Analysis for flexible operation of supercritical CO2 Brayton cycle integrated with solar thermal systems

In this paper, we perform a comprehensive parametric study for supercritical CO2 (sCO2) Brayton cycle integration with a concentrated solar thermal (CST) plant. The main focus is to develop operational strategies for the cycle to adapt to fluctuations in solar energy availability. Several cycle layouts are analysed and a 'combined' cycle comprising recompression, reheat and intercool is found to be the most efficient cycle. Two key cycle parameters (sCO2 circulation rate and splitting fraction) are sensitized while net shaft power (NSP) output is controlled at 11.5 MW. By manipulating these two parameters, the cycle can adapt to heat input variations without affecting NSP output. This finding leads to two operational modes: flexible temperature mode (FTM) and constant temperature mode (CTM). A solar-assisted case study is used to test the proposed strategies, and an auxiliary fossil-fuelled back-up (AFB) unit is utilised when solar energy is insufficient. In general, the proposed solar-assisted cycles (CTM and FTM) were able to achieve the highest fossil fuel savings of 28.9% and 31.2%, respectively compared to a conventional cycle without solar. It is found that both FTM and CTM can deal with fluctuations in solar thermal energy supply. However, FTM is more effective in tolerating drops in solar energy supply and considerably outperforms CTM in lowering contributions from AFB unit. Thus, even at the most efficient conditions, CTM still underperformed FTM by 4.5% in term of fossil fuel saving.