Metamaterials — Part III
How metamaterials are changing power transmission, wireless charging, and energy harvesting
In the first blog post of this series on Metamaterials, we introduced you to these fascinating materials and discussed applications in thermal management, noise and vibration reduction, and seismic-proof buildings. In the second blog post, we discussed how metamaterials are transforming consumer electronics, optics, and laser applications, including automotive Light Detection and Ranging (lidars). Today we will focus on power and energy applications.
As a reminder or for those who are new to this series, here is a quick summary on Metamaterials. They derive their name from the Greek word meta, meaning “beyond” and the Latin word materia, meaning “matter” or “material”; they are artificially structured materials with the objective to overcome the limitations associated with conventional materials, changing the boundaries of materials science and opening up a plethora of new prospects for innovation and economic progress. The materials consist of assembled structures that repeat at specific patterns smaller than the wavelength of the phenomena that they can sway. The size, shape, geometry, orientation, and arrangement of these structures, made of composite materials such as plastic, metal, or ceramic, give them their unique properties to manipulate the electromagnetic or mechanical waves of the phenomena they influence.
Power transmission
Wireless transmission has always been a dream since Nikola Tesla.
Companies like Metapower in the United States and Emrod in New Zealand are developing wireless power transfer transmitting systems.
Metapower developed a power-beaming system that shoots microwaves at a metamaterials-based reflective array the size of a chalkboard, which focuses the waves on their intended target. The reflector can be shifted electronically to track a moving target and provide power across distances of several hundreds of feet, through dust or fog. Metapower’s system operates within an unlicensed spectrum band, i.e. industrial, scientific, and medical (ISM), and requires little bandwidth, so that wireless communication protocols like Wi-fi or Bluetooth can continue to operate in its presence. The advantage of using the metamaterial components is that the microwave can be focused and steered with high efficiency, and no mechanical moving parts. The biggest challenge for this type of system is that it requires safety measures to limit the exposure of humans to the microwave beam in accordance with federal safety guidelines. This could be achieved with a monitoring system that switches off the beam when a person walks close to the beam.
Emrod is a New Zealand company that has developed the first long-range, high-power wireless transmission system, eliminating the need for traditional copper wiring infrastructure to support the power grid. The company is doing a commercial pilot with PowerCo, the country’s second-largest power distributor company.
Emrod uses beam shaping and metamaterials technology to create columnated beams that safely transmit power over many kilometers with no radiation around the beam, as there is with high-voltage wire transmission.
The great advantage of this wireless technology compared to traditional wired ones is reliability — it has fewer failure points and it is not affected by bad weather, it has lower infrastructure and maintenance costs, and it has an “eco-friendly” status — replacing lines and underwater cables minimizes the human footprint on the environment.
In fig. 4, you can see three components: 1. a transmitting antenna, 2. a relay that is essentially lossless, doesn’t require any power and acts as a lens refocusing the beam extending the travel range, and 3. a rectenna that receives and rectifies the beam back to electricity. Metamaterials allow us to convert wireless energy back into electricity efficiently.
At the moment a one-square-meter (10.7-sq-ft) transmitter could send about 10 kW for about 10 meters (33 ft), and a 40-square-meter (430.5-sq-ft) transmitter could provide about a 30-km (18.6-mi) range, which is enough for the vast majority of applications. The efficiency of the system is approximately 70% and the loss is mostly due to the transmitting side. Traditional copper wire transmission has an efficiency of 85–90%. Despite the lower efficiency, there is room for improvement especially with the advancement of technologies related to communication, such as 5g. Also, there are already use cases where this application is already economically viable, for example where there is difficult terrain, mountains, forests, or national reserve.
Wireless charging
An interesting application for metamaterials is wireless charging. Metamaterials can be cleverly used to improve the efficiency of low-power wireless charging systems. Wireless power transfer (WPT) technologies have attracted attention in the past years for a broad range of applications: for example, low-power consumer electronics implanted medical devices, industrial and electric vehicle applications. Magnetic coupling (also called inductive coupling) is used to charge your phones or the electric toothbrushes which are placed on a charging dock or pad without the need for any alignment or electrical contact.
Another name for wireless charging is inductive charging because the transfer of energy is happening via inductive coupling. The mechanism is simple: an alternating current flows in an induction coil placed in a charging station or pad, that can be considered the transmitting coil. The alternating current generates a magnetic field, which varies in strength over time. This changing magnetic field generates an alternating electric current in the device’s induction coil (also known as receiving coil), which is then converted to a direct current by a rectifier. Finally, direct current charges a battery or supplies power to a device. We can only achieve very high efficiency of magnetic coupling at small distances, usually distances less than a few centimeters. A way to increase the distance between the transmitting and receiving coils is to use resonant inductive coupling, where a capacitor is added to each coil in order to create two inductor, capacitor (LC) circuits with a specific resonance frequency.
Negative-index metamaterials (NIMs) can greatly improve the efficiency of wireless charging. A flat slab of NIM has negative refraction happening at both interfaces. When a transmitting object is in front of such a slab, the propagating wave components of the object can be focused inside the NIM slab and refocused on the other side of the slab. Moreover, evanescent wave components (the part of the electromagnetic wave that is formed when waves traveling in a medium undergo total internal reflection at its boundary) can be enhanced inside a NIM slab to levels similar to the levels adjacent to the original electrical conductor. Substantially, the NIM slab recovers both propagating waves and evanescent waves of an object and makes a “perfect magnetic lens” (see fig. 6).
A research group at Mitsubishi Electric Research Laboratories has fabricated the wireless power transfer system with a NIM lens. The published results show that the wireless power transmission system with metamaterials has roughly double the efficiency compared to the conventional without metamaterials (see fig. 7).
Companies innovating in this space are Metaboards based in Oxford (UK) and Metamaterials Inc. (Meta) in Canada.
Metaboards is developing innovative and very flexible techniques to integrate wireless charging into surfaces so that several products can be charged at the same time without the use of numerous power supplies or close alignment. Using a metamaterial layer that can be easily integrated into practically any surface (furniture, clothing, carpets, wallpaper, etc.), consumer devices (smartphones, tablets, laptops, cameras) can all be charged from the same surface.
Energy harvesting
The sun is a priceless source of life, providing energy to our planet in the form of light and heat. Global human population growth poses a serious threat in terms of energy scarcity and this problem is becoming an important global challenge that humankind is trying to solve with technological solutions. One approach to solve energy scarcity is to harvest free and clean energy from the sun.
Solar energy harvesting can be done in three ways: 1. The photovoltaic approach converts photon energy into electricity 2. The photochemical approach converts solar energy into storable chemical fuels like hydrogen 3. The photothermal approach converts photon energy into thermal energy by solar-thermal absorbers. The third approach has the advantage that it exploits a broader bandwidth of the solar spectrum, enabling higher conversion efficiency and the smallest carbon footprint.
An interesting application of metamaterials has been investigated by an Australian research group at the Swinburne University of Technology, which developed a solar-thermal absorber with an impressive solar to thermal conversion efficiency of 90.1%. An ideal solar absorber requires a selective and almost total absorption in the entire solar spectrum, minimized energy dissipation in the near and mid-infrared, and a tunable cut-off frequency. The group demonstrated a three-dimensional structured graphene metamaterial (SGM) that takes advantage of wavelength selectivity from metallic trench-like structures and broadband dispersionless nature and excellent thermal conductivity from the ultrathin graphene metamaterial film. The results were published in Nature Communications.
The second approach mentioned above transforms photons, i.e. visible light into energy. Thermovoltaic devices absorb specific wavelengths in the infrared spectrum instead of the visible spectrum, thus transforming heat into electricity. A group of metamaterials experts at Duke University used machine learning to optimize all-dielectric metasurfaces (ADMs), that absorb and emit specific frequencies of terahertz radiation. The ADM supercell consists of four cylindrical unit cells, as shown in fig. 9. The heights, diameter, and inter-space of the cylinders affect the frequency of light the metamaterial interacts with. The team used machine learning to find the right set of parameters to produce a certain system’s response (the frequency at which the system absorbs radiation).
Face International Corporation in Norfolk, VA., patented a new coating that appears completely opaque when applied to a surface but actually allows 80% or more of the incoming light to pass through it. The coating technology, called Spectral, can be applied like paint in virtually any color, pattern, or texture, making light-harvesting devices such as thin-film photovoltaic (PV) panels essentially invisible. Applications could be: 1. Solar-powered roof or other parts of the building which appear as the conventional counterpart (see fig. 10) 2. Electronic devices or cars being solar powered without any compromise to their aesthetics 3. Military or surveillance devices that can be completely hidden 4. Solar-powered commercial or private buildings can be energy efficient and blend with the surroundings.
In the next installment of this series, we will discuss metamaterial applications related to Telecommunications.
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