Winning Solutions to Enhance Cooling System Efficiency

It is also important to look at ambient temperature, and whether cooling can be achieved during cooler parts of using outside air. This is possible where the temperature of the room/chamber or fluid being cooled is relatively high compared to ambient. Average UK temperatures for a quarter of the year are below 10°C, and less than 15°C for half of the year. Through careful selection of air handling equipment, it is possible to use free cooling where room temperatures are above 15°C (e.g. office and ambient warehouses). It is also possible to avoid the need for mechanical cooling for high water temperature applications, including data centres.

Defrosting

This is necessary when operating air coolers at low temperatures close to zero or below. Defrosting puts energy into the environment, which then has to be removed by the refrigeration system. It is often carried out on a timed basis, irrespective of whether the cooler has a frost build up or not. By measuring the time run since the last defrost, or by measuring the frost level on the cooler surface, it is possible to eliminate defrosts and therefore reduce the heat load on refrigeration systems.

Operating Temperature

A refrigeration plant removes energy at one temperature and rejects it at a higher temperature. Reducing this temperature difference results in lower energy consumption and increased efficiency. Here are some ideas to help achieve this:

• Maximum cooling temperature – Raising the temperature at which the cooling is provided by 1K improves compressor efficiency by as much as 5% in a low temperature application (e.g. freezers), 4% in a chilled environment and 2% to 3% for air conditioning. By challenging traditional operating temperatures and determining what the maximum allowable temperature for a given application is, it is possible to improve the energy efficiency of the plant. For a recent warehouse project involving five distribution warehouses, the temperature at which the freezers and chill chambers operated was raised by 5 and 2 degrees respectively. In a year on year comparison, figures showed improvements of up to 21% in terms of the energy consumption.

• Minimum heat rejection temperature – Reducing the temperature at which heat is rejected provides energy improvements. Typical savings are 3% for every 1K reduction in condensing temperature. Effective ways of improving the condensing temperature include avoiding air recirculation and keeping condensers free of dirt. A significant benefit can also be achieved by allowing the condensing temperature to follow changes in ambient temperature. For direct expansion systems, this may be possible through the installation of electronic expansions valves in place of the traditional thermostatic type.

Equipment Selection

Ensuring the use of high efficiency equipment and then developing a suitable control that integrates these components together for each specific application is essential to achieving energy savings over the life cycle of a system.

In a typical refrigeration system, the main electrical consumer is the compressors, using 70% to 90% of the total energy, with the remaining 10% to 30% being consumed by fans, pumps, heaters etc. Choosing the most efficient type of compressor is essential – and this means looking not just at the design conditions, but at the profile operation throughout the year. Here are some examples:

• Variable Speed Centrifugal Compressor – Moving from the standard screw compressor to a variable speed centrifugal compressor for an air conditioning application where the load is varying can increase efficiency by 25%. Further benefits can also be achieved by raising the water temperature. For data centre applications, savings of £100,000 per year per 1MW of cooling capacity are possible.

• Variable Speed Reciprocating Compressor – In a brewery system, switching from a fixed to variable speed reciprocating compressor – which matched speed in conjunction with the varying load of the brewery – saved 40,000KW hours per week following the installation.

Variable speed fans also offer an opportunity for reducing energy consumption. For example, by moving away from on/off control for condenser fans to variable speed control, it is possible to match the speed of the ambient conditions and heat rejection requirement. Measured fans power savings of 25% when making this change.

In summary, improving efficiency involves eliminating unnecessary loads and determining the optimum temperatures of cooling heat rejection. Choosing the right equipment for the application is also crucial. Most efficient equipment may cost more initially, but the benefits to the customer could be counted in millions of pounds over a 28 year life cycle.

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