Advanced Photovoltaic Power System Development at the U.S. Air Force Research Laboratory

Abstract

Photovoltaics continue to be the primary source of electrical power for most near-Sun space missions. The desire to enhance or enable new space missions through higher efficiency, increased specific power (W/kg), increased volumetric power density (W/m3) and improved radiation resistance, along with decreased costs, continues to push the development of novel solar cell and array technologies. To meet present and future space power requirements, advanced multijunction solar cells and novel cell technologies are being pursued. These efforts have resulted in a continual advancement in performance, but new paradigms will be required to continue that performance trend. Similarly, new array technologies are being investigated and developed to meet the ever increasing power system performance requirements.

Full text

ADVANCED PHOTOVOLTAIC POWER SYSTEM DEVELOPMENT AT THE U.S. AIR
FORCE RESEARCH LABORATORY
John Merrill(1), David Wilt(1), David Chapman(1), Geoff Bradshaw(1), Kyle Montgomery(1), Nathan Gapp(1) and
Bernie Carpenter(2)
(1) US Air Force Research Laboratory, 3550 Aberdeen Ave SE, KAFB, NM 87117, Email:
AFRL.VSSVOrgMailbox@us.af.mil
(2) Aerospace Corporation, Email: Bernie.F.Carpenter@aero.org
ABSTRACT
Photovoltaics continue to be the primary source of
electrical power for most near-Sun space missions. The
desire to enhance or enable new space missions through
higher efficiency, increased specific power (W/kg),
increased volumetric power density (W/m3) and
improved radiation resistance, along with decreased
costs, continues to push the development of novel solar
cell and array technologies. To meet present and future
space power requirements, advanced multijunction solar
cells and novel cell technologies are being pursued.
These efforts have resulted in a continual advancement
in performance, but new paradigms will be required to
continue that performance trend. Similarly, new array
technologies are being investigated and developed to
meet the ever increasing power system performance
requirements.
1. INTRODUCTION
Space photovoltaic (PV) development has made
continual and remarkable progress, averaging roughly
0.5% per year improvement in absolute efficiency over
many decades (fig. 1). Early solar cell development
activities were driven solely to meet spacecraft
performance needs, as PV materials costs were
considered to be too high for terrestrial applications.
Beginning in the early 1970’s, investment in space PV
development began to wane as the conventional wisdom
of the day asserted that PV generation capability was
limited to approximately 1 kW of spacecraft power and
beyond that nuclear power systems would be utilized.
In addition, financial forecasts indicated that
photovoltaics would always be too expensive for
terrestrial application. Fortunately, these apparent
constraints turned out to be inaccurate, and PV
development and applications have flourished for both
terrestrial and spacecraft needs.
A wide variety of PV technologies have been
investigated and developed in order to attain the
continual efficiency improvements demonstrated, with
current 1-Sun space solar cells projected to attain >35%
efficiency in the near future. History has shown that
advancements in cell efficiency, as long as the new
technologies have a comparable or better environmental
durability than current state of the art, are quickly
adopted for widespread use in space for both
commercial and government spacecraft. This speaks to
the leveraging impact increases in photovoltaic system
performance has on the rest of the spacecraft design.
This paper focuses on photovoltaic power system
development efforts within the Space Vehicles
Directorate of the Air Force Research Laboratory
(AFRL) specifically focusing on advanced solar cell,
blanket and array technologies. AFRL has interest and
efforts in energy storage and power management
technologies however this presentation will focus on the
energy generation component of the power system.
Figure 1. Historical performance trend for space solar
cells.
2. PHOTOVOLTAIC DEVICE DEVELOPMENT
Conventional lattice matched germanium (Ge) based
triple junction solar cells have largely reached their
practical efficiency limit of ~30%. Moving to higher
efficiencies has driven solar cell developers to
investigate a range of alternative cell technologies,
including metamorphic structures, novel materials (e.g.
dilute nitride) and novel structures (e.g. quantum
enhanced devices). Within the AFRL research portfolio,
the emphasis in next generation cells has focused
primarily on Inverted Metamorphic Multijunction
(IMM) devices [1]. IMM devices consist of
conventional III-V materials, but are grown inverted
such that the critical high bandgap cells are produced
lattice matched to the epitaxial substrate, thus
preserving their high performance. The lower bandgap
cells in the multjunction stack are then grown lattice
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© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative
Commons Attribution
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mismatched in order to enable the optimum bandgap
combinations to be achieved. This approach has resulted
in air mass zero (AM0) beginning of life (BOL)
efficiencies ranging from 33% to 37%.
AFRL/MANTECH is nearing the completion of an
IMM manufacturing development program with an
IMMX+ cell expected to begin AIAA S-111
qualification in the near future [2]. The IMMX+ cell
offers ~33% BOL efficiency and perhaps more
importantly, offers a 10% performance benefit at end-
of-life (EOL) and at operating temperature, compared to
conventional 30% class triple junction solar cells.
In addition to the performance advantage, the IMMX+
is being developed to be delivered to blanket and array
manufacturers as a pre-integrated string of solar cells,
known as Flex String Arrays (FSAs) (fig. 2). FSAs will
be available in custom cell length strings and custom
configurations and should offer significantly lower
blanket and panel integration costs, compared to
conventional discrete cells. A full cell
repair/replacement process for the FSAs has been
developed and has passed preliminary qualification
confidence testing.
Figure 2. Four 4-cell IMM FSAs assembled into 16-cell
string (l), three cells being replaced (center) and string
following re-work (r) [2].
AFRL is also investigating a range of other cell
technologies which offers high performance. These
include dilute nitride based multijunction solar cells [3],
GaAs based quantum dot photovoltaics [4],
mechanically bonded multijunction solar cells [5] and
solar concentrator solar cells [6]. The mechanically
bonded and concentrator cells are intended for use in
space solar concentrator solar arrays that will be
described in the array section.
3. CELL AND BLANKET TECHNOLOGIES
In addition to new solar cells, AFRL is also developing
ancillary technologies of benefit to a wide range of cell
and array options. The first is a solar cell coverglass
replacement technology known as pseudomorphic glass
(PMG) [7]. PMG consists of small glass beads (e.g.
fused silica) imbedded in space qualified silicone
adhesive and offers a flexible, fully encapsulation
alternative to conventional coverglass. PMG is currently
produced in thin sheets via a doctor blade casting
process, coated with a UV rejection coating (Optical
Coating Solutions) and then bonded to panels once the
cell strings have been integrated. This technology has
been validated in space and is being used on multiple
commercial LEO satellites.
With the interest of both space and terrestrial solar cell
developers in thinner crystalline cells, in order to reduce
mass, cost and/or increase performance, the potential for
cells to fracture and lose performance may be increased.
To address this potential issue, AFRL has developed an
advanced solar cell metal matrix composite (MMC)
metallization consisting of low-cost, low-purity, multi-
wall carbon nanotubes imbedded within the bulk silver
metallization [8]. This technology has demonstrated the
ability to electrically bridge >40 micron fractures in the
underlying semiconductor material and was recently
integrated with triple junction space solar cells. That
work demonstrated that the Jsc of the cells with MMC
metallization was maintained even after the cells were
intentionally fractured, whereas cells with conventional
metallization demonstrated ~50% reduction in Jsc under
similar fracturing tests.
4. SOLAR ARRAY TECHNOLOGIES
In contrast to the widespread adoption of new solar cell
technologies, the transition of new solar array
technologies has historically been very difficult.
Recently however, several new array technologies have
been developed which are bucking that trend and
moving towards commercial acceptance.
The MOSAIC solar array, under development by
Vanguard Space Technologies [9], is based on a
modular (e.g. Lego-like) architecture whereby modules
containing a single string of solar cells are mechanically
attached to a pre-wired frame to form completed panels
(fig 3). The MOSAIC frames contain internal
mechanisms that enable synchronised deployment with
a simple single actuator. The MOSAIC system was
designed to be compatible with smaller spacecraft (e.g.
ESPA class) utilizing 28Vdc direct energy transfer bus
topology and a total power of approximately 1.5 kW.
MOSAIC arrays offer the potential for lower cost
(reduced non-reoccurring engineering and custom array
fabrication costs), faster development (all components
are modular and can be warehoused) and the ability to
test and fly new cell and panel technologies as part of
the operational array. This testing feature is a new
capability that is being utilized in two upcoming AFRL
sponsored MOSAIC flight experiments (SurreySat and
STPSAT IV).
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Figure 3. MOSAIC module (l) and pre-wired MOSAIC
panel frame capable of holding six modules (r).
In the area of small arrays, AFRL is also working with
Deployable Space Systems (DSS) on the Aladdin solar
array (fig 4). This array design leverages recent
advancements in strained composite technology and has
the potential to offer very low cost and high reliability
given the very low part count. The array consists of a
strained composite structure (similar to tape measure
but with a wider and flatter mid-section). In the folded
configuration, cells are placed in patches separated by
areas which enable the structure to fold. When the
structure is flattened, it is able to be folded and stowed
for launch. Once on-orbit, the array deploys using the
stored strain energy in the composite structure.
The largest solar array currently being commercialized
is the DSS ROSA solar array [10]. ROSA (Roll Out
Solar Array) uses two strained composite slit-tube
booms to deploy and tension a flexible solar array
blanket. ROSA is currently under qualification
development testing by Space Systems Loral (SSL) for
their next generation of GEO spacecraft. In parallel with
the SSL activity, AFRL is working with NASA and
DSS to perform a flight demonstration of ROSA aboard
the ISS (fig 5). The ISS flight experiment is primarily a
structural validation of the ROSA (deployment,
stiffness, etc.) although the experiment will include
multiple active solar cell strings.
Figure 4. The DSS Aladdin solar array shown in a
rolled configuration (top) and a fold-out configuration
(bottom).
Figure 5. The DSS ROSA solar array ISS experiment (l)
and shown during testing aboard the ISS at the end of
the robotic arm (r).
The ROSA ISS hardware has been delivered to NASA
for integration and is expected to be launched to ISS in
the trunk of the DRAGON in early 2017. Once on-orbit,
ROSA will be grappled by the robotic arm, deployed
and characterized for approximately 7-days. The base
on the ROSA experiment includes mechanical actuators
to induce loads to the deployed ROSA hardware. The
response of ROSA to these disturbances will be
assessed using a variety of on-board sensors and
photogrammetry tools. Results of the on-orbit testing
will be used to validate ground models. The active solar
cell strings will also be characterized via I-V testing. At
the conclusion of the experiment, ROSA will be
retracted and re-stowed in the DRAGON trunk which
will burn up upon re-entry.
AFRL is also active in the development of solar
concentrator technologies as they offer potential for
cost, performance and environmental durability
benefits. Current efforts are primarily considering
refractive concentrator technology, given their benefits
related to shape error tolerance and soiling degradation.
In addition, AFRL is investigating the potential to
modify high performance terrestrial concentrator
technologies (Semprius) for use in space. These systems
have well developed manufacturing process and utilize
very small (~600 micron) cells enabling very thin
packaging, high environmental shielding and potentially
low cost. Given the past challenges of space solar
concentrators and their requirements for precise
pointing, these systems will have to show considerable
benefit in order to transition to operational use.
Fortunately there have been a number of successful uses
and demonstrations of space solar concentrators, thus
there is hope that this technology may be adopted in the
near future.
5. CONCLUSIONS
Improvements in solar cell and array technology
continue to transition to operational use given the
tremendous benefit these technologies offer to the
overall spacecraft. Rather than diminishing, the number
of solar cell technologies that have the potential to
provide >30% efficiency is expanding. Given the
limited size of the space marketplace, it may be
challenging for all of the potential technologies to be
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commercially viable.
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