Embed Size (px)
Citation preview
A Sunlight-to-Microwave Power Transmission Module Prototype for
Space Solar
FISO telecon 2017-05-31
Paul Jaffe, PhDU.S. Naval Research Laboratory
Overview
Space Solar and the “Sandwich” Approach
Thermal Considerations & the Tile and Step Sandwich Module Concepts
Progression Through Layer Designs & Implementations:– Solar Array– Electronics– Antenna
Testing Methodology
Results2
Motivation
Climate change demands new energy sources
Alternatives to fossil fuels often suffer from:– Intermittency– Lack of scalability– Locale dependence– Safety risks
Solar energy has a long history:– Pro: The sun is an effectively unlimited energy supply – Con: Ground solar collection suffers from night and
atmospheric losses
Recent studies of Space Solar suggest research that may clarify its technology challenges & economic feasibility– Technological advances may increase its prospects– Niche applications may tolerate higher energy cost,
such as remote military bases
4
What is Space Solar?
Collection of solar energy in space and its wireless transmission for use on earth
– Overcomes atmospheric and diurnal limitations associated with terrestrial solar power
– Could offer energy security, environmental, and technological advantages to initial developers
– Has been criticized as economically infeasible, but there is not an empirical basis from which to create a realistic detailed analysis
5
Functions a Solar Power Satellite Must Perform
Energy Collection: – Photovoltaics (PV)– Solar thermal (Heat Engine)– Sun-pumped lasers
Power Transmission: – Microwave– Laser– Reflection
NASA Reference Design, circa 1981
Aerospace Corp. Laser Concept, circa 2002
For this discussion, focus will be on the most commonly proposed
combination, PV/Microwave
6
System Blocks and EstimatedCurrently Achievable Efficiencies
Segment Efficiency Notes
Photovoltaics 30% Efficiencies >40% in lab under concentration
DC-to-RF Conversion 85% Varies with conversion
method & implementation
Antenna 90% Includes conduction and scan losses
Atmospheric Transmission 98% Weather & frequency
dependentRF Collection
Area 90% Function of rectenna array size & transmit taper
Rectenna Elements 91% Demonstrated at 2.45
GHz
TOTAL 17%Energy is available essentially 24 hours a day, all year round
Satellite
Ground Station
Modular Architectures
Each PV/Microwave architecture has:– Solar panel area
• Affects power collected– Antenna transmission area
• With frequency, affects power beam directivity
Some architectures match these two areas and increase power collected using concentrating reflectors– Reduces wiring mass and
avoids slip rings
Modular Symmetrical Concentrator, circa 2007 (NSSO)
Thousands of adjacent sandwich modules form this surface
SPS-ALPHA, circa 2012 (Artemis Innovations)
Thousands of adjacent sandwich modules form this surface
8
Solar Concentration
Concentration advantages: – Improves solar cell efficiency– Reduces the required panel
area– Has the potential to reduce
launch mass for given power, since reflectors tend to be lighter than solar cells per unit area
– Reflectors used for concentration may also be used to redirect energy to simplify onboard power distribution
Concentration disadvantages:– Compounds thermal challenges
because of the additional heat needing to be dissipated
– Requires additional structure to implement reflectors
– Requires higher pointing accuracy
Integrated Symmetrical Concentrator, circa 1998 (NASA)
Light
μWaves
Photovoltaics
DC to RF conversion
Antenna
The “Sandwich” Module
9
Prior Sandwich Module Efforts
Hiroshi Matsumoto, Kyoto University, with SPRITZ –Solar Power Radio Integrated Transmitter, 2001
Owen Maynard Solid State Sandwich Report, 1980
2002 and 2001 Sandwich Reference Models, JAXA Contractor Report, 2003, URSI ICWG Report, 2007
Nobuyuki Kaya, Kobe University, and John Mankins with sandwich prototype, photovoltaics removed, 2009 Photovoltaics removed to show
phase control electronics
Key Problems Low PV & DC-RF efficiency Implementing retrodirective
control of beam Layer integration & thermal
dissipation
10
11
Objectives of the Research
(2) Perform the First Test of a Sandwich Module for Space Solar
Power Under Space-like Conditions
(1) Design, Fabricate, and Test the Highest Specific Power, Highest
Efficiency Sandwich Module to Date
(3) Characterize and Compare the Performance of Two Different Types
of Sandwich Modules
12
Figures of Merit (FOMs) for Sandwich Modules
Mass per unit area [kg/m2] Specific power [W/kg] Combined conversion efficiency [%] Sun concentration ratio acceptance [# suns] Survival temperature range [°C] Continuous operation duration [hours]
Other considerations: – Adaptability for use with a retrodirective control scheme, Susceptibility to space radiation
environmental effects, Susceptibility to solar wind and space weather effects, Solar UV degradation tolerance, Space environment charging behavior, Susceptibility to parts aging effects, Avoidance of multipactor effects, Launch acoustic and vibration environment tolerance, Electromagnetic compatibility and interference susceptibility, Manufacturability, Ease of integration with other modules in space, Ability to transfer heat from other modules, Ability to transfer electrical power from other modules, Outgassing qualities, Structural rigidity, Reliability, Durability, Serviceability
Of Primary Interest
Light (about 7% of incident light is reflected)
μWaves
Photovoltaics ~30% efficient
DC to RF ~80% efficient
Antenna ~95% efficient
Summary of the Thermal ChallengeUsing Idealized Efficiency Figures
13
P is the heat power radiatedε is the emissivity of the materialσ is the Stefan-Boltzmann constantA is the radiating areaT is the temperature
Stefan-Boltzmann Law:P = εσAT4
HeatTOTAL MODULE
EFFICIENCY: ~23%
For every 100W of incident sunlight,about 72W must be radiated as heat power
14
Temperature Considerations
Solar cell and solid state power amplifier efficiencies decrease with rising temperature
Options to maintain acceptable operating temperatures:
P = εσAT4
– Increase total module efficiency to reduce heat power• PV is limiting factor, efficiency increase beyond scope
– Reduce sun concentration• Reduces potential system mass savings
– Use high emissivity materials (≈1)• Limited by black body radiator
– Increase device operating temperature• Beyond project scope
– Increase radiator area• Means a departure from the flat module approach
(constant)
15
Radiator Area Required to Maintain Temperature Equilibrium for a Flat, 28 cm x 28 cm Square Module at 23% Efficiency
16
Temperature of Flat 28 cm x 28 cm Square Module with Both Sides as Black Body Radiators for Various Module Efficiencies
17
Using a “Tile” Sandwich Module
The top and bottom sides of tile module are available to radiate heat, sides connect to adjacent identical modules which also need to radiate heat.
18
Using a “Step” Sandwich Module
Additional area on the step module for radiating heat versus the tile module allows cooler operating temperatures and/or higher sun concentration levels
19
Simulation Shows Step Module Max Temp Runs ~60° Cooler at 3 Suns vs. Tile Module
RF & power electronics go here to lower heat exposure; note electronics temp is ~20°cooler than tile
SOLAR ARRAY FACE
TRANSMIT ANTENNA
FACE
SOLAR ARRAY FACE
TRANSMIT ANTENNA
FACE
Photovoltaics
DC to RF Electronics
Antenna
Tile Sandwich Module Layer Implementations
20
Solar Array: 28 Cells in Two Strings
Array has two 14 cell strings in parallel– 28.3% efficient Spectrolab UTJ cells used,
mounted on FR4
1.59mm aluminum support substrate– Step module utilizes a continuous piece of
pyrolytic graphite sheeting for heat spreading
Nusil RTV for bonding
21
AM0, 1 Sun, 70°CVoc (V) 33.8Isc (A) 0.919Vmp (V) 29.1Imp (A) 0.870Pmp (W) 25.3Power @ 28V (W) 24.4
Output current scales nearly linearly with sun concentration for a fixed temperature
Tile Module: 0.30m x 0.29m (12.6” x 11.3”)
Step Module: 0.30m x 0.29m (12.6” x 11.3”) with 0.29m (11.5”) radiators
Tile Module Solar Array I-V Curve Testing
22
4000W Xenon light source with different combinations of light attenuating screens used for measuring power output of each panel string
Electronics: 2.45 GHz RF Amplifier Chain
RF chain matched for solar array is about 47% efficient Tile module uses a single chain, Step module uses three
chains in parallel that are power combined
23
Electronics: Power Conversion
24
Power electronics was designed to support both tile and step modules
Power electronics measured efficiency ~96% or better
Power and RF Electronics on Tile Module Baseplate Prior to Thermal Feature Installation
25
Power Electronics
BoardVoltage
ControlledOscillator
DriverStage RFAmplifier
FinalStage RFAmplifier
26
Power and RF Electronics on Tile Module Baseplate After Thermal Feature Installation
BlanketingCovering
Power Electronics
Board
Thermocouple Wire Bundle
Black Kapton
Tape
Antenna: Short Backfire Design
Flat reflector version used
Max published gain ~ 18.1 dBi
Quoted efficiency ~ 91-95%
Electronics module output connected to dipole feed port (linear-polarized)
To be Measured:– VSWR– Radiation Patterns & Gain– Efficiency (Wheeler Cap method)
Gain Pattern2.45 GHz16.5 dBi peak
E-plane
H-plane
27
Dia: 292mmHgt: 61.2mm
28
Integrated Tile Module with Antenna Mockup
Tile Module Solar Array & Power and RF Electronics Testing
29
DC to RF Electronics
Step Sandwich Module Layer Implementations
30
Photovoltaics
Antenna
31
Integrated Step Module with Antenna Mockup
Step Module Solar Array & Power and RF Electronics Testing
32
33
Testing Apparatus – Tile Module Configuration
Vacuum ChamberVacuum ChamberTest WorkstationTest Workstation
ProtectiveShroud
ProtectiveShroud
SunSimulator
andAttenuating
Screens
SunSimulator
andAttenuating
Screens
34
Tile Module Illumination Testing –Electronics Powered by Solar Array
Illumination Testing at Ambient Pressure on Lab Bench
Illumination Testing Under Vacuum in Thermal Vacuum Chamber
The gobo prevents excess light from entering and unnecessarily heating the chamber itself, rather than the test article
Tile Module RF Conversion Efficiency and Solar Array Temp at Ambient Pressure Under Various Illumination Conditions
Screen A No Screen E D C B
Tile module RF Conversion Efficiency, Solar Array Power, and RF Output Power Under Various Illumination
Conditions at Ambient Pressure
Tile Module Data Show Vacuum Correlateswith Reduced Output Power
Ambient,Pwr Sim
Ambient,Light
Vacuum,Light
Vacuum,Light+
Vacuum,Light++
Each cluster of 3 points represents (in order) the mean, min, and max
Chamber window(not used for ambient)incurs ~5% power loss
Light,Light+, & Light++ correspond increased light intensity & degraded field uniformity
Data was collected over a 30 minute equilibrium period for each condition (σ<0.4°C for every temperature point)
Tile Module Data Show Vacuum Correlateswith Higher Module Temperatures
Ambient,Pwr Sim
Ambient,Light
Vacuum,Light
Vacuum,Light+
Vacuum,Light++
Each cluster of 3 points represents the mean, min, and max
Data was collected over a 30 minute equilibrium period for each condition (σ<0.4°C for every temperature point)
Tile & Step Module Figures of Merit Mass per unit area (Lower is better)
– Antenna mockup rather than antenna used– Tile Module: 21.9 kg/m2
• 1.91kg/(0.286m * 0.305m = 0.0872m2)– Step Module: 36.5 kg/m2
• 3.33kg/(0.286m * 0.319m = 0.0913m2)– Results fall within 4 kg/m2 to 40 kg/m2 predicted range found in the
literature
Specific power (Higher is better)– Antenna and miscellaneous small parts masses are estimated– Tile Module: 4.5 W/kg measured @ minimum 1.0 sun illumination in
vacuum• Solar array temps 122-150°C, 9W RF output / 1.91kg module mass
– Step Module: 5.8 W/kg measured @ minimum 2.2 sun illumination in vacuum
• Solar array temps > 103-130°C, 19W RF output / 3.33kg module mass
40
Tile Module Efficiency in Vacuum
Module conversion efficiency with minimum one sun incident on module (>117 W over 0.0872 m2)
Solar Panel: power measured during integrated module under vacuum and solar illumination, solar array temps in range 122-150°C as seen in plot for case “Light++”. Note cell voltage at peak power drops ~6.5mV/°C.
Power Electronics: power measured during electronics board standalone test under loading conditions similar to integrated module test
RF Chain: power measured during integrated module test under vacuum and solar illumination, driver stage amp @ 80°C, final stage amp @ 83°C
Antenna: *efficiency calculated from simulation
**Combined figure use simulated antenna efficiency value.
Element Goal Achieved Power Out (W)
Solar Panel 24% 19% 22Power Electronics 95% 97% 22RF Chain 50% 44% 9Antenna 95% 95%* 9
COMBINED MODULE 11% 8%** 9
(Combined efficiency and power out at ambient under illumination with no chamber
window were 11% and 14W)
41
Step Module Efficiency in Vacuum
Module conversion efficiency with minimum 2.2 suns incident on module (>275 W over 0.0913 m2)
Solar Panel: power measured during integrated module under vacuum and solar illumination, solar array temps in range >103-130°C. Note cell voltage at peak power drops ~6.5mV/°C.
Power Electronics: power measured during electronics board standalone test under loading conditions similar to integrated module test
RF Chains: power measured during integrated module test under vacuum and solar illumination, driver stage amps in range 105-107°C, final stage amps 95-101°C
Antenna: *efficiency calculated from simulation
**Combined figure use simulated antenna efficiency value.
Element Goal Achieved Power Out (W)
Solar Panel 20% 17% 46Power Electronics 95% 97% 44RF Chains 50% 44% 19Antenna 95% 95%* 18
COMBINED MODULE 9% 7%** 18
42
Summary
Trade studies, analyses, and simulations were performed in the design and production of sandwich module prototypes for space solar power
A novel approach for increasing thermal dissipation capabilities in modular space solar architectures was explored
The first-ever sandwich module testing under space-like conditions was conducted
This work provides an empirical basis for informing technical and economic analyses for a prominent class of space solar systems
44
Backup Charts
45
Historical Survey of Some SSP Concepts
NASA/DOE SPS Reference System, circa 1978
SPS 2000 Japanese LEO concept, circa 1994
SunTower LEO/MEO/GEO concept, circa 1999
Peter Glaser GEO concept, circa 1968
46
System Blocks and Historical Efficiencies
Image from 1980 DOE/NASA report
23 W107 W 30 W 24 W
1 W6 W
47
Heat Dissipation for One Sun Incident28 cm x 28 cm Module Area
70 W
Solar Cells30% efficient
DC-to-RF80% efficient
Antenna95% efficient
Reflected light
Incomingsunlight
Power sentto the ground
Total heat power to be dissipated: 77 W
Combined module efficiency: 23%
Efficiency estimates are optimistic, especially for DC-to-RF, and neglect power distribution and other losses
Module Architecture Trade Study
Shape should tessellate in order to form arbitrarily large surfaces. Candidates: triangles, squares, hexagons
Hexagons make best volumetric use of a cylindrical payload fairing. Cross-sectional area coverage: triangle ~41%, square ~64%; hexagon ~83%
However, as PV cells are generally available as rectangular shapes, higher module percentage coverage is provided by a square vs. a hexagon
Additionally, since launch vehicles tend to be mass-limited instead of volume-limited for a given payload fairing accomodation, optimizing the use of volume is important only for the very lowest density payloads
Thus, a square module shape is likely favored
48
49
An Approach To Increase Radiator Area
The “sandwich module” disc is replaced by an open-top conical graphite structure
“Stepped” Sidewall
Reflective inner film
PV panel
Waste heat radiator area is increased
Structure wall is high-conductivity graphite composite
Use an aperture constructed of step-shaped modules Increased radiator area vs. flat module, giving lower operating temps Heat rejection area can be increased arbitrarily, but at the cost of structure
mass and increasing distance from the primary heat source Two-phase heat pipes could be used for heat transport within and between
modules, but complexity would increase
Photovoltaics Trade Study
Trade factors: Temperature performance characteristics, Concentration ratio tolerance, Efficiency, Power output/mass, High voltage capabilities, Can I actually buy it?
– 43.5% efficient cells under hundreds of suns for a fraction of a second in the lab are not applicable to the prototype; 35.8% one-sun efficiency cells are likewise not commercially available, even for space
Higher efficiency cells (~30%) are likely worth the cost– PV is the most inefficient link in the chain, want to minimize loss– Higher efficiency PV also helps reduce the thermal problem– Now possible to get very lightweight triple junction cells
Manufacturers: Emcore and Spectrolab
50
DC-RF Conversion Trade Study
Solid State Power Amps are light, available,and easy to phase control
51
Method GaN SSPA Magnetron TWT MBKEfficiency 43-70% 44-73% 66-70% 50%*Mass (kg) <0.1 0.9-4.3 0.7-3.0 1.0*Power Output (W) 25-220 900-5,000 20-300 1,000*Input voltage (V) 28-50 4,000-20,500 5,000-20,000 2,000-4,000*Manufacturers Cree, TriQuint Toshiba, Hitachi L3, Thales CCR
SSPA=Solid State Power Amplifier, TWT = Traveling Wave Tube, MBK = Multiple Beam Klystron. Values (except for MBK) taken from data sheets of potential models in the 2-10GHz frequency range, some available from Richardson Electronics. Masses exclude voltage conversion components. *rough estimates
Trade factors: Mass, Usability as thermal radiator, Efficiency, Ease of use in an array, Compatibilitywith Mechanical Design
Array element options– Type
• Patch, helix, slots, dipole, X-dipole, etc.– Polarization– Spacing of elements– Number of elements per module– Sub array characteristics
Beam forming considerations– Signal distribution
• Coax, waveguideDiagram Source: Kawasaki, S., "A Unit Plate of a Thin, Multilayered Active Integrated Antenna for a Space Solar Power System," URSI Radio Science Bulletin, No. 310, September 2004, pp. 15-22
Antenna Trade Study
52
53
Characterization of Power Added Efficiency Performance of Final Stage Amplifier
Region of Interest
Data Sheet for Cells Used for Modules
Power Electronics Block Diagram
55
56
Back of Solar Array with Thermal Features
Black Kapton
Tape
Thermocouple Wire Bundle
57
Tile Module Power and RF Electronics Baseplate Integrated with Solar Array
Test Workstation
58
Solar Array Simulator
PC with LabView
RF Attenuator
USBPowerMeter
Test Box Data Acquisition Unit
SpectrumAnalyzer
Thermal Vacuum Chamber
Tile Module Power and RF Electronics Testing
59
Sandwich Module Functional Diagram
60
Sandwich Module Functional Diagram Showing Elements Implemented
61
Temperature Effect on I-V Curves at About One Sun
62
Tile Module Integration and Testing Flow
63
Xenon Lamp & Solar Spectral Power Distribution
64
Fused Silica Spectral Transmissivity
Beam Uniformity Maps
65
One Lamp Two Lamps
Beam uniformity varies with lamp focus setting and other factors
Goubau and Schwering Method of Finding Beam Collection Efficiency
66
0%10%20%30%40%50%60%70%80%90%100%
0 0.5 1 1.5 2 2.5 3
Collection Efficiency (%
)
Calculated
Measured
Using GEO (36,000km), 1500 m Tx diameter, and 2.45 GHz assumptions with a 7.5 km diameter receiving area provides a τ of about 2, > 95% collection efficiency
One-way Sea Level to Zenith Attenuations in Clear Sky Conditions
67
0.001
0.01
0.1
1
10
100
1000
0 20 40 60 80 100 120 140
Zeni
th A
ttenu
atio
n (d
B)
Frequency (GHz)
Total Water Vapor Dry Air
1013 hPa pressure15°C temperature7.5 g/m³ water vapor density
9435
5.82.45
SPS Systems Designs Considered in URSI Report
68
Simplified Levelized Cost of Energy (LCOE)for Space Solar Power
69
Comparison of Levelized Cost of Energyfor Various Means of Power Generation
70
Comparison of JP-8 Cost per Gallon with $/kWh Equivalents and SPS Cases
71Range of reported “Fully Burdened Cost of Fuel” values is $3 to $400 per gallon
