what if you took a constant flow of water, but rather than change the flow you change the temperature of water in a recyclable sealed system. So if the probe in the wine comes up too high then a compressor could turn on and cool the water down, or heat it up.
Actually a better idea is using a peltier system. Thus you don't have a compressor and it can heat or cool as needed with the same system. Since you will need electricity anyway, you could use the peltier system to heat or cool the wine.
Furthermore, peltier systems are very energy efficient and since most wines take 12-24 hours to settle before one adds yeast it would have enough time to get the juice to a suitable starting temperature.
If you really wanted to get very high tech, you could have two thermostats that use a CPU to controll the temperature. one could be measuring the water at 'intake' into the wine cooling/heating system and the other at the 'exit'. It could determine the difference in temperature and make changes via heating or cooling respectively.
This has great potential for the home winemaker, with the fact that you could ultimately use the CPU for a large number of testing references and effectively have it measure CO2 ouput. Thus you could actually graph out a 'fermentation curve' with the CPU and have wines start quicker, take longer to ferment (enjoy more contact time with certain strains of yeast) and know when they are done. It could also keep track of alcohol content and raise or lower the temp to stop the process when the winemaker hits their target ABV or sugar level.
Oh crap, my wine glass is empty. Hold on while I program my robot dog to go get me another glass....
Awesome idea, Let us know what you find out. (though, I will admit we seemed to hijack your post from what you intially wanted it to be).
Okay back to what you asked: I think it would depend. I would suspect that different juices/sugar levels have a different heat transfering rate. Therefore depending on the yeast strain, level of yeast actually used to innoculate, and the stage of fermentation that number could change a lot. I would suggest you test with some sugar water vs yeast. set up a program that logs the temperature changes and ambiant temp and it should give you a reasonable baseline.
Ok I'll stop now, before I hurt myself with another bad idea.
Cheers,
-Ryan