Hi all, I'm looking into a small scale hydro project just to get my feet wet with renewable energy. Does anyone have any ideas to share on how to design an efficient alternator in the 100 to 250 watt range? I know I can use motors etc, but I'm interested in knowing how to get the maximum performance from the little bit of water energy I've got available. Especially, since I have other plans for what to do with low power alternators :)
Any sources of high efficiency alternators I can simply buy? If nothing else, suggested reading material?
A guy I know used an unmodified car alternator for his hydro project. He had considered having it rewound for 24 or 48V, but it turned out to be cheaper (in his case) to use a 12V system and have rather heavy cables from the alternator back to his house.
If I can find his web page (with how-to guide), I'll send you the URL.
From what I have read, and based on my knowledge of motors and generators, one of the most difficult things is to find the most efficient conversion of the mechanical energy (water flow or wind), by running the device at its optimal speed. This is especially true for wind power, which varies so greatly. Water flow is usually more consistent, but you might want to get more power when it is raining and there is a high volume of flow. You must adjust the load on the device so that it allows the blades to turn at a rate that generates the maximum power. As you increase the load, you get more current but usually less voltage as the speed drops, and at some point you will have maximum power. You can also do things like changing the pitch on the turbine blades to minimize turbulance, which wastes power. You can store energy during times of high water flow by storing it in a dam, or you can just pull as much energy out as possible and store it in a battery.
Automotive alternators might be a good choice, but you will need to adjust field current to get optimum charging current out, and there is a lower limit of RPMs where they don't work very well. For 250 watts, you can find a
1/3 HP tool motor that should work well and be cheap and simple. It will probably help to design a DC to DC converter to maintain an optimal charging current on the battery under various conditions, and have the battery close to the generator, and then use an inverter to make 120 VAC at 2 amps or so, which can be run a long distance on ordinary extension cord wire or UF cable, which can be buried.
Paul brought up some good points about how complicated stuff can get. I don't have any alternative energy hands-on experience but I like to go to
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and read the posts. There's a lot of experience on that board, people that build their own permanent magnet alternators and carve their own wind turbine blades. There's some discussion of hydro too. You would get some good advice there.
Motor and alternator design are a compromise of size, weight, cost and efficiency. One designed to operate efficiently at relatively high speed may not give you good system efficiency because of losses in the mechanisms (belts, gears etc) necessary to drive it at speed from wind or hydro. Efficiency is a matter of how much iron, how much copper and how powerful the permanent magnets. Large copper windings have low IR drop and loss. Lots of iron and strong magnets reduce the number of turns per winding to produce given back EMF at given speed.
Automotive alternators use an excited field, which is a built in inefficiency. That loss doesn't matter much in an automobile.
I would explore electric trolling motors. They are all at least 250 watts. Larger ones, as 700 watts or more, are typically 24-volt motors. They use very strong magnets and multipole designs because they are designed to run fairly slowly (direct drive to a propellor), and efficiency is valued above cost in this application. You can probably scrounge them for little or nothing if you live near a repair shop. Many motors brought in for repair have either bent support shafts or burnt speed control elex, both of which are uneconomical to repair and neither of which is necessary for your application. The best time to look for them is midsummer to late summer, because they often clean up shop toward the end of the season.
Most are brush DC motors, but a few may be brushless now. Brushless would be better because they are intrinsically polyphase motors with electronic commutation so would work well as alternators just by removing the commutation elex. But even one with brushes and a commutator might serve very well in your application. Electronic conversion of DC to AC at that power level (if even necessary) can be done very efficiently.
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