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Perry Tsao, Matt Senesky, Seth SandersUniversity of California, Berkeley Perry’s thesis defense presented www-power.eecs.berkeley.edu May 15, 2003

A Homopolar Inductor Motor/Generator andA Homopolar Inductor Motor/Generator and Six-step Drive Flywheel Energy Storage Six-step Drive Flywheel Energy Storage

System System

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Flywheel Energy Storage SystemFlywheel 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 StatorBearings

Containment

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FlywheelsFlywheels

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 StatorBearings

Containment

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Homopolar Inductor Motors (HIM)Homopolar Inductor Motors (HIM)

S id e v iew

To p v iew

B o tto m v iew

C ro ss-sec tio n s

Rotor for HIM

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Armature Winding ConstructionArmature Winding Construction

Bladder

FR4

Arm. Windings FR4

Stator Inner Bore

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Six-Step DriveSix-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

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Six-Step DriveSix-Step Drive

Charging (motoring)

Discharging (generating)

25,000 rpm, 1kW operating point

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Efficiency TestsEfficiency Tests

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Efficiency MeasurementsEfficiency Measurements

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MEMS REPS ProjectMEMS REPS Project

MEMS Rotary Engine Power System

Concept– Replace conventional batteries

with rotary engine and generator plus fuel

Specifications– Goal is to provide 10-100mW– Need ~10% system efficiency

with octane fuel to beat batteries

Engine/ Generator Package

Concept Unit

Generator

Matthew SeneskyMatthew SeneskySeth Sanders, Al PisanoSeth Sanders, Al Pisano

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DesignDesign 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

1 2 3 4 5 6 7 8 9millimeters

Bottom Plate

Top Plate

Side Plate

Side Plate

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ConstructionConstruction

Stator pole faces cut with EDM

Stator core, coil (with bobbin) and toroid.

250 m

2.2 mmPartial stator assembly

Steel test rotor Microfabricated Si rotor

1 cm

2.4 mm2.4 mmDr. A. Knobloch, 2003

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

Low-Cost Distributed Low-Cost Distributed Solar-Thermal-Electric Solar-Thermal-Electric Power GenerationPower Generation

A. Der Minassians, K. H. Aschenbach,

S. R. Sanders

Power Electronics Research GroupUniversity of California, Berkeley

IntroductionIntroduction 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 PlanPossible Plan Solar-Thermal Collection

Low-concentration non-imaging collector Low maintenance Low cost: sheet metal, glass cover, plumbing Proven technology Low temperatureLow 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 DiagramRepresentative Diagram

Stirling EngineInsulated Pipe

Collectors

Pump

Heater

Cooler

System EfficiencySystem Efficiency

Collector (linearized)Engine (2/3 Carnot eff.)

System (overall)

Collector (nonlinear)

Comparative Cost AnalysisComparative Cost Analysis

Cost goal set by PV is under $5/W !!!Cost goal set by PV is under $5/W !!!

Peak insolation = 800 W/m2

System optimal efficiency = 10%

ignore engine cost

Cost of collector must be less than $400/m2

For solar-thermal-electric system…

Market Available CollectorsMarket Available Collectors

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

Collector Model

U

[W/m2K]T m(opt)

[oC]

sys(opt)

[%]CPA STC

[$/m2]CPW sys

[$/W]

Thermo Dynamics G Series 74 5.247 79 3.9 194 6.27

Arcon HT 79 3.796 101 5.8 142 3.07

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

Flate Plate Collectors

CPC-based Collectors

Cost Analysis: CollectorCost Analysis: CollectorCost 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 16.50Sheet Copper 1.26 6.35 8.00

Fiberglass Insulation 1.2 0.83 1.00Total 13 N/A 40.10

Material cost is $0.71/W;High-volume manuf. cost?

Based on a complete system efficiency of 6.9%...

Stirling Engine: BasicsStirling Engine: Basics

Closed gas circuitWorking fluid: air, hydrogen, heliumCompress – Displace – Expand – Displace

Skewed phase expansion and compression spaces neededHeater / Cooler: wire screensRegenerator: woven wire screens

Stirling Engine: LossesStirling Engine: LossesHeater / Cooler

Fluid flow frictionIneffectiveness (temperature drop)

RegeneratorFluid flow frictionIneffectiveness (extra thermal load)Static heat loss (extra thermal load)

Use “free” diaphragms as pistons = No surface friction, No leakage, No mechanical coupling!

CarnotEngine

Rejected heat atcooler

(294.6 W @ 300 K)246.3 W

5 K temp. drop332.7 W

8 K temp. drop

Injected heat atheater

(366.9 W @ 420 K)

86.4 W

Leakage throughregenerator housing

(13.9 W)

Leakage due toregenerator ineffectiveness

(27.8 W)

Output power(72.3 W)

Eff.=19.7%

Stirling Engine: Power BalanceStirling Engine: Power Balance

E xpansion space (H ot)H eater (H ot)R egeneratorC ooler (C old)C om pression space (C old)D iaphragm pis tonR igid L inkageC antilever beam (spring)D iaphragm

Single S tirling engine in three-phase system

Stirling Engine: Multiple-PhaseStirling Engine: Multiple-Phase

Stirling EngineStirling Engine:: Simulation Simulation

Stirling EngineStirling Engine:: Simulation Simulation

Cost Analysis: Stirling EngineCost Analysis: Stirling EngineCost for a representative 200W Stirling engine

Engine Material Mass [kg] Specific Cost [$/kg] Cost [$]Cast Aluminum 4.8 5.50 26.40Copper Wire 3.5 10.00 35.00

Total 8.3 N/A 61.40

Engine cost is $0.31/W

System cost: about $1/WSystem cost: about $1/W

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Prototype 3-Phase Stirling MachinePrototype 3-Phase Stirling Machine

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Heater/Cooler and RegeneratorHeater/Cooler and Regenerator

ConclusionConclusion Low-cost distributed solar-thermal-

electricity possible with standard solar hot water collectors and low temperature Stirling heat engine

Prototype experiments in progress