The air-conditioning systems that operate during off-peak and partial-peak hours for thermal storage, or those that use mainly natural gas to provide cooling to avoid higher electric demand and rates during on-peak hours, are called off-peak air-conditioning systems. These systems include ice-storage and chilled-water storage systems, desiccant cooling systems, absorption systems, and gas engine chillers. In an ice-thermal-storage system, or simply an ice-storage system, ice is stored in a tank or other containers to provide cooling for buildings in on-peak hours or on- and partial-peak hours. Ice crystals are formed in glycol-water when its temperature drops below its freezing point during normal operation. Chilled-water storage uses the same water chiller and a similar coolant distribution system, except for additional water storage tanks and corresponding piping, additional storage pumps, and controls. The larger the chilled-water storage system, the lower the installation cost per ton.hr storage capacity.