For the past 2% years, our team has been developing a unique photovoltaic concentrator array for collection and conversion of infrared laser light. This laser-receiving array has evolved from the solar-receiving Stretched Lens Array (SLA). The laser-receiving version of SLA is being developed for space power applications when or where sunlight is not available (e.g., the eternally dark lunar polar craters). The laser-receiving SLA can efficiently collect and convert beamed laser power from orbiting spacecraft or other sources (e.g., solar-powered lasers on the permanently illuminated ridges of lunar polar craters). A dual-use version of SLA can produce power from sunlight during sunlit portions of the mission, and from beamed laser light during dark portions of the mission. SLA minimizes the cost and mass of photovoltaic cells by using gossamer-like Fresnel lenses to capture and focus incoming light (solar or laser) by a factor of 8.5X, thereby providing a cost-effective, ultra-light space power system

Aiken, Dan

Fikes, John

Fork, Richard

Howell, Joe

McDanal, A. J.

O'Neill, Mark

Phillips, Dane

NASA Technical Reports Server

Source of Acquisition I 
NASA Marshall Space Flight Center! 
Stretched Lens Array (SLA) for Collection and Conversion of Infrared Laser Light: 
45% Efficiency Demonstrated for Near-Term 800 W/kg Space Power System 
Mark O’Neill’, Joe Howell2, John Fikes2, Richard Fork3, Dane Phillips3, Dan Aiken4, A.J. McDanal’ 
1. ENTECH, Inc., Keller, TX 76248 USA 
2. NASA Marshall Space Flight Center, Huntsville, AL 35812 USA 
3. University of Alabama-Huntsville, Huntsville, AL 35899 USA 
4. EMCORE Photovoltaics, Albuquerque, NM 87123 USA 
ABSTRACT 
For the past 2% years, our team has been developing a 
unique photovoltaic concentrator array for collection and 
conversion of infrared laser light. This laser-receiving 
array has evolved from the solar-receiving Stretched Lens 
Array (SLA). The laser-receiving version of SLA is being 
developed for space power applications when or where 
sunlight is not available (e.g., the eternally dark lunar polar 
craters). The laser-receiving SLA can efficiently collect 
and convert beamed laser power from orbiting spacecraft 
or other sources (e.g., solar-powered lasers on the 
permanently illuminated ridges of lunar polar craters). A 
dual-use version of SLA can produce power from sunlight 
during sunlit portions of the mission, and from beamed 
laser light during dark portions of the mission. SLA 
minimizes the cost and mass of photovoltaic cells by using 
gossamer-like Fresnel lenses to capture and focus 
incoming light (solar or laser) by a factor of 8.5X, thereby 
providing a cost-effective, ultra-light space power system. 
INTRODUCTION AND BACKGROUND 
ENTECH, NASA, and other team members have been 
developing refractive photovoltaic concentrator systems 
for producing space power from sunlight since the middle 
1980’s [I]. The first such technology developed and 
successfully flown in space was the point-focus mini-dome 
lens array, shown in Fig. 1. This array used mechanically 
stacked GaAslGaSb cells from Boeing in the focal point of 
. .=. , . ..... .. --. , ,” -”. .” , .. ,.-, .-. _. - . . .-. . .-- . ..=. .J 
Test (1994-1995). 
ENTECH’s silicone mini-dome Fresnel lens concentrators. 
The lenses were coated with a multi-layer oxide coating to 
protect the silicone lens material from solar ultraviolet (UV) 
radiation and monatomic oxygen (AO). The mini-dome 
lens array in Fig. 1 flew in 1994-95 on the NASNUSAF 
year-long PASP-Plus flight test in a very high radiation 
elliptical orbit (363 km by 2,550 km at 70-degree 
inclination). Of the 12 advanced photovoltaic array types 
included on PASP Plus, the minidome lens provided the 
highest performance and the least degradation [2]. 
After the minidome lens array success, ENTECH, NASA, 
and other team members next developed the line-focus 
arched lens array, which evolved into the SCARLET array 
that performed flawlessly for the full thirty-eight-month 
mission on NASA’s Deep Space 1 probe, shown in Fig. 2. 
SCARLET (acronym for Solar Concentrator Array using 
Refractive Linear Element Technology) employed silicone 
Fresnel lens material made by 3M using a high-speed 
continuous process. ENTECH laminated this silicone lens 
material to 75-micron-thick ceria-doped glass arches, 
which provided support and UV protection for the lenses. 
Monolithic triple-junction (GalnPIGaAslGe) cells were 
placed in the focal lines of the SCARLET lenses. The 
SCARLET array powered both the spacecraft and the ion 
engine on Deep Space 1 and performed as predicted on 
this highly successful mission [3]. 
Fig. 2. SCARLET Array on NASNJPL 
Deep Space 1 Probe (1998-2001). 
770 FNTFCH I ensea nn I 
https://ntrs.nasa.gov/search.jsp?R=20060024813 2019-08-29T22:01:52+00:00Z
Shortly after the SCARLET array delivery, ENTECH 
discovered a simpler means of deploying and supporting 
the line-focus silicone lenses, thereby eliminating the 
fragility, mass, and cost of the glass arches used on 
SCARLET. The new approach uses simple lengthwise 
tensioning of the lens material between end arches for 
lens deployment and support on orbit, as shown in Fig. 3. 
Called the Stretched Lens Array (SLA), the new ultra-light 
concentrator array also enables a very compact stowage 
volume for launch [4]. Over the past 2% years, our team 
has been developing a new version of SLA which can 
provide power in the absence of sunlight, by collecting and 
converting infrared laser light beamed from another 
location. Results to date have been outstanding, as 
further discussed below. 
I 
rig. a. arrercneo Lens nrray ( 3 ~ )  rrororype. 
LASER VERSION OF STRETCHED LENS ARRAY (SLA) 
The laser version of SLA uses single-junction photovoltaic 
cells in place of the multijunction cells used in the solar 
version of SLA. For infrared laser light in the wavelength 
range of 0.80-0.85 microns, GaAs is an ideal photovoltaic 
cell. To demonstrate the achievable conversion efficiency 
of the laser version of SIA, a number of prototype SLA 
modules were developed using GaAs cells made by 
EMCORE. Fig. 4 shows one of these SLA modules during 
outdoor testing at ENTECH under sunlight irradiance. Of 
course, under sunlight irradiance, the SLA performance is 
significantly lower with single-junction GaAs cells than with 
triple-junction GalnPIGaAslGe cells. Fig. 5 shows a 
typical IV curve and corresponding net SLA module 
efficiency curve for the SLA module of Fig. 4 under 
terrestrial sunlight irradiance. The net SLA module 
efficiency of 22% with a GaAs cell is much lower than the 
typical 30% net SLA module efficiency measured under 
terrestrial sunlight with a triple-junction cell. Testing under 
sunlight was done to ensure that the prototype SLA 
modules were fully functional prior to delivering them to 
UAH for testing under laser irradiance. Fig. 6 shows a 
typical UAH measurement of the SLA module of Fig. 4 
under 0.805 micron laser irradiance. Note that the IV 
curve of Fig. 6 is very similar to the IV curve of Fig. 5, but 
that the overall net SLA module conversion efficiency has 
more than doubled for the monochromatic laser light 
compared to sunlight. The 45.5% net SLA module 
conversion efficiency corresponds to the product of about 
50% cell efficiency times about 92% lens optical efficiency. 
The much higher SLA module efficiency under laser 
irradiance is due to the' module's much better current 
response for laser light (about 0.52 ANV) than for solar 
irradiance (about 0.25 ANV), since the laser wavelength is 
very near the peak spectral response of the GaAs cell. 
SingleJunction GaAs CeliUnder Outdoor Sunlight 
0.70 I I I I I I 20% 
Testing. 
z4Ya 
22% E 
2% 5 
10% 
:: 1 
12% 
0% 2 
6% 
4% 
2% 
10% f 
0.0 0.2 0.4 0.6 0.8 
Voltage M 
I 0% 
1.0 1.2 
Fig. 5. Measured IV Curve for SLA Prototype 
Under Outdoor Sunlight Irradiance. 
1 .o 
0.9 
0.8 
0.7 
5 0.6 
2 0.5 
* 
L 
0' 0.4 
0.3 
0.2 
9.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 
Voltage (V) 
The UAH laser measurements were conducted with 
collimated laser light using a rectangular aperture slightly 
smaller than the SLA module's aperture. The laser input 
power was measured by removing the SLA module and 
2 
>- 1 , 
then measuring the total power of the laser light 
transmitted through the rectangular aperture. 
Parametric performance measurements were made for the 
SLA concentrator modules by varying the 805 nm laser 
power input to the stretched lens, and the lens-tocell 
spacing, with typical results for one module shown in 
Fig. 7. Note that once the laser power exceeds a value of 
about 0.75 W, the net SLA module conversion efficiency is 
relatively constant for all laser power levels and all lens-to- 
cell spacings between 8.8 cm and 9.1 cm. 
Efficiency (K) 43. 
Lens-toCell Laser Power Input to Lens 
u1 
I ,  
Fig. 7. Stretched Lens Array (SLA) Prototype 
Performance for Various Laser Input Power Levels and 
Lens-to-Cell Spacing Variations. 
In addition to measurements on the SLA concentrator 
module as an assembly, the lens was removed and direct 
laser irradiance tests were performed on the cell alone. 
One of the key measurements for the cell was the current 
response as a function of laser wavelength, as shown in 
Fig. 8. The measured current response results were well 
bracketed by calculated curves for quantum efficiency 
values of 86% and 88%. Dividing the current response at 
805 nm with (0.52 A/W) and without (0.56A/W) the lens 
Cell Current Response Versus Wavelength with Stretched Lens 
Removed 
805 810 815 820 825 830 835 040 
Laser Wavelength (nrn) 
Fig. 8. GaAs Cell Current Response for Different Laser 
Wavelengths. 
verifies that the lens net optical efficiency is about 92%. 
All of the UAH measurements on the SLA assembly, 
including those presented in Figs. 6 and Fig. 7 above, 
were made with a 30 W laser operating at a wavelength of 
805 nm. Inspection of Fig. 8 shows that significantly 
higher current response is obtainable with a longer 
wavelength laser. For example, at 835 nm wavelength, 
the current response is 4.6% higher than at 808 nm. This 
implies that the 45.5% net SLA module conversion 
efficiency at room temperature, shown in Fig. 6 and Fig. 7, 
would be over 47.5% for a future flight system using 
835 nm laser input instead of 805 nm laser input. 
SYSTEM-LEVEL PERFORMANCE METRICS 
The laser version of SLA is being developed to utilize the 
same deployment and support structures as the solar 
version of SLA. One very attractive SLA deployment and 
support structure approach is ATK Space Systems’ 
SquareRigger platform. SLA on SquareRigger (SLASR) 
has an unprecedented portfolio of performance metrics [5]. 
A full-scale prototype of one rectangular building block of 
SLASR is shown in Fig. 9. Large SLASR arrays will be 
comprised of multiple rectangular building blocks, or bays, 
of the relatively large size (about 2.5 m x 5.0 m) shown in 
Fig. 9. SLASR is particularly well suited to large space 
power applications. 
Due to the much higher conversion efficiency of the laser 
version of the SLA compared to the solar version, 
proportionally less waste heat is generated by the laser 
version. Therefore, the irradiance of the laser can be 
increased to a value significantly higher than the normal 
space solar constant of 1,366 W/m2 while maintaining the 
cell temperature at the same value as for the solar version 
of SLA. With higher input irradiance and higher 
conversion efficiency, the areal power density of the laser 
version of SLA is very much higher than for the solar 
version of SLA. 
A direct comparison of the key performance metrics of 
both versions of the SLA on the SquareRigger platform is 
3 
shown in Table 1. These performance metrics are based 
on already demonstrated cell and SLA concentrator 
module efficiency levels for both versions of SLA. These 
performance metrics are also based on today’s lens, 
radiator, cell, structure, and deployment mechanism 
materials and mass estimates. For a nominal 100 kW 
solar version of SLA on SquareRigger, the total areal 
mass density of the SLASR array has been estimated by 
ATK Space and ENTECH at 0.86 kg/m2. Using these 
current values for performance and mass, note that the 
key performance metrics for the solar version of SLASR 
are outstanding at 309 W/m2, 359 Wlkg, and 80 kWlm3. 
Note that the key performance metrics for the laser 
version of SLASR are extraordinary at 690 Wlm2, 
803 Wlkg, and 179 kW/m3. 
11 
12 
13 
14 
15 
16 
CONCLUSION 
SLA SquareRigger Gross Areal Power Density 343 W/sq.m. 767 W/sq.m. Item 7 Times Item 9 
WiringlMismatchlPacking Knockdown Factor 90% 90% Typical Factor for Other Losses 
SLA SquareRigger Net Areal Power Density 309 W/sq.m. 690 W1sq.m. Item 11 Times Item 12 
SLA SquareRigger Wing Mass Density 0.86 kg/sq.m. 
SLA SquareRigger Net Specific Power 359 W/kg 803 W/kg Bottom-Line Wing-Level Specific Power 
SLA SquareRigger Stowed Power Density 
0.86 kglsqm. Based on 100 kW Solar Point Design 
,79 kW leu. m. Solar Item 16 Times Laser Item 13 Divided by kw/cu.m. Solar Item 13 
A new version of the Stretched Lens Array (SLA) has been 
developed for collecting and converting infrared laser light 
instead of sunlight. The new laser version of SLA extends 
the unprecedented performance metrics of the solar 
version of SLA to much higher values due to the higher 
conversion efficiency and allowable irradiance of the laser 
version. The laser version of SLA represents an excellent 
candidate for providing lunar surface power for NASA 
lunar exploration missions. Many of these missions will 
require power in locations without sunlight, including the 
lunar polar craters. In addition, even for lower lunar 
latitudes, the long lunar night (about two weeks long) 
presents huge energy storage challenges for conventional 
solar power systems. Beamed infrared laser light from 
lunar-orbiting spacecraft could be efficiently collected and 
converted by the laser version of SLA on the lunar surface 
to provide power for such missions. 
ACKNOWLEDGEMENT 
The authors gratefully acknowledge NASA’s support of the 
work presented in this paper. 
REFERENCES 
[ I ]  Piszczor, M.F. and ONeill, M.J., “Development of a 
Dome Fresnel LenslGaAs Photovoltaic Concentrator for 
Space Applications,” 19th IEEE Photovoltaic Specialists 
Conference (PVSC), New Orleans, 1987. 
[2] Curtis, H. and Marvin, D., “Final Results from the 
PASP Plus Flight Experiment,” 25th IEEE PVSC, 
Washington, 1996. 
[3] Murphy, D.M., “The SCARLET Solar Array: 
Technology Validation and Flight Results,” Deep Space I 
Technology Validation Symposium, Pasadena, 2000. 
[4] O’Neill, M.J., et al., “Development of the Ultra-Light 
Stretched Lens Array,” 29th IEEE PVSC, New Orleans, 
2002. 
[5] O’Neill, M.J. et al., “Stretched Lens Array 
SquareRigger (SLASR) Technology Maturation,” 19th 
Space Photovoltaic Research and Technology (SPRAT) 
Conference, Cleveland, 2005. 
Table 1. Comparison of the Laser Version and the Solar Version of SLA on the ATK Space Systems’ SquareRigger 
Platform Operating on Geostationary Earth Orbit (GEO) with the Same Cell Temperature of 71 C. 


Stretched Lens Array (SLA) for Collection and Conversion of Infrared Laser Light: 45% Efficiency Demonstrated for Near-Term 800 W/kg Space Power System

Stretched Lens Array (SLA) for Collection and Conversion of Infrared Laser Light: 45% Efficiency Demonstrated for Near-Term 800 W/kg Space Power System

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