96/00870 World supply/demand survey of A/Cs in '94

Abstract

This chapter analyzes limits on multistage production or consumption of mechanical energy (work) in sequential heat-mechanical operations characterized by finite rates. The benchmark system, for which these limits are evaluated, is a cascade of imperfect stages through which a resource fluid flows at a finite rate. Each stage consists of a fluid at flow, an imperfect work generator or consumer, and the environment. Cascade models serve to analyze the dynamical behavior of engines and heat-pumps when the resource reservoir is finite and the power yield cannot be sustained at a steady state. Based on equations of nonequilibrium work, enhanced bounds are found for real processes. An essential decrease of the maximum work received from an engine system and an increase of the minimal work added to a heat-pump system occurs in the high-rate regimes and for short durations. These works are quantified in terms of finite-rate exergies, which exclude some evolutions allowed by the classical exergy. The practical value of multistage solutions refers to energy limits enhanced due to the inclusion of imperfect thermal machines with a small number of stages and internal dissipation. For short durations the heat-pump mode exergy, which defines the lower bound on the work consumption, can be significantly higher than the minimal work of classical thermodynamics. It explains the restrictive applicability of classical thermodynamic bounds when they are applied to real processes, and it shows that these bounds should be replaced by stronger bounds obtained from nonequilibrium thermodynamics.