Function-integrated KERS in the context of the energy policy quadrangle
Motivation
The turn away from fossil fuels in the generation of electrical energy to CO2-free renewable energies requires new approaches to supply energy continuously. If using sun and wind power then it must be ensured that even when the sun is not shining or the wind is not blowing, electricity is constantly available for households and industry. This can only be achieved by storing surplus energy and using it at times when renewable power sources are not available, such as at night or during doldrums. This storage is essentially mechanical, chemical or electrical, but can also take place in combination. The amount of energy stored over the period of time determines the application purpose (long-term or short-term storage).
Possibilities to store mechanical energy
For decades, energy has already been stored in pumped storage plants. In times of low grid load, water is pumped into a higher storage reservoir using electrically operating pumps. If required, it is drained again using turbines and the stored energy is converted into electricity. As a result, grid peak loads can be compensated.
Alternatively mechanical energy can be stored using a flywheel similar to the principle of a potter’s wheel. In this process, the flywheel is set into a rotating motion, which prevents rapid braking and results in a uniform motion. Of course, the motion works only for a limited time and is also largely dependent on how the flywheel is mounted. A low-friction suspension with limited losses is advantageous. If this is not enough, there are several ways to increase the storage capacity: So the rotational speed can be increased or the mass, moreover the flywheel shape and geometry can be adapted. However, all this is limited by the flywheel material properties. High storage capacities result in high speeds with large dimensions and requires particularly strong material; even steel reaches its limits here.
Solution approach
Carbon fiber-reinforced plastic CFRP has proven to be particularly suitable for such high loads. Carbon fibers are embedded in synthetic resin. The mechanical properties of the matrix like tensile strength and stiffness are particularly high. The matrix enables high rotational speeds and high circumferential speeds. However, the density is much lower compared to steel. Therefore, the challenge to achieve a high energy density is to design the flywheel geometry for high speeds in an optimal way and in accordance with the load. A manufacturing technology has to be developed that guarantees the defect-free production of the CFRP rotor and thus safe and reliable operation of the flywheel at the highest speeds. Additionally sensors can be integrated into the fiber composite to monitor the dynamically highly stressed flywheel.
If the flywheel mass integrates its magnetic bearings and the motor-generator unit then the result is a compact arrangement as shown in the picture. The modular design allows the interconnection of several storage systems in a grid and thus an adaptation of the system capacity to the user's requirements.
Project partners
The KERS is developed in collaboration between the Fraunhofer IWU and the IPM of the HSZG, both institutions are located on the university campus in Zittau. The main objective of the IWU is the development, design and construction of a suitable fiber composite material. The manufacturing technology as well as the integration of the metallic rotor parts into the fiber composite matrix is a further objective. The IPM is responsible for the design and development of a suitable low-loss magnetic bearing system and the selection of a motor-generator unit suitable for the targeted speed range. The sensor network integration suitable for the online condition monitoring and the required control technology are further challenges.
Be a part of the mission
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Funding
The project is funded by the BMWi as part of the program to strengthen the transformation dynamics and start in the areas and at the coal-fired power plant sites STARK. The results serve to achieve the international and national climate protection targets and to strengthen the ecologically sustainable and resource-efficient structure of the regions affected by the coal phase-out.
