## 5 Conclusions  
Within this paper, a basic sensitivity analysis is performed to understand the sources of uncertainties within a CFD simulation of a vertical shell-and-tube LTESS during charging and discharging. First,  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Study** & **Output** & **1. ** & **2. ** & **3. ** & **4. ** & **5. ** \\ \hline \multirow{2}{*}{[PERSON] et al. [27]} & \multirow{2}{*}{mean power deviation} & \(T_{\text{m}}\) & \(\dot{V}\) & \(T_{\text{in}}\) & \(d\) & \(L\) \\  & & \(\pm 1\) K & \(\pm 6.14\%\) & \(\pm 0.6\) K & \(\pm 10\%\) & \(\pm 10\%\) \\ \hline \multirow{2}{*}{[PERSON] et al. [30]} & \multirow{2}{*}{mean absolute power deviation} & \(T_{\text{in}}\) & \(T_{\text{m}}\) & \(\rho\)* & \(\frac{\partial h}{\partial T}\)* & \(\alpha\) \\  & & \(\pm 1\) K & \(\pm 1\) K & \(\pm 5\%\) & \(\pm 5\%\) & \(\pm 30\%\) \\  & & \(T_{\text{in}}\) & \(\rho\)** & \(\frac{\partial h}{\partial T}\)** & \(T_{\text{m}}\) & \the influence of several numerical parameters was investigated, and it was concluded that the major part of the uncertainties must be found elsewhere. Consequently, a sensitivity analysis of the material properties, as well as boundary and initial conditions and measurement positions, was carried out. In order to keep the simulation effort within limits, a simple analysis was performed in which the parameters were varied by a certain value in relation to a reference case. Ten material properties were varied by either \(\pm 10\%\) or \(\pm 1\) K, and six boundary and initial conditions by either \(\pm 10\%\), \(\pm 1\) K, or in an on/off manner, and the resulting deviation of the mean power, as well as the maximum deviation of the power and the global liquid fraction, were analyzed. In addition, the position of the temperature measurements within the simulation was varied by \(\pm 1\) mm. The most important results are:  
* The variation of the material properties had a very large effect on the power and the global liquid fraction. In conclusion, uncertainties in the material properties may affect the results of a CFD simulation of a LTESS significantly. * The material properties can be subdivided into three groups:
* Material properties that have no or only little effect: only \(C_{1}\) has an influence that is always below \(1\%\) (maximum of \(0.12\%\)), but one has to keep in mind that it is varied by several orders of magnitude in literature. * Material properties that have an influence on the power and the global liquid fraction slope, but none on the mean power: the material properties \(\eta\), \(\beta\), \(\lambda\) and \(T_{1}-T_{\rm s}\) mainly influence the maximum deviations and not the mean power. * Material properties that also influence the mean power: all remaining material properties (\(\rho\), \(L\), \(c_{l}\), \(c_{s}\), \(T_{m}\)) also affect the mean power when varied, but \(c_{l}\), \(c_{s}\) and \(T_{m}\) only slightly. * \(\dot{Q}_{\rm loss}\) (up to \(21.4\%\)), \(\rho\) (up to \(9.2\%\)) and \(L\) (up to \(6.4\%\)) have the largest influence on the mean power. * The ranking of the parameters and the magnitude of the influence is sometimes distinctively different in studies found in literature. * Comparing the maximum deviation of the power can be misleading as the deviation may only occur as a short peak, and has no effect on the overall performance. * For the investigated storage configuration and orientation, a radial variation of the temperature measurement position had a much higher impact than a vertical variation. A displacement of the measuring point in radial direction by \(\pm 1\) mm already led to a maximum deviation in the measured temperature of more than \(5\) K.  
The above points indicate that the results of numerical models of latent thermal energy storage units are subject to substantial uncertainties.