Summary and conclusions

The focus of this work was the identification of suitable PCMs in the temperature range 120 to 250°C. First, literature of organic PCMs in this temperature range was reviewed. In summary, it can be pointed out that the long-term thermal stability, the reactivity with air oxygen and the high vapour pressure of organic materials are the major critical aspects. All discussed organic PCMs require hermetically sealed storage systems and this can be considered disadvantageous.

The thermal stability of inorganic materials is typically higher. Besides long-term stability, PCM selected for the steam storage application need to fulfill other requirements such as suitable properties regarding handling and economics. Another aspect is that the entire temperature range 120 to 250°C should be covered with different melting temperatures. Taking these aspects into account, the combination of two alkali nitrate/nitrites to form binary systems is a suitable option. This work selected three alkali metal nitrates (LiNO3, NaNO3, KNO3), two alkaline earth nitrates (Sr(NO3)2, Ba(NO3)2) and two alkali metal nitrites (NaNO2, KNO2). Ternary systems, such as the system KNO3-NaNO2-NaNO3, which offer a further potential but also some complexity were not considered. For the considered seven alkali nitrate/nitrites, the minimum melting temperature of all binary combinations was identified either from secondary literature or through measurements presented in this work (Table 4). It can be concluded that the considered temperature range 120 to
250°C can be covered by these systems with a maximum temperature gap of 20°C (Fig. 5). Known values of the latent heat factors were in a range from 14 to 33 J/(mol K), where LiNO3 systems had exceptionally high values of 36 and 43 J/(mol K). In this work unknown minimum melting temperatures and systems around 170°C were assessed in more detail by phase diagram determinations and melting enthalpy measurements. Although KNO2 can be regarded as an less ideal candidate material, because it is a uncommon substance and strongly hygroscopic, it is the only material of the selected candidates, which forms binary systems with a melting temperature in the range 150 to 190°C. Of the two candidates at around 170°C, KNO2-Sr(NO3)2 has been identified as the favorable system due to its higher melting enthalpy and flatter liquidus line rather than KNO2-Ba(NO3)2.

It can be pointed out that the knowledge about the binary systems varies widely. For some systems essential values such as the eutectic temperature, composition and enthalpy are unknown. The characterization of other systems has progressed much further due to their application as heat transfer carrier, salt bath fluids and molten sensible heat storage media. Examples are the KNO3- NaNO3 and KNO3-LiNO3 systems, where work on aspects such as thermophysical properties, thermal stability and steel corrosion has been reported. Hence, after the selection of PCM with a suitable melting temperature, reported in this work, full qualification of PCMs for

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