One of the largest problems facing the aerospace industry involves size, weight, and power (SWaP) analysis and the increasing size of payloads with typically a finite amount of power to drive ever more demanding systems. This issue plagues all platforms, regardless of the energy storage medium, the type of propulsion, or system design. Only recently has an opportunity to augment these systems with on-board power generation been deemed not only possible but practical. Much like the space industry, which has benefited from solar technology on satellites that have been aloft for decades, it is now possible for both small and large fixed wing unmanned platforms to drastically increase flight time and payload capabilities.

Solar Technology

The Bramor missions include surveying and remote sensing for large scale agriculture. (Image courtesy of Alta Devices)

The solar industry has been expanding significantly over the past decade and while most solar technologies are too heavy or rigid for aerospace, there are several types that are extremely well-suited for fixed wing applications. Wafer-based and thin film solar technologies are now being actively used by aerospace designers to transform a wing from a passive mechanical element into a significant power source. These technologies are thin, lightweight and highly efficient.

An example of one of these advanced new technologies is a thin, flexible solar panel that can be integrated directly into wing skins in order to augment —and sometimes fully power — fixed wing systems. The base technology is a 2×5 cm cell which produces approximately 0.2 watts of power at close to 1 volt. This cell is then connected in series to match the power systems of the platform, and in parallel to maximize the coverage across the wing surface. Multiple panels can be built up to further increase power using available wing surface. This solar will add a minimal amount of weight (less than 1 gram per watt) if retrofitted, but if a manufacturer is proactive and designs the solar into the wing skin, a layer of carbon fiber or fiberglass structure can be removed resulting in zero additional weight for a significant power boost.

Solar UAV Case Study

World record efficiency is now flexible enough to be added to many wing structures. (Image courtesy of Alta Devices)

On top of the high power-to-weight and power-to-area ratios for appropriate thin film solar, the ease of integration in practice is really what makes for a game changing technology. Using the example of a fixed wing UAV the size of the Aerovironment Puma, the wing-span is approximately 3m and the chord is about half a meter at the body intersection sweeping to about a quarter meter at the wing tips. Important to note is that the size of the UAV does not matter as much; the AV Raven, a much smaller system, uses considerably less power to stay aloft and has less space for a sensor payload. With this available surface area, and given this is an existing craft being retrofitted, the battery system is examined to determine the size and quantity of solar capable of being added to the UAV. Additionally, some gas-powered systems can benefit from added regulated power being fed to the payload allowing for more consistent power or reducing the size of the battery for other SWaP considerations.

Battery Selection

An example of a typical battery for UAVs would be a 6S LiPo, which is to say it is a lithium polymer battery with 6 cells in series. These cells each have a nominal voltage of about 3.7V and placing them in series leads to a nominal battery voltage of around 22.2V. This is the voltage that flight controllers would monitor for warnings, speed controllers would use to drive the prop, and lighting and payload systems would use to keep the craft seen and doing good work.

The power draw of all of these systems depends on their efficiency, what the UAV's flight parameters are, and how the UAV is actually designed, be it fast and power hungry using speed to generate sufficient lift, or slow and efficient with a larger wing surface. For this, example we have a typical 11,000 mAh battery which would mean the finite battery capacity of the system is about 244 watt hours. On a system like the Puma a battery like this would allow for an average of 2.5 hours of normal operations including telemetry, payload collection, and propulsion for the initial climb, cruise, and any required maneuvering. The power draw of the system can be determined to be an average of 97.6W which will give us an idea of how much solar will contribute to the end solution.

System Layout and Power

Continuing with this example, a solar layout is able to be built where the voltage at maximum power is designed to be as close as possible to the fully charged battery voltage, so after climb-out the batteries are at a lower voltage than the solar and physics allows for preferential draw of power from the higher voltage source, in this case, the solar. With each solar cell generating .96V at Vmp, a solution of 26 cells in series is used providing 24.96V, which is close to the batteries’ fully charged voltage of 25.2V. Using this 26-cell strip, the largest most densely packed panels possible are built.

Since this technology involves shingling the cells together to increase the voltage, some of the surface area of the cells overlap each other, which achieves the targeted voltage in a more compact space., Each shingled cell is about 1.7cm in the series direction, so each strip is about 45cm long. On a Puma or similar UAV, a total of 24 strips across both wing halves or about 124 watts of power across the entire wing can be fitted. This additional power will be available to greatly increase performance of the system such as to use when the draw is less than average, maintain flight during cruise, or allow for greater pay-loads to be installed.