Podcast

Listen to the Podcast (8-minute mp3)
Also available on iTunes
RSS Feed for Energy Priorities podcasts (What's this?)
Music by Chris Keister

Cooling inefficiencies are costly

Commercial building owners and tenants in the U.S. spend over $14 billion a year on space cooling alone, according to the Department of Energy. Cooling accounts for eleven percent of an average building's energy consumption and cost -- considerably more in hotter regions -- and it's responsible for 300 million metric tons of carbon emissions from power stations each year.

Centralized chiller plants like this one refrigerate water and distribute it through the building for space cooling. They typically run at full speed, even when it's not necessary, which wastes energy. Optimum Energy's control system varies the operating speed of the air conditioning system based on the demand for cooling. (Optimum Energy photo)

A common form of air conditioning uses a centralized plant to chill water and distribute it to air handlers throughout the building. A cooling plant big enough to cool a building on a hot day is not very energy-efficient on an average day. The plant is either on or off, even though the cooling demand may be for only a fraction of the system's capacity. Unfortunately, these plants run at less than full capacity (but still at full speed) about 90 percent of the time.

Control systems match speed to demand

Recent developments are making it possible to retrofit these chiller plants and make them significantly more efficient. One approach is to vary the speed of the components according to the cooling demand. On a pleasant day, when a building needs only 50 percent of the system's cooling capacity, the plant runs at 50 percent speed. It uses less energy as a result.

Optimum Energy has developed a system based on a control technology called the Hartman LOOP.

"Our technique looks at the demand for cooling on the plant and vary the operation of all of the plant components, so they're only using the minimum amount of energy necessary to meet that demand," says Jim Hanna, vice president of product development for the Seattle, Washington company.

Some HVAC systems already use variable-frequency drive (VFD) couplers to modulate the operating speed of some parts of the plant. The Hartman technology uses VFDs, and synchronizes them with the other parts of the plant for efficiency.

"The important thing about VFDs is that you have to determine what speed the equipment runs at," Hanna says. "The more traditional control approaches take a rough guess, so they may help a little bit. You have to base the speed on the demand on the plant, and adjust every component based on that demand, to make the best use of VFDs."

Efficiency doesn't mean compromising comfort

Property managers worry when owners start talking about saving energy through changes in space cooling. Hanna points out the energy efficiency should not be confused with energy conservation. Making the plant more efficient does not mean there will be more hot and cold complaints.

"You don't have to change your thermostat to get efficiencies with our technology," Hanna explains. "We achieve efficiency on the chilled water side of the plant, and it doesn't really affect the comfort level. We supply sufficient chilled water to meet demand, but we do not affect the comfort settings. "

Three-year payback after incentives

EXAMPLE

Plant:	750 tons
Final cost after incentives	$135,000
Energy savings	327,000 kW
Peak reduction	37 kW
Estimated annual cost savings	$42,000

Hanna says a control system can pay for itself in energy savings in 18 to 36 months. He gives an example of a building in the San Diego area where the payback was about three years (see table).

Incentives can cover half the cost

Utilities in southern California offer generous incentives that apply to control systems. Two utilities -- San Diego Gas & Electric, and Southern California Edison -- offer incentives of eight to sixteen cents per annual kWh saved.

"That works out to $50,000 to $100,000 of incentives, which can cover more than half the implementation cost," says Hanna.