Alec Cameron said the following to Craig Healy on the subject of
solar power in apt? (15 Nov 97  13:00:16)
 AC> On (12 Nov 97) Craig Healy wrote to Alec Cameron...
 CH> That 30 volts is open circuit, no?  So if you charge a 12v battery,
 CH> you need about 13.7v at some practical current flow.  All this current
 CH> flows through the diode.  Say you have a 5 amp current.  Five amps
 CH> times the .7 volts (Ohms Law power formula) is 3.5 watts.
 AC> This is a tricky one, a guy called Kirchoff would explain it better
 AC> than me [Kirchoff's Laws]. The 0.7 volts is an emf [electromotive
 AC> force] not a resistive drop. The diode behaves *like* a wee battery and
 AC> series switch, connected in series opposing the battery voltage. So
 AC> what is "lost" is 0.7 of the 30 volts available on open cct. .7 div by
 AC> 30 =   2.33%. Less than a wee cloud passing by.
 AC> If you find the concept of 0.7 being an emf unconvincing, then put a
 AC> voltmeter across the diode while increasing/ decreasing the panel
 AC> output eg by shading it. You will see that the 0.7v does not change.
 CH> If you are looking to wring every last bit out of the system, then
 CH> removing the diode and substituting some switch-type system may
 CH> help.  As I said, I'm not really satisfied with less than 101%
 CH> efficiency.  
 AC> That could make you very unhappy!! I am real happy with 50%
Gotta disagree a bit on the .7 volt diode value. If you look at the spec
sheets of common rectifier diodes you'll see numbers closer to 1 volt at
a stated current. This is a much more appreciable loss.
In theory the drop should be about .7 volts but the difference is due to
the bulk semiconductor material itself has some resistivity between the
junction area and the connection. At rated current this is an additional
loss.
What he should be using is a Schottky type diode. This has a forward
junction voltage of about .25 volts. With bulk resistive losses this
comes up to about .5 volts in practice or about half the loss of an
common rectifier diode. The difference isn't much but if you're trying
wring all the efficiency possible then it's another option.
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