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In building the school, the Birdville Independent School District (BISD) wanted an HVAC system that would be economical and also ensure good indoor air quality (IAQ) for the 1,100 students, teachers, and staff. In addition, the system had to be designed for minimal noise impact on the classroom environment.
BISD already had some successful experience with geothermal heating and cooling. “In 1993, the school district had a bond program that included some $10 million for HVAC replacement at the 20-plus schools,” said BISD consultant Don Penn, P.E., Don Penn Consulting Engineer, Grapevine, TX. “The budget was based on replacing existing systems with like systems, i.e., rooftop unit for rooftop unit, split system for split system, etc. We took these funds and put geothermal in most of these schools.
“Because of budget constraints, we did ‘hybrid’ installations at numerous campuses to cut costs. These hybrids were geothermal in the classroom and support areas, with conventional rooftop units or split systems in auditoriums, cafeterias, and other areas that are not used in the day-to-day educational process.”
When construction on Birdville High School was initiated in the late 1990s, BISD decided to go with a complete geothermal system, using Trane ground-source heat pumps and a Tracer Summit™ building management system. The facility was completed in June 1999.
TEXAS-SIZE SCHOOLAt approximately 300,000 square feet, the high school is one of the largest single buildings in the country heated and cooled by a geothermal system. Penn designed the system to reduce energy consumption, provide a positive solution to IAQ concerns, and maintain aesthetics by keeping all equipment within the building. He worked with David Pickard, sales engineer with the Trane Fort Worth commercial sales office, to develop the design specifications.
The total system includes:
The vertical ground loops are zoned into 27 well fields, dug 250 feet deep, stretching across the 70 acres of land around the school. Zoning provides efficient energy management, said Penn, “the strategy being to only run what you need to run when you need it.”
Spacing the loops into zones also allows greater heat transfer to the earth and helps eliminate hot spots. For those areas of the school that have a strong demand for cooling, the ground loops are spaced at wider intervals to ensure sufficient heat transfer. In areas with a low heat transfer load, the loops are placed closer together.
Penn’s use of a distributed pumping approach, rather than central pumping, also increases efficiency. “This allows the system to individually support the pumping requirements of each unit rather than run a central pump to feed all units,” he said. “The schools we have seen with central pumping schemes are penalized approximately 20% on energy savings.”
The zoned ground loops are arranged in a reverse-return configuration, providing uniform pressure drop across the system, and are connected to the ground-source heat pumps through the individual pump modules. The pumps contain a shut-off valve to stop water flow to the heat pump to ease maintenance.
The horizontal ground-source heat pumps are installed above the ceiling in the school hallways for classroom heating and cooling. There are 105 units ranging in size from 1.5 to 5 tons capacity.
Vertical heat pump units of up to 25 tons are installed in mechanical rooms to condition larger areas such as the atrium, auditorium, and gymnasium.
ENERGY-EFFICIENT VENTILATIONThe ventilation system for the building uses high-efficiency energy recovery ventilators to treat and temper fresh air to the building. The Mitsubishi units contain an energy exchange core that exhausts stale air and allows fresh ventilation air to pass through multiple air passages separated by a composite resin partition plate. Sensible heat is transferred from the warmer to the cooler airstream, while latent energy (water vapor in the gas state) transfers from the wetter to the drier airstream.
The fresh incoming air is thus automatically preheated or precooled, depending on the season, and ducted directly to the ground-source heat pumps, helping to reduce energy costs. “We’ve had little trouble meeting air quality standards,” Penn stated.
DDC controls tie the mechanical system to the building management system, to automate and optimize overall operation.
QUIET, PLEASEThe mechanical system was designed to meet NC35 indoor sound levels. By mounting the horizontal heat pumps in the hallways adjacent to the classrooms, noise interference is minimized. Equipping each unit with its own pumping module eliminated the need for a central pumping station and its associated noise. The vertical heat pumps are in cinder block-lined mechanical rooms.
Penn noted that there have been no complaints from the staff regarding noise.
SUBSTANTIAL SAVINGSWith this system, said Penn, “We’re seeing nearly a 50% reduction in costs over comparable gas and electric HVAC systems used with other similarly designed schools in the district.”
At just over 305,000 square feet, Haltom High School is experiencing annual heating costs of approximately $316,000 or $1.04 per square foot. Richland High School employs a central and split system powered by both gas and electricity. At 274,000 square feet, annual costs are approximately $252,000 or $0.92 per square foot.
Birdville High School, on the other hand, has heating/cooling costs of approximately $174,000 annually or $0.58 per square foot.
The capital costs of geothermal installations have also been studied by BISD. When compared to the costs of constructing a central pumping plant for a traditional HVAC system, the school district determined that the overall costs are almost identical.
BISD also expects to see savings in reduced maintenance and longer service life.
The heat pump units are seeing less “wear and tear” because, on average, entering water temperatures are 85 degrees vs. 130 degrees F ambient temperatures with rooftop- or ground-mounted systems, translating into less compressor loss and reduced stress on condenser coils. Having the geothermal system housed within the building has eliminated damage caused by hailstorms and other inclement weather, vandalism, and fire ant infestation.
Penn said that he is expecting 25 to 30 years of service from the school’s geothermal system, as opposed to 10 to 15 years from conventional HVAC systems.
Publication date: 08/26/2002