Bringing It Home: AC Efficiency

March 30, 2006
U.S. federal and state agencies are serious about appliance efficiency. California's latest Appliance Efficiency Regulations (CEC-400-2006-002) were released in January. The standards specify ?energy efficiency ratio? (EER) for room air conditioners and

U.S. federal and state agencies are serious about appliance efficiency. California's latest Appliance Efficiency Regulations (CEC-400-2006-002) were released in January.

The standards specify "energy efficiency ratio" (EER) for room air conditioners and "seasonal energy efficiency ratio" (SEER) for central-air-conditioning equipment. EER is "the cooling capacity of an air conditioner in Btu per hour divided by the total electrical input in Watts."

Metric countries, of course, figure EER in terms of W/W. The multiplication factor for converting Btu/hr to watts is 0.293, so a U.S. EER of 10.8 Btu/hr/W is about 3.16 W/W, or 32%.

SEER amounts to the same thing during the central air conditioner's "normal annual usage period for cooling." There's also "heating seasonal performance factor" (HSPF) and "coefficient of performance" (COP), which apply to heat pumps.

The testing methodology comes from U.S. Department of Energy test procedures in Part 10 of the Code of Federal Regulations (CFR). The section relating to room air conditioners is 10 CFR part 430.23(f).

In the California regulations, room air conditioners (with louvered sides) rated from 6000 to 19,999 Btu/hr must demonstrate EERs of 9.7 or 9.8. Those particular requirements haven't changed since late 2000.

For the most part, the industry is ahead of the curve. Better Energy Star-compliant room air conditioners have EERs of 10.7 and 10.8. (Energy Star is a cooperative industry/government program under the auspices of the Environmental Protection Agency. It's not prescriptive, though it provides a common standard for claims of energy efficiency.

So how does IR's iMotion platform contribute to overall air-conditioner efficiency? The figure shows performance data measured by a Japanese air conditioning company and verified by IR. Both companies were surprised by the magnitude of improvement.

Typical baseline values for the most efficient Japanese designs were 90% to 91%. Tests showed peak values for the iMotion-based test bed above 95%. The boost came in three areas: more efficient operation of the interior-permanent-magnet (IPM) motor, more efficient IGBT power switches, and PFC accomplished at a lower frequency with smaller-value inductances.

Going back to the real-world overall efficiency numbers for air conditioners, as noted above, an EER of 10.8 Btu/hr/W represents about 32% overall efficiency. In other words, there are lots of mechanical and thermodynamic losses.

There is no simple calculation that tallies all the effects of the control system on all those sources of efficiency loss. A control system like IR's enables system-level efficiency improvements, such as allowing the fan and compressor to run at lower speeds and power levels while maintaining a constant temperature. So, the only valid assessments of the true value of new iMotion controllers will be done empirically after production air conditioners are available for test.

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