nasa proof-of-concept 1-w stirling convertor development

NASA Proof-of-Concept 1-W Stirling Convertor

Development for Small RPS

AIAA Propulsion and Energy (P&E) Forum and

Exposition, International Energy Conversion and

Engineering Conference (IECEC)

August 19-22, 2019, Indianapolis, IN

National Aeronautics and Space Administration

www.nasa.gov

Session: ECD-02

Authors: Nick Schifer1, Scott Wilson1, Daniel Goodell1, Michael

Casciani2

1. NASA Glenn Research Center, 2. Vantage Partners, LLC

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2

Small nuclear power systems that would provide electricity to

probes, landers, rovers, or communication repeaters for space

missions• Operate in vacuum or on planetary surface (ie. Moon, Mars, more...)

• Use conversion technology to convert heat to electricity for powering spacecraft

sensors and communications • Fractional GPHS (General Purpose Heat Source) offers around 60 watts of thermal input

• LWRHU (Light Weight Radioisotope Heater Unit, often called RHU) offers around 1 watt of

thermal input for each unit and multiple units could be used

Why Low Power RPS?

Development Goals• Sufficient power for spacecraft functions

• Long-life and low degradation to ensure power at

EOM

• Robust to critical environments (vibration, shock,

constant acceleration, radiation)

• Thermal capability and high efficiency

Dynamic Power Conversion• 12-16% overall system efficiency possible

from 1 to 10 watts electrical power output [Ref 1] Conceptualization of Seismic Monitoring

Stations Being Deployed from Rover [JPL Pub 04-

10, Sept-2004]


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Design Goals• Long life design (no wear mechanisms)

• 3 kg system mass

• Envelope of 11 cm diameter X 32 cm length

• Performance• Heat from multiple LWRHU

• At least 1 We power output

• At least 12% system efficiency

• Maximum of 400 ºC acceptor temperature

• Maximum of 50 ºC rejection temperature

• Robustness• Overstroke collision tolerant (limited time)

• Operates in vacuum or atmosphere

• Launch vibration

• Constant accelerations

• Shock

• Compliance• Minimize exported force

• EMI

Low Power Dynamic RPS Concept


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Low Power Dynamic RPS Concept

Electrical

Controller

Stirling

Engine

Multi-Layer

Metal Insulation

Heat

Source

Linear

Alternator

Stirling Convertor

Radiative Coupling Heat Rejection Flange

: -----------------' ' ' ' ' ' ' ' ' ' -------

' ' ' ' ' ' ' ' ' ' ' __ ,


Proof of Concept – 1 We design

• Split-Stirling, gas duct between engine and alternator compression space

• Gap regenerator – no porous matrix

• Flexure bearings for piston and displacer

• Laboratory design did not minimize mass

• Simulating heat from 8x RHUs using electric heater, 350 ºC hot end temp

• Fluid loop heat rejection, 50 ºC cold end

• 100 Hz, 94 psig helium, 4.0 mm Xp, 2mm Xd

5

Test Setup(insulation not shown)

Stirling Convertor

Heat addition

Engine

Heat rejection

Gas duct

Alternator


Convertor Instrumentation

6

Instrumentation

• Piston hall effect sensor

• Displacer hall effect sensors

• Dynamic CS pressure transducer

• Hot end temperature (1x)

• Cold end temperature (1x)

• Alternator housing temperature (1x)

• Electrical heat input

• Alternator output

Displacer Hall sensor

Piston Hall sensor

....----Displacer

----- Tait housing


Testing Sequence

7

• Flexure Stiffness Characterization

• Displacer & Piston Resonance Characterization

• Displacer & Piston Position Sensor Calibration

• Convertor Characterization


Displacer Flexure Stiffness

Characterization

8

Finite element model over predicted

displacer flexure stiffness by 7% at

full 2 mm amplitude

Force applied using

calibrated masses

> 4x flexures {installed as it will

operate)

Displacement measured using laser sensor

z

3.00

2.50

2.00

~- 1.50 '-0

LL

1.00

0.50

0.00 0

y = 0.1714x2 + 0.7683x + 0.0028

y = 0.1374x2 + 0.7601x + 0.0109

y = 0.1434x2 + 0.7464x + 0.012

.. -~:::. .... :·.:·::::::::~------·····

✓~-··

...........

......... .:~·· --..................

... -····· ......

0.5 1 1.5

Displacement, mm

• FEA Results • Test 1 • Test 2

2

.... ···

2.5


Piston Flexure Stiffness

Characterization

9

Finite element model over predicted

piston flexure stiffness by 13% at full

4 mm amplitude

Force applied using cal ibrated masses

l 2x flexures {insta lled as it w ill

pera te)

Displacement measured using laser sensor

14.00

12.00

10.00

z 8 .00 Q) u ....

6 .00 0 u..

4.00

2.00

0.00

0

y = 0.2137x2 + 1.8025x + 0.1865 .. ····

.·· y = 0.1471x2 + 1.7781x + 0.0915 .. ••· •

y = o.mx' + 1837x ~:.~::~;_::.'.:~t:::::::-::::1/

•···

.. -;::•· . ,,,,,::• .. ... ,.:.::.:--1 2 3

Displacement, mm

• FEA Results • Test 1 • Test 2

4

•

5


Displacer Resonance Characterization

10Displacer amplitude (Xd) versus frequency.

Test setup used for characterizing displacer

resonance.

Goal: Achieve 102-103 Hz at 2 mm amplitude

Procedure:

• 1 W linear alternator was used as an exciter

driven by an AC source

• Frequency swept from 90 to 104.75 Hz

• Displacer (mass-spring) assembly allowed to

resonate

• Adjust number of flexures and mass as needed.

2.50

E E 2.00 (l)-

"C :::,

:t= 1.50 a. E <t: ,_ 1.00 (l) u ro a. -~ 0.50 0

0.00 90.0

y = 0.1552x-13.994

I .... .... • I ... •·•···1··

•······· I

92.0 94.0 96.0

.•... •

98.0

.... .•

. •

r

•••

100.0 102.0

Frequency, Hz

.. -·• .• •••

104.0 106.0


Piston Resonance Characterization

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Goal: Achieve 95-98 Hz at 4 mm amplitude.

Two Approaches:

• Resonant approach (used for displacer), requires 2x alternators

• Ringdown

• Drive alternator to 4 mm, go open circuit on the alternator.

- A free piston should ring down for >1 second.

- Frequency of oscillation equates to resonance

throughout the ringdown.

~6 seconds

1.5

1

0 .5

E E 0

c 0

·.;;;; -0.5 "cii

0 Cl. C 0 -1 'tii Cl.

-1 .5

-2

-2.5

110 111 112 113 114

Time, Seoonds

115 116 117 118


Displacer & Piston

Position Sensor Characterization

12

Procedure:

• Displacer was excited via harmonic resonance.

• 1 W linear alternator was driven via AC source.

• A laser displacement sensor was used to measure position.

• All signals were recorded and monitored via LabVIEW.

• Correlations of hall sensor voltage amplitude to laser amplitude (in mm) were derived.

Signals are linear over and beyond entire operating range.

10.00

> 9.00 a.,' 8.00

"C

_.; 7.00

E 6.oo <l'. 5.00 fili ro 4.00

.:!: ;; 3.00

ro 2.00 I 1.00

0.00

--. ·----1

0.00 0.50

t-. •

• •

1.00 1.50

Displacer Amplitude, mm

T •

2.00 2.50

>

5.00

4.50

a.,' 4.00 "C _.; 3.50

t 3.00

<l'. 2.50 Q)

~ 2.00 .:!: g 1.50

ro 1.00 I

0.50

0.00 • 0.00

•• • • • • ---+.-• • ~-~

~- -- •~ - - -• - - - -•-- - - - - -+- - - -• - - • - - - - - - -t- - - -• I -- I

1.00 2.00 3.00 4.00 5.00

Piston Amplitude, mm


Convertor Characterization

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Test process:

- Engine and alternator assemblies were integrated

- Convertor filled with helium

- Used and AC source to drive the piston

- Motor at piston amplitude of 2 mm at frequencies of 95-103 Hz

- Motor at piston amplitude of 4 mm at frequencies of 95-103 Hz

Observations:

- Round 1 of testing w/ non-rigid mount

- Measured case motion: 0.1 mm

- Round 2 of testing w/ rigid mount

- Displacer leads piston by 50-80 degrees

at frequencies of 95-99 Hz.

- Cooling of hot-end observed

- 3.5 W to drive the cooler (rub discovered)

Mode 1 Mode 2

Hot-end Heating Cooling

Xp-Xd Phase Angle ~170 deg ~0 deg

Non-rigid mount

Rigid mount


Objective: High performance required (~0.001 W/m-K effective thermal conductivity)

- Peregrine Falcon Corp. designed and fabricated multi-layered metal insulation (MLMI)

- The prototype is currently under test at GRC.

Current Challenge: Low conductance of the evacuation port requires long evacuation time.

Insulation – Functional Test

14

Fluid Rejector

Stirling Thermal SimulatorInsulation

Evacuation

Port

Multiple layersHeat Source


Controller design and functionality

• Linear AC regulator controller using a MOSFET H-bridge with analog circuit to control FETs for AC to DC rectification and load control

• Constant power load monitoring allows for load control and shunting of unused power

15

ControllerDesign Progression

• LTspice model contains a linear alternator, H-bridge rectifier, constant power circuit, and waveform smoothing circuit for power factor and Total Harmonic Distortion correction

• Model validated with breadboard testing.

• Design finalized and incorporated into a printed circuit board design. Assembly in progress

Alternator Voltage, Vp-p 25.6

Alternator Power, We 1.24

Controller Voltage, Vdc 11.1

Controller Power, We 1.16

AC-DC Conversion Efficiency 93%

Controller Breadboard Testing Results

P P :Cll1 0 ~/OA P P :Clll i¥&, :Cll10 103 WoA RIIS :Cll1

· s1oppe<1----4932 2018/12/13 15 :06 :05 .02974723

2"J_b3V Av1,

137 Ol4nA RHS

10ai1 :11 .11

11 1398V q OH?IV

Ac<lbio •: • Hor lld l--200kS/s 5os/div

Reallime Math

Filter /Delay SetllJ

I ·l Next 1/2

fdge-- c111 •r -----------~ :Hlo ___ _ Auto O .020A 201 8/12/13 15 :07 :•6


Summary

• Small RPS are being considered for small spacecraft missions

• Enables long-life power for use in darkness

• 1-W Stirling RPS is in development at NASA GRC

• Testing & Demonstration of Subcomponents is Underway:

• Convertor

• High-performance insulation

• Controller

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Special thanks to contributors

• Barry Penswick

• Jonathan Metscher

• Malcolm Robbie

• Cheryl Bowman

• Paul Schmitz

• Roy Tew

Thank you for attending

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nasa proof-of-concept 1-w stirling convertor development

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