Thermal energy storage

District heating accumulation tower from Theiss near Krems an der Donau in Lower Austria with a thermal capacity of 2 GWh

Thermal energy storage comprises a number of technologies that store thermal energy in energy storage reservoirs for later use. 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) that of the ambient environment. The applications today include the production of ice, chilled water, or eutectic solution at night, or hot water which is then used to cool / heat environments during the day.

Thermal energy is often accumulated from active solar collector or more often combined heat and power plants, and transferred to insulated repositories for use later in various applications, such as space heating, domestic or process water heating.

Solar energy storage

Main article: Solar hot water storage tank

Most practical active solar heating systems have storage for a few hours to a day's worth of energy collected. There are also a small but growing number of seasonal thermal stores, used to store summer energy for space heating during winter. Molten salt is now in use as a means to retain a high temperature thermal store, in conjunction with concentrated solar power for later use in electricity generation, to allow solar power to provide electricity on a continuous basis, as base load energy. These molten salts (Potassium nitrate, Calcium nitrate, Sodium nitrate, Lithium nitrate, etc) have the property to absorb and store the heat energy that is released to the water, to transfer energy when needed. To improve the salt properties it must be mixed in a eutectic mixture.

Economics

High peak 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^{[citation needed]}, 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 Storage

Main article: Hot water storage tank

Large stores are widely used in Scandinavia to store heat for several days, to decouple heat and power production and to help meet peak demands. Interseasonal stores have been investigated and appear to be economic, based on rock caverns.^[1]

Ice-based technology

Thermal energy storage using ice makes use of the large heat of fusion of water. One metric ton of water, one cubic meter, 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 m²) house in Boston in the summer. This definition has since been replaced by less archaic units: one ton HVAC capacity = 12,000 BTU/hour (~3.5 kW). 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.

As such there are developing and developed applications where ice is produced during off peak periods and used for cooling at later time.

Molten salt technology

Molten salt can be employed as a thermal energy storage method to retain thermal energy 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 Two project from 1995-1999. The system is predicted to have an annual efficiency of 99%, a reference to the energy lost by storing heat before turning it into electricity, versus converting heat directly into electricity.^[2][3][4] The molten salt is a mixture of 60 percent sodium nitrate and 40 percent potassium nitrate, commonly called saltpeter. 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 addition of other nitrates like Calcium nitrate reduces the melting point (133 ºC).

The salt melts at 221 °C (430 °F). It is kept liquid at 288 °C (550 °F) in an insulated "cold" storage tank. The liquid salt is pumped through panels in a solar collector where the focused sun heats it to 566 °C (1,051 °F). 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.^{[citation needed]}

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 a tank of about 30 feet (9.1 m) tall and 80 feet (24 m) in diameter to drive it for four hours by this design.

Several parabolic trough power plants in Spain^[5] and solar power tower developer SolarReserve use this thermal energy storage concept.

Research

Storing energy in molecular bonds is being investigated. Energy densities equivalent to Lithium ion batteries have been achieved.^[6]

See also

	Sustainable development portal
	Energy portal

• District heating
• Eutectic system
• Fireless locomotive
• ice storage air conditioning
• Liquid nitrogen economy
• Phase change material
• Pumpable ice technology
• Seasonal thermal store
• Steam accumulator

References

1. ^ Gebremedhin, Alemayehu; Heimo Zinko. "SEASONAL HEAT STORAGES IN DISTRICT HEATING SYSTEMS" (PDF). Linköping University, Linköping, Sweden. Archived from the original on 2011-07-13. http://www.webcitation.org/609jELkwS. Retrieved 2011-07-13. 
2. ^ Mancini, Tom (10 January 2006). "Advantages of Using Molten Salt". Sandia National Laboratories. Archived from the original on 2011-07-14. http://www.webcitation.org/60AE7heEZ. Retrieved 2011-07-14. 
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. ^ Biello, David. "How to Use Solar Energy at Night". Scientific American. Scientific American, a Division of Nature America, Inc.. http://www.scientificamerican.com/article.cfm?id=how-to-use-solar-energy-at-night. Retrieved 19 June 2011. 
5. ^ Parabolic Trough Thermal Energy Storage Technology Parabolic Trough Solar Power Network. April 04, 2007. Accessed December 2007
6. ^ Kolpak, Alexie M. (20 June 2011). "Azobenzene-Functionalized Carbon Nanotubes As High-Energy Density Solar Thermal Fuels". NANO Letters. American Chemical Society. http://pubs.acs.org/doi/full/10.1021/nl201357n. Retrieved 14 July 2011. 

• "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.

External links

• ASHRAE white paper on the economies of load shifting
• MSN article on Ice Storage Air Conditioning