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Energy Harvesting: Powering a Greener Future

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by Barry Manz, Editor, Microwave Product Digest

The concept of harvesting RF and microwave energy has been around for decades, with early experiments dating back to the 1960s. However, significant progress has only been made in recent years in improving the efficiency and practicality of this technology. Ambient energy harvesting is being explored as a potential method for generating DC power for IoT devices. IoT devices often require low power consumption and may be deployed where regular battery replacements or wired power connections are impractical.

One of the main driving forces behind the search for new energy harvesting devices is the desire to power sensor networks and mobile devices without batteries that need external charging or service. Batteries have several limitations, such as limited lifespan, environmental impact, size, weight, and cost. Energy harvesting devices can provide an alternative or complementary power source for applications that require low power consumption, such as remote sensing, wearable electronics, condition monitoring, and wireless sensor networks.  Energy harvesting devices can also extend the battery life or enable the batteryless operation of some applications.

At the heart of RF and microwave energy harvesting lies the rectenna, short for “rectifying antenna,” which is a device that converts electromagnetic energy into direct current (Figures 1 and 2). It consists of two main components: an antenna and a rectifier. The antenna captures the RF energy and the rectifier converts it to a DC voltage. The process is typically accomplished with diodes, which only allow current to flow in one direction. The simplest crystal radios are rectennas, using a long antenna and a diode to capture radio waves and convert them into a signal strong enough to drive headphones.

Figure 1: A wearable millimeter-wave textile rectenna fabricated on a textile substrate for harvesting power in the 5G K-bands (20–26.5 GHz). Source: Wikipedia, MW AHM, CC BY-SA 4.0

The efficiency of a rectenna depends on factors such as the antenna design, impedance matching between the antenna and rectifying circuit, and the efficiency of the rectifying elements. Researchers continually improve rectenna designs to increase efficiency and adapt them for various frequencies and applications.

Figure 2: A printed meshed rectenna lighting an LED from a Powercast 915 MHz transmitter. Source: Wikipedia, MW AHM, CC BY-SA 4.0 , via Wikimedia Commons

One of the biggest impediments to achieving usable energy harvesting systems is the low power density of the signals to be rectified, less than what other energy sources like solar or thermal approaches can provide. Consequently, proponents of gleaning usable amounts of DC power from over-the-air signals have focused on making rectennas as efficient as possible, from optimizing antenna designs to improving impedance-matching techniques and exploring novel materials for rectifying circuits.

Recent advancements in rectenna technology have shown promising results. In 2021, researchers from the National University of Singapore and Japan’s Tohoku University developed a rectenna that can harvest energy from ambient Wi-Fi signals in the unlicensed 2.4 GHz and 5 GHz bands. Their design achieved a maximum DC output power of 40 uW. Another notable development came from a Georgia Institute of Technology team in 2020 that designed a rectenna that harvested energy from LTE signals in the 700 MHz to 900 MHz range. It achieved a peak DC output power of 105 uW.  In 2019, a team from the University of South Florida developed a rectenna array capable of harvesting energy from a 5.8 GHz RF transmitter that produced a DC output power of 5.4 mW 1 m away from the antenna. While this approach requires a dedicated RF source (in this case, the 5.8 GHz transmitter), it showed what might be achievable as rectenna technology becomes more efficient.

An example of the wireless power device is the Powercast Powerharvester® receiver chipset (Figure 3), The Powercast P1110B Powerharvester receiver is an RF energy harvesting device that converts RF to DC and operates from 850 to 950 MHz. It’s housed in an SMD package and provides RF energy harvesting and power management for battery and capacitor recharging. The P111B converts RF energy to DC and provides the energy to the attached storage element. When an adjustable voltage threshold on the storage element is achieved, the P1110B automatically disables charging. A microprocessor can be used to obtain data from the component for improving overall system operation.

Figure 3: A Powercast Powerharvester RF-to-DC converter chip. Source: Powercast

Researchers are also investigating hybrid energy harvesting systems that combine rectennas with other energy harvesting methods, such as solar cells or piezoelectric generators. These hybrid systems can harvest energy from multiple sources, increasing energy harvesting efficiency and reliability. For example, a study published in 2021 by researchers from the University of Cambridge and the University of Bedfordshire proposed a hybrid energy harvesting system that integrates a rectenna with a solar cell. Their simulations showed that the hybrid system could achieve a higher overall energy harvesting efficiency than the rectenna or solar cell alone.

Storage Is Essential

Capacitors and supercapacitors can be viable storage solutions for energy harvesting applications. Traditional capacitors have a limited energy density compared to batteries, but they offer high power density, meaning they can charge and discharge quickly. This makes them suitable for applications that require frequent, rapid charge-discharge cycles, such as capturing intermittent energy from vibrations or RF sources.

On the other hand, supercapacitors, also known as ultracapacitors or electrochemical double-layer capacitors (EDLCs), have a much higher energy density than regular capacitors, bridging the gap between capacitors and batteries (Figure 4). They can store more energy and provide stable power output over longer periods, making them attractive for energy harvesting applications with intermittent sources like solar or wind.

Figure 4: Construction of a wound supercapacitor: terminals (1), safety vent (2), sealing disc (3), aluminum can (4), positive pole (5), separator (6), carbon electrode (7) collector (8), carbon electrode (9), negative pole (10). Source: Wikipedia, Tosaka, CC BY 3.0

Capacitors and supercapacitors offer several advantages for energy harvesting, including a long lifespan with millions of charge-discharge cycles, high power density for quick charging and discharging, a wide operating temperature range, and environmentally friendly materials. However, there are some limitations to consider. Capacitors and supercapacitors have lower energy density than batteries (especially for regular capacitors), higher self-discharge rates than batteries, and higher cost per unit of energy stored (especially for supercapacitors).

In energy harvesting applications, capacitors and supercapacitors are often used with batteries or other energy storage devices to create a hybrid system. The capacitors or supercapacitors can quickly store and release energy, while batteries provide long-term storage and stable power output. The choice between capacitors, supercapacitors, and other storage solutions depends on the specific requirements of the energy harvesting application, such as the power profile, energy density needs, cost constraints, and environmental conditions.

Harvesting at Optical Frequencies

An optical rectenna (Figure 5) is a rectenna that works with visible or infrared light. While rectennas have long been used for radio waves or microwaves, an optical rectenna would operate similarly but with infrared or visible light, turning it into electricity.

Figure 5: An optical rectenna directly converts electromagnetic waves at optical frequencies to DC. Source “A Carbon Nanotube Optical Rectenna,” Asha Sharma, Virendra Singh, Thomas Bougher, and Baratunde Cola, Georgia Institute of Technology.

While traditional (radio and microwave) rectennas are fundamentally similar to optical rectennas, making an optical rectenna is more challenging. One challenge is that light has such a high frequency—hundreds of terahertz for visible light—that only a few specialized diodes can switch quickly enough to rectify it. Another challenge is that antennas tend to be similar in wavelength, so a tiny optical antenna requires a challenging nanotechnology fabrication process.

A third challenge is that being very small, an optical antenna typically absorbs very little power and, therefore, tends to produce a tiny voltage in the diode, which leads to low diode nonlinearity and, hence, low efficiency. Due to these and other challenges, optical rectennas have so far been restricted to laboratory demonstrations, typically with intensely focused laser light producing a tiny but measurable amount of power.

The Nantenna

The term nantenna (nano-antenna) is sometimes used to refer to either an optical rectenna, or an optical antenna. In 2008, it was reported that Idaho National Laboratories designed an optical antenna to absorb wavelengths in the 3–15 μm range. These wavelengths correspond to photon energies of 0.4 eV down to 0.08 eV. Based on antenna theory, an optical antenna can absorb any wavelength of light efficiently, provided that the antenna size is optimized for that specific wavelength. Ideally, antennas would be used to absorb light at wavelengths between 0.4 and 1.6 μm because these wavelengths have higher energy than far-infrared (longer wavelengths) and make up about 85% of the solar radiation spectrum.

In 2015, Baratunde A. Cola’s research team at the Georgia Institute of Technology developed a solar energy collector that can convert optical light to DC, using an optical rectenna of carbon nanotubes. Vertical arrays of multiwall carbon nanotubes (MWCNTs) grown on a metal-coated substrate were coated with insulating aluminum oxide and capped with a metal electrode layer. The small dimensions of the nanotubes act as antennae, capable of capturing optical wavelengths. The MWCNT also doubles as one layer of a metal-insulator-metal (MIM) tunneling diode. Due to the small diameter of MWCNT tips, this combination forms a diode capable of rectifying the high-frequency optical radiation.

The primary drawback of these carbon nanotube rectenna devices is a lack of air stability. The device structure originally reported by Cola used calcium as a semitransparent top electrode because the low work function of calcium (2.9 eV) relative to MWCNTs (5 eV) creates the diode asymmetry needed for optical rectification. However, metallic calcium is highly unstable in air and oxidizes rapidly. To prevent device breakdown, measurements had to be made within a glovebox under an inert environment. This limited the practical application of the devices. Cola and his team later solved the challenges of device instability by modifying the diode structure with multiple layers of oxide. In 2018, they reported the first air-stable optical rectenna and efficiency improvements.

Improving the diode is an important challenge. There are two challenging requirements: speed and nonlinearity. First, the diode must have sufficient speed to rectify visible light. Second, unless the incoming light is extremely intense, the diode needs to be extremely nonlinear (much higher forward current than reverse current), in order to avoid “reverse-bias leakage.” An assessment for solar energy collection found that to get high efficiency, the diode would need a (dark) current much lower than 1μA at 1V reverse bias.

This assessment assumed (optimistically) that the antenna was a directional antenna array pointing directly at the sun; a rectenna that collects light from the whole sky, like a typical silicon solar cell does, would need the reverse-bias current to be even lower still, by orders of magnitude. (The diode simultaneously needs a high forward-bias current, related to impedance-matching to the antenna.) There are special diodes for high speed (e.g., the metal-insulator-metal tunnel diodes discussed above), and there are special diodes for high nonlinearity. Still, it is quite difficult to find a diode that is outstanding in both respects at once.

Future Trends

While the advancements in RF and microwave energy harvesting are promising, there are still challenges to overcome for widespread adoption. One of the main challenges is the limited amount of power that can be harvested from ambient signals due to their low power density. This may restrict the applications to ultra-low-power devices or require multiple rectennas to generate sufficient power. Additionally, the overall efficiency of rectennas in converting RF energy to DC power needs further improvement to make them more practical for real-world applications.

Another consideration is the potential interference and regulatory issues associated with RF and microwave energy harvesting. Rectennas must comply with RF spectrum use regulations and should not interfere with existing wireless communication systems. Researchers and engineers must work closely with regulatory bodies to ensure that energy harvesting technologies can coexist harmoniously with other wireless services.

Despite these challenges, the future of RF and microwave energy harvesting looks bright. With ongoing research and development efforts focusing on improving rectenna designs, increasing conversion efficiency, and exploring new materials and techniques, we can expect to see more efficient and practical energy harvesting solutions in the coming years. The potential applications of this technology are vast, ranging from powering wireless sensors and IoT devices to enabling self-sustainable electronic systems.

In conclusion, RF and microwave energy harvesting, mainly through rectenna technology, is promising for generating sustainable DC power. Recent advancements have demonstrated the ability to harvest energy from ambient Wi-Fi signals, cellular networks, and dedicated RF sources, with DC output powers reaching tens to hundreds of microwatts.

As research continues to push the boundaries of rectenna efficiency and explore hybrid energy harvesting approaches, we can anticipate a future where ambient electromagnetic energy is effectively harnessed to power a wide range of electronic devices. By leveraging the ubiquity of RF and microwave signals in our environment, energy harvesting technology has the potential to contribute to a more sustainable and energy-efficient future.

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