On low-grade waste heat utilization from a supercritical steam power plant using an ORC-bottoming cycle coupled with two sources of heat

Highlights: • Detailed presentation of little-known binary vapor cycle invented in 1960s by polish professor Robert Szewalski. • Size reduction of supercritical steam power unit by implementation of cascade ORC. • Investigation of a novel modification utilizing great amount of low temperature waste heat (200 MWt, 90 °C). • Parametric optimization of proposed cycles for four low boiling point fluids using Computational Fluid Mechanics (CFM) code. - Abstract: This paper analyzes a waste heat recovery system based on a binary vapor cycle composed of an organic Rankine cycle (ORC) bottoming a supercritical steam cycle. The organic Rankine cycle is supplied by two heat sources. The first one is waste heat from a steam boiler, which condenses flue gases to 200 MWt at 90 °C and preheats the fluid with a low boiling point. The second one is a steam condenser, which also acts as a low-boiling-point fluid vapor generator. Steam condensation temperatures was tested in the range 55–115 °C. Usage of a low-boiling-point fluid instead of steam in range of the low temperature (below 100 °C) has several advantages. One advantage is the possibility for the effective utilization of a large amount of low-grade waste heat from a supercritical steam cycle. For the most efficient configuration, 22.92 MW of additional electrical energy is generated. The thermal efficiency of the waste heat recovery system is 11.46%, which is 71.75% of the Carnot efficiency. Usage of an organic Rankine cycle for bottoming the supercritical steam cycle also provides cubature reduction of the power plant. For the most efficient case, a steam volume flow at the new steam turbine outlet is reduced by 88% compared to a reference stream turbine. The volume flow at the ORC turbine outflow is reduced by 54%. Numerical analyses of the thermodynamic cycles, before and after modifications, are carried out using computational flow mechanics, mainly, with in-house code.