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
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. Thermoelectric transducers are very thin (few millimeter) and these can be easily attached between a metal enclosure and a heatsink.
Piezoelectric: When piezoelectric transducers are stressed, they generate electrical energy. These transducers are used as vibration sensors to detect vibration of aircraft 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.Piezoelectric elements can produce in the order of 100 of µW/cm2 power depending on their shape and size.
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. Compared to the other energy sources, RF energy provides a relatively low energy density of 0.2 nW/cm2 –1 𝜇W/cm2.
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.
RF Energy Harvesting System
RF energy harvesting (RFEH) is very suitable choice for IoT devices mostly for cases where solar energy is not available. However, there are several RFEH system design challenges as the overall conversion efficiency, form factor and bandwidth. Power available from energy harvesting sources and energy harvesting technique is shown below in table.
|Source||Source Power||Harvested Power||Energy Harvesting Technique||Efficiency|
|Light||Indoor||0.1 mW/cm²||10 µW/cm²||Photovoltaic||32+-1.5%|
|Outdoor||100 mW/cm²||10 mW/cm²||25+-1.5%|
|Vibration/ Motion||Human||0.5m @1 Hz, 1m/s² @50 Hz||4 µW/cm²||Piezoelectric||50-100 uW/cm2|
|Machine||1m @5 Hz, 10m/s² @1 kHz||100 µW/cm²|
|Thermal||Human||20 mW/cm²||30 uW/cm²||Thermoelectric||0.1+-3.5%|
|Industrial||100 mW/cm²||1-10 mW/cm²|
|RF||Cell Phones||0.3 µW/cm²||0.1 µW/cm²||RF||50%|
Wireless power transfer (WPT) in IoT applications
IoT, 5G Wireless Systems, Wearable Electronics, etc. requires extensive placement of sensors often positioned at remote places. Replacing several numbers of batteries over the time in these systems is a tedious task. Wireless energy harvesting (WEH) provides a sustainable as well as green energy solution to this challenge. 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. The power density from the various radio-frequency sources ranges from 0.1-1000 μW/cm2.
Power Loss Calculation:
Using the free space path loss Friis transmission formula, the signal strength at any point in free space is calculated. It depends on several factors like antenna gain, operating frequency and distance from the transmitting antenna. The received power at a distance is calculated using Friis free-space formula;
where λ is the wavelength, is the power transmitted by the transmitter, is the gain of the transmitting antenna, is the gain of the receiving antenna, R is the distance between transmitting and receiving antenna. The free space path loss is calculated as:
Where Pl is the free space path loos.
The RF energy harvesting can be based on either far-field or near-field energy transfer. Near-field RFEH are designed by either magnetic resonance coupling or by using inductive coupling. In magnetic resonator, two coils resonate at the same frequency through magnetic coupling while in inductive coupling system. When two systems resonate at same frequency then the energy is transferred through coupling between these systems. Inductive coupling is used for in wearable electronics and in mobile charging while magnetic resonance coupling is used for charging commercial applications and for charging consumer electronics.
The RF power transfer beyond λ/2π distance from transmitting antenna is considered as far field energy transfer. This allows devices spread in a wide area to be powered by RF energy harvesting. The far field transfer is still having many research challenges like low power conversion efficiency.
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 has been widely used for WPT and RFEH systems.
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. Basic requirement(s) of rectennas are:
- Compact antenna
- Multiband or broadband design can gather more power over the wide frequency-band and hence produces more output
- Ability to receive RF energy in any plane. Circularly polarized (CP) antenna are more suitable for RFEH system.
- Reconfigurable antennas using polarization and frequency diversity
- High radiation efficiency
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. The antenna receives RF signals and to obtain high efficiency, a matching circuit is used to match the impedance of the circuit to the antenna. The rectifier converts the AC signals into DC. In multiplier circuit, the number of stages determines the output voltage. Voltage multipliers are applied to boost the output DC because the amplitude of the DC output voltage is lower in comparison to the received RF signal amplitude. Power conversion efficiency (PEC) is one of the key parameters and there are two methods to achieve high PCE. Frequently used method is maximum power absorption and its distribution to the rectifier circuit that can be realized by using broadband and large antenna arrays. Second approach is to apply a low pass filter (LPF) in between the antenna and rectifier or by using a harmonic rejection antenna. The RF signal to DC power conversion efficiency (PEC) of the rectenna is given by:
where PDC, is the output DC power and PRF is the input RF power.
In antenna design one of the basic requirements is small antenna size but there are many challenges in designing miniaturized antennas without any significant performance degradation. Below are some methods to achieve antenna miniaturization:
- Use of high dielectric constant materials with low loss
- Use slits, slots, geometric optimization or shorting posts
- By using meandering slit in antenna
- Antenna design based on Electromagnetic Band Gap (EBG) structures, Metamaterials, and defective grounded structures (DGS)
- Fractal antennas
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 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.