Quality & Standards: The Efficiency Equation (May 2008)
May 1, 2008
The refrigeration industry currently has a number of standards and methods of measurement in place to provide a helpful guide for determining the energy efficiency of equipment. These methods, such as the Energy Efficiency Ratio (EER), allow engineers to compare different compressors and operating conditions, ultimately finding a combination that fits the application at hand.
However, a new approach takes these useful measurement tools a step further. The Annual Energy Efficiency Ratio (AEER) allows for a better representation of how refrigeration systems actually operate, taking into account factors that don’t always figure into other simple measurements. The level of accuracy brought about by this new AEER measurement will help system design and application engineers provide the best suitable products for their end-users’ refrigeration equipment needs.
Over the years, the Air-Conditioning, Heating and Refrigeration Institute (AHRI) has set various standards for the industry. Standard 540-2004 spells out the conditions at which commercial refrigeration and air-conditioning compressors must be tested for measuring and reporting important compressor performance data, such as capacity and energy efficiency. These AHRI test conditions are different for high-, medium-, and low-temperature applications, as seen in Table 1. The table also shows the appropriate pressure and temperature settings for different operating conditions.
EER and SEERThe current Energy Efficiency Ratio (EER) method of measurement is calculated according to these AHRI standards and is a single point efficiency measurement for an air-conditioning or refrigeration compressor at high-, medium-, and low-temperature operating conditions. It is based on one rating point, at one evaporating temperature, and one condensing temperature. For low-temperature applications, this is –25 DegF evaporating temperature and 105 DegF condensing temperature to calculate the capacity, power and EER of the compressors. For medium temperature applications, manufacturers use 20 DegF evaporating and 120 DegF condensing temperature. As a general rule, a compressor that has a higher EER at the rating condition could be expected to perform better in a system than one that has a lower EER.
When EER is calculated using the AHRI standard, it is a useful method of comparing equipment and operating-condition choices, offering the benefit of standardization by utilizing one universal measure of compressor performance. It has become a helpful tool for both engineers and manufacturers and is currently a common method for calculation of refrigeration equipment energy efficiency. However, in any given location, the ambient temperatures vary greatly from the single condensing-temperature rating point used with the EER method. In fact, in some regions, temperatures never even reach the ambient temperature corresponding to the EER rating-point condition. (See Figs. 1, 2, 3, 4, 5.) Therefore, EER does not give a complete representation of refrigeration system operation under other temperatures and conditions.
Related to EER is another current efficiency measure, the Seasonal Energy Efficiency Ratio (SEER), which is specifically used to rate only air conditioning equipment. It expands upon the EER analysis and is calculated over a range of expected external temperatures. SEER measures the BTU of cooling output during a typical cooling season divided by the total electric energy input in watt-hours (W-Hr) during the same period. The higher a SEER rating of a unit, the more energy efficient it is.
Because SEER incorporates additional factors, such as seasonality, it is an effective way to compare air-conditioning equipment and is a trusted standard for air-conditioning applications. However, there is a need in the refrigeration industry for a method that takes the EER and SEER concepts one step further.
A new methodThe AEER measure takes the EER and SEER concepts and expands upon them. By using varying condensing temperatures tied to the actual weather data in a particular location, AEER offers the most precise indication of capacity demand and energy usage for engineers and other professionals in the refrigeration industry.
AEER is a weighted average efficiency for a system that utilizes a concept similar to that of bin analysis. Bin analysis examines the performance of a refrigeration system in a specific geographical location, and is calculated by using the annual temperature profile of a geographical location to vary the condensing temperature of the system. With that, annual power consumption and average efficiency are calculated to provide an estimate on the system performance. Although bin analysis is never exactly the same as the real performance of the system, it is extremely useful in comparing different system and compressor options to select the optimum design.
AEER uses this bin analysis concept to create a weighted energy efficiency ratio that represents the entire year in a specific location. AEER is determined by taking the percentage of time spent at each condensing temperature and multiplying it by a weighted energy efficiency ratio. Then the results are summed to give an annual EER number for a compressor at a certain location with specific evaporating and return-gas temperatures. (Table 2 shows the AEER calculation for a ZB45KCE, a medium-temperature scroll compressor running R-404A in Phoenix, Ariz.)
Because AEER is based on the bin hours of different ambient temperatures, it brings an additional component to the table. It uses the bin analysis concept to more precisely take seasonality into account for a fixed-load refrigeration system in a specific location.
BenefitsThe AEER method of measurement offers great benefits to refrigeration system design and application engineers. Because AEER is a single number that represents an average performance for the whole year, it allows engineers to make better equipment decisions. AEER also lends itself easily to calculations of total annual power and energy cost. By using the AEER evaluation equation, found in Table 3, engineers can also better determine the energy cost for the year associated with a particular piece of equipment in a specific application. Because of these benefits, system design and application engineers increasingly support the AEER concept of selecting a compressor at design conditions for capacity and on a weighted averaged annual basis for efficiency. In the end, it provides the customer with the best product for their application.
FutureAEER offers many advantages to the industry. However, there will always be room for further refinement. Future enhancements of the AEER method may include provisions for handling variable-capacity compressors, variable loads and variable energy rates. There might also be a potential need for incorporating more system parameters into the analysis, as system configuration can significantly affect actual system performance.
Although the refrigeration industry’s current measures for calculating the performance of commercial refrigeration systems have been in place for several years and continue to provide helpful assistance to industry engineers, there is still a need for a measure that addresses the various additional conditions that impact refrigeration systems. The AEER approach ultimately meets this need, allowing engineers to better serve their customers and the industry as a whole.