On Sat, 19 Sep 2009 14:18:54 +0200, you wrote: >John, sorry for the late answer. > >Unwanted induction heating on rings necklaces etc: they say it doesn't >happen because you need very fine tuning to receive (see the TED video >I linked to, the guy walks happily through the power beam, same thing >for the original MIT research team photographed while sitting in the >beam, photo shown in the video)
--- As far as I know, the ring or necklace would act like a shorted single-turn secondary of a transformer and would heat up without regard to the frequency of the field, the heating depending only on the turn's resistance, its orientation relative to the field it was in, and the intensity of the field. I don't know the intensity of the fields shown in the videos, but my concern would be that in a field of sufficient intensity to charge a cell phone battery would also be capable of heating rings, necklaces, and the like. --- >Turning witricity off when not loaded: yes, good idea if they haven't >thought of that already. If absence of loading can't be detected >easily it might be done by communicating via bluetooth or wifi, e.g.: >"hi there can you send so many watts my way", the emitter would then >deliver witricity until the receiving device stops responding to "do >you still need power" enquiries. --- For a small device, say a pad upon which you'd place a cell phone to charge, detecting a no-load condition would be easy by having the transmitter turn the field on periodically and measure the current into the coil, which would be lower than if a load was present. That would obviate the need for data communications between the transmitter and receiver, reducing costs as well as eliminating the electrical power needed for that function. In a large system, such as the one you describe where walking through or being immersed in the field would charge the cell phone's batteries, the simple load detection scheme wouldn't work and some sort of data communications scheme would need to be employed. Either that or the transmitter would have to stay on forever, wasting power, which seems more plausible given the current propensity to lower up-front manufacturing costs at the expense of long term power waste. --- >But anyway that's the idea I gather, your cellphone, PDA or tablet >will be charged while in your pocket, which will extend (by at least a >factor of two I would guess) its battery life, which would mean a >global saving in energy. It makes sense to me. --- Shooting from the hip is one thing, but having enough reliable data to successfully predict the path of the bullet is quite another. Do you have any actual numbers relating the extension of battery life and its savings on replacements to the cost in manufacturing and operating the large charging system you envisage? --- >What I would disagree with would be using witricity for TVs and other >stuff which could be powered by wire, which I am afraid is in the >pipes... --- Well, we can all vote with our wallets. What I disagree with is that any system designed to send electricity "wirelessly" will ever exceed the efficiency of a properly designed and operated wired system, and will, consequently, waste power. Unfortunately, the convenience of being able to just mindlessly drop a cell phone on a pad will probably win out over any effort made to tout conservation of our resources.
