The use of unmanned aerial vehicles (UAVs) has been increasing worldwide as demands for agile, rapid, targeted response forces have grown. UAVs provide surveillance and detection capabilities to these smaller forces that are unmatched by other technologies. As the reliance on UAVs increases, the short endurance times of existing UAV solutions become a significant factor. Existing commercial solutions have endurance ranging from 45 minutes to 4 hours, limiting both operational range and loiter capabilities. To increase endurance, UAV manufacturers have begun to target technologies ranging from increased performance batteries, improved control algorithms, fuel cells and other alternative power sources. While solar powered UAVs have long been a desired goal, existing solar technologies have characteristics that have made solar integration difficult and limited their integration.

Challenges

GaAs device fabrication process.
The most basic challenge is the limited surface area with which to generate solar energy. While most small UAVs fly at altitudes of a few thousand feet or less, the available solar resource is typically between 900 and 1000W/m2 on bright, clear days. As such, using 20% efficient solar cells, a UAV with 0.5m2 of wing area and 80% packing density (ratio of active area to inactive area) would generate 80W in optimal conditions (20% * 1000W/m2* 0.5m2 * 80% packing density). The solar resource will significantly reduce in overcast weather or during the hours close to sunrise or sunset. These factors demonstrate how important conversion efficiency as well as packing density is for the selected solar technology.

Another major challenge is weight. As the lift force required for flight has to be equal to the weight of all the elements constituting the UAV, every component used has been engineered to provide all necessary functionality and the lowest possible weight. Solar solutions are no different and can be evaluated on a simplistic “grams per W” basis for first order comparisons. While evaluating weight, encapsulation requirements (materials required to protect the cells for reliability or durability purposes) must not be ignored. Often these materials can add multiples of the weight of the solar cells themselves.

Beyond these basic challenges lie more complex questions. How will the solar integrate into the existing electrical system? How will the control surfaces on the UAV or its flight pattern shade the solar cells and how tolerant are they to this? What electronics are required to maximize the production of the solar system under all conditions? How robust is this material to rough field handling?

Existing Solutions

The most optimal solar solution is a material that optimizes and responds to all demands: highly efficient, lightweight, easily integrated and robust to varying illumination and handling. This has been the major shortcoming of previous solar solutions. They are capable of answering one or two of the demands but fall short in other areas, leaving the overall solution lacking.

Standard monocrystalline silicon cells have reached efficiencies as high as 24% and are produced by a wide number of vendors. However, the physical form of the cells (130 to 200um thick, 150 x 150mm rigid wafers) makes them difficult to integrate into complex aerodynamic surfaces, fairly weighty and difficult to pack densely into small UAV wings. Typical research airplanes using crystalline silicon cells have focused on designing a plane around the cells rather than integrating the cells into a known, proven UAV platform.

aSi (amorphous silicon) cells are lightweight and flexible, allowing for easier integration. However, their low efficiencies (ranging from 6-8%) make them unsuitable for use. Some of the CIGS (copper indium gallium selenide) cell technologies represent a middle-of-the-road solution. With efficiencies up to 16% and flexible substrates, they seem to be decent options but are still limited by lower efficiencies and added weight to protect them from moisture.

Mobile Power Solution

Sample of a flexble solar cell based on GaAs technology.
Recently, Alta Devices commercialized a new type of mobile power technology that is uniquely qualified for use in UAV applications. This new technology is based on GaAs (gallium arsenide) rather than silicon or CIGS and optimized to provide record breaking single junction efficiencies (28.8%) significantly higher than those capable in any other material. Additionally, due to the manufacturing techniques used, these cells are light and flexible, allowing for easy integration directly into existing UAV designs with little to no modification necessary.

The device fabrication process is illustrated schematically in Figures a thru d. First, a reusable GaAs growth substrate is introduced into a metal-organic chemical vapor deposition (MOCVD) chamber. A buffer layer of GaAs material is grown, followed by a thin release layer, on top of which the PV device structure was grown. What later becomes the sunfacing side of the device is grown first, and the back side of the device is grown last (Figure a). A back contact metal is deposited and this metal-on-semiconductor stack is then attached to a flexible handle, using an adhesive (Figure b).

The handle-metal-semiconductor stack is then introduced to a heated bath of aqueous acid. The acid etches the release layer, but leaves the remainder of the device intact. The etch completes leaving a semiconductor thin-film supported by the back metal composite and flexible handle ready for device processing (Figure c).

Front metallization is deposited using a plating process. An anti-reflective coating (ARC) is applied (Figure d) and the device is separated into cells to complete the process.

By interconnecting these cells, Alta Devices can generate large area assemblies with efficiencies greater than 24% and packing densities of over 90% on most existing UAV layouts. The cell material can be directly integrated into existing layup materials, eliminating additional encapsulation materials and resulting in a minimal weight gain of ~240g/m2. Combined, these characteristics provide a power to weight ratio of roughly 1g/W under optimal illumination conditions (typically 4 to 5 times higher than alternative solutions) that can be used for preliminary energy balance calculations.

There are additional advantages to the Alta Devices technology. Due to the high voltage and small size of the cells, voltage can be built in small areas, allowing the solar system voltage to quickly reach the bus voltage of the UAV and minimize demands on voltage matching electronics. The use of shingling to electrically interconnect the cells minimizes the impact of partial shading without significant additional electrical circuitry. Finally, the GaAs material has a significantly lower thermal coefficient than silicon (0.1% degradation /°C vs 0.4%/°C for silicon), maintaining maximum output even when in extremely hot operating environments.

Summary

Solar powered UAVs promise significant advantages to the operational theater. Real world flight times can be increased from one to over six hours, providing significant increases in loiter time and range. This additional power can also be used to enable new, high-power sensors or communication abilities without degrading existing flight times. While improving ROI on existing UAVs and missions, these capability improvements can fundamentally change how UAVs are used in the field, increasing soldier safety and effectiveness.

This article was written by Sam Cowley, Director of Product Engineering, Alta Devices (Sunnyvale, CA). For more information, contact Mr. Cowley at This email address is being protected from spambots. You need JavaScript enabled to view it., or visit http://info.hotims.com/45601-201.


Photonics Tech Briefs Magazine

This article first appeared in the March, 2013 issue of Photonics Tech Briefs Magazine.

Read more articles from this issue here.

Read more articles from the archives here.