Lots of the super-duper duty ice storage systems have a thermal store that resembles a plastic rain barrel with a wort chiller coil inside. Made of the same thing as milk jugs and igloo coolers. A few companies have designed residential size units in the 2 to 5 ton range that provide for around 4 hours of compressor-free operation. This allows homeowners to shift the electrical consumption away from peak rate hours and save lots of money. Most of these units are guaranteed for decades, as water is going to freeze the same in 20 years as it does today. Battery and brine-based solutions degrade in capacity over time.
Look around the web at the wort chiller coils. Many of them are rated the same as conventional heat exchangers, and the prices for some of them are not much more than rolling your own. Obviously they are not going to be rated for high pressure refrigerant duty, but if you are going to be running an antifreeze loop, that won't matter. The specs will give you a good idea of how to size the heat exchange pipes.
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