The role of chillers in reducing a building’s carbon footprint
Among design features to increase the energy efficiency of Carrier Aquaforce chillers is the use of micro-channel heat exchangers.
Andrew Keogh explores how chiller technology is taking on the energy efficiency challenge.The current version of Part L of the Building Regulations requires a minimum 20% improvement in the energy performance of the services installed in an air-conditioned building. As one of the major energy consumers the chillers could, arguably, make a significant contribution to make to this saving — so what are the possibilities for improving energy efficiency? Measures of energy efficiency
Perhaps the most significant ‘improvement’ stems from adopting a more realistic measure of energy efficiency. In the past, chillers were specified on their full load EER (energy efficiency ratio) at the project design operating conditions. Typically this would mean an EER of around three. The Building Regulations have now introduced the concept of a seasonal energy efficiency ratio (SEER) which combines the efficiency of the chiller at four different loads/operating conditions to generate a more representative measure of ‘real-life’ performance. The SSER will typically be around 4.3 — already an apparent improvement of 43%. Is this enough? Of course, many buildings are served by systems comprising several chillers — possibly of different sizes and maybe even of differing technologies. There is a trend to use absorption chillers powered by waste heat from CHP as the base-load cooling provider, topped up with conventional vapour compression. Determining the overall SEER for a such a system requires much more detailed analysis, which should include the following. • Degree of over-sizing of the total installed capacity.
• Sizing of individual chillers.
• EERs of individual chillers.
• Control mode of the multiple chillers. • Load profile of the proposed cooling load. Carrier has software capable of undertaking this analysis. The results can be very worthwhile, as the system SEER can often be in excess 5.5, over 80% better than the basic EER for a chiller. Other, relatively simple changes can further improve system efficiencies. Whenever a packaged air-cooled chiller is running it is rejecting large quantities of heat to the surrounding air. It is quite likely that a gas or electric heater is being used at the same time to provide hot water for toilets and kitchens. Chillers are now available with heat-recovery exchangers that allow heat previously rejected to atmosphere to be ‘recycled’ to satisfy some or all of this demand. Variable-speed drives
Further advances in technology mean that it is now cost effective to specify variable-speed drives for key parts of an air-conditioning system. For large buildings, a chiller with a centrifugal compressor and a variable-speed drive can offer part-load COPs in excess of 10 — a huge improvement over fixed-speed units. More generally, the use of variable-speed pumps for distributing chilled water can yield tremendous savings — not least because these devices run continuously, whatever the size of the required cooling load. Most chillers will tolerate a variable evaporator flow without problems, provided the maximum and minimum flow rates are respected and the rate of change of the flow rate is not excessive. Interestingly, variable-speed drives do not always yield large energy savings. Varying the speed of positive-displacement compressors (screws and scrolls) yields only modest savings because whilst the refrigerant flow rate reduces, the compression ratio is fixed by the design of the compressor. There are interesting developments in applying variable-speed centrifugal compressors to packaged air-cooled chillers, but there still appear to be significant practical obstacles to overcome.
Measures of energy efficiency — comparing the seasonal energy efficiency ratio of a chiller against its energy efficiency ratio at design operating conditions immediately shows an improvement in efficiency of 43%. Using two chillers instead of one enables the SEER to be raised by a further 35%.
Improvements in the other major components of chillers tend to be incremental. The efficiency of screw and scroll compressors improves slightly with each new generation, as does the performance of the chiller water heat exchangers. There is, however, a significant development in condenser coil construction, introduced last year by Carrier, that now looks set to become the industry standard — micro-channel heat exchangers (MCHX). This technology removes the one great disadvantage of conventional air-cooled condensers, the combination of two dissimilar metals, which can lead to premature corrosion of the coil. Micro-channel heat exchangers offer improved efficiencies, reduced refrigerant charges, lower weight and improved corrosion resistance — all for no increase in cost. Free cooling
The other technology to receive renewed attention as a consequence of the new Part L is free cooling. A building in the UK may have significant demand for cooling (from lights, office equipment and people) even when the ambient temperature is relatively low, and it may be feasible to satisfy at least part of this load with some form of free cooling. This may be achieved through the air-handling units, or through the chillers. Two different technologies are available for achieving free cooling with chillers — hydronic or refrigerant thermo-siphon. Hydronic uses an additional heat exchanger to reject heat directly from the chilled water to ambient. Refrigerant thermo-siphon uses the natural tendency of refrigerant to migrate to the coldest part of a system to transfer heat from the chilled water to the ambient. Each system has its own characteristics and application. Whilst hydronic can offer significantly more free cooling, most hydronic systems of this type will require the addition of glycol (for freeze protection), which will de-rate the performance of the fan coils and air-handling units. The thermo-siphon system produces less free cooling, but is significantly cheaper and does not require the addition of glycol. The appropriate choice will depend on the particular application. In fact, this point is true of nearly all the efficiency enhancements described; the running cost savings, the emissions reductions and the cost effectiveness are all extremely application dependent. This means that it is more important than ever for system designers to collaborate with equipment suppliers with the necessary expertise and analytical tools to be able to evaluate the possible options to find the most appropriate and efficient solution in any particular building or circumstance. Andrew Keogh is UK engineering manager with Carrier Air Conditioning.