Investigation on the cycle performance and the combustion characteristic of two CO2-based binary mixtures for the transcritical power cycle

Abstract

It is difficult to condense subcritical CO2 in the CO2 transcritical power cycle by the conventional cooling, because of CO2's low critical temperature. With the aim to solve this problem, a new transcritical power cycle is proposed, using a CO2-based binary mixture as working fluid. Two mixtures are considered, namely, n-butane/CO2 and isobutane/CO2. Because n-butane and isobutane are flammable, the flammability of the mixtures are worth of note as well as their cycle performance. A laminar flame combustion rate experimental platform is established to investigate the combustion characteristics of both mixtures under different mixing ratio. The results show that their critical temperature increases with increasing the organic fraction and their critical pressure shows a peak value in considered conditions. The critical temperature can reach 40 °C with the organic fraction of 0.0711 for n-butane/CO2 and 0.0806 for isobutane/CO2. Under these mixing ratios, the transcritical power cycle can run by the conventional water cooling. The thermal efficiency reaches the highest value of 12.78% under the mole ratio of 0.28/0.72 for n-butane/CO2. Isobutane/CO2 gives the best condition under the mole ratio of 0.32/0.68, with the thermal efficiency reaching 12.97%. The flammable critical mole ratios for n-butane/CO2 and isobutane/CO2 are 0.04/0.96 and 0.09/0.91, respectively.