The key to concentrator cost effectiveness is the proper design of the reflector surface panels. The low cost concentrator reflective surface design is based  on use of a thin, backsilvered mirror glass reflector bonded to a molded structural plastic substrate. This combination of reflective panel material offers excellent optical performance at low cost. The design approach, rationale for the selected configuration, and the development status are described. Reflective panel development and demonstration results are also presented

Bedard, R. J., Jr.

Overly, P.

English

NASA Technical Reports Server

LOW COST CONCENTRATOR 
G 
J *  
R. dard 
P: R e r l y  
Acurex Corporat ion 
Mountain View, C a l i f o r n i a  
ABSTRACT 
The Acurex Corporat ion i s  under con t rac t  t o  t he  Jet  Propuls ion Laboratory 
t o  design, fab r i ca te ,  i n s t a l l ,  and t e s t  a cos t - e f f ec t i ve  p o i n t  focus s o l a r  
concentrator.  The key t o  concent ra tor  cos t  e f fec t iveness i s  t he  proper  
design o f  t h e  r e f  l e c t o r  surface panels. The low cos t  concen t ra to r  
r e f l e c t i v e  surface design i s  based on t he  use of a t h i n ,  backs i l ve red  
m i r r o r  g lass r e f  l e c t o r  bonded t o  a molded s t r u c t u r a l  p l a s t  i c  subst ra te .  
This combination o f  r e f l e c t i v e  panel ma te r i a l  o f f e r s  exce l l en t  o p t i c a l  
performance a t  low cost.  This paper b r i e f l y  descr ibes t h e  design approach, 
r a t i o n a l e  f o r  t he  se lected con f igu ra t ion ,  and t he  development s ta tus.  
Re f lec t i ve  panel development and demonstration r e s u l t s  a re  a l s o  presented. 
INTRODUCTION 
The o v e r a l l  ob j ec t i ve  of the  low cos t  concent ra tor  p r o j e c t  i s  t o  develop 
and demonstrate a state-of- the-art  technology concent ra tor  which i s  cos t  
e f f ec t i ve  i n  h igh volume product ion and has a 30-year l i f e  under wide 
environmental extremes. The development p r o j e c t  i s  s t r uc tu red  i n t o  a 
three-phase e f f o r t .  Phase I, completed i n  March 1979, encompassed t he  
concept se lect ion,  p re l im ina ry  design and cos t  assessment, and demonstrat ion 
of t he  mass product ion r e f l e c t i v e  panel f a b r i c a t i o n  approach. The Phase I 1  
e f fo r t s ,  which began i n  September 1980 and a re  c u r r e n t l y  underway, encompass 
de ta i  led design and ana lys is  and demonstrat i on  o f  t he  p ro to t ype  r e f l e c t i v e  
panel f a b r i c a t i o n  approach. Phase I 1 1  inc ludes f a b r i c a t i m ,  i n s t a l  l a t i o n ,  
and t e s t i n g  of th ree  proto type concentrators and i s  scheduled f o r  cornplet i on  
i n  May o f  1982. 
DESIGN SUMMARY 
The design of the  11 meter diameter (95  m2 gross aper ture area) Low Cost 
Concentrator i s  shown i n  F igure 1. The concent ra tor  i s  a two-axis t r a c k i n g  
system designed t o  i n t e r f a c e  w i t h  a 1,500 l b  thermal rece iver lpower  
conversion u n i t  package. Pred ic ted performance o f  t h e  concen t ra to r  i s  
63 kWt a t  the  r ece i ve r  aper ture based on the  f o l l o w i n g  design cond i t i ons :  
a 800 ~ l m 2  i n s o l a t i o n  
0 1,7000F rece i ve r  operat ing temperature 
a 95 percent re f lec tance  
a 30 mph operat ing wind 
https://ntrs.nasa.gov/search.jsp?R=19810021998 2020-03-21T09:46:24+00:00Z
Figure 1. Design Desc r i p t i on  
Tile major design fea tu res  o f  each o f  the  subassemblies of the  mass 
product ion concent ra tor  are discussed i n  t he  f o l l o w i n g  paragraphs. 
Proto type-spec i f ic  mod i f i ca t i ons  f o r  t he  r e f l e c t i v e  panel  subassembly are 
a1 so presented. 
Re f l ec t i ve  Panel Subassembly 
The r e f l e c t i v e  panel subassembly cons is ts  o f  i nner  and ou te r  groups o f  
r e f l e c t o r  gores forming a  complete but  p h y s i c a l l y  d iscont inuous r e f l e c t i v e  
surface. As shown i n  F igure  2, a  concent ra tor  cons is ts  of 40 ou te r  and 
24 inner  ores. The r e f l e c t i v e  gores are a  composite cons t ruc t i on  o f  t h i n  
(0.028 ing, backsi  lvered m i r r o r  g lass w i t h  a  sheet molding compound (SMC)  
support ing substrate.  A t h i n  g lass r e f l e c t o r  was chosen because o f  h igh  
performance and long l i f e  cha rac te r i s t i c s .  I n  terms o f  performance, 
backs i lvered m i r r o r  g lass provides the  h ighest  p r a c t i c a l  s o l a r  hemispher ical  
re f lec tance (0.95) and has exce l l en t  specu la r i t y .  Glass i s  h i g h l y  abrasion 
r e s i s t a n t  and env i ronmenta l ly  durable.  The r e f l e c t i v e  panel subs t ra te  i s  a  
compression molded ma te r i a l  gene r i ca l l y  r e fe r red  t o  as SMC. SMC i s  a  
r eady - t oao ld  po lyes te r  r e s i n  ma te r i a l  w i t h  chipped f i b e r g l a s s  reinforcement 
processed i n  continuous sheet form. Par ts  of SMC are  t y p i c a l l y  molded a t  
3000F and 1,000 p s i  i n  3 t o  5  min cyc l e  times. SMC molding i s  a  h igh 
vnluine product ion process and o f fe rs  t he  p o t e n t i a l  f o r  low cos t  r e f l e c t i v e  
panel substratss.  The r e f l e c t i v e  panel subst ra te  design cons i s t s  o f  a  t h i n  
(0.15 i n )  face sheet w i t h  an i n t e r g r a l l y  molded r i b  s t r uc tu re .  The g lass 
m i r r o r  i s  bonded t o  the  SMC substrate.  
a. Re f l ec t i ve  Surface Conf i g u r a t  i on  
glass 
substrate 
\ 
Ribs 
b. Typ ica l  Gore Conf igurat ion 
F igure 2. R e f l e c t i v e  Panel Design 
S u ~ ~ o r t  S t ruc tu re  Subassembl ies  
The three support s t r u c t u r e  subassemblies are:  
e Panel support s t r uc tu re  
e Receiver support s t r u c t u r e  
In termediate  support s t r u c t u r e  
The 1 ightweight  space frame subdssembl i e s  f ea tu re  welded s t e e l  shop 
subassembly cons t ruc t  i on  us ing standard s ize,  comnercial l y  avai  l a b l e  s t e e l  
tubing. F i n i t e  element a n z l j s i s  techniques were Used t o  opt imize t h e  
support s t r uc tu re  f o r  minimum weight. 
Foundation and Dr ive  Subassemblies 
The foundat ion design fea td res  simple i n s t a l l a t i o n  and a d a p t a b i l i t y  t o  
s lop ing  o r  rough t e r ra i ns .  The foundat ion cons i s t s  c? a s i n g l e  
cast- in-place, re in force0 concrete  p i e r  w i t h  an azimuth t u r r e t  mount. The 
s i ng le  p i - r  foundat ion was se lected i n  order t o  minimize s i t e  p repara t ion  
and foundat ion i n s t a l l a t i o n  labor  costs.  I t  does r e s u l t  i n  a  s l i g h t l y  
h igher  weight concent ra tor  than would r e s u l t  w i t h  a  wide base foundat ion.  
However, because o f  reduced i n s t a l l a t i o n  labor, t o t a l  i n s t a l l e d  cos t  i s  
minimized. Hydraul ic power u n i t s  were se lected f o r  both azimuth and 
e l eva t i on  d r i v e  systems. The azimuth d r i v e  i s  a  hyd rau l i ca l  ly-powered 
gear d r i ve .  The e l eva t i on  d r i v e  i s  a  s i n g l e  stage, double-act ing 
hydrau l i c  c y l i n d e r  actuator .  Emergency power i s  prov ided hy a  pressur ized 
gas accumu 1 ator .  
Tracking and Contro l  System 
A hybr id,  two-ax is,  sun t r a c k i n g  c o n t r o l  system based on microprocessor 
technology, has been selected. Coarse syn the t i c  t r a c k i n g  i s  achieved 
through a microcomputer based c o n t r o l  system t o  c a l c u l a t e  sun p o s i t i o n  f o r  
t r ans i en t  per iods of c loud  cover as w e l l  as sundown and sunup pos i t i on i ng .  
Accurate a c t i v e  t r a c k i n g  i s  achieved by two-axis sun sensors. 
Re f l ec t i ve  Panel Proto type Mod i f i ca t ions  
Prototype-speci f  i c  mod i f i ca t i ons  t o  the  mass p roduc ib le  r e f l e c t i v e  panel 
design are being made t o  reduce p ro to t yp i ng  cost .  The most s i g n i f i c a n t  
mod i f i ca t ion  i s  i n  t he  area o f  t he  compression molded SMC substrate.  The 
cos t  of a f u l l - s i z e  mold i s  p r o h i b i t i v e  f o r  p ro to t yp i ng  purposes. The 
p ro to type  panels w i  11 be fabr i ca ted  by hand layup o f  g lass - re i  nforced 
po lyes te r  (GRP) on a contoured epoxy t o o l .  The panel face  sheet w i l l  be 
fabr icated on t h i s  t o o l  i n  a s i m i l a r  manner as boat h u l l s .  The r i b s  w i l l  
be c u t  from GRP sheet stock, assembled, and bonded t o  t h e  face  sheet. The 
m i r r o r  g lass w i l l  be bonded t o  t he  assembled subst ra te .  
Re f l ec t i ve  Panel Develo~ment and Demonstration 
Two-foot square compression molded SMC-Mirror Glass panels were f ab r i ca ted  
and tes ted  i n  the  Phase I e f f o r t .  Compliance w i t h  t he  requirements o f  t h e  
low cost concent ra tor  has been successful l y  demonstrated. Eoth subsize 
and f u l l - s i z e  hand layup GRP-Mirror Glass p ro to type  panels w i l l  be 
fabr icated and tes ted  i n  Phase I 1  o f  t h i s  p ro j ec t .  Panel t e s t i n g  w i l l  
cons is t  of dimensicnal v e r i f i c a t i o n ,  s lope e r ro r ,  h a i l  impact, thermal 
cyc l ing,  and s t r u c t u r a l  d e f l e c t i o n  tes ts .  
The pr imary o b j e c t i v e  of the  Phase I compression molded SMC-Mirror Glass 
t e s t  panels was t o  demonstrate the  o p t i c a l  surface q u a l i t y  a t t a i n a b l e  w i t h  
present s t a te  o f  t he  a r t .  Test panels were f ab r i ca ted  w i t h  bo th  a 
s ingle-step molding process and a two-step, molding-bonding approach. The 
single-r 'ep process i n t e g r a l l y  molded the  SMC-Mirrnr Glass panel i n  one 
molding cycle.  The two-step process invo lved molding of t he  SMC i n t e g r a l  
face sheet-r ibbed subs t ra te  fo l l owed by adhesively bonding t he  m i r r o r  
glass using the  female p o r t i o n  o f  t he  mold as t he  bonding f i x t u r e .  
Q u a l i t a t i v e  and q u a n t i t a t i v e  eva lua t ion  of t e s t  panel o p t i c a l  q u a l i t y  was 
performed. Hepresentat i v e  panels produced w i t h  each manufacturing method 
are shown i n  F igure  3. The r e f l e c t e d  l i g h t  pa t t e rns  from each panel 
prov ide a very s e n s i t i v e  q u a l i t a t i v e  eva lua t i on  o f  m i r r o r  sur face 
topography. The s ing le-s tep molded panel exh ib i t ed  d i s c e r n i b l e  r i b  
pr in t - through ( t h e  diagonal  1 i ne pa t te rns  c r i s sc ross i ng  the  m i r r o r  
surface). This e f f e c t  i s  r e l a ted  t o  ma te r i a l  shr inkage a t  t he  r i b l f a c e  
sheet j unc t i on  dur ing  molding and c u r i i g .  A second observable f ea tu re  i n  
the  s ing le-s tep molded panel i s  a system of concen t r i c  r i p p l e s  progress ing 
outward from the  cen te r  of t he  panel. This p a t t e r n  was t raced t o  a system 
of concent r ic  r i p p l e s  i n  t he  t o o l .  The pa t t e rns  were impressed i n t o  t he  
glass sheet by the  h i gh  molding pressure of t he  compression molded 



Low cost concentrator

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