Total Energy Recovery Wheels: Integral to Sustainability

Heat recovery wheels have been an integral part of HVAC systems in large commercial, government, and education buildings for decades. But, with escalating energy costs, coupled with a renewed focus on environmental sustainability, architects and engineers are turning their attention to total energy recovery wheels as part of HVAC systems with increased energy efficiency. This state-of-the-art technology, which differs from the heat recovery wheel (the total energy recovery wheel also captures and reuses latent heat [moisture]), currently exists in maybe 12 buildings nationwide. Two of those buildings are laboratories – a building type that traditionally consumes tremendous amounts of energy.

HGA first designed and integrated total energy recovery wheels into the HVAC systems of a high-security government agricultural and public health facility. So, when work began on Leonard A. Ford Hall, the new 67,000-square-foot addition to Mankato State University’s Trafton Science Center, the total energy recovery wheel was a natural solution to energy-conservation requirements as well.

Ford Hall is the first building within the Minnesota State Colleges and Universities (MnSCU) system to fulfill the protocol of the State of Minnesota's Sustainable Building Guidelines, known as B3 (Buildings, Benchmarks and Beyond). B3 specifies that completed, operational buildings exceed existing energy code by at least 30 percent. The total energy recovery wheel systems in Ford Hall, however, result in energy savings far above that benchmark and save MnSCU hundreds of thousands of dollars annually in energy costs.

Ford Hall incorporates four energy recovery wheel systems to minimize the penthouse footprint and ensure that three systems continue their operation while one is being maintained. As mentioned before, the total energy recovery wheel captures latent heat – latent being the moisture (from occupants' breath or perspiration or humidity) in the air or heat – which is actually half of the load required for cooling purposes (the non-moisture half is called sensible heat).

As the wheel turns, exhaust passes through holes on one side as the incoming air passes through the other side, while a silica-gel desiccant absorbs moisture from one airstream and transfers it to another. The simple system removes up to 80 percent of the energy from the exhaust airstream and reuses it before the air is discharged into the atmosphere. The process recovers energy throughout the year, thus reducing heating and cooling costs by more than 15 percent.

In addition, two mixed-flow, constant-volume exhaust fans per system (with premium-efficiency motors) maintain an acceptable plume height by mixing unconditioned outside air with the varying volume of building exhaust air. We were also able to reduce the building’s nighttime exhaust rate by 65 percent (compared to daytime flow), thus reducing mechanical costs by 25 percent. Similarly, the 100 variable-volume chemical-fume hoods incorporated into the system maintain flow velocity through the sash opening by varying the volume of air being exhausted – an efficiency of more than 80 percent that results in more than $100,000 in energy savings.

Water temperature is based on outside air temperatures and monitored remotely over the Internet via a building-automation system, as are water pumps on variable-frequency drives that operate according to demand. In both cases, the building-automation system selects the smarter, lower-energy use. As a result, the system is seldom required to run at peak load. Likewise, HGA designed hot and chilled water piping and components to minimize transport energy and reduce pump horsepower. Because an upgraded campus chilled-water system has increased pressure in the campus loop, no secondary chilled water pumps were necessary in Ford Hall.

Many of these systems are exposed and on view throughout the building to show students the ways in which Ford Hall incorporates groundbreaking sustainable-design initiatives. Major piping for water, compressed air, and natural gas, as well as duct systems, are visible in corridors. Virgin polypropylene pipe that provides pure water for the labs is also on view. Racks with domestic water systems line hallway walls. All are tagged for easy identification.

Incorporating the total energy recovery wheels and their ancillary systems, in addition to sustainable lighting and materials, added nearly $1 million to the total project cost. But, the energy savings from sustainable strategies will allow the school to recover its investment in 4.5 years. Mankato State University had the foresight to make that investment, especially in the total energy recovery wheel. As a result, says the project designer Bill Blanski, “All of our clients now want to know the metrics on the return on investment and cost-benefit ratio delivered by such energy-saving strategies, and Ford Hall is an excellent example.”

Bob Vestal is a design engineer and senior associate at Minneapolis-based HGA Architects & Engineers.