Insight on electrical and thermal powers mix with a Gen2 PWR: Rankine cycle performances under low to high temperature grade cogeneration
Résumé
In the challenging framework for a carbon free energy mix achievement, performances of a Rankine cycle exemplary of Power Conversion Systems (PCS) from the Gen2 Pressurised Water Reactors (PWR) fleet, are mapped under heat cogeneration. Noticeably, cycle performances are investigated using the THERMOFLEX software that allows taking into account the thermodynamic irreversibilities balance within the PCS that arise from turbine part-load operation, with regards to the PCS performance at design which is optimized for a 100% electrical duty. First, the technical scope of the paper is discussed together with the methodology. A peculiar methodological point is to investigate how deviations of thermodynamic irreversibilities under part-load alter calculation accuracy of cycle performances, depending on temperature grade and thermal load that are considered for cogeneration. To start with the simulation work, THERMOFLEX results are verified by comparison with CYCLOP in-house code, on a reference case chosen as exemplary of a 1300 MWe PWR's PCS. Then, several cogeneration scenarios are computed with THERMOFLEX, differing from each other by location of the line proving heat for cogeneration (recovered as latent heat of steam condensation), to cover a 146 to 285°C range. Thermal load for cogeneration is also varied and rates up to 20% of Steam Generators (SG) ones. Hence, spanned cases extend the range of load and temperature grade from common cogeneration applications, such as district heating or water desalination. Inline, electrical power to the grid is mapped as a function of cogeneration thermal load, providing first engineering guideline about the mix performance. It comes out from the most stringent cogeneration case which is studied, that cycle electrical efficiency drops from 2.7 points. Extrapolation of this result to a 50% load would significantly rise the loss to 6.9 points. Calculations are repeated with CYCLOP, which, contrary to THERMOFLEX (but similarly to a common approach in the applied engineering literature) performs thermodynamic calculations while disregarding part-load thermodynamic irreversibility aspects. Main mechanisms responsible for the discrepancy between both approaches, are analysed. It comes out that isenthalpic throttling valve which govern steam admission in the turbine, play a key role while degradation of turbine efficiency remain low in the investigated range. Turbine expansion lines are indeed shifted to lower pressures, by so decreasing turbine stage losses contribution due to steam moisture content and balancing other mechanisms of losses. Finally computational methodology and multi-objectives optimization relevancy, are discussed paving the way to next investigation of an extended range for electrical and thermal powers mix.
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