While the deployment of fuel-cell cars in the European fleet will take decades (it normally takes more than 20 years for standard functions to reach a 90% fleet penetration), CO2 problems are present and demanding; the automotive industry favours solutions offering future potential when coupled with innovative powertrains as well as with the possible realisation of short-term benefits in combination with state-of-the-art powertrain technology.
In this regard, it is necessary to stress the fact that automotive technology has grown to be more and more complex in recent years through the addition of an increasing number of functionalities. OEMs (original equipment manufacturers) addressed this challenge by decreasing the production of in-house parts and by the supply of black box-like system components, the integration of which still constitutes a big challenge in terms of handling complexity. This is why the HyHEELS consortium considered it to be appropriate to focus on providing an UltraCap storage function comprising all the properties necessary to make it an integrative component. This is the unanimous view of both the supplier and the OEM regarding manageable interfaces.
The overall goal of this project was to provide an UltraCap energy storage system for use in hybrid and fuel cell vehicles, which satisfies all properties necessary to make an integrative component. Therefore, the development work comprised the optimisation of the electric properties of the basic cap, its combination into scalable modules with integrated power balancing within the modules, power prediction and the communication interface with the drivetrain.
Technical objectives were:
- the development of an improved energy supply concept for fuel cells based on advanced, powerful Ultra Capacitors (UltraCaps);
- development of an advanced UltraCap module for integration into fuel cell vehicle architecture;
- advanced research, simulation, installation, and evaluation of UltraCap modules on test benches and existing Hybrid Vehicles.
The steps in order to achieve an improved cost-efficient energy supply concept for hybrid vehicles based on an advanced, powerful UltraCap, were as follows:
- increasing the maximum operating voltage of UltraCaps from 2.5V to 2.7V. High-cell voltage requires an electrochemical stability of the electrode, the electrolyte and the packaging materials;
- cost reduction of the electrodes by new production technologies;
- cost reduction of cells and modules by industrialisation;
- advanced UltraCap component electrode and packaging. All the materials need to have a high electrochemical stability in order to operate the components at a higher voltage over a longer period of time. The component packaging weight must be minimised. Special attention must be paid to the packaging tightness and to the mechanical resistance;
- advanced UltraCap module packaging with optimised thermal behaviour, weight and cost;
- development of an UltraCap controller, including a single cell voltage measurement and cell balancing, providing extended UltraCap information to the fuel-cell system supervisor.
The final goal of the project was the installation of an advanced, reliable and cost-efficient UltraCap module, providing all necessary information, which would enable the integration into the fuel-cell vehicle architecture.
Within the project a newly cell / stack technology was developed and a new module was designed. The stack was made of one positive stack and one negative. On one side the positive electrode was connected to the lid and the other side to the bottom can. A plated connection was laser welded between the two cells to give a very low resistive connection for voltage measurement. A thermal shrink tube insulated the stack.
UltraCaps have become available on the market with restrictions regarding automotive applications when looking on maximum voltage and working temperature and packaging requirements. As the maximum voltage of a single capacitor is only 2.5 V, several capacitors had to be connected in serial to a module if higher supplying voltages were required. This made it necessary to develop an advanced UltraCap module packaging with optimised thermal behaviour, weight and cost. Furthermore, caused by different self-discharge of the single capacitors, the individual voltages of the module would be drifting away. Finally, the capacitor module would be mismatched in voltage. Battery systems would be usually overcharged to keep it balanced in charge and voltage. However, capacitors could not be overcharged. Therefore, special charge balancing systems were developed in the past. These charge-balancing systems exchanged the energy between the single capacitors in such a manner, that all capacitors achieved equal voltages.
The performance as well as the mechanical design of the controller ensured the reliable operation of the module and satisfied the future requirements of all automotive applications in this area.
New simulation models were developed to design and configure the UltraCap modules for different vehicles. The simulations were validated by experimental results.
The test platform at Vrije Universiteit Brussel was applied to verify the configuration the super capacitor based energy storages for passenger cars and heavy duty vehicles, with respect to the voltage variation, maximum charging / discharging current, power losses in speed cycles.
The driving cycle tests confirmed very good electrical stability even in the pre-series UltraCap modules. As a result, the super capacitor modules developed in the HYHEELs project were suitable for the automotive applications.
A Life cycle assessment (LCA) was done to compare the environmental impact of UltraCaps with batteries. The uncertainty analysis was performed through a Monte Carlo analysis. The UltraCap scored better