Averaged polytropic efficiency for wet steam expansion cycles

Historically wet steam expansion cycles were used in the first reciprocating steam engines. Then such cycles disappeared in favor of superheated cycles. However, wet cycles had a remarkable revival and dominate today’s nuclear power plants with around 10% of the world’s electricity production from 440 reactors in the year 2023. And this is combined with great future growth prospects. Such plants with electric power output of up to 1700MW have complicated arrangements for managing the large specific volumes (with multi-pass solutions), steam and condensed water extractions for both managing the water droplet content in the working steam and for feed water preheating. The intention of this paper is exploring a concept to assess the averaged thermodynamic quality of the used steam expansion system architecture. This is offered by the definition of an “averaged polytropic expansion efficiency”, which can be derived from five given quantities of a plant. This paper shows how to derive such numbers, which represent exactly the achieved share of the maximal thermodynamically possible power output for the given steam admission state and the condensation condition in contrast to any other efficiency definition. The historic background of such cycles is summarized and example numbers for understanding its “averaged polytropic expansion efficiency” with the need-to-know hints are indicated.