In reply to John Berry's message of Tue, 5 Feb 2013 11:12:21 +1300: Hi John,
>Ok, I think there is something that David and I myself are unclear on... > >Let's say you take IR radiation, so what is that? > >A high frequency EM wave in the Teraherz range, if you have a nano antenna >and diodes suited it could maybe be rectified directly to usable power, >since this can be done with microwave energy I do not see any issues with >doing it at IR at least in theory. Agreed. > >Now I do know anything really about photovoltaic cells, but I imagine they >do something like this, they rectify terahertz to DC. No exactly, but the result is the same. Electrons in solar cells absorb the energy of the incoming EM freeing them from their atoms, and giving them the energy to cross the semi-conductor diode junction. > >So is a temperature differential required to convert EM into DC electrical >power? I don't think so. However .... there is also a leakage current in solar cells, which presumably goes in the wrong direction, and is temperature dependent. When no temperature difference exists, do the forward and backward currents equal out? I don't know the answer, and suspect that an experiment is the only way to find out. One interesting point to note however is that sunlight has an effective blackbody temperature of about 91 ºC by the time it reaches the surface of the Earth, so there is definitely a temperature difference with the surroundings. (i.e. a black body of that temperature would radiate the same amount of power / m^2 as we receive from the Sun; about 1 kW/m^2.) > >If you had a solar cell on the sun that somehow could survive such radiant >temperatures, would it have no DC output? See above. > >I guess it just seems (perhaps incorrectly?) that solar cells are somehow >getting past the normal thermodynamic laws because they tap the EM >radiation (which can be focused, polarized and interacted with >electromagnetically) and not thermal energy directly. > >So are you saying that EM flux can not be tapped without a temperature >differential? See above. >That if IR Emectromagnetic radiation were too homogeneous you could not >rectify it??? First, I'm not sure what you mean by homogeneous in this context, but I suspect you mean the opposite of coherent. The answer in that case is that I don't think it makes any difference whether it is coherent of not. Second, as mentioned above this is not exactly rectification that is taking place, though that may just be a semantic argument. > >John > > >On Tue, Feb 5, 2013 at 9:05 AM, <[email protected]> wrote: > >> In reply to Jones Beene's message of Tue, 29 Jan 2013 11:14:17 -0800: >> Hi Jones, >> [snip] >> >In the end - if you want to find a practical and gainful >> heat-to-electricity device close to ambient, then provide the virtual sink >> well below ambient. That may be difficult, but Dirac permits it and I >> would never argue with PAM. >> >> ..wasn't it you who first mentioned mercury based semiconductors with a >> very low >> bandgap on this list? >> >> Quite apart from that however consider that the kinetic energy of molecules >> tends to be distributed across all energy levels, so if energy can be >> withdrawn >> at *any* level, then that level will eventually be replenished by energy >> from >> the other levels (the sum of which will become depleted by the amount >> withdrawn). This is essentially what happens with wind-chill. There is a >> specific amount of energy required to break the hydrogen bonds between >> water >> molecules, and this is supplied by thermal energy of those molecules with >> sufficient kinetic energy, with the temperature of the liquid dropping to >> compensate for the lost energy, as the energy of the other molecules is >> redistributed. >> >> The implication of this is that a semiconductor with any bandgap should >> work, >> though I would think that those with a smaller bandgap would probably work >> faster as there is a larger population of low energy electrons than of high >> energy electrons. >> >> Regards, >> >> Robin van Spaandonk >> >> http://rvanspaa.freehostia.com/project.html >> >> Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/project.html
