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 Author Topic: recovering waste heat using a hexane steam engine?  (Read 3347 times)
speckman
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 July 10, 2011, 06:19:46 AM

I'm hoping someone can do the math on this:

What if you liquid cooled your GPUs with hexane or water that heats up the hexane?  Make hexane steam (it has a boiling point of about 60C, depending on the type) to power a steam engine.  How much psi do you think you could get for an average high-end ATI card's waste heat?  I'm wondering how much electricity you could generate off of that heat.  I figure 1kw off of 150psi, just from looking around at kits and diy projects, which is roughly how much I'm using with my 4 gpus.  I'm rounding off a lot of numbers here though.

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Pentium100
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 July 10, 2011, 07:03:36 AM

You won't get a lot of power.

The efficiency of a heat engine depends on the temperature difference between the hot and the cold sides.

Assuming your room temperature is Tc=25C=298.2K and the hot side is Th=80C=353.2K, we get:

For an ideal engine:

efficiency = 1 - Tc/Th = 1 - 298.2/353.2 = 0.156 = 15.6%

For a more realistic engine has this efficiency (matches quite well with that observed in power plants) [source]:

efficiency = 1 - Sqrt(Tc/Th) = 1 - Sqrt(298.2/353.2) = 0.081 = 8.1%

So, for every 1kW of heat you put in, you'll get 81W out at best (disregarding the losses in the alternator and so on).

I don't think you'll recover the money you invest in the engine.

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rethaw
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 July 12, 2011, 12:14:42 AM

I don't think you'll recover the money you invest in the engine.

But it would look freaking amazing. You could also experiment with gathering some of the heat back using a peltier, but expect to light up a few LEDs, not another card.

deslok
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 July 12, 2011, 12:31:44 AM

perhaps something closer to a stirling engine would be better for this

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speckman
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 July 12, 2011, 05:29:23 AM

Yeah, I was thinking stirling too.  There's also some pretty swanky new steam engines out there.  It *would* look freaking amazing.    There's just so much juice going into my cards, and so much heat going out, it's such a shame to not boil something with it.
rethaw
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 July 12, 2011, 05:30:33 AM

How about heating a hot tub?

Pentium100
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 July 12, 2011, 07:24:56 AM

But the problem is that cards will not work in high temperatures and there is no way of heating something up to a higher temperature than the source temp without wasting a lot of energy.

You'd be better off building you own coal power plant (or whatever fuel you can get cheaply)

And any heat engine is subject to the calculations I posted earlier. Do not forget that you will need to cool the "cold" side too, so you may end up with all electricity produced going to the fans that cool your engine.

For example, in 25C room temperature, 80C hot side temperature and 1kW heat input, you will get 81W of electricity, provided you can remove the 919W of heat from the cold side of the engine and make it as close to room temperature (25C) as possible, so within a few degrees, or you will get even less power.

Now, if you had a vacuum tube transmitter, with tubes that can be allowed to heat up to, say, 250C, you would get more power, about 245W from 1kW, again, this excludes the power needed to cool the cold side.

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rocksalt
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 July 12, 2011, 07:53:21 AM

thats a really spanking concept

To turn the heat by product into something else.
in winter it's not so bad as you could duct the hot air back into your house and warm via convection, summer is a real b!tch though.
I know that my own little setup currently makes my room feel like a dry sauna, so im looking forward to reduced heating bills in the winter

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speckman
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 July 12, 2011, 03:02:44 PM

Oh well.  Yeah, I didn't realize it was such a poor efficiency.  For reference, this is what you're talking about, I think:  http://en.wikipedia.org/wiki/Carnot%27s_theorem_%28thermodynamics%29.

Well, it seems like that's enough energy to power a liquid cooling system, so that you could have a self-contained cooler requiring no extra juice, powered solely off of the waste heat.  It'd be cool.  Not terribly useful but pretty badass.

Oh yeah.  I can't wait till winter.  I use so little heat in the winter, that I think what I'm generating right now by mining will be plenty.
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