Measuring performance for a modulated system is a very challenging task. For a fixed speed air-conditioning system, performance is measured as energy efficiency ratio (EER) or coefficient of performance (COP). To calculate EER or COP, the air-conditioning equipment has to be tested in a psychometric room per the ARI550/590 standard. The standard cooling test for ARI550/590 specifies that the outdoor should be maintained at 35°C and indoor should be maintained at 27°C dry bulb and 19°C wet bulb. Under such operating conditions, the full load cooling capacity and input power is measured. Dividing the cooling capacity by input power gives the EER of the machine. As can be seen, this is a fairly straight forward measurement as only one operating point is involved.
A capacity modulated system can operate at various levels of cooling capacity. Depending on the compressor technology used, the cooling capacity can range from 10% of full load capacity to 100% capacity. Given the wide operating range, the questions would be: At which point do you measure the cooling capacity? Should we select the 100% capacity point, the 50% capacity point or any other point? Similarly, at what ambient should the machine be tested? Since capacity modulated systems are sold on the value of seasonal efficiency, most of the times they operate at ambient conditions that are much lower than the 35°C (as used in the single point EER measurement).
So the following indexes can be used to comprehensively measure energy efficiency of VRF systems
Energy Efficiency Ratio (EER)
Energy Efficiency Ratio (EER) refers to the air-conditioning cooling performance at a single point. The higher the EER value, the better the air-conditioning efficiency. The units are W/W or kW/h/W.
EER =Cooling capacity/cooling power consumption
Coefficient of Performance (COP)
Coefficient of Performance is similar to EER, but measures the heating efficiency of a system. A higher COP represents a more efficient system. The units are W/W. COP = heating capacity/heating power consumption.
Overall System Efficiency
While the specific details in calculating the overall system efficiency vary from country to country, the general principle is generally the same. The regulation selects 3-4 specific points to measure EER or COP and then weights the results based on the amount of time that the system is likely to operate at or near that point. This system efficiency can be calculated separately for heating and cooling or combined into a single number. A common formula for system efficiency is listed below.
System Efficiency=a × A + b × B + c × C + d × D
A: Performance efficiency at 100% Capacity Operation
B: Performance efficiency at 75% Capacity Operation
C: Performance efficiency at 50% Capacity Operation
D: Performance efficiency at 25% Capacity Operation a, b, c, and d are Weight Coefficients based upon the average ambient conditions in a country or region.
A few examples of specific standards for overall system efficiency are listed below.
ANSI/AHRI Standard 1230, Performance Rating of Variable Refrigerant Flow Multi-Split Air-Conditioning and Heat Pump Equipment (2010)
GB21454, VRF Air-conditioning (Heat Pump) Energy Efficiency Rating (2008)
JRA 4048-2006. List calculation methods of Cooling Season Total Load (CSTL), Cooling Season Total Electrics (CSTE), Heating Season Total Load (HSTL), Heating Season Total Electrics (HSTE), Cooling Season Performance Factor(CSPF), Heating Season Performance Factor(HSPF) and Annual Performance Foctor (APF).
South Korea Multi Heat Pump EER Standard was effective from April.1st. 2012. In this standard, IEER, COP and EERa are measure indexes of multi heat pump EER.