The idea is to use cheap off-peak energy at night to freeze a tank of water or “distributed energy storage system” then use that great block of ice to cool your data centre in the daytime. Refrigerant would circulate from the tank to the Data Aire equipment eliminating the need to run the energy-intensive compressor and condenser during peak daytime hours.

Apparently this can save up to 45% on energy requirements for the cooling.

via Ice air con system aims for cool on the cheap • The Register.