As building operators gear up to achieve ongoing energy efficiency improvements they will need to take both short and long term measures. Dean Ward of Walter Meier (Climate UK) explains how this strategy can be applied to chillers.
By now, those organisations qualifying for the Carbon Reduction Commitment Energy Efficiency Scheme (CRC EES) have - or should have - registered their carbon emissions with the Environment Agency. So the next priority for these organisations is to look at how they can implement the ongoing improvements in energy efficiency that the CRC ESS encourages.

Similarly, those that are currently below the CRC EES threshold will want to stay there and will also be looking at how they can minimise their energy consumption. Many such organisations will be looking to building services engineers to help them make the most appropriate decisions and investments.


Such decisions are likely to fall into two main timescales. Of immediate importance will be quick-win measures which will yield tangible improvements with relatively low investment in a short space of time. Then there will be longer-term measures that may be initiated in the near future but will take longer to complete and reduce energy. This staggered approach will result in ongoing improvements, year-on-year.
These principles can be illustrated by looking at how they might affect chiller plant.

Quick wins

There are several ways in which chiller plant can lose efficiency with time, even when it is maintained regularly. For example, as building usage changes both the level and pattern of demand for cooling may also change. So the controls strategy may need to be optimised to ensure chillers are being controlled in a way that suits current demand, rather than what the demand was when the system was installed.

Also, in a typical chiller system a layer of non-conductive compressor oil will gradually build up on the internal surfaces of the evaporator and condenser. This creates laminar friction which decreases refrigerant flow rates and heat transfer efficiency. It can be quickly and easily remedied by using a special additive which will reduce oil contamination and increase the thermal conductivity of the heat exchangers.

Longer-term wins

The examples given above show just two ways in which chiller efficiency can be increased quickly with relatively little investment. There may be situations, though, where a life cycle cost assessment indicates the most cost-effective option in the longer-term will be to replace the chiller plant.

When such assessments are made, it's important to base them on the latest performance figures for chillers, as advances in compressor design have introduced major improvements in efficiency and responsiveness.


In the vast majority of applications, chillers operate at full capacity only for relatively short periods of time, therefore generally efficient performance at part-loads will achieve a high Seasonal Energy Efficiency Ratio (SEER).

In parallel, with many buildings now having high heat loads throughout the year, there is considerable scope for using free- cooling at ambient temperatures below 15ºC.

Typically, the level of mechanical cooling is gradually reduced from this level, so that 100 per cent free-cooling is achieved at ambient temperatures of 0ºC or less. Field trials show free-cooling can deliver a 35 per cent energy saving over traditional air cooled chillers where the chillers are running 24 hours per day.

Increased use of free-cooling also serves to extend the periods the chiller operates at part-load. So chillers which offer free-cooling should also offer high efficiency at part-loads.

To that end, it's no longer appropriate to think only of screw compressors when it comes to efficiency.

Scroll compressors now offer higher part load performance and higher capacity multiple scroll compressor chillers are now a viable alternative to screw compressor chillers in many projects up to 1000 kW cooling capacity. This is because multiple scroll compressors deliver more efficient step adjustment in response to variable loads, resulting in higher efficiencies at part-loads. In fact, compared with traditional semi-hermetic compressors, reduction in electrical consumption can be as high as 40 per cent depending on loads.

This isn't to say that scroll compressors are the best answer to every project; simply that they have developed rapidly in recent years, as have other types of
compressor.

For instance, oil-free centrifugal chillers with a magnetic rising motor not only offer high efficiencies at part-loads, they also have reduced maintenance requirements for lower cost of ownership. Screw compressors have undergone similar advancements and now offer higher energy ratings.

And then there are the other things that can make a big difference, such as DC or EC fan motors, energy-saving defrost cycles and control options. Many of these features can be selected to suit the project when opting for modular chillers, so that the specification can be tailored to the needs of the building, while also building-in sufficient flexibility for the system to be adapted in line with changes in the building.

The key is be aware of all the options and, perhaps, call on specialist advice that will help building operators make the best choice for them, in terms of commercial viability and reduction of carbon emissions.