Chillers: efficiency takes off
Chillers are key refrigerating systems since, according to the Department of Energy, in the US, they account for about 13% of the power consumed by the nation's buildings and 9% of the overall demand for electric power. Examples below illustrate recent progress achieved regarding chillers in terms of energy efficiency. - The "free cooling" chiller concept is gaining importance in Information Technology (IT) and commercial building applications. It specifically refers to package air-cooled chillers with an integral free cooling circuit and a self-contained control system. Ambient air, temperature permitting, is used to cool the chilled water supply as in a conventional air-cooled chiller, but with the addition of an integral dry cooler circuit - built into the same mechanical assembly that supports the condenser coils and fans, - thereby reducing power consumption and direct expansion mechanical cooling. According to K. Carpenter, Sales Manager at Emerson Network Power, in applications where the load is relatively constant, such as data centres and offices highly populated with IT, a free cooling chiller will typically reduce energy consumption by about 35%. When designing a free cooling system, it is important to bear in mind that the higher the chilled water temperatures, the higher will be the energy savings. An efficient free cooling chiller requires the use of peak efficiency twin rotor screw or fully hermetic scroll compressors. Coil areas are maximized so that airflow rates and fan power requirements are similar to those of conventional chillers. - Chillers based on the oil-less centrifugal Turbocor compressor are known to deliver high efficiency thanks to variable speed capability, magnetic bearings and compact design. Two recent projects in the UK, using the Turbomiser chiller - developed by Geoclima and harnessing the benefits of this compressor - have been reported to show significant energy savings. The chiller's outstanding energy performance is due to the use of a combination of flooded evaporators to maximize heat exchange and control technology fully integrated with the compressor control, plus high-efficiency fans. This approach ensures that high COP is delivered at all times, optimizing chiller performance in response to changing ambient temperatures and load. A 3-MW-installation at the Dorchester Hotel in London has replaced the existing water-cooled chillers and cooling towers and is expected to deliver a payback in the first year on the premium over conventional chillers through improved energy performance and use of an air-cooled condenser, saving on water and chemical treatment costs. The second installation at the Hilton Hotel in Stansted Airport is anticipated to deliver 30% energy savings compared with traditional designs based on reciprocating, screw or scroll-based compressors thanks to a high part-load performance. - Nanotechnology used in a refrigeration lubricant additive could significantly boost the energy efficiency of chillers, according to the US National Institute of Standards and Technology (NIST). NIST has found that dispersing the right amount of copper oxide particles in a standard polyester refrigerant lubricant and combining it with R-134a improves heat transfer by between 50% and 275%. The particles, which are 30 nanometres in diameter, are thought to stimulate double bubbles - secondary bubbles that form on top of bubbles initiated at the boiling site. Bubbles can carry heat away from the surface, and the fact they are being formed more efficiently means the heat gets transferred more readily. Just how nanomaterial additives to lubricants improve the dynamics of heat transfer in refrigerant/lubricant mixtures is not thoroughly understood, but the right concentration of nanoparticles seems to be crucial.