 50 to 400 feet. Groups of 6 to 12 loops are manifolded together at the surface into hydraulic circuits. Water-well or geotechnical rigs drill 4 to 6 inch holes using standard mud rotary, air-hammer or sonic techniques. Individual bore holes are pressure-grouted from bottom to top with 20% bentonite (by weight) or with thermally-enhanced grouts wherein very high silica-content sand is mixed with bentonite. Cement is not suitable for loop grouting, except for localized formation plugging, as HDPE pipes flex with recurrent temperature changes. The design goal is to place a total length of pipe into the ground, subject to thermal properties and drilling character of native soil or rock. Earth temperature is presumed uniform from 20 to 400 feet as gradient considerations are offset by migrating ground water. Therefore, if design calls for 60,000 of total drilling, thermal transfer is similar by 200 loops of 300 feet each or by 300 loops reaching only 200 feet. Drilling tools and techniques seek speed and uniformity and adapt to local conditions in pursuit of both. Thermal characteristics of conductivity and diffusivity are measured prior to design by 36- to 48-hour, in-situ thermal conductivity tests. Monitoring of energy (expressed as water temperature) introduced to and recovered from a production loop indicates how readily soil accepts or gives up heat and how quickly it moves heat away from the loop. This is a primary determinant of how many feet of pipe are needed for each “ton” of heating or cooling load to be served. |