Alkali polyphosphates as new potential materials for thermal energy storage

Until now, molten salts (nitrate-based salts) are the main materials for sensible heat storage at industrial scale. The working temperature range of these materials is limited below 500 °C. There is not yet a solution for sensible heat storage at high temperature. This paper aimed to investigate alkali polyphosphates ((MPO3)n, with M = Li, Na or K) as new promising materials for sensible heat storage at high temperature. Alkali polyphosphates could be formed by dehydration of monoalkali dihydrogenphosphates (MH2PO4, with M = Li, Na or K), which occurs below 400 °C. Alkali polyphosphates resulted from this dehydration melted at 628, 657 and 812 °C for sodium, lithium and potassium polyphosphate, respectively. All these liquids evaporated above 900 °C but no destruction of their chemical structure was recorded. Sodium polyphosphate seemed to be the most promising material for sensible heat storage at high temperature because of its large potential working temperature (628-900 °C), its availability and its low cost compared to lithium and potassium polyphosphates. The results open new prospects for the development of the thermal energy storage field.