From: geoffrey_dot_landis@nasa.gov   
      
   In  Keith F. Lynch asked:   
      
   > Why not use *vaccum* cells?  Light hits the cathode, and knocks   
   > electrons out of it with enough momentum to reach the anode against a   
   > moderate voltage gradient.   
      
   Because they are terribly inefficient.   
      
   A related concept, thermionic conversion, has some potential: you heat a   
   cathode, and it emits thermal electrons, which you pick up at an anode.   
      
   However, it's *also* terribly inefficient, although it has the advantage   
   of converting heat to electricity with no moving parts (other than   
   electrons).   
      
   >  Should work at any temperature, so long as the components aren't   
   >  white hot.   
      
   Actually, no; the anode has to be cold or the thermal back-current will   
   kill you.   
      
   > Perhaps the best "material" for space manufacture is the stuff there's   
   > no shortage of in space -- vacuum.   
      
   Vacuum microelectronics is making somewhat of a comeback with modern   
   field emitters and MEMS processing, but unfortunately it's way down on   
   the learning curve compared to semiconductor electronics, so there's a   
   lot of catch up to do.   
      
   --   
   Geoffrey A. Landis   
   Mars Guy and photovoltaic scientist   
   Any opinions here are my own, and not official announcements of any   
   agency I may work for   
   http://www.sff.net/people/geoffrey.landis   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)