Nelson Ramon and I led a discussion in Sustainable Energy class last week focussing on wireless power. The concept was originally conceived by Tesla about a zillion years ago, but was halted by the FBI. It has been poked at a number of times since then but a solid technique was never fully realized until it got picked up by a team of theoretical physicists at MIT, lead by Dr. Sljacic. Since their successful development of the technique, a handful of initiatives and ventures have commenced, giving this technology quite the explosive re-birth. Here are the resources Nelson and I used to lead the class conversation.
Thanks to our Prof. Jeff Feddersen for his insight and the comments from the class (especially Colombo who brought up a number of great points).
http://www.ted.com/talks/lang/eng/eric_giler_demos_wireless_electricity.html http://web.mit.edu/newsoffice/2007/wireless-0607.html http://www.amazon.com/s/ref=nb_sb_ss_i_2_18?url=search-alias%3Delectronics&field-keywords=inductive+charging&x=0&y=0&sprefix=inductive+charging http://www.wiredstore.net/product/202--em-energizer-em-sub-reg-sub-nbsp-inductive-charger.html http://www.powermat.com/us/portable-mat.html http://www.energizer.com/inductive/product-tech.aspx http://www.energizer.com/inductive/press.aspx http://www.wirelesspowerconsortium.com/ http://www.wirelesspowerconsortium.com/technology/how-it-works.html http://www.wirelesspowerconsortium.com/technology/total-energy-consumption.html http://www.nist.gov/el/isd/upload/Neurostimulators_Chiao.pdf ---------------------- To understand the effect, it can be compared to mechanical resonances. Consider a string tuned to a certain tone as mechanical resonator. Even a far away and low level sound generator can excite the string to vibration, if the tone pitch is matched. ------------------------------------------------- electronic products with wireless charging capability are anticipated to increase from 3.6 million units in 2010, to 234.9 million units in 2014. ----------------------- An estimate of power consumption by wireless chargers. POWER CONSUMPTION OF WIRED CHARGERS Let’s first look at the power consumption of a classic mobile phone charger. These chargers are simple so-called “external power adapters”. A good source for data is the ENERGY STAR website. Here you will see that Energy Start compliant AC-DC adapters typically rate: • Efficiency @ max load: 72% on average for 5 Watt adaptors • Power consumption @ no load: 0.12W on average for 5 Watt adapters with a few exceptionally good adapters going down to 0.01 W. Suppose that you use the adapter for 1 hour per day, and that it remains plugged in for the rest of the day. That is not a good practice, but it is quite common to leave power adapters and cradles continuously connected to the mains. You see that the total energy consumption is: • charging: 1 hour * 2 W / 72% = 2.8 Wh (this assumes that 5 W charger will supply, on average, 2 W during a complete charging cycle) • standby (no load): 23 hours * 0.12 W = 2.8 Wh You see that standby power contributes significantly to the total energy consumption of a mobile phone charger. WHAT ABOUT WIRELESS CHARGERS? Our wireless chargers also contain an AC-DC power adapter. Let’s assume that is has the same efficiency (72%). Let’s also assumes that it has the same standby power (0.12 W). [footnote: Wireless chargers can have a much lower standby power, but this keeps the comparison easier.] The transfer efficiency of the wireless power link is typically 70%. And assume that the wireless charger replaces 2 wired chargers. The total energy consumption is: • charging: 1 hours * 4 W / 72% / 70% = 7.9 Wh (we are now charging 2 devices simultaneously) • standby (no load): 23 hours * 0.12 W = 2.8 Wh HOW DOES THAT COMPARE WITH THE WIRED CHARGERS? Total power consumption of two wired chargers: 2 * ( 2.8 + 2.8 ) = 11.2 Wh Total power consumption of one wireless charger with two receivers: 7.9 + 2.8 = 10.7 Wh You see that the total energy consumption is comparable. Although wireless transfer is obviously not as efficient as transport over a copper wire, wireless power transmitters saves standby power energy when the wireless transmitter replaces multiple external power adapters.