30 May 2005

CO2 THERMOSYPHONS AS HEAT SOURCE SYSTEM FOR HEAT PUMPS – 4 YEARS OF MARKET EXPERIENCE


In Austria ground-coupled heat pumps are mainly used for heating only purpose in domestic
applications. In principal two heat source systems are used: direct evaporation and secondary loop
systems. These systems have been discussed to be harmful to the environment in the case of leakage and
the later one needs an electrically driven circulation pump which reduces the system COP significantly.
A CO2 probe based on the closed two-phase thermosyphon principle is a very promising alternative
to conventional vertical systems. After a detailed experimental as well as theoretical investigation the first
commercial system has been installed in 2001/02. Up to now about 100 systems are in operation: The
efficiency of the heat pump system is higher for this ‘advanced’ system because of the natural circulation
of the working fluid (CO2) instead of a forced circulation in case of brine as the heat carrier.
The paper describes the principle design of the system, measured results as well as market
experience collected so far with the CO2 probes. Finally, a naturally circulating CO2 collector, i.e. for
horizontal installation, is introduced.