Small Home Electrical Generating Windmill - Squeezing out Some heat

November 18, 2015 7:42 AM

I have a small home windmill and wish to extract even the tiniest bit of heat from the 12v DC electricity it generates. The heat would be used to help warm water.

In my 10 x 16 greenhouse I have a 500 gallon 'trough" of water as a heat-sink and source of moisture. I also have a small home-sized windmill that I've been experimenting with.

When the wind blows hard, this small windmill generates enough electricity to make a 12-vol automotive brake light bulb glow very brightly.

What could I use, or how I can convert that wee bit of electricity into something that could provide even the tiniest bit of 'warm' to the water?

Can I hook up a standard aquarium heater to the 12-volt DC?

Is there a 'thing' designed for this?

It doesn't matter that the electricity generated in unreliable. I just want to try and harvest some wind and convert it to heat.

Thoughts?

posted by bricksNmortar to Science & Nature (15 answers total)

you can hook up the brake bulb, as long as it's not blowing. put it in a largish lightproof box if you want to be perfectly efficient. old fashioned light bulbs (with filaments) are basically heat generators.

edit: i am not sure what a wind generator "looks like" in a circuit. is it a variable voltage source? is there some kind of regulator that fixes output at 12V? this might alter the answer above slightly.
posted by andrewcooke at 7:52 AM on November 18, 2015

Can I hook up a standard aquarium heater to the 12-volt DC?

Nope, those work at mains voltage.

Bear in mind that it'll take around 2 kilowatts to heat that mass of water by a single degree (C) in an hour. A decent-sized domestic wind turbine will put out about that amount when the wind is blowing, but I suspect yours isn't that powerful.
posted by pipeski at 7:53 AM on November 18, 2015 [1 favorite]

Best answer: Drop in a 12V immersion heater like this one?
posted by ftm at 7:56 AM on November 18, 2015 [2 favorites]

Response by poster: edit: i am not sure what a wind generator "looks like" in a circuit. is it a variable voltage source? is there some kind of regulator that fixes output at 12V? this might alter the answer above slightly.

It must be variable as sometimes the bulb barely glows, and other times it's really bright. There is no regulator, just two wires coming out of the generator and connected to the light bulb 75 ft away.

Would that alter your suggestion?
posted by bricksNmortar at 7:57 AM on November 18, 2015

Your link is more geographically appropriate but it looks like almost exactly the same thing that I just googled.

A car sized 12v immersion heater will turn whatever amount of power you can get into heat. It probably won't do very much, but it'd be interesting to see how much heat you can get.
posted by Just this guy, y'know at 7:59 AM on November 18, 2015

The variability is probably not going to be a problem. If you hook two wires up to that immersion heater any voltage that you get is going to be turned into the equivalent amount of heat, simply because there's nowhere else it can go.
Although, there must be a rectifier in there to turn the AC into DC.
posted by Just this guy, y'know at 8:08 AM on November 18, 2015

There are 12v immersion elements, just search amazon, $30 something. This is used by boaters that have wind and solar as a power sink at the point that batteries are fully charged, that requires a smart charge controller. But 500 gallons is a lot of sink and it's unlikely there will be more than the barest change on the thermometer where a 5 gallon insulated water heater is not expected to get hot. Now during a long windstorm there might be a lot of amps but unless there's a lot of insulation that wind will be grabbing most of the heat.

If you need hot water... propane!
posted by sammyo at 8:58 AM on November 18, 2015

Use that power to pump up water into a gravity based solar water heater? Glass / black paint / tubing? That kind of tech?
posted by Freedomboy at 9:22 AM on November 18, 2015 [2 favorites]

I get the impression that the idea is to heat the water by whatever comes in and use it as a heatsink. So it doesn't need to get hot. It's just going to sit there and store anything that comes in and slowly radiate it away.

The wind is just a little extra top up.
posted by Just this guy, y'know at 9:23 AM on November 18, 2015

voltage varying isn't a problem in itself, i was asking because i was trying to remember old information about electronics that takes a while to come back to me.

anyway, two general points:

1 - in a sense, anything will do. a light bulb or a heater, or even a TV. in the end, the electricity is converted to heat (you might lose a tiny amount as sound or light, but not if you put it in a sound-proof, light-proof box).

2 - but the efficiency varies. and it varies in a very weird and unintuitive way that depends on resistance. the best way i can think to explain this is that if you connected up something with a huge resistance, then the turbine would have so much difficulty pushing electricity through it would be useless. but similarly if you connect up something with a small resistance (like just shorting the outputs together) it will still not be optimal - in that case, it's "too easy" in a sense, and the generator in the turbine will get hot itself (wasting the electricity there, and possibly damaging the generator).

what (2) means is that optimally, you want to connect something (eg a bulb or heater) whose resistance is the same as the "internal resistance" of the generator. sometimes called "internal impedance". and this trick, to get the most energy possible, is called "impedance matching". it's like the goldilocks value - not too small or too large.

i tried searching for "wind turbine impedance matching" and "window turbine internal resistance", but couldn't find any good numbers. if your turbine comes with a manual it might say somewhere what its impedance or resistance is. if it does, then you could then work out what to connect.

alternatively, if it gives a maximum current then we could also use that as a rough guide.

without either of those, connecting up a heater is a reasonable approach. it might not be optimal, but likely it's a high enough resistance to not damage the turbine.
posted by andrewcooke at 9:33 AM on November 18, 2015

Hey! I agree that the 12V immersion heater that ftm recommends is probably the way to go. However do read the reviews as those things tend to break irreversibly if they are turned on without being completely submerged in water. I don't know your exact setup, but I imagine the level of your 500 gallon trough is pretty variable over the course of days or weeks? To save yourself some headache, keep that in mind as you figure out how to install it. Maybe put a 5 gallon bucket right under the inlet, which fills preferentially and overflows into the trough, and install the heater in THERE?
posted by Joey Buttafoucault at 9:40 AM on November 18, 2015 [2 favorites]

Just to follow up on pipeski's information about the scale you are talking about, assuming your generator can run a brake light, that is about 25 watts. To heat up a 500 gallon tank of water by 1 degree Fahrenheit with your generator would take 60 hours, about two and a half days if the wind were blowing constantly. This is assuming that the tank is perfectly insulated, which it is not. In reality, heat would leak out of the tank about as fast as you put it in. So in other words you may as well just run the light bulb in the open air of the greenhouse for the little amount of heat it would provide and save yourself the cost of an immersion heater.
posted by JackFlash at 10:01 AM on November 18, 2015 [4 favorites]

Just this guy, y'know: "Although, there must be a rectifier in there to turn the AC into DC."

For a pure restive load with supplied watts well under the rating of the heater A/C or D/C won't make any difference.
posted by Mitheral at 4:15 PM on November 18, 2015

True. But I didn't say must to mean that it's required for this, just that they said it was a DC output and a windmill would produce AC, so there's going to be a rectifier in there.
posted by Just this guy, y'know at 2:22 AM on November 19, 2015

This is getting a bit far a field however there are direct DC generators available for windmills. If you want DC as your output they can be more efficient and they can be cheap.
posted by Mitheral at 9:59 AM on November 19, 2015