 Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

 Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

Document Version Peer reviewed version Link back to DTU Orbit Citation (APA):Dannemand, M., Schultz, J. M., Johansen, J. B., & Furbo, S. (2015). Long term thermal energy storage with stable supercooled sodium acetate trihydrate. Applied Thermal Engineering, 91, 671-678. DOI: 10.1016/j.applthermaleng.2015 requires that the sodium acetate trihydrate is heated to a temperature somewhat higher than the melting 20 temperature of 58°C before it is cooled down. Expansion and contraction of the phase change material in a 21 closed tank compromises the stability of the supercooling. An expansion device that allows for the phase 22 change material to expand and contract without pressure changes in a closed chamber makes it possible to 23 achieve stable supercooling. Initializing the crystallization of the supercooled sodium acetate trihydrate can 24 be done by cooling the phase change material locally to the maximum level of supercooling or by 25 mechanically providing a seed crystal. Supercooled sodium acetate trihydrate at 20˚C stores up to 230 26 kJ/kg of thermal energy. The energy discharged is affected by the storage temperature of the supercooled 27 phase change material before crystallization and the final discharge temperature. Internal cavities are 28 formed during the contraction of the phase change material when crystallizing decreasing the heat transfer 29 rate. A thermally conductive liquid that does not mix with the phase change material can be added to fill 30 the cavities and enhance the heat transfer. Graphite powder can improve the thermal conductivity of the 31 sodium acetate trihydrate and needs to be kept dispersed evenly in the phase change material by a 32 thickening agent that is stable at the maximum temperature of the storage during the charging. A TRNSYS 33 simulation of a solar thermal combi system including a storage utilizing stable supercooling of sodium 34 acetate trihydrate elucidates the system size to achieve 80 % solar fraction of the low energy house in 35Danish climatic conditions. 36

Sodium acetate trihydrate is a phase change material that can be used for long term heat storage in solar heating systems because of its relatively high heat of fusion, a melting temperature of 58˚C and its ability to supercool stable. In practical applications sodium acetate trihydrate tend to suffer from phase separation which is the phenomenon where anhydrous salt settles to the bottom over time. This happens especially in supercooled state. The heat released from the crystallization of supercooled sodium acetate trihydrate with phase separation will be lower than the heat released from sodium acetate trihydrate without phase separation. Possible ways of avoiding or reducing the problem of phase separation were investigated. A wide variety of composites of sodium acetate trihydrate with additives including extra water, thickening agents, solid and liquid polymers have been experimentally investigated by a simple heat loss method. The aim was to find compositions of maximum heat released from the crystallization of supercooled sodium acetate trihydrate samples at ambient temperature. It was found that samples of sodium acetate trihydrate with 0.5% to 2% (wt.%) Carboxy-Methyl Cellulose, 0.3% to 0.5 % (wt.%) Xanthan Gum or 1% to 2% (wt.%) of some solid or liquid polymers as additives had significantly higher heat contents compared to samples of sodium acetate trihydrate suffering from phase separation.

No abstract