Hydraulic Flywheel Accumulator for Mobile Energy Storage Paul Cronk University of Minnesota October 14 th, 2015
I. Overview Outline I. Background on Mobile Energy Storage II. Hydraulic Flywheel Accumulator Concept III. Research Goals and Progress II. Modeling I. Architecture II. Hydraulic Losses III. Kinetic Losses III. Prototype and Experimental Setup IV. Preliminary Experimental Results V. Future Work 2
OVERVIEW 3
Why Hybridize a Powertrain? Recapture kinetic energy otherwise dissipated as heat during braking events Downsize engine and run at peak efficiency point without compromising vehicle performance With plug-in capability, utilize cleaner energy sources for vehicle propulsion Accumulator Engine Pump Pump/Motor Axle 4
Hydraulic vs. Electric Powertrain Component Weight Component Cost ESS Lifetime ESS Power Density ESS Energy Storage Density Hydraulic Powertrain Electric Powertrain Li-Ion Battery Energy Density: 432 kj kg kj Hydraulic Accumulator Energy Density: 6 kg 5
Pressure vs. SOC In a traditional accumulator, pressure varies with state-of-charge. 50 Dimensionless Pressure vs. Dimensionless Energy 45 40 35 Pressure [P/P charge ] 30 25 20 15 10 5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Energy [E/(P charge V charge )] 6
Flywheel-Accumulator Concept Rotating Pressure Vessel Piston Separates Compressed Gas and Oil Torque is applied at the gas side Two Energy Storage Domains: Hydro-Pneumatic Rotating Kinetic 7
Analyzing Fluid Pressure FBD of Infinitesimal Volume: Y r P dp A 2 i PA s P dp A 2 o X P r r 2 2 2 P S dθ PA s l dr Influences on System Pressure: Adding Oil Increases Pressure Increasing Angular Velocity Decreases Pressure 8
Flywheel-Accumulator Concept P r r 2 2 2 P S Pressure rises as flywheel kinetic energy is extracted More pneumatic energy is stored for the same system pressure. Less kinetic energy is required to maintain pressure. Potential for higher efficiency than a spatially separated combination of flywheel and accumulator 9
Research Goals Specify physically feasible design Model performance and optimize design parameters to maximize energy capacity and efficiency Build and test medium energy prototype Refine model and for higher energy levels. 10
MODELING 11
Architecture Steel liner with circumferential composite filament winding Composite provides high hoop strength Liner ensures piston sealing and alleviates radial tensile stresses in composite 12
Architecture Axial and radial ports in the axle facilitate transport of oil The axle, housing, and end caps are radially unconstrained to one another Retainers and radial pins provide axial and tangential constraints between components 13
Hydraulic Losses Axle Port Losses HSRU Leakage 14
Kinetic Losses Bearing losses Aerodynamic drag HSRU Viscous Dissipation Pump/Motor losses Use modified McCandlish and Dorey model Assume one set of loss coefficients provides roughly accurate loss estimates for a range of PM sizes Spin-Up Losses Strohmaier, K.G. et al., 2014. Experimental Studies of Viscous Loss in a Hydraulic Flywheel Accumulator. In 2014 Proceedings of the 52nd National Conference on Fluid Power. 15
Optimization Method Multi-Objective Genetic Optimization 1 0.8 0.6 Vehicle Velocity and State-of-Charge vs. Time state-of-charge vehicle velocity (normalized) 0.4 0.2 0 0 200 400 600 800 1000 1200 1400 time (s) Energy density Drive cycle efficiency 16
Selection of a Prototype Design E d (W-h) 96 94 92 90 88 86 84 82 (%) 94 92 90 88 86 E d m sys 100 90 80 70 60 50 40 30 m sys (kg) Heavy solutions incur high rolling resistance Very energy-dense solutions incur high losses Minimize W dc minimize E d 80 2 4 6 8 10 12 14 16 20 u d (kj/kg) 84 82 Drive cycle losses, W loss 79.2 kj Drive cycle efficiency, η 86.8 % 80Usage ratio, R u 1.97 2 4 6 8 10 12 14 16 u d (kj/kg) Pressure fraction, f pressure 48.0 % (+26.8 % / -21.2 %) System mass, m sys 39.3 kg Energy density, u d 8.77 kj/kg Energy capacity, E d 81.8 W-h Mass, excluding PMs 32.3 kg Capacity ratio, R c 76.3 Housing safety factor 7.35 Storage PM displacement, D Packaging volume (approx.) 0.63 cc/rev 48.6 liters 17
PROTOTYPE AND EXPERIMENTAL SETUP 18
Prototype Components 19
Chamber and Drive Section 20
Test Circuit 21
PRELIMINARY EXPERIMENTAL RESULTS 22
Flywheel Mechanical Efficiency η m,m = T flywheel T ideal = I designα flywheel T ideal η m,p = T ideal T fw = T ideal I design α flywheel 23
Hydraulic Pump/Motor Volumetric Efficiency η v = V a V i η v = V i V a 24
Rotary Union Leakage W l = P s 2 πd s c s 3 12μ o l s 26
Future Work Explore loss mechanisms at higher flywheel speeds Explore the effect of fluid spin-up on HFA performance Implement the HFA prototype in a simulated drive cycle Use validated models to explore benefits of scale 27
Thank You 28