MadSci Network: Engineering

Re: could a PV cell be manufactured with an IC so that it occilates at 50 hertz

Date: Sun Jun 24 04:49:18 2001
Posted By: Karl Kolbus, Staff, Data processing, Mequon Consulting Corp.
Area of science: Engineering
ID: 992689815.Eg

G'day Paul,

To answer your question in one word, that word would be "yes". Yes it 
could be done. Would it be feasible and practical? That requires a little 
more thought. Let me illustrate what I think would be the major hurdle 
with a short story:
When I was in the U.S. Air Force, our unit ran both ground communications 
and radar equipment to control our fighter aircraft. This was a 24 hour, 7 
day a week operation. Even when we were stateside, we would run our 
generators in parallel with the commercial grid, just in case the 
commercial power would fail. We had two sets of generators and would run 
one for 24 hours, then switch to the second unit and run it for 24 hours 
while performing any necessary maintenance on the first, then back to the 
first, and so on. To bring one unit online before shutting down the 
second, you watched two incandescent light bulbs, cross connected between 
the 2 generators. When BOTH bulbs were dark, the generators were in phase 
and you could safely throw the cut-over switch. If EITHER bulb was lit, 
even a tiny bit, the generators were not in phase and you had to adjust 
the speed, little by little, until they were both dark. At this point, you 
may have guessed what happened. One night, while adjusting the speed, one 
of the bulbs decided to burn out. Seeing that both bulbs were dark, the 
Airman threw the switch (he didn't know that the one of the bulbs had 
burned out). What resulted was the complete and utter destruction of BOTH  
generators and, if that weren't enough, it also took down the whole 
southeastern grid of the state of Georgia! Fortunately, there were no 

The point is that it would be difficult, if not impossible, to phase 
match the outputs of the many photovoltaic/sine wave devices that would be 
needed. The problem is not the oscillators; which you could clock 
synchronously with the grid current, but lies in the delay differences 
between devices. Even if all oscillators were running in perfect 
synchronization, each device would present the resulting sine wave at the 
output at a slightly different time; this being seen as a phase shift. 
While the result may not be as spectacular as in the above story, each 
tiny phase difference would equate to a loss of efficiency, or noise on 
the line, or shortened lifespan of some of the devices, or all of these. 
Consider two lead/acid batteries; one fully charged, the other only half 
charged. Does the half-charged battery add its' power to the fully charged 
one? No, it drains the fully charged one until they are more or less 
equal. The same would most likely happen because no two resistors or 
capacitors have exactly the same value, no 2 transistors have exactly the 
same gain, no two wires have exactly the same impedance, etc. Even if they 
did, they probably wouldn't remain so under varying loads, ambient 
conditions, etc. Most importantly, however, would be the requirement to 
continuously monitor the output delay of each device and apply an 
appropriate delay factor to the clocking of its' oscillator to insure a 
(nearly) perfect phase relationship with the grid; no trivial matter! 
Failing this, you would never be able to connect to the grid. Why a delay? 
Because you can't advance the peak of a cycle that is already there; you 
have to delay the next one.

 As distasteful as an inverter may be to you, think about the positive 
aspects of it. You would only need one or several; not hundreds or 
thousands, phase matching would be greatly simplified (watch out for those 
light bulbs!), design/fabrication/testing costs would be less, and 
maintenance would be less frequent and simpler. You could probably improve 
on the design of the inverter by using a torroid instead of a transformer, 
which would reduce heat losses and magnetic radiation, and put diodes in 
series with the photovoltaic cells to eliminate current sapping between 
the hard-working cells and the lazy ones. I'm sure you can think of many 
more ways to improve it. You have the advantage of knowing the exact 
requirements that need to be met and can design a system accordingly.

Let me know if I can be of any further help, and Good Luck with whatever 
method you choose!


Your not-so-mad scientist,


p.s. - you sent your question at 04:32 on Sunday, June 24th and I answered 
it at 03:44 on Sunday, June 24th. How could I answer it before you sent 
it? Do you think we have surpassed the speed of light? :~)     

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