Thin Film Solar Cells for the "PV for UAV" Technology
Materials Science and photovoltaic research topics in photovoltaics (PV) for PV systems intended to power UAVs are still some of my R&D areas of interest. This research started with an earlier projects in 2005-2006 funded by DoD-NAVAIR. The focus was for powering unmanned aerial vehicles (UAVs) with high efficiency solar cells. The research them was: use of Multi-Source energy system and on-board energy management for extending the mission of Unmanned Aerial Vehicles in duration and functionalities. The title of the NAVAIR sponsored project was "Photovoltaic Power Supply for autonomous Sensors".
The research started with cooperation with Advanced Ceramic Research Inc., which allowed using their Firefox UAV, as a model for the PV of UAV project. Thanks to a number of important features the plane had, it was, at that time, the best UAV platform for a photovoltaic R&D project. The main goal was to provide the maximum power for propelling the UAV and for powering the payload (i.e., the on-board electronic and electro-mechanic devices).
There were several challenging technical problems that were researched, some are due to the geometry of the machine. But, independent of the UAV platform itself, I have solved numerous science and technology problems related to choices of: the various semiconductor and polymer materials, the lightweight requirement, the type of solar cells, the conformal coverage, the solar cell encapsulation, the power management and distribution, the methods for the on-bard energy mixing, the storage of excess of energy, multi-source and multi-load electrical impedance matching coupled to the energy management, and the on-flight energy conservation. Several of the worked solutions make the technology applicable to other vehicles (hybrid electric-mechanical cars, bicycles,...). One of the crucial problems is the choice of the photovoltaic technology that can enable flying UAVs over a long period of time.
Till today, researchers are still working on the "PV for UAV" science and technology not just to improve the performance of existing systems but for solving specific technology and materials issues. In the following article the authors discuss the use of thin film solar cells for UAV, again under the sponsorship of the Navy. This paper is reproduced in this space because it is extremely relevant to the UAV technology and just in case it will no longer be available in PV-Tech website, the article copyright belongs fully to the owner of the paper published by PV-Tech.org.
What the US Navy’s solar drones tell us about thin-film solar’s potential
By Edgar A. Gunther, published in PV-Tech website on Jul 04, 2018 4:35 AM BST
Sources: PV-Tech.Org
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When the US Naval Research Laboratory (NRL) selected Alta Devices, a Hanergy Company, as the solar solution for the prototype Hybrid Tiger UAV (Unmanned Aerial Vehicle) Project, I decided to see what I could find out from NRL. After some discussion, an NRL spokesperson said:
“Due to sponsor considerations our lab is not inclined to discuss Hybrid Tiger. However, NRL's Solar-Soaring UAV uses photovoltaic technology and we are very open to discussing its capabilities.”
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Sponsored by the Office of the Deputy Assistant Secretary of Defense for Operational Energy and the US Marine Corps Expeditionary Energy Office, the Hybrid Tiger Group 2 UAV project integrates Alta’s high efficiency solar cells, a hydrogen fuel cell, and autonomous cooperative soaring algorithms to achieve over 3.5 days of flight endurance, even during the winter solstice at up to 50° north latitude. |
Source: Alta Devices. |
According to Jane’s last year, Hybrid Tiger will be powered by the hydrogen fuel cell at night and solar cells during the day while carrying a small surveillance and reconnaissance payload, and first flight was expected last month, May 2018.
In the prior Solar-Soaring UAV programme, NRL tested a range of custom built solar wing sets to extend the endurance of a battery-powered, electric motor on a modified SBXC sailplane kit as detailed in the IEEE PVSC-44 paper, “Enhanced Endurance of a Unmanned Aerial Vehicles Using High-Efficiency Si and III-V Solar Cells” by David A Scheiman, Raymond Hoheisel, et al. NRL built custom SBXC solar wing sets using the following:
- SunPower monocrystalline silicon Interdigitated Back Contact (IBC) cells
- Microlink Devices' Inverted Metamorphic Multijunction (IMM) cells
- Alta Devices' thin-film Gallium Arsenide (GaAs) single junction cells
- SolAero Technologies' thinned ZTJ lattice-matched triple junction cells
Another wing set using Sharp IMM cells was still in process at the time of the paper.
Without solar, the PV-SBXC UAV can fly about four hours using the 400 Watt-hour, 1.91 kg battery pack arranged as six series by six parallel (6s6p) Panasonic NCR18650B batteries. Each of the wing sets was ground tested and test flown for at least one hour. Extended endurance test flights for SunPower and Alta Devices have only been reported by NRL thus far. All the tests were flown with a fixed rectangular pattern and altitude range, and the autonomous soaring software algorithm was disabled for the two endurance flights to determine solar performance though passive soaring was permitted. The endurance flights began near sunrise with the batteries charged to at least 90% state of charge (SOC).
With the SunPower wing, the PV-SBXC UAV flew for 10.9 hours and about half of the power used during the flight was generated by the solar wing not including passive soaring. The SunPower flight benefited from two one-hour periods of thermal soaring and 40% of the flight was spent with the motor off. Correlated to a second SunPower wing on the ground, the UAV wing generated about 10% less power perhaps related to MPPT (Maximum Power Point Tracking) adjustments during aircraft motion and orientation to the sun.
The PV-SBXC UAV with the Alta Devices wing flew for 11.2 hours, and the solar wing generated almost two-thirds of the power used during the flight without any benefit from soaring. Correlated to a second Alta Devices wing on the ground, this time the UAV wing generated about 15% less power perhaps again related to MPPT adjustments during aircraft motion and orientation to the sun but also the higher Alta Devices array voltage. Alta Devices further outperformed by using only 66% of the battery capacity versus 89% by SunPower.
NRL concluded the two endurance flights demonstrated how solar power can extend UAV flight duration by over two times and even longer when paired with autonomous soaring algorithms. NRL Aerospace engineer Dr. Dan Edwards said:
“The experiments confirm significant endurance gains are possible by leveraging thermal updrafts and incident solar radiation, rather than ignoring these free sources of energy. Future testing will focus on quantifying the trade space between improvements in solar cell efficiency and combining with autonomous soaring for improved solar-recharging.”
Solar Performance Index
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Alta Devices has proposed a new metric, the Solar Performance Index (PUAV), seeking to rank solar technologies by capturing the dual concerns of power per unit |
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area and power per unit mass in UAV designs. PUAV is defined as the square root of the product of the power to mass ratio, Pm, and the power to area ratio, PA. |
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Where:
Pm is the power to mass ratio (Watts/kilogram)
PA: is the power to area ratio (Watts/square metre)
The units of PUAV work out to be Watts per the product of metres and the square root of mass and do not have a physical meaning other than translating a plot of the power to mass ratio versus the power to area ratio into a single index value.
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I calculated the Solar Performance Index for cells from SunPower, Alta Devices, and SolAero in tabular and bar chart form as shown below with a few caveats. SunPower cell weight only available for a bare GenII cell (~6.5 grams). Estimated correction factors for interconnect (0.3g) and lamination (110 g/m2) were applied. Alta Devices and SolAero power to mass |
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ratios include top encapsulation. SolAero efficiency data for AM1.5G @ 28 °C instead of the standard 25°C.SunPower performs well considering their price to performance ratio; therefore high-efficiency crystalline silicon cells would be the preferred solution for very cost sensitive UAV applications. |
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Alta Devices and SolAero GaAs cells would be preferred in UAVs when endurance and weight are paramount and price is secondary.
Gen4
Alta Devices launched their fourth-generation, single junction, solar cell technology (“Gen4”) last month with the key benefits of 30% weight reduction to 170 g/m2 (grams per square metre) ratio at the matrix level, including cell-to-cell interconnects, ribbons and protective diodes, and reduced thickness perfect for UAV and other autonomous power applications.
Alta Devices high-efficiency, thin-film GaAs solar cells are 100% made in the USA at their Sunnyvale, California, facility. Over the next eighteen months to two years, Alta expects to reach around 3MW of production capacity maxing out the available space and permitted capacity of the building. Alta is considering other locations for future expansion based on market demand or collocation with potential large partnerships.
Meanwhile, Alta said “more tools are landing every day” as the capacity ramp is underway through expansion and debottlenecking. Alta’s mass production process uses three unique tools for metal organic chemical vapour deposition (MOCVD), Epitaxial Lift Off (ELO), and matrix assembly. Alta Devices has created significant new Intellectual Property (IP) concerning the processes, tooling, packing factor, and weight reduction.
Alta has been using their own design MOCVD reactor and is in the process of commissioning over the next few months their latest custom designed MOCVD reactor described as much bigger than anything available. Alta’s MOCVD reactor is optimized for gas material utilization and the area based deposition required by solar cells. As a result, Alta has simplified the reactor design compared to commercial MOCVD reactors used for laser and LED production.
Alta developed another large tool to handle ELO or separation of the Gen4 GaAs thin-films from the wafer with high yield and high throughput. Alta did not disclose the current average number of lifts per wafer; wafer reuse is critical for ongoing cost reduction and capital equipment improvements.
And the Matrix Assembly tool, fed from a cell magazine, builds customized solar modules to match customer requirements via serial and parallel cell configurations. In the typical UAV application, the cell matrix built by Alta for customers includes top encapsulation for direct integration into wing materials such as carbon fibre or metal.
When I asked Alta Devices about thin-film solar cell pricing, I wasn’t sure I would get a response. However, Alta said the solar cells are UAV market value priced lower than substrate based multijunction solar cells offered by Boeing Spectrolab and SolAero, for example. At this time, Alta’s capacity is sold out to projects driven by the growth of their aerospace business.
Alta Devices future ambitions extend to the automotive market where much more competitive pricing will be required to enable range extension though still at a premium to high-efficiency crystalline silicon solar solutions.
Dual junction
As a two time, dual junction (non-concentrator) solar cell world record holder, Alta Devices updated the dual junction datasheet around the Gen4 launch and is shipping small quantities of dual junction solar cells to key customers. Dual junction cell production is planned for yearend 2018 with single junction cell production continuing in parallel.
Priced at a premium to single junction, dual junction solar cells have found two use cases in UAV applications. For the non-endurance flying market, the higher efficiency dual junction solar cells can reduce battery size and weight for the intended application enabling additional payloads. In contrast, as ongoing battery improvements increase capacity, more solar power is needed to recharge the battery favouring the improved power density of Alta’s dual junction solar cells in endurance UAVs.
Solar is poised to become a standard feature on low altitude UAVs with any reasonable surface area, typically wings, to extend endurance or increases payloads or both.
Boeing Spectrolab declined to discuss the application of their multijunction solar cells in UAV applications.
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PV-Tech.Org : The organization maintains a website that informs readers on the latest photovoltaic technologies under development or made it to the market, as well as the market performances. https://www.pv-tech.org/editors-blog/list Also, the organization arranges for periodic talks on photovoltaics, via Tech Brief and IEEE Spectrum, and publishes several journals, two are on solar cells: PV-Tech Newsletter: US Edition and the Photovoltaics International. The latter focuses on technology and the PV supply chain, including materials, components, equipment, manufacturing and large-scale utility installation. https://www.pv-tech.org/photovoltaics-international/overview