The development of an advanced photovoltaic power system that would have application for a manned lunar base is currently planned under the Surface Power element of Pathfinder. Significant mass savings over state-of-the-art photovoltaic/battery systems are possible with the use of advanced lightweight solar arrays coupled with regenerative fuel cell storage. The solar blanket, using either ultrathin GaAs or amorphous silicon solar cells, would be integrated with a reduced-g structure. Regenerative fuel cells with high-pressure gas storage in filament-wound tanks are planned for energy storage. An advanced PV/RFC power system is a leading candidate for a manned lunar base as it offers a tremendous weight advantage over state-of-the-art photovoltaic/battery systems and is comparable in mass to other advanced power generation technologies

Brinker, David J.

Flood, Dennis J.

English

NASA Technical Reports Server

PRECEDING P_,GE i3!..ANK :,"T F!LMED
N9 3-14
ADVANCED PHOTOVOLTAIC POWER
SYSTEM TECHNOLOGY FOR
LUNAR BASE APPLICATIONS
DavidJ. Brinker and DennisJ. Flood
NASA Lewis Research Center
Cleveland OH 44135
593
004
INTRODUCTION
The establishment of a permanently manned presence on the
lunar surface represents a formidable challenge to a broad
spectrum of space technologies. While all the technologies that
will be required to sustain the evolution of a lunar base from its
initial establishment as an outpost to its final manifestation as a
permanent, life-sustaining, and productive habitat are essential, the
pacing technology for it all is the production of power. A new
aspect of such an endeavor is that the "mission" requirements are
no longer fixed, but will evolve over time. It is now necessary
to examine and develop a time-dependent set of requirements for
the power system, and to put into place an adequately supported
research and development program that is properly phased to
produce the needed technology at the right time. The Lewis
Research Center, as the lead center for space power for the Office
of Aeronautics and Space Technology (OAST), has taken the first
steps in that direction with the implementation of a program in
High Capacity Power and the impending implementation of
programs in Surface Power and Rover Power. All the preceding
initiatives are the outgrowth of planning activities that have been
conducted by OAST over the past few years, and which have
culminated in the establishment of the Civil Space Technology
Initiative (CSTI) and the Pathfinder program. The High-Capacity
Power program is an element of CSTI, and the Surface Power and
Rover Power programs are elements of Pathiinder.
POWER SYSTEMS MASS COMPARISON
While the definition of a complete set of time-dependent re-
quirements is an unfinished task, an understanding of key issues
has been developed to help guide the focused technology
programs mentioned above. Technologies intended for application
on the lunar surface will be driven by mass considerations,
primarily because of the high cost of payload delivery to the
Moon. Even if the assumption is made that low operational cost
cargo vessels will be available for transit from low Earth orbit
(LEO) to the Moon, there will still be a high cost for delivery
to LEO that must be considered. For comparison purposes the
cost can be represented by a payload mass multiplication factor
that takes into account the total hunch mass required to deliver
the intended lunar bases elements to LEO. Although a universally
agreed-on value for such a multiplier does not exist, primarily
because the exact nature of future heavy-lift hunch capabilities
is not known, a value of 5 has been assumed for this discussion,
along with an assumed heavy-lift vehicle (HLV) payload capability
of 92,000kg (200,0001b) to LEO. Such assumptions are not
unreasonable with respect to future launch ,systems. No further
justification for using them will be provided except to point ont
that doing so allows a quantitative comparison of power system
alternatives in terms of "operational" impact--the number of
launch vehicles required to deliver the system elements to LEO
for subsequent trans'port to the lunar surface.
The key figure of merit for a photovoltaic array is the power
per unit ma,ss in watts per kilogram (W/kg). For a storage system
the appropriate figure of merit is the amount of available energy
per unit mass in watt-hours per kilogram (Whr/kg). The advanced
power system uses an ultralightweight photovoltaic array and an
advanced hydrogen-oxygen regenerative fuel cell (RFC) for
storage. The figures of merit for both ,systems are listed in Table 1.
Table 2 compares the system masses for a state-of-the-art
photovoltaic generation/battery storage system sized to deliver
lOOkW to a lunar base to that performance projected for an
advanced version of such a system. Two cases are considered for
the 336-hr luna_ night: a 100% duty cycle and a 20% duty cycle.
Also shown is the mass saved in delivering the advanced system
to LEO, along with the resulting number of HLV launches saved,
under the assumptions given above. The final column of the table
shows the additional number of HLV launches that would be saved
by using the SP-100 nuclear power system currently under de-
velopment, and intended to have a specific power of 33 W/kg.
The table provides compelling evidence that there is a substantial
payoff to be had in developing the advanced PV/RFC technology,
particularly when placed in the "operational" context of the
weight saved at lEO. A third case also exists, that in which the
astronauts' stay would be limited to the 336-hr lunar day with
a night duty cycle of zero, or close enough to zero so that lander
energy storage would be sufficient. In this scenario, only a
photovoltaic array would have to be delivered to the lunar suface.
A state-of.the-art PV array to supply lOOkWe has a mass of
1515kg, while an advanced array would weigh only 333 kg, a
significant savings under a restricted mass budget.
TABLE 1. Figure of merit comparisons for photovoltaic/electrochemical
technology options.
State-of-the-art Advanced
Array 66 W/kg, OAST-I 300 W/kg, ultralightweight
Storage 14 Whr/kg, NiH battery 1000 Whr/kg, H-O RFC
https://ntrs.nasa.gov/search.jsp?R=19930004816 2020-03-17T08:49:58+00:00Z
594 2nd Conference on Lunar Bases and Space Activities
TABLE 2. Comparison of current and advanced photovoltaic power systems for a manned lunar base.
Power Level Night Duty SOA PV/banery ADV PV/RFC Weight Saved HLV Launches Additional HLVS
(KWe) Cycle Mass (kg) Mass (kg) At LEO (kg) Saved Saved with SP-IO()
100 100% 1,680,000 34, 350 7,910,000 87 1.6
1O0 20% 336,420 7,133 1,580,000 17.4 0.2
Figure 1 provides a more graphic comparison between the mass
of the SOA phovoltaic/battery system, the advanced PV/RFC
system, and the SP-IO0 nuclear power system. As can be clearly
seen, the advanced PV/RFC technology has the potential to reduce
the mass of a l OO-kWe lunar stLrface power system using state-
of-the-art technology by more than a factor of 45, to a value less
than 2.5% of the mass of the latter. (The SP-IO0 system, even
though projected to be lighter than the advanced PV/RFC ,system
by a factor of 10, will only save a little more than another 2%
of the SOA system mass.) The long lunar night is clearly the major
issue in determining the mass of the lunar base photovoltaic-
electrochemical storage system. The key feature that allows such
a large mass reduction is that the stored energy in an advanced
RFC system is in the form of gaseous reactants stored in high-
pressure tanks, with the result that the RFC can approach
1000 Whr/kg, a factor of 4 or 5 better than that projected for
advanced batteries, and a factor of more than 60 better than SOA
batteries (NiH, for example). The remainder of this paper
contains a more detailed description of the technology that will
be pursued in the Sur_ce Power program to achieve these gains.
PHOTOVOLTAIC ARRAY TECHNOLOGY
The key figure of merit for a photovoltaic array is the power
per unit mass, also referred to as the specific power. A
photovoltaic array consists of a number of solar cells intercon-
nected to provide the required voltage and current levels to the
electrical load, usually through a power management and
distribution system. The cells are mounted on a substrate that can
be either rigid, such as honeycomb panels, or flexible, such as
kapton. The cells, substrate, protective diodes, and wiring harness
'2,O0 --
1.60
I. 2O
tc
.8o
.4O
_////////
_///////
_H/H///
_////////
Y////_
SOA PV/NIH
tlO
_ _o
_ 1o
o
/ \
/ \
/ \
\
//
ADV PV/R,_ C
SYSTEM TYPE
SP-IO0
Fig. 1. Power systems rn_ss comparison.
constitute the blanket. The remaining portion of the photovoltaic
array is the mechanical structure, which includes the stowage
container, the deployment mechanism, and the struts to maintain
the blanket in a planar configuration pointed at the sun.
Improvement in the specific power can be achieved through two
different, although often coupled, approaches: increasing the
conversion eflficlency of the solar cell and reduction of the cell/
blanket mass and/or array structure mass. Improvements in cell
efficiency not only increase the array specific power, but also
decrease array area if a fixed power level is required. For a system
such as that envisioned for a rover vehicle, reduction in array area
can be critical.
The program objective in the Surface Power program of
PathlYmder is an array specific power of 300 W/kg at air mass zero
(AMO) insolation (solar insolation at 1 AU). At present, light-
weight photovoltaic arrays have been demonstrated on a space
shuttle experiment (OAST-1 ) at 66 W/kg. A recent design, under
development at the Jet Propulsion Laboratory for OAST was
established at 130W/kg (Scott-Monck and Stell_ 1986). This
design, the Advanced Photovoltaic Solar Array (APSA), is based
on 2-mil thick silicon cells. These two array designs are intended
for the zero gravity conditions of LEO and geosynchronous Earth
orbit (GEO). For lunar base applications, the array structure must
be rugged enough to withstand the 1/6 g of the lunar surface.
To achieve the 300 W/kg specific power goal, two solar cell
technologies have been identified for further development. These
candidate cell types are nltrathin gallium arsenide (GaAs) and
amorphous silicon (a-Si). Table3 summarizes the current
performance of technologies to be developed for a lunar base
power system and their current performance. Gallium-arsenide
cells are currently manufactured for space use at an efficiency of
about 18%, with research devices achieving 21%. However, the
current cell is too thick at 200-250/_m to give the performance
needed for lunar base application. Fortunately, because it is a
direct-gap semiconductor, GaAs absorbs all photons available for
energy conversion within 3-4 #m of the impinging surface. This
allows, unlike crystaUine silicon, for an ultrathin, high-efficiency
cell to be produced. Gallium-arsenide cells 5.5 #m thick have
been fabricated using the cleaved lateral epitaxy for film transfer
(CLEFT) process (Fan et al., 1984), a technique in which a
single-crystal thin GaAs layer is grown on a masked GaAs substrate
and mechanically removed. Other processes, such as chemical
thinning of the substrate, have also been successfully demon-
strated as capable of producing high-quality, ultrathin layers and
cells. Basic research and development in cell interconnectors and
cell incorporation into a space-compatible blanket will be critical
becaLLse (ff the brittleness of the ultrathin GaAs cells.
Amorphous silicon is primarily a terrestrial photovoltaic
material; however, 9% space performance has been measured. The
electronic structure of the disordered, amorphous material allows
for total cell thickness of less than 1 #m and the use of flexible
substrates. This is compatible with a very high blanket specific
power and low-volume storage requirements. An extensive
Brinker and Flood.. Adt,anced photovoltaic power technology 595
TABLE 3. Technology status and design projections.
Lunar Base Design Current Performance
Photovoltaic Devices
Gallium Arsenide 25% AMO Eft. 21%
Amorphous Silicon 15% AMO Eft. 9%
Array Structure
Specific Power 300 W/kg (APSA) 66 W/kg (OAST-1 )
Energy Storage
High-Pressure Gas 1000 Whr/kg 300 Whr/kg (Primary
Regenerative Fuel Cell 60% Eft. 60%Eff. Fuel Cell)
manufacturing base already exists for a-Si terrestrial solar cells;
however, several major hurdles must be overcome before it can
be considered as a viable space cell candidate. Among these are
low conversion efficiency and cell performance degradation under
constant illumination. Although terrestrial arrays are manufactured
on flexible, rugged substrates, few of the materials used are com-
patible with space requirements, necessitating basic studies in
blanket materials and design.
Additional improvement in the photovoltaic array specific
power can be achieved by minimizing the mass of the array struc-
ture. For the APSA design, the structure, blanket box, and deploy-
ment mechanism constitute more than 50% of the mas,s of the
entire array. Research and development on the array structure are
also warranted by the need, for the first time, for a space solar
array to operate in a continuous gravity field. An APSA wing is
pictured in Fig. 2, along with a detailed cross section of its blanket.
Its design specific power of 130W/kg is met with 13.5% efficient,
63-/_m-thick silicon cells. Replacing the silicon cells with GaAs
cells of 25% efficiency, assuming the same blanket mass and
eliminating the 5% mass contingency built into the design, yields
a specific power of 260 W/kg, quickly approaching the lunar base
goal. This also assumes that a reduced gravity structure will weigh
no more than the zero-g APSA structure, which is possible since
manual deployment or erection is an option for a manned lunar
base and could eliminate the deployment motor and mast.
Figure 3 shows the approach taken by NASA toward a 300-W/kg
zero-g array. Improvements in the structure and cell interconnec-
tor wiring, coupled with a high-efficiency cell, will enable
attainment of this performance level. These improvements, as well
as the overall design experience gained with zero-g arrays, will
be incorporated into the hmar base array structure.
REGENERATIVE FUEL CELL TECHNOLOGY
At present only primary fuel cells exist, and regenerative cells,
which do not limit mission time or power availability by the
amount of hydrogen and oxygen that can be carried along, have
not been designed. The primary focus of RFC research for a lunar
base power system will be on fuel cell stack configurations
including oxygen electrode catalysts, thermal and gas manage-
merit, and lightweight, high-pressure, robust tank technologies.
The principal effect of the 336-hr duration of the lunar night is
the requirement for a very large fuel cell reactant ma_s. Therefore,
significant mass gains can be made by reduction of the storage
tank ma,ss. Figure 4 illustrates the effect of storage duration on
300 --
250
2OO
150
2o.SI CELL- - -
5o
STRUCTURE
INTERCO#INECTOR- - -
GRIDDED BACK CONTACT--
- LOW ALPHA (O.GS) - -- --
13.5Z CELL .........
62 pM CELL-
SO _M COVER - - -
-
BASELINE - -
EARLY 80's RID 80'S LATE 80's/
EARLY SO's
Fig. 3. High-performance solar array research and technology.
---GO0 ]NCHES----------'_V_ BLANKET PLUS 3
PANEL
LEADER PANELS
..... i=
r _ MD T_R_L ERITTANCE
I CO_I_
]F CRX GLASS CO.R, _ pMSIL_R-PLATED
i
INV_ I/
INTERCONNECTOR7 I,/r u
I -- SlLIC_ AD_ESI_. 38 rM
/
/
,, I;/IIF 63 pM SILIC_ _L_ CELL
__ SILIC_ ADHESI_, _ UM
xX
x--S gM KAPT_ SU_TRATE
d_
uJ
1500 --
1200 --
900 --
600 --
300 --
CRYO TANKS
I
I
I
I
l
I/
,7
//
!
I
/
FILAMENT WOUND TANKS
X SHUTTLE ORBITER
/ METALTANKS
/ _ SODIUM SULFUR BATTERY
1 lO lOO lOOO
LOG STORAGE DURATION, HR
Fig. 4. Approximate energy density characteristic of fuel cell _tems _LS
Fig. 2. Advanced photovoltaic solar array (AI_) wing and blanket, a function of tank _T)e and storage duration.
596 2nd Conference otl Lunar Bases and Space Activities
RFC system energy density for several tank types (L. H. Thaller,
personal communication, 1988). For the high-pressure gas storage
system chosen for the lunar base, the use of filament-wound tanks
enables the storage system energy density to approach 1000 Whr/
kg. This is exceeded only by cryogenic reactant storage, which
at present has application for primary fuel cells only and is not
viable for the lunar base mission.
SUMMARY
The development of an advanced photovoltaic power system
that would have application for a manned lunar base is currently
planned under the Surface Power element of Pathfinder. Sig-
nificant mass savings over state-of-the-art photovoltaic/battery
systems are possible with the use _ff advanced lightweight solar
arrays coupled with regenerative fuel cell storage. The solar
blanket, using either ultrathin GaAs or amorphous silicon solar
cells, would be integrated with a reduced-g structure. Regener-
ative fuel cells with high-pressure gas storage in filament-wound
tanks are planned for energy storage.
In conclusion, an advanced PV/RFC power system is a leading
candidate for a manned lunar base as it offers a tremendous
weight advantage over state-of-the-art photovoltaic/battery s3_ems
and is comparable in mass to other advanced power generation
technologies.
REFERENCES
F.,n J. C. C., McClelland R. W., and King B. D. (1984) _ cleft solar
cells for space applications. In Proceeda'ngs of the 17th Photovoltaic
Specialists Conference, pp. 31-35. IEEE, Piscataway, New Jersey.
Scott-Monck J. and Stella P. (1986) Advanced photovoltaic solar array"
program--A preliminary assessment. In 21st lntersociety Energy
Conversion Engineenng Conference, VoL 3, pp. 1458-1461. American
Chemical Society, Washington, DC.


Advanced photovoltaic power system technology for lunar base applications

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