1 Perry Tsao, Matt Senesky, Seth Sanders University of California, BerkeleyPerry’s thesis defense presented www-power.eecs.berkeley.edu May 15, 2003 A Homopolar Inductor Motor/Generator and Six-step Drive Flywheel Energy Storage System

2 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 Bearings Containment

3Flywheels Integrated flywheel – Single-piece solid steel rotor – Combines energy storage and electromagnetic rotor – Motor housing provides Vacuum containment Burst containment Integrated Flywheel Flywheel Rotor Motor Stator Bearings Containment

4 Homopolar Inductor Motors (HIM) Rotor for HIM

5 Armature Winding Construction Bladder FR4 Arm. Windings FR4 Stator Inner Bore

6 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

7 Six-Step Drive Charging (motoring) Discharging (generating) 25,000 rpm, 1kW operating point

8 Efficiency Tests

9 Efficiency Measurements

10 MEMS REPS Project MEMS Rotary Engine Power System Concept – Replace conventional batteries with rotary engine and generator plus fuel Specifications – Goal is to provide mW – Need ~10% system efficiency with octane fuel to beat batteries Engine/ Generator Package Concept Unit Generator Matthew Senesky Seth Sanders, Al Pisano

11Design Electroplated NiFe poles allow engine rotor to be used as generator rotor Axial-flux configuration Claw pole stator made from powdered iron Toroid Core Pole Faces Rotor Coil Permanent Magnet millimeters Bottom Plate Top Plate Side Plate

12Construction Stator pole faces cut with EDM Stator core, coil (with bobbin) and toroid. 250  m 2.2 mm Partial stator assembly Steel test rotorMicrofabricated Si rotor 1 cm 2.4 mm Dr. A. Knobloch, 2003

13 Preliminary Results Open circuit voltage of 150  V/turn in 112  coil at 500 Hz Expect to improve this by factor of 4-5

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/W peak – 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

System Efficiency Collector (linearized) Engine (2/3 Carnot eff.) System (overall) Collector (nonlinear)

Comparative Cost Analysis Cost goal set by PV is under $5/W !!! Peak insolation = 800 W/m 2 System optimal efficiency = 10% ignore engine cost Cost of collector must be less than $400/m 2 For solar-thermal-electric system…

Market Available Collectors Assumes engine achieves 2/3 Carnot, ambient is 27 º C, and engine cost is negligible Even at retail (500 m 2 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 Material cost is $0.71/W; High-volume manuf. cost? Based on a complete system efficiency of 6.9%...

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

Stirling Engine: Multiple-Phase

Stirling Engine: Simulation

Cost Analysis: Stirling Engine Cost for a representative 200W Stirling engine Engine cost is $0.31/W System cost: about $1/W

29 Prototype 3-Phase Stirling Machine

30 Heater/Cooler and Regenerator

Conclusion Low-cost distributed solar-thermal-electricity possible with standard solar hot water collectors and low temperature Stirling heat engine Prototype experiments in progress