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 Home Brew Forums > How much heat does a fermentation produce
11-08-2009, 03:14 PM   #1
silverbrewer
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 How much heat does a fermentation produce

I need to get a handle on how much heat a fermentation produces so I can work out how much cooling power I can get away with.

My cunning plan is to use one or two of those Peltier effect membranes that heat one side and cool the other side when a DC voltage is applied, and reverse the effect if the voltages polarity is reversed. These are the things that 12 volt cooler boxes have in them, but they are not all that powerfull. The advantage of using these things would be they can double up as a heater and a cooler, you just reverse the polarity........Oh, and I can get hold of them for free....

So, does anybody know how many watts of heat a 5 gallon fermentation kicks out at full pelt? (pun intended)

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11-08-2009, 04:13 PM   #2
rocketman768
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So, looks like yeast crank out 118 kJ per mol of glucose.

http://en.citizendium.org/wiki/Ferme..._(biochemistry)

Let's assume 5 gal (19L), 12 plato, OG = 1.050, and that all of the sugar is consumed. We have 19 * 1.050 * 0.12 = 2.394 kg of equivalent glucose which is 2394/180 = 13.3 mol. That means we are going to release 13.3 * 118 = 1,569.4 kJ. Assume that your fermentation is quick and it finishes in 24 hours. That means the average rate of energy release is 1,569,400 / (24*3600) = 18 W.

This is average, and not instantaneous. Who knows what the maximum wattage could be. You will probably be more concerned with the watts you are losing through the walls of your fermentation chamber. There are lots of threads around here with people trying out peltiers. You just have to understand they suck a lot of power for the amount of cooling they do, and that you need large heatsinks with fans on both the hot and cool side.

11-08-2009, 05:08 PM   #3
silverbrewer
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Wow, that's some calculation. That doesn't seem much power at all, so as long as you don't let the heat rise and then take off things might work out after all.
Thanks for the input.

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11-09-2009, 12:36 AM   #4
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uuuummmmmmmmmmmm....... yea, what he said. More platos, with my beer please.

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11-09-2009, 01:30 AM   #5
RunBikeBrew
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In my experience, it seems that warmer fermentation generates a ton of extra heat, whereas cooler fermentation generates less heat. I makes sense if you consider that the heat is a product of yeast eating sugar (i.e heat, alcohol, and CO2). Faster eating=faster heating (and it rhymes!).

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03-01-2013, 03:22 AM   #6
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I know I'm raising a zombie here, but since I've been going over this problem over and over again for a peltier build I thought I'd share with people what I think is a better answer to this question.

Gibbs free energy of fermentation is actually -235 kJ/mol. Almost all of the energy will be released as heat eventually, so that is a better estimate of the real heat output of fermentation. This gets a little more complicated if you want to consider different malt compositions with maltose, sucrose, maltotriose, etc. All my calculations all put glucose as the upper bound. Otherwise this calculation is sound. I get 37W with rounding.

Here is my citation if anybody wants to check my work. http://whitman.myweb.uga.edu/coursed...%20al%2077.pdf

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03-01-2013, 08:31 AM   #7
StMarcos
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235kJ/mol of what? Glucose? This is all quite interesting. Still, it's really the peak power that we'd need to deal with, and that's obviously much more complicated as it depends on so many parameters that are hard to reproduce on a hb scale. Amazing to think that they ferment 20,000L of wash in Scotland, with no cooling jackets, and they don't climb above 90degF. I'd have thought they'd overheat and kill the yeast. I did some tests years back on some 14.5gal stainless conicals, and found that with an ambient controlled to 64degF, the beer would hit 70degF with most yeasts.

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03-01-2013, 08:30 PM   #8
rickyspeak
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I guess I wasn't clear. Yes, I mean glucose.

C6H12O6 -> 2 C2H5OH + 2 CO2 + 235 kJ/mol

Sucrose is:

C12H22O11 + H2O -> 4 C2H5OH + 4 CO2 + 425 kJ/mol

You can do any other sugar you want and get similar calculations. Glucose seems to have the highest energy density at ~1.3 kJ/g. So that forms a nice upper bound for your expectations but the real energy per point of sugar is actually lower than this.

I agree that the real question is: "What is the steepest heat output of a fermenting beer?" That depends on the yeast strain, pitching rate, gravity, fermentable composition of the wort, temperature, water chemistry, ad nauseum. Any graph of SG over time I've never seem to drop faster than half the OG over a 24 hour period. It could be fermenting faster than that at some point in the 24 hour period, so this still isn't an exact calculation.

I agree with rocketman768's assumption that 100% of the sugar over 24 hours is a good guess for the fastest possible rate of fermentation. Or at least a good guess at the maximum. 50% of the sugar over 24 hours might be closer to the real value though.

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03-02-2013, 01:00 AM   #9
Wynne-R
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gives it as 140 kcal/kg

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03-02-2013, 02:50 AM   #10
BigFloyd
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If you're just looking for the temp differential between the bucket and the surrounding air, I've measured 7*F higher on the side of the bucket.
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