Batteries not included: How small-scale energy harvesting will power the future

The venerable crystal radio has been around since the early 1900s, but for one very unique reason, people still build and use them today. It isn’t particularly loud, it won’t grab faraway stations, and its esthetics certainly won’t impress your hoity-toity buddies. However, this old school marvel offers a single perk no other radio can: It functions without traditional power sources. That’s right – no cord, no batteries, no hamster wheels.

How is this possible? Because crystal radios grab all the power they need from the radio waves themselves.

Pretty cool, right?

Radio waves, you see, are energy. It works like this: Radio stations convert audio into radio waves that then travel, at the speed of light no less, omnidirectionally from the transmitter. These waves are, in essence, electromagnetic fields – forms of electrical energy not too dissimilar from the power that flows to your AC outlets, only they’re sprayed in all directions. The fractional amount that actually reaches your home is not very potent at all.

Bereft of other power sources, the crystal radio needs to “harvest” as much of the approaching electromagnetic field as possible. It does so with a rather considerable antenna (usually a long stretch of copper wire), a “coil” that’s tuned to the frequency (number of waves per second) of the desired station, a “detector” to extract the audio signal, and an earphone to convert the audio signal back into sound waves.

Though the crystal radio itself fell out of vogue some time ago, the “energy harvesting” part of the equation – whereby power is gobbled, both literally and figuratively, out of thin air – is now in the midst of a notable renaissance. From harnessing the frenetic energy of a dance club to utilizing the heat given off by the human body, engineers are looking to the world around us for untapped sources of energy. And getting results. Here’s why power outlets and batteries are no longer good enough, and what scientists are doing about it.

It’s all about our hunger for power. We begin our days operating various devices around our homes – alarm clocks, toasters, coffee pots. We then hop in our car, which itself has an unquenchable thirst for even more energy. Or maybe we grab the bus, toying with our notebook or tablet or smartphone or music player or handheld gaming system along the way. Ultimately, our entire day plays out as it began – dependent on various forms of electricity to do what we need to do. 

• Consumption overload

• Radio wave harvesting

• Piezoelectric energy harvesting

• Thermoelectric energy harvesting

• Human motion energy harvesting

• Back from the future

Consumption overload

There are more than a few things wrong with this picture. For one, whether we’re continually buying new batteries, recharging rechargeables, or merely running devices on AC current, power is an expensive, wallet-draining resource that we never seem to stop consuming.

Besides the monetary cost of staying juiced up, there are costs to the planet as well. According to the Environmental Protection Agency (EPA), Americans purchase nearly three billion dry cell batteries annually to run radios, toys, cell phones, watches, mobile computers, and portable power tools. And even in today’s somewhat enlightened society, the vast majority of these are eventually tossed into landfills. Here they’ll decompose over the course of decades, leeching all sorts of caustic substances into the surrounding soil and groundwater. And that’s just the tip of the iceberg – from hydroelectric dams to nuclear power plants and diesel generators, our thirst for watts hammers the planet.

Which is where energy harvesting may save the day. Wind turbines and solar power immediately come to mind as examples of ways to tap the environment around us for clean power, but both are limited by the availability of the power source they harvest. You can’t grab nearly as much energy when the wind isn’t blowing or the sun isn’t shining.

Yet there’s a whole other level of energy harvesting on a smaller scale. Because many of the personal, portable electronics we now use don’t require much electricity to begin with, opportunities for harvesting previously negligible amounts of energy are popping up everywhere.

And they could mean phones, music players and laptops you’ll never have to charge again.

Strap on the antennas: Radio wave harvesting

One of the most prominent approaches to pulling electricity from thin air clearly gives a nod to the venerable crystal radio. It harvests energy, not surprisingly, from radio waves.

At the University of Bedfordshire’s Centre for Wireless Research in Luton, UK a trio of bright people have been working on a radio-wave-harvesting solution they believe will one day not only reduce consumer reliance on batteries and AC power, but will also reduce the impact on our environment.We spoke with Ben Allen, who heads up the team, to find out more. Allen and his associates gained international attention back in February when they announced they’d developed the technology and had filed a patent application for radio wave harvesting. Some wags suggested it was a world’s first, though Allen’s group is but one of many worldwide looking into the very same prospects. Indeed, noted Serbian-American scientist and certified brainiac Nikola Tesla demonstrated the phenomenon of wireless energy transmission more than a century ago. Nevertheless, Allen and his team have taken the technology to a point few others have.According to Allen, the team’s proposed solution focuses on the frequency band “which is around 1MHz and is sometimes called the ‘AM band’. It doesn’t have to be these signals that we harvest energy from, but we’ve been focusing on medium waves as we believe they have advantages compared to high-frequency waves.”At the heart of the concept is an antenna – an antenna Allen likens to a windmill. “The (radio) wave induces a current into the antenna, which we convert to DC and apply to the device requiring power. The bigger the antenna, the more power is available. The antenna is like a windmill — the bigger the windmill, the more power is available.”

Allen is also quick to boast that the technology can be retrofitted to existing devices and is comparatively compact. As for concerns that widespread radio wave harvesting might eventually vacuum up so much energy that nothing is left, Allen again plays the windmill card. “We use antennas that are very small and have a negligible effect on the signals – a bit like the effect a child’s windmill has on the wind. If we had a very large device then it would be difficult to receive a signal behind it, just like the shortage of wind behind a large wind generator.”The obvious upside is the constancy of the harvested source. As Allen explains, radio wave harvesting does not depend on sunlight or wind. Nor does it rely on heat differential (the concept behind thermoelectric energy harvesting, discussed later in this article). Radio waves are always out there, more so in metropolitan areas. Moreover, says Allen, “In principal, it operates well in rural locations and the radio waves should have relatively good penetration into buildings compared to those at higher frequencies.”

Radio waves don’t pack a ton of harvestable juice, so Allen and his team are currently targeting ultra-low power devices – products such as wireless sensor network nodes that only occasionally execute power-demanding actions and otherwise remain in wait mode. Future candidates include remote controls and clocks.

“Some applications will require rechargeable batteries to help with energy supply, but (even they will be) charged from harvested energy,” Allen says. “This would limit the life of any device and eventually the battery will degrade and need replacing. This may be after a few years, but does vary due to battery technology, temperature and charge cycle.”

Still, high-drain current-gobblers such as tablets and flashlights may never be part of the picture. And if they are – and we’re looking way down the road here – radio wave harvesting almost certainly won’t be the only power source. Picture a rechargeable battery that depends on “regular” recharging but is potentially also topped up by an amalgamation of energy harvesting alternatives, and you begin to get an idea what the future might hold.

Good vibrations: Piezoelectric energy harvesting

Meanwhile at dance clubs all over the globe, patrons gleefully bop the night away. Apparently there’ll always be a place in this world for those who have a fixation for the gyration. But we digress. The point here is that some of these clubs are different than others. Seems there’s a movement afoot to “green up” our dance halls. And from re-used water to rooftop turbines, that movement grows stronger by the day.

Indeed, it extends all the way down to the floor. The dance floor, that is.

You see, there’s a tremendous amount of energy generated when dozens or hundreds of people repeatedly bounce to the beat. Until now, that energy dissipated into the floor and whatever lay beneath it. But what if you could somehow harness it?

With “piezoelectricity,” you can. Seems there are certain materials in this world, crystal- and ceramic-based, that generate wattage when put under pressure. If you place those materials in a spot where they’ll receive a constant pounding – say, in the floor of a dance club – you have a way of tapping into that pounding and producing useable electricity. In fact, companies such as Rotterdam’s Sustainable Dance Club are already doing just that, installing illuminated dance floors that are, in effect, self-powering.

The piezoelectric push is not confined to dance clubs. There’s a railway station in Tokyo that uses the effect to power display boards and ticket gates, and a sidewalk in Paris that juices the streetlights.

At the Cornell Nanoscale Science and Technology in Ithaca, New York, plans are underway to bring all this groovy piezoelectric stuff into the consumer world, albeit with a somewhat different approach. The name of the outfit is MicroGen Systems, Inc, and the people involved are seemingly in it for the long haul. CEO Mike Perrotta tells us the company was founded in 2007 and incorporated in 2010 after a “major investment contract was signed.” Perrotta estimates man hours thus far to be in the 20,000 range.At the heart of the push is MicroGen’s proprietary Piezoelectric Vibrational Energy Harvester (PZEH) technology. According to MicroGen, the concept will extend rechargeable battery lifetimes or eliminate the need for batteries altogether. An early permutation is the “BOLT060 MicroPower Generator,” a teeny-tiny gizmo that looks like a computer CPU. It operates purely by applied vibration, and should theoretically function for 20 years or more.

Why a miniature harvesting technology based on vibrations?

“Vibrations are everywhere, and do not depend on temperature differential, light, radio frequency, or other types of sources,” says Perrotta. “Everything that is plugged in vibrates, and many things have a natural harmonic even if they are not plugged into electric. We have even had conversations about placing these devices in a cow’s stomach, with a temperature sensor and wireless radio, to monitor livestock health and conditions. I haven’t seen anything plugged into livestock yet.”

Bovines aside, Perrotta envisions MicroGen piezoelectric technology in a wide variety of low-draw applications. “Think of us as a micro power plant, thus the name MicroGen. Flashlights and the like will require too much power (for our technology) to recharge. However, you can have a flashlight app on your mobile and that works pretty good.”

“Clothes dryers are another example, where current sensor technology only measures the average humidity of the entire load, thus requiring more energy. The sensor can now be relocated with our device powering it, and therefore more accurately indicate the humidity of the clothing, and stop the dryer more accurately. This will save significant energy consumption. Tire-pressure monitoring systems, now in every vehicle in the US, Canada and European markets, may be able to go battery-less. If not, our systems will last the life of the car, thus reducing the number of batteries in a landfill. Many more examples, especially in the industrial and commercial arenas, are targeted to reduce energy consumption, improve safety, and security as well.”

Perrotta claims the technology is currently capable of delivering a rather mild 200 microwatts, though he expects that figure to double and perhaps triple in the short term. The increasing efficiency of mobile devices could also help. “We’ve certainly seen a great degree of reduction, in the order of 50 to 80 percent, on the power needs of these devices, even over the last couple of years,” Perotta says. “We expect that will continue, and our power output per square millimeter to increase. Our target in the consumer space is ‘trickle charging’ a mobile device, so that the batteries do not wind down as quickly after the last plug-in charge.”

In the near term MicroGen devices will be coupled with either a solid-state battery or a super capacitor. But the eventual goal, he says, “is to be battery-less. However, most of that will depend upon the trends around sensors and wireless radio power needs.”

Whether MicroGen’s solution achieves those goals remains to be seen, though the mere potential was enough to compel the New York State Energy Research and Development Authority to award the company a $1.2 million grant just a month ago.  

Canned heat: Thermoelectric energy harvesting

If harnessing energy from radio waves and vibrations seems a might far-fetched – did the Jetsons ever do that? – try this notion on for size: Power from body heat.

Yet that’s precisely what they’re working on at Wake Forest University’s Centre for Nanotechnology and Molecular Materials. The technology is called “thermoelectric,” and the Wake Forest approach to thermoelectric is labelled “Power Felt.” Seems that merely by touching this mysterious fabric, body heat is converted into electric current.

Next they’ll tell us the moon’s not made of cheese.

So, what exactly is this Power Felt? For starters, it really does look like fabric. Futuristic fabric, sure, but fabric nonetheless. Made of carbon nanotubes locked in flexible plastic fibers, it can seemingly be “wrapped” around virtually anything. The folks at Wake Forest call it “wearable power.” And then they jump in their spaceships and fly off to their home galaxy.

We’ll let PhD-bearer David Carroll, Professor of Physics at Wake Forest and inventor of Power Felt, detail the intricacies.

“Our materials work the way any thermoelectric module works. Imagine that you hold in your hand a metal bar. You grasp it tightly at one end of the bar while the other end is free. Now the electrons that make the metal up are free to move, and your hand is heating them. So under your hand the electrons move more rapidly than say the electrons at the other end of the bar. This means that these electrons will spread out quickly and move away from the heat source. By moving to the cold end, they leave behind a deficit of electron in the hot end. They create a surplus of electrons in the cold end. This establishes a voltage, called the thermoelectric voltage, and as long as there is a difference in temperature, this voltages exists.”

Picture a rechargeable battery that depends on “regular” recharging but is potentially also topped up by an amalgamation of energy harvesting alternatives, and you begin to get an idea what the future might hold”Trouble is that eventually the other end heats up because of the thermoconductivity of the metal. By using lots of nanofibers in a plastic matrix, instead of the metal bar, the electrons can still move down the metallic pathways of the fibers, but the heat is blocked because it is not transported across the junctions from fiber to fiber. This is how we have made our fabrics. Our ‘metal’ fibers are carbon nanotubes.  And within the fabrics are layers upon layers of electronic nanofibers, allowing for electrons and holes to flow freely.”

Carroll extols the virtues of his invention but he’s just as realistic. It will not, he says, replace batteries. Not yet anyway. Nor will it work unless “large areas of temperature gradient exist.” The human body is a workable spot. So too is the hood of a car or the seats in an airplane.

“It will make the use of less expensive batteries more attractive from a market perspective. Generally, for market insertion, you don’t want to change too much too quickly, so it will be coupled with existing battery technologies. Your cell phones will last longer on one charge. A passenger jet may be able to use smaller internal generators, saving weight and money.”

Though Power Felt will never run an electric car or energy-hungry appliances like refrigerators (Carroll tells us a square centimeter produces “nanowatts to tenths of microwatts, depending on thickness”), it will, apparently, be quite capable of augmenting current power structures in such applications. “Imagine,” says Carroll, “reducing the charging time for your hybrid car because the heat reclamation comes from the cooling engine, or the passengers inside.” 

Carroll also looks to house construction, saying Power Felt could conceivably take the place of Tyvek house-wrapping to “generate as much power an inexpensive solar array.”

Carroll makes a case for Power Felt in the mobile world too, suggesting a swatch of it could be included in the covering of batteries at the point of manufacture. By merely placing said batteries on “something warm,” they would partially self-charge. But once again, thus far anyway, it’s a matter of augmenting rather than supplanting regular battery charging. Carroll asks, “Ever been at the airport and had your phone run out of power? Wouldn’t it be nice to make that one last call for someone to pick you up? The power from your body’s heat could do this.” 

Running man: Human motion energy harvesting

So far we’ve seen a trio of concepts that are apparently ready to help reduce our dependence on traditional power sources but generally do not have the oomph to fully replace those sources. That trend continues for our fourth and final entry, biomechanical “human motion” energy harvesting. That’s right, this technology harnesses power from the mere act of moving. Wanna charge up your Android? Better start swigging that beer a little faster.

Jesting aside, it turns out that biomechanical energy harvesting isn’t quite the novel idea it may initially seem. Remember those old-time bicycle headlights that would pull power from a generator tapping into the rotation of your tires? That was a perfectly decent – if not absurdly tiring – example of the same process.

But way down in the southern hemisphere, they have a slightly different take on the subject. Here, at the Biometrics Lab in the Auckland Bioengineering Institute, dedicated folk huddle in the darkness (and in the brightness too – this is no witch’s coven) to devise a better way. They think they’ve found it.

Their idea, at least initially, does away with the bicycle element altogether and asks that you get walking (or running) instead. Ben O’Brien of SoftGen, the company that’s currently springing to life around the concept, provides more insight.

“For portable electronic devices, we want to capture otherwise wasted energy so that the user doesn’t feel the extra load. For example, when we walk around, the soles of our shoes compress. This compression requires energy, energy that is lost as heat. If instead we replace part of the sole with a soft generator we can capture this energy and convert it to electricity.”

Like all the technologies we’ve showcased, the fundamental SoftGen idea has been with us for some time already. In the case of “heel strike generators,” as they’re called, that time frame stretches back decades. But SoftGen has added a new wrinkle in the form of “artificial muscle generators.” Invented in California at the turn of the millennium, artificial muscle generators are, O’Brien and company believe, the spark that takes heel strike generators into the future.

“The basic idea,” explains O’Brien, “is to apply electrical charge to a deformed elastomer membrane. When the deformation is relaxed, the charge is boosted to a higher energy state. By cycling the deformation and controlling when you put the charge on and off, you can generate power with approximately 10 times the energy density of competing technologies. All this with something as simple as a piece of rubber.” “The trick, and this is where our group comes in, lies in controlling these generators to produce useful energy using circuits that are small enough and light enough for portable applications.”

O’Brien plays down the concept of a completely battery-free future, saying that energy needs to be “smoothed out” and stored for periods of lesser activity. As for “simple” early targets for SoftGen’s brand of energy harvesting, O’Brien says heel strike generators “could power bright lights for safety at night, electronics embedded in the shoe (a la the Nike+ product range), and medical monitoring for podiatrists,” adding that, “Our particular niche is low-powered applications. We’re working to have the technology in a real-world consumer product soon.”

There seems to be no shortage of accessible energy on tap when foot strikes ground. In The Energetics of Running and Running Shoes, Martyn R. Shorten claims there may be as much as 10 joules (1 joule = the work required to produce one watt of power for one second) of wasted energy per each running step. This, of course, is music to the ears of folks like O’Brien. “Theoretically, if you could capture all of this, you could fully charge a smart phone off a single shoe in a half hour run. And your shoes wouldn’t feel any different than they do now.”

O’Brien welcomes alternative energy harvesting technologies, recognizing that what might be a great fit in certain situations or with certain people won’t be panacea for everyone. “The great thing about human motion power is that it’s always available where we are. This might not be a big deal when you go home at the end of the day and have easy access to a wall socket, but as the number of electronic devices we carry grows, it becomes a nuisance to charge them all. Now consider all the times you’re not next to a wall socket, or if you’re travelling in a country with different plugs, or you’ve gone off the grid – tramping or hiking – or due to poor infrastructure or after a disaster. In all these cases, human motion power becomes a very attractive concept.” 

[Image credit: Auckland Bioengineering Institute]

Back from the future

Much earlier in this article we discussed the media hubbub surrounding the U of Bedfordshire’s radio-wave harvesting model. We’ve since learned that Ben Allen and his team are but one of scores of such teams worldwide working on a variety of small-scale ambient energy harvesting technologies.

Yet most of those concepts are still a long way from being fully prime time. Indeed, there’s no shortage of people who claim certain approaches to micro energy harvesting will never be a seriously viable solution. Mosey on over to spots such as the Physics Forum, for example, where you’ll find a gaggle of hyper-scientific types pontificating on the subject – many with less than favorable conclusions.

The primary issue is, as we covered earlier, rather weak source material. When you need watts and the harvesting device is generating mere nanowatts or microwatts, a huge gap clearly exists. This gap can be closed some with further technological (harvesting) advances, by pairing multiple technologies together, or by keeping a dedicated energy storage device (battery) of some description permanently in the equation. But all of these solutions add to the bulk, the complexity, the cost, and the R&D time.

Moreover, the notion that any of the technologies or combination of technologies we’ve featured will completely replace traditional power in high-draw devices (flashlights, tablets, smartphones, etc.) is fanciful at best.

And in the interim, other already entrenched technologies continue to evolve – solar power arguably being the best example. Large-scale solar applications are now commonplace, but small-scale solar is already here – and seemingly doing just fine. One need only explore the sheer number of solar-equipped radios and flashlights and even battery chargers currently on the market to see how many companies have already jumped in the game.

But in a world that chews through power like a dog through a juicy steak, there’ll clearly be an increasing need for “alternative” power that doesn’t rely on the sun (or the wind) – even if those solutions are augmentative to traditional sources, or to each other. Considering that each of these technologies is apparently suited to such a wide variety of applications – and when taken as a group seem to cover virtually all the bases – we see a bright long-term future for those who can best adapt their ideas across the spectrum.