Supercritical and real gas Brayton cycles operating with mixtures of carbon dioxide and hydrocarbons

Closed Brayton gas and supercritical cycles operating with mixtures of carbon dioxide and hydrocarbons, in particular mixture with a low content (5–15%) of benzene, were studied. Totally, supercritical cycles and condensation cycles were looked at and a comparison was made with pure carbon dioxide cycles with a minimal temperature around 40–50 °C (typical minimum temperatures of air cooled radiators). First and second law cycle efficiencies were considered and analysed. Critical point calculations of several mixtures were performed by means of an accurate model for the thermodynamic properties and compared with experimental data from literature. For the cycle calculations, a simpler model with classical mixing rules was used because the results were in sufficient agreement. The cycles operating with mixtures showed lower maximum pressures and higher cycle efficiencies compared to the pure carbon dioxide cycles. Taken into account, the high global warming potential of fluorinated fluids and the high flammability and high volume expansion ratios of comparable Rankine toluene cycles, mixtures of carbon dioxides and hydrocarbons exposed promising features.