high-speed centrifugal compressors, refrigerants, gas bearings
Demand for heating and cooling continues to grow as populations and living standards continue to increase around the globe. At the same time, persistent uncertainty in energy prices and increasing awareness of the environmental impacts associated with the use of fossil fuels - including climate change and poor air quality - are motivating an interest in reducing energy use, in general, and fossil energy, in particular. Heat pumps with scroll or reciprocating compressors have been gaining ground for residential, commercial and industrial heating and cooling applications on a scale up to several tenths of kWmech (over 100 kWtherm). They usually reduce energy use relative to conventional approaches (e.g. fossil-fuel heating in conjunction with electrically-driven cooling) but require higher first costs. Micro-centrifugal compressors with impeller diameters as small as 10-20 mm operating at rotational speeds exceeding 100,000 rpm are now feasible. They are assembled with precisely machined components and they are driven directly (i.e. without gears) by efficient high-speed motors. They have impellers suspended without contact with solid surfaces through bearings lubricated by refrigerant vapor. Thus, they realize high efficiencies and eliminate well-known disadvantages resulting from the use and management of conventional lubricants in positive-displacement systems such as having to: i) ensure adequate lubricant circulation (especially for two stage systems), so as to prevent lubricant depletion at the compressor and associated increased wear and lubricant accumulation in heat exchangers (especially cold evaporators) and associated reduced rates of heat transfer (especially from enhanced surfaces); and ii) limit the maximum operating temperature of high-temperature heat pumps, so as to maintain long-term lubricant chemical stability. HFO-1336mzz(Z) (CF3CH=CHCF3) is a hydro-fluorolefin with a normal boiling point of 33.4 oC and an A1 safety classification (non-flammable, lower toxicity) according to ASHRAE standard 34. It has an ultra-low global warming potential over a 100 years of 2, which virtually eliminates business risk from climate protection regulations emerging around the world and allows R&D investments for technology development. It has been commercialized for use as a component in R-514A, a replacement for HCFC-123 in centrifugal chillers, as a working fluid for high temperature heat pumps and Rankine power cycles and as a foam expansion agent. This paper proposes novel heat pump systems based on micro-centrifugal compressors for selected heating and cooling applications, discusses refrigerant selection, evaluates the performance of systems with HFO-1336mzz(Z) as the refrigerant and identifies promising applications with potentially attractive economics for further development.