Comparing the partial cooling and recompression cycles for a 50 MWe sCO2 CSP plant using detailed recuperator models

Abstract

Supercritical carbon dioxide (sCO2) power cycles are attractive for renewable energy technologies as they exhibit high thermal efficiencies, compact components, and reduced cycle complexity and levelized cost of energy (LCOE) relative to traditional steam power cycles. Consequently, numerous studies have investigated different layouts and operating parameters in search of an optimal cycle configuration. Two cycle layouts provide promising results, namely the partial cooling with reheating (PCRH) cycle, and the recompression with intercooling and reheating (RCICRH) cycle. Both cycles utilise recuperators with high heat transfer rates and literature suggests that the cost for the recuperators may account for a significant portion of the total plant capital expenditure. When modelling the recuperator, most cycle comparative studies employ simplified recuperator models where heat exchanger effectiveness or conductance is utilised to approximate heat transfer. These models do not consider the effect of recuperator size and geometry on heat transfer and pressure drop in detail. In this work, for a concentrated solar power (CSP) application, the performance and component size requirements of the cycles are evaluated parametrically for different cycle mass flow split ratios, pressure ratios, and ratio of pressure ratios across the compressors using detailed discretised one-dimensional (1D) recuperator models. Furthermore, the geometry of these models are optimised (by volume) for straight and zigzag channel printed circuit heat exchangers (PCHEs), widely accepted as the most suitable for this application. Finally, a high-level cost comparison is conducted. The results show that if appropriate pressure drop assumptions are made, a simplified model approach yields valid cycle level results when compared with the results obtained using detailed models which consider practical recuperator geometry. However, the pressures and temperatures within the recuperator may not be predicted to a sufficient level of accuracy. In addition to providing clarity regarding the interplay between key cycle process parameters and cycle performance, straight channel PCHEs provide better thermofluid performance than zigzag channel PCHEs. Furthermore, the RCICRH cycle requires larger turbomachinery and a marginally higher capital outlay for the power cycle, but offers superior thermal efficiencies and requires smaller heat exchangers. For sCO2-CSP applications employing dry cooling, this suggests that the RCICRH cycle requires a smaller solar field and cooling system, and may therefore offer increased revenue in adverse weather conditions where direct normal irradiance (DNI) is low and/or ambient temperatures are high. However, the PCRH cycle requires a smaller solar receiver system, a smaller thermal energy storage (TES) system, and smaller turbomachinery, the cost savings of which may outweigh the benefits of the RCICRH cycle.