A summary design analysis of a GaAs power conversion system for the solar power satellite (SPS) is presented. Eight different satellite configuration options for the solar arrays are compared. Solar cell annealing effects after proton irradiation are considered. Mass estimates for the SPS and the effect of solar cell parameters on SPS array design are discussed

Nussberger, A. A.

English

NASA Technical Reports Server

GALLIUM ARSENIDE (GaAs) POWER CONVERSION CONCEPT
A.A. Nussberger, Rockwell International
...
_. '_ 8%' 2 _1_ "]" _ 12214 Lakewood Boulevard, Downey, CA 90241
The Rockwell Satellite Power System concept utilizes gallium arsenide (GaAs) solar cells and flat
plate concentrators (CR=2) to generate 9.52 GW of power at the array sufficient for the satellite
microwave antenna system to deliver 5 GW at the utility interface. The,solar array bay configuration
and design factors are shown in Figure 1. This concept shows a 3-bay by 10-bay matrix 3,900 m wide
by 16,000 m long exclusive of the antenna. Each bay contains two panels 600 m by 750 m, providing
a voltage string of 45.7 kV. The 600-m width consists of 24 rolls each 25-m wide.
The solar array is sized for worst conditions using summer solstice values (1311.5 W/m2), end of
life reflector values (CR effective = 1.83), solar cell degradation allowance (4% non-annealable loss),
operating temperature of 113°C at summer soltice (solar cell temperature coefficient of Ar//ZIT =
0.0282%/oc), north-south seasonal inclination (altitude tipping of the SPS configuration accounts for
9.05 ° of the nominal misorientation of 23.5 °, resulting in a seasonal_factor of 0.968), packaging and
array voltage mismatch factor of 0.89, and switch gear factor of 0.997. Array power output is
calculated to be 352.6 W/m 2. The solar cell array area of 27:<-106 m 2 provides a 1.7% margin.
Key functional requirements include: delivery of 5 GW at constant power (except during solar
eclipse) to the utility network; operation in geosynchronous orbit for 30 years (size for end of life);
and cost-competitiveness with ground-based power generation. The last requirement (cost competitive
with ground-based power generation) has driven the Rockwell design toward use of higher technology
hardware.
The solar cell used in the satellite system design is a GaAs cell having a nominal efficiency of
20% (AMO, 28°C). Based on today's technology, 20% cell efficiency is expected by the year 1990.
The best laboratory GaAs cells are presently around 18% (Hughes, Rockwell International). The basic
SPS cell concept is an inverted GaAs/sapphire design having a specific mass of 0.252 kg/m 2
(Figure 2). This cell design has a 20 Mm sapphire (AI20 3) substrate upon which is grown a 5 /_m
single crystal GaAs junction. The Electronic Research Center (ERC) of Rockwell has supported this
effort with investigations of the development and mass producibility requirements of the baseline
GaAs/AI20 3 cells using a metallic oxide-chemical vapor deposition (MO-CVD) process. Figure 3
shows a production model of inverted structure GaAs/AI203 continuous ribbon solar cell. Trade
studies by Rockwell on the system level have shown GaAsto be the-I_referred cell material compared
to silicon. This is based on its higher efficiency (20% versus 17.3%); potential for cell efficiency
improvements (the multi-bandgap concept is essentially a gallium arsenide cell with potential of
25-30% or greater); lower space radiation degradation damage (125oc threshold temperature for
annealing versus >500oc); lower specific mass (0.252 kg/m 2 versus 0.427 kg/m2); better
compatibility with concentrators (improved temperature coefficient, _r//_T = O.028%/oc versus
0.043%/oc}; and lower overall SPS cost.
A comparison of GaAs solar cell annealing effects after proton irradiation is presented in
• Figure 4. Over 400 small-area (0.4-cm-square) solar cells were tested by Rockwell. 1 Both typical and
best cell annealing results are shown in Table 1. The SPS design assumes that nearly all radiation
' damage can be self-annealed out or annealed out with sufficient time and proper temperature.
A cost comparison-was made of single_:rystal GaAs, single-crystal silicon (Si), and amorphous
silicon (A-Si). The baseline GaAs configuration was utilized at a mass of 0.252 kg/m z, the
single-crystal silicon cell stack configuration was taken from the DOE/NASA reference system report 2
0.427 kg/m 2, and an amorphous silicon configuration was modeled frnm an RCA^paDer. 3 It was
assumed that the A-Si cell stack weight was equivalent to i-mil g/ass (0:i43 kg/mZ) - and t-hat,he
blanket configuration was the same as in the baseline GaAs. Figure 5 summarizes results and shows
that A-Si must achieve near theoretical efficiency (_-15%) and low cell cost (--_$20/m 2) to provide a
cost-competitive SPS system. Single-crystal silicon (even at the high efficiency of 17.3%) appears to
result in p signifi_cantly higher SPS_cost (A cost _2.13 B_)compared to the GaAs CR=2 baseline.
1Gallium Arsenide Solar Concentrator Hardness Study, Rockwell International, Technical Report AFAPL-TR-78-30 (May 1978).
2Concept Development and Evaluation Program, U.S. Department of Energy and NASA Reference System Report (October 1978).
3Twelfth IEEE Photovoltaic Specialist Conference, p 893.
246
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SOLAR NIRAY DESIGN FACTOIS
Sgi.M llflJT 1311.S WATTS/Iq 2
[NENY glfl'O CELL (CR - 1.83) 2400.1
OPERATIONS TBIfERA_ 113 C
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N_GI# (0.9¢1) 3_.6
$OtkR ARRAY POWEr OUTINJT - 3_i2.6 W/M 2 X 22.0 (1063 M 2
- 9.52 GW
Figure 1. Solar Array Configuration
_%?'G_ _ t"-- 4:C_ELLS/t4eTER
ITEM WEIGHT (MG/CM2) I
_._"/20 ,M AI20 2 ,.-- "_ 5.0
_ ,.T.CO.NEc_ 3, o
_'_ _ & TOP GRIO
%% _ .._, ^. / / CONTACTS
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-7....,....__/ % \\ O._CCONTACr
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SPACER \ %
25 I_M KAPTON 3.8
(BLANKET)
8 _. M POLYMER 0J
THERMAL 2S,2S (.252 KG/M2)
COATING
Figure 2. GaAs on Sapphire Solar Cell Blanket Cross-Section
IkCt|lOi ¢ONTF_I LAY|R ..,_.[%\\ "_m.,_-_ _...m
ILLWMIIATIOII
IMLOR JMIiCT iONII[ &L ._.
' _'_' FRONT GI_ LAYER
._._ __ _ .,_re._ '_ _*_'|xPO$|O i) _A4 FRONT LAY|R FOR CONTACT ACC[_
I_ - 1RcmWI°[. I - IMm LONGI
Figure 3. Production Mode/lnverted StTucture GaA$/AI203
Continuous Ribbon Solar Ceil
• I I
.7 _c . .- "" °_•'I-L J .-- ,
,,. " |
Ji r-_,/-/lllOC I_lC_,_l*" J
1 I I
.1
lEO _0 _00
ANNEALING TIME -- HOURS
*ARNEALIRO PARAMLrTER PA * POWER AFTER AIIWEAL
eR " POm,AFTEEImmlATIO.
ANNEALING TIME
Pl " PR If
RECOVERY TIME
f RECOVERY FACTOR
Figure 4. GaAs Solar Cell Annealing Effects
COST FACTORS (I_/KG)
7.01- _ 2 J AL KAPTON - 143 (REFLECTORS)
_L . $37.0/M J STRUCTURE - 80.7
'_/ A- SiLiCON i CONDUCTOR - 1.5
i SWITCH GEAR - 65
I OTHEeS - 2o
6.C I J TRANSPORTATION " 37.5 (TO GEO)
.. ]_ ,-- $47.3/M 2
"1' _L ._ SILiCON (SINGLE CRYSTAL)
527.0/M 2 q _k '_ (.5257 M)
A - 5i "_ _ : _(4703 M). CR " I
2
4.0 ' S20.O/M _. _ $70,8I/M2
A - S; _ _lb 'q GoAIAS
(SING LE CIIYSTAL)
3.0--
(3130 M)
1'o 2'o
SOLAR CELL EFFICIENCY
Figure 5. Solar Array Cost Comparison
247
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Table 1. Proton Test Data
Test
Initial
Voc (V)
Isc(MA)
After Irradiation
Voc (%)
Isc (%)
Recovery
Voc (%)
Isc (%)
Confil_ration*
1 2 3
Best Typ Best Typ But Typ
0.987 0.87 0.90 0.85 0.89 0.65
0.62 0.61 0.66 0.63 0.34 0.44
98.6 93.7 198.5 96.0 68.1 64.0
90.1 76.4 91.3 85.3 54.1 45.8
99.3
95.1
*Test Configurations:
1
Radiation 10 MEV
(protons/cm2) (6.6x1010)
Annealing 130
Temp (°C)
Annealing 65
Time (hr)
97.2 99.2 97.8 83.6
88.3 94.2 93.1 80.0
2 I
3.iv I
13.3x1010)i
1130 i
65
1 MEV
(lx1012)
180
17
73.1
75.2
Figure 6. Configuration Options
Eight different satellite configuration options (Figure6) were studied to obtain a better
understanding of the impact of solar cell selection (GaAs versus Si), antenna mounting location (end
versus center), number of troughs (range 3 to 10), concentration ratio (CR=2 versus non-
concentrated), and radiation degradation assumption (annealable versus non-annealable). For these
studies, solar cell and power distribution efficiencies were held constant, as was antenna mass. The
data are summarized in Table 2.
Very little SPS mass difference was calculated between configurations with different numt)ers of
troughs; however, construction considerations strongly favor a narrow configuration. A relatively
small mass savings is indicated for a center-mounted antenna (0.4 kg/kWut); similarly, a relatively
small difference in mass was shown between GaAs annealable and non-annealable CR=I configura-
tions (0.36 kg/kWut) and between GaAs CR=I and CR=2 (0.89 kg/kWut).
Figure 7 shows a plot of SPS mass estimates made over the last few years. A mass curve was
prepared which normalized to an early estimate made by Dr. Peter Glaser in 1974 (--2.3 kg/kW utility
power). As shown, SPS mass estimates have grown by a factor of approximately 2.3 for GaAs
: configurations and 3.5 for Si configurations (NASA reference concepts). The GaAs concept falls near
the nominal range of uncertainty established initially by NASA/JSC in-house studies conducted in
1975. Various alternative concepts are compared, including solid-state (SS) configurations which
replace klystrons with solid-state power amplifiers for the dc to RF microwave system and
multi-bandgap (MBG) solar cells replacing the reference GaAs single function cells. The impact felt by
oell efficiency improvements is demonstrated by the MBG concepts, which use a 30% nominal cell
efficiency.
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GaAs solar array major technology needs for the SPS program are identified in Table 3. Assumed
values are given for critical parameters used in the satellite concept definition. Impacts on the SPS
design from a failure to achieve the design values also are given in the table. These technology
requirements are to be addressed as part of the Ground-Based Exploratory Development (GBED)
activities. 4
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I-I S_ END MT
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UNCE RTAINTY Lr MSG S$ ENO MT
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O/ Oo IREFERENCEI
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. _ _.'JMIGSSSANDWICH
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t.4 I-1 . _o I NO WEIGHT O_X
I? ,_- I PENALTIES INCLUDED
t+_ .t+,_ _m_ocsu 1 INSmS!_7 I ,,=-.ul.+,.*.o_
1.0 _'_,_ _ GLASE R _ 2.3 KO/KW [ I_S" ¢OLID STATE
_._ - FEB 74 I • "
0 _- I I I I '
_3 2 4 E E 10 12 14 16 18 20 22 24 ,
1970 1_) 1900 I_l_) lIB0 I_PSI
YEARS
Figure 7. SPS Weight Uncertainty
Table 3. SPS Solar Cell Parameters as Design Drivers
SPS Gaps ImpKt on Design
Pammlter Design Valuss Description (failure to lob/eve .lies)
_11 efficiency
Weight
Operating
temporeturw
Colt
Cell thickness
20% (AMO,
28oc)
4%
' 0.252 ki/m 2
;_125Oc
$70.8/m 2
=olaf calls
5 pm active
GaPs region
20 pm sapphire
substrata
SPS corpept (CR=2) requir_
27.0 x 106m2 of solar cells;
gtray output: 352.6 W/m 2
Non-anneallble ellowince is
4% errly ire; currant design
assumes se/f-annealing at
;_125°C
Total SPS array weight =
7.536 x 10o kg; ~25% of
total ,-tellite waist
Gaps performance it
operating temperltura
.17.6_
Total array cost $3130.2M
(pesic call/reflector cost
$23201_) par satellite*
Gallium requirement for SPS
_375 metric tons (_ GW)
Lower efficiency penalizes
array area, weight, array
cost, transportation cost,
and construction schedule; Si
parformince could bl = low
as 123.6 W/m 2
Failure to achieve annealing
will penalize Irray area 16%
in GEe and 40% EOTV; Si
degradation penalties Ire
greater
Substitution of Si penalize
system by 22,2 x 106 kg or
more
Lower performance could
penalize system by forcing
nonconcent_eted SPS
_4.66 x IO ° kg; 18.8 x 106
m 2 solar ceils
Si call COil plnllty adds
$2126.7i to array cost °
Thicker materials affect
weight, cost, and availability
*Reference: Satellite Power System (SPS) Concept Definition Stud, (Exhibit C)
Fits1 Quarterly Review. Rockwell International, SO 76-AP-0075 (June 21-22, 1978)
4SatellitePower SystemGround-BasedExploratory Development(GBED) SystemAnalysisandTechnology Plan.National Aeronautics
and SpaceAdministration.RoughDraft (December1, 1979).
249


Gallium arsenide (GaAs) power conversion concept

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