Since 1986, ENTECH and the NASA Lewis Research Center have been developing a new photovoltaic concentrator system for space power applications. The unique refractive system uses small, dome shaped Fresnel lenses to focus sunlight onto high efficiency photovoltaic concentrator cells which use prismatic cell covers to further increase their performance. Highlights of the five-year development include near Air Mass Zero (AM0) Lear Jet flight testing of mini-dome lenses (90 pct. net optical efficiency achieved); tests verifying sun-pointing error tolerance with negligible power loss; simulator testing of prism-covered GaAs concentrator cells (24 pct. AM0 efficiency); testing of prism-covered Boeing GaAs/GaSb tandem cells (31 pct. AM0 efficiency); and fabrication and outdoor testing of a 36-lens/cell element panel. These test results have confirmed previous analytical predictions which indicate substantial performance improvements for this technology over current array systems. Based on program results to date, it appears than an array power density of 300 watts/sq m and a specific power of 100 watts/kg can be achieved in the near term. All components of the array appear to be readily manufacturable from space-durable materials at reasonable cost. A concise review is presented of the key results leading to the current array, and further development plans for the future are briefly discussed

Fraas, Lewis M.

Oneill, Mark J.

Piszczor, Michael F.

English

NASA Technical Reports Server

N91-30223
KEY RESULTS OF THE MINI-DOME FRESNEL LENS
CONCENTRATOR ARRAY DEVELOPMENT PROGRAM
UNDER RECENTLY COMPLETED NASA & SDIO SBIR PROJECTS
Mark J. O'Neill
ENTECH, Inc.
DFW Airport, TX
Michael F. Piszczor
NASA Lewis Research Center
Cleveland, OH
Lewis M. Fraas
Boeing High Technology Center
Seattle, WA
INTRODUCTION
Since 1986, ENTECH and the NASA Lewis Research Center have been developing a new
photovoltaic concentrator system for space power applications. The unique
refractive system uses small, dome-shaped Fresnel lenses to focus sunlight onto
high-efficiency photovoltaic concentrator cells which use prismatic cell covers
to further increase their performance. Under Small Business Innovation Research
(SBIR) funding provided by both NASA and SDIO, the mini-dome Fresnel lens
concentrator array has progressed from a paper concept in 1986 to functional
array hardware in 1990-91. Since 1989, Boeing has been a key participant in the
development of this concept, providing both record-breaking GaAs/GaSb tandem cell
technology and significant expertise in the development of the panel structure
and related manufacturing techniques. Other project participants include 3M
Company (lens tooling); Fresnel Optics (prism cover tooling); and Varian
Associates (GaAs cells).
Highlights of the five-year development include near-AMO Lear Jet flight testing
of mini-dome lenses (90% net optical efficiency achieved); tests verifying
sun-pointing error tolerance with negligible power loss; simulator testing of
prism-covered GaAs concentrator cells (24% AMO efficiency); testing of
prism-covered Boeing GaAs/GaSb tandem cells (31% AMO efficiency); and
fabrication and outdoor testing of a 36-1ens/cell element panel. These test
results have confirmed previous analytical predictions which indicate substantial
performance improvements for this technology over current array systems. Based
on program results to date, it appears that an array power density of 300
watts/square meter and a specific power of 100 watts/kilogram can be achieved in
the near term. All components of the array appear to be readily manufacturable
from space-durable materials at reasonable cost. This paper presents a concise
review of the key results leading to the current array, and briefly discusses
further development plans for the future.
SYSTEM DESCRIPTION
Figures 1 through 4 show the basic mini-dome Fresnel lens space concentrator
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https://ntrs.nasa.gov/search.jsp?R=19910020909 2020-03-19T17:29:37+00:00Z
array concept. Small, square-aperture, thin, dome-shaped Fresnel lenses focus
incident sunlight by a factor of about i00 onto circular photovoltaic cells. The
cells are mounted to a backplane radiator for waste heat rejection. Individual
lenses are placed within slots in a honeycomb panel, which is structurally
integrated with the backplane radiator. Cells are interconnected in
series/parallel circuits to build up the desired voltage, current, and power
values for the panel. Panels are mounted onto automatically deploying support
structures to form large, multi-kilowatt arrays.
Material selection has been one of the key issues in the development of the
mini-dome lens array. The current materials have been chosen based on previous
successful space use, ease-of-fabrication, and cost. The lens is a laminated
assembly of ceria-doped microglass over clear silicone rubber, as shown in Figure
5. The honeycomb and radiator are both made from aluminum. The cell is a tandem
structure of gallium arsenide over gallium antimonide, to maximize array
performance. The cells use silicone rubber prismatic covers to eliminate grid
shading losses, thereby enhancing performance. As discussed in the following
section, prototype lenses, cells, prismatic covers, and panels have all been
successfully fabricated and tested.
KEY RESULTS
The unique dome lens design is shown in Figure 6. While every prism in the lens
is different from all others, each prism is configured for symmetrical
refraction. Specifically, the angle of incidence of the solar rays on the outer
smooth surface of the lens is equal to the angle of emergence of these solar rays
on the faceted inner surface of the lens. This symmetry minimizes reflection
losses, thereby maximizing efficiency. Furthermore, this symmetry greatly
improves image quality compared to conventional flat Fresnel lenses. Even more
importantly, this refraction symmetry vastly expands allowable inaccuracies
encountered in both initial manufacture and long-term operation. Remarkably, the
slope error tolerance of the mini-dome lens is more than iO0 times larger than
for a flat Fresnel lens, and more than 200 times larger than for a reflective
concentrator, for equal image defocussing.
By "tweaking" the angles of the individual prisms making up the Fresnel pattern,
the dome lens has been designed to focus the sunlight into a circular spot about
2.6 mm in diameter, which is smaller than the cell diameter of 4.0 mm by an
amount which was selected to allow a sun-pointing error of 1 degree without loss
of power output. Performance goals for the lens were >90% net optical efficiency
and !i degree tracking error tolerance with negligible loss of power.
Measurements on a pure silicone lens (no glass superstrate) with a square
aperture mask coupled with a gallium arsenide cell are shown in Figure 7. Note
that the lens indeed achieved 90% efficiency. Note also that the power loss at ]
degree tracking error is only I%. Later lenses with prototype glass superstrates
have achieved about 85% optical efficiency with less than 5% power loss at I
degree tracking error. Further improvement in the glass superstrates is expected
to raise the laminated lens performance back to the pure silicone lens levels.
Still higher performance should be achievable through the use of antireflection
coatings on the glass superstrate.
Figure 8 shows the Boeing-developed tandem cell approach. The prism-covered
gallium arsenide top cell converts about 24% of the available sunlight to
electricity. The top cell energy conversion occurs for that portion of the solar
20-2
spectrum below about 0.9 micron in wavelength. Longer, infrared wavelengths pass
through the top cell onto the prism-covered gallium antimonide bottom cell. The
bottom cell converts another 7% of the available sunlight to electricity, for a
total tandem cell efficiency of 31%. This value has been confirmed by NASA-Lewis
via Lear Jet flight tests coupled with flash solar simulator tests. Higher
efficiency values are anticipated in the future, as the newly developed gallium
antimonide cell technology matures.
Thermal analyses have been conducted to predict on-orbit cell operating
temperature. Figure 9 shows a typical thermal analysis result for the hottest
portion of a low earth orbit (LEO) mission. The radiator temperature just
beneath the cell is about 96C. Thus, with a well designed cell-to-radiator mount
(with a 4C gradient), the cell temperature should be about 100C. Figure i0 shows
a similar result for a geosynchronous earth orbit (GEO) mission. The cell
temperature will be about 76C for GEO operation.
Mass analyses have been conducted to estimate mass per unit area for the baseline
panel, as shown in Figure ii. A value of about 2.4 kg/sq.m, appears achievable
in the very near term. Furthermore, automatically deploying support structures
designed by others have been identified for use with the mini-dome lens panels.
These structures have a mass of about 0.7 kg/sq.m., for a total array mass
density of 3.1 kg/sq.m. This array mass density is approximately equivalent to
the planned one-sun Kapton blanket array for the Space Station Freedom. Thus,
the mini-dome lens array is extremely light-weight.
Figure 12 summarizes the near-term significance of the previously discussed
performance and mass parameters. With single junction cells, power density
values of 250-260 W/sq.m. will be achieved. With tandem cells, power density
values of 300-330 W/sq.m. will be achieved. With single-junction cells, specific
power values above 80 W/kg will be achieved. With tandem cells, specific power
values above i00 W/kg will be achieved.
PROTOTYPE PANELS
Over the past year, several prototype panels have been successfully made and
tested. The most recent panel is shown in Figure 13. Boeing has developed a
computer-controlled milling process for rapidly producing extremely rigid,
light-weight, thermally efficient radiator/honeycomb assemblies from a plate of
aluminum. Cell assemblies are mounted directly to the panel backplane, while
individual lenses are attached to the front of the panel structure. Outdoor
testing of these panels has shown performance levels close to expectations for
the lenses and cells utilized. These prototype panels have convinced the project
participants of the practicality of the mini-dome lens panel concept.
CONCLUSION
The mini-dome lens array development has progressed successfully to the prototype
hardware stage. Performance measurements have closely matched expectations. A
small array space flight test is planned for 1992 in conjunction with the PASP+
program (as discussed by Guidice et al in another paper at this conference).
Independent comparative array analyses are confirming the relative merits of the
new array technology (e.g., as discussed by Kraus in another paper at this
conference). Figure 14 summarizes the key features and advantages of the
mini-dome Fresnel lens space concentrator approach.
20-3
Fig. i
DOME LENS PV MODULE
CONCEPTUAL DESIGN
Fig. 2 ENTECH DOME LENS PV CONCENTRATOR
PANEL CONCEPTUAL DESIGN
i i:
. _- _._ CALUUkl _01E CELL
20-4
Fig. 3 CROSS - SECTIONAL VIEWS OF
DOME LENS PV PANEL
SlOE_EW
_i F:i__......I z_";_l__J
trjdiator
TOP _qEW WtIIIOUI L[N$ HONEYCOMB dAT_
375 _ ....
SOLA_ CELL
Fig. 4
DOME LENS
PV ARRAYS ON ESS SYSTEM
ATTACHED TO SPACE STATION
-'1 I-" 0,4 m
i
S]OWED CONFIGURATION ..,,OkC
PARTIALLY DEPLOYED l[
$01i_CE: NASACR 176394 1.5 •
6. ] im L/ DEPLOYEO CONFIGURATION
20-5
Fig 5 BASELINE LENS DESIGN FOR LOW-EARD4-ORBIT (LEO) APPLICATIONS
• (MICROGLASS SHIELDS POLYMERIC LENS FROM ArOMIC OXYGEN)
LAMINATED CERIA MICROGLASS/SILICONE RTV
MINI-DOME FRESNEL LENS
iI# ..<,Z; L(_h-(_k...........
....:: \"k.. I //7/---_3<..-" / s.,_o... R_v_- L_ \ _!
......_.....-_.-J_ ....\ _\_
el Rotational Symmetry)(^xI, "--_--" ",, _........
".. Focal .-_" . .'""
""-. Plane --'" ""........ ........
.. . I° •...................... ..'"
"4 I
Fig. 6
MAGNIFIED VIEW OF SEVERAL PRISMS WIIIIIN
TIIE ENIECII DOME LENS, SIIOWING REFRACIION
SYMME7RY AND BLUNT TIP IOLERANCE
1111S SYHMETRICAL REFRACTION CONDITION HINIMIZES REFLECTION LOSSES
FOR A GIVEN RAY TURNING ANGLE, THEREBY HAXIHIZING IRANSHITTANC[.
TIlE SYMMETRICAL REFRACTION CONDITION ALSO MINIMIZES IMAGE SIZE
AND HAXIHIZES TOLERANCE FOR MANUFACTURING ERROR5 AND _ERR_TIONS,
-_ ,42k!'II;'l#_,n
_, I_,
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20-6
NASA LEWIS LEAR JET HIGH ALTITUDE TEST FACILITY
MEASURED LENS PERFORMANCE FOR MODULE #1
0.91
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0.90
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AIR MASS
(Prototype Silicone Rubber Lens,
Masked to Simulate Square Aperture
Flown March 1990)
TRACKING ERROR PERFORMANCE TEST
.J
:>UJ
1.0
FOR PROTOTYPE MODULE #1
0.8 ¸
0.6
0.4 ¸
m m',_ : : : : -- : . i
0.2 . . .,,- , • . - . . - , .... , . , . _
-3 -2 -1 0 1 2 3
TRACKING ERROR (DEGREE)
LENS/CELL ELEMENT DESIGNED
FOR 1 DEGREE TRACKING ERROR TOLERANCE
Fig. 7
(Prototype Silicone Rubber Lens,
Masked to Simulate Square Aperture)
20 -7
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20-8
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for Low Earth Orbit (LEO)
for 200 Hlcron (8 nit)
Radiator Thickness
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61.67_ it. Rid 94.2O4 el.lie9 6|.Tilr 60.920 60,3_6 19.119R _JS. lll IO.O_k_
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5_.5Q4 99.4_0 mo.34+ 3_._1_ 5_.057 se,es_ me. PSI 50.B32 50.545 5B++_l
20-9
Fig. i0 Thermal Analysis Results
for Ceosynchronous Earth
Orhlt ((IEO) for 200 Micron
(8 roll) Radiator Thickness
Fig. ii
MINI-DOME LENS SPACE PHOIOVOLIAtC CONCENIRATOR
NEAR-TERM BASELINE PANEL MASS BREAKDOWN
M__AlR_QT.A4Ak IHICKNESS _LR_rACE AREA MASS/PANEL AREA
PANEL AREA(G/CU.CM.) (CM) (KO/SO.M.)
LENS SUPERSIRAI[
LENS PRISMS
RAOIAIOR
CELL/CovER/MOUNt
HONEYCOMB
RADIAIOR COATING
MISCELLANEOUS
TOTAL
MICROGLASS 2.50 0.O15 1.30 0.49
SILICONE 1.00 0.015" 1.30 O.19
ALUMINUM 2.11 0.020 1.00 O.55
GAAS ET AL 5.10 0.046 0.02 0.05
ALUMINUM 2.17 0.015 2.20 O.91
ALUMINA 3.88 O.001 2.00 O,O8
................. 7.5% or ABOVE ]DIAL ..................
2.44
" SILICONE BASE THICKNESS - O.010 CM
SILICONE PRISM THICKNESS - O.0_O CM (BUT HALF VOID)
EFFECTIVE SILICONE THICKNESS • O.015 CM
Fig. 12
MINI-D(_ FRESNEL LENS ARRAY - NEAR-TERM PERFORMANCE ESTIMATES
BASED ON RECENT TEST RESULTS FOR PROTOTYPE CELLS AND LENSES
IT___ NEAR-TERM GmAs NEAR-TERM TANDEM
Lens Type Glass/Silicone Glass/Silicone
Panel Type 0.02 cm Alum. 0.02 cm Alum.
Cell Type GaAe GaAs + GaSb
Cell Elf. at 25C 2&__Z_ 2&% + 7Z - 31%
Max. LEO Cell Oper. Temp. IOOC IOOC & IOOC
Cell Elf. at Max, LEO Temp, 22..__% 22_._%%÷5% = 27X
Max. GEO Cell Oper. Temp. 76C 76C & 76C
Cell Eff. at Max. GEO Temp. 23___% 23___Z+ 6% - 29%
Lens Efficiency 90___Z 90___ZZ
Packing Factor 97% 97%
Mismatch/Wiring Factor 93Z 93%
LEO Array Efficiency 18% 2_%
LEO Power Density (w/sq.m.) 247 302
CEO Array Efficiency 19% 24Z
GEO Power Density (w/sq.m.) 260 329
Panel Mass (kg/sq.m.) 2.& 2.4
S_ruc_uce Mass (kB/sq.m.) 0.7 0.7
Array Mass (kB/sq.m.) 3.1 3.1
LEO Specific Power (w/kg) 80 97
GEO Specific Power (w/k8) 84 106
Not__e: Measured Performance Parameters for Prototype Cells and Lenses Are Underlined.
20-10
iiiiii!_!iii}ii_
20-11
Fig. 14
MINt-DOME LENS SPACE Pt_IOVOLTAIC CONCENTRATOR
KEY FEATURES AND ADVANTAGES
UNIQUE LENS: THE TRANSMITTANCE-OPTIMIZED DOME LENS PROVIDES _0_ NET OPTICAL EFFICIENCY
(WITHOUT THE NEED FOR SECONDARY OR TERTIARY CONCENTRATORS).
EXCEPTIONAL TOLERANCES FOR MANUFACTURING AND OPERATIONAL INACCURACIES
(E.G., 200 TIMES THE SLOPE ERROR TOLERANCE OF REFLECTIVE CONCENTRATORS,
A_D 100 TIMES THE SLOPE ERROR TOLERANCE OF FLAT FRESNEL LENSES), AND
EXCELLENT AND SELECTABLE TRACKING ERROR TOLERANCE
(1 DEGREE FOR 4 MM CELL, 2 DEGREES FOR S.4 MM CELL. ETC.)
CELL USAGE: VARIOUS CELLS CAN BE USED IN TttE DOME LENS CONCENTRATOR, INCLUDING
BOEING'S GAAS/GASB, VARIAN'S GAAS, NASA's INP, ET AL.
(DUE TO HIGH CONCENTRATION. ONLY I% OF NORMAL CELL AREA IS NEEDED).
PRISMATIC COVERS: ALLOW HEAVY GRID COVERAGE FOR EFFICIENT CURRENT COLLECTION.
HEAT REJECTION: CELLS ARE MOUNTED DIRECTLY TO A BACKSIDE RADIATOR.
PACKING FACTOR: LENSES CAN BE CUT SQUARE (OR HEX) IN APERTURE TO MAXIMIZE
LENS APERTURE/PANEL AREA RATIO (97_ IS EASILY ACHIEVED).
MODULARITY: THE NUMBER OF LENS/CELL ELEMENTS CAN BE SELECTED FOR OPTIMAL PANEL ourPuI.
MAIERIALS: READILY AVAILABLE LIGIITWEIGHT MATERIALS ARE USED THROUGHOUT IIIE PANEL.
MANUFACTURABILITY: ALL PANEL ELEMENTS APPEAR TO BE READILY MANUFACIURABLE.
DEPLOYABLILITY: AUTOMATICALLY DEPLOYING STRUCTURES BEING DEVELOPED FOR OTHER
CONCENTRATORS CAN DE EASILY ADAPTED TO THE MINI-DOME PANELS.
(E.G.. THE ASIRO-AEROSPACE ESS OR STACBEAH STRUCTURES).
COST: DUE TO TIIE SMALL CELL AREA REQUIREMENT. THE MASS-PRODUCIBILITY OF ALL
ARRAY COMPONENTS. AND THE LARGE ALLOWABLE TOLERANCES, THE MINI-DOME LENS
ARRAY OFFERS SIGNIFICANT COST REDUCTION POTENTIAL.
RADIATION HARDNESS: [I_E PANEL CONFIGURATION DAN BE TAILORED TO PROVIDE
AN APPROPRIATE LEVEL OF PARTICULATE RADIATION SIIIELDING
(I.E., ELECTRONS AND PROTONS). MINIMIZING CELL DEGRADATION.
20-12


Key results of the mini-dome Fresnel lens concentrator array development program under recently completed NASA and SDIO SBIR projects

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