Efficiency is cool
The move towards lower-carbon buildings need to be seen in the light of the practical considerations of maintaining comfort levels. So where comfort cooling is required, it needs to be efficient, says Dean Ward.
It is now generally accepted within the building-services community that while natural ventilation is a highly desirable way of maintaining comfortable temperatures, it simply is not viable for many modern buildings. Even where the building fabric is minimising the impact of solar heat gains, the technology-dependent nature of the modern office environment generates significant internal heat gains that necessitate comfort cooling.
At the same time, the push towards low-carbon buildings continues — so it is essential to ensure that where comfort cooling is required, it is delivered efficiently. Furthermore, that efficiency needs to be maintained at all times of day, matching loads to demand as those internal heat gains vary with occupancy levels and other variables.
As is so often the case, new technologies go some way towards facilitating this, backed by tried-and-tested engineering principles and innovative solutions from building-services engineers.
One of the more recent technologies that shows significant potential is the Turbocor centrifugal compressor that is now being used in some air-cooled and water-chillers. These variable-speed centrifugal compressors can respond very precisely to variations in demand and deliver major energy savings at part and full loads. In fact, extensive testing has shown that chillers with Turbocor compressors can achieve a European Seasonal Energy Efficiency Ratio (ESEER) of up to 9.0.
In terms of life-cycle costs, which are also an important consideration to building operators, the use of oil-free, non-contact magnetic bearings in Turbocor compressors eliminates wear and reduces noise and vibration. So these are additional reasons for specifying an inherently efficient compressor design.
However, efficiency does not stop at the compressor, and to achieve maximum efficiency it is important to consider every aspect. For example, the efficiency of heat exchange is clearly important and, again, new designs are enabling higher efficiencies. A case in point is the introduction of micro-channel condensing coils with parallel flows to increase heat-exchange performance by up to 45%, compared to traditional coils. These heat exchangers can also be combined with Turbocor compressors to arrive at a very efficient solution.
Of course, Turbocor chillers will not be right for every project and there are many other ways that specifiers can optimise energy performance. For example, scroll compressors offer very efficient part-load performance and we are now seeing higher capacity (up to nearly 1 MW) multiple scroll compressors with very precise step adjustment entering the market. In the right circumstances these will often offer the most efficient solution with the best carbon and energy savings and fastest return on investment. However, in other circumstances, systems with semi-hermetic screw compressors may be a more appropriate choice. So, as always, the important thing is to choose the best chiller to suit the specific requirements of each project.
As noted earlier, innovative technologies need to be combined with sound engineering principles and many specifiers are now keen to make maximum use of free cooling. With high internal heat gains all year round, there is clearly a lot of potential in the winter for using outside air to cool the chilled water flow and avoid running the chillers’ fans. At other times of year it may be possible to pre-chill the water so that less energy is consumed in reducing the chilled water to its design temperature.
A key element of the design is the set point temperature in the space. In many work environments, raising the set point by as little as 1 K will greatly increase the number of hours through the year that free cooling can be exploited. In fact, this principle can be taken a step further by raising the temperature of the chilled water even further and distributing higher volumes of cooled air around the space. In a properly configured system the energy savings from not running the chillers will more than compensate for the extra fan power. Plus it may be possible to recover heat from the higher-temperature return water for use elsewhere.
Again, making maximum use of free cooling is facilitated by newer chiller designs. For instance, some chillers now enable some free cooling at ambient temperatures as high as 15°C. Field tests indicate that in typical UK ambient temperatures this free cooling will deliver around 35% energy savings over a year in situations where chillers are operating 24 hours a day, compared to conventional air-cooled chillers.
In combining energy-saving technologies with their own engineering expertise, specifiers have the opportunity to deliver low-carbon buildings for their clients without compromising on comfort levels. And that has to be the best all-round solution!
Dean Ward is applied products manager with Walter Meier (Climate UK).