Dear Dennis... analogZONE's Dennis Feucht answers your design queries in his new Circuit Design Clinic! June 2006

analogZONE presents a new, interactive analog design service to readers! Send us your design questions (with relevant data; schematics in JPEG or GIF, please) for some free engineering advice from analogZONE's circuit consultant, Dennis Feucht, on how you might solve a design problem or improve circuit performance. Submissions may be edited for clarity or brevity, and submitters and their email addresses will remain anonymous (unless otherwise indicated). Please send your questions to Dennis here.

Solar Energy System Design and Installation

Design engineers in laboratories often do not regard installation as a significant concern. The following dialog illustrates how easily technical and social problems intertwine, requiring that the engineer also have some people skills (that is, use some tact).

This problem involves a resort on a cay in the tropics. The manager has hired a local solar-energy installer to maintain his solar- and wind-powered system. He does not have enough power and wants to expand by adding more solar panels. I visited the resort and evaluated the installation, leaving the manager a list of suggestions on how to proceed. The first was to put the solar-panel wiring in conduits, to protect it from the occasional high winds along the beach. The technician then proceeded to debunk my comments, as reported to me by the manager:

"I showed Eddy [the maintenance man] your writing and he said: That's all b[*]s[*]. Concerning getting the wires into pipes: 'Yeah, you know Americans always have too much money and that's why they wire everything. There's no need. You wire it properly, wrap it around the lumber and that's it. No wind can make any harm.' All the rest: 'He writes one whole page and didn't say anything.'"

Different people have different ways of doing things. Some people are neat and do things well and others are haphazard and consider it good enough. I won't argue with Eddy (who I do not know) but I've been doing electronics engineering for 35 years and have some idea about causes for electrical failures. The biggest problem with the current scheme is leaving wire nuts exposed. The copper leads on the wires are not hermetically sealed and will oxidize, or more likely form copper chloride with the salt air. Then they start becoming intermittent and sometimes the system doesn't seem to have quite as much power and it takes longer to charge the batteries.

As for all the rest, it's my perspective as an electronics engineer. I hope Eddy is competent in maintaining these systems, but I kind of wonder what his track record is, leaving lead-acid batteries in an uncharged (yet electrolyzed) state for a long time. Check what the battery literature says about that; none of them recommend it. They warn against it.

"He told me that it's not true that the 10 kW tower is too big for us. 'Keep it -- it's the only way to get your batteries charged.'"

Then he must think that the amount of wind there is less than my guess. Maybe he is right, since he lives there. The generator is not 10 kW; it is 7.5 kW. The 10 kW rating is only for cogeneration applications. Yet 7.5 kW is about twice the power you are generating now. That's a big increase.

The cays would be a good place for the Bergy generator except for the problem of putting it up and maintaining it. You'll probably need an 80 foot crane.

"About the batteries: Yes, they self-destruct. But your system is too weak to charge all batteries and to keep them charged. If you connect your 10 kW wind mill then you could connect the rest of the batteries."

If that is what he said, I think you were right instead. Think of the batteries as an electric charge reservoir. You might need to use your diesel generator to charge them all, but once charged, your power usage will discharge 7 of them only 3/7 the amount of three of them. The amount of charge taken out of them is the same as before, and it takes the same amount of charge from the solar panels to recharge them. The panels cannot charge them if they are too deeply discharged, but it can under normal usage.

"He told me (now after I invested $2500 into the big diesel generator) to buy a new one. He knows somebody who sells a 7 kW diesel generator for $3500 and a 10 kW diesel generator for $5000. (I must explain to you that he says that our diesel generator has 45 kW -- I do not know if that's true.)"

This makes no sense to me at all. A 45 kW generator is huge compared to what he is recommending. You can certainly do with it whatever a 5 kW or 7 kW generator can do. Perhaps he is thinking that it is too large and that the efficiency is low because it is not running anywhere near its full capacity. That would be true, but it is also a hassle to have to change generators, especially if you have one that already works and is there at the resort. With more solar panels, you would not need to run the generator often to maintain fully charged batteries, so why change it?

"Eddy connected the generator, measured the power that is produced and discovered that there is no power produced -- the engine works but we do not produce any power. So it's obvious that we returned the generator on the island in vain! Eddy said that one of the main wires was connected to ground and that could have caused a damage. Isn't that the biggest b[*]s[*] on earth?"

Here is something simple you can do. Disconnect the shaft from the engine to the generator and spin the generator shaft. At 45 kW you might need to use a wrench for leverage to do this. Make sure that the wires coming out of the generator are open (not connected to each other). If there is an internal short in the generator it will be very hard to turn the shaft. Try connecting two or more of the generator wires together and then try to spin the shaft and you'll see what I mean. If it is much easier with them disconnected from each other (open) then the generator is probably not shorted inside.

The next step in testing it: have Eddy bring his oscilloscope (hope he has one) and monitor the generator output as the shaft is turned. If he knows anything about electric machines, he will know how to calculate the electromechanical flux (voltage constant) of the motor from the waveform and determine if it is within the right range for that generator. If it is, then the generator is probably okay.

"But Dennis, it is not yet over: Eddy connected the solar panels. He was a little upset as I told him that he has to connect 2 panels to go from 12 to 24 V and he told me that he knows to do his job. He finished yesterday at 9 pm and left to town. This morning, as soon as the sun went up, all of a sudden the battery charger started to disconnect. We turned it off and then on again. The red light (the light that shows that it's charging) came back and then it started to spark, some fire and smoke came out of the charger and it turned off. It looks as we have to run the generator half of the day during Christmas season as we have just one charger charging our batteries."

Oh, no! This is awful! Solar panels are not complicated at all, and connecting them correctly should be easy. You could do it! I drew the diagram on the back of the yellow page of notes.

"Did Eddy connect the solar panels the wrong way?"

I hope he had enough sense to measure the output voltage of the new panels before connecting them into the existing system. One possibility is that too many of the panels are wired in series and it has overvoltaged the charger input -- not good.

"Dennis, after all this unbelievable b[*]s[*], have you got an idea what I should do?"

Smoke is definitely not a good symptom from electronics. I would do the following:

1. Disconnect the new panels and put the old system back like it was for now. See if the charger still works on the old system. If not, it will have to be repaired or replaced.
2. Check the new panel section wiring against my diagram. Then measure with a voltmeter the output of it (not connected) and make sure it is what it should be for a 24-V system. The panels say on them what the "open circuit voltage" should be. The voltmeter reading should be within 20% or so.
3. The Trace Engineering (Xantrex) inverter problem with the recurring error light needs to be solved. During my visit, I looked at the manual and recommended that the parameters be checked. If they are okay, then something deeper is wrong. I recommend calling the Trace Engineering (Xantrex) service center and ask them what it means. I doubt if anything is wrong with the inverters because it is happening to both of them. Something is not configured right. Somebody needs to go slowly through the manual and check the inverter configuration.

The next round is as follows:

"Since our system broke down partly, the guy who did this and who should take care in fixing it told me he should get a new battery charger."

I agree that it would be good to have a spare battery charger in case of catastrophic failure. A smoking charger is possibly a failed charger. That could seriously impair the resort electricity system, causing you to revert to the diesel generator until the charger is repaired. I would get a spare unit for such occasions. The chargers are far cheaper in price than inverters: about $200 US.

"Would you please be so kind and tell me what I have to do now. Do I need a new battery charger? Where do I get this from if I need one? Could he tell just by phone that I need one or is it absolutely clear?"

There's an old American saying that "If you want it done right, do it yourself." I recommend that you:

1. Get the charger documentation and from it get the website of the company who made your existing charger.
2. Do a web search for similar models (for compatibility in swapping in a spare unit) and buy one directly yourself from the distributor. Check http://www.ebay.com for possible good deals on used chargers of the make and model that you already have.
3. If # 2 does not work out, contact the company directly and tell them you want to buy a spare, and where can you get your first unit fixed? Also, if you contact them, ask them how you can get the technical manual, with schematic diagrams.

"I tried what you did, disconnected the new units and tried to run the charger without the new units and it didn't work. The red control light lights for one second and then shuts down, relights again and shuts down and so on (continuously). Could you please advise what I should do?"

I suspect that the charger has failed and that the batteries are consequently not being recharged. They have been discharged to the limit by the inverter and it won't discharge them farther (which is good). It is probably indicating that by the blinking light. You might call the energy distributor for Trace Engineering and see what they can do.

Eventually, I hope to train some local people to be competent in helping people with their power problems, but that won't help you at the moment.