Reducing the costs of chiller plant
Reducing the lifetime costs of chillers — Wayne Perrins.
Understanding the life cycle costs of a chiller installation requires a clear view of the full picture. Wayne Perrins explores the features of chillers that can make all the difference.When considering the energy consumption of an air conditioned building, it is vital to take a close look at the chiller plant — not just its capital cost but also its performance during the life of the installation. Here, the devil is very much in the detail; simply assessing the capital costs and co-efficient of performance (COP) at maximum load provides a partial picture that will do the end client no favours. This cost of ownership approach is, of course, a well established principle that many industry pundits talk about at length — and quite rightly so. Unfortunately, the enthusiasm for this principle that is prevalent in seminars does not always manifest itself in real projects. Walk the talk
However, all the talk has certainly made end users more aware of these issues, and there is now far more pressure on the building-services industry to walk that talk. So we need to get away from sizing chillers on the basis of maximum ambient temperatures that may or may not occur for a few days every year and start to think about whole-life costs. That approach means giving higher priority to criteria such as COP at part-load and the system’s ability to respond accurately and efficiently to variable loads. Compressors with continuous capacity control achieve a 15 to 20% energy saving over step control and, along with an electronic control system, enable chillers to precisely maintain a constant outlet temperature independent of cooling load. This method of control benefits both air conditioning and industrial process use. Chillers with this type of control exhibit a much higher COP when operating at part load. For example, a Hitachi Samurai chiller operating in an ambient of 20°C and around 70% capacity will provide a coefficient of performance almost 175% better than the same chiller operating at 35°C at 100% capacity. The power input is more than halved and running costs are similarly reduced. With chillers having more than one compressor, the typical control configuration will wait for the first compressor to reach load before starting the second — and so on. However, starting each compressor unloaded and then modulating them together to meet the cooling load will use less power at start-up and avoid the need to oversize cables to allow for peak currents. If the chillers have a high power factor, it will not be necessary to install a series of capacitors to compensate for the reactive energy consumed by the motors. Fine detail As noted earlier, the key to assessing the real cost of ownership is to get to grips with the fine detail. For example, the efficiency of oil separation in the refrigeration circuit can make a significant difference to life-cycle costs. In particular, discharging too much oil to the refrigerant cycle will increase the maintenance requirements of the chiller and reduce performance. Ideally, technologies such as more advanced cyclone oil separators should be used to ensure the oil is no more than 1% of the refrigerant weight. The minimum water volume that must be maintained to prevent excessive stops and starts is another consideration that should not be ignored. Keeping compressor cycling to a minimum increases overall efficiency of the system by reducing electrical demand. The same is true for the refrigerant volume, which can be minimised by using a compact plate heat exchanger to obtain maximum efficiency with the least possible amount of refrigerant.
Chillers with continuous capacity controls, such as these Hitachi Samurai chillers, have a much higher COP at part load, a feature that can be exploited to reduce whole-life costs.
Paying attention to other chiller components can also make a difference. Using DC fan motors, for example, will enable finer, linear control, with up to 10% reduction of power losses compared to AC motors. These are some of the key factors that ought to be considered when selecting chiller plant on the basis of whole-life costs. It is also important to have a good understanding of the characteristics of the chillers being used — generally by working closely with a knowledgeable supplier. With the support of specialist expertise it is possible to ensure not only that the system is designed to give optimum performance but also to devise a control strategy that takes account of a chiller’s characteristics to maintain optimum performance on a day to day basis. All these issues can be addressed properly simply by taking a step back so the full picture comes into view and fine-tuning designs to suit the characteristics of the chillers being used. Wayne Perrins is applied products manager with Axair Climate.