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Comment: Re:Wireless charging hit mainstream ~ 1-2 years ag (Score 2) 184

by dan42 (#49256903) Attached to: Why Apple Won't Adopt a Wireless Charging Standard
Efficiency is all about what you are willing to pay for. Most low power and small transformers are much lower efficiency.

20W output at 66% full load efficiency

Some transformers can have high efficiency when running at light loads - but that means you just paid extra for an oversized transformer. In general, copper losses go up with the square of load current and iron (core) losses go up with the square of voltage. Of course a 30MW transformer will be designed for higher efficiencies - no one wants to pay for 10MW of losses or deal with all that heat.

Comment: not worth it (Score 1) 221

by dan42 (#49212517) Attached to: New Concept Tire Could Recharge Car Battery
Super capacitors are a super pain to design for. The charge/discharge curve is far worse than any battery. They have permanent capacitance degradation if held at rated voltage or at high temp, so you can't top them up and hold the energy for a rainy day. Its expensive/challenging to design circuits to pull the last few volts out, so typical designs have them stacked in series for high voltages with degraded peak load efficiency (ESR). Poor part tolerance combined with charge/discharge cycles requires active balancing ($$$). In the end designs need WAY more total energy storage than useful/usable energy storage.

Compared to gasoline (according to wikipedia), 300kJ is about 0.01 Liters of gasoline = about 1 cent worth. So 1 penny worth of kinetic energy is lost every time you have to emergency stop on the freeway with your SUV? At normal highway speeds its less than half that energy (E=1/2 m v^2).

Comment: Re:miniscule amount of power makes it pointless (Score 1) 40

by dan42 (#49212245) Attached to: Energy-Generating Fabric Set To Power Battery-Free Wearables
"the technology still producing electricity after over 12,000 compression and release cycles" So, assuming an average walking speed that's about *2 hours* before you need new pants. Washing the clothes would most definitely wear them out! And think about what detergent would do to this...

Comment: Re:VC1 (Score 1) 193

by dan42 (#22836642) Attached to: What will be done about hi-def DVDs for the 360?
I believe you're thinking of encoding HD resolution using MPEG2 on DVD? Yes, the bit rate for that is prohibitive (can't fit a whole move), and it will probably need to spin at 8x or faster.
That's the beauty of MPEG4/DivX/VC1,etc - they are much more efficient than MPEG2 (standard DVD encoding) - you can actually fit HD quality movies on a regular DVD (as mentioned earlier).

You cannot have a science without measurement. -- R. W. Hamming

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