Economizers and Energy Recovery Wheels: Heads and/or Tails?

Economizers and energy recovery wheels are alternative energy strategies, but choosing between them isn't like flipping a coin — the two components are actually two sides of the same coin.

Both technologies have enormous potential for energy savings: economizers because they utilize outside air, and energy recovery wheels (also called "heat wheels") because they don't waste exhaust air. But that potential will be maximized depending on the status of your existing HVAC system, the climate of your location, and your building's efficiency.

"Energy recovery wheels and economizers go hand in hand," says David P. Callan, senior vice president of consulting engineering firm Environmental Systems Design, Inc. "But every building is different. They're like people – no two are the same. To implement either system, there's a lot to be done."

To Retrofit or Replace?
The energy savings alone of either technology can justify a retrofit, although doing so gets complicated. Retrofits require significant space considerations for either type of equipment. Economizers also need special fan arrangements and new holes in the exterior, Callan explains, and energy recovery wheels would necessitate more ductwork.

Both likely require more sophisticated controls, such as a digitally controlled system with a computer front end to perform calculations that indicate when to turn the equipment on or off, Callan adds.

"If you want to implement either system, you should look for an opportunity when you're at the end of your HVAC system's useful life," he says. "If you already have to replace that system, then the cost of integrating these technologies is much less, the payback is maximized, and it's less complicated than retrofitting an existing system."

Pick Low-Hanging Fruit First
Conduct an energy audit or evaluation to begin the decision process of whether to implement these technologies. That will tell you whether it's feasible to take such a significant step toward energy efficiency or if there are smaller hurdles to jump first.

"For new buildings, ASHRAE 90.1 and many codes already require economizers and to investigate and install energy recovery in a lot of cases," Callan says. "For existing buildings that don't have these technologies and are considering them, it's a serious commitment."

Your HVAC load depends on several other building systems, such as how efficient the lighting or envelope is. The first step is likely making sure your entire building system is up to snuff. A consulting engineer can tell you whether there is low-hanging fruit to pick first.

"An engineer will know if the system has five years of usable life left and if you should replace ballasts, sensors, or a couple of valves first," Callan says. "Engineers can perform an evaluation for little or no cost, spend an hour with the owner, look through the building and at some climate charts, and determine if the case warrants further investigation."

Once your building is tweaked and fine-tuned, this investigation will analyze your existing HVAC system and perhaps take an energy model of the building. After that, you can determine if an economizer or an energy recovery wheel – or both – is right for your facility.PageBreak

Consider Economizer Cycles
Economizers draw cool outside air into buildings to reduce mechanical refrigeration, and they can produce energy savings of 24-35% and cost savings around 38%, according to research from the Pacific Northwest National Laboratory.

Economizers work best in cool, dry climates because utilizing outdoor air also means bringing in humidity, which makes the environment uncomfortable for occupants and can be detrimental to building materials. They typically aren't effective or economically justifiable in southern, humid climates.

An economizer cycle strategy requires many motorized parts that change the building pressurization and modulate the outside air dampers, return air dampers, and supply air. It also depends on sensors and setpoints.

"An enthalpy sensor tells you when the total heat content of the outside air is lower than the interior air. At that point, your chiller would be doing less work if that outside air is brought in," Callan explains, adding that economizer strategies are thus referred to as "free cooling."

However, if the economizer cannot accommodate the full load, you may need to supplement it with additional mechanical cooling. But the energy savings associated with free cooling can spur paybacks of two years, says Richard Lord, engineering fellow with manufacturer Carrier Corporation.

"Depending on where you are in the country, you can save anywhere from 15 to 40%," Lord adds. "That assumes the system is running well, was properly commissioned, and has been maintained."

Economizer systems should be inspected at least twice a year – every time the season changes from heating to cooling or vice versa, recommends Xiaohui Zhou, building energy efficiency program manager at the Iowa Energy Center, a nonprofit research organization.

"Make sure the damper isn't stuck and the sensor is accurate. If the system isn't working correctly, not only are you not saving energy, you also could be wasting energy by letting warmer air in," explains Zhou. "Don't just set it and forget it."PageBreak

Weigh Energy Recovery Wheels

Think of the energy recovery wheel as the opposite or reciprocal of an economizer from a climate standpoint, Callan says. Instead of throwing out exhaust air and its energy, heat wheels recover energy from the exhaust stream and transfer it to the ventilation air. It's not as expensive to condition recycled air as it is outside air.

An energy recovery wheel rotates with half of its area in the exhaust air and the other half in the ventilation air. The wheel uses desiccant – which absorbs moisture at atmospheric pressure – to transfer energy instead of actual air.

"Transferring air is something you want to avoid because the exhaust air might be dirtier," explains Skip Ernst, project manager with manufacturer Daikin McQuay.

The ability to recover latent energy (moisture) provides important benefits all year. During the cooling season, the outdoor air is dehumidified. In the heating season, the costly humidification load is reduced through moisture recovery.

An energy recovery wheel can eliminate about 70% of energy costs on the ventilation system, which makes up anywhere from 25-50% of the total HVAC cost for a building, Ernst adds.

Payback depends heavily on the amount of ventilation air you have and the type of application. A densely occupied school requires more ventilation than other commercial buildings and could see a payback of three years, Ernst estimates.

Maintenance of energy recovery wheels is relatively simple, explains Matt Pemberton, regional manager of desiccant wheel products for manufacturer SEMCO.

The air flow in both directions has a self-cleaning effect on the wheel, but the filters need to be kept clean, and some people don't realize the wheel has its own filters, according to Pemberton. "Use compressed air to clean if the components are dirty," he adds.

Energy recovery wheels work much better in hotter, humid climates because you need to bring in the outside air but want to avoid the expense of having to dehumidify it, Callan says.

Finding the Happy Middle
There are many areas of the country that have a temperate, mixed climate – such as the Midwest, East Coast, central mid-Atlantic region, and some parts of Texas. These areas experience both cool and humid periods. Thus, you could economically justify having both of these technologies in the same HVAC system because there would be enough opportunity in the winter to take advantage of free cooling and also times to utilize energy recovery.

"That's why these technologies are two sides of the same coin. It's all part of the same discussion. Are you hot and humid or cold and dry?" Callan says. "Most of the country is not in either one of those extremes."

Engineers use software that predicts the weather every day of the year and can determine when either or both of these technologies is cost-justifiable. If you're somewhere in the middle, enlist outside help to diagnose your circumstances.

"Installing big HVAC components typically isn't low-hanging fruit," Callan says. "These technologies are the next significant step toward energy efficiency."

Chris Curtland [email protected] is assistant editor of BUILDINGS.