electric fans as generators??

[hertfordnc]

I have a Dodge 3500 with Cummins diesel. THinking about electric fans.

Reading all the arguments about how much energy the clutch fan uses versus the amp load on the alternaotor versus "I'm old and I know everythign and that will never work becasue it does not work on my truck"

SO i got thinking about a seperate battery to power the fans.

THis is a 7000 lb truck so a couple of golfcart batteries in the bed would not be felt.

So what if the fans could generate a charging voltage when they are not running?

THen add some sort of low voltage switch to get charging power off the alternator.

So the whole fan system works like a hybird and recovers it's energy whenever the engine is below 195 degrees.

I hope this generates soem discussion.

[Daox]

Spinning a fan is going to create aerodynamic drag. That drag has to be overcome by your engine's output. So, you'd be using more fuel. Best to let the alternator do its job, or use those deep cycles and disconnect the alternator completely and charge at home.

[hertfordnc]

Not sure about the drag- there will be some but the air on the fan will have already pushed through the radatior before it hits the fan so the drag becomes hard to measure.

I don't seee charging at home as a good option. Of course if I'm coasting my 3.5 ton truck the drag is negligble and the charging power is free.

I'm thinking I could open the charging circuit whenever the engine is pushing so it only charges when coasting.

The questions is whether or not cooling fans have Permenant magnets and whether or not they could charge the batteries.

[spacemanspif]

It would probably be easier to rig up a solar charger above the "battery box" in the bed to tend the batteries while the fans are not running and while the truck is parked, parking outside would be best, of course. But on a truck that already runs dual batteries why would you want to add more? I would be a damn shame to be stuck in traffic with the AC on when your batteries die and the fans cut out. Running them off the engine's electrical system would be a safer bet since the fans will keep running as long as the alternator works. With an engine putting out 800lb-ft fo torque, is the extra load on the alternator going to make that big of a difference?

[hertfordnc]

THis is really more of a theoretical discussion.

Just wiring the fans to the alternaotr would probably bring some benefit. But the naysayers love to point out that the alternator will drag delivering the load to power the fans.

So my first thought was to have a secod battery to power the fans then charge that battery from the alternator but only when the truck is coasting in gear. That would be some pretty good energy recovery.

THen i thought, maybe the fans themselves could produce charging voltage. If it worked it would be pretty cool.

Solar is an option if it were small, too expensive and ugly to put a big panel on the truck.

[Daox]

Most radiator fan motors are PM type. So, lets assuming you did hook up your fans as a charger. Lets also say they spin at say 2000 rpm @ 13V (based off this). That means you have to spin the fan FASTER then that to produce more than 13V, and thus charge your battery. Having the fan behind the radiator isn't going to do that. You'd have to put it on to of your roof or something.

[hertfordnc]

That's just another problem to solve. If the fans generate any decent amount of voltatge then you could get a charging circuit by either pulsing the power through a capacitor or using a pair of 6 volt batteries and a relay to turn it from a parallel (6 volt) to a series (12 volt) circuit)

[Daox]

True, you can use a boost converter, or some other form of voltage increasing circuit. However, once you put a load on that fan, its rpm is going to plummet. That fan I linked to creates an airflow of 1730 cfm @ 12.7A. How much airflow do you think you have? Say you get a very generous 1000 cfm through your radiator at 60 mph. The fan is only going to capture a fraction of that airflow for energy generation. A DIY wind turbines is about 30% efficient, so you get 300 cfm captured for generation. That would give you 2.2A of charge (assuming no other losses, and there will be other losses). Is it worth the hassle for 2.2A? That is a right around 30W. A 30W solar panel isn't that big, goes for about $65 shipped to your door, and it would be way easier to hook up, plus it charges when you aren't going 60 mph, and it even charges when you aren't even driving...

I'm not saying it can't be done. I'm saying its not worth the time. There are other things that would be much better alternatives and give you larger gains.

[hertfordnc]

THose are some very good points. But i think the airflow is higher. THeoretically the front of the truck is feeling 5000 CFM for every sq foot of area(at 60 mph). Hard to figure how much of that force would make it throgh the radiator to turn a fan.

Solar could work but really, this idea is just cooler to me if it's feasable.

[Daox]

If you wanted to test it, disconnect your rad fan connector and put a voltmeter on the pins and go for a spin to get some idea of what kind of voltage is produced. If its producing good voltage, put a load on it (a resistor), and put an ammeter on it to see what kind of power its capable of producing.