Optical power transmission lights up remote possibilities
When the US military sets up an operation in a remote locale, one of its major challenges is securing enough power to run equipment, from portable radios, to computers, to lights. It can be a risky and time-consuming task, often involving shipping fuel for a generator across an ocean, then perhaps transporting the fuel via helicopter across hostile areas. It would be convenient if the military could just beam power through the air to its encampments.
In fact, such power beaming is the aim of a program of the Defense Advanced Research Projects Agency (DARPA), which recently issued a call to contractors to develop equipment for the Persistent Optical Wireless Energy Relay (POWER) project. While sending power through the air via beams of light has been demonstrated before, “the idea with POWER is to extend the distance over which we can effectively do power beaming by orders of magnitude,” says program director Paul Jaffe.
The project will use one of the lasers at the Defense Department’s High Energy Laser Systems Test Facility in New Mexico and fire a beam up to an airborne drone equipped with an optical receiver. That drone would relay the beam to another and another, until finally it is sent down to a receiver at its destination. Phase one of the project, scheduled to be completed by March 2025, has contractors such as Draper and RTX Corporation working on the optical relays. If it’s successful, DARPA will go on to test the relays on aircraft, and then finally demonstrate the concept by delivering 10 kW of optical power, enough to run an average house for a few hours, to a receiver 200 km away. “The long-term vision is, instead of having a fossil-fuel-powered aircraft, you would have an electric aircraft that could be powered by siphoning off a little bit of the energy that is passing through the relay,” Jaffe says.
At its most basic, optical power transmission is the idea of converting electrical energy to optical energy, such as a laser or fiber-optic, and back again to electric, where it can be used by anything that needs to be plugged into a power supply.
The concept of using lasers to beam energy to where it’s needed has been around since the 1970s, when people imagined satellites that would collect solar power from the Sun and transmit it from orbit to the ground. “After that, there was not much active work on it until the early 2010s,” says Tomoyuki Miyamoto, an associate professor at Tokyo Institute of Technology, who this year chaired the 6th Optical Wireless and Fiber Power Transmission Conference in Japan. Interest in the area has been increasing, however, and academics, startup companies, and telecoms are exploring optical-power transmission for a variety of uses, from the high-power, long-distance aims of the DARPA program to shorter-range, lower-power optical chargers for Internet-of-Things (IoT) devices, as well as power delivered over optical fiber.
Some researchers predict that practical optical power systems could begin to show up commercially within a few years. All the pieces of such systems exist—lasers or LEDs to act as the light source, monochromatic photovoltaics to receive the beams, and methods to aim the beam, keep people safe, and cool the components. Most of the development lies in improving the components to increase efficiency and extend the distance and power that the systems could handle, and to make them smaller and less expensive.
The main niches for optical power would be places where plugging something into a wall, swapping out batteries, or fuel delivery wouldn’t be practical. People envision providing power optically to drones, which could stay aloft indefinitely, instead of the 10 to 30 minutes they currently last. Optical power could be useful for shelf-mounted displays in stores or for sensors in warehouses, where changing batteries is labor intensive and expensive. It could power remote equipment, such as field sensors. Optical power transmitted over fiber would be useful in environments where electricity delivered over copper might suffer from electromagnetic interference or pose a risk because of the possibility of sparks in, say, a sensor inside a fuel tank. Light in the blue wavelength might even be used to supply energy to underwater robots used for research or inspection.
Optical power beamed to airborne drones could power them indefinitely. Photo credit: PowerLight.
PowerLight Technologies is a company that sees a wide variety of uses for optical power transmission, such as using portable equipment to supply temporary power to an area hit by a hurricane or earthquake. They’ve done a pilot project with telecom Ericsson on powering 5G wireless base stations, allowing the company to quickly expand its 5G network with less time and expense than installing traditional cell towers.
PowerLight worked on another DARPA-funded project, the 10-year Lunar Architecture study, to explore using light beaming to send power to lunar rovers looking for ice in the permanent darkness of Moon craters. One end of the system would be on the crater’s rim, collecting energy from sunlight, and fire its laser toward a photovoltaic array on the rover. The lack of air on the Moon provides an advantage, because the beam wouldn’t have to deal with atmospheric effects or water vapor that challenge other Earth-based light-beaming setups, says Tom Nugent, co-founder and chief technical officer at PowerLight. “But the lack of atmosphere also means thermal management is more difficult. There’s no air to blow over your stuff to cool it.”
Of course, optical power transmission is inherently inefficient. There’s loss when the energy is converted from electrical to optical, and an equivalent loss when it’s converted back. There’s loss when the light travels through air or fiber. And there are energy costs in the overall system, such as cooling the laser. Jaffe says DARPA is aiming for a system efficiency of about 10 percent. A test carried out by the US Naval Research Laboratory on the International Space Station last year, the Space Wireless Energy Laser Link, beamed 1.5 W over a distance of 1.45 m across space with an end-to-end efficiency of 11 percent. Though the numbers may seem small, the NRL says it’s the highest power, longest distance, and most efficient power beaming demonstration in orbit, and that the efficiency was 10 times the team’s goal.
But flying fuel into combat sites is not exactly efficient either, Jaffe says, and in cases where the choices are inefficient power transfer or no power at all, the question becomes moot. “If you want to power the sensor up on the mountaintop, the Marine is not going to be like, ‘Well, is this going to be efficient?’ They’re like, ‘Is it going to power the sensor?’ As long as we’re accomplishing the mission, that’s the key success metric,” he says.
“Real systems may have 5, 10, 15 percent efficiency,” says Henning Helmers, a photovoltaics expert at the Fraunhofer Institute for Solar Energy Systems in Germany. Normally, he says, losing that much energy wouldn’t make sense. “But it does make sense where you enable applications that are simply impossible otherwise.”
In a setup built with today’s commercially available lasers and photovoltaic receivers, a system efficiency of roughly 16 percent is about the best that can be achieved, Miyamoto says. Researchers have experimentally shown efficiency improvements for both sources and receivers that might get a system up to 50 percent overall efficiency in the near future. That’s likely pushing the limits, though. “I believe that an efficiency of 70 percent or more is impossible” for optical power transmission, Miyamoto says. PowerLight, for its part, has a roadmap to get to 30 percent system efficiency over the next five years.
The amount of power transmitted and the distance it has to travel can vary widely, depending on the application. Where DARPA is aiming for kilowatts across kilometers, IoT devices would require only a few milliwatts sent across a few meters. The Japanese telecom company NTT reported last year it had delivered what it called a record-breaking (for the type of system) 1 W across 30 km of fiber, whereas Motoharu Matsuura, a professor at the Institute of Electro-Communications in Tokyo, says his group has sent more than 40 W of electrical power a distance of 300 m using a double-clad fiber. For context, a phone charger requires about 5 to 10 W, whereas a laptop can use 100 or 200 W.
Low-power transmission over standard optical fiber is fairly easy to achieve, says Nugent. That could be useful in places where telecom companies want to provide new optical communications but don’t want the added burden of installing a power supply, or where they need to avoid electromagnetic interference, perhaps around sensors. But optical losses as the beam travels through the glass fiber make long distances a challenge, especially since the glass is less transparent at the 800 to 1,100 nm wavelengths coming from high-power lasers. “It’s probably not practical to go more than eight or so kilometers if you’re trying to do high power,” he says.
Optical power transmission would eliminate the need for bulky cables for future Moon missions. Photo credit: PowerLight Technologies.
Though there are probably billions of kilometers of optical fiber installed around the world, that fiber is not very practical for power transmission, Nugent adds. It carries only a single mode of light and cannot handle much power. The NTT work relied on a multicore fiber, able to carry more light along the same cable. And researchers such as Matsuura are testing systems with hollow-core fiber, where the beam travels through empty space, but inside a fiber. “Since the core is not made of glass, nonlinear effects are extremely small,” he says, and power can be transmitted at a choice of wavelengths. But it’s still difficult to manufacture large quantities of hollow-core fiber.
Different setups also use different laser sources. Jinyong Ha, a professor in the Center for Photonic Systems at Sejong University works with lasers emitting at 1,550 nm, the popular telecom wavelength, for systems that will transmit 300 mW across up to 30 m. PowerLight Technologies researchers prefer lasers in the 800 to 1,100 nm range for their higher power applications. “When we’re focusing on kilowatts of power, you can buy multi-kilowatt diode lasers and multi-kilowatt fiber lasers,” Nugent says. “You can’t really buy multi-kilowatt 1550 lasers nowadays. Not ones that have any useful lifetime.”
The longer infrared wavelength, on the other hand, is safer for indoor use, where a misdirected laser beam could strike someone’s eye. Eye safety is the main concern with any laser beams traveling through free space, where, depending on wavelength and power, damage can occur in microseconds. The intensities of some systems—again, depending on their design—can exceed skin safety thresholds, but the exposure time for damage to occur is much longer, giving people time to react. Any system will need to manage how long a human is exposed to the laser beam and shut it down immediately if there is a safety issue.
The same applies to high-power systems for airborne charging. The DARPA setup includes a way to monitor something entering the beam, such as a bird, and shut it down immediately. In an industrial setting, where there might be beams sending power to sensors mounted on the ceiling, the system can be designed to be safer by aiming its beams over the heads of any people in the room. Still, says Nugent, safety remains a paramount concern.
On the receiving end of the system, William Scheideler, an electrical engineer at Dartmouth University, is looking at making photovoltaics with perovskites, a technology that solar cell researchers have been heavily pursuing, to act as receivers for IoT devices. Perovskites are a family of materials that share a crystalline structure with a common mineral. That structure makes it more efficient at converting solar energy into electricity than the widely used silicon solar cells, and at the single wavelengths required for optical power, they can have efficiencies in the range of 60 to 70 percent, Scheideler says.
The energy characteristics of the perovskite also allow the creation of extremely thin absorber films, less than 1 µm thick, which is 100 to 200 times thinner than a silicon solar cell. That would allow for a lightweight, flexible light receiver appropriate for the small devices of the IoT. The anticipated tens of billions of sensors and transceivers, powering everything from product tracking to smart refrigerators, could clearly benefit from optical power transmission, allowing them to be installed in more places and objects. “There’s just not really an option to go around and change the batteries in all of these devices,” Scheideler says. “If you can start to fix some of those power constraints by charging them with light, then I think we can get a lot closer to that initial vision that people had for the Internet of Things.”
Wi Charge is selling a system that plugs into a wall socket and emits infrared light to charge nearby devices, such as advertising displays for store shelves. But Miyamoto says optical power needs another three to five years of technological work to get to the point where many customers might start to find it appealing, after which companies would start developing new products. “I think it will take five to 10 years for [optical power transmission] to have a major impact on the economy and society,” he says.
Nugent, though, is optimistic. He says PowerLight will have a prototype product out within a year or so. “Delivering power through the air—real, useful amounts of power over serious distances—is much closer to reality than a lot of people realize,” he says.
Neil Savage writes about science and technology in Lowell, Massachusetts.
