Rectifying AC Hydro Generator

[Ray Perreault]

New member here. Greetings to you all. Didn't see a general discussion forum for introducing myself so thought the next best thing is the forum where I will probably have most of my questions and discussions. I currently have a solar system consisting of 4 260w panels, MS4024PAE Inverter/Charger, Outback FM60 Charge Controller charging 8 L16 6 Volt Batteries wired in series/parallel for 740AH @ 24 Volt. I have had this system up and running since Aug 2017 and have been pleased considering the amount of sun we get in North Central BC in the winter time. I am looking to possibly add a small hydro system such as a Watter Buddy turbine to help offset the times when we go extended times without decent solar and also in the winter months with the shorter charge times. Still researching whether a homemade turbine would be just as good as buying the Watter Buddy because we would really only need to have about a continuous 75-100 watt system to run our household appliances. The one thing that has always confused me a little with commercially bought turbines is where they rectify them. As I understand it, most, if not all, hydro turbines naturally create AC power and then a bridge rectifier is used to turn that into usable DC current for charging batteries. My question is, why do the manufacturers rectify the AC power right at the turbine and then require you to run a very large gauge wire to the destination where your batteries reside. From what I understand if you are running a 48 volt turbine the farthest you can transmit the power with a large gauge wire is 500'. It would make more sense to me to leave the turbine AC and sending AC as far as you want using much smaller (less expensive) wire and then rectifying it right at the battery bank. Is there some reason this can't be done? Maybe someone on here who is way more familiar with the electricity side of things can enlighten me? Thanks

[Rob Beckers]

Hi Ray,

Welcome to the forums!

Line losses are a factor of distance, wire gauge, and current. Most water/wind turbines produce 3-phase AC, and if that's at the same Voltage as the batteries there's not much to be gained by running 3-phase lines (3 wires) and rectifying at the end, or first rectifying and running DC (2 wires).

Rough numbers: A 500 Watt turbine into a battery bank at 60 Volt (charging a 48V bank). On the DC side that's 8.3 Amp (through 2 wires). On the AC side it'll be around 4.8 Amp (going through 3 wires). Let's assume 500 feet wire length, and for the AC wiring 12 AWG, while for DC we'll use 10 AWG. That makes it so the 3 AC wires will have about the same weight in copper as the 2 DC wires, which means you'll spend about the same money on wiring in both cases.

500 feet of 10 AWG has a resistance of 0.500 Ohm, while 500 feet of 12 AWG has a resistance of 0.794 Ohm. Now on to losses, first for 3 phase AC through 12 AWG:

AC-Loss = 3 x 4.8 x 4.8 x 0.794 = 55 Watt, or 11% of total power

For the DC lines, made from 10 AWG, the losses are:

DC-Loss = 2 x 8.3 x 8.3 x 0.500 = 69 Watt, or 14% of total power

The bottom line: Yes, AC works out slightly better, but we only counted copper, and adding in insulation likely means you'll pay more for those three 3 wires vs. the 2 DC wires, making it a wash.

Where AC does have an advantage is if you can transform it to a higher Voltage, transport that over the wires, and then transform back to the Voltage you need. Higher Voltage means lower current, and line losses go with the current squared. That has been done with hydro and wind turbines, though it has its own set of issues (and cost).

-RoB-

[Ray Perreault]

Thanks for the welcome Rob. As for wire size/gauge, I would have thought to go 500' moving DC current would have taken a lot bigger gauge than AWG 10/2. If that is the case then I can see there isn't really too much difference between AC and DC. Thanks

[Rob Beckers]

Ray, I didn't look at what wire size it takes. All I tried to show is that wiring AC or DC makes no difference. For the same amount of money in wire you get very similar losses, whether it is AC or DC wiring.

You mentioned both 24 and 48V battery banks. If you narrow it down and let me know what peak power from the turbine, and how far you need to run the wires, is I'll tell you what wire size vs. losses will be.

-RoB-

[Ray Perreault]

I don't have a hydro system yet, just doing research to see what options I have. I would be using a 24 Volt turbine and the charge controller and batteries would probably be about 200' away. The turbine would probably be outputting 200 watts max.

[Rob Beckers]

200 Watt at 24V would be 8.3 Amp DC (I have a deja-vu feeling here! :weird:). In 3-phase AC that would be 4.9 Amp per wire.

With 8 AWG wire the losses for 200 feet would be 17 Watt (8.7%) for DC with 2 wires, 8.7 Watt (4.4%) for AC with 3 wires. Generally you want to keep peak-loss under 4 percent, but you can pick another number if you like.

For 6 AWG the losses are 11W (5.4%) for DC, 5.5W (2.7%) for AC.
For 4 AWG the losses are 7W (3.4%) for DC, 3.5W (1.7%) for AC.

-RoB-