Comparative dynamic performance of SCO₂ cycle and combined cycle with transiently modelled printed circuit heat exchangers

The dynamic behavior of SCO₂ (supercritical carbon dioxide) cycles and combined cycles is an important part of researching SCO₂ cycle performance. However, the current research on the dynamic analysis of SCO₂ cycles mostly focuses on standalone SCO₂ cycles, with limited exploration of the SCO₂ combined cycles. To further enhance the understanding of the performance of the SCO₂ cycle and the combined cycle, the performance of SCO₂ cycle and ORC (organic Rankine cycle) / SCO₂ combined cycle under different transient conditions is thoroughly investigated. For this purpose, the multi-pressure evaporation ORC/SCO₂ Brayton cycle layout is selected. First, a detailed modeling of the SCO₂ cycle and its PCHE (printed circuit heat exchanger) is conducted. A novel transient model for PCHE is proposed and its solution method is provided. Subsequently, the SCO₂ cycle and PCHE models are validated. Finally, the impact of transient behaviors commonly encountered in three practical scenarios, namely heat source power variations, load changes, and heat source power fluctuations in the SCO₂ and ORC/SCO₂ combined cycles, is analyzed. The main results show that both SCO₂ and SCO₂ /ORC combined cycles exhibit similar dynamic characteristics. Also, adding an ORC bottom cycle to construct SCO₂/ORC combined cycle does not significantly impact the dynamic behavior of the SCO₂ cycle. The specific results are as follows: During heat source variations, the settling times for SCO₂ cycle and SCO₂/ORC combined cycle are 1200 s and 1150 s, respectively. During load variations, the settling times are 560 s and 590 s, respectively. The dynamic characteristics of both SCO₂ cycle and the combined cycle are relatively stable during reactor power fluctuations. Results also reveal a phase lag in the net output power and efficiency, with the efficiency phase lag being more pronounced in the combined cycle compared to the standalone SCO₂ cycle. The high regenerative requirement of SCO₂ cycle affects system stability positively by buffering fluid temperature changes due to reactor power variations but also leads to delays in net output power response. The latter negatively impacts the overall performance of the cycle. Both cycles exhibit non-minimum phase behavior, with efficiency showing negative response in the initial stage. Achieving rapid changes in net output power requires complex reactor adjustment strategies and advanced controller's design methods.