Storage of Thermal Energy

1)        Sensible Heat Storage

To use the sensible heat of the material is the simplest way to store energy. In applications, water, sand, carpolite and soil can be used as the material for storage, among which the specific heat capacity of water ranks the highest with widespread use. In 1970s and 1980s, there was once report on storage of solar energy across the season by use of water and soil. But the heating on the material was comparably lower and the energy storage was limited.

2)        Latent Heat Storage

To use the latent heat of the material absorbed and released during the variable of phase is another method of energy storage, which can keep a larger capacity of energy and release heat with the constant of temperature.

It is generally applied salts with crystal water to store energy in low temperature, such as Na₂So₄¡¤10H₂O, CaCl₂¡¤6H₂O or NaHPO₃¡¤12H₂O. But super cooling and demixing concerns should be tackled while utilizing so as to ensure the working temperature and service life.

Moderate temperature storage of solar energy is generally ranging from100¡ãC to 500¡ãC, approximately around 300¡ãC. The enabling materials include: pressured water, organic fluids, eutectic crystal salts.

High temperature storage of solar energy is generally above 500¡ãC. The materials under the experiment include sodium metal and fused salt.

Extreme temperature storage above 1000¡ãC can employ alumina and zirconia fireproof ball.

3)        Chemical Storage

Heat storing by use of chemical reaction results in large capacity, compact volume and lightweight. The products derived from chemical reaction can be stored separately, which are to release heat if needed with a longer storage time period.

The chemical reactions, which are actually applied to heat storing, must satisfy the following criteria: sound reversibility in reaction without any side reaction; the resultant from reaction is likely to be separated and can be stored steadily; the reactant and the resultant are of no poison, no corrosion and no combustibility; large amount of heating, lower price in reactant. Now, some chemical reactions have been selected to satisfy the above-mentioned criteria, such as the thermal decomposition reaction of Ca(OH)₂, Ca(OH)₂+63.6kj = CaO+H₂O.

For the heat storage by above-mentioned absorption reaction, thermal energy can be released through exothermic reaction if needed. But the temperature of Ca (OH) ₂ in dehydration reaction under barometric pressure is still higher above 500¡ãC, under which the dehydration is difficult to be realized. The reaction temperature can be declined with catalyzer, but is still comparably higher. Therefore, research on thermal storage by chemical reaction remains to be tapping and cannot be realized in the short term.

Other enabling chemical reactions include thermal decomposition reaction of metal hydride and circulating reaction of ammonium hydrogen sulfate.

4)        Plastic Crystal Thermal Storage

In 1984, a kind of household plastic crystal heating material appeared in the US market. Its scientific name is called NPG, which possessed 3-dimension periodicity of crystal similar to liquid crystal, but its mechanical property resembles plastic. It can store and release heat under the constant temperature. The thermal is to be stored by the phase varying of solid-solid produced through the formations of molecule of plastic crystal instead of depending upon the phase varying of solid-liquid. Under the constant temperature of 44¡ãC, plastic crystal stores thermal energy by absorbing solar energy in the day, and releases the stored thermal energy at night.

Extensive research on NPG¡¯s heat storage performance and its applications was made in the US, among which plastic crystal can be applied to heat storage by melting to the glass and organic fiber wall plate, and can be refrigerated for cooling through adding the adjusted plastic crystal into the glass and organic fiber wall plate.

Research on plastic crystal was also made in China without any applications.

5)        Thermal Storage by Solar Tank

Solar tank is a kind of salt-water tank with a certain salinity gradient, which can be used for solar energy collecting and storing. Due to its simplicity, low cost and accessibility to widespread use, it catches people¡¯s attention. After the1960s, many countries started its research and development on solar tank. 3 sets of solar tank power station were built up in Israel.

After the 1970s, China also tapped into the research of solar tank and achieved preliminary applications.