Well we have learned a lot about the wind generator and issues related to low voltage power transfer. We know why the wind generator mentioned in these previous articles, here and here, has failed to work for us. We also have a plan and some interesting experiments to try. More after the jump.
The key problem is that the wiring we used to install the wind generator is too small. We used about 150 feet of 12 Gauge wire, the same wire you typically find in your home for 120V applications. The voltage drop on this wire would be about 5V at 10 amps. The generator system simply can not support this much drop.
Ross Taylor provided some comments at the original article here and via email. He provided some links to wire gauge information and calculators to help determine what size wiring would be required to make things work well.
Since the loss occurs in both sides (positive and negative) we have the equivalent of 300 feet of 12 Gauge wire for the purposes of determining loss. The wire table linked above shows the resistance of 12 Gauge wire as 1.6 Ohms per 1000 feet. So our cable run should have about 0.5 Ohms of resistance. Using Ohm’s law we can see that the voltage drop would be about 5V at 10 amps. This means that to deliver charging current to our battery (14V) we would need about 19V at the wind generator. The extra 5 volts are required to compensate the losses in the wire.
The simple solution is to use larger gauge wire but this may not be practical. It will drive the cost of this system way beyond the initial cost of the wind generator. The wire size and loss calculators tend to focus on how much you are willing to lose in your wiring but in this case the key is that the wind generator will not charge the battery if it sees too much drop. If we want to get 10 amps of charging current then we can determine what gauge of wire is necessary in a given installation.
We have learned that the wind generator will stop charging the battery if the voltage it sees is above some threshold. We don’t yet know what that limit is.
Assuming they intended to support one (1) volt of drop we can calculate the wire size. Using Ohm’s law again we can find the maximum allowable wire resistance at R=E/I or 1/10, 0.1 Ohms. If this is true, we need wire that is about five times larger then what we have. We need wire with 0.3 Ohms per 1000 feet of resistance. Looking at the wire table we see that 4 gauge is 0.25 Ohms per 1000 feet and 5 is 0.31 Ohms per 1000 feet. 5 Gauge is close but 4 gauge would be required if we have to have less then 1 volt drop.
The 12 AWG wire we used cost about $50 per 100 feet or say about $75. It was outdoor Romex cable with three conductors. A quick online check for pricing on 4AWG wire turns a cost of about 0.50 per foot for a single conductor. We would need 300 feet (150 feet and two conductors) which works out to about $150 for the wiring.
Again this all assumes that one volt of drop is acceptable to the wind generator design limits. If we need less drop, then larger wire will be required.
The extra $150 to install this wind generator is significant. It seems reasonable to expect that users would need to make long wiring runs between the generator which might be on a tower to the area where the battery’s and other power systems are located.
There are a few ways to mitigate this extra cost. One is to move the charge controller from inside the wind generator to the other end, near the batteries. A much higher voltage could be generated directly at the generator there by reducing the current in the feed lines to keep the looses down. The charge controller would sense the battery charge state and regulate this higher voltage. This is similar to how our home power systems work. The power plant generates a high voltage so that the line losses due to current draw are reduced. The wind generator could output three phase AC at a higher voltage and would then work with typical low cost house type wire. This AC signal could be rectified and controlled at the batteries. A separate electronics box would be required and may drive the wind generator cost up a bit but the overall installation cost could be reduced.
Another answer is to run a pair of sense wires. These sense wires would measure the battery voltage without significant voltage drop as virtually no current would be flowing in the sense lines. The integrated charge controller could automatically increase the voltage to compensate for line losses this way. The system would adapt to the losses gracefully. Of course there would be some limits. The system would need to support 5 volts of drop to work in our configuration. Also, you would need to run another pair of wires from the wind generator. These two wires could be very small as significant current is not passed though them.
For our experiments we have opened the generator and disabled the control system so that we get a loosely rectified output directly from the generator. Unfortunately the voltage at the generators output is not high enough to cure the voltage drop issues. We may need to build a charge pump to dump energy into the battery at much lower input voltages. This is not too difficult to do and maybe of value to other people getting into wind or solar energy where the cost of large gauge copper wire may be avoided, at the cost of some efficiency.
We have also been playing with solar panels. My current impression is that they are a better way to get started with alternate energy. I have a bunch of those solar yard lights and some Malibu lights on my deck. I want to convert all of these to use solar from a central system. I hope to have some write ups about this soon. Let us know if you have a recommended source for large solar panels.
Any ideas or comments?