This past August 12th was the first day of the new school year for ~ 1,000 students at Lookout Mountain Elementary School in Phoenix, Arizona. The high temperature that day was a torrid 109 degrees Fahrenheit while inside the newly constructed 50,000 square foot wing of the school, temperatures were a balmy 76 degrees Fahrenheit, thanks to a new ground-source geothermal system that exchanges heat with the cooler earth hundreds of feet below the ground surface.
Figure 1. Foreman Tom Greary of Adolpson and Peterson Construction addressing the representatives of Arizona utilities and state agencies during the August 6, 2013, tour of Lookout Mountain Elementary School. Principal Heller-Johnson is standing directly behind Tom. Figure 1a. The resurfaced well field directly behind the chain link fence will soon host athletic grounds - note the backstop.
On August 6, Lookout Mountain Principal Tricia Heller-Johnson and Assistant Principal Audrey Barrett engaged about 2 dozen individuals from utility companies (Salt River Project and Arizona Public Service), the Governor’s Office of Energy Policy, Arizona Oil and Gas Conservation Commission, state agencies (Arizona Division of Water Resources and Arizona Geological Survey), and the Washington School District on a tour of the new facility and its closed-loop, ground-source system. Don Penn of Image Engineering Group, Ltd, and construction foreman Tom Greary of Adolpson and Peterson Construction were on hand to discuss plant design and construction (Figures 1 and 1a).
The idea behind heat pumps or geothermal ground-source energy is a simple one that involves leveraging year-round ground temperatures, typically between 54 – 75 degrees Fahrenheit, for cooling in the summer and heating in the winter. An experimental bore well at Lookout Mountain showed a range of undisturbed formation temperatures from 77.8 to 84.2 degrees Fahrenheit. Higher formation temperatures in Arizona have long discouraged growth of ground-source technology. But according to Don Penn, PE and Certified GeoExchange Designer (CGD), a simple and effective solution is to enlarge the footprint of the well field and increase the number of well bores to provide the necessary heating and cooling.
To heat and cool 50,000 square feet, a well field of approximately the same footprint is required. At Lookout Mountain, contractors drilled 191 vertical wells (Figure 2), arranged 20 feet on center, along the south perimeter of the complex. Boreholes in excess of 100 feet deep, or those encountering shallow groundwater, require permitting by the Arizona Dept. of Water Resources. Multiple boreholes can be drilled under a single well permit, provided they are all on the same parcel, or located in the same section (Michael Lacey, ADWR Deputy Director, personal communication). Each well was drilled through several hundred feet of unconsolidated sand and gravel before bottoming out in granite bedrock between 300 to 400 feet deep. The thermal conductivity (measured in watts per unit Kelvin) of unconsolidated sediments in this north Phoenix area was about 0.82, while the granite was markedly higher at ~2.0.; higher conductivity values result in more efficient heat exchange. The well field was emplaced in just 10 weeks.
Figure 3. A typical vertical well bore for a closed-loop, ground-source geothermal system. Nearly 200 such well bores were deployed at Lookout Mountain. Note that well bore heads at Lookout Mountain were buried six feet deep. (Image courtesy of Don Penn, Image Engineering, Ltd.)
High density polyethylene piping was then installed in each bore hole, and each well was backfilled with an admixture of bentonite clays and sand to assure solid contact between the polyethylene pipes and bore hole, and thus promote effective heat transfer. Each well included a supply and return head and U-bend assembly for circulating water that exchanges heat with the surrounding well bore (Figure 3). The head of each well bore resides at a depth of six to eight feet below ground surface to insulate the header against ambient air temperature.
Figure 4. Components of a standard geothermal heat pump: fan, compressor and heat exchangers. At Lookout Mountain, most of the heat pumps reside above a false ceiling in each classroom. (Image courtesy of Don Penn, Image Engineering, Ltd.)
Lookout Mountain uses a closed-loop, ground-source system with simple tap water as the circulating fluid. Because this is a closed-loop system, there is no additional water requirement once in service. Eight-five geothermal heat pump units deployed throughout the complex circulate water and exchange heat with the surroundings (Figure 4).
Image Engineering Group, Ltd., out of Dallas, Texas, designed the Lookout Mountain ground-source system, which was then build by Phoenix-based Adolpson and Peterson Construction. Up-front costs to install the ground-source system is about 10 to 15 percent greater than conventional heating and cooling technology. Conversely, the estimated annual cost of operating the ground-source system is 65 to 70 percent that of a conventional system. The annual savings on energy bills yields payback in a mere two to seven years.
Lookout Mountain’s ground-source geothermal heating and cooling system conditions about 50% of the school complex, including classrooms, the cafeteria and gym. Ground-source heating and cooling are part of the Washington School District’s program to operate sustainable and environmentally efficient schools.
An excellent 2.5 minute video, “Energy 101 – Geothermal Heat Pumps” (2.5 minutes long), interlacing footage of trenching and pipe laying with animated sequences showing fluid circulation and heat exchange via a heat pump illustrates how ground-source geothermal works. As the film points out, “geothermal heat loops can be almost anywhere in the U.S. because all areas have nearly constant shallow ground temperatures. “ When it comes to ground-source installations, Arizona is behind the curve, but a surge in new ground-source geothermal construction in Arizona schools and government buildings is blazing a path for those in the construction and the energy industries to follow.
Acknowledgments. I thank Don Penn and Tom Greary for their patience and fortitude in helping me grasp the intricacies of how ground-source geothermal works during the Lookout Mountain tour. Don graciously supplied some of the graphics used in this article. Principal Heller-Johnson and Assistant-Principal Audrey Barrett were gracious hosts. Michael Lacey (ADWR) kindly supplied information on the role ADWR plays in permitting ground-source geothermal well bores. Last, Frank Thorwald of the Arizona Oil and Gas Conservation Commission brought this event to our attention.