18 May 2017

O.3.5.2 Absorption of CO2-NH3-H2O mixture in mini-channel heat exchangers

Compression resorption heat pumps (CRHP) are a promising option to upgrade waste heat from industry.
Alternative working fluids can further improve the efficiency of CRHP. The ternary mixture NH3-CO2-H2O has
been identified as a promising working fluid for CRHP and has the potential to further enhance the coefficient
of performance (COP) of the cycle compared to the traditionally used ammonia water mixture. So far the
studies on the NH3-CO2-H2O mixture have focused mainly on carbon capture applications. But the desired
operating conditions are different than for CRHP applications, e.g. the NH3 concentration. Additionally the
absorption process with the mixture in tubular absorbers has not yet been reported. The focus of this study is
therefore to investigate experimentally the absorption process of a CRHP with this ternary mixture. To reach
this goal a model is developed for ammonia-water that takes into account the kinetics and mass transfer during
the absorption process. To validate the model, experiments were performed for an absorption process in a mini
channel heat exchanger with NH3 concentration of 35 wt%. The results show a good match between the model
and the experiments. Additionally CO2 has been added to the solution and the experimental performance was
compared with the experimental performance of the NH3-H2O mixture. A concentration of 2 wt% CO2 resulted
in a performance increase of up to 5% however the working fluid flow became limited by pumping instabilities.