Decentralized low-cost flywheel energy storage for photovoltaic systems

This publication demonstrates that flywheel energy storage systems (FESS) are a valid alternative to batteries for storing energy generated by decentralized rooftop photovoltaic systems. The increasing number of private PV arrays calls out for high energy storage capacities in order not to overload the grid. Despite being the current storage technology of choice, chemical batteries are still too expensive and have certain disadvantages compared to FESS, such as capacity fade over time and currently still difficult recycling. Within a research project at the Graz University of Technology a feasibility study for a low-cost, low-loss FESS was conducted. Energetic dimensioning was performed using actual PV power and electric load data recorded at a building in Austria with 6 apartment units. A low-cost flywheel system with an energy content of 5.0 kWh and 2.2 kW maximum rated power using a steel rotor and economic off-the shelf components was designed and investigated. Self-discharge of the proposed FESS design was significantly reduced using a cast silicone bearing seat, which allows supercritical rotor operation. Axial bearing loads were compensated by nearly 100% via repelling permanent magnets allowing drastic down-sizing of the bearings and further reduction of torque loss. The concept was validated by a small-scale test setup, which showed promising results. Finally, an improved design option is compared to the initially proposed FESS in terms of costs and self-discharge.