Like many I've been struggling with fermentation temperatures for a while now. I've been dealing with this like many people and placing a carboy in a tub of water and cycling frozen water bottles through in order to keep the temperature low enough. This is better than some alternatives but it is not practical for styles that require lower temperatures because unless you can change bottles out every 4 to 6 hours your going to see large temperature fluctuations. What I came up with is basically just a modification of what I have been using that makes it more efficient and automated.
10 gallon Igloo Cooler $60 (I salvaged one and you could use your mash tun too)
Cooler of any size $30 (I had one like most people will)
Submersion Water Pump $20 (Nothing too fancy here)
Renco 111000 Temperature Controller $65
Misc Tubing $10?
Total Cost: $175
This could be less depending on what you have.
The basic idea here is that the carboy is placed in the 10 gallon cooler which is filled with water to just bellow the wort line (which is important unless you want your carboy to float). The temperature sensor is placed in the 10 gal cooler and as this temperature rises above a set point it will turn the pump on. The pump will transfer water from the cold water cooler, filled with frozen bottles, to the 10 gal igloo which will drain the warm water back to the cold water cooler.
1. Get a temperature controller. There are plenty of threads on this so just do a search.
2. Get a Cooler that will fit your primary. A 10 gallon igloo works perfect for a better bottle and a carboy so if your doing all grain you might use your mash tun.
3. Get a pond pump.
4. Turn the spigot on the cooler around so the spigot faces up on the inside. This will create the drain for the inside of the fermenter cooler.
5. Attach tubing on the outside of the cooler to drain into the chilling cooler.
6. Attach tubing on the inside to drain. This tube should be cut off at slightly bellow the level of your wort in your primary so it won't float. I stuck a thermometer in it to keep it staight.
7. Put cold water in the cold water cooler with the pump. Pump tubing from cold water cooler goes to the fermenter cooler attached with a clamp above the water line. If don't don't keep this above the water line it will siphon back into the cold water cooler when the pump shuts off.
8. Place primary in the 10 gal igloo cooler.
9. Place temperature controller probe in the cooler and plug the pump in.
The chilling cooler will hold temp for quite a long time as long as you put some ice in every couple of days. My first try I had the cooler from 43 to 45 in two days without adding ice.
I'll keep you posted.