What is EER Value?
"EER" represents the energy efficiency ratio and rates the efficiency of mechanical cooling systems (e.g. chilled water generators). EER describes the ratio of cooling capacity (in kW) to power input (in kW). Thus, the higher the EER value, the more efficiently the machine operates.
For chilled water generators, DIN EN 14511 specifies certain design parameters to ensure comparability between different manufacturers.
In precision air conditioning technology, there is only a basic definition of such design parameters. As a result, EER data mostly depends on operating points and standard design parameters used by various manufacturers. This makes comparison difficult, especially since some manufacturers use "total" cooling capacity as their reference point, while others use "reasonable" cooling capacity.
What is PUE Value?
"PUE" refers to the effectiveness of power usage. PUE is the quotient based on the ratio of the total power consumed by a data center to the total power consumed by the IT equipment. The total power consumed by the data center is the sum of all energy consumed by its key components (IT equipment, cooling, lighting, UPS systems, batteries, power distribution equipment, generators, fire protection systems, etc.).
Therefore the calculated value is always higher than 1. The lower the PUE value, the more efficient the data center. For example, if the PUE is "3.0", this means that the total power consumed by the entire data center is three times greater than the power consumed by the IT equipment.
A highly efficient and well-planned data center can achieve PUE values of 1.3 to 1.6. However, on average, the PUE is between 1.8 and 2.5 (depending on which study you look at).
To determine the relevant measurement data and calculate the PUE exactly, several prerequisites must be met. These include the time at which measurements were taken and the location of the measurement sensors.
PUE describes the average annual energy consumption of a data center. It is not used to compare different data centers, but to compare or evaluate the recovery/decline of a data center over its service life.
The attainable PUE also depends on the location and therefore the climatic conditions. Since PUE represents the annual average, different PUE values can be obtained at different times of the year (e.g. when Free Cooling is used more for air conditioning).
In the field of data center air conditioning, in particular, the term "PUE" is not always used correctly. At various times, manufacturers of air conditioning solutions have declared PUE values below 1.1. But based on the official definition of PUE outlined above, such values are almost impossible to achieve.
These manufacturers actually calculate what's called "pPUE" – partial power usage efficiency, which only tells part of the story. This partial value is calculated basically as described above, but does not cover all the components required for the data center. Instead, it only takes into account the power consumed by the air conditioner.
These calculations are correct and allowable. However, it should be absolutely clear to the reader that the value is a "pPUE" and not a "PUE".
Example:
PUE : Power usage effectiveness (Energy Consumption Efficiency)
PUE = TOTAL CONSUMPTION / IT CONSUMPTION
DCIE: Data center infrastructure efficiency
DCIE: IT CONSUMPTION / TOTAL CONSUMPTION (%)
or DCIE = 1/PUE
Let's make an example calculation
Total Energy Consumption of the data center 1 MW (1000 KW)
Let the energy consumed by IT equipment be 600 KW.
PUE = 1000/600 = 1.67
DCIE= 600/1000
= 0.60 = 60%
What is AER Value?
"AER" refers to the airflow efficiency ratio and is a value dependent on the operating points (airflow and external pressure). AER (invented by Stulz) describes the ratio of the fan's power consumption to the airflow rate of an air conditioning unit.
This value measures the effectiveness of the "movement of air through the data center" function, regardless of how much cooling capacity is achieved with this airflow.
The unit used is watts per cubic meter per hour or W/(m³/s). For example, if an air conditioning unit moves 30,000 m3/h of air at an external pressure of 20 Pa and requires 4,500 watts of fan power consumption to do so, AER = 4,500 / 30,000 = 0.15.
What is ESEER Value?
"ESEER" stands for European Seasonal Energy Efficiency Ratio and defines the annual energy rate of a chilled water producer. ESEER is calculated by combining EER full load and part load values. The exact calculation is defined by the "Eurovent Certification Company" (Document RS6 / C003-2011). This value is specified for both air-cooled and liquid-cooled chilled water generators.
It focuses specifically on the machine in part load mode and takes into account that the chilled water generator must reach full cooling capacity for a few hours per year, since in most applications the required power is in the part load range between 25% and 75% for the vast majority of the time. The calculation on which ESEER is based (see table and see formula) is a European compromise based on extremely diverse load requirements and outdoor temperature profiles. Therefore, ESEER is a comparative value that has little in common with the real-life load profile. As a result, the calculation is based on different heat loads and various external and chilled water temperatures, which are then weighed. The basis of the calculation is a temperature of 7 ° C for water leaving the chilled water generator.
ESEER rating and weighting table:
Load Outside air temperature Chilled water temperature (inlet) Weight factor
(air cooled) (liquid cooled)
100% 35 °C 30 °C 3%
75% 30 °C 26 °C 33%
50% 25 °C 22 °C 41%
25% 20 °C 18 °C 23%
ESEER = 0.03x EER (100%) + 0.33x EER (75%) + 0.41x EER (50%) + 0.23x EER (25%)
As explained above, ESEER is a comparative value based on average load and temperature profiles, which are always the same.
In other words, this value does not provide meaningful data for a "data center" application because this makes completely different demands on the chilled water producer. The load in the data center is more or less constant, but the water temperature is much higher to enable the system to operate with energy-saving Free Cooling. The ESEER calculation also ignores the Free Cooling option.
What is COP Value?
"COP" stands for coefficient of performance and basically applies only to heat pumps or chilled water generators with a recirculated refrigerant circuit in heating mode. COP shows the ratio of heating capacity provided (in kW) to electrical energy consumed (in kW). Electrical energy consumed includes power consumed by auxiliary systems such as pumps for heating systems and/or groundwater pumps. COP is calculated under defined test conditions that govern temperature and set times, among other parameters. The test method is defined by DIN EN 255.
Basically the following applies: the higher the COP, the more efficiently the machine operates.
Some compressor manufacturers provide a "COP", which they also call a "coefficient of performance". However, this figure differs fundamentally from the COP value described above for heat pumps and basically indicates "only" the efficiency of the compressor at one operating point. That is, the ratio of the compressor's cooling capacity (in kW) to the compressor's electrical power consumption (in kW) as a function of the refrigerant, evaporation and condensation temperature, and superheating and subcooling.
These two values may share the same name, but are not calculated in the same way and apply to different applications and/or equipment/components. Therefore, they should not be confused or compared with each other.
