In the quest to improve building performance, energy recovery ventilation (ERV) is not the shiniest tool in the HVAC kit. It is, however, a proven technology with more than two decades of use to its name. With the growing adoption of codes mandating energy efficiency and the expectation that all building typologies should be designed with an eye on their environmental footprints, architects may soon find building owners requesting energy recovery alongside more visible measures such as daylighting.
ERV improves the performance of a building’s mechanical system by transferring energy, typically in the form of heat, between the exhaust and supply airstreams. During the heating season, energy from the outgoing conditioned air is captured to warm up and humidify the incoming fresh air. Come summertime, the cooler, drier exhaust air alleviates the energy needed to temper the hot, humid outdoor air.
Air-to-air recovery devices, such as rotary or plate heat exchangers, heat pipes, runaround coil systems, and water-to-water recovery devices can capture energy. Rotary heat exchangers, or enthalpy wheels, are popular due to their reasonable cost—roughly $1 per cubic foot per minute (CFM) of capacity—and effectiveness at recovering energy, upward of 70 percent. They can transfer sensible energy (measurable by a temperature differential), latent energy (contained in water vapor), or both. By preconditioning the supply air, energy recovery devices reduce the load imposed on conventional heating and cooling equipment, such as chillers and boilers, as well as the size of these systems, creating savings in capital and operating costs.
Laboratories, a Tough Typology
Laboratory buildings generally require 100 percent outdoor air, which means that “the heating and cooling energy needed to condition and move this outside air can be five to 10 times greater than the amount of energy used in most office buildings,” according to “Energy Recovery in Laboratory Facilities,” a report produced by Laboratories for the 21st Century (Labs 21), a joint program of the U.S. Environmental Protection Agency and Department of Energy.
In the first of five buildings in the University of Miami Life Science and Technology Park, ZGF Architects and mechanical engineer Ballinger employed chilled beams and four 30,000-CFM air handling units, each with a 136-inch-diameter total-energy-recovery wheel. By tempering the humidity of the supply air, the enthalpy wheels reduce the risk of condensate forming on the chilled beams.
ERV not only reduced the load of this energy-intensive building typology, but it also boosted another asset: space. “The biggest savings [came from] the ability to reduce the floor-to-floor [height] of the building,” says ZGF partner Ted Hyman, FAIA. Laboratory spaces in the six-story building average 14 feet 6 inches tall—shorter than usual due to the absence of space needed for large ductwork—which saved millions of dollars in design and construction costs. Energy recovery devices will produce an estimated 37 percent savings in HVAC energy costs.
At the Consolidated Forensic Laboratory (CFL) in Washington, D.C., HOK (with R.G. Vanderweil Engineers) chose a glycol runaround system with two separate air streams and heat coils for the laboratory portion of the building; the office portion uses chilled beams and enthalpy wheels. Though the risk for cross-contamination between airflows in enthalpy wheel systems is nearly nonexistent, any risk was still too much for the CFL, says Vanderweil principal Michael Walsh, who also authored the update to the aforementioned Labs 21 guidelines.
Even with the energy recovery devices, Walsh wanted more savings. “The latest thing is finding a way to recover the heat that an enthalpy wheel can’t recover,” he says. Vanderweil’s team added a water-to-water recovery device that shifts heat collected within the laboratory spaces from sources, such as the extensive equipment, to other spaces in the CFL that need heating. “By keeping the heat in the building, we have a significant amount of heating savings and carbon reductions, even after addressing the electricity to run the [heat shift] chiller,” Walsh says. The 351,000-square-foot CFL is expected to use 30 percent less energy than that of a baseline laboratory building.
Recovery in Renovations
The energy and space savings proffered by chilled beams and energy recovery devices were also critical to the renovation of the University of Arkansas’s Peabody Hall. The historic structure had limited space for modern mechanical systems, says Matthew Cabe, AIA, a project architect at Allison Architects. In lieu of bulky ductwork, the team used chilled beams, which required the less-challenging task of routing an 8-inch, low-velocity duct through the three-story, 27,230-square-foot building with original stamped tin ceilings.
A “beefy” dehumidification wheel delivers “more moderate, constant-temperature air to the chilled beams,” Cabe says. Without energy recovery, he says, the project would have never met the institution’s goal of 80 percent of ASHRAE 90.1’s baseline for classroom buildings.
In sweltering Atlanta, Perkins+Will turned a 1980s office building into a LEED Platinum record setter. The 79,000-square-foot building, now the firm’s own office, literally embodies an efficient HVAC system, with components such as radiant heating and cooling, underfloor ventilation, chilled beams, an enthalpy wheel, and an adsorption chiller that leverages waste heat from the natural-gas-powered, electricity-generating microturbines. Holistically, the system is expected to reduce energy usage by 58 percent from that of a baseline building. Associate principal and senior project designer Bruce McEvoy, AIA, emphasizes that the key to the savings was not in the equipment, but in the planning. “We dropped all the [energy] loads as much as we could … before we had to tackle it,” he says.
Driven by the Code
Despite its longstanding use by building professions, ERV is finally gaining public awareness. Ryan Hoger, director of corporate training at Temperature Equipment Corp. (TEC), sees two things driving the market: building codes and free money. The International Energy Conservation Code 2012 and ASHRAE 90.1-2010 have mandated energy recovery systems for commercial and residential buildings that exceed prescribed supply airflows. “If you have this amount of air on and this much airflow, you have no choice in the matter,” Hoger says.
Paul Shapiro, an associate principal at Ballinger Engineers who worked with ZGF on the University of Miami project, says the International Building Code and International Mechanical Code have required energy recovery in commercial buildings taking in more than 50 percent of outdoor air for the past decade. “It’s helping the energy recovery business,” he says.
State governments are also requiring utility companies to offer rebate programs to reward projects that reduce the energy load on the overall grid. The rebates can decrease the approximate two-to-seven-year payback period for an energy recovery device to just over one year, Hoger says.
Regardless of their maturity, energy recovery technologies still have room for improvement. Swiss manufacturer Konvekta now offers a high-efficiency glycol runaround system that the company monitors during the system’s operating life to optimize performance, Walsh says. TEC’s Hoger anticipates that more manufacturers will improve the devices’ controls to enable monitoring and communication with building automation systems.
Energy recovery devices are also getting smaller. Whereas they were once only an option for projects able to fit “a large recovery unit on the rooftop the size of a school bus,” McEvoy says, modern units can fit inside interior mechanical spaces. Some manufacturers, including Trane and Carrier, are offering combined air handling and energy recovery units, Hoger says. “You don’t have to have two devices on the roof.”?Even with the tinkering, the technology behind ERV still withstands the test of time. In fact, some of Walsh’s past projects have performed 10 to 15 percent better than predicted. “It’s always good to know [that the system] is operating and performing better than what we modeled,” he says.