Study of Flywheel Energy Storage in a Pure EV Powertrain in a Parallel Hybrid Setup and Development of a Novel Flywheel Design for Regeneration Efficiency Improvement 2021-01-0721

In electric vehicles, there is a continuous shift in the charging and discharging of the battery due to energy generation and regeneration. This adds up to the total number of charging-discharging cycles of the battery. This fluctuation amounts to faster battery degradation and life-cycle reduction. Also, we are exploring solutions to improve the low regeneration efficiency of EVs. For example, overall regeneration efficiency from wheels-to-wheels is only around 64% in Tesla Roadster. Even in current EV powertrains, the regeneration efficiency only reaches up to around 75%, which is much lower compared to the potential efficiency of flywheel-based energy storage (FES) as no energy conversion takes place from one form to another. We implemented FESS in a parallel hybrid setup solely for regenerative braking. Based on the power requirements from the vehicle, the drivetrain smartly switches its power source between the Electric motor and flywheel during the drive cycle. It uses a high-efficiency power transmission system such as half-toroidal CVT or CFT. Based on our simulations using QSS Toolbox on Simulink, an improvement of 2.64% in the range is achieved in the NEDC drive-cycle. We studied another setup where the flywheel is coupled with a generator and performs energy conversion only when the flywheel rotates at rpm range close to its rated-rpm. With a synchronous generator specially designed, an improvement of 2.93% in range is achieved in the NEDC cycle. We developed a novel flywheel design called ‘Centrifugal Flywheel’ similar to a centrifugal clutch with masses and springs. Its moment of inertia reduces with the reduction in kinetic energy and so, the angular velocity reduction is less steep until a point and then reduces more steeply. Based on our simulation, centrifugal flywheel rotates at a high-efficiency energy conversion rpm range for 75% longer time than a conventional flywheel. Graphical comparisons between the conventional and centrifugal flywheel are shown.