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This article was originally featured in the edition: Volume 24 Issue 8

Sending CPV Into Space

Satellites could be powered by incredibly efficient modules featuring five-junction micro-cells and tiny glass lenses BY MATTHEW LUMB FROM GEORGE WASHINGTON UNIVERSITY

By far the most efficient class of technology for converting
sunlight into electrical power is the high concentration photovoltaic (HCPV).
Modules that are based on this technology use low cost lenses or mirrors to
focus sunlight onto multi-junction cells with three or four junctions, to
realise conversion efficiencies at the cell level that are nearing 50 percent.
Note that the focusing of sunlight by factors of typically 500 to 1,000 is
critical to increasing the bang-per-buck of this technology: it spawns a hike
in conversion efficiency, as well as slashing cell costs, due to a dramatic
reduction in the amount of compound semiconductor material required to deliver
a given output.



The commercial success of HCPV has been thwarted by several
factors, limiting this field to a handful of companies. Arguably the most
damaging of thorns has been the rampant success of low-cost, large-area solar
cell technologies, particularly crystalline silicon. But there are other
issues, such as the requirement for precise two-axis tracking, which adds cost
and complexity to an installation, while limiting packing density due to
shadowing. Another impediment is that it is impossible to focus diffuse light
with conventional optics, so utility-scale CPV is restricted to desert-like
locations, where the diffuse sunlight fraction is small.



One glimmer of hope is that conventional, large-area silicon
solar panels are now beginning to push firmly against their fundamental
efficiency limits. So, if a large boost in HCPV efficiency came along at low
cost, and were combined with the remarkably high energy yield available from
two-axis tracking of the sun, this solar technology could be highly competitive
once again.



Further cause for optimism is the emergence of myriad niche
applications where the key requirement is maximising the number of watts per
square meter, rather than the cost per watt. The former metric is the priority
in remote and space-confined applications, such as bicycle and car-charging
stations; in temporary DC microgrids, such as those used for disaster response;
and in the application we will focus on in this article - space power.



Cell considerations





During the last decade, many CPV companies have closed up
shop. But the majority of cell suppliers have not suffered the same fate,
thanks to a healthy market for III-V multi-junction solar cells in space. Here
they are the established industry leader, with numerous multi-junction solar
cells on the market. Success stems from a different set of priorities:
cost-per-watt is a secondary consideration, and topping the list is the
specific power, judged in watts-per-kilogram, and the volumetric power density,
evaluated in terms of watts-per-cubic-metre.







Figure 1. (a)-(e) The main features of the micro CPV design,
which has been developed by a team that is led by researchers at George
Washington University. Modules incorporating state of the art CPV microcells,
all-glass optics and diffuse collection using off-the-shelf crystalline silicon
solar panels. (f) A GaAs-based dual-junction microcell transfer printed on to a
glass substrate. (g) A hexagonal array of dual-junction microcells under
forward bias, operating as LEDs.




The need for a high specific power reflects the very high
launch cost for putting anything in space. Regardless of what it is, it costs
in the region of $10,000 per kilogram. So, to drive down launch costs, solar
cell suppliers try to produce the lightest weight cells possible.



The other key consideration is the volume of the payload. To
minimise this, extremely creative and hi-tech photovoltaic panel deployment
systems are employed on satellites. They feature low profile modules and
large-area, III-V multi-junction solar cells with industry leading performance.



Devices that work well on earth may not do so in space, due
to the far higher levels of radiation that degrade material quality and reduce
efficiency. To minimise this over the lifetime of the cell, suppliers
encapsulate their devices in tens to hundreds of microns of cover glass. This
coating shields the semiconductor layers from radiation, but adds significant
weight to the final product.







Figure 2. Predicted DC global energy harvest efficiency
(annual kWh production / annual global resource on a two-axis tracker) in
locations with different diffuse contents for conventional HCPV and a micro-CPV
array.




CPV in space?



There have been several notable, successful experiments
based on taking CPV panels into space. These efforts, aimed at exploiting the
high efficiency of the technology to increase specific power, have focused on
low concentration demonstrations that have failed to gain a strong foothold.
But this time it could be different, thanks to the advent of a new type of CPV technology
- micro CPV. Its timing could not be much better, given that multi-junction
cells operating at one sun are reaching practical limits for efficiency and
specific power; and the advent of reusable rockets, coupled with large
increases in space traffic, could drive down launch costs.



Helping to develop a new generation of CPV technology for
space is the US Department of Energy's Advanced Research Projects Agency-Energy
(ARPA-E). Through the programme MOSAIC - Micro-scale Optimized Solar-cell
Arrays with Integrated Concentration - it is funding 11 teams with a total of
$24 million.



The efforts that ARPA-E are supporting include those by our
team at George Washington University, working in partnership with researchers
at the US Naval Research Laboratory, Veeco, Northwestern University, MIT and
X-Celeprint. Together, our collaboration is aiming to address some of the main
obstacles facing conventional HCPV by marrying ground-breaking,
ultra-high-performance five-junction cells with all-glass lens arrays and a
glass backplane (see Figure 1 for an overview). Our glass lenses feature a
novel, bifacial anti-reflection coating that provides an extremely high optical
efficiency while concentrating sunlight by a factor of approximately 500.



One of the great strengths of our modules is that they can
capture diffuse light, thanks to the incorporation of a bifacial, crystalline
silicon solar panel. Attaching this to the CPV module injects a significant
boost to the overall power output, especially in locations where a large
fraction of the total solar resource arrives as diffuse illumination (see
Figure 2).







Figure 3. Specific power and volumetric power density for
different lens designs for the five-junction CPV cell developed by the team led
by researchers at George Washington University.




Our micro CPV module is also able to excel in efficiency.
The apertures for our CPV cells are just 170 μm by 170 μm, so the devices are
small enough to avoid the use of metal grid fingers when efficiently extracting
current from the cell. Instead, a single metal contact is employed at the edge
of the aperture, eliminating shadowing loss. Another benefit of using
microscale cells is that they simplify thermal management. Thanks to this,
cells can operate at lower temperatures, thereby churning out power at higher
efficiencies.



We assemble our microcells with a micro-transfer printing
process, a technology commercialized for PV by former CPV company and team
partner Semprius. Note that this is also the core technology of our industry
partner, X-Celeprint: it applies micro-transfer printing to a range of
micro-optoelectronic applications.



Micro-transfer printing allows precise, parallel assembly of
microscale devices that are removed from their native substrate with a wet etch
procedure. In addition, we use micro transfer printing to heterogeneously
integrate dual-junction, GaAs-based solar cells with triple-junction InP-based
solar cells. The five-junction cells that result - featuring high quality,
lattice-matched materials - capture a broad range of the solar spectrum. Do
this at low cost, and it can be disruptive to conventional PV technology.



Our CPV module is well-suited for use in space. Its merits
include: a very high radiation tolerance, thanks to shielding by the glass lens
array; a reduced arcing risk between cells, due to their large separation; and
a low profile module, enabled by focal lengths of just a few millimetres. Those
dimensions allow units to be drop-in replacements for conventional panels, and
be compatible with existing deployment techniques.



In missions to deep space, it's actually an advantage to
operate multi-junction cells at high concentration. That's because this negates
many of the difficulties associated with low-temperature, low-intensity
environments. These issues, which include complications arising from
hetero-barriers and trap states, are encountered on missions to parts of the
solar system much farther from the sun.



Yet another strength of our CPV modules is that they have
the potential to be far cheaper than large-area III-V solar cells. This benefit
will become increasingly important in the future, due to decreasing launch
costs.








Figure 4. Optical throughput versus incident angle for glass
lenses with a 1 mm by 1 mm aperture and 5 mm focal length for different
CPV cell dimensions.







Sweet spots



Modelling suggest that the efficiency of our CPV cell in
space peaks at roughly 200 suns. Go any higher and performances falls due to
temperature-related losses and series resistance. Note that lower values of
concentration are used in space than on earth, due to the lack of convective
cooling, which impairs thermal management. A higher cell efficiency does help
thermal management though, as less wasted power is required to be dissipated.



We have evaluated the specific power and volumetric power
density of our module for different lens aperture sizes, while assuming
all-glass lens arrays with square apertures (see Figure 3). In this study, our
1 mm by 1 mm lens aperture provides a concentration of about 35 suns, while the
4 mm by 4 mm aperture offers a concentration of 550.



The smaller lenses are compromised by a lower optical
efficiency, due to a ‘cusp' region that extends for about 50 μm along the
perimeter of each lenslet. This flaw, resulting from the surface tension of
glass as it cools in the mould, hampers the focusing of light from the cusp
region. However, even with this imperfection and the lower concentration, the
smaller lens yields a higher specific power. That's because these lenses are
lighter, thanks to the combination of their smaller apertures, and their lower
lens curvature, due to a longer focal length.



Unfortunately, from the perspective of maximising the
volumetric power density, it is better to use a lens with a shorter focal
length. The highest density values coincide with concentrations that correspond
to the peak cell efficiency.



This creates a conundrum that is not faced by the designers
of conventional flat panel designs, which produce highest specific power when
delivering their highest volumetric power density. However, there is a happy
medium for micro-CPV modules in space: small aperture lenses with a short focal
length. In future, it may be possible to make even lighter lens arrays using
Fresnel lens concepts - this would improve the specific power even more.








Figure 5. A reflective microcell CPV array is hardly any
thicker than a conventional coverglass-integrated space PV cell (CIC).




Wherever CPV systems are used, arrays must be pointed
directly at the sun to maximise power. This is not as daunting to realise in
space as it might first appear, as most satellites can track the sun to within
a degree or so, using a variety of attitude control systems. However, the
danger is that if the design employs a narrow acceptance angle, a significant
pointing inaccuracy runs the risk of a power outage.



The good news is that it is easy to avoid this scenario,
because the economics of space are in our favour. To highlight the way forward,
we have considered the optical throughput efficiency for broadband illumination
at varying incident angles, for a 1 mm square lens, 5 mm module thickness and a
variety of CPV cell sizes (see Figure 4). This study shows that by oversizing
the cell, a much wider angular acceptance is possible. There are penalties to
pay, in the form of a slightly reduced cell performance - solar cells generally
work best when uniformly illuminated - and, of course, a higher overall cost,
due to an increase in semiconductor content. However, even with those oversized
cells, the cost of the panel is still an order of magnitude lower than an
equivalent one with no concentration.



The work described so far considers refractive optics for a
space concentrator system. But that is not the only promising option: there is
also much potential with a reflective concentrator geometry (see Figure 5).
Those of us at George Washington University are pursuing this as well, in a
partnership with a team at Penn State University, led by Chris Giebink. Modules
are formed by printing microcells on a thin sheet of glass, which is
subsequently bonded to a reflective lenslet array.



One of the primary merits of CPV modules with reflective
optics is that they enable cells to operate near the thermodynamic limit of
concentration, while the optics have a far lower aspect ratio - the thickness,
divided by the aperture width, may be as low as 0.25. With such a design, mass
can reach a new low, while increasing the angular acceptance for a given
concentration ratio.



For example, for 170 μm by 170 μm cells operating at a
concentration of roughly 35 suns, the total thickness of a practical
concentrator can be as low as 0.5 mm - that is comparable to the thickness of
existing coverglass-integrated space PV cells - while the angular acceptance
can be as high as approximately ±5˚.



A practical challenge with this type of module is the
extraction of heat from the cells, because they are embedded in low thermal
conductivity glass.



This should be manageable, according to thermal modelling
and experimental measurements on a similar, terrestrial microcell CPV. However,
we can only be certain of this after we have experimentally tested this design.



We have no doubt that micro CPV offers exciting new
opportunities for lightweight, low-cost, low-profile CPV arrays that set a new
benchmark for efficiency. Armed with funding from ARPA-E, the new, state of the
art CPV modules that we are developing using micro transfer printing promise to
have benefits on earth and up in space.







CS International 26-27 March 2019, Sheraton Airport Hotel, Brussels

In its ninth year, CS International will continue to provide timely, comprehensive coverage of every important sector within the compound semiconductor industry. Presentations are split into 5 key themes and each industry key theme will be delivered by a keynote presentation from a leading industry figure as well as a market analyst presentation tailored to the theme. Together, the talks will detail breakthroughs in device technology; offer insights into the current status and the evolution of compound semiconductor devices; and provide details of advances in tools and processes that will help to drive up fab yields and throughputs. Attendees at the two-day conference will gain an up-to-date overview of the status of the CS industry, and have opportunities to meet many other key players within this community.

Places will be limited, so register your place today: https://cs-international.net

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