Energy Harvesting and Wireless Power Transfer Antenna for IoT Applications

Several processes that include energy conversion, there is energy waste in the form of heat, like power amplifiers, automobile engines, light bulb and electrical motor get hot and energy is wasted. Energy harvesting devices capture some of these wasted energy from one or more renewable energy sources (like light, vibration, RF, motion, heat etc.), convert it to back into electrical energy. Energy harvesting (EH) is also known as micro energy harvesting or energy scavenging, is defined as the process of capturing energy from the surroundings of a system and converting it into usable electrical energy. The electrical energy is conditioned for either direct use or accumulated and stored for later use. This provides an alternative source of power for applications in locations where there is no grid power and it is inefficient to install wind turbines or solar panels. Based of energy source, there are several energy harvesting types:

Light: From sunlight or indoor lighting
Kinetic energy: From mechanical stress/strain or vibration
Thermal: Energy from thermal sources like heaters, engines, friction, furnaces, etc.
RF: From radio-frequency signals[image error]

Solar: Solar energy is converted into electrical energy by using photovoltaic (PV). These polycrystalline silicon or thin-film cells convert photons to electrons with a typical efficiency of about 10-20% for polycrystalline and 5-12% for thin film cells. Solar power can provide an unlimited level of energy to IoT and embedded platforms if there’s sunlight but since light sources tend to be irregular, solar cells are used to charge batteries to provide a stable energy source.

 Thermoelectric: Thermoelectric harvesters are based on the Seebeck effect, where a voltage is generated when there is a temperature difference at the junction of two dissimilar metals. Generally, thermoelectric generators (TEG) contains silicon nanowires of 10-100 nm length and these wires are suspended over a cavity. An array of these TEGs are connected in series to a common heat source such as water heater or engine. Total output power depends on the temperature differential and the size of the TEG array.[image error]

Piezoelectric: When piezoelectric transducers are stressed, they generate electrical energy. These transducers are used as vibration sensors to detect vibration of aircraft [image error]wings, sound, motor bearing noise or any other kinetic energy. Because of kinetic energy, the movement of cantilever generates an AC output voltage. This voltage is rectified, regulated, and stored in a thin-film battery or super capacitor. Piezoelectric sensors are put inside the car tire where they do energy harvesting to monitor tyre pressure and transmit the information.

Radio Frequency – RF: Radio frequency (RF) energy harvester captures RF signals and generate useful electrical power. These are very attractive for low-power electronic devices and wireless sensor networks (WSNs). A typical RF energy harvesting system consists of a receiving antenna, peak detector, matching circuit, and voltage elevator. Antenna captures the electromagnetic waves and converts into RF signal. By using a matching circuit and peak detector, this signal is converted to a voltage value. Finally, this voltage output is adjusted using the voltage elevator.

Energy Harvesting for IoT Devices:

Low power is one of most important electronic design criteria special in IoT devices. These devices often consume in active operating mode and nano-watts in standby or non-operating mode. It is required to extend the battery life by harvesting environmental energy sources – most often available as heat, light, heat, motion, vibration, or ambient RF. For low power IoT devices if battery replacement is difficult or expensive then it is possible to rely completely on harvesting ambient energy sources for power without any battery. The energy harvesting together with ultra-low-power microcontroller unit (MCU) for open the door for many applications that previously were not possible. Even though the power is usually harvested in small amounts, it is adequate for various low-power applications. The power-management integrated circuits (PMC ICs) manages ultra-low-power applications and ensure efficient use of the harvested energy.

Wireless power transfer (WPT) in IoT applications

Wireless energy harvesting (WEH) or green energy harvesting is used to convert RF energy into electrical energy. In WPT system, electric energy is transmitted without conducting wires. A transmitter driven by electric power source, generates electromagnetic field. This electromagnetic field (power) is transmitted in the free space and a receiving device extracts this power from the electromagnetic field and convert back to electrical power. Nikola Tesla first worked on WPT technology in 1890 and he was able to transmit electric energy from one coil to another coil. WPT is an alternate solution to energy harvesting when the environmental energy not enough. For IoT devices, WPT technologies and energy harvesting are very good renewable and clean power source. Implantable electronic devices and RFID tags are also used as RF power harvesting method. Selection of frequency-band is a vital consideration in RF power harvesting systems. For energy harvesting systems any frequency band can be used but most readily available bands are Wi-Fi hotspots, cellular (850/900 MHz band), PCS (1800/1900 MHz band), WiMAX (3.5/2.3 GHz) and 2.4 GHz network transmitters.

Rectenna:

In case of IoT applications, an antenna system (called rectifying antenna or rectenna) is used to charge remotely placed batteries wirelessly. The radio-frequency (RF) energy from neighboring sources, such as nearby wireless local area networks (WLANs), cellphones, Wi-Fi, DTV, and FM/AM radio signals, is collected by a receiving antenna and rectified into DC voltage. Rectenna is has two main elements: (1) the antenna that receives the electromagnetic waves and convert in a RF/microwave signal and (2) the rectifier that changes RF signal into direct current (DC). DC power depends on several factor such as the available RF power, the choice of antenna, frequency band, and the energy harvesting technique.

[image error]

For rectenna design, microstrip patch antennas are widely used because these are planar, low profile and lightweight structure. Design of rectenna with wide bandwidth and high gain is crucial to maximize the received power. Efficiency of the overall system greatly depends upon the matching between the rectifier and antenna. System efficiency is further limited by variable input impedance with frequency of the rectifier.

[image error]

Nantenna:

A nantenna (nanoantenna) is a nanoscopic rectifying antenna. At high-frequency, semiconductor-based solar cells and very small (nanoscale) antennas for power harvesting applications are gaining research interest. These nanoantennas transform thermal energy into electrical energy. Nanoantennas are designed at the infrared (IR) wavelengths. At these frequencies (terahertz radiation), traditional photo-voltaic cells are not very efficient. Nantenna is an electromagnetic collector that absorb specific wavelengths those are proportional to the size of the nanoantenna. The MBE (molecular[image error] beam epitaxy) technique is used to AlGaAs/GaAs-stamps which are used to fabricate nano antennas. MBE-fabricated MOM (metal-oxide-metal) diodes having spatial dimensions in the nanometer range, is use for in nano antennas. For rectifying THz electromagnetic radiation, MOM tunneling diodes are printed having an ultrathin dielectric.