The test and development of the 7-meter Shenandoah parabolic dish collector incorporating an FEK-244 film reflective surface and cavity receiver are described. Four prototypes tested in the midtemperature Solar System Test Facility indicate, with changes incorporated from these development tests, that the improvements should lead to predicted performance levels in the production collectors

Kinoshita, G. S.

English

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DOE PARABCjLIC DISH S O W  THERMAL 
ANNUAL PRCGRAM REVIEW 
DEVELQFXENT AND TESTING OF THE 
SHENANDOAH COLLECTOR* 
Ceorcje S. Kinoshita 
Sandia National Laboratories 
Albuquerque, N e w  Mexico 87185 
The t e s t  and developnent of the GE-designed 7-meter Shenandoah parabo:lic 
dish collector  incorporating an FEK-244 film re f l ec t ive  surface and cavi ty  
receiver is described. Four prototypes tested in  the Midtemperature 
,So!.ar System '&st Faci l i ty  indicate,  w i t h  charyes incorporated £ran these 
developnent t e s t s ,  that. the improvements should lead t o  predj cted perforrance 
levels  rn the production collectors .  
*This work supported by the U.S. Department of Energy, SAblD81-0028A 
https://ntrs.nasa.gov/search.jsp?R=19810022000 2020-03-21T09:46:36+00:00Z
A parabolic dish solar  col lec tor  was selected for  the Shenandoah Solar Total 
Fnergy Project applicat ion because it could supply the  design .:oads throughout 
the peak e l e c t r i c a l  demand period of the u t i l i t y  and do t h i s  from a limited 
(5-acre) f i e l d  and under moderate (Atlanta a rea )  insolat ion conditions. ?he 
collector  was designed by the General Elec t r ic  Company under a DOE contract  for  
the  design of the  Shenandoah Solar 'Jbtal EZlergy System. 
'ihe i n i t i a l  rrrodel upon which the f i n a l  col lec tor  design is based evolved from 
a 5-meter diameter caronmications dish antenna which Sc ien t i f i c  Atlanta had 
developed. A so lar  col lec tor ,  which was cal led the engineering prototype 
col lec tor ,  EPC, was fabricated by the expedient of applying a ref lec t ive  film 
to the "petals" of the  c m u n i c a t i o n  antenna and attaching a solar  receiver 
where the c a s s q r a i n  ref lec tor  was normally located. lhis EPC model was 
evaluated a t  t h e  Sandia Solar Cbllectmr Module '&st Fac i l i ty ,  and it indicated 
the  f eas ib i l i ty  of adapting the low cos t  fabricat ing techrique of die-stamping 
petal sections t o  produce solar ref lec tors .  These t e s t s  a lso  led t o  modifications 
t o  the original  receiver design result ing in improved receiver operation. 
?he ref lec tor  surface was o r ig ina l ly  conceived t o  be a g lass  surface over 
polished aluminum. The aluminum was a magnesium al loy which w u l d  polish t o  
a high re f l ec t iv i ty .  Alternatives were investigated, and an RTV s i l i cone  
subs t i tu te  for g las s  was developed when proprietary issues could not be resolved 
with the u.se of the GE g lass  process. An anodization scheme was carr ied  on a s  
an a l te rnat ive .  Both ref lec tor  approaches were eventually replaced by a r e f l ec t ive  
f i l r r  (FEK-244, 2 3M product). mis  c h a ~ y e  provided an improvement in  r e f l e c t i v i t y ,  
enhancing the col lec t ion  of solar  energy t o  provide the thermal energy needs of 
the project.  The change a lso  indicated a protracted wash cycle could be considered 
over the other ref lec tor  approaches, making reduced operat ion and maintenance 
(O&M) cos t s  possible. Since the aluminum was no longer the ref lec t ing  surface,  
the aluminum was changed t o  a lower cos t  al loy.  
A key element in adapting the  ref lec tor  film for dish col lec tor  use was the  
process develapent  for applying FM-244 t o  a compound curved surface. (31 the 
e a r l i e r  WC, the f i l m  was applied to the  individual "petal"  sect ions u s i ~ y  the 
squeege/'detergent hand application method which is r e c m n d e d  by 3M for  laminating 
the film to  f lat .  panels. This was t h e  f i r s t  time the film had been applied to a 
canpound curved surface so  no h i s to r i ca l  p r e c d e n t  could be c i t ed  which w u l d  
provide confidence a s  to  the lc ry  term in tegr i ty  of the film ( remin ing  attached 
t o  the subs t ra te)  under a l l  environmental conditions. ?he film was only a 
temporary expedient to convert a ccmnunications antenna a solar  col lec tor .  
'I%us, a l t e rna te  approaches f c r  a ref lec tor  were encouraged. 
When it became evident,  however, tha t  the FEK-244 film of fei  ed s ign i f i can t  
advantages over the R l V  o r  anodized a l ternat ives ,  the problem of applying the 
film to  a canpound surface was readdressed. m e  solution turned out t o  be 
r e l a t ive ly  simple. ?he FEK f i lm was laminatc-d t o  the f l a t  aluminum subst ra te  
material prior  to die-stamping in to  the  "peta l"  s h a p .  'Jb protect  the  r e f l ec t ive  
film, an opaque premask film was laminated over the FEK. An addit ional  benefi t  
accruing fran the e a s i l y  peeled premask is tha t  is a lso  permits col lec tor  assenbly 
outdmrs  without creat ing a concern over eye hazards. Both film and premask a r e  
applied using a roller applicator which reduces the time and labor over that 
associat-ed with the hand application method. Enviromntal t e s t s  of two petals 
proceised by the roller met!!od disclosed the tendency of the FM film to "tunnel. " 
"l'tmnelling" is a consequence of FEK expansion when exposed to hot, high humidity 
conditions and is the term applied to  localized r idge l ike  l i f t ing  which occurs, 
especially a t  s t ress  s i tes .  FM has a coefficient of thermal expansion of about 
45 microinch/inch/OF. Fesolution of th is  problem was effected by cutting the FEK 
every two feet to  reduce the size of the laminated film sections. Subsequent 
environmental tes ts  on petals with enlarged film sections (3-foot cuts) indicate 
no tendency toward tunnelling, ?his w i l l  reduce the number of cuts required in 
each "petal." Whether th is  phenomenon is associated pr imr i ly  with the double 
curvature of a parabolic dish surface or is commn even i n  a planar configuration, 
i f  the film sections are large enoilgh, or whether roller application causes 
differences from hand applications is not known. 
Four 7-meter diameter pre-production prototrpe dish collectors were fabricated 
for testing and evaluation in a quadrant of the Sandia Midtemperature Solar System 
'&st Facility (MSSTF). In i t ia l ly ,  the collectors had RTV-coated reflectors. Che 
of the four was subsequently replaced with an anodized surfice and another with 
an FEK surface. A l l  of the reflectors were assentdies of 21, 8-foot long "petals" 
and a 29-inch wide center annulus section. The *part reflector was a consequence 
of the petal fabricator being limited to a press s ize which would only accanmodate 
an 8-foot die. With the acquisition of a 90G-ton press the fabricator can now 
stamp full-length petals, eliminating the need for the annulus section. m e  
annulus was fabricated by a spinning operation. AII improvement i n  the collector 
efficiency i s  expected with the extended ptal design. 0-1 the quadrant t e s t  
coliectors, the annulus accounts for a b u t  10 percent of the reflector area but 
contributes much less  than the expected reflected energy due to the non-spxularity 
of the spinning. 
The collector to be installed a t  Shenandi&~ w i l l  incorporate several design changes 
as a result of the quadrant 'ests. 
Difficulties evidenced i n  tne assembly of the reflector to the declination axis 
prompted the change fran trying to align two horizontal boles for attachment to 
the frame, to  mating the f l a t  surfaces to effect assembly. 
The large ,mount of field welding of the frame assembly led to t h 2  use of a base 
s u p p r t  frame to permit the f r m  assembly to be shop welded and be field installed 
as  a finished section. This procedure also permits the p l a r  drive motors and 
jackscrews to !x shop wlded to the collector frame assembly and the ent i re  
assedly  checked for propr  p l a r  roQtion prior to shipment. 
The diff icul t ies  encountered in maintaining the reference orientation for the 
position indicating ptentiometers has led to a redesign of the mounting bracket 
and a change i n  the attachment to  the rotating axes. 
?he mechanical stop on the jackscrews w i l l  be strengthened to prevent the gear 
motors fran driving through the stops and causing the reflector to freely pivot 
about the p l a r  axis. 
Each time the seceiver was brought into or taken out of f x u s ,  the aperture plate 
(made of stainless s t e e l )  r ece iv f~  a healthy thermal input c a u s i q  the apx tu re  
plate to  buckle. l%e heating 6lso lead to the malfunctioning of the optical I 
f ibre solar tracking system. A thicker s teel  sheet was not totally satisfactory. i I 
A quartz refractory pad is now used to  insultate the aperture plate. ! 
f 
The receiver coi l  through which the heat transfer fluid is circulated has been j 
changed £ran a double coil  to  a single coil. A t  flow rates slightly less  than 
1 gal/min through the double coi l  receiver, it was noted that a transition to 
laminar flow appeared to  be occurring. f i e  tubing diameter for the single coi l  
has been enlarged to maintain the pressure drop a t  about 15 psi while maintaining 
the tube wall to fluid AT a t  less  than 100°F a t  the minimum flow rates to  keep 
the Reynolds nrrmber above 8200. Zhe new coi l  was tested in a quadrant t e s t  
collector and indicated improved operation in effecting heat transfer a t  low 
flow rates. 
The hub, which is the centrally located element to  which the reflector petals 
are attached, had kcen changed frun an aluminum weldment to a s tee l  weldment as  
a cost saving measure. Solicitations f ram potential fabricators now indicate 
that the hub can be made fran an aluminun casting a t  an even greater cost savings, 
so this  avenue is being explored further. 
'Ihe collector was designed tcr m e t  the requirements indicated in Table 1. An 
operational characteristic ikich is distinctive to th is  d i s h  collector is that 
the fu l l  temprature differential  (fran 500°F input to 750°F output) is accanmodated 
i n  contrast to troughs where a number o collectors make up a ST string. The 5 minimum operation level of 50 Btuhr-f t  is the level a t  which the system losses 
(parasit ic ard thermal) are  j u s t  inet. The other requirements l is ted are c m n  
to other concentrating distributed collector systems. 'Zhe design requirements 
were translated into collector optical and receiver thermal parameters and 
incorporated into a cvllectcr systm analysis model. This model was used to  
analyze the c o l l e c ~ r  prformnce i n  terms of key variables. lhese variables 
are shown i n  Figure 1. 
The £,Id ra t io  was selected on the basis of optimizing the concentration rat io  
without an undue increase i n  the receiver heat losses. Figure 2 shows t h e  
efficiency was maximized a t  a f/d ra t io  of 0.5 
The sensit ivity of the concentration ra t io  (CR)  from 250 is shown in Figure 3. 
The collector for Shenandoah w i l l  have a CR of 234 w i t h  an 18-inch diameter 
receiver aperture. 
The indicated reflectivity,  Figure 4,  is the level which, in conjuncticn w i t h  
the intercept  factxr 3rd receiver efficiency, was thought to be required to 
provide the overall collector efficiency needed to meet the collector design 
requirements. The FEK-L44 surface on the environnentally tested p e l s  has 
maifested a reflectance of about 0.85 af ter  washing after degrading to  about 
0.82. 01 the Quad '&st units, exposure to the elements for 3 months resulted 
in a reduction in the spec:~lar reflectivity (35 mr), but the lclvel was recovered 
af ter  washing. 

f/d 
Concw tra  t ion Ratio 
Reflectivity 
Slope Error 
Tracking Error 
Focal Length to  Dish Diameter Ratio 
Collector Aperture/Weiver Aperture 
Tbtal Hemispherical and Specular Distribution 
Deviation fran a Paraboloid 
Receiver Offset from Solar F l u  
FIGURE 1. COLLECTOR ESIm CaRIABLES 
fID FOCAL LENGTH 
'DIAMEI ia 
FIGURE 2. FOCAL LENGTH OP'I XMIZATION 
LSE RANGE -, 
, , CRlCR 
1.8 2.0 REFEHENCE 
CONCF tJTRATION 
FIGURE 3. CONCfNI'F!ATION RATIO SENSITIVITY %YSIS 
FIGURE 4. REFLECTOR SURFACE PARAMETER SESITIVITY W Y S I S  
'Ihe intercept factor, which is defined a s  the percentage of the reflected energy 
incident a t  the receiver aperture, is a function of the specularity, s l o p  errors, 
and tracking errors associated with the collector and is recpired to  be about 0.96 
to achieve the collector performance requirements. A slope error of 1/2 degree 
was considered a design p a r m t e r  and its sensit ivity relative to  energy collection 
is shown in Figure 5. 
FIGURE 5. SLLlPE ERROR SENSITrVI'IY AEUYSIS 
?he sensit ivity of the tracking error on the energy collection is shown in Fiqure 6. 
'I%e tracking bias of 1 /4  degree was used as the collector design parameter. 
?he d i s h  diameter of 7 meters was selected on the bdsis of being the best 
canpromise considering collector cost, f ield cost, collector efficiency, and 
fluid heat losses. ?he diameter optimization results are shcnm in Figure 7. 
A collector field cost per unit of delivered energy versus collector diameter 
plot can be cowtructed for various projected collector costs. Fbr our case, 
the optimal diameter l i e s  i n  the 7-meter range. I f  collector costs can be 
reduced, other field canpnent costs become mre important, and the trend 
is toward optimizing a t  larger diameters. 
These collector design parameters are shown i n  Figure 8,  and the collectcr 
performance curves are indicated to show the expected off-design characteristics. 
Quadrant test results fran the FM-244 collector indicate that these early 
prototypes are achieving operational levels very close to design levels. Pro- 
duction  collector^,, incorporating improvements suggested fran the quadrant tes t s  
are expected to provide performance levels predicted. 
FIGURE 6. TRACKING BIAS ERQR SENSITIVITY ANALYSIS 
\ COLLECTOR E T  
FIGURE 7. OPTIMIZATION RESULZ3 - COL- DIAMliTER VS. COGT/B?U ELIVERFA 



Development and testing of the Shenandoah collector

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