Transparent polymer films are currently considered for use as solar concentrating lenses for spacecraft power and propulsion systems. These polymer films concentrate solar energy onto energy conversion devices such as solar cells and thermal energy systems. Conversion efficiency is directly related to the polymer transmission. Space environmental effects will decrease the transmission and thus reduce the conversion efficiency. This investigation focuses on the effects of ultraviolet and charged particle radiation on the transmission of selected transparent polymers. Multiple candidate polymer samples were exposed to near ultraviolet (NUV) radiation to screen the materials and select optimum materials for further study. All materials experienced transmission degradation of varying degree. A method was developed to normalize the transmission loss and thus rank the materials according to their tolerance of NUV. Teflon(Tm) FEP and Teflon(Tm) PFA were selected for further study. These materials were subjected to a combined charged particle dose equivalent to 5 years in a typical geosynchronous Earth orbit (GEO). Results from these NUV screening tests and the 5 year GEO equivalent dose are presented

Bowden, Mary L.

Edwards, David L.

Hubbs, Whitney C.

Piszczor, Michael F., Jr.

Willowby, Douglas J.

English

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Reprinted From
Solar Engineering
Editor=: D. E. Claridga, and
J. E. Pache¢o
Book No. G01050- 1997
Space Environmental Effects on the Optical Properties
of Selected Transparent Polymers
David L. Edwards
Physical Sciences and Environmental
Effects Branch, NASA, MSFC, AL 35812
Phone: 205-544--4081/Fax: 205-544-5103
E-mail: david.edwards @ msfc.nasa.gov
Whitney C. Hubbs
Physical Sciences and Environmental
Effects Branch, NASA, MSFC, AL 35812
Phone: 205-544-8375/Fax: 205-544-5103
E-mail: whitney.hubbs@ msfc.nasa.gov
Douglas J. Willowby
Electrical Power Branch, NASA, MSFC, AL 35812
Phone: 205-544--3334/Fax: 205-544-5841
E-mail: douglas.willowby@ msfc.nasa.gov
Michael F. Piszczor, Jr.
Photovoltaic and Space Environments Branch
NASA, LeRC, Cleveland, OH 44135
Phone: 216-433-2237/Fax: 216-433-6108
E-mail: michaeLpiszczor@ lerc.nasa.gov
Dr. Mary L. Bowden
AEC Able Eng'ineering, Inc., Goleta, CA 93116-0588
Phone: 301-405-0011/Fax: 301-314-9001
(University of Maryland)
ABSTRACT
Transparent polymer films are currently considered for use as solar
concentrating lenses for spacecraft power and propulsion systems.
These polymer films concentrate solar energy onto energy conversion
devices such as solar cells and thermal energy systems. Conversion
efficiency is directly related to the polymer transmission. Space
environmental effects will decrease the transmission and thus reduce
the conversion efficiency. This investigation focuses on the effects of
ultraviolet and charged particle radiation on the transmission of
selected transparent polymers. Multiple candidate polymer samples
were exposed to near ultraviolet (NUV) radiation to screen the
materials and select optimum materials for further study. All materials
experienced transmission degradation of varying degree. A method was
developed to normalize the transmission loss and thus rank the
materials according to their tolerance of NUV. Teflon TM FEP and
Teflon TM PFA were selected for further study. These materials were
subjected to a combined charged particle dose equivalent to 5 years in a
typical geosynchronous Earth orbit (GEO). Results from these NUV
screening tests and the 5 year GEO equivalent dose are presented.
INTRODUCTION
Recently, much emphasis has been placed on the concentration of
solar energy for spacecraft propulsion and electrical power systems.
Increasing interest in lightweight structures that possess small stowage
space and deploy on orbit are driving development towards flexible
solar concentrators.
199
The testing reported in this paper was a joint effort between the
National Aeronautics and Space Administration (NASA) Marshall
Space Flight Center (MSFC) and Lewis Research Center (LeRC), and
AEC Able Engineering Co., Inc., to evaluate candidate materials for the
Solar Concentrator Arrays with Refractive Linear Element Technology
(SCARLET) program. SCARLET is a joint Ballistic Missile Defense
Organization (BMDO) and NASA program to develop and demonstrate
advanced photovoltaic concentrator array technology that is applicable
to a variety of government and commercial space missions (Jones and
Murphy, 1996). Under this program, AEC Able will fabricate a 2.6 kW
concentrator solar array as the primary power source for NASA's New
Millennium Program Deep Space One (DS-I) spacecraft (Casani).
A variety of transparent polymers were initially studied for use as
the material of choice for the arched linear Fresnel lens optics.
However, schedule demands and the lack of extensive environmental
test data precluded the use of these materials for the DS-1 mission.
Still, these materials offer a number of significant advantages for the
further implementation of this technology and will continue to be
studied.
The Physical Sciences and Environmental Effects Branch was
selected to characterize the candidate transparent polymers provided by
AEC Able. The testing conducted under this effort did not include any
effect of solar concentration due to Fresnel optics, but examined the
total transmission degradation of the polymer. The ultimate goal was to
determine an optimum transparent polymer from a large selection of
commercially available candidate materials. Transmission was
determined to be the characterizing property for optimum material
https://ntrs.nasa.gov/search.jsp?R=19970026598 2020-06-16T01:27:29+00:00Z
selectionsi celoss of transmission of the polymer lens leads to a drop
in solar cell power output. Clearly, maintaining high optical trans-
mission throughout the life of the mission is essential to the
performance of the solar cells.
This investigation centers on the transmission evaluation of
commercially available polymers when exposed to a simulated space
environment. In order to compare the various candidates, a technique
was developed that normalized the transmission degradation and thus
allowed the polymers to be ranked according to transmission tolerance
to space environmental effects.
EXPERIMENTAL PROCEDURE
Description of the Facility
The MSFC Physical Sciences and Environmental Effects Branch
investigates the effects of the space environment on materials. This
branch operates several high vacuum NUV and vacuum ultraviolet
(VUV) exposure systems, atomic oxygen (AO) exposure systems, and a
unique combined environmental effects (CEE) test system. The CEE
system offers the capability to irradiate samples to a simultaneous or
sequential exposure to protons, low-energy electrons, high-energy
electrons, NUV, and VUV (Edwards et al., 1996). The CEE system also
allows in-vacuum reflectance measurements of the temperature
controlled samples.
These test systems are verified to be contamination-free prior to
any testing by exposing an optical witness sample (OWS) to NUV for
1 week in vacuum. The NUV is included because it enhances
contaminant photodeposition (e.g., photofixing). The OWS of greatest
sensitivity to contamination photodeposition is an MgF2/AI mirror. This
OWS mirror is characterized, pretest, using a VUV reflectance system.
The mirror is then exposed to NUV as described above and analyzed,
posttest, using the same VUV reflectance system. The criterion to pass
the contamination-free verification test is less than +_3% AR/R for all
wavelengths in the range from 121 to 200 nanometers (nm).
Screening Test
Eleven transparent polymers (see Table I) were selected by AEC
Able for the first screening test. This screening test consisted of
simultaneously exposing all samples to an equivalent dose of NUV and
then selecting the most NUV-tolerant materials for further testing.
Pre-exposure transmission spectra were obtained on these i 1
samples using a Lambda 19 spectrometer analysis system. Trans-
mission spectra were obtained over the wavelength range from 185 to
2200 nm. The analytical normalization techniques, discussed in the
Analytical section, were applied over the wavelength range from 300 to
1000 nm, because this range encompasses the operating range of
several types of solar cells.
The samples were placed in the vacuum test system and the NUV
exposure was initiated when the vacuum level reached 5x10 -_ torr.
Based on pretesting characterization, the NUV intensity was
determined to be, on average, 2.75 NUV Suns. This intensity was deter-
mined by integrating the NUV source output over the wavelength range
from 250 to 400 nm and comparing the results to the accepted values
for a solar constant at air mass zero (AM0) (ASTM E 490-73a, 1992).
The samples were continuously exposed to NUV for 474.5 hours and
with the NUV intensity at 2.75 NUV Suns, which yields an exposure
duration of approximately 1300 equivalent Sun hours (ESH). Figure 1
shows the transmission spectra of one of the 11 samples exposed to
! 300 equivalent Sun hours of NUV.
TASLE 1. Eleven transparent polymers selected for the first screening test, an exposure to 1300 ESH NUM.
Solar transmissivity values are shown for control and exposed polymers.
Material
Control Exposed Transmission
Transmissivity Transmissivity Degradation
(Tc) % (Te) % (Td) %
Lexan (7 mils) 77.92 56.41 21.51
Triton COR (2 mils) 85.30 59.43 25.87
UVR T2 Tefzel (2.5 mils) 84.33 77.55 6.78
Acrylic (10 mils) 89.28 65.34 23.94
LaRC CP1 (1 mil) 74.12 64.64 9.48
Tefzel 500 17 (5 mils) 90.60 74.60 16.00
T2 Teflon TM PFA (4 mils) 94.66 83.71 10.95
T2 Tefzel 500 ZM (5 mils) 90.38 78.35 12.03
Teflon TM PFA ( 3 mils) 94.31 76.09 18.22
UVR T2 Tefzel (5 mils) 82.25 74.84 7.41
UVR Coated PET (7 mils) 78.57 55.52 23.05
200
100
C
0
°_
E
e-
l=
80
60
40
20
400
•---- Control Transmission
.......... Exposed Transmission
0 I I I i I r ! I i
200 600 800 1,000
Wavelength (nm)
FIGURE1.Transmission spectra of LaRC CP1 before and after exposure to 1300 ESH NUV in vacuum.
AEC Able identified several more candidate samples and
requested a second NUV screening test. Pre-exposure transmission
spectra were obtained and the sample exposure was initiated exactly as
in the first screening test. Approximately 153 hours into this test, the
NUV source failed and the test was prematurely terminated. The
samples were analyzed and the test duration was recorded as a 420
equivalent Sun hours of NUV exposure. These samples and the test data
are shown in Table 2.
Two materials, Teflon TM FEP (10 mils) and Teflon TM PFA (10
mils), were selected for an exposure to a combined effects environment,
This test was a combined charged particle exposure equivalent to
5 years in a typical GEO. This dose was determined by calculating the
radiation dose as a function of depth in Teflon TM for a 5 year external
exposure in a typical GEO (Coggi, 1996). This 5 year GEO dose-depth
profile was compared to calculated dose-depth profiles in Teflon TM that
were obtained using charged particle energies available in the CEE test
system. The energy and fluence parameters of the CEE-available
sources were analytically varied to produce a test dose-depth profile
that best fit the 5 year GEO dose-depth profile obtained by Coggi
(1996) (see Fig, 2).
TABLE 2. Polymers exposed to the second screening test, an exposure to 420 ESH NUV.
Solar transmissivity values are shown for control and exposed polymers.
Material
Control Exposed Transmission
Transmissivity Transmissivity Degradation
(Tc) % ('re) % ('rd) %
Teflon TM PFA (10 mils) 92.50 89.15 3.35
Teflon TM FEP (10 mils) 93.34 92.95 0.39
Tefze1500 LZ (5 mils) 90.40 88.11 2.29
Aclar (5 mils) 92.86 92.40 0.46
UVR Teflon TM PFA (5 mils) 76.72 74.53 2.19
201
50KeVelectrons:2.0 E15 e--/sq.cm
220 KeV electrons: 1.35 E15 e-/sq.cm
500 KeV electrons: 5.0 E14 e-/sq.cm
100
100 KeV protons: 1.05 E15 p+/sq.cm
200 KeV protons: 1.05 E14 p+/sq.cm
300 KeV protons: 2.5 E13 p+/sq.cm
400 KeV protons: 1.0 E13 p+/sq.cm
500 KeV protons: 5.0 E12 p+/sq.cm
700 KeV protons: 1.7 E12 p+/sq.cm
NUV: 100 ESH
O
E
¢0
C
I-
90
80
70
60
5O
40
30
20
10
0
200
,/
I/
............. Ex_s_
l Contml(FEP)
.... Ex_s_
--Control(PFA)
FEP T c = 93.28%
FEP T e = 86.72%
FEPTd = 6.56%
PFA T c = 92.22%
PFA T e = 87.42%
PFA T_ = 4.8%i
400 600 800 1,000
Wavelength (nm)
FIGURE2. Transmission spectra of Teflon TM FEP andTeflon TM PFA before and after exposure to a 5 year equivalent GEO dose.
Transmission specwa of Teflon TM FEP and Teflon TM PFA were
obtained prior to CEE exposure. The two samples were placed in the
CEE test chamber and a base vacuum of 5xl0-S tort was established.
The sample holder was maintained at 20 °C throughout the 5 year
equivalent GEO charged particle exposure. Sample exposures varied
from day to day but each source maintained a flux of I nA/cm 2 during
irradiation. NUV exposure was included during each charged particle
irradiation. The Teflon TM FEP and Teflon TM PFA samples were
removed from the CEE system once the 5 year equivalent GEO charged
particle dose was achieved and transmission spectra were obtained (see
Fig. 2).
ANALYTICAL
The values for solar transmissivity, Tc and "re as shown in Tables 1
and 2, were calculated using a technique modified from the procedure
described by Wilkes (1969). The ASTM document E 490-73a lists the
solar irradiance at AM0 over the wavelength range from 115 nm to
Ix 106 nm. The numerically integrated solar irradiance is quoted as
1353 W/m 2 over this wavelength range and is referred to as l solar
constant at AM0. The modified technique applied in this effort utilized
102 specific wavelengths ranging from 300 nm to I000 nm. This
wavelength range was selected because it encompasses the operating
range of several types of solar cells. Also, data obtained by reducing the
transmission spectra in this wavelength range is applicable for solar
thermal efforts. The individual irradiance values were determined using
a linear interpolation which reduced the irradiance values to units of
W/cm2*nm.
The solar transmittance, in W/cm2, of the polymer was determined
by multiplying the transmission of the polymer (T'k) at a specific
wavelength, and the solar irradiance (SZ) at the same specific
wavelength. This calculation yields the solar transmittance, in W/cm 2,
of the polymer at a specific wavelength. This is expressed as:
(Ts)x=(TOx (so (l)
The values of (Ts)_ are summed over the wavelength range from
300 to 1000 nm as:
1000
_ (Ts);t= Ts
Z=300
(2)
It is important to note that this summation utilizes only 102 specific
wavelengths out of the 700 wavelengths that are possible in this range.
The total solar irradiance (S) is defined, in this technique, as the
sum of the 102 specific wavelengths over the range from
300 to I000 nm. This is expressed as:
1000
,__S_ =s
k=300
(3)
The value of S is constant and was determined to be 0.01413 W/cm 2.
202
The solar transmittance of the polymer was determined by the
following:
TslS x 100% = T (4)
Tables 1 and 2 list the values of Tc and Te where Tc =- solar
transmittance of the polymer before exposure and Te -- solar
transmittance of the polymer after exposure. These solar transmittance
values can be used to analytically determine solar cell output provided
the conversion efficiency of the solar cell is known. A GaAs solar cell
was selected to demonstrate this procedure. The conversion efficiency
of a standard GaAs solar cell is shown in Fig. 3 (lies, 1990).
The power generated by the solar cell at each wavelength (P'L) can
be determined by
PX = OX x SX (5)
where e_. - conversion efficiency at a specific wavelength _. and S_. ---
solar irradiance at AM0 at wavelength _.. Summing the values of P'_
over the wavelength range from 500 nm to 900 nm for GaAs yields a
total power generated of 0.0254 W/cm 2.
The power generated by a solar cell with a transparent polymer in
the optical path can be determined by the following,
P_. = O k x (Ts)z. (6)
The total power generated by the solar cell with a transparent polymer
in the optical path illuminated by 1 solar constant at AM0 is given by
900
_P_ =P
_.=500
(7)
Solar cell power values, with the effect of a transparent polymer in the
optical path, were calculated for several polymers shown in Table 1.
These values of solar cell power are shown in Table 3.
60
' I i t i i I i i i i i
600 700 800
Wavelength (nm)
9OO
FIGURE3. Conversion efficiency versus source wavelength
for GaAs solar cells with an input power of I W/cm2.
TABLE3. GaAs solar cell power calculated with transparent polymer material in the optical path.
Solar Cell Power Calculated With Material In Optical Path
Material Material
Pre-Exposure Post-Exposure Exposure
Material W/cm 2 W/cm2 Environment
Lexan 0.0226 0.02 1300 ESH NUV
LaRC CP1 0.0227 0.0215 1300 ESH NUV
Tefzel 500 ]7 0.0235 0.0217 1300 ESH NUV
Acrylic 0.0246 0.0221 1300 ESH NUV
UVR T2 Tefzel 0.023 0.0221 1300 ESH NUV
Teflon TM PFA 0.0242 0.0217 1300 ESH NUV
Triton COR 0.0225 0.0201 1300 ESH NUV
Teflon TM FEP 0.024 0.0236 5 year GEO
203
CONCLUSIONS
Transparent polymer usage as a solar concentrating lens is
dependent on the ability of the polymer to maintain optical integrity
while in the space environment. These flexible polymer lenses offer a
number of distinct advantages from both the power system cost and
manufacturability perspectives. Even with these advantages, these
materials must demonstrate long-term optical performance in the
relevant space environment.
A large number of commercially available transparent polymers
have been investigated to determine the effect of various simulated
space environmental exposures on the polymer transmission. A method
was presented that calculates solar transmissivity over the wavelength
range from 300 to 1000 rim. This method allows the polymers to be
ranked according to the tolerance of the space environmental exposure.
A technique was also described that demonstrates the effect of
transmission degradation on the power output of a standard GaAs solar
cell. This technique can be used first to choose an optimum polymer/
cell pair and second, to determine the solar cell surface area needed to
maintain spacecraft power requirements throughout the mission
lifetimes.
The results obtained in this study provide a small, but significant,
contribution to the database necessary to further the development and
implementation of transparent polymers for space. Continued material
development and quantitative testing is expected as the applications for
these materials continue to expand.
REFERENCES
ASTM E 490--73a, "Standard Solar Constant and Air Mass Zero
Solar Spectral lrradiance Tables," 1992.
Casani, Kane, Program Manager, NASA/Jet Propulsion
Laboratory, the New Millennium Program Website: http://
nmp.jpl.nasa.gov.
Coggi, John, The Aerospace Corporation, work performed, 1996.
Edwards, D.L., et al., NASA TM-108518, "Radiation Induced
Degradation of the White Thermal Control Paints Z-93 and Z--93P,"
Oct. 1996.
Iles, Peter, Solar Cell Conversion Efficiency Source Material,
1990.
Jones, P.A., and Murphy, D.M., "Linear Refractive Photovoltaic
Solar Array Flight Experiment," AIAA Journal of Propulsion and
Power, Sept.--Oct. 1996, pp. 859-865.
Wilkes, Donald R., NASA TM-53918, "A Numerical Integration
Scheme to Determine Hemispheric Emittance, Solar Absorptance and
Earth Infrared Absorptance from Spectral Reflectance Data,"
Sept. 16, 1969.
204


Space Environmental Effects on the Optical Properties of Selected Transparent Polymers

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