A high-performance, high-voltage VMJ photovoltaic cell enables
high-wattage transmission of power via laser light for applications
including remote powering of small UAVs and remote sensing.
MICO PERALES
Laser power transmission
involves the transmission of power from a laser source either through
free space (power beaming or PB) or via a fiber-optic cable (
power over fiber
or PoF) to a photovoltaic (PV) receiver. The PV receiver includes a PV
cell, or an array of PV cells, optimized to convert a specific laser
wavelength to electricity at high efficiencies, typically in the range
of 30% to near 50%. Some uses include remotely powering unmanned aerial
vehicles (
UAVs), robotic devices, and hazardous environment or remote sensor applications.
MH GoPower (MHGP) produces a high-performance silicon-based vertical
multijunction (VMJ) PV cell that enables high-wattage laser power
transmission. The 3D design of the VMJ PV cell manages heat during
high-wattage laser power transmission through the use of the bulk
property of silicon, which includes fins integrated into the device.
Here, we explore the applications and value proposition of free-space PB
and PoF, as well as review the technology behind the high-performance
VMJ PV cell.
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| FRONTIS. MHGP VMJ cell design provides for versatility. |
Why power beaming?
When compared to battery-derived power, the advantages of free-space
PB include increased operating time, a reduction or elimination of
recharging events, and a potential reduction in weight. We illustrate
these advantages by examining the UAV application. The flight time of a
UAV can be extended in theory to indefinite duration, so long as the
power beamed to the PV receiver on the UAV and converted to electricity
exceeds the power usage.
By extending flight time, the need for the operationally sensitive
and technically risky maneuver of landing for recharging is reduced.
Also, if extended flight duration is required, rather than attaching
additional batteries and adding weight to the UAV, a lighter-weight PV
receiver can be deployed to achieve the extended-duration requirements;
this may also allow for additional payload capacity.
Why power over fiber?
There are several advantages of sending power over fiber rather than
through copper. Power delivery through copper is susceptible to RF and
magnetic field interference, which can result in power surges or other
signal anomalies that can damage equipment, making systems unreliable.
Furthermore, copper wiring serves as a ground path for lightning strikes
and can induce sparks in high voltage or poorly insulated environments.
Power by laser light suffers none of these problems.
Some examples of applications that benefit from these advantages
include sensors and other devices operating in hazardous, explosive, or
high-voltage environments (current sensors), devices exposed to harsh
weather such as lightning (outdoor video surveillance cameras as an
example), and devices requiring noise immunity (medical monitoring
equipment and military applications).
Barriers to market acceptance
Laser power transmission applications have been primarily limited by
system costs. The metric for these costs is cost per watt of delivered
electrical power. Driven by shrinking device sizes and improved
efficiencies, costs per watt for laser systems have been coming down
dramatically and are expected to continue declining. However, lasers are
only part of the cost of a laser power transmission solution. Other key
components are the fiber (for PoF), the optics, and the PV receiver
(cell array) subsystems.
The optics for PoF is a small part of the overall system cost.
However, for free-space PB, the focusing optics cost increases as the
application distance increases due to an increase in lens size. As a
result, optics costs can quickly dominate the free-space PB system
costs, and thus become the limiting factor on the practical range.
Still, ranges of a few kilometers are economically and technically
viable. Similarly, for PoF applications, the cost of the fiber begins to
dominate costs over longer distances, generally limiting practical
applications to a few kilometers.
Finally, PV receiver power density can play a significant factor in
the viability of the laser power transmission solution. In order to
ultimately reduce system costs, higher receiver power densities are
required, and this can be achieved in two ways: 1) by increasing the
power of the laser, thus increasing the intensity of light on the PV
receiver; or 2) by increasing the efficiency of the PV receiver.
In the first scenario, the increased power of the laser results in a
lower incremental cost per watt-up to a point. For example, in a
free-space UAV application, the tracking hardware and software, laser
mounting hardware, optics, UAV, and receiver size are all fixed, and
thus an increase in the laser power does not increase the cost of these
systems. In fact, it could mean that smaller batteries are needed,
further reducing the incremental cost of adding this additional power.
Consequently, as long as the PV receiver can perform well in the
increased intensity laser light and can manage the heat, it is
advantageous to increase the power of the laser source (the analysis
similarly holds true for a PoF application).
In the second scenario, improving PV receiver efficiency results in
more power output for a given PV receiver footprint and balance of
system costs. Alternatively, it allows for the use of a lower-power and
lower-cost laser system. A more-efficient PV receiver also means that
there is less waste energy to dissipate, with the result that cells will
operate cooler (a positive performance feedback loop) or that a
lower-cost and often lower-weight heat sink can be used. Another
advantage of more-efficient PV receivers is that the overall system
efficiency will be enhanced, which is important for some applications in
terms of both cost and system operation.
How the VMJ PV cell achieves high power density
The MHGP silicon-based VMJ PV cell (VMJ cell) is a cost-competitive
concentrating PV cell, which works most effectively with laser systems
with wavelengths in the 900 to 1000 nm range. The VMJ cell's material
composition and device structure serve as the foundation of the cell's
competitiveness for laser power transmission applications.
The VMJ cell is an integrally bonded series-connected array of
miniature silicon vertical-junction unit cells. The VMJ cell is
fabricated by bonding a stack of diffused and metalized silicon wafers
together and then dicing the stack into thin slices, resulting in
high-voltage, low-current cells that each contain approximately 50 unit
cells (junctions) per centimeter of length, resulting in near 30 V/cm.
Electrical leads are attached to the end contacts, so that current flows
from the ends of the cells (see Fig. 1).
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| FIGURE 1. The MHGP VMJ cell has the potential to
deliver higher power density than conventional cells through both
application of greater light intensity and higher PV receiver
efficiency. |
Increasing intensity on conventional PV cells will increase output to
a point. Gallium arsenide (GaAs) concentrating PV cells designed for
laser light conversion require dense grid lines on the front and back
surface of the cell to manage higher concentrations of light. This has
the effect of reducing cell performance as intensities go up. The MHGP
VMJ cell has been shown to maintain constant performance in efficiency
(based on flash test results) up to intensities greater than 100 W/cm
2.
The major challenge for PV cells under highly concentrated light is
managing the waste thermal energy. As intensities increase, cell
temperatures rise, resulting in lower performance, which results in
still proportionally greater waste energy. In other words, high
intensities have a compound negative impact on cell performance.
The barrier to efficiently removing heat from PV arrays is the
thermal interface layer (TIM), which bonds the PV cells onto the
thermally conductive heat sink. Typically, this TIM must utilize a
thermally conductive, yet electrically insulating material to prevent
the current-carrying, electrically active cell surfaces or leads from
shorting. As such, these TIM materials are typically not ideal thermal
conductors. The result is that a barrier exists to heat transfer,
resulting in a limit to the rate of heat transfer away from the PV cell.
To improve heat dissipation, we have incorporated a number of
innovations into our VMJ cell and receiver designs, including: a) thick
cell architectures (up to 1 cm thick); b) end-contact heat dissipation;
and c) embedded heat-sinking structures in the PV device. In
combination, these features enable some novel heat-dissipation
solutions.
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| FIGURE 2. WiFins are integrated fins that dissipate
heat from a PV cell via a heat path that does not need to pass through
the receiver's thermal interface layer (TIM; the layer that bonds the PV
cell onto its thermally conductive heat sink). |
As an example, thick VMJ cells with integrated "fins" (called WiFins;
see Fig. 2) improve heat-sinking by increasing the cell's surface area
for radiant and/or convective cooling, allowing heat to be dissipated
without having to pass through the TIM bottleneck. Thick VMJ cells with
metalized end contacts use a metallic, bonding material with high
thermal conductivity to attach the large-surface-area end contacts to an
electrically conductive heat sink, which doubles as the device's anode
and cathode. These innovations enable the VMJ cell to more effectively
dissipate heat, leading to higher-intensity power operation.
PV receiver efficiency
Finally, the MGHP VMJ cell also improves PV receiver efficiency. This
is accomplished through the VMJ cell's customizable size (up to 7 x 7
cm in width/length) and high voltage characteristics. The customizable
size allows the cell to be matched in size to the expected spatial
nonuniformity of the application. In other words, a highly spatially
nonuniform beam pattern would require smaller cells, which would be
paralleled together, avoiding series losses. A beam pattern that varies
more slowly spatially would allow for larger cells, again paralleled
together.
This completely parallel architecture is viable due to the
high-voltage nature of the VMJ cell, which along with the customizable
cell size, allows the PV array to easily approximate the voltage
requirements of the end application, thus helping to reduce the costs,
inefficiencies, and additional weight requirements of step-up
converters.
The benefits of delivering power via fiber or through free-space are
spurring the growth of new applications. To further accelerate the
adoption of these applications, additional system cost reductions must
be achieved. This will continue to occur due to the expected drop in
laser prices, but can be further spurred by the incorporation of the
MHGP VMJ cell into PV receiver designs, resulting in greater PV receiver
power densities that will further lower laser power transmission system
costs.
Mico Perales is business development manager at MH GoPower Co. Ltd., Kaohsiung, Taiwan; email:
mico.perales@mhgopower.com;
www.mhgopower.com.
