A Homopolar Inductor Motor/Generator and Six-step Drive Flywheel Energy Storage System Perry Tsao, Matt Senesky, Seth Sanders University of California, Berkeley Perry’s thesis defense presented www-power.eecs.berkeley.edu May 15, 2003 1 Flywheel Energy Storage System Prototype design goals – 30 kW (40 hp) – 15 s discharge – 500 kJ (140 W-hr) – 1 kW/kg (30 kg, 66 lbs.) Integrated Flywheel Flywheel Rotor Motor Stator Containment Bearings 2 Flywheels Integrated flywheel – Single-piece solid steel rotor – Combines energy storage and electromagnetic rotor – Motor housing provides Vacuum containment Integrated Burst containment Flywheel Flywheel Rotor Motor Stator Containment Bearings 3 Homopolar Inductor Motors (HIM) Cross-sections Top view Side view Bottom view Rotor for HIM 4 Armature Winding Construction Bladder FR4 Arm. Windings FR4 Stator Inner Bore 5 Six-Step Drive Six-step – PWM impractical at max speed (6.7 kHz) – Lower switching losses – Field winding compensates for fixed voltage Potential problems – Harmonic currents – Harmonic rotor core losses Controlled by adjusting armature inductance 6 Six-Step Drive Charging Discharging (motoring) (generating) 25,000 rpm, 1kW operating point 7 Efficiency Tests 8 Efficiency Measurements 9 MEMS REPS Project Matthew Senesky Seth Sanders, Al Pisano MEMS Rotary Engine Power System Concept – Replace conventional batteries with rotary engine and generator plus fuel Concept Unit Specifications Engine/ Generator Package – Goal is to provide 10-100mW – Need ~10% system efficiency with octane fuel to beat batteries Generator10 Design Top Plate Electroplated NiFe poles allow Side Plate Core engine rotor to be used as generator Coil rotor Axial-flux Permanent Toroid configuration Magnet Claw pole stator made from Pole Faces powdered iron Side Rotor Plate Bottom Plate 123456789 millimeters 11 Construction 2.4 mm 2.4 mm 250 m Dr. A. Knobloch, 2003 Steel test rotor Microfabricated Si rotor Stator pole faces cut with EDM 1 cm 2.2 mm Partial stator assembly 12 Stator core, coil (with bobbin) and toroid. Preliminary Results Open circuit voltage of 150V/turn in 112 coil at 500 Hz Expect to improve this by factor of 4-5 13 Low-Cost Distributed Solar-Thermal-Electric Power Generation A. Der Minassians, K. H. Aschenbach, S. R. Sanders Power Electronics Research Group University of California, Berkeley Introduction Photovoltaic (PV) technology – Efficiency: up to about 15% – Cost: about $5/Wpeak – Materials cost: about $5/W (with a low profit margin) – Cost reduction limited by cost of silicon area – No alternative for small-scale off-grid applications Technology similar to PV but at lower cost would see widespread acceptance View is that unit cost ($/W) is paramount Many untapped siting opportunities Possible Plan Solar-Thermal Collection Low-concentration non-imaging collector Low maintenance Low cost: sheet metal, glass cover, plumbing Proven technology Low temperature Thermal-Electric Conversion Stirling heat engine: Theoretically achieves Carnot efficiency, can achieve large fraction of Carnot eff. Low cost: Bulk metal and plastic Linear electric generator (high efficiency & low cost) Representative Diagram Cooler Pump Insulated Pipe Heater Stirling Engine Collectors System Efficiency Collector (nonlinear) Collector (linearized) Engine (2/3 Carnot eff.) System (overall) Comparative Cost Analysis Cost goal set by PV is under $5/W !!! For solar-thermal-electric system… Peak insolation = 800 W/m2 System optimal efficiency = 10% ignore engine cost Cost of collector must be less than $400/m2 Market Available Collectors U T m(opt) CPA STC CPW Collector Model sys( op t ) sys [W/m2K] [oC] [%] [$/m2] [$/W] Flate Plate Collectors Thermo Dynamics G Series 74 5.247 79 3.9 194 6.27 Arcon HT 79 3.796 101 5.8 142 3.07 CPC-based Collectors AOSOL CPC 1.5X 75 4.280 90 4.7 158 4.16 SOLEL CPC 2000 1.2X 91 4.080 106 6.9 193 3.49 Assumes engine achieves 2/3 Carnot, ambient is 27 ºC, and engine cost is negligible Even at retail (500 m2 qty) prices and low system efficiency, some collectors achieve costs less than $5/W Cost Analysis: Collector Cost breakdown of commercial collector for hot water Collector Material Mass [kg/m2] Specific Cost [$/kg] Cost [$/m2] Low-Iron Cover Glazing 7.8 1.87 14.60 Sheet Aluminum 2.75 6.00 16.50 Sheet Copper 1.26 6.35 8.00 Fiberglass Insulation 1.2 0.83 1.00 Total 13 N/A 40.10 Based on a complete system efficiency of 6.9%... Material cost is $0.71/W; High-volume manuf. cost? Stirling Engine: Basics Closed gas circuit Working fluid: air, hydrogen, helium Compress – Displace – Expand – Displace Skewed phase expansion and compression spaces needed Heater / Cooler: wire screens Regenerator: woven wire screens Stirling Engine: Losses Heater / Cooler Fluid flow friction Ineffectiveness (temperature drop) Regenerator Fluid flow friction Ineffectiveness (extra thermal load) Static heat loss (extra thermal load) Use “free” diaphragms as pistons = No surface friction, No leakage, No mechanical coupling! Stirling Engine: Power Balance Leakage due to regenerator ineffectiveness (27.8 W) Leakage through regenerator housing (13.9 W) Injected heat at Rejected heat at heater 332.7 W 246.3 W cooler (366.9 W @ 420 K) 8 K temp. drop 5 K temp. drop (294.6 W @ 300 K) Carnot Engine 8 6 f H . lo e 4 d ) w a i t W lu W lo er f s f r .9 s lu le (0 ( id oo s f H 1. s lu a 8 C lo r id lf W to ) f re ra W lo g ) flow e .7 w en en (5 lo e g s s r re s s at f lo ( o al 5. r H ow 7 fl W id ) flu Output power (72.3 W) Eff.=19.7% Stirling Engine: Multiple-Phase Expansion space (Hot) Heater (Hot) Regenerator Cooler (Cold) Compression space (Cold) Diaphragm piston Rigid Linkage Cantilever beam (spring) Diaphragm Single Stirling engine in three-phase system Stirling Engine: Simulation Stirling Engine: Simulation Cost Analysis: Stirling Engine Cost for a representative 200W Stirling engine Engine Material Mass [kg] Specific Cost [$/kg] Cost [$] Cast Aluminum 4.8 5.50 26.40 Copper Wire 3.5 10.00 35.00 Total 8.3 N/A 61.40 Engine cost is $0.31/W System cost: about $1/W Prototype 3-Phase Stirling Machine 29 Heater/Cooler and Regenerator 30 Conclusion Low-cost distributed solar-thermal-electricity possible with standard solar hot water collectors and low temperature Stirling heat engine Prototype experiments in progress.