Reducing wireless energy transfer to practice through resonant evanescent coupling by Prof. Marin Soljacic and others at MIT has raised some interesting criticisms. For example, my blog post, “MIT Team Invents ‘WiTricity’ Wireless Energy Transfer” received the following reader comment from a fellow named Larry:
“How does this sit with the world’s energy problems? On the scale that this is envisaged to be applied, the losses are enormous,” said Larry. “Further, resonant coupling is nothing new; e.g. RFID. The MIT folks should be embarrassed, putting out nonsense like this.”
The later portion of Larry’s comment was echoed by others who wrote into Physics Today after WiTricity was announced to punch holes in the concept. Resonant coupling, they say, is not new. Once such letter representing this criticism appeared in “Unwired energy questions asked, answered” in September 2007.
Physics Today allowed the MIT researchers to respond: “Resonant-electromagnetic-induction technology has been inadequate to efficiently power, over mid-range distances, devices that require on the order of tens of watts of power or more. Our work demonstrates that it is the physics of strong coupling, for which resonance is a prerequisite, that enables the efficient wireless energy transfer needed for larger power applications.”
As a scientist, I stand by my original blog commentary. The resonant evanescent coupling work by Soljacic, et al. is fundamentally different than any pre-existing wireless energy transfer technique. However, as an engineer, I cannot counter the first portion of Larry’s criticism; WiTricity wastes energy.
According to Soljacic’s original article, the energy transfer efficiency of the WiTricity technique is about 40% over a 2-meter distance. Achieving global energy sustainability is a problem that will only be solved by attacking on two fronts: 1) improving renewable generation technology AND 2) improving end-use efficiency of energy utilization.
Unfortunately, I am forced to side with Larry on WiTricity. If this technique truly dissipates 60% of the input energy, we must elect to pass on WiTricity and be content to continue charging our laptops with hard-wired connections.