Thermal energy storage using ice is practical because of the large heat of fusion of water. One metric ton of water, one cubic metre, can store 334 million joules (MJ) or 317,000 BTUs (93kWh or 26.4 ton-hours). In fact, ice was originally transported from mountains to cities for use as a coolant, and the original definition of a "ton" of cooling capacity (heat flow) was the heat to melt one ton of ice every 24 hours. This is the heat flow one would expect in a 3,000-square-foot (280 m) house in Boston in the summer. This definition has since been replaced by less archaic units: one ton HVAC capacity = 12,000 BTU/hour. Either way, an agreeably small storage facility can hold enough ice to cool a large building for a day or a week, whether that ice is produced by anhydrous ammonia chillers or hauled in by horse-drawn carts.
The most widely used form of this technology is in large building or campus-wide air conditioning or chilled water systems. Air conditioning systems, especially in commercial buildings this technique shifts generation capacity to the day. To generate the ice at night, the turbine is often mechanically connected to the compressor of a large chiller. During peak daytime loads, water is circulated between the ice pile and a heat exchanger in front of the turbine air intake, cooling the intake air to near freezing temperatures. Because the air is colder, the turbine can compress more air with a given amount of compressor power. Typically, both the generated electrical power and turbine efficiency rise when the inlet cooling system is activated. This system is similar to the compressed air energy storage system.
