Energous wireless power transfer over 5.8 GHz RF - Page 1
Forum section: electronics / projects

author: Danseur
author_rank: Contributor
timestamp: November 23, 2015, 04:40:24 am
content: This is my assessment of this article.http://spectrum.ieee.org/energywise/energy/the-smarter-grid/can-energous-deliver-on-wireless-power-promisesThis can only be true when you’re very close to an array transmitter. It’s the near field effect of a line or planar source. Beyond the dimensions of the array transmitter, it reverts back to inverse-square losses. That means you double the distance and the power diminishes by 4. Triple the distance and power diminishes by 9.It most certainly *does* go through your skin. Some of it will bounce off your skin but most of the radio waves that hit your body goes through your skin while the rest of your body’s water content will block/absorb most of the 5.8 GHz energy as heat. I’m not saying this is medically dangerous, but it should be noted and they are still obligated to stay within the FCC’s SAR (Specific Absorption Rate) safety standards. So if your phone is in your clothing pocket, it is conceivable that they may have problems passing the SAR safety limits because of the proximity of the RF “pocket”.Also, it’s extremely doubtful they can eventually hit 25% best case end-to-end efficiency at 10 feet range. Energous was likely talking only about the percentage of the radio wave energy that hits the receiver, and that’s only when the receiver is in an optimal orientation. In fact if Energous supports 4 devices concurrently, by definition they’re splitting the energy 4 ways which means 25% per device at best. This does not include AC-to-DC conversion which is at best 85% efficiency. It does not include the losses of converting DC electricity to Radio Frequency (RF) which is likely around 85% efficiency. It does not include RF to DC conversion efficiency which is 79.5% efficiency at 5.8 GHzhttp://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=6712143&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel7%2F22%2F4359079%2F06712143.pdf%3Farnumber%3D6712143.That means the total best-case efficiency for 10-foot range is 14.4% and that’s if all the stars are aligned. Moreover, their 10-foot range may be limited to certain coverage angles at the transmitter.The reality is that they won’t be able to consistently capture 25% of the RF energy because the receiver won’t always be aligned correctly. Energous might be able to direct up to 25% of the radio energy to the phone, but they have no control over the orientation of the receiver. That means their real efficiency could be less than half the 14.4%. So their typical end-to-end efficiency could be well under 7%.The article is also too pessimistic on the efficiency of existing solutions. Here’s a good source of information below.http://batteryuniversity.com/learn/article/charging_without_wiresNote that the 93% to 95% at 8-inch gap is just the wireless efficiency. It does not include the 15% loss at the AC-DC brick. When included, they’re down to 79 to 80.7% efficiency. If we made a gas station analogy, this is kind of like filling your car up from 10 feet away and aiming the gas hose at your car. For every 8 gallons of oil that goes into your car, 2 gallons of gas misses the fuel port and goes into the gutter. I suspect people would not be enthusiastic about this when it comes time to pay.As far as charging cell phones go, many people I talked to actually don’t care if they have to waste 99 watts to deliver 1 watt to their phone. I suspect many people are willing to tolerate 99 watt waste or even 999 watt waste if it is a short amount of time. Many people will simply justify this by saying they already waste far more energy. In my personal opinion this is irresponsible design, but it does indicate a market willing to waste 99% of the energy if a product could actually charge their phones at 10 feet range.The big question is whether this device will get FCC certification when there are hundreds of transmitters with each transmitting close to 1 watt. I suspect Energous will attempt to justify it by using the loophole that each radio is under the FCC limit of 1 watt, but that’s not how the FCC certifies devices. The FCC expects realistic end-to-end tests that include factors like antenna amplification/focusing, even losses due to antenna cable length, and the fact that there are multiple transmitters on the same circuit board. Will the FCC permit a single product to emit 100 watts of power? I highly doubt they would permit this and if Energous tried to force the issue with lawyers, the FCC is no pushover. In fact the FCC has recently shown much willingness to exceed their legal authority by forcing Hotels to accept $700K settlements/fines because they sent out WiFi deauthentication packets to block personal WiFi devices.I suppose anything is possible if enough lobbying is done, but this seems to open a gaping loophole in the FCC 5.8 GHz unlicensed spectrum.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 24, 2015, 02:51:42 am
content: While it's unlikely to apply to a 5.8GHz transmitter, it is possible to build a wireless power transmitter that runs directly from rectified mains. In that case, it is possible to build a wireless power solution with a higher efficiency than many of the wall warts on the market.

author: Danseur
author_rank: Contributor
timestamp: November 24, 2015, 07:44:21 am
content: Quote from: NiHaoMike on November 24, 2015, 02:51:42 amThat only addresses the transmitter portion. It does not deal with all the other losses involved.

author: ConKbot
author_rank: Super Contributor
timestamp: November 24, 2015, 12:01:27 pm
content: Fcc should tell them to pound sand. I like my 5 ghz Wi-Fi working thank you very much. It's not bad enough that 2.4 is crowded to the point of non working, you're gonna waste spectrum on a wireless power farse? Using non inductive coupled wireless power is garbage for efficiency anyway.As far as being safe? 2.4ghz penetrates the body enough to cause subdermal damage, mm wave is used in active denial "heat ray" systems. Last time I checked, 5.8 was above 2.4 and below millimeter wave, so it'll still be absorbed into the body.