Combined sensible and latent heat storage with binary anhydrous salt mixtures with melting temperatures in the range 300 to 500 °C

Kurzfassung

For solar thermal power plants, large scale two-tank systems filled with a mixture of molten sodium nitrate and potassium nitrate are commercially used. Such sensible storage media have fundamental limits in terms of their energy density due to their thermo-physical properties (density and heat capacity). Assuming a narrow temperature interval, phase change materials (PCMs) show much higher energy densities compared to sensible media. In the temperature range from 300 to 500 °C, typical PCMs have latent heat values from 100 to 500 J/g, compared to a sensible heat capacity of molten nitrate salt of around 1.5 J/(g∙K). However, the utilization of PCMs in a wide temperature range (e.g. 200 K) with additional sensible heat storage is not straightforward. For such applications, the cascaded PCM storage concept has been proposed. In this paper, we discuss fundamental aspects of an alternative concept using a melting range rather than the melting point of binary anhydrous salt mixtures. This concept requires different mixtures with a melting range within the temperature span 300 to 500 °C. The presented paper focuses on the identification and characterization of potential salt mixtures in this temperature range. Besides a detailed examination of the phase change behaviour with respect to the melting range and the enthalpy, the work includes other selection criteria such as thermal stability, handling and economic aspects. In addition some fundamental experiments on the melting and crystallization behavior of the well known system potassium nitrate and sodium nitrate are reported. For the examination, three different setups are utilized and they result in a wide range of sample masses: differential scanning calorimeter (about 20 mg), thermal apparatus (about 2 kg) and lab-scale storage (156 kg). The impact of thermal cycling between different phases (solid, melting range, liquid) on the enthalpies is examined. Finally, the consistency of the results by the three methods is assessed.