Supercapacitors, also known as supercaps, have a very high power density, i.e., they can provide a great amount of power in a very short time period without sacrificing their service life. However, they also have a disadvantage: as their energy density is low, their capacity is exhausted quickly. Supercapacitors are unbeatable in a sprint, but then run out of breath very fast.

Lithium-ion batteries and other battery storage systems are different. These long-distance runners boast a high level of endurance due to their large capacity. However, they do not cope with short-term load peaks as well as supercaps. Among other things, this causes heat stress, which makes the batteries age even faster.

“Hybrid storage systems combine the advantages of batteries with those of supercapacitors, while eliminating their respective disadvantages. We expect a significant increase in service life with such hybrid storage systems. That’s something we want to investigate in more detail in the project. That work could lead to significantly lower system costs,” explains Matthias Puchta, Business Unit Manager, Hardware-in-the-Loop Systems at Fraunhofer IEE.

Fully exploiting the potential of hybrid storage

However, there is still a lack of suitable tools for designing these hybrid storage systems optimally in terms of their size, cost and service life. The main reason for this is that available models do not include battery aging. Furthermore, there are no management and control systems for hybrid supercap/battery systems.

These gaps are now being closed by the joint project “Supercapacitors for optimizing the lifetime of battery hybrid storage systems—design methods and control algorithms” (SuKoBa). The project, which is funded by the German Federal Ministry for Economic Affairs and Energy (BMWi), has been running since the beginning of the year and is scheduled to be completed by the end of 2023.

“With SuKoBa, we are creating the prerequisite for fully exploiting the potential of hybrid storage technologies,” says Puchta. “Our research project is making a significant contribution to the economical use of battery storage systems even in places where very high power is required for short periods.”

One possible field of application for hybrid storage systems is their stationary use in the grid. To support the batteries which are already used today for primary power control, supercaps can stabilize the voltage and frequency in the grid in a few seconds or less if a major disruption occurs. Furthermore, hybrid storage systems can be put to good use in electromobility. Supercaps charge and discharge considerably faster than batteries, which is particularly advantageous during acceleration and recuperation. Moreover, they provide power efficiently even at low temperatures, for example when starting up in the winter.

Supercapacitor extends battery system life

The first step the research partners are taking is to develop a software tool that can be used to design a hybrid supercap/battery storage system with the objective of maximizing the service life. This optimization is being achieved in line with the requirements for the power density, energy density, capacity, cost, weight and volume of the storage system. The various scenarios for its use are represented by typical load time series.

The partners are investing a great deal of experimental effort in verifying this process. Demonstrator models are to be set up in the laboratory and tested for various applications.

As part of the SuKoBa project, the partners are developing a modular, parameterizable management system that controls and manages the interaction of the individual storage components and simultaneously monitors the overall system. The system is intended to allow the relevant parameters to be adjusted during operation to match specific applications and situations.

Among other things, Fraunhofer IEE is contributing to the project with its many years of experience in the field of battery simulation. Simulation models from the institute are used here, in particular for predicting aging processes. In addition, the Fraunhofer researchers have a great deal of expertise in technology for controlling complex energy systems. Last but not least, Fraunhofer IEE is providing much of the test infrastructure required for testing the hybrid storage systems and monitoring their aging.  

Consortium leader Skeleton Technologies is contributing its extensive real-world experience in the use of supercapacitors for various high-performance applications, including electric-powered commercial vehicles, grid stability systems, hybrid diesel drives for buses, and harbor cranes.  

The automotive supplier AVL brings in its competencies and methods for developing, optimizing, testing and validating storage systems, as well as developing application-specific control solutions. AVL also has extensive application-specific software tools and models libraries.