Figure 1: Schematics of the FlyGrid system from grid to vehicle.The FlyGrid system is governed by a high-level controller, which receives information from the following modules: · Electric vehicle: AC vs. DC charging type, state of charge, power demand etc. · Flywheel: State of charge, state of health · Electric grid: Voltage, current and frequency · Electricity supplier: Demand side management commands With this information the controller can operate the system in such way that grid loads are minimized and renewables are used in the most efficient way.
Another benefit is that - if the electricity supplier demands it - the FESS can either feed power back into the grid or be charged when excess energy is available. The ultra-fast reaction time of the flywheel power electronics (~milliseconds) allows nearly instantaneous compensation when the grid frequency drops, which can be crucial for avoiding blackouts.
Figure 2: Electromechanical Flywheel Energy Storage SystemEnhanced grid stability and power quality through the introduction of inertia in the grid are positive side-effects of FlyGrid. One may argue that with only limited power available to charge up the FESS, it would take long to recharge after the flywheel has spun down during the EV charging cycle. While this is true to a certain extent, it must be mentioned that the EV charge power is only at its maximum at the very beginning of the charge cycle (“constant current mode”) and then drops (“constant voltage mode”) until it reaches “trickle charging” during the last 5 ~15 % of the EV battery’s state of charge. From this, three important conclusions can be drawn: a) The actual energy content that must be stored in the FESS is much lower than the energy content of the EV battery b) The FESS can be charged up again, even when the EV is still plugged in by using the excess power between trickle charging and maximum available grid power c) As a consequence of b), the load on the grid will be completely uniform!
Figure 3: Charging cycle of a FlyGrid facility in a low power distributor grid.