Advanced solar cell power systems to reduce weight and withstand thermal stress conditions of Interplanetary Monitoring Platform /IMP-D

Cherry, W. R.

Slifer, L. W., Jr.

English

NASA Technical Reports Server

. '  ' 
. .  
* .  
ADVANCED SOLAR CELL POWER SYSTEMS VO'A SPACE 
W i l l i a m  R. Cherry and Lu the r  W. Slifer 
As'space m i s s i o n s  have become mare sophls t i -  
cated, t h e  demands fo r  electrical  power have in-  
creased. 
weight allowed fo r  t h e  power s y s t e m  is t i g h t l y  re- 
stricted o r  even reduced. 
manned spacecraft has a l l o t t e d  between 20 and 25% 
of its weight for the  power system.' They have -- 
A l l  too o f t e n  t h e  percentage of payload 
Today$the  average  un- 
. - -  ._ - -  
ranged from as l i t t l e  as 9% t o  as much as 44%. 
Missions such  as the  Pioneer  sun  probe and 
o r i e n t e d  sa te l l i t es  l i k e  t h e  new Radio Astronomy 
Explorer (RAE) and t h e  App l i ca t ions  Technology 
Satell i te (ATS) w i l l  develop solar array tempera- 
t u r e s  i n  e x c e s s  of 70 t o  90°C. To meet t h e  power 
requi rements  of these and similar mis s ions  and s t a y  
w i t h i n  t h e  permissible yeight i i m i t s ,  improvements 
i n  t h e  solar ce l l  array des ign  are necessary.  
of t he  methods used t o  reduce weight and t o  with- 
s t a n d  t h e  s e v e r e  thermal stress which were adopted 
between the d e s i g n  of t h e  I n t e r p l a n e t a r y  Monitor ing 
a 
Some 
.. . 
/- 
0 
m 
0 
1 
I 
I (THRUI -1833 
P e /7 / 
I mkz/ 72 
IACCESSION NUMBER) 
(CODE1 (PAGES) 
(NASA CR OR T M F O R  AD NUMBER) 
https://ntrs.nasa.gov/search.jsp?R=19660009049 2020-03-24T04:07:21+00:00Z
Pla t form (IMP) A and t h e  new l u n a r  anchored IMP-D 
are d i s c u s s e d  i n  t h e  fo l lowing  paragraphs.  
The IMP series of s a t e l l i t e s ,  shown i n  F igu re  1, 
is in tended  t o  assess the  P r o j e c t  Apollo r a d i a t i o n  
hazard ,  s t u d y  the  solar  f l a r e  p r e d i c t i o n  c a p a b i l i t y ,  
measure solar/ terrestrial  r e l a t i o n s h i p s ,  and record 
t h e  r e l a t i o n  of magnetic f i e l d  t o  t h e  s u n ' s  p a r t i c l e  
f l u x e s  i n  c i s l u n a r  space.  
o f  about 106,000 n a u t i c a l  m i l e s  and a p e r i g e e  of  
n e a r l y  200. IMP-D is in tended  t o  o r b i t  t h e  moon 
with an apogee of 1600 t o  5400 n a u t i c a l  miles and 
a p e r i g e e  from 270 t o  800. 
I M P - A  a t t a i n e d  a n  apogee 
- _ _  
Table 1 shows a comparison between the  I M P - A  
and 1MP.D so lar  cel ls  and cover  g l a s s e s .  The g r e a t l y  
improved r a d i a t i o n - r e s i s t a n t  2 x 2 c m  N/P conf igura-  
t i o n  is used on IMF-D as w e l l  as a much t h i n n e r  
solderless cell.  Th i s  decreased t h e  ce l l  weight 
fo r  an e q u i v a l e n t  4 c m 2  c e l l  area from 0.46 t o  0.30 
gram. The s t r o n g e r  t i t a n i u m - s i l v e r  c o n t a c t s  are 
a lso used t o  better wi ths t and  t h e  thermal  stresses 
associated w i t h  t h e  l u n a r  o r b i t  where t empera tu res  
, are expected t o  range from +4OoC t o  w e l l  below -lOO°C, 
. two or  three t i m e s  i n  24 hours. This  compares with *. ' 
+25OC and -9OOC for  IMP-A which e c l i p s e d  i n  t h e  
ear th ' s  shadow once eve ry  4-1/2 days.  
The r a d i a t i o n  environment is expec ted  t o  be 
q u i t e  a b i t  less s e v e r e  f o r  IMP-D t h a n  I M P - A ,  which 
has  pe rmi t t ed  a r e l a x a t i o n  o f  t h e  cover  g l a s s  
t h i c k n e s s  from 12  t o  6 m i l s .  I M P - A  showed a 25% 
degrada t ion  i n  its power gene ra t ing  c a p a b i l i t y  i n  
e x a c t l y  one yea r  of  space  l i f e .  I f  N / P  cel ls  were 
used,  t h i s  probably would be observed a f t e r  about  
10 years .  To i n s u r e  maximum t r a n s m i s s i v i t y  of t h e .  
cover glass, I M P 4  w i l l 1  use 7940 type fused silica 
i n s t e a d  of microsheet 0211. The same a n t i r e f l e c t i o n  
and b lue  f i l t e r s  w i l l  be employed on IMP-D as on 
IMP-A.  There is i n s u f f i c i e n t  ev idence  of  any bene- 
fits of us ing  a red f i l t e r  t o  j u s t i f y  t h e  added 
e 
cost f o r  t h i s  c o a t i n g  a t  t h i s  t i m e .  
P r i o r  t o  t h e  s e l e c t i o n  of  2 x 2 c m  ce l l s  f o r  
t he  I M S D  s a t e l l i t e ,  ex tens ive  ground environmental  
t es t s  were performed t o  e s t a b l i s h  a c c e p t a b i l i t y .  
The advantages sought  i n  us ing  t h e  larger ce l l s  
were t h e  i n h e r e n t l y  fewer cells ,  i n t e r c o n n e c t i o n s ,  
. .. 
cove r  s l i p s ,  and lower cost per u n i t  area of a c t i v e  
surface. F igu re  2 shows a mechanical t e s t  paddle 
w i t h  2 x 2 c m  cel ls  and 3 x 3 c m  s i l i c o n  blanks.  
T h i s  paddle is designed t o  meet a l l  t h e  mechanical 
requi rements  of shock, v i b r a t i o n ,  a c c e l e r a t i o n ,  
tempera ture ,  vacuum, etc., fo r  the expec ted  launch  
c o n d i t i o n s  of t h e  Thrus t  Augmented Delta booster 
and the expected o r b i t a l  l i fe  of one year. Should 
3 x 3 c m  cells  e v e n t u a l l y  become c o m p e t i t i v e l y  
a v a i l a b l e ,  there is hope these can a lso be used fo r  
such  miss ions  w i t h  f u r t h e r  r e d u c t i o n  i n  t h e  number- 
of s o l a r  cells ,  cove r  glasses, i n t e r c o n n e c t i o n s ,  
- . _ _  - - . _. 
- __ 
and cost. 
Reduction of c e l l  t h i c k n e s s  is an  obvious way 
t o  s a v e  weight,  b u t  there are c e r t a i n  l i m i t s  t o  
which one may go. F i r s t ,  t h e  t h i n n e r  t h e  s i n g l e  
c rys ta l  wafer becomes, t h e  higher  the  breakage rate. 
Also, as the,cell  t h i c k n e s s  is reduced below 1 2  m i l s ,  
t h e  electrical p r o p e r t i e s  of t h e  ce l l  begin  t o  
s u f f e r  apprec i ab ly .  Figure 3 shows the  r e s u l t s  of 
a s t u d y  made by Wolf & Ralph' on the effects of 
._.. 
so la r  ce l l  t h i c k n e s s  on s h o r t  c i r c u i t  c u r r e n t .  
Assuming a c e l l  w i t h  a minor i ty  carrier l ifetime of 
6 microseconds,  i t  is s e e n  t h a t  a r e d u c t i o n  i n  Is, 
by 2 t o  3% can  be expected a t  a i r  mass zero fo r  a 
12 m i l  t h i c k  c e l l  and as much as 8 or 10% when re- 
duced t o  8 m i l  t h i c k n e s s .  As can  be s e e n  from t h e  
f i g u r e ,  t h i s  is cons ide rab ly  more s e v e r e  t h a n  one 
would expec t  theoret ical ly .  
S i g n i f i c a n t  improvements i n  t h e  paddle sub- 
s t ra te  c o n s t r u c t i o n  have also c o n t r i b u t e d  t o  a more 
f a v o r a b l e  weight  d e n s i t y  i n  the  IMP-D s p a c e c r a f t .  
F igu re  4 shows a cutaway view of t h e  paddle. The 
aluminum honeycomb is covered w i t h  t w o  0.0015 i n c h  
th i ck  epoxy-impregnated F i b e r g l a s  sheets which act 
as  an  i n s u l a t e d  s u b s t r a t e  for  the  paddle. Next, an  
expanded s i l v e r  metal mesh conductor  is l a i d  down 
d i r e c t l y  beneath where t h e  solar  ce l l  modules are 
ts be placed..  This i r , m v a t i o n  acts as t h e  r e t u r n  
path t o  t h e  prime c o n v e r t e r  f o r  t he  so la r  c e l l  cur- 
r e n t .  Also, it is d i s t r i b u t e d  ove r  a r eg ion  equa l  
t o  t h e  solar ce l l  a c t i v e  area, t h u s  n e u t r a l i z i n g  any 
i 
5. 
./ 
s t r a y  magnetic f i e l d s  which might o the rwise  be in- 
duced when u s i n g  convent iona l  wi r ing .  
Another two-ply l a y e r  of 1.5 m i l  epoxy- 
impregnated F i b e r g l a s  is placed over  t h e  expanded 
metal mesh; t h e n  t h e  cel ls  are bonded t o  t h i s  l a y e r  
w i t h  RTV-40 adhesive.  Connections from t h e  ends of 
t h e  s o l a r  ce l l  modules t o  t h e  s i l v e r  mesh are made 
by p e n e t r a t i n g  through t h e  t w o  upper layers o f  
F i b e r g l a s  and s o l d e r i n g .  
, 
A comparison between t h e  p r o p e r t i e s  of  t h e  
IMP-A and IMP-D paddles  is shown i n  Table  2. Even 
though t h e  paddle  area on  IMP-D is 1.2 f t .  p e r  
s i d e  l a r g e r  t h a n  on I M P - A ,  t h e  paddle weight is 
0.4 l b .  less. IMP-D w i l l  gene ra t e  45 watts p e r  
paddle  f a c e  compared w i t h  33.6 f o r  IMP-A a t  3OoC 
2 
and normal inc idence .  Thus, t h e  IMP-D weighs .1.3 
2 l b . / f t .  on one s i d e  o r  0.15 l b . / f t O 2  coun t ing  bo th  
s i d e s  of t h e  ,paddle. While t h e  e f f i c i e n c y  o f  t h e  
cel ls  has  changed very  l i t t l e  over  t h e  l a s t  t w o  
y e a r s ,  g i v i n g  about  9.3 t o  9.4 w a t t s / f t O 2  of 
i l l u m i n a t e d  s u r f a c e ,  t h e  IMP-D w i l l  produce 7.3 
w a t t s  p e r  pound of paddle,  wh i l e  t h e  1 M F . A  developed 
6 .  
.- . . -. .. . . 
on ly  5.1. T h i s  is a 43% improvement i n  performance 
p e r  u n i t  weight.  
Developments are c u r r e n t l y  under way i n  a t t e m p t s  
t o  f u r t h e r  reduce t h e  w e i g h t  d e n s i t y  of solar  paddles. 
One of t h e  most r e c e n t  is t h e  w r a p a r o u n d  2 x 2 c m  
so la r  ce l l  which d e l i v e r s  5% more power t h a n  the  
conven t iona l  dev ice  because of the a d d i t i o n a l  act ive 
area formerly covered by t h e  bus bar. 
I n  many miss ions ,  very t h i n  cover  glasses of 
1 t o  3 m i l s  would be s u i t a b l e .  Handling conven t iona l  
cover  glasses of t h i s  t h i c k n e s s  is p r e s e n t l y  pro- 
h i b i t i v e  from a breakage s t andpo in t .  I n t e g r a l  cove r  
glasses, now i n  ear ly  stages of development, appear 
to be f e a s i b l e  and should  indeed r e p r e s e n t  a weight 
s a v i n g s  i n  f u t u r e  space  a p p l i c a t i o n s .  
Welded i n t e r c o n n e c t i o n s  between ce l l s  and 
modules o f f e r  promise of t h e  complete e l i m i n a t i o n  
c?f the soldez now used and should  i n c r e a s e  t he  
o p e r a t i n g  o r  peak temperature  t o  which so la r  cel ls  
may be sub jec t ed .  Th i s ,  a l s o ,  is expec ted  t o  per- 
m i t  a more rel iable  and rugged connec t ion .  
7. 
Dendr i te  webs are now be ing  produced i n  w i d t h s  
of 1 e m  by many centimeters long. With p r e s e n t  
semiconductor technology,  s t r i p s  of ser ies -connec ted  
solar cel ls ,30 c m  o r  more i n  l eng th ,deve lop ing  
s e v e r a l  v o l t s  p e r  un i t , shou ld  become a v a i l a b l e  and 
a p p l i e d  as needed. Many i n t e r c o n n e c t i o n s  would be 
f u r t h e r  e l imina ted .  
The research and development performed o v e r  
t h e  l a s t  year and a h a l f  h a s  permitted the  r e d u c t i o n  
of paddle  weights  from 1.8 l b . / f t O 2  t o  1.3 l b . / f t .  2 
t h i s  reduces  from 0.9 l b . / f t O 2  t o  0.65 lb . / f t .  2 
Counting t o t a l  a c t i v e  a r e a  (both sides of t h e  paddle), 
Thus, o r i e n t e d  p a n e l s  under 3/4 l b . / f t O 2  can  now be ', 
b u i l t  t o  wi ths t and  t h e  r i g o r s  of launch  and space.  
F u r t h e r  developments p r e s e n t l y  under way shou ld  
make f u r t h e r  r e d u c t i o n s  p o s s i b l e  i n  t h e  n e a r  f u t u r e .  
i 
8.  
REFERENCES 
1,  Balent ,  R . ,  Heathco, C.W., N . A . A .  Atomics 
I n t e r n a t i o n a l ,  AI-64-207, "Standard U. S. 
l a u n c h  Vehic l e s  and Space Nuclear Power 
Systems," Sept.  21, 1964 
2. Wolf, M . ,  Ralph, E . L . ,  "Effect of Thickness  
on Short C i r c u i t  Current," 4th  Annual 
Photovo l ta i c  Conference, C l E l m h i o ,  
June 2-3 8 lg7 
\ -  
, .  
IMP SOLAR CELL PHYSICAL CHARACTERISTICS 
C e l l  Type 
C e l l  S i z e  (cm) 
C e l l  Weight (gm) 
C e l l  Thickness (mil)  
Contacts 
Cover Glass Type 
Cover Glass 
'Thickness ( m i l )  
Coatings 
IMP-A&B 
P/N 
1 x 2  
0.23 
20 
s o l d e r )  
( inc luding 
Ni p l a t e d  
0211 
12 
Blue & AR 
Table 1 
\ 
IMP-D 
N/P 
2 x 2  
0.30 
12.5 
( s o l d e r l e s s )  
TiAg 
7940 
6 
Blue & AR 
. I .  
. ,  . .  
SOLAR PADDLE CHARACTERISTICS 
IMP-A&B IMP-D -
3.6 4.8 Paddle Area One Side (ft. 2 ) 
Paddle Weight (lbs.) 6.6 6.2 
Power Output One Side, AM0 (watts) 33.6* 45.0* 
Paddle Weight/Area (1b./fto2) 1.8 1.3 
Paddle Power/Area (watts/ft. 2 ) 9.3 9.4 
7.3 Paddle Power/Weight (watts/lb.) 5.1 
*at 3OoC 
Table 2 
i 
, 
. 
. .  . .  . . .  
c 
DEVELOPMENTS UNDER WAY 
TO FURTHER IMPROVE 
SOLAR CELL SYSTEMS 
A .  
Be 
C. 
D. 
Wrap-Around C e l l s  
Integral  C e l l  Covers 
Welded Interconnections 
Webbed Dendrites  
Table 3 
B C  . .  . .  
I .  
t 
" .  . C  . P  , L I  
I 1 
i 
SUBJECT CATEGORIES FOR GODDARD JQURNAL , 
(To be used for classifying a l l  NASA publications, journal articles, and other papers 
for inclusion in the Goddard Journal.) 
. a  
i Part A'-  Space Sciences . .  
i 
. .  A 1. Astronomy and Astrophysics 
A 2. Celestial Mechanics and Geodesy 
A 3. Solar Physics 
A 4. Ionosphere and Radio Physics 
A 5. Fields and Particles 
A 6. Planetology 
A 7. Planetary Atmospheres 
7 
A 8. General (subjects not clearly belonging in any 
of categories 1-7) 
Part B - Space Technology 
B 1. Projects and Programs 
B 2. Space Dynamics and Control Systems 
B 3. Spacecraft and Subsystems 
B 4. Vehicle Technology 
B 5. Sounding Rockets 
B 6. Sensors 
B 7. General Electronics 
B 8. Environmental Testing 
B 9. Tracking Systems 
B 10. General (subjects not clearly belonging in any 
of  categories 1-9) 


Advanced solar cell power systems for space

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server