Thermal energy storage

Thermal energy storage

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Encyclopedia
Thermal energy storage comprises a number of technologies that store thermal energy
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....

 in energy storage
Energy storage
Energy storage is accomplished by devices or physical media that store some form of energy to perform some useful operation at a later time. A device that stores energy is sometimes called an accumulator....

 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
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

, 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
Solar collector
-See also:*Solar thermal collector*Solar water heating*Solar air heating*Photovoltaic module*Renewable heat*Concentrating solar power...

 or more often combined heat and power plants, and transferred to insulated
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...

 repositories for use later in various applications, such as space heating, domestic or process water heating.

Solar energy storage



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 store
Seasonal thermal store
A seasonal thermal store is a store designed to retain heat deposited during the hot summer months for use during colder winter weather...

s, used to store summer energy for space heating during winter. Molten salt
Molten salt
Molten salt refers to a salt that is in the liquid phase that is normally a solid at standard temperature and pressure . A salt which is normally liquid at STP is usually called a room temperature ionic liquid, although technically molten salts are a class of ionic liquids.-Uses:Molten salts have...

 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
Potassium nitrate
Potassium nitrate is a chemical compound with the formula KNO3. It is an ionic salt of potassium ions K+ and nitrate ions NO3−.It occurs as a mineral niter and is a natural solid source of nitrogen. Its common names include saltpetre , from medieval Latin sal petræ: "stone salt" or possibly "Salt...

, Calcium nitrate
Calcium nitrate
Calcium nitrate, also called Norgessalpeter , is the inorganic compound with the formula Ca2. This colourless salt absorbs moisture from the air and is commonly found as a tetrahydrate. It is mainly used as a component in fertilizers but is found other applications...

, Sodium nitrate
Sodium nitrate
Sodium nitrate is the chemical compound with the formula NaNO3. This salt, also known as Chile saltpeter or Peru saltpeter to distinguish it from ordinary saltpeter, potassium nitrate, is a white solid which is very soluble in water...

, Lithium nitrate
Lithium nitrate
Lithium nitrate is an inorganic compound with the formula LiNO3. It is the lithium salt of nitric acid. It is made by reacting lithium carbonate or lithium hydroxide with nitric acid.-Uses:...

, 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
Electricity generation
Electricity generation is the process of generating electric energy from other forms of energy.The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday...

 industry. The industry meets these peak loads with low-efficiency peaking power plant
Peaking power plant
Peaking power plants, also known as peaker plants, and occasionally just "peakers," are power plants that generally run only when there is a high demand, known as peak demand, for electricity.-Peak hours:...

s, usually gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

s, which have lower capital cost
Capital cost
Capital costs are costs incurred on the purchase of land, buildings, construction and equipment to be used in the production of goods or the rendering of services, in other words, the total cost needed to bring a project to a commercially operable status. However, capital costs are not limited to...

s 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 Storage



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.

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 joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

s (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
Cooling capacity
Cooling capacity is the measure of a cooling system's ability to remove heat. Common units include tons, and tonnes, which describe the amount of water at a given temperature that can be frozen in a given amount of time...

 (heat flow) was the heat to melt one ton of ice every 24 hours. This is the heat flow one would expect in a 3000 square feet (278.7 m²) house in Boston in the summer. This definition has since been replaced by less archaic units: one ton HVAC
HVAC
HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...

 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
Solar power tower
The solar power tower is a type of solar furnace using a tower to receive the focused sunlight. It uses an array of flat, movable mirrors to focus the sun's rays upon a collector 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. The molten salt is a mixture of 60 percent sodium nitrate
Sodium nitrate
Sodium nitrate is the chemical compound with the formula NaNO3. This salt, also known as Chile saltpeter or Peru saltpeter to distinguish it from ordinary saltpeter, potassium nitrate, is a white solid which is very soluble in water...

 and 40 percent potassium nitrate
Potassium nitrate
Potassium nitrate is a chemical compound with the formula KNO3. It is an ionic salt of potassium ions K+ and nitrate ions NO3−.It occurs as a mineral niter and is a natural solid source of nitrogen. Its common names include saltpetre , from medieval Latin sal petræ: "stone salt" or possibly "Salt...

, commonly called saltpeter
Saltpeter
Saltpeter or saltpetre often refers to:*Potassium nitrate, or the mineral niter, the critical oxidizing component of gunpowder, and a food preservative.It may also refer to:...

. 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
Calcium nitrate
Calcium nitrate, also called Norgessalpeter , is the inorganic compound with the formula Ca2. This colourless salt absorbs moisture from the air and is commonly found as a tetrahydrate. It is mainly used as a component in fertilizers but is found other applications...

 reduces the melting point (133 ºC).

The salt melts at 221 °C (429.8 °F). It is kept liquid at 288 °C (550.4 °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 (1,050.8 °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.

When electricity is needed, the hot salt is pumped to a conventional steam-generator to produce superheated steam
Superheated steam
Superheated steam is steam at a temperature higher than water's boiling point. If saturated steam is heated at constant pressure, its temperature will also remain constant as the steam quality increases towards 100% Dry Saturated Steam. Continued heat input will then generate 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 (24.4 m) in diameter to drive it for four hours by this design.

Several parabolic trough power plants in Spain and solar power tower
Solar power tower
The solar power tower is a type of solar furnace using a tower to receive the focused sunlight. It uses an array of flat, movable mirrors to focus the sun's rays upon a collector tower...

 developer SolarReserve
SolarReserve
SolarReserve is a Santa Monica, California based company that develops utility-scale concentrating solar power plants with integrated molten salt thermal energy storage technology. It was initially founded in early 2008 by US Renewables Group in partnership with United Technologies Corporation...

 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.

See also


  • District heating
    District heating
    District heating is a system for distributing heat generated in a centralized location for residential and commercial heating requirements such as space heating and water heating...

  • Eutectic system
    Eutectic system
    A eutectic system is a mixture of chemical compounds or elements that has a single chemical composition that solidifies at a lower temperature than any other composition. This composition is known as the eutectic composition and the temperature is known as the eutectic temperature...

  • Fireless locomotive
    Fireless locomotive
    A fireless locomotive is a type of locomotive designed for use under conditions restricted by either the presence of flammable material or the need for cleanliness...

  • ice storage air conditioning
    Ice storage air conditioning
    Thermal energy storage using ice is practical because of the large heat of fusion of water. One metric ton of water, one cubic metre, can store 334 million joules or 317,000 BTUs...

  • Liquid nitrogen economy
  • Phase change material
    Phase Change Material
    A phase-change material is a substance with a high heat of fusion which, melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy...

  • Pumpable ice technology
    Pumpable ice technology
    Pumpable ice technology is a technology to produce fluids or secondary refrigerants also called coolants with a viscosity of water or jelly and the cooling capacity of ice...

  • Seasonal thermal store
    Seasonal thermal store
    A seasonal thermal store is a store designed to retain heat deposited during the hot summer months for use during colder winter weather...

  • Steam accumulator
    Steam accumulator
    A Steam accumulator is an insulated steel pressure tank containing hot water and steam under pressure. It is a type of energy storage device. It can be used to smooth out peaks and troughs in demand for steam. Steam accumulators may take on a significance for energy storage in solar thermal...


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