Wireless power transfer (WPT), wireless power transmission, wireless energy transmission (WET), or electromagnetic power transfer is the transmission of electrical energy without wires as a physical link. In a wireless power transmission system, a transmitter device, driven by electric power from a power source, generates a time-varying electromagnetic field, which transmits power across space to a receiver device, which extracts power from the field and supplies it to an electrical load. The ability to power devices remotely to eliminate the need for replaceable batteries or reliance on environmental inputs such as sunlight to provide energy offers attractive benefits.
Wireless power techniques mainly fall into two categories, near field and far-field. In near field or non-radiative techniques, power is transferred over short distances by magnetic fields using inductive coupling between coils of wire, or by electric fields using capacitive coupling between metal electrodes.
In far-field or radiative techniques, also called power beaming, power is transferred by beams of electromagnetic radiation, like microwaves or laser beams. These techniques can transport energy longer distances but must be aimed at the receiver. Proposed applications for this type are solar power satellites and wireless powered drone aircraft.
In the case of electromagnetic radiation closer to the visible region of the spectrum (tens of micrometers to tens of nanometers), power can be transmitted by converting electricity into a laser beam that is then pointed at a photovoltaic cell. This mechanism is generally known as “power beaming” because the power is beamed at a receiver that can convert it to electrical energy. At the receiver, special photovoltaic laser power converters which are optimized for monochromatic light conversion are applied.
Laser-based wireless power systems are characterized as having high energy density, highly directive beams, long transmission distances, and small transmission apertures. Furthering the trend towards the development of laser-based WPT, DARPA placed a small business technology transfer (STTR) opportunity announcement in Nov 2021 seeking proposals to develop a “wireless energy web.” WPT has the unique potential to transform war fighting of the future and alleviate the battlefield battery burden for both soldiers and manned and unmanned vehicles on land, air, and undersea.
The desired energy web network consists of ground laser sources providing power to airborne nodes that use this power for own ship requirements through conversion and relay the remaining power without conversion to other energy web nodes. Successful proposals will address a portion of one or more of the conversion or relay technical challenges, with a planned component demonstration that substantiates the system level improvement. Designs should support low size, weight, and power (SWAP) implementation for the airborne portions of the power beaming system.
The end goal is to attain systems that have an energy flux of 1 kW/m2 scalable up to 100 kW/m2, along with relay systems that operate optically (without electrical conversion) between nodes and have “rigorously assured photon containment” to prevent accidental injury to personnel. A wireless energy web consisting of multiple dynamic nodes will significantly improve military capabilities.
To realize these gains, this effort seeks proposals that address at least one of the key challenges in the following focus areas.
- Develop high-energy-flux, high-efficiency optical energy conversion or relay technologies.
- Develop safe optical power beaming systems that are compatible with dynamic airborne platforms acting as receivers and relays of optical energy. The links must provide rigorous photon containment and intrusion monitoring to ensure optical energy is delivered to the intended receiver without harming human bystanders or objects.
Optical power beaming system effectiveness is a combination of energy throughput and system efficiency. Traditional photovoltaic-based power beaming systems lose efficiency at high energy fluxes due to high temperatures. Novel conversion methods such as thermoelectrics are one approach to achieving higher energy fluxes but typically have
poor efficiency. This study should optimize system efficiency, accounting for losses associated with cooling, for systems with an energy flux of 1 kW/m2 scalable up to 100 kW/m2 or more.
Wireless beaming of optical energy will require at least 10 kW to 100 kW of sustained beamed optical power. These power levels are inherently dangerous and pose substantial systems design challenges, particularly for dynamic platforms operating in real world environments.
Of particular concern is the danger to bystanders who may be subject to “splash” glints or “spillover” reflections that are inadvertently directed toward unintended locations. Therefore, there is strong interest in technologies that substantially address these safety concerns through rigorously assured photon containment
Any proposed method of assured photon containment may tackle a portion of the overall problem, for example sensing glints over a nearly-spherical field of regard, designing surface morphologies that capture reflections, antireflective coatings or absorptive surfaces. Additionally, low SWAP solutions to provide continuous path monitoring to detect and react to mobile intruders into the beam are desired.
Relays are elements of the optical chain that retransmit optical energy without first converting it back to electricity. These optical waveguides could be as simple as a mirror, but will likely involve multiple components to flexibly redirect power to the intended receivers. They will need to demonstrate high efficiency and beam quality while accounting for losses such as fiber coupling inefficiencies and wavefront aberrations due to turbulence in the propagation path. Proposals should address a specific novel method for correcting beam aberrations or mitigating losses in optical chain relays. An objective system must be able to provide arbitrary beam steering over a range of 2Pi Steradians between input and output beams so component improvements should be designed to fit within this flexible architecture. Expected end-to-end relay efficiency accounting for propagation losses and any required cooling should be addressed as a metric of interest.
The systems required for efficient power beaming can be applied to many other applications such as free-space laser communications, high-energy laser propagation, LIDAR, or other high-pulse-energy or continuous power laser application. Optical systems with high tolerance for thermal loads may also be applied to systems that operate in challenging environments (not just high laser power) involving nearby sources of heat.
The outdoor use of lasers has always come with strong concern of risk to bystanders, and the methods to be developed on this effort pertaining to photon containment could be applied to a wide variety of uses of lasers outdoors.