Optimal design of supercapacitor stacks at APEC 2024

We were pleased to present our paper, detailing our research on supercapacitor storage sizing within the framework of the project, and titled ‘Optimal design of supercapacitor stacks for size-critical applications’, at the Applied Power Electronics Conference at Long Beach, USA on 28th February, 2024.

The size of the supercapacitor storage system is an essential consideration for supercapacitor integration into a power converter, which the project aims to achieve. Since the loads fed by the power converter have well-defined requirements of power and energy, the question of sizing translates to the following: How should supercapacitor storage be designed and operated such that it meets the requirements of the load while having minimal size?

This size reduction and minimization requires the appropriate selection of parameters for the supercapacitor stack. One such parameter is the discharge ratio, which quantifies to what fraction, of the nominal (charged) condition, the supercapacitor is discharged during operation. We show that the discharge ratio selection strongly affects the size of the supercapacitor stack. If the stack is discharged down to an almost uncharged condition, almost all of its stored energy is extracted, and such designs are suitable for meeting the load energy demands. However, discharging the stack in this manner leads to a very low final voltage across the stack; extracting the required power from such a low terminal voltage may not be feasible.

Hence, the power and energy delivery capabilities of the supercapacitor stack trade off as the discharge ratio is varied. Using a simplified model of a supercapacitor stack, we quantify this trade-off. We also show that, for a given energy and power requirement of a load, it is possible to make an optimal choice of discharge ratio of a supercapacitor stack, such that the required size of the stack is minimized. This is true for both continuous operation of the stack (as may be required in electric transportation where the stack is repeatedly charged and discharged) and quite intermittent operation (such as an uninterruptible power supply, UPS).

The optimal choice of discharge ratio, which we call the size-optimal discharge ratio or SODR, is derived considering the simplified model of the supercapacitor stack. This SODR can be derived using only the load energy and power requirements, and the parameters of the supercapacitor cell (part) constituting the stack. Hence, this design method is compatible with other algorithms. A sizing algorithm for supercapacitor stacks is proposed and validated. A size reduction of more than 40% is achieved compared to existing approaches of sizing the stack. This reduction in stack size reduces the size overheads that result from the integration of supercapacitors into the power converter.