WIRES have a lot going for them to move electric power around. But they also have their limitations. People are not uncommon to get tired of unplugging their phones and other rechargeable gadgets. It is a hassle.
Electric utilities also face challenges from wires: They must ensure that the voltage applied to transmission cables is very high to prevent dissipating most of the power. When powering public transport, such as electric trams and trains, wires must be used in conjunction with sliding or rolling contacts. These contacts can spark and, in certain settings, can generate dangerous contaminants.
These issues are urgently needed. Wireless charging has been widely adopted over the past decade, not only for consumer electronics but also for vehicles. Wireless charging is convenient because you don’t have to disconnect and disconnect cables often. However, it can deliver energy over a very short distance. It is difficult to charge or power a device if the distance between them is less than a few centimetres. Are wires really necessary to transmit power across greater distances?
Some people associate wireless power transmission with Nikola Tesla’s lightning bolts and high-voltage coils. This would not be a stupid connection to make. Tesla was interested in using the atmosphere and ground as conduits for long-distance power transmission. However, this plan did not work out. However, his dream to send electric power over great distances and without wires has remained.
Guglielmo Marking, Tesla’s contemporary, discovered how to use “Hertzian waves,” or electromagnetic waves, to transmit signals over long distances. This breakthrough allowed the use of the same type of waves to transport energy from one place. This is, after all, how all the energy stored in wood, coal, oil, and natural gas originally got here: It was transmitted 150 million kilometres through space as electromagnetic waves–sunlight–most of it millions of years ago.
Is it possible to harness the same fundamental physics to replace wires? Here are some reasons my colleagues and I believe so at the U.S. Naval Research Laboratory in Washington, D.C.
Although there have been some attempts over the last century to use electromagnetic waves for wireless power transmission, these efforts produced mixed results. 1975 was perhaps the most important year in wireless power transmission research. Richard Dickinson, a retired NASA scientist, and William Brown (of Raytheon) used microwaves to transmit power through a laboratory with more than 50% efficiency. They could also transmit more than 30 kilowatts for a distance of approximately a mile (1.6 km span>).
These demonstrations were part of a larger NASA/U.S. program. These demonstrations were part of a larger NASA and U.S. Department of Energy campaign to investigate the feasibility of solar power satellites. They would, it was suggested, harvest sunlight from space one day and beam it down to Earth in microwaves. This research was largely motivated by the 1970s energy crisis. Interest in solar-power satellites declined over the next decades.
While researchers are still interested in the possibility of solar-power satellites, actual power beaming demonstrations have not been able to exceed the 1975 high water mark for efficiency, distance and power. Fortunately, this situation is changing thanks to recent transmission and reception technology advancements.
Early efforts to beam power used microwave frequencies. This is the same electromagnetic spectrum that hosts Wi-Fi and Bluetooth today. This was partly due to efficient microwave receiving and transmitting equipment.
However, there has been an improvement in the efficiency and availability of devices operating at higher frequencies. Researchers have concentrated on optical, microwave and millimetre-wave frequencies to overcome limitations in transmitting electromagnetic energy within certain parts of the electromagnetic spectrum. Although microwave frequencies are more efficient than other frequencies, larger antennas are required. For many applications, optical links or millimetre-wave frequencies work better.
Systems that use microwaves or millimetre waves typically have solid-state electronic amplifiers and phased array, parabolic, and metamaterial antennas. Rectennas are an array of elements that make up the receiver for millimetre-wave or microwave waves. This term, a combination of antenna and rectifier, describes how each element converts electromagnetic waves into direct current electricity.
A laser is the most common type of laser used in optical power transmission systems. It has a narrow beam like a fibre laser. Specialized photovoltaic cells are used to convert one wavelength of light into electricity with high efficiency. Indeed, efficiency can exceed 70%, more than twice that of a standard solar cell.
The U.S. The U.S. Naval Research Laboratory has spent 15 years investigating power beaming options and exploring potential applications. These include increasing the flight time and payload capacity of drones, powering satellites in darkness, powering lunar rovers in permanently shadowed areas of the moon, sending energy from space to Earth’s surface, and providing energy for troops on the battlefield.
A device that sends large amounts of energy through the air in a narrow beam might sound like a deathray. This brings us to the core of an important consideration: power density. Technically, there are many power density options. They can range from too low for practical use to dangerously high. It is possible to find a happy middle ground between these extremes. Clever ways to allow beams with high power density to be safely used. This is exactly what I was part of in 2019, and we have successfully expanded this work.
PowerLight Technologies (formerly known as LaserMotive) is one of our industry partners. They have been creating laser-based power beaming systems for over a decade. This company is well-known for its success in powering fixed-wing drones, quadcopters, and robotic tether climbers. It also won the NASA Power Beaming Challenge 2009. This is crucial because while many research groups have demonstrated laser beaming over the years, including teams at the Naval Research Laboratory and Kindai University, Beijing Institute of Technology, The University of Colorado Boulder, JAXA and Airbus, only a handful have been able to do it safely in every possible circumstance.