Annual energy efficiency of chillers – a comparison
The energy efficiency of chillers has in the past mainly been indicated by manufacturers by stationary maps at full load operating points or the EER = Energy Efficiency Ratio, which only represent a fraction of the real operating behaviour during the course of the year. The energy losses due to partial load operation were not considered. With the introduction of the ESEER (European Seasonal Energy Efficiency Ratio, Eurovent) and SEER (Seasonal Energy Efficiency Ratio, DIN EN 14825), an attempt was made to take this shortcoming into account and enable more realistic assessments.
EER = Energy Efficiency Ratio
The EER is a parameter for quantifying the energy efficiency of heat pumps and chillers or of systems containing such units. It is defined as the ratio of the “refrigerating capacity produced” to the “electrical capacity used”. A high coefficient of performance thus indicates high energy efficiency.
It should be noted that cooling capacity is usually not only understood as the sensible cooling capacity, which is associated with a reduction in air temperature; rather, it also includes the latent cooling capacity, which is associated with dehumidifying the air (removal of condensation): The condensation heat of the water in the air (moisture) must also be dissipated by the cooling unit.
If the proportion of latent cooling capacity is high – for example, when cooling incoming fresh air on a humid summer day – the sensitive cooling capacity is correspondingly lower.
For the EER, however, the total cooling capacity is always considered. The degree of dehumidification and thus the proportion of latent cooling capacity depends not only on the operating conditions but also on the design of the unit: a larger surface area of the heat exchanger at the evaporator and a higher air flow tend to result in weaker dehumidification and correspondingly stronger sensitive cooling capacity.
The EER of a chiller depends on two relevant temperature levels, i.e. the temperature at which the cooling power is delivered and the ambient temperature of to which the waste heat is transferred. EER values are measured under standardized conditions, i.e. at given values of temperature and humidity and at full load, i.e. maximum cooling capacity. In case of an air conditioner, the EER is measured at 27 °C indoor temperature and 35 °C outdoor temperature according to the standard. If the same unit had to cool a room to 22 °C, it would achieve a significantly lower EER value due to the higher temperature difference.
There are no specific agreed conditions for EER, and it is based on a single specific nominal condition as specified by the manufacturer, so therefore it can be difficult to compare.
ESEER = European Seasonal Energy Efficiency Ratio
The ESEER is defined and certified by the Eurovent Certification Company. It is not specified in any standard but is based on DIN EN 14825 and DIN EN 14511. The certification is voluntary. The manufacturer sends the technical data to the certification body. Devices are randomly selected to check the data and are then handed over to an independent test laboratory. There, measurements are then carried out according to the standards of DIN EN 14511. For air and water-cooled units, the characteristic value is composed of the weighted EER at four defined operating points.
ESEER combines full and part load operating energy efficiency ratios for differential seasonal conditions and appropriate load weighting factors. This is correct for air conditioning applications, where the load varies with ambient temperature but not for arbitrary applications e.g. not for data centres.
The outlet temperature to be set at the evaporator varies depending on the application. In case of cooling via cooling ceilings, it is 18°C and in case of air conditioning, for example with fan coils, it is 7°C. There are also other temperature settings for use in commercial refrigeration. The ESEER is calculated according to equation
ESEER = 0,03 ∙ EER100% + 0,33 ∙ EER75% + 0,41 EER50% + 0,23 EER25%
where, for example, ESEER75% stands for the EER at 75% part load condition.
For pure chillers there is also the indication of the ESEER for European SEER. Corresponding values can be significantly higher than the SEER values (see below) because a simplified procedure is used which does not take standby losses into account. The SEER values are always relevant for air conditioning units.
SEER = Seasonal Energy Efficiency Ratio (for EcoDesign evaluations)
For the estimation of energy efficiency in practical operation, pure EER values, which are measured e.g. only at 27 °C inside and 35 °C outside and at full load, are not very meaningful. The average EER value under the temperature conditions actually occurring during operation would be relevant for practical purposes.
In order to better reflect this, the standards have been further developed accordingly. Since 2013, the provisions of LOT 10 of the Eco-Design Directive have been in force in the EU for air conditioning systems with a cooling capacity of up to 12 kW. Since then, manufacturers of air conditioners have been required to specify the Seasonal EER (SEER) (coefficient of performance in cooling mode) in accordance with EN 14825. This is a seasonally averaged value calculated from the measured EER values for different outdoor temperatures (20, 25, 30 and 35 °C). The weighting of the values for these measured temperatures is made according to the climatic conditions in Strasbourg, which are reasonably representative for use in Central Europe. It is important to note that the calculation of the SEER values largely takes partial load operation into account: Full cooling capacity is only required at 35 °C, but at lower outside temperatures correspondingly reduced capacities down to 21.1 % at 20 °C are required.
The consideration of SEER values (instead of only EER values) has important advantages for a fair comparison of different devices. Not only is it good that different operating temperatures are considered at all, but manufacturers have an incentive to optimize their equipment for a wide range of operating conditions. The EcoDesign Directive is a regulatory requirement in the EU and minimum levels need to be achieved to be compliant.
In addition, the considerable efficiency advantages of appliances with speed-controlled compressors (inverter air conditioners) are now adequately captured. In fact, such inverter units run at a fairly efficient partial load operation at not too high outdoor temperatures, achieving SEER values that are much better than EER values at full load. Old units (with non-power controlled compressors), on the other hand, have to adjust the power by cyclic operation, i.e. they alternately run at full load and not at all. Standby losses are also considered in the SEER, although standby consumption is already strongly limited.
The SEER value of an air conditioner must also be shown on the corresponding EU energy label – together with the rated cooling capacity, the annual power consumption (for an operating time of 350 hours per year) and the noise level. In addition, an energy efficiency class is assigned on the basis of the SEER value, with class limits depending on the cooling capacity and type of appliance (split, single or dual-hose mobile).
Under full load (100%), the performance of a device as defined in the standard corresponds to the (maximum) cooling load of the building. The annual power consumption is the quotient of the reference annual cooling load and the weighted seasonal performance factor in cooling or heating mode called SEERon and the electrical energy consumption in various other operating modes of the unit.
To determine the SEERon, 24 temperature steps between 17 and 40°C ambient air temperature are defined for the recooling. There is also a weight distribution of the steps in relation to the total cooling period. The standard as of August 2016 only reflects the Strasbourg site, others are to follow. Exemplary calculations with other location data in northern Europe show only minor deviations (less than ± 5%), i.e. Strasbourg is well suited as a reference for locations.
For each stage, an EERbin value is determined by interpolation between the four partial load EER values EERd measured or specified by the manufacturer. For the temperature steps below 20°C and above 35°C the EERd- value for 20°C or for 35°C is used.
All in all, the calculation of the SEER considers much more realistic air and condenser temperatures than the ESEER, which, in addition to the water outlet temperature, have a significant influence on the machine performance. In addition, the SEER considers operating conditions with electrical power consumption and simultaneously unavailable cooling capacity (passive mode, e.g. crankcase heater, etc.), which are not considered in the calculation of the ESEER, thus significantly increasing the informative value compared to the ESEER.
The previous practice of specifying the EER at one to a few operating points does not adequately characterize the efficiency of a cooling unit over the course of a year.
In particular, the most frequently specified operating point at 13 hours per year, is very rare for the Strasbourg site according to the standard. The EER of cooling units tends to decrease with increasing air temperature, which means that the specified EER indicates a rather poor stationary operating condition. Furthermore, the EER of individual appliances varies – conclusions from one operating point to the rest of the stationary operating map are not meaningful.
The EER does not provide any information about the behaviour in partial load operation. The ESEER, on the other hand, combines EER values at different partial load conditions, which are weighted by their frequency in the year. It is therefore more meaningful than the EER at a single operating point.
Finally, the SEER allows an evaluation of efficiency considering 24 outdoor temperature levels, weighted by their frequency, as well as an approximate cycle operation, which is represented by the power consumption in different modes during which the unit is not cooling. The SEER thus represents the most meaningful of the key figures presented. Nevertheless, such key figures have their limits.
They serve the purpose of comparability of devices, but for this purpose generalizations have to be made. In order to make more precise statements about a specific device at a certain location, in a known system, at a defined load, simulations are necessary. Especially the partial load and cycle behaviour can be mapped much better. However, the more meaningful a key figure or process is, the more time and effort it takes to determine it. In case of simulations, simulation models are needed which are not widely available.
There are other parameters which are subject to the regulatory requirements of the EcoDesign Directive which need to be achieved to be compliant. Examples are the coefficient of performance (COP) for heat pumps and the SEPR for process cooling.
The key figure EER used to describe the stationary performance of chillers is insufficient for a characterisation of the annual operation. The introduction of the key figures ESEER and especially SEER has contributed to a significant improvement regarding the consideration of the partial load operation of chillers. However, neither SEER nor ESEER are able to show the influence of different control strategies on the overall performance of a chiller. Simulations provide detailed results and thus allow a reliable planning for the selection of a suitable cooling solution.