Spin Your Wheels for Energy Savings

Enthalpy wheels shrink cooling loads by as much as 80%

by C.C. Sullivan

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 while slashing energy use.

Curious? It’s the enthalpy wheel, an unsung hero of green building but one that’s eminently worthy of consideration for any project, new or retrofit.

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.

How much more efficiently? 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, whether it’s 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.

Enthalpy wheels often work by rotating between two air streams, moving heat and moisture energy from one duct to another. Imagine two ducts, one supply and one exhaust, stacked on top of each other: 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 public 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, which has no operable windows.

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. Clark University’s LEED Gold Lasry Center for Bioscience in Worcester, MA, captures heat from ventilation exhaust air with enthalpy wheels, and its nearby Kneller Athletic Center was retrofitted with energy recovery units. At Georgia Tech, a number of residence halls and academic buildings have enthalpy wheels hidden within their walls.

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.

Beyond educational facilities, a number of commercial projects are also employing enthalpy wheels. They are highly effective for office buildings, as found by Guaynabo, Puerto Rico-based Standard Refrigeration Co., which built the first LEED Platinum project in the Caribbean. The enthalpy wheel was sized to handle more than enough outside air – 125% of the required load. According to Juan S. Quintana, president of Standard Refrigeration, electricity savings alone from the enthalpy wheel and a few other green features will pay for the full cost of the new building in 10 to 15 years.

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.

Can we offer due praise to this unsung hero of sustainability?

C.C. Sullivan (chris@ccsullivan.com) is a marketing communications consultant and writer specializing in architecture, design, and construction technology.