This is my assessment of this article.
http://spectrum.ieee.org/energywise/energy/the-smarter-grid/can-energous-deliver-on-wireless-power-promises“First, the energy that can be received is proportional to the distance between the transmitter and the receiver”
This 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.
"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)"
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 GHz
http://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_wiresFor household and business use, the California Energy Commission (CEC) Level V mandates that AC adapters must meet a minimum efficiency of 85 percent; Energy Star Level V requires 87 percent (European CE uses CEC as a base). Adding the losses of the AC adapter to wireless charging brings the overall efficiency further down as the inductive transfer efficiency of inductive charging is only 75–80 percent.
Resonance charging is not limited to high wattage wireless chargers; it is used at all power levels. While a 3kW-system for EV charging achieves a reported efficiency of 93–95 percent with a 20cm (8”) air gap, a 100W system is better than 90 percent efficient; however the low-power 5W systems remains in the 75–80 percent efficiency range. Resonance charging is still in experimental stages with an agreed standard.
Note 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.
Home
Help
Search
Login
Register
