The prototype fabrication of a lightweight, high-quality cellular glass substrate reflective panel for use in an advanced point-focusing solar concentrator was completed. The reflective panel is a gore shaped segment of an 11-m paraboloidal dish. The overall concentrator design and the design of the reflective panels are described. prototype-specific panel design modifications are discussed and the fabrication approach and procedure outlined

Bedard, R. J., Jr.

Bell, D. M.

English

NASA Technical Reports Server

ADVANCED CONCENTRATOR PANELS 
, . 
i /  0: M. B e l l  
R.' J. Bedard, Jr. 
Acurex Corpo ra t i on  
Mountain View, C a l i f o r n i a  94042 
ABSTRACT 
Acurex Corpora t ion ,  under c o n t r a c t  t o  che J e t  Propu 1  s  i o n  Labora to ry  (JPL ) , 
has completed t h e  p r o t o t y p e  f a b r i c a t i o n  o f  a  l i g h t w e i g h t ,  h i g h - q u a l i t y  
c e l l u l a r  g lass  s u b s t r a t e  r e f l e c t i v e  panel  f o r  use i n  an advanced 
p o i n t - f o c u s i n g  s o l a r  concen t ra to r .  The r e f l e c t i v e  pane l  i s  a  gore shaped 
segment of a  ll-m p a r a b o l o i d a l  d ish .  
Th is  paper b r i e f l y  descr iSes t h e  o v e r a l l  c o n c e n t r a t o r  des ign  and t h e  
des ign  of t h e  r e f l e c t i v e  panels.  P r o t z ~ t y p e - s p e c i f  i c  pane l  des ign  
m o d i f i c a t i o n s  a re  d iscussed and t h e  f a b r i c a t i o n  approach and procedure 
o u t l i n e d .  The o p t i c a l  q u a l i t y  of t h e  p r o t o  
e x c e l l e n t ,  a l though no q u a n t i t a t i v e  r e s u l t s  
BACKGROUND 
JPL f i r s t  d e v e l o ~ e d  t h e  c o n c e ~ t  o f  u s i n a  ce 
ype pane ls  appears t o  be 
Ere y e t  a v a i l a b l e .  
l l u l a r  g las;  i n  c o n j u n c t i o n  
t h  t h i n  b a c k s i l v e r e d  m i r r o r '  g l ass  t o  fo rm 1  i gh twe igh t ,  s t r u c t u r a l l y  
f i c i e n t  r e f l e c t i v e  pane ls  f o r  h i g h - f l u x  s@:ar c o n c e n t r a t o r s .  C e l l u l a r  
g l a s s  i s  a low-cost,  n o n c r i t i c a l  m a t e r i a l  w i t h  a  v e r y  h i g h  s t i f f n e s s - t o -  
we ight  r a t i c .  I t  i s  e a s i l y  machinable and can be f o r m u l a t e d  t o  p r o v i d e  an 
exce l  l e n t  c o e f f i c i e n t  o f  thermal  expansion match t o  most g l a s s  types .  
Gore shaped r e f l e c t i v e  pane ls  ( F i a l ~ r e  1) f a b r i c a t e d  f r o m  a  composi te o f  
c e l l u l a r  g lass  and sheet  g ' . r q  form t h e  b a s i s  o f  t h e  JPL Advanced 
C ~ n c e n t r a t o r  concept  f i r s t  proposed i n  1977. The l a r g e l y  s e l f  s u p p o r t i n g  
gores are  used t o  d i s p l a c e  much o f  t h e  s t r u c t u r a l  framework n o r m a l l y  
r e q u i r e d  t o  m a i n t a i n  an adequate d i s h  s t i f f n e s s .  
9 
." 
fi 
4' 
Gore support  r i n g  t r u s r  
FIGURE 1. CELLULAR GLASS GORE 
https://ntrs.nasa.gov/search.jsp?R=19810021999 2020-03-21T09:46:19+00:00Z

REFLECTIVE PANEL DESIGN 
The key element o f  the  Advanced Concentrator i s  c l e a r l y  t he  c e l l u l a r  g lass 
subst ra te  r e f l e c t i v e  gore. As shown i n  F igure 3, each gore i s  fabr icated 
from a composite of 1.0- Corning Glass Works 7809 b o r o s i l i c a t e  glass and 
a P i t t sburgh  Corning FoamsilB 75 c e l  i u l a r  g lass core. The ~ o a m s i l ~  75 has 
been s p e c i a l l y  formulated t o  match t he  thermal expansion cha rac te r i s t 4cs  
of t he  7809 sheet glass. A s i n g l e  sheet o f  backs i lvered t h i n  glass i s  
cont inuously  bonded t o  a  contoured subs t ra te  of t he  c e l l u l a r  g ldss 
mater ia l .  A narrow s t r i p  o f  uns i l ve red  t h i n  glass i s  bonded t c  the outer  
face of the  c e l l u l a r  glass spar running l o n g i t u d i n a l l y  along the  backside 
of t he  gore. The face sheets and the  c e l l u l a r  g lass core form a composite 
s t r uc tu re  i n  which the  m i r r o r  g lass and t he  spar cap c a r r y  a  s i g ~ i f i c a n t  
p o r t  i on  o f  the aerodynamic and g r a v i t a t i o n a l  l y  induce0 bending loads. 
Three compress i o n  molded g lass re i n fo r ced  po lyes te r  (GRP) pads are bonded 
t o  t he  gore t o  serve as a t t i 1  hnent po in t s  f9r  the i n t e r f a c e  w i t h  the 
support s t ruc tu re .  
t 83 .R  cm marimun w i d t h  -t; 
f o a ~ s  i;m 75 core 1.0 m mi-rot 
ylass 
1 . 0  mn, u n s ~ l v e r e d  Lorl torn~al  c o a t  i r s  
n la s s  spar c a p  ovev  a! ! 
n c n r e f l e i ? i :  . 
s u r f ~ c e :  
FIGURE 3. OUTER GORE CROSS SECTION 
Two panel types f o r a  the parabolo ida l  surface. Fo r t y  ou te r  gores and 
twenty-four inner  gore5 are required. The masses o f  t he  ou te r  and inner  
gores ( l ess  attachments; are 23.2 kg and 15.8 kg, respec t i ve ly .  The w id th  
of each gore type i s  l i m i t e d  by t he  maximum steady-state curvature s t ress  
which the sheet gldss can withstand. A maximum panel w id th  of 84 cm f o r  
the ou te r  and inner  gore 1 i m i t s  t he  steady-state s t resses t o  14.9 MPa. 
A d e t a i l e d  design was developed fo t  the  au te r  gore type only.  The 
r e s u l t i n g  gore i s  s t ress  l i m i t e d  w i t h  a  5 percent p r o b a b i l i t y  o f  f a i l u r n  
i n  t he  c e l l u l a r  glass core under a  governing load cond i t i on  of a 1 minute 
cumulative exposure t o  a  110 kmlhr wind a t  the  worst-case o r i en ta t i on .  
The peak t e n s i l e  core s t ress  i s  275 k3a under t h i s  cond i t i on  w i t h  a  
corresponding m i r r o r  g lass s t ress  o f  20.1 MPa. Under worst-case operat ing 
condit ions, the outer  gore panel y i e l d s  a peak d e f l e c t i o n  s lopc e r r o r  o f  
less  than 0.3 mrad and an area weighted rms de f l ec t i on  slope e r r o r  of less  
than 0.2 mrad. 
Due to current manufacturing 1 imitations, the maximum block size for the 
Foamsi l'@ 75 material is 46 cm by 61 cm by 10 cm. Near-term production 
therefore requires the bonding of several blocks of cellular glass into a 
large core blank prior to machioing. Future developments in cellular 
glass production may lead to full size mono1 ithic core blanks or even 
foamed to shape cores. 
PROTOTYPE PANEL FABRICATION 
To verify the fabricability and integrity of the gore design, Acurex has 
fabricated several full-scale prototype gores. These gores wi 11  be tested 
by JPL to determine the structural and optical characteristics of the 
design. 
Prototype Design Modifications 
Several prototype-specif ic design modifications were incorp~rated to 
reduce cost. Due to limited availability of the 1.0 mm Corning 7809 sheet 
glzss and the Pittsburgh Corning Foamsil@ 75, the prototype gores were 
fabricated from 1.5  TI Corning 0317 glass and Pittsburgh Corning's 
standard Foamglas*material. While these materials are not ideally 
thermally matched, and the thicker sheet glass provides a shorter panel 
life, much insight into the gore design has still been gained. Steel 
weldments were substituted for the compression molded GRP attachment pads 
at a penalty of approximately 2.3 kg ger gore. 
In addition to these prototype material changes, two significant 
dimensio~~al changes were also incorporated. To simplify the core 
machining operation, the rearside contour was modified from a constant 
edge thickness configuration to a cons'ant contour angle design and the 
spar depth was increased to avoid a local bond joint problem. This change 
ddded approximately 10 percent to the core mass, but allowed the use of a 
sivplifiet contouring scheme. The frontside contour was also modified to 
simplify t, 2 prototype machining operation. In .lieu of the more perfect 
paraboloidal contour, a compromise of a parabolic contour in the radial 
directior and a constant radius of curvature in the circumferential 
direction was selected. The effective area-weighted slope error impact of 
this modification is approximately 0.3 mrad rms. 
Fabrication Approach 
To minimize prototype fabrication cost, Acurex developed a simple 
contouring scheme which allows accurate, repeatable substrate fabrication 
with a minimal investment in tooling. The prototype gore fabrication 
procedure is essentially a ten step operation: 
Cut cellular glass blocks 
@ Bond blocks to form core blank 
Cut core blank to planform 
Machine core backside 
Bond sheet glass spar cap 
@ Machine ccre frontside 
@ Bond mirror glass 
Bond attachment pads 



Advanced concentrator panels

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