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   XPost: sci.physics, sci.electronics.design, sci.chem   
      
   On May 2, 9:50 am, dagmargoodb...@yahoo.com wrote:   
   > On May 2, 9:10 am, Phil Hobbs    
   > wrote:   
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   > > On 5/2/2013 7:04 AM, Jasen Betts wrote:   
   >   
   > > > On 2013-04-30, Tim Wescott  wrote:   
   > > >> On Mon, 29 Apr 2013 21:36:22 -0700, RichD wrote:   
   >   
   > > >> I'm not sure why quantum efficiency would suffer as recombination time   
   > > >> goes down unless (a) they're referring to the overall quantum efficiency   
   > > >> (in which case power efficiency and quantum efficiency are just   
   synonyms)   
   >   
   > > > I can't see power efficiency being synonymous with quantum efficiency   
   > > > solars photons go in with an mean energy of around 2eV and you get   
   > > > electron current out (of a silicon photocell) at less than a third of   
   that.   
   >   
   > > Solar cells are diodes working in forward bias.  If you short-circuit   
   > > the cell, you lose practically nothing to recombination (i.e. forward   
   > > conduction), so you get all of the photocurrent, and therefore the   
   > > maximum operating quantum efficiency.  Unfortunately you get zero power,   
   > > because P = VI.   
   >   
   > > If you open-circuit it, you get the maximum terminal voltage, i.e. the   
   > > maximum energy per electron, but you waste all of the photocurrent   
   > > forward biasing the diode, i.e. the operating quantum efficiency is zero.   
   >   
   > > In between, you get less than maximum voltage and less than maximum   
   > > current, but since both are nonzero you also deliver power to the load.   
   >   
   > > The maximum power point is where d(VI)/dV = 0, i.e.   
   >   
   > > I + V dI/dV =0    so   I/V = - dI/dV   
   >   
   > > If you increase the temperature, the forward voltage of the diode   
   > > decreases just like any other diode, so if you keep the same operating   
   > > voltage you start to lose current (i.e. the operating quantum efficiency   
   > > goes down).  To maintain maximum power, you have to reduce the operating   
   > > voltage, which will increase the current some, but not all the way back   
   > > to its lower-temperature value.   
   >   
   > You can get higher quantum efficiency by throwing a tarp over it,   
   > reducing the temperature.  Makes 'em last longer too.   
   >   
   > > So if you want to maintain maximum power, you have to run at lower   
   > > operating QE as the temperature increases, and the power you get is   
   reduced.   
      
   I designed a really nifty solar space-heater a few years ago, only to   
   realize the winter I designed it was running overcast/cloudy 8.5 days   
   out of 10.  (I measured "cloudy" intensity at 3-to-5% of clear, on   
   typical days, so that's a lot of cold days.)   
      
   Fast-forwarding to today, there's a guy locally with a lot of solar   
   panels selling for $1/W.  This inspires an idea--air blown up under   
   the panels could cool the panels, harvest the waste heat for heating,   
   and I could be without heat or electricity both, all winter long.   
      
   --   
   Cheers,   
   James Arthur   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)