23 September 2011


: Oil management is more difficult in transcritical CO2 heat pumps than in subcritical
heat pumps. Inadequate oil management can damage the compressor very quickly, so that
suppliers’ products use mainly standardised modules of complete CO2 heat pumps and
specially engineered large-scale installations are rare. The current project focuses on the
development of an oil-free turbo-compressor for large-scale CO2 heat pumps, which avoids
the challenge of oil management, allows much simpler heat pump layouts and, for various
reasons explained in the paper, leads to more efficient heat pumps. This paper presents
design steps for a CO2 micro-compressor, which operates at very high rotational speeds,
resulting in high electrical efficiencies, a high power density and a compact machine.
Optimization results and calculated performance data of a single stage are shown for a wide
range of conditions required in this specific application. It is concluded that the stage
presented allows the production of a highly efficient oil-free CO2 compressor for application in
heat pumps, resulting in a coefficient of performance (COP) of 4 at an adequately chosen
reference point.