This study deals with the off-design operation and controller tuning of a 5 MWth, air-cooled recuperated supercritical carbon dioxide (sCO₂) Brayton Cycle. Previous studies indicated that implemented PID controllers require gain scheduling for efficient operation under fluctuating air temperatures or load levels. However, regarding sCO₂ cycles, limited literature explicitly covers controller tuning, with a majority relying on manual methods. Therefore, this study presents and implements a gain scheduling approach for the air-cooled heat sink of the cycle in a systematic and automized manner, includes the following steps: Calculation of the steady-state boundary conditions, determination of transfer functions from step tests, controller tuning using Matlab and Internal Model Control and, finally, controller testing.

It was found that the Internal Model Control approach, with a closed loop time constant equal to 1/10 of the open loop one, yields a stable and efficient controller performance for all boundary conditions, except cases with a low air mass flow rate. Consequently, it is recommended to avoid such extreme part-load conditions, e.g. by cooler modularization. Finally, important considerations for the following tuning of the inventory control are presented, highlighting the interaction of component design and controllability.