in my opinion i would not do it that way.
first off, starting and stopping a 1/2 horsepower chiller that much is going to be expensive for one, hard on the machine for another.
lastly, would you even consider do that to a heater? with the chance it may over heat your tank? this is the opposite side of that coin. The reason I am saying this is 1/2 hp is pretty big for a tank even up to a 180 no matter what the chiller rating is.
I would think seriously about buying a proper controller, or a proper sized chiller.
What are your tank params now that you feel you need a chiller? I only ask b/c alot of people misconceive the physics at work here.