Sustainability comes in many forms, and one of the least celebrated – yet most effective – can shrink both the physical footprint of the HVAC system and the carbon footprint of the facility. It comes in a box that remains hidden for years, working quietly and diligently to boost indoor air quality and slash energy use.
Curious? It's the enthalpy wheel, which is eminently worthy of consideration for new and retrofit projects.
Philip C. Bartholomew, P.E., a LEED-accredited expert in HVAC system design and laboratories with HDR CUH2A, explains, "Enthalpy wheels, unlike other heat recovery devices, are designed to recover latent loads – moisture and humidity loads – as well as temperature-sensible loads." In practical terms, latent transfer means that heat can be removed from one air stream and added to another – for example, subtracted from supply air and heaped into an exhaust vent. That cools the interiors more efficiently.
The Threshold for Savings
In some climates, as much as half the cooling load for outdoor make-up air may be latent. If it's 15% or more, an investment in enthalpy wheels is probably well worth it for a new or upgraded HVAC system.
In applications where good ventilation is needed, energy-recovery components, including enthalpy wheels, assist in efficiently cooling air and controlling humidity, according to Stanley A. Mumma, Ph.D, P.E., a mechanical engineer and professor emeritus at Penn State University. For 100% outside air in a conditioned space, Mumma endorses the dedicated outside-air system (DOAS), which employs an enthalpy wheel to cool and dehumidify the fresh air supply. The wheel eases the load on the cooling coil by as much as 80% and can shrink cooling and heating systems by 40% or so.
This downsizing of the HVAC plant often will offset the first cost of the enthalpy wheel application, says Innergy tech, the company that markets the ERW3000 wheels for air-to-air energy recovery. "In other situations, the amortization periods can be as short as six months," says the Quebec-based company. Other manufacturers include NovelAire, which markets the ECW, as well as Lennox Industries, Richardson, TX, and Minneapolis-based XeteX, Inc.
Summer and Winter Efficiency
Enthalpy wheels work by rotating between two air streams, moving heat and moisture energy from one duct to another. The enthalpy wheel is inserted upright spanning both ducts, and the spinning captures heat from the outdoor air stream, and then rotates it to the exhaust where it is pushed back outside. In the winter, the indoor air can preheat the fresh-air supply.
Two functioning enthalpy wheels are on display at Ohlone College's Center for Health Sciences and Technology in Newark, CA. Set behind glass on a mezzanine, the 16-foot-diameter wheels improve the efficiency of the LEED Platinum facility.
While few can boast an "enthalpy wheel portal" like the one students enjoy at Ohlone College, many other schools have also adopted heat recovery as part of campus expansions and upgrades. At Georgia Tech, some residence halls and academic buildings have enthalpy wheels hidden within their walls. They are highly effective for office buildings, as found by Puerto Rico-based Standard Refrigeration Co., which built the first LEED Platinum project in the Caribbean.
According to Mumma, Penn State University has installed a DOAS in a 3,200-square-foot architecture studio, for use in peak summer and winter conditions. On a hot, humid August day, the system cut humidity levels by about 45%; in the winter, it held an operative temperature of 68 degrees F., even as outdoor air temperatures plunged to zero.
Thanks to the finely engineered enthalpy wheel, all of these organizations have bought less expensive HVAC systems – smaller compressors and condensers, and other vapor compression cycle components – substantially lowering the first cost of the physical plant. The kicker is that their zero-payback period is followed by years of energy savings, making it a life-cycle cost cutter, too.
C.C. Sullivan (firstname.lastname@example.org) is a marketing communications consultant and writer specializing in architecture, design, and construction technology.