From: David Makin <makinmagic@tiscali.co.uk> To: math-fun <math-fun@mailman.xmission.com> Sent: Mon, February 21, 2011 3:32:54 PM Subject: Re: [math-fun] Time-reversed laser absorbs coherent light
That's not the only issue, surely it depends on what form the absorbed energy takes on absorption, or I should say the overall efficiency of going from light
to accessible/transferable power ? _______________________________________________ The NYT article seems to say that, at least so far, absorption in an anti-laser
goes into heat. But a standard absorber can be designed to produce useful work,
as for example a solar cell. The front surface of a solar cell can be etched into tiny pyramids, subwavelength laterally, several wavelengths tall, to gradually shift the refractive index and thus reduce reflection.
-- Gene
I know, but what's the efficiency rating of state-of-the-art solar cells ? Also, probably more relevant, what's the efficiency rating of light->solar->electric heater->heat ((Equal to efficiency of solar cell, the remaining power being wasted in heating the solar cell.)) compared to light->anti-laser->heat ? ((Equals 100% if we take into account the power needed to operate the anti-laser.)) I realise it's all hypothetical because I'm making the massive assumptions that somehow sunlight could be used with anti-lasers *and* that the method used would be very efficient ;) I suspect that even if possible then converting sunlight to light coherent enough to work with the anti-lasers would be somewhat inefficient anyway. _______________________________________________ In general the efficiency of any passive heating device is 100% -- all the power going in comes out as heat. However, a heat pump can achieve >100%. Recall that a heat engine can absorb heat Q2 from a high temperature T2, discharge heat Q1 to a low temperature T1, and produce useful work Q2-Q1. If the engine is reversible, Q2/T2 = Q1/T1 (i.e. no entropy production). Then the efficiency is (Q2-Q1)/Q2 = (T2-T1)/T2. Now operate the engine in reverse. Do work Q2-Q1 in order to extract heat Q1 from a cold environment at T1 and provide heat Q2 to a high temperature environment at T2. The efficiency is Q2/(Q2-Q1) = T2/(T2-T1), the equality holding for a reversible heat pump. An air conditioner pumps heat from your cool indoors to the hot outdoors. These efficiencies are the maximum possible consistent with thermodynamics, and they will be lower for real engines and pumps. -- Gene