Can Solar, Thermal, and RF Harvesting Ever Offer a Good Energy ROI?

Earlier this year, Swiss smartwatch maker SEQUENT announced a completely self-powered smartwatch using "patented autonomous micromechanical movement technology." SEQUENT isn’t alone in its quest to ditch charger-reliant wearables either. 

 

SEQUENT’s comparison of its smartwatch lifespan—a bold “infinite” claim—against other common wearables on the market. Image used courtesy of SEQUENT

 

In recent years, engineers have gotten creative when it comes to powering wearables, innovating new methods for sustainable and self-powered devices. One overarching issue that energy harvesting presents, however, is a low energy return on investment (EROI). The concept of EROI (introduced by systems ecologist Charles A.S. Hall in the 1980s) is one that's familiar to EEs: energy sources are only useful insofar as you get more energy out than you put in.

In the context of wearables, EROI specifically nods to the ratio between the harvested energy and the energy actively invested in the harvesters. It’s no surprise then that researchers are in the hot pursuit of high EROI harvesters for self-charging wearables—and many have turned to human and ambient energy sources as the answer.

 

Energy Harvesters—and the Struggle for EROI

There are a plethora of energy-harvesting methods on the market, such as solar, RF, thermal, mechanical, and chemical. Each of these methods has benefits and tradeoffs, particularly when it comes to an energy return on investment. Here’s a brief rundown of each harvesting technology, along with recent research that may support EROI advancements in the future.

 

Solar Energy Harvesters

One of the most attractive options for EROI-savvy wearables is solar harvesters. When light that is above a certain energy threshold hits the surface of select materials, electrons that were previously bound to that material are released. This process is called the photoelectric effect and is the basis of solar to electrical energy generation.

Photovoltaics can be used for powering wearables as they don’t draw too much current and operate at fairly low voltage levels. The main drawback of solar is the inability to operate inside, at night, or on overcast days—an issue usually fixed by pairing this technology with batteries or supercapacitors.

Recently, scientists at Australian National University found that ultra-thin 2D materials can convert sunlight into electricity by “twisting” the angle between the material's two layers.

 

The material is said to be hundreds of times thinner than a human hair. Image used courtesy of Australian National University and PV Magazine

 

With further research, the scientists believe this material, when coated on an object’s surface, can allow devices to power themselves—from phone screens to windshields.

 

RF Energy Harvesters

Radiofrequency energy harvesters convert signals that fall within the radio and microwave bands into electrical energy using specifically tuned antennas.

Every type of data transfer to and from our phones, computers, and other electronics uses some type of signal that belongs to the radio spectrum. However, the energy that can be harvested from random sources is extremely low because of the proximity and range of the sources. To properly power a device, the source’s emitters need to be either targeted, close by, or very large.

Researchers from ETH Zurich posit that harvested RF energy is not sufficient to power even single components in a wearable device, as depicted below.

 

Typical current requirements for specific components in a wearable. Image used courtesy of ETH Zurich and Research Gate

 

Not all researchers agree, however. For instance, Israeli startup Wiliot recently made headlines when it raised $200 million to create an RF energy-harvesting sensor. Wiliot’s sensor contains a Bluetooth Low Energy Arm MCU that is said to draw power from surrounding RF signals. This 1 MHz Cortex-M0+ chip features sensors for temperature, proximity, and humidity along with 1Mbits of NVM and a self-power management unit.

 

Thermal Energy Harvesters

Thermoelectric devices convert heat—or more accurately, differences in temperature—directly into electric energy via semiconductor transducers. Wearable designers have often tapped into the thermoelectric effect to power small wearable devices because the human body effectively acts as a continuous heat source.

While the human body works well as a continuous source, some scientists are looking to excess heat from machinery like ovens and factory smokestacks for thermal energy. Researchers at the University of Colorado Boulder have recently designed a solution that captures excess heat from its environment using rectifying antennas or “rectennas.” These devices operate similarly to car antennas, except instead of converting radio waves into sound, optical rectennas capture light and heat and turn it into energy.

 

Image of the optical “rectennas” developed by CU Boulder. Image used courtesy of CU Boulder Today

 

While regular antennas require electrons to pass through an insulator to harvest energy, the CU researchers’ device uses two insulators to promote something called “resonant tunneling,” a process by which electrons hit a quantum well with just the right energy to tunnel through the two insulators without resistance.

The researchers are hopeful that this so-called breakthrough can eventually be used on a large scale—even to capture energy radiating from Earth into outer space.

 

Mechanical Energy Harvesters

Researchers have explored multiple ways to convert mechanical energy into electrical energy. For wearables, piezoelectricity is a widely researched energy-harvesting method. This method uses piezoelectric elements that can accumulate an electrical charge when certain mechanical stress is applied to them.

These elements can be embedded within clothing such as shoes or mounted on organs such as the heart, powering themselves continuously from different types of muscle motions that can range from voluntary movements to idle human functions.

Recently, scientists in India created piezoelectric molecular crystals that produce power under mechanical impact.

 

Depiction of how the molecular crystals promote autonomous self-repair. Image used courtesy of The Hindustan Times

 

Using these crystals, the researchers have forged a way for broken components to generate electrical charges at the crack junction. The device can then autonomously repair itself at the point of fracture when the damaged parts attract each other.  

“The material may find application in high-end micro-chips, high-precision mechanical sensors, actuators, micro-robotics, and so on,” the department explained. “Further research into such materials may eventually lead to the development of smart gadgets that self-repair cracks or scratches.”

 

Chemical Energy Harvesters

Chemical to electrical energy is most commonly associated with batteries. This is because batteries are manufactured out of materials with special chemical characteristics that allow them to retain and discharge an electrical current.

Some researchers, however, are developing so-called biofuel cells to generate power from human perspiration. This technology discharges electrical energy in place of the internal battery chemistry. By connecting the circuitry of the wearable directly to the human skin, this chemical process can allow the wearable to continuously harvest energy from its user and independently power itself.

 

Diagram of the device self-powered by biofuel cells. Image used courtesy of Joule 

 

Collecting hundreds of millijoules of energy during sleep, this Band-Aid-like device proved more efficient than other on-skin harvesters. The researchers also integrated piezoelectric generators beneath the biofuel cell to garner additional mechanical energy when a user pressed his or her finger against it. The study proposed that this energy-harvesting wearable might be useful for powering electrochromic displays and sensors, providing a “remarkably high energy return on investment.”

 

Engineering Challenges for Self-powered Wearables

In the span of just a few years, wearables from smartwatches and smart glasses to fitness trackers and medical monitors have become integral to a lot of people’s lives.

It’s possible that with continued research in embedded energy harvesting, these devices may one day be completely self-powered. While many embedded designers have historically focused on low-power processors, sensors, and displays in this pursuit, high EROI energy harvesters could be the final piece of the puzzle.

To maximize EROI, it’s likely that engineers will continue to combine these harvesting methods: using multiple sources during a singular activity may be the key to develop sustainable and self-powered systems.

 

Featured image used courtesy of SEQUENT