An efficiency boost for chillers

Published:  01 June, 2004

Madley
Energy savings on the central chiller plant at the BT Madley Earth Station are being boosted by variable-speed drives on condenser fans and the use of Excalibur’s Liquid Pressure Amplification.

With air-conditioning being such an energy-demanding service, achieving and maintaining an efficient system is vital. Gareth Holden gives some pointers to making an immediate impact on the efficiency of chiller plant.

Air conditioning in the modern office building is, in many cases, unavoidable. It is an energy-intensive service, with air-conditioned offices typically consuming 226 kWh/m2 year compared to 85 kWh/m2 for offices using natural ventilation.

Control costs

However, there is scope to control these costs

In large central air-conditioning systems, the chiller consumes a large proportion of this power. Typically they receive an annual or bi-annual service. The quality of this service is variable (normally in proportion to the costs) and due to the general lack of understanding by the client it can be difficult to know if value for money is being achieved. Provided that space temperatures are maintained and occupiers are happy, it is assumed that an efficient system is being achieved.

This could be far from the truth. Many factors influence the efficiency of packaged liquid chillers, and a high price could be being paid in terms of electricity consumption and emissions. A poorly maintained/operated chiller could easily consume 20% more electricity than necessary.

Many factors which affect energy efficiency can be determined from a visual inspection. The condenser on an air-cooled chiller should be cleaned regularly to ensure a good airflow and efficient heat exchange between the refrigerant and ambient air. Condenser fan operation should also be checked; it is not unusual to see several faulty fans on one condenser. Many systems will continue to operate in this condition until the height of summer, but when it is considered that for every 1 K increase in condensing temperature, refrigeration efficiency is reduced by about 2%. The faults described above could easily raise condensing temperatures by 5 to 10 K.

A shortage in refrigerant charge can have a drastic effect on refrigeration efficiency. A 15% shortfall could reduce capacity by up to 50%, resulting in the chiller operating for longer periods to satisfy comfort conditions. A good indication of refrigerant charge is to check the sight glass that is located in the liquid line between the condenser and expansion valve. If there is a constant flow of vapour (bubbles) visible, the system could well be undercharged.

The effective control of chillers can also impact on energy efficiency. It is not unusual to see two chillers on the same system, each operating at part load. A recent example I encountered was two screw chillers, each operating at 35 to 40% of full capacity. Both chillers were consuming 70% of full-load current. In this case the manufacturer was able to provide a standard control which ensured the first chiller was fully loaded before the second was enabled — a similar solution could be achieved through most building-management systems.

Improve efficiency

Once chillers are operating efficiently, there is scope to further improve efficiency. My company, for example, specialises in the installation of liquid-pressure amplification (LPA) and variable-speed drives to condenser fans and associated pumping systems. The application of this technology can further improve efficiency by 30 to 40%. With LPA in place, the compressor is only required to raise the refrigerant condensing temperature to 5 to 15 K above ambient. Recent examples include a contract to retrofit both these technologies to all of the air-conditioning chillers at Heathrow Airport, which is reducing carbon-dioxide emissions by 2248 t a year.

Gareth Holden is with Excalibur LPA Ltd, Pelican House, 83 New Street, Andover, Hants SP10 1DR.



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