Control algorithms for hybrid battery and supercapacitor systems
A consortium in Germany has developed a verified toolbox for evaluating and developing hybrid supercapacitor and lithium ion battery designs.
Skeleton Technologies, AVL Deutschland and Fraunhofer-Gesellschaft joined forces to explore combinations of lithium-ion batteries and supercapacitors in mining trucks and passenger vehicles to industrial machinery.
The SuKoBa research project funded by the German Federal Ministry for Economic Affairs and Climate Action set out to develop cross-application methods and tools for the development and optimization of hybrid lithium-ion batteries (LIB) and supercapacitor (SC) systems.
The two technologies are complementary but the widespread adoption of hybrid systems where both LIBs and supercapacitors are coupled in parallel has been stalled, says Skeleton. This is because there is a lack of data on the costs and benefits of incorporating supercapacitors, compared to a pure LIB design.
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The consortium developed a software toolbox that can determine the lifetime- and cost-optimal combination of Skeleton’s supercapacitors and LIBs, particularly NMC, for a given system design and operational use. This includes design methods and control algorithms for the cross-application of hybrid energy storage systems.
The consortium used a combination of theory with simulation and experimental data from a demonstrator for the toolbox.
LIB systems for applications with high power peaks are often economically disadvantageous because they are either very large or reach the end of their life span very quickly. In such use cases, batteries could be combined with supercapacitors to filter the power peaks out.
The design methods were created to automate individual configurations of supercapacitor-battery-hybrids based on application profiles and considering auxiliary conditions.
The control algorithms manage hybrid energy storage systems using application-specific control strategies. Both design methods and control algorithms were then tested in use cases and verified with the demonstrator. Both the demonstrator and the software toolbox were designed flexibly, so that new trends in batteries, supercapacitors and power electronics can be integrated.
AVL in Graz, Austria, contributed its expertise in the development, optimization and validation of storage systems and application-specific control solutions and developed the control strategy and simulation of the hybrid system using the AVL Cruise M simulation tool.
Fraunhofer Institute for Energy Economics and Energy System Technology (IEE) in Kassel worke don the battery simulation, including precise aging predictions, as well as extensive expertise in the control of complex energy systems and has developed the demonstrator.
A semi-active topology was used in the SuKoBa project where the LIB is connected to the load and the SC is connected behind the DC-DC converter. This allows unrestricted access to the supercapacitor because its voltage level is separated from the load by the converter.
The results showed that hybrid battery systems appear to be particularly advantageous when they include regenerating braking energy, especially in larger energy storage systems with high peak power requirements. For example, a mining truck with 700 kW power compared to a telehandler with 75 kW demonstrates this well.
The mining truck requires slightly less than 10 times the peak power of the telehandler. However, it is not nearly 10 times larger in size. Here, the advantage of the high-power density of supercapacitors (>10 kW/kg and >12 kW/L) is a very promising solution.
In a mining truck, a battery lifetime extension of more than 20% by a hybrid system compared to a standalone battery at a competitive system cost was demonstrated. By decreasing electrical losses by 6% and thermal losses by 10%, the overall system efficiency increases significantly.
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