Adding AEER To Your Acronyms

May 12, 2008
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Technicians quite familiar with terms such as EER and SEER may want to make room for yet another in the seemingly endless lists of industry acronyms: AEER.

Emerson Climate Technologies is proposing annual energy efficiency ratio (AEER) as “a new measure for refrigeration applications” to go along with the more familiar energy efficiency ratio (EER) and seasonal energy efficiency ratio (SEER).

According to Rajan Rajendran, director of applications engineering for Emerson, “Our industry currently has measures in place to calculate the performance of commercial refrigeration systems. However, none of these fully address the various conditions that impact refrigeration systems.

“With the introduction of AEER, Emerson is proposing a new measure that truly addresses the industry’s needs.”

The information is in a 24-page white paper called “Annual Energy Efficiency Ratio: A Better Measure of Refrigeration System Performance,” published this past fall by Emerson.

In the paper, Rajendran and his colleagues - Brian Buynacek, Emerson senior consultant with the Design Service Network, and Autumn Nicholson, a student majoring in mechanical engineering at the University of Dayton - noted that the EER for a/c and refrigeration compressors is calculated based on Air-Conditioning, Heating and Refrigeration Institute (AHRI) Standard 540-2004.

The authors said, “EER has its benefit, since it is a standardized measure of compressor performance. (But) EER is a single-point efficiency measurement for an a/c or refrigeration compressor at a specific condition. EER is based on one rating point and one evaporating temperature and one condensing temperature.

“Therefore, EER is not representative of how a system actually operates under other temperatures and conditions.”

The researchers also looked at SEER, noting it is “an efficiency measure used to rate air conditioning equipment; the higher the SEER rating of the unit, the more energy efficient it is.” They added, “The relationship between SEER and EER is relative, depending on location, because equipment performance is dependent on air temperature, humidity, and atmospheric pressure.”

The AEER acronym was described as “an efficiency measure for refrigeration systems that takes the concept of SEER in a/c a step further. While SEER neglects the high load on hot summer days in a/c, AEER takes seasonality into account in a fixed load refrigeration system.”

Rajendran said AEER is more representative for refrigeration applications because it “is a weighted average performance for a refrigeration system, using varying condensing temperatures tied to the actual weather data for a location. The AEER is a single number that represents an average performance for a whole year and leads itself easily to calculations of total annual power and energy cost.”

As part of the research, a case study was done with EER and AEER to evaluate the efficiency of walk-in refrigeration compressors.

In one situation in the case study, a refrigeration system designer in Atlanta wanted to select a compressor for a medium-temperature outdoor walk-in cooler. The system requirements were 17,500 Btuh at 20°F evaporating temperature and 120° condensing temperature using R-404A refrigerant at 60 Hz.

An evaluation showed the option of either a hermetic compressor with a 7.1 EER or a high-efficiency refrigeration scroll with 6.6 EER.

The report said, “At first glance, it looks like the hermetic reciprocating compressor is 7 percent more efficient than the high-efficient refrigeration scroll. But even though Atlanta is hot most of the time, the system will not be operating at 120° condensing temperature.

“The compressor performance should be evaluated through its entire operating range.”

By using the AEER evaluation, the high-efficiency refrigeration scroll was shown to be 8 percent more efficient than the hermetic.

“The AEER evaluation was done for both compressors holding the minimum condensing temperature at 70°, the lowest allowable condensing temperature for the reciprocating compressor. Some scroll compressor models are actually approved for a lower condensing-temperature.

“AEER can be improved further by allowing for the lowest approved condensing temperature, but always check the approved compressor operating envelope for restrictions.”

The white paper also reported on the selection of a compressor for a medium-temperature distributed refrigeration system in a supermarket.

In both cases, “The results of the study show that although one compressor received a higher EER than another compressor, the compressor with the lower EER could actually be more efficient when the performance of both compressors is evaluated through the entire operating range, using the AEER analysis.”

The actual calculation of AEER was described by Emerson as “using the concept of bin analysis to create a weighted energy efficiency ratio that represents the whole year in a specific location. Taking the percentage of time spent at each condensing temperature and multiplying it by EER at that condition determines it. Then the results are summed to give an annual EER (AEER) number for a compressor at a certain location with specific evaporating and return-gas temperatures.

For more information, the entire report can be downloaded at www.emersonclimate.com/energy.htm.

Publication Date: 05/12/2008

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