Optimized operation of recompression sCO₂ Brayton cycle based on adjustable recompression fraction under variable conditions

The use of supercritical carbon dioxide (sCO₂) cycle has been proposed as a promising alternative to replace conventional steam Rankine cycle. This study entails the development of a power cycle model to assess the impact of fluctuations on the heat source and environmental conditions on a recompression sCO₂ Brayton cycle during off-design operation. Two operational strategies are tested during off-design operation, including fixed recompression fraction and adjusted recompression fraction. It is found that a superior performance is obtained when the recompression fraction is adjusted according to heat addition and ambient temperature variations. The variations of the heat addition have a greater impact than ambient temperature on the cycle's performance, showing up to 70% greater cycle efficiency when the heat addition ratio is reduced to 30%. In some conditions, the recompression cycle operates similarly to a regenerative cycle, hence no recompression fraction is required when the heat addition ratio is lower than 55%. The influence of the ambient temperature is more relevant when a dry cooler is used, and in this case, it is important to include a detailed cooler's model in order to account for the variability of the thermophysical properties of the carbon dioxide close to its critical point.