Geothermal water source heat pumps: no Trojan horse at Michigan school
The sign in the Whitmore Lake High School gymnasium says “The Home of the Trojans.” Just like the Trojans of Homer’s Iliad, these Trojans of Michigan have constructed an edifice to be envied. The new 150,000-sq-ft high school, which opened in August 2006, is one of the first high schools in Michigan to achieve LEED® Silver certification. Among its many sustainable features, a geoexchange system for HVAC includes 67 McQuay water source heat pumps. The 430-ton HVAC system delivers conditioned air to classrooms and other spaces, helping to reduce energy costs by an estimated 35% compared to a conventional system.
A Need To DistinguishWhitmore Lake High School, the only high school in the school district located near Ann Arbor, serves about 420 students. One of the project goals was to build an exceptional school that would stand out in the community. “There are several private schools in the area that local students could choose to attend,” said Tom Dekeyser, principal of Whitmore Lake High School. “Plus, Ann Arbor public school students can choose to attend Whitmore Lake because we’re so close. So we wanted to put our best foot forward and show that Whitmore Lake can make a difference. Achieving LEED certification was one way to distinguish ourselves, and we decided we wouldn’t settle for anything less than LEED Silver. In fact, it was non-negotiable.”
In addition to establishing a community icon, school officials wanted to minimize operating costs - especially as the cost of natural gas rose during the planning stages. The school board recognized that any additional first costs for the geoexchange system would be offset by energy cost savings. As a result of their foresight, the school anticipates saving about $80,000/yr. In the first year alone, that’s more than double the added cost to construct the geoexchange system vs. a conventional HVAC system.
Although geoexchange systems reduce energy costs compared to traditional systems, Whitmore Lake’s geoexchange system goes even farther by combining a horizontal loop and pond system. Approximately 47 miles of pipe are laid in a horizontal field and a 15-acre pond. The system has two horizontal layers of piping trenches dug 8-ft deep. The top pipe is 6-ft below the surface and loops back with the second pipe 2-ft below that.
“We could have put the whole pipe system in one horizontal field, and still had an efficient system,” said Bob Roop, mechanical engineer with Peter Basso & Associates. “We had plenty of real estate. But we had to create the pond to provide storm water retention and a fire protection water supply. The water in the ponds is a better heat transfer media than the soil, so we took advantage of what we had to construct anyway to further improve the efficiency of the geoexchange system.”
28% To 33% Less Energy Consumption“When Whitmore Lake told us they wanted LEED certification, we did some comparative cost information and came back with some favorable numbers for a geoexchange system,” said Roop. “Our boiler tower systems consume about 28% less energy than traditional systems; we expect that Whitmore Lake, with its geoexchange system, will use about 33% less in energy consumption compared to a conventional building. Those numbers, combined with the ample real estate available, convinced school officials to go ahead with the system.”
All of the McQuay geoexchange heat pumps, located throughout the school, are fed a pre-treated, measured amount of outside air from one of two pre-packaged energy recovery units. “All of the relief air from the building goes through the energy recovery unit,” said Roop. “It has a plate-type heat exchanger and pretreats all the outside air. It maintains a discharge air temperature in winter of 60°F, so all of the outside air is pre-treated to 60°.
“The 67 individual water source heat pumps are basically responsible for space tempering, and don’t have to deal with the outside air load. We’ve found this to be very successful for supplying heat in a northern climate in the dead of winter. This is integral to all of our designs for the past 10 years; it’s a keystone for making these systems more energy efficient.” ES