When undertaking a chiller selection for a customer recently we were asked a question about the energy efficiency of chillers as given by the coefficient of performance (COP) and how COP changes with the ambient air temperature.
COP is defined as kW refrigeration / kW electrical input.For example a 100kW chiller using 30kW power has a COP of 100/30 = 3.33.
We were also asked to compare this between different brandsof chillers and realised that there was a lack of published data available onthe performance of different refrigerant gasses when it comes to changes inambient air temperatures.
Just because a particular chiller is rated at 38 deg Cambient air, does this make it better? Or more efficient? The answer is clearly no.
We set out to prove if there was a relationship between energy efficiency and ambient air temperatures and to see if this changes between refrigerant gasses.
As a local chiller manufacturer Summit Matsu Chillers uses a number of powerful vendor software selection tools from some of the world's largest manufacturers of compressors, evaporators, and condensers. Summit Matsu Chillers goes to great lengths to buy from the same companies that supply to the worlds largest chiller and refrigeration companies.
Three refrigerants were chosen for the study: R407c, R134a, and R410a.
For R407c and R134a the selection program from RefcompCompressors was used
(data measured based on standard ISO 917, and 2000/14/ECDirective TUV Italy)and as a cross check the selection program from Hanbell Compressors was alsoused. A single, semi hermetic screw compressor was used for the measurement.
For R410a we used the published data available from acompetitive chiller manufacturer. In the competitive chiller two hermeticscroll compressors were used.
The chiller size chosen in each case was nominal 100kW.
Although screw compressors are more efficient than scrollcompressors, what we were interested in was the trend in refrigerant gas COPrather than the performance of different compressor technology.
For R407c and R134a we held water leaving temperatureconstant (7 deg C) and the calculation was run at ambient air temperaturesof 25 deg C, 30 deg C, 35 deg C, 40 degC and 45 deg C.
For R410a the published data was used at the same points.
The same method was used for a higher water outlettemperature of 12 deg C.
The results of COP efficiency vs. ambient temperature are given below:
R407c is the most efficient refrigerant (highest COP) at most ambient conditions.
R134a becomes more efficient as ambient air increase above 30deg C.
R410a is the worst performer and is only more efficient thanR407c when the air temperature is above 37.5 deg C.
When the same method is run at a higher leaving watertemperature of 12 deg C the result is the same.
Data from the Bureau of Meteorology was used to evaluate which refrigeration gas is the best for the Sydney market. The details of maximum ambient air temperatures for Sydney Airport for 2010 are given below.
When customers are assessing chiller options it is important to look at the COP over the entire range of operating temperatures they will encounter.
Customers in most parts of Australia get HOT days in Summer,but if the average temperature of all 8760 hours per year are used, most of thetime R407c is more efficient.
R407c is the most efficient gas for chillers for thislocation, and most locations in Australia,given that most of the time the ambient air temperature is below 25 deg C.
Although R134a is more efficient than R407c the vessels within the chiller such as compressors, condensers, and evaporators need to be larger to affect the same heat exchange. This has a larger carbon footprint for production and this was not taken into account.