|MadSci Network: Engineering|
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 injuries. 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! Regards, Your not-so-mad scientist, Karl KarlKolbus@ameritech.net 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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