Alternative Power Sources Boost Small Drone Endurance
When using small drones — which typically run on lithium-ion batteries — there are several shortfalls that need to be overcome in order to make the systems more effective for warfighters, said John Jennings, the director for innovation in the office of the assistant secretary of defense for operational energy plans and programs (OEPP).
“Typically what people want is greater range and endurance, basically being able to stay up longer and fly farther,” he said. Additionally, “they want them to have greater onboard power. You’re not flying them just to fly them, you’re flying them because you have a payload and those payloads usually [need] power.”
Reliability is also a factor because many of the smaller systems were not designed to run for long stretches of time, he noted.
OEPP is tasked with “maximizing the military capability we get out of energy while reducing the burdens and risks that our energy supply lines create,” Jennings said. While the office doesn’t solely focus on alternative power sources in order to achieve that goal, it is one of the tools the Defense Department uses to squeeze every drop of energy out of its systems, he said.
The operational energy capabilities improvement fund (OECIF) is one of the ways the department promotes technology that will improve operational energy performance. It contributes science and technology research funding to several military projects and encourages the services to take risks and move in new directions, Jennings said.
To decide which programs will receive that funding, OEPP asks the military to propose programs. From those submissions the office chooses which projects to fund, and the services themselves execute and manage them, Jennings explained. For fiscal year 2016, the Obama administration requested $37.42 million for the fund.
One program that currently receives seed funding through OECIF — though the Navy and Marine Corps are financing the majority of the project — looks at harvesting both solar and thermal energy to power a small unmanned aerial vehicle for an extended period of time.
“The Navy and the Marine Corps have gaps with regard to UAV capabilities,” said Capt. Anthony Ripley, the science and technology lead for the Marine Corps’ expeditionary energy office. “This effort will enable us to close a lot of those gaps.”
The program participants are: the expeditionary energy office, the Naval Research Lab, the Naval Postgraduate School and industry partners — Semprius, a solar array manufacturer from North Carolina and Packet Digital, a Fargo, North Dakota-based company that specializes in power management.
The capability will enable small units to have a hand-launched group one UAV — a system weighing 20 pounds or less — that can fly for days, providing a persistent ISR capability on zero fuel, Ripley said.
The system will use thermal updrafts to remain in the air all day, collaborating with other drones to pinpoint current and potential thermals, he said. A thermal is a rising current of warm air that is produced when the sun heats an area of the ground. The research laboratory developed the algorithm that locates those currents. A collaborative soaring algorithm developed by the postgraduate school will enable the systems to communicate with each other, he said.
In conjunction with thermal soaring, the system will harvest and store solar power through high efficiency photovoltaic cells developed by the research laboratory, enabled in part by Semprius’ manufacturing process for printing multi-junction solar cells on top of each other. According to the program’s executive summary, the novel, stacked multi-junction solar cell has the potential to achieve 40 percent efficiency after one day spent under the sun, compared to the approximate 33 percent limit for present technologies.
Jennings said thermal and solar-powered UAVs reduce soldier load by eliminating the need to carry extra batteries and fuel into the battlefield. “If you can just collect the energy that you need in order to run your aircraft from the sun — the solar environment or thermals — then that’s energy you don’t have to bring in,” he said. “You don’t have to haul it, you don’t have to protect it and nobody can cut that energy supply line.”
Ripley said initial modeling for the system demonstrates an endurance of 19 to 20 hours. “Once the higher efficiency solar is added, we believe we can make the endurance nearly indefinite or multiple day-night cycles.”
The limiting factor for the program will be the efficiency of the solar panels and the amount of energy that can be harvested and stored throughout the day, he said. “The amount of energy stored determines if the aircraft will be able to extend through the night when thermal updrafts are typically not available.”
The individual pieces of technology are being developed for initial integration and testing this November, he said. The first test demonstrating the consolidated concept is scheduled for the end of 2015. A full systems demonstration will take place sometime in 2016, Ripley noted. Operational testing is expected by fiscal year 2017.
Members of industry are also researching alternative solutions for powering small UAVs.
In July, Cella Energy Ltd., a United Kingdom-based company that specializes in high-performance, low-cost hydrogen energy, announced a partnership with Israel Aerospace Industries to develop and evaluate a fuel cell power system for IAI’s BirdEye 650 mini UAV. The system will be based on Cella’s solid hydrogen fuel system that uses the company’s patented hydrogen storage material.
The material is based on ammonia borane — a chemical compound that was identified by the Department of Energy as a leading candidate for solid hydrogen storage, said Alex Sorokin, CEO of Cella Energy.
“The advantage with solid hydrogen — obviously done in the right way — is a safety factor,” he said. “In other words you can store it without having to worry about it discharging like lithium-ion batteries, or … if the casing itself ends up being punctured, you don’t have to worry about the material catching fire the same way you do with lithium-ion batteries.”
It also has a safety and logistical advantage over compressed hydrogen, he noted. The material, which looks and feels like plastic, has a low toxicity level and although it is flammable, it is no more dangerous than gasoline, according to the company.
Another advantage is the weight of the material, which is three times lighter than a lithium-ion battery, Sorokin said.
Cella’s product could more effectively use space compared to traditional batteries, according to Sorokin. “Our material doesn’t necessarily have to be confined to a very specific geometry or shape that batteries do.” It can be conformed to any size or dimension, he noted.
The company’s objective is to reach an endurance of up to 12 hours, which can vary depending on the model and characteristics of the UAV being used, he said.
Airbus Defence and Space, a division of European aircraft manufacturer Airbus Group, has also been involved in exploring alternative propulsion methods for lightweight UAVs.
The company is currently developing the Zephyr Z8, a solar-powered electric UAV that can fly at altitudes over 65,000 feet. The Zephyr system is powered by the sun during the day through lightweight solar arrays that drive electric motors and charge reusable batteries, which enable the aircraft to stay up overnight, said Paul Brooks, principal consultant for Airbus high altitude pseudo-satellites.
Zephyr Z8 will look to solve problems with persistent high altitude surveillance, a task currently performed by satellites, he said.
“When you’re using satellites, they’re flying around in low-Earth orbits at seven kilometers a second. They spend very little time over the areas of interest that you want to look at,” he said. “You then have to put a lot of satellites into orbit to enable you to catch what you want to see and monitor things persistently, so the idea of a vehicle that can stay at high altitude and just stay above an area and continue to look around that area” is attractive.
The Zephyr platform could also be valuable for tactical communications, he said. Current satellites can adequately perform communications tasks but they are not as flexible as drones, he said. “If you suddenly have a need for communications in an area — a disaster occurs or you need to put something in place — you just can’t reconfigure satellites very quickly,” he said. “The idea of putting up something which can allow communications over a wide area … 400 miles across or so from a platform you can just fly into position” would provide added value to both military and commercial customers.
The Zephyr Z8 will be a lighter and more efficient version of Zephyr 7, the company’s prototype platform.
Zephyr 7 has a mass of 110 pounds — including its payload, structure, batteries and solar array — making the system too large to be categorized as a group one or group two UAV, which weigh 55 pounds or less. However, in comparison to similar systems, it is very light, Brooks said. The Zephyr 7 holds the world record for UAV endurance, flying continuously for two weeks in July 2010, he said. In 2014 the platform completed a test flight staying up for 11 days in winter conditions, a more demanding task due to the shorter days and longer nights, which limit exposure to the sun, a company press release stated.
“What we’re looking to do is be able to fly year round over a wide range of altitudes,” Brooks said. The Zephyr 7 currently has to come down to 20,000 feet overnight, an elevation where high winds can push the system out of position. The Zephyr Z8 will be able to stay up at 50,000 feet, high enough to remain above strong winds and provide a more persistent surveillance capability, he said.
Airbus is developing first production vehicles of the Zephyr Z8, which is scheduled to have its first flight tests in 2016. Between 2017 and 2018, the company plans to make the platform available in the commercial market, Brooks said.