Real gas Brayton cycles for organic working fluids

Abstract

Organizing a closed Brayton cycle in such a way that the compression process is performed in the vicinity of the critical point where specific volumes are a fraction of those of an ideal gas yields performance indices particularly attractive, mainly at moderate top temperatures. Cycle thermodynamic analysis requires the development of adequate methods for the computation of thermodynamic properties above the vapour saturation curve about the critical point. Working fluids suitable for the proposed cycle can be found in the class of organics, in particular among the newly developed, zero ozone depletion potential, chlorine-free compounds. The numerous technical and environmental requirements which a fluid must meet for practical use combined with the peculiar thermodynamic restraints limit the number of suitable fluids. Mixing two substances of different critical temperatures yields an indefinite number of fluids with tailor-made thermodynamic properties. One such mixture 0.93 HFC23 + 0.07 HFC125 (molar fraction), having tcr = 30°C, at tmax = 400°C, pmax = 150 bar, gives an efficiency above 27 per cent with heat rejection temperatures between 89 and 33°C. With a different mixture composition with a 50°C critical temperature, at the same tmax and pmax, an efficiency of 25.1 per cent is attained in a combined heat and power generation cycle with heat available in the range 53-103°C. An experimental programme to test the thermal stability of organic fluids showed that top temperatures of 380-450°C are achievable with some commercially available fluoro-substituted hydrocarbons. In view of practical applications a conversion unit based on a reciprocating engine could handle without problems the pressures and temperatures involved. The use of turbomachinery would lead to power plant of large capacity for the usual rotor dimensions or to micro-turbines at high rotating speed in the low power range.