Ice storage air conditioning
Ice storage air conditioning is the process of using ice for thermal energy storage. This is practical because of water's large heat of fusion: one metric ton of water (one cubic metre) can store 334 megajoules (MJ) (317,000 BTU) of energy, equivalent to 93 kWh (26.4 ton-hours).
Ice was originally obtained from mountains or cut from frozen lakes and transported to cities for use as a coolant. The original definition of a "ton of cooling capacity" (heat flow) was the heat needed to melt one ton of ice in a 24-hour period. This heat flow is what one would expect in a 3,000-square-foot (280 m2) house in Boston in the summer. This definition has since been replaced by less archaic units: one ton HVAC capacity is equal to 12,000 BTU per hour. A small storage facility can hold enough ice to cool a large building from one day to one week, whether that ice is produced by anhydrous ammonia chillers or hauled in by horse-drawn carts.
Ground freezing can also be utilized; this may be done in ice form where the ground is saturated. Systems will also work with pure rock. Wherever ice forms, the ice formation's heat of fusion is not used, as the ice remains solid throughout the process. The method based on ground freezing is widely used for mining and tunneling to solidify unstable ground during excavations. The ground is frozen using bore holes with concentric pipes that carry brine from a chiller at the surface. Cold is extracted in a similar way using brine and used in the same way as for conventional ice storage, normally with a brine-to-liquid heat exchanger, to bring the working temperatures up to usable levels at higher volumes. The frozen ground can stay cold for months or longer, allowing cold storage for extended periods at negligible structure cost.
Replacing existing air conditioning systems with ice storage offers a cost-effective energy storage method, enabling surplus wind energy and other such intermittent energy sources to be stored for use in chilling at a later time, possibly months later.
The most widely used form of this technology can be found in campus-wide air conditioning or chilled water systems of large buildings. Air conditioning systems, especially in commercial buildings, are the biggest contributors to peak electrical loads seen on hot summer days in various countries. In this application, a standard chiller runs at night to produce an ice pile. Water then circulates through the pile during the day to produce chilled water that would normally be the chiller's daytime output.
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