How can you make ice at 15 degrees higher than freezing? You're looking at it.
This tub pictured here, made by Transphase Phoenix, is designed to hold water permeated with various salts so that it will freeze at 47 degrees rather than the standard 32. That means ice can be made at night without as much electricity when power is cheaper. The idea is to reduce peak demand in the afternoon, when electricity is most expensive, and obviate the need for "peaker" plants.
The ice melts throughout the day, providing air conditioning in as many tubs as needed. (They're designed to be stacked.)