The principal application today is the production of ice or chilled water or eutectic solution at night which is then used to cool environments during the day.

Proposed Thermal energy storage technologies envision the storage of heat, usually from active solar collectors, in an insulated repository for later use in space heating, domestic or process hot water, or to generate electricity.

Molten salt has been proposed as a means to retain a high temperature thermal store for later use in electricity generation. Solar Tres Project Molten salt as an energy store

Most practical active solar heating systems have storage for a few hours to a day's worth of heat collected. There are also a small but growing number of seasonal thermal stores, used to store summer heat for use during winter.

Economics
High peak summertime loads drive the capital expenditures of the electricity generation industry. The industry meets these peak loads with low-efficiency peaking power plants, usually gas turbines, which have lower capital costs but higher fuel costs. A kilowatt-hour of electricity consumed at night can be produced at much lower marginal cost. Utilities have begun to pass these lower costs to consumers, in the form of Time of Use (TOU) rates, or Real Time Pricing (RTP) Rates.

Water based technology
Thermal energy storage is made practical by the large heat of fusion of water. One metric ton of water, just one cubic meter, can store 334 MJ (317 k BTUs, 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 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 This definition has since been replaced by less archaic units: one ton HVAC capacity = 12,000 BTU/hour.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.

Air conditioning
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, are the most significant contributors to the peak electrical loads seen on hot summer days. In this application a relatively standard chiller is run at night to produce a pile of ice. Water is circulated through the pile during the day to produce chilled water that would normally be the daytime output of the chillers.

A partial storage system minimizes capital investment by running the chillers 24 hours a day. At night they produce ice for storage, and during the day they chill water for the air conditioning system, their production augmented by water circulating through the melting ice. Such a system usually runs in ice-making mode for 16 to 18 hours a day, and in ice-melting mode for 6 hours a day. Capital expenditures are minimized because the chillers can be just 40 to 50% of the size needed for a conventional design. Ice storage sufficient for storing half a day's heat rejected will do.

A full storage system minimizes the cost of energy to run the system by shutting off the chillers entirely during peak load hours. Such a system requires chillers somewhat larger than a partial storage system, and a larger ice storage system, so that the capital cost is higher. Ice storage systems are inexpensive enough that full storage systems are often competitive with conventional air conditioning designs.

The efficiency of air conditioning chillers is measured by their coeffcient of performance (COP). In theory, thermal storage systems could make chillers more efficient because heat is discharged into colder nighttime air rather than warmer daytime air. In practice, this advantage is overcome by the heat losses while making and melting the ice.

There are still some advantages to society from air conditioning thermal storage. The coal or uranium used at night to produce electricity is a domestic resource in most countries, so that less imported fuel is used. This process also has been shown in studies to significantly reduce the emissions associated with producing the power for air conditioners, since inefficient "peaker" plants are replaced by low emission base load facilities in the evening. The plants that produce this power are often more efficient than the gas turbines that provide peaking power during the day. And because the load factor on the plants is higher, fewer plants are needed to service the load.

A new twist on this technology uses ice as a condensing medium for refrigerant. In this case, regular refrigerant is pumped to coils where it is used. Instead of needing a compressor to convert it back in to a liquid however, the low temperature of the ice is used to chill the refrigerant back in to a liquid. This type of system allows existing refrigerant based HVAC equipment to be converted to Thermal Energy Storage systems, something that could not previously be easily done with chill water technology. In addition, unlike water cooled chill water systems that do not experience a tremendous difference in efficiency from day to night, this new class of equipment typically displaces daytime operation of air cooled condensing units. In areas where there is a significant difference between peak daytime temperatures and off peak temperatures, this type of unit is typically more energy efficient than the equipment it is upgrading.

Combustion gas turbine air inlet cooling
Thermal energy storage is also used for combustion gas turbine air inlet cooling. Instead of shifting electrical demand to the night, 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.

External links
ASHRAE white paper on the economies of load shifting
Thermal energy storage links at dmoz.org
Molten Salt Energy storage Sandia National Laboratories
[1] MSN article on Ice Storage Air Conditioning

References
"Prepared for the Thermal Energy Storage Systems Collaborative of the California Energy Commission" and "Source Energy and Environmental Impacts of Thermal Energy Storage." Tabors Caramanis & Assoc.