30 May 2005

MODELING OF A CO2 THERMOSYPHON FOR A GROUND SOURCE HEAT PUMP APPLICATION


Ground source heat pumps provide high efficiency operation during the space heating season, even
during time periods of very low or rapidly varying outdoor temperatures, due to the use of the ground as a
stable-temperature heat source. Up to now most ground source heat pump systems rely on a brine solution
in a secondary fluid loop to extract heat from the soil. Recently, a new concept has gained interest which
uses a closed, self-circulating CO2 thermosyphon to extract heat from the soil. Advantages of this system
are the higher efficiency compared to secondary fluid loops and environmentally friendliness. The
disadvantage of this system is that it only works in heating and not in cooling mode. The scope of the
analysis presented in this paper is to model the two-phase counter-current annular flow behavior of the
coupled system, given the required amount of heat output and thermal characteristics of the soil. Using
the theoretical model, a parameter study with respect to tube diameter was conducted to investigate trends
applicable to design optimization. The results show that a major part of the pressure drop is caused by
gravitation, whereas friction only needs to be considered for small diameter tubes.