Wireless power sounds like an absolutely fantastic idea, in theory. All of your devices, charging themselves all the time, without you having to plug them in or even think about their batteries. Wearables you never have to take off. Cellphones that work when you need them to. Remote controls that never need the batteries changed. But, beaming power through the air in a safe, effective, and efficient way is hard, and so far, the technology isn’t always living up to the promises that it’s making.
The promises made by a company called Energous include multi-device charging using radio frequency energy out to a range of 4.5 meters, relying on transmitters integrated into home appliances and receivers that can easily fit into portable electronics. Last week, Energous released a report from Underwriters Laboratories (UL) that tested and verified the wireless power delivery provided by Energous’ WattUp transmitters and receivers. It answered some of our questions, but raised a few more, and we spoke with Energous founder and CTO Michael Leabman to get things all figured out.
Energous’ WattUp wireless charging technology depends on several pieces of hardware and a lot of fancy software, so let's start with describing the physical setup. Fundamentally, it’s very similar to the Wi-Fi that you more than likely have all over your house right now: there’s a transmitter (like a router) equipped with antennas that send signals to multiple devices, all with receiving antennas of their own. Instead of transmitting data, however, Energous’ system transmits power.
A WattUp transmitter consists of multiple small antennas (tens to hundreds of them) arranged in a two dimensional array. Here’s a picture of a portion of an array, with just a few (24) antennas.
Photo: Evan Ackerman/IEEE Spectrum
These antennas broadcast radio frequency (RF) energy at 5.8 GHz, a band reserved for industrial, scientific, and medical use. Unlike the antennas in your Wi-Fi router, the antenna array in a WattUp transmitter can be used to focus and steer the RF waves into a beam (or multiple beams) pointing in any direction by carefully varying the phase of the RF wave being emitted by each individual antenna. This enables the transmitter to target an area of three-dimensional space with multiple “micro beams” of RF energy, which combine with each other inside a “pocket” a few inches in diameter. While each individual micro beam doesn’t represent very much power, enough of them combine inside the pocket to provide a useful, harvestable amount of energy, just in that one small space.
Photo: Evan Ackerman/IEEE Spectrum
To harvest this energy, devices require a special WattUp “metamaterial array” of receiving antennas. The receiving antennas are substantially smaller than the transmitting antennas, and they’re flat, so that you can easily integrate them into other devices, like the back cover of a cellphone (pictured above). When the receiver is inside the pocket of RF energy created by the transmitter, it starts capturing all of those RF waves and converting them into electricity, which it feeds into the battery of your device.
The third part of Energous’ wireless power system is the software that provides coordination between the receiver on your device and the transmitter. Communication between the two is done through Bluetooth, providing the transmitter with information about your device, how low the battery is, and what level of charge it can accept, for example. The transmitter also uses the Bluetooth signal to pinpoint the location of the device (within an inch or so) so that it knows where to form the RF energy pocket for charging. To do so it uses a process that's “essentially triangulation” that Energous is in the process of patenting. Transmitters can deliver power to as many as four devices simultaneously, and power delivery continues even if the receiver is in motion.
WattUp transmitters can be integrated into consumer electronics, such as the bezel of a TV or inside speaker systems. They can also be stand-alone devices that look a lot like a wireless router as in the photo shown.
Photo: Celia Gorman/IEEE Spectrum
It’s important to note that all of this wireless power coolness comes with some very important caveats. First, the energy that can be received is proportional to the distance between the transmitter and the receiver. Energous says that it can deliver 4 watts per device (to up to four devices) at 1.5 meters or or less, 2 watts between 1.5 meters and 3 meters, and 1 watt between 3 and 4.5 meters. This equates to approximately the same charging rate as a wall charger within 1.5 meters, the same charging rate as a USB port within 3 meters, and trickle charging out to 4.5 meters.
Also, 5.8 GHz RF cannot penetrate your skin, or most other things. This makes it safe to be around (it’s less penetrative than your cellphone), but it also means that you can easily block RF energy with your own body (or other objects). Energous does have a solution for this, or at least, a way to mitigate it. Rather than targeting the receiver directly, the transmitter can bounce microbeams off of walls, the ceiling, or the floor, avoiding obstacles to try and find a better path to the receiver. Since the transmitter is getting continuous feedback from the receiver over Bluetooth, it can keep trying different beam paths until it finds the best one.
Photo: Evan Ackerman/IEEE Spectrum
WattUp technology is certainly real: we’ve seen this all working in the form of a very functional (and very polished) demo, at CES back in January. The question is not whether Energous can do what they’re claiming to be able to do, because they definitely can: We held a cellphone in the middle of a suite at the Hard Rock Hotel and watched the battery slowly tick up (it was kind of amazing). The question at this point is whether Energous’ perfectly functional technology can be implemented realistically for consumers, taking into consideration things like efficiency, size, and cost.
Energous does discuss efficiency on its FAQ page; here’s the answer to the “Is It Efficient” question:
WattUp™ transmitters sense and communicate to authorized receiver devices via Bluetooth Low Energy (BLE) and only send power when needed, and requested by those devices. WattUp is software controlled, intelligently determining which devices receive power, when, and in what priority. If there are no authorized devices within range, the WattUp transmitter becomes idle and no power is wasted.
So, yes, it’s not just beaming power out into your living room all the time, but not wasting power doesn’t really have anything to do with whether or not WattUp transmitters and receivers are efficient. The fact that Energous asks the question and then deliberately (we have to assume) doesn’t answer it is worrisome.
It won’t surprise you to hear that the Underwriters Laboratories report released last week showed better than expected performance at all testing ranges between 0 and 4.5 meters. However, the report didn’t include such information as the size of the transmitter, the size of the receiver, or the amount of power that the transmitter was consuming to reach the delivery targets. We asked Energous for clarification on these questions, and were able to speak with founder and CTO Michael Leabman to get some additional details on the testing.
IEEE Spectrum: Can you tell us how large the transmitters and receivers were during the UL testing?
Michael Leabman: The transmitter is based on one from a year ago, and we’ve come a lot farther along in the last year. But, that transmitter is roughly on the order of the speaker that you saw at CES. It may be a little bit wider, but it’s roughly that size. The receiver was cellphone-back sized.
Approximate size of the transmitter used in the UL testing is that of a stereo speaker.Photo: Celia Gorman/IEEE Spectrum
IEEE Spectrum: Can you talk to us about the end-to-end efficiency of the WattUp system?
Michael Leabman: We typically don’t talk about efficiency because it really depends on the situation, meaning: how many antennas do you have on the transmitter? What’s the shape? And how far away are you? And you have the same issues on the receiver. So, we usually don’t talk about end-to-end, we just talk about what we deliver, because there are too many factors in play.
IEEE Spectrum: Recognizing that the UL testing was done in an ideal situation, can you use that to give us a sense of what the potential efficiency is?
Michael Leabman: I can tell you our target. Again, we’re not deploying equipment, we’re a technology company. We’re giving reference designs to different customers, and we’re getting a lot of feedback on what they believe is the right combination of efficiency and price point. So for example, the system at UL is a little dated, and we’re much farther past that right now, but as an example, we could go as high as 25% efficiency within 10 feet [3 meters]. We can go even higher as you get closer. If you want to do five feet [1.5 meters] and in, we can be a lot more efficient, but you’re limiting your use case.
IEEE Spectrum: So in general, your target is 25% efficiency? What’s coming out of the wall, a quarter of that is usable by the device?
Michael Leabman: That’s our target, yeah. It’s really dependent on what customers want. Do you need five watts? Does it need to be the same as plugging into a wall? Is it okay having two watts at ten feet [3 meters]? Does it need to be two watts at 15 feet [4.5 meters]? Can we do it? Absolutely. It’s just a cost tradeoff, whether someone wants to enter the market with a $100 transmitter versus a couple hundred dollar transmitter.
IEEE Spectrum: How do you plan to achieve that 25% efficiency target?
Michael Leabman: At some point, the pocket will get smaller than the receiver. Right now, it’s not. The pocket is bigger than the receiver, and that energy is wasted. But we’re working on technology such that in the future, our pocket will get smaller and smaller so that [the receiver] is a lot more efficient at capturing energy. That’s probably a year and a half away for us, not today. This is one of the ways of getting [to 25% efficiency]; that level of control is in our roadmap, but we want to get a product out first. We don’t want to be constantly stretching for something that we know we can achieve, but isn’t necessarily needed right away.
That 25% target is really the only specific number that Leabman was willing to give us, and it’s a target for future efficiency as opposed to a measurement of the efficiency of the system right now. We followed up with Energous by email, asking them specifically what the power draw from of the transmitter was at each of the three ranges during the UL test, and received this response:
WattUp is still very much under development and likewise the efficiencies are still being worked on. We are making strides, as shown by the UL testing results. Because Energous is a licensing company, it will work with manufacturers to integrate the tech into products. As such, efficiency rates will be dependent upon how they integrate it into products and many environmental factors such as what type of device it is, distance from the transmitter and the situation it’s in.
I’m not at all sure what “we are making strides, as shown by the UL testing results” means in reference to “the efficiencies are still being worked on,” since the UL report did not include any efficiency information. We followed up again, pointing out that this was not an answer to our question, but Energous declined to provide any additional information.
Photo: Evan Ackerman/IEEE Spectrum
The point that Energous keeps making about efficiency is certainly a valid one: Because their technology can be adapted to all kinds of different form factors and used in all sorts of different situations, there’s really no way for them to say, “this is how efficient our system will be for you.” Are you charging your phone right next to a transmitter? Are you charging a wearable with your hand in your pocket? Are you charging a tablet that’s flat on a table, or that you’re holding up? The efficiency could be (and very likely is) significantly different in each of these cases.
Another thing to consider is that WattUp could change how consumers use devices. If your phone is charging all the time while you’re at home, you might not need a very fast or efficient system in the first place: a continuous trickle could be plenty. And it’s certainly possible that consumers may decide that they’re willing to deal with an inefficient system if it means that they don’t have to worry about taking off their wearables or plugging stuff in every night. We should also point out that wireless charging pads (the ones that require contact) are only about 50% efficient, and even plugging something directly into the wall is often only about 75% efficient due to the conversion from AC to DC current.
While we’d be a lot more comfortable with Energous’ technology if they’d be more forthcoming about some of the very basic (and very relevant) details—and we're having a hard time not being suspicious about why exactly they aren’t willing to communicate—it’s hard to tell how much of an effect (if any) this will have on the acceptance of their technology. We’re expecting more details over the next several months as we get closer to CES 2016, and if you have any specific questions about Energous or WattUp, post a comment below and we’ll see if we can get an answer for you.
Evan Ackerman is a senior editor at IEEE Spectrum. Since 2007, he has written over 6,000 articles on robotics and technology. He has a degree in Martian geology and is excellent at playing bagpipes.