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Thermodynamic mapping of power cycles working around the critical point

González-Portillo, Luis F., Muñoz-Antón, Javier, Martínez-Val, José M.
Energy conversion and management 2019 v.192 pp. 359-373
enthalpy, entropy, heat transfer, temperature
A new thermodynamic coefficient, called logarithmic factor of isobaric expansion, is defined for a better guidance in the cycle characterization of regenerative cycles working totally or partially at supercritical conditions. The logarithmic factor of isobaric expansion allows to generalize the characterization of cycles working around the critical point, agglutinating supercritical and transcritical cycles under a unique type of cycle: the pericritical cycle. The analysis of pericritical cycles allows to analyze all the cycles working around the critical point together, without the limitation that may involve the study of supercritical and transcritical cycles separately. Enthalpy and entropy balances give the internal relations among the cycle relevant variables in a thermodynamic mapping. The new thermodynamic coefficients defined in the paper identify the regions where the compression work is lower. But the lower compression work is counteracted by some limitations in the heat transfer regenerative phase, where the hot fluid of the low-pressure side heats the (same) fluid at the high-pressure side. The logarithmic factor of isobaric expansion in the compressor indicates the relevance of the compression work and the heat regeneration in the cycle, which will affect the thermal efficiency. Pericritical cycles with lower compression work and so higher limitations in the regenerative phase, handle better the inefficiencies of the cycle components at the expense of larger pressure ratios. Moreover, cycles with lower compressor inlet temperatures and higher high-side pressures take more advantage of the benefits of reducing compression work, which involves higher thermal efficiencies at the expense of larger pressure ratios.