From: Geoffrey_dot_landis@sff.net   
      
   Allen Meece  wrote:   
   >>Heat is what we need to better utilize for outer space power, not so   
   >>much the visible part of solar energy. Hi tech steam engines?   
      
   [for what it's worth, thermal conversion systems are referred to in the   
   business as "Dynamic conversion".  Nobody would run steam engines per se;   
   it would be either Brayton or Stirling converters.  (Heck, nobody runs   
   old fashinoned steam engines on Earth for electrical power any more,   
   it's all turbines.]   
      
   In  Henry Spencer replied:   
   > Solar-dynamic power systems -- concentrating sunlight to heat a   
   > working fluid to drive a turbine (etc.) -- have been seriously   
   > proposed, not least for solar power satellites.  Efficiency is higher   
   > than solar arrays,   
      
   Can be higher.  This is very dependent on the details of the sytem...   
   and also dependant on which solar arrays you're comparing it to.   
      
   > at the cost of greater complexity, moving parts,   
      
   Moving parts is always a big problem.   
      
   > and the need for large waste-heat radiators.  Interest has been   
   > limited because of those costs and the lack of experience with them,   
   > but they have potential for some applications.   
      
   and lack of flight heritage is always another big problem.   
      
   > The space station *almost* had Brayton-cycle (working fluid always a   
   > gas, no boiling or condensation) solar-dynamic power.  The higher   
   > efficiency   
      
   Actually, the Brayton wasn't significantly better in efficiency, if you   
   compared it to the solar cells you would have gotten if you updated the   
   solar array with later technology.  However, the phase change material   
   helped quite a bit on the power storage.   
      
   > would have meant smaller collecting areas and hence less   
   > air drag, while phase-change materials would have stored heat to run   
   > the turbines during orbital night, eliminating the heavy and short-   
   > lived batteries.   
      
   Exactly.   
      
   > The idea died during one of the assorted cutback cycles.   
      
   Exactly.   
      
   (In fact, there was a project to fly a test model on Mir.  It died when   
   the Russian partners said (paraphrasing) "and, of course, the existence   
   of this test project means that the United States is committed to   
   extending the lifetime of the Mir space station."  The answer,   
   unfortunately, was "no, we have funding to fly the test, but we're not   
   authorized to change policy.")   
      
   > For systems on the scale of powersats, the necessary turbomachinery is   
   > large and heavy, and almost certainly has to come from Earth rather   
   > than being made from lunar/asteroidal materials, which reduces the   
   > appeal.   
      
   Dynamic conversion SPS has been proposed.  In general, it comes out   
   heavier than photovoltaic.  Remember, the big win of dynamic systems is   
   that heat storage is simpler than battery storage for large systems.  (   
   it loses badly for small-scale power sytems).   
   The mirrors could easily be lunar in origin.   
      
   --   
   Geoffrey A. Landis   
   Mars Guy and Solar Power Scientist   
   Disclaimer: any opinions here are my personal opinions, and not the   
   official announcements of any agency I may work for or have affiliation   
   with in any way.   
   http://www.sff.net/people/geoffrey.landis   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)