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Thermal energy storage may refer to a number of technologies that store energy in a thermal reservoir for later reuse. They can be employed to balance energy demand between day time and night time. The thermal reservoir may be maintained at a temperature above (hotter) or below (colder) than that of the ambient environment.

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

Thermal energy storage technologies store 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. 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 space heating during winter.

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


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. Thermal energy is cheaper than any other energy source.

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, 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 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 This definition has since been replaced by less archaic units: one ton HVAC capacity = 12,000 BTU/hour.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 rejected heat 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 coefficient 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 fuel 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 replacing.

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.

Molten salt technology

Molten salt can be employed as a heat store to retain heat collected by a solar tower or solar trough so that it can be used to generate electricity in bad weather or at night. It was demonstrated in the Solar Twomarker project from 1995-1999. The system is predicted to have an annual efficiency of 99%, although it is not clear if this is the second law efficiency.The molten salt is a mixture of 60 percent sodium nitrate and 40 percent potassium nitrate, commonly called saltpetre. It is non-flammable and nontoxic, and has already been used in the chemical and metals industries as a heat-transport fluid, so experience with such systems exists in non-solar applications.

The salt melts at . It is kept liquid at in an insulated "cold" storage tank. The liquid salt is pumped through panels in a solar collector where the focused sun heats it to . It is then sent to a hot storage tank. This is so well insulated that the thermal energy can be usefully stored for up to a week.

When electricity is needed, the hot salt is pumped to a conventional steam-generator to produce superheated steam for a turbine/generator as used in any conventional coal, oil or nuclear power plant. A 100-megawatt turbine would need tanks of about tall and in diameter to drive it for four hours by this design.

Several parabolic trough power plants under development in Spain and solar power tower developer SolarReserve plan to use this thermal energy storage concept.

See also


  1. Seasonal Thermal Energy Storage in Thermal Banks.
  2. Advantages of Using Molten Salt Tom Mancini, Sandia National Laboratories, Albuquerque, NM . Accessed December 2007
  3. Molten salt energy storage system - A feasibility study Jones, B. G.; Roy, R. P.; Bohl, R. W. (1977) - Smithsonian/NASA ADS Physics Abstract Service. Abstract accessed December 2007
  4. Parabolic Trough Thermal Energy Storage Technology Parabolic Trough Solar Power Network. April 04, 2007. Accessed December 2007

  • "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.

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