## 2 Special Issue Findings
The increase in froth height is a parameter that provides information about how large the interface is between the gaseous and liquid phases. Finally, the authors found that the hydrodynamic properties of the shell are crucial for the choice of optimal absorber operating conditions. * The main contribution of the tenth article is related to molten salt energy storage systems. The authors ([PERSON]; [PERSON]; [PERSON]; and [PERSON], from the Technical University of Darmstadt, Germany, and the University of Technology, Iraq) created a numerical model of a molten salt energy storage system used in existing Andasol II solar power plant in Spain [15]. The storage model was validated by comparing the results with measured data from the real power plant. A system analysis and system optimisation were then performed, and the stand-alone concept of the thermal storage system was introduced. Stand-alone means an isolated deployment of the storage system without a solar power plant. At lower peak power demands, the storage medium was heated with surplus electrical energy and later fed back into the electrical grid via a steam cycle at higher peak power demands (see Figure 4). The system was then further optimised by modelling four different types of storage medium (Andasol 2 (base), SSalt max, Hitec, and carbonate salt) with different temperature differences in the cold and hot tanks. From the comparison, the most preferred storage medium was found to be carbonate salt, as it increased both the efficiency and the capacity. The largest increase in efficiency in terms of electricity generation could also be achieved with carbonate salt (18.2%), while the increase for SSalt max and Hitec was 9.5% and 7.4%, respectively. * In the eleventh article of this Special Issue, the authors [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; and [PERSON], from the Technical University of Darmstadt, Germany, and the University of Technology, Iraq, prepared a new nanofluid that has the potential to be used in various applications such as working fluids in cooling systems [16]. A novel, two-step technique (co-precipitation method) was used to prepare an efficient magnesium oxide-deionised water (MgO-DW) nanofluid without using surfactants and/or organic base fluids. Then, the influence of the volume concentration of the nanopowder and the duration of the ultrasonic treatment on the stability and thermophysical properties of the MgO-DW nanofluid at room temperature was investigated. Based on the experimental assessment, the co-precipitation method was successfully applied to prepare crystalline and  
Figure 4: Schematic flow diagram of stand-alone molten salt energy storage system: (**a**) during charge mode and (**b**) discharge mode [15]. pure MgO nanopowder, with an average particle size of 33 nm. Scanning electron microscopy (SEM) images demonstrated unique feathery or fluffy-like nanostructures of the MgO nanopowder that was prepared with volume concentrations ranging from 0.05% to 0.25% and ultrasonic treatment times ranging from 45 to 180 min at ambient temperature. The introduction of MgO nanoparticles to conventional liquid such as deionised water improved the thermal conductivity, with the highest value of thermal conductivity improvement of 25.08% observed at a volume concentration of 0.25% and ultrasonic treatment time of 180 min. The impact of ultrasonic treatment duration on the improvement of thermal conductivity was relatively like the impact of nanoparticle volume concentration. However, this conductivity improvement was limited after an ultrasonication duration of 135 min. The measurements of dynamic viscosity showed that it was directly proportional to the volume concentration of MgO-DW nanofluid until the highest value of 0.0052 [Pa s] was reached at a higher volume solid content of 0.25% of MgO-DW nanofluid. By contrast, increasing the ultrasonic treatment time resulted in a sharp decrease in the dynamic viscosity of the nanofluid samples.