Up to now, batteries have been used as a major energy source for wearable or portable devices, an approach presenting several intrinsic drawbacks, including size and weight, since they occupy a significant fraction of a system. Following the advances in low power design, lower power consumption of electronics opens the possibility to harvest energy from the environment to power directly the electronics or recharge a secondary battery. This kind of new techniques is called “Energy Harvesting (EH)”. The key parameter of an energy harvesting (EH) device is its efficiency, which strongly depends on the conversion medium. In recent years, metamaterials (MMs), artificial materials and structures with exotic properties (such as negative stiffness, mass, Poisson's ratio, and refractive index), have been introduced for EH applications. However, the current resonant MM technologies do not generate enough power, especially under the existing size and weight restrictions, while, the power harvesting bandwidth is usually narrow, down to a few Hz around their resonance frequency. As a result, even a slight frequency shift results in a steep power output reduction. Within the proposed project, we will explore wireless power transmission (WPT) devices based on metamaterials, aiming to develop efficient energy harvesters with extended range and improved flexibility. For the first time, we will simulate, design, fabricate and verify experimentally the use of MM-based WPT systems, with a range of some hundreds of MHz, or even GHz, by changing their geometry. Moreover, we will study, both theoretically and experimentally the use of transparent conductive MMs, suitable for energy harvesters in buildings’ glasses employed in smart rooms, hospitals etc. Finally, mechanically flexible polymeric MM-based WPT systems will be demonstrated in novel applications, such as wearable devices, human body sensors, and everyday use electronic devices such as mobile phones etc.
Wireless power transfer (WPT) has a long history of over 100 years, dating back at least to Tesla in 1893. In recent years, WPT is re-emerging due to rapidly increasing demands in new applications. For example, WPT technology is deployed to provide wireless charging for batteries of smart phones and wearable devices, requiring frequent recharging, as well as in cases where a mechanical charging socket may wear out in normal use. WPT is promising in many areas with different power levels, from implantable medical devices (few mW) to electric vehicles (few kW to tens of kW). The rapid research progress of functional metamaterials has provided a new approach for electromagnetic (EM) energy reception. Metamaterials, consisting of sub-wavelength periodic particles, are manmade materials that possess desired bulk effective permittivity (εr) and permeability (μr) enabling a variety of functional applications. Among other applications, the metamaterial-based WPT systems have been proposed in recent years as a novel EM energy harvester. Quite different from the rectenna, the MM-based WPT systems collect the EM power by an array of periodic metamaterial particles instead of a receiving antenna, having at the same time much smaller electrical dimensions (usually 1/14-1/4 wavelength in contrast to 1/2 wavelength in antennas). In the MM-WPT systems, the metamaterial particles are designed to have identical εr and μr to yield the same impedance as free space, expecting to completely capture incident EM waves with no reflection. By properly integrating rectifiers, the MM-WPT system is able to further convert the captured EM power to DC. Just as with the rectenna, the overall efficiency (here called harvesting efficiency) of a WPT system is defined as the product of two efficiencies: the capture efficiency and the conversion efficiency. The former is the ratio of captured EM power to spatial EM power incident on the surface, and the latter is the ratio of DC output power to captured EM power. For maximum harvesting efficiency, the WPT system needs precise design to realize perfect impedance matching (between free space and the surface, as well as between the metamaterial particles and the rectifiers), which is difficult due to the use of nonlinear components like diodes. Additionally, existing MM-based WPT systems usually expose the diodes in high power EM incidence, so the diodes’ performance may be hindered. Hence, the design of an efficient WPT system is still a major challenge.
Within the META-ENERGY project, we will perform simulations on the implementation of metamaterial-based WPT systems with different designs. Moreover, we will fabricate samples and show experimentally that the power transfer efficiency can be improved significantly by a metamaterial.
The META-ENERGY project deals with novel applications of Metamaterials in energy harvesting. It identifies the main obstacles, proposes specific approaches to overcome them, and investigates unexplored capabilities of those materials. The project objectives are focused on: Energy harvesting in microwaves/converting the microwave signal to electrical power, and are based on the following tasks.
- Simulation, design, fabrication and verification experimentally of the use of metallic MM-based WPT systems, with a range to some hundreds of MHz, or even to GHz, by changing their geometry.
- Demonstration of the feasibility of radio frequency energy harvesting (RFEH) with metamaterials, using an array of MMs to enhanced electromagnetic (EM) energy to create DC energy. Radio frequency energy is emitted by sources that generate high EM fields such as TV signals, wireless radio networks and cell phone towers, but through using a power generating circuit linked to a receiving antenna, this free flowing energy can be captured and converted into usable DC voltage.
- Design, fabrication and verification experimentally of the use of resonant-coupling metamaterial-based energy harvesting systems operating in the frequency region of cellular phones and Wi-Fi internet bands (825-2690MHz), as well as existing 3G and 4G bands.
- Additionally, we are going to study theoretically and fabricate metamaterial-based WPT systems made of transparent conductive oxides’ (TCOs) meta-atoms, suitable for typical use as energy harvesters in buildings’ glasses, in smart rooms, hospitals etc.
- Finally, mechanically flexible conductive polymers using simple approaches such as 3D printing, which can be used in novel applications, such as wearable devices, human body sensors etc.
Our work will show that, besides communication and other low-power applications, metamaterials can also be used in high-power energy applications.
In all the cases, besides performing theoretical simulations, we will demonstrate energy harvesting also experimentally through structure fabrication and characterization.
In the proposed research project, we will explore WPT devices based on metamaterials-related ideas, aiming to extend the range and improve the flexibility of an efficient energy harvester system. Within the proposed project, we will fill the gap in the field of WPT applications, using rigorous simulations, design, fabricate and verify experimentally the use of MM-based WPT systems, extending their range to some hundreds of MHz, or even to GHz, by changing their geometry. We will study for the first time, both theoretically and experimentally, the use of transparent conductive MMs, suitable for typical use as energy harvesters in buildings’ glasses, in smart rooms, hospitals etc. Finally, mechanically flexible polymeric MM-based WPT systems will be demonstrated in novel applications, such as wearable devices, human body sensors, autonomous everyday use electronic devices such as mobile phones etc.
A MM-based WPT prototype will be designed and fabricated, providing energy to a typical mobile phone. The main concept is that by attaching a flexible WPT system to a mobile phone, it will be transformed to an autonomous energy system by taking advantage of the available 3G or 4G EM band that is either way necessary for its everyday use. Moreover, several low-power body sensors could be attached to the WPT device providing biometric measurements of the user without energy cost.
The scientific methodology is based on the synergy of rigorous full-wave simulations and actual measurements, in consecutive design steps to achieve an optimal working solution.
Regarding the theoretical design methodology for efficient metamaterial-based energy harvesting systems, two main goals are identified;
- First, the design of a reflectionless metasurface which at the same time fosters high electromagnetic fields in the resonant meta-atoms. This goal can be successfully met by relying on interference effects between different resonances, so that the aggregate scattered field in the far zone is cancelled. As a result, we achieve complete absence of reflections and at the same time high power density on the metasurface due to the acting resonances. In addition, the absence of reflections allows us to cascade multiple metasurfaces and further enhance the overall efficiency of the energy harvesting process. For selecting the appropriate metasurface design, we will identify the pertinent physical considerations and comparatively assess the wide range of available resonant meta-atoms in the literature. The frequency of operation critically depends on the physical/electrical size of the resonant meta-atom. We will, thus, produce designs that can cover the different regimes of operating frequencies in a directly scalable manner.
- The second design consideration concerns the rectifying circuit design and its integration with the metasurface. More specifically, an important goal is to maximize the power transfer to the rectifying element, by engineering the radiation resistance of the meta-atom and the rectifier load impedance. Different rectifying circuits will be considered. In addition, different integration approaches with the metasurface will be investigated. An initial approach will be based in integrating PN junction diodes in each meta-atom at suitable positions and avoiding the use of biasing circuits that further complicate the system.
After completing simulations, and once we have determined suitable designs, we will fabricate the resulting planar and 3D MM samples following standard PCB, PVD, and CVD techniques, as well as 3D printing.
As already reported, there are two main proposed categories of planar metamaterial-based WPT systems: (i) metallic, and (ii) TCO-based ones, on either glass, or polymeric flexible substrates.
- Typical screen printing circuit technology will be adopted, in order to fabricate copper-based MMs on PCBs, for prototyping. The same approach will be applied on polymer membranes in order to build flexible WPT devices. For the final demonstrator WPT structures PCB fabrication may be outsourced to specialized companies in case that low-loss substrates and/or finer fabrication resolution (linewidth/spacewidth) are required.
- Thermal evaporation (PVD technique) letting us create thin films and patterns (such as MMs) from metals or semiconductors on several substrates such as glass or polymeric sheets.
- Chemical techniques such as CVD, spray and ultrasonic spray pyrolysis, giving us the possibility to fabricate TCO-based WPT systems on several substrates such as glass, flexible polymers, etc.
In order to build 3D MM-based WPT devices we are going to produce polymer based conductive filaments using either commercial or homemade extruders available in IESL-FORTH. Considering that nanoparticles keep their own properties (e.g. conductivity) unaffected, after the blending with the polymer matrix, the produced filament could exhibit corresponding functionalities. This way, polymer based filaments doped with graphene, carbon nanotubes, metal nanoparticles or metal oxides are going to be used in our 3D printers, in order to fabricate 3 dimensional polymer-based WPT systems.
Once the samples mentioned above are completed, we will proceed with their characterization.
- The metallic metamaterial platforms and WPT devices will be measured by means of a microwave network analyzer, in order to verify that they exhibit the simulated resonance at the required frequency. Afterwards, the produced DC voltage of the fabricated WPT systems is going to be measured using typical Volt-Ampere-Ohm meters, to more sophisticated multi-meters with high sensitivity, in order to verify the efficiency of the WPT devices.
As regards the TCO-based and polymeric-based MM platforms and WPT devices, one more set of characterization should be provided in order to verify their structural and chemical identities:
(a) FT-IR, Raman and Multispectral spectroscopy measurements will be performed in order to check the chemical bonds of the as grown semiconductors, their purity, and their distribution in a 3D printed polymeric matrix (using mapping measurements)
(b) X-ray diffraction (XRD) measurements will be performed to both TCOs’ and 3D printed samples in order to reveal their crystal structure.
Especially for the 3D printed MM platforms, their mechanical properties will be studied using typical tension and compression tests, according to international standards (such as ASTM 638/95 Type V). This way we will indeed verify that the mechanical properties of the flexible and polymeric samples of the META-ENERGY project would be such that they can be easily incorporated in realistic devices.
The proposed research and scientific methodology feature distinct novel aspects as listed below:
- The demonstration of TCO-based energy harvesting metamaterials, besides conventional copper (PCB) metamaterial systems.
- The investigation of novel interference phenomena for achieving at the same time reflectionless operation and high local fields, thus enhancing the efficiency of energy harvesting
- The demonstration of flexible metamaterial-based energy harvesting systems that can find use in important everyday applications, such as wearables, consumer electronics and health monitoring devices.
- The use of 3D-printing techniques to allow for deviating from planar structures and using tailored material solutions for constructing the harvesting metasurface/metamaterial structure.
The work packages (WPs) of the META-ENERGY project, the Deliverables, and Milestones are:
WP1 (Literature review, Monthv1-Month 3): We will explore the existing resonant MMs in energy harvesting applications and study their failure for WPT applications.
WP2 (Simulations, Month 1 - Month 12): Simulations on the implementation of metallic metamaterials in WPT systems with different designs.
Milestone 1 (Month 12): Demonstration of metallic MM-WPT wide range systems (hundreds of MHz).
WP3 (Experiments, Month 6 - Month 18): Experimental verification of radio frequency energy harvesting (RFEH) with MMs.
Milestone 2 (Month 18): Demonstration of radio frequency energy harvesting (RFEH) with MMs.
WP4 (Simulations & Experiments, Month 12 - Month 24): Design, fabrication and demonstration of MM-based WPT systems operating in frequency region of cellular phones and Wi-Fi internet bands.
Milestone 3 (Month 24): Demonstration of mobile and Wi-Fi bands energy harvesting with MMs.
WP5 (Simulations & Experiments, Month 20 - Month 36): Design, fabrication and demonstration of WPT systems based on TCOs’ meta-atoms.
Milestone 4 (Month 30): Demonstration of WPT systems based on TCOs.
WP6 (Simulations & Experiments, Month 20 - Month 36): Design, fabrication and demonstration of mechanically flexible WPT systems based on polymeric meta-atoms, through 3D printing etc.
Milestone 5 (Month 36): Demonstration of mechanically flexible WPT systems based on polymers.