Wireless Power Transmission (WPT)

The idea of using the electromagnetic field as energy source dates back to the end of the nineteenth century when Nikola Tesla demonstrated for the first time the transmission over a certain distance of electrical energy without any electrical conductors.
Recently, thanks to costs reduction and development of low-power architectures, Wireless Power Transmission (WPT) has become the enabling technology for implementation of Energy Autonomous Systems (EASs), namely electronic systems designed to operate without on-board batteries or being connected to a power grid.
There are many advantages coming from the theoretically inexhaustible energy supply of these systems, especially in those applications where environmental conditions are harsh or the devices to be powered (for instance, a node of a Wireless Sensor Network – WSN)  are inaccessible once deployed.
Although the most common applications of the WPT now are charging mobile electronic devices (mobile phones, laptops), an important class of devices taking advantage of this technique is represented by implantable medical devices for the treatment of serious diseases such as acute diabetes, epilepsy, heart disease. One of the major challenges of these implantable devices is supplying them with the power necessary to perform operations. Early devices used battery-type power sources, such as, small lithium ion batteries. Although this solution allows the implanted device to work without a physical connection with an external power supply, it has the disadvantage of requiring invasive maintenance operations. In fact, due to the limited lifetime of modern batteries, it is necessary to operate on patients, with consequent risks to the health and additional expenses.
A viable solution to overcome this drawback is to wirelessly supply the necessary power. This can be obtained both by directly providing power to the implantable device or by using rechargeable batteries.
Therefore, it is extremely important to focus research efforts on efficient wireless powering of implantable medical (and not only) devices. In fact, the quality of life of people affected by debilitating diseases such as acute diabetes, epilepsy, heart disease and so on would undoubtedly benefit from the widespread adoption of WPT techniques.

WPT can be implemented by using resonant systems communicating by means of their electromagnetic field. Two main strategies can be identified depending on whether the communication uses the far or the near electromagnetic field. In a far-field communication, antennas are used to transmit and receive power; whereas, in a near-field communication electrically or magnetically coupled systems are used resulting in a so-called Wireless Resonant Energy Link (WREL).
In both cases, the electromagnetic energy collected by either the resonator or by the antenna, is converted into electrical current by a rectification circuit. For this reason, the whole system will be named rectenna (rectifying antenna) in the case of a far-field power link (Fig. 1).

Within the EML2 there is a division, coordinated by Eng. Giuseppina Monti, which develops both near-field and far-field wireless power transmission systems. Emphasis is on investigation of design strategies using non-conventional supports (such as tissues), development of wearable devices and systems for biotelemetry applications (namely, wireless power transmission for implantable medical devices). Other application fields of major interest are the development of systems for solar energy conversion into DC current (nanorectenna devices) and energy automation of sensors for domotic applications (home automation, smart home).
WPT_ENG1
Figure 1. Wireless Power Transmission: a. near-field link; b. far-field link.

WPT: Near-field wireless link
Among the research lines which are carried within the EML2 WPT division, particularly intriguing is the adoption of magnetically coupled resonant systems to establish a near field link (inductive link). Specifically, this approach is thoroughly investigated for wirelessly powering implantable medical devices, such as a pacemaker, which will no longer need batteries or wires connecting the device to an external power source (serious infections could arise from the use of wires).
A schematic representation of the inductive link is given in Figure 2. Each resonator can be represented as a resonant LC circuit; the primary resonator is connected to a power source while the secondary resonator is connected to the load. The two resonators are inductively coupled so that a current supplied to the primary resonator induces a current in the secondary resonator.
WPT2
Figure 2. Schematic representation of an inductive link.

In Fig. 3 an inductive link designed and fabricated at EML2 is reported. Specifically, this link, working at 500 MHz, is made up of two planar resonators fabricated on a low cost copper clad: the primary resonator is connected to the power source while the secondary resonator is placed inside the body and it is connected to an implanted device (Fig. 3a, 3b).
In order to experimentally evaluate the performance of the proposed inductive link in the presence of human tissues, we used minced pork (Fig. 3c). In fact, in the frequency range of interest minced pork exhibits values of electric parameters very close to the ones corresponding to the human skin and muscle.
In Fig. 3d the power delivered by the secondary resonator when the transmitted power is 1 W is reported. Received power was obtained by varying the distance of the primary resonator with respect to the minced pork surface.
Reported experimental data demonstrate that the maximum power received by the secondary resonator  is enough to feed a medical implantable device whose typical power consumption is between hundreds of µW and a few mW.
WPT3
Figure 3. Proposed inductive link: a. primary resonator prototype, b. secondary resonator prototype, c. Experimental setup adopted to verify the performance of the proposed inductive link in presence of human tissues, d. power delivered by the secondary resonator to the implantable device obtained by varying the working frequency.

WPT: Far-field wireless link
Far-field wireless links guarantee larger operating distance than near-field ones (up to some meters in the case of working frequencies of some MHz). A far-field link is implemented by means of a rectifying antenna (rectenna) that can be also utilized to scavenge the electromagnetic energy emitted by common telecommunications systems, such as mobile phones, WiFi, radio transmissions. This process is known as energy harvesting or energy scavenging.

WPT or, in general, electromagnetic energy harvesting through rectennas is a low cost and no environmental impact technology with a huge number of applications. Furthermore, new recent breakthroughs in nanotechnology would render solar nano-rectennas  (namely, rectennas that absorb sunlight and directly turned it into a DC current) a valid alternative to conventional semiconductor photovoltaics. In fact, a solar rectenna would have an efficiency which dwarfs all other solar cells. Yet another advantage with respect to a photovoltaic cell is their ease of installation as well as low production costs.

Some devices designed and fabricated at EML2

 

WPT4
Figure 4. Rectenna prototype fabricated at EML2: a. antenna; b. rectifier; c. the antenna connected to the rectifier; d conversion efficiency for different values of the frequency of the RF input signal.

A rectenna prototype designed and fabricated at EMLfor the collection of the electromagnetic energy coming from UHF RFID systems is reported in Fig. 4. The proposed rectenna consists of a compact planar bow-tie antenna and a simple rectifying circuit. Experimental results demonstrate a conversion efficiency higher than 60% over a wide bandwidth, as illustrated in Fig 4d.
Another frequency range of interest for WPT is definitely the ISM (Industrial, Scientific and Medical) band centred at 2.45 GHz. A prototype of an ISM-band rectenna developed at EML2 is reported in Fig 5. In this case, the antenna (a planar monopole) and the rectifier are integrated on the same substrate guaranteeing this way further compactness to the whole system. The maximum  conversion efficiency (45%) was achieved at 2.45 GHz (see Fig. 5c).

WPT5
Figure 5. ISM-band rectenna prototype fabricated at EML2: a. rectenna; b. detail of the rectifier; c. conversion efficiency for different values of the frequency of the RF input signal.

The development of ultra-low power architectures has minimized the power consumptions of the nodes of a WSN. Therefore, the rectenna technology promisingly fits the energy demand of a Wireless Body Area Network  (WBAN), namely a “short-range” telecommunication network made up of one or more wireless devices with power consumptions between 0.1 and 50 mW. This devices can be placed within (implantable WBAN) or in close contact (wearable WBAN) with the human body. To this regard, the EML2 deals with the design and fabrication of textile rectennas, which are rectenna fabricated on non-conventional substrates (such as wearable or paper materials). The final goal is to develop smart textiles equipped with sensing ability to monitor the environment and react to specific alarms or to monitor and measure patient health over distance (telemedicine).
WPT6
Figure 6. Wearable rectenna prototype designed and fabricated at EML2.

Examples of wireless power transmission

Wirelessly powered clock

WPT7

Wirelessly powered LED light

WPT8

EML2 on RAI 3

On March 12, 2012 Luciano Tarricone, Giuseppina Monti and Fabrizio Congedo were guests on the TV program “Geo & Geo” (RAI 3) on an episode dedicated to rectenna devices and their use for the harvesting of electromagnetic energy and wireless power transmission.

GeoScienzaRectenne

On April 23, 2013 Luciano Tarricone, Giuseppina Monti and Fabrizio Congedo were guests on the TV program “Geo & Geo” (RAI 3) on an episode dedicated to inductive wireless power transmission for implantable medical devices such as a pacemaker.

GeoScienzaPacemaker

References
1.   Giuseppina Monti, Luciano Tarricone, Marco Dionigi, Mauro Mongiardo, “Magnetically Coupled Resonant Wireless Power Transmission: An Artificial Transmission Line Approach”, in proceeding of 42th European Microwave Conference (EuMC2012), Oct. 28th – Nov. 2nd 2012, Amsterdam, pp. 233-236.
2.   Fabrizio Congedo, Giuseppina Monti, Luciano Tarricone, Broadband Bowtie Antenna for RF Energy Scavenging Applications,4rd European Conference on Antennas and Propagation (EuCAP), 11-15 April 2011, Roma. 3.     F. Congedo, G. Monti, L. Tarricone, V. Bella, “A 2.45-GHz Vivaldi Rectenna for the Remote Activation of an End Device Radio Node,” accepted for publication in Trans. On Sensors Journal, may 2013.
4.     G. Monti, L. Corchia, L. Tarricone, “UHF Wearable Rectenna on Textile Materials,” accepted for publication in IEEE Transactions on Antennas and Propagation, Vol. 61 No. 07, 2013.
5.     Leonardo Sileo, Luigi Martiradonna, Paola Arcuti, Giuseppina Monti, Vittorianna Tasco, Marco Dal Maschio, Giacomo Pruzzo, Benedetto Bozzini, Luciano Tarricone, Massimo De Vittorio, “Wireless system for biological signal recording with Gallium Arsenide High Electron Mobility Transistors as sensing elements Microelectronic Engineering”, accepted for publication in Journal of Microelectronic Engineering, 2013.
6.  G. Monti, L. Tarricone, C. Trane, “Experimental Characterization of a 434 MHz Wireless Energy Link For medical Applications”, Progress In Electromagnetics Research C, Vol. 30, 53-64, may 2012.
7.     G. Monti, F. Congedo, D. De Donno, L. Tarricone, MONOPOLE-BASED RECTENNA FOR MICROWAVE ENERGY HARVESTING OF UHF RFID  YSTEMS, Progress In Electromagnetics Research C, Vol. 31, pp.: 109-121, 2012.
8.     Monti, G.Corchia, L.Tarricone, L., “ISM band rectenna using a ring loaded monopole,” Progress In Electromagnetics Research C
, Vol. 33, pp.:1-15, 2012.
9.  G. Monti, L. Tarricone, M. Spartano, “X-Band Planar rectenna”, Antennas and Wireless Propagation Letters, Volume: 10, pp.: 1116-1119, ISSN: 1536-1225, 2011. 10. G. Monti, F. Congedo, “UHF Rectenna Using a Bowtie Antenna”,Progress In Electromagnetics Research C, Vol. 26, pp. 181-192, 2012.