A wider utilization of energy storages is one of the most important steps towards the 100% renewable energy systems. By integrating thermal energy storage (TES) systems with concentrated solar power (CSP) plants, it is possible to directly store heat and transform it into electricity later, therefore avoiding additional energy transformations. High-temperature sensible TES systems are already commercially utilized, mostly as twotank molten salt design, together with large CSP plants. High-temperature latent heat storages (HTLES), on the other hand, have numerous advantages such as higher energy density and (near) isothermal energy transfer, but due to some inherited drawbacks which are characteristic for high temperature phase change materials (PCM), they are not yet commercially used in CSP plants. Most PCMs suitable for HTLES are salts. Salts are characterized by low thermal conductivity which severely limits achievable heat transfer rates in HTLES. To increase commercial appeal of the high temperature PCM, achievable heat transfer rates need to be increased, either by improving the effective thermal conductivity of PCMs (material modifications), improving heat exchanger geometry or by PCM encapsulation. Traditional heat exchanger design methods are limited by high non-linearity and transient nature of the problem, therefore, detailed numerical models of the high temperature PCMs are used. As a result, there are different models which can be distinguished based on dimensionality (1D, 2D or 3D), heat transfer mechanism (conduction or conduction + convection), usage of fixed or adaptive grid, phase boundary modeling, etc. However, an increasing number of papers concerning high temperature LTES numerical modeling is not accompanied by the increase of experimental papers that can be used for model validation. These few papers with experimental results are limited to temperature data for very specific geometries while model validation with that data usually results in relatively large temperature errors which occur during the period of melting front propagation. In this thesis a finite volume numerical model based on the enthalpy-porosity approach is proposed and implemented in open source computational fluid dynamics toolbox OpenFOAM. A dedicated experimental setup for more reliable measurements of high temperature PCMs is developed. Setup allows both melting front propagation measurements and temperature measurements at different points during the melting. New model is successfully validated by acquired data. In the final part of the thesis, the validated model is used for analysis of different parameters of finned shell-and-tube storages and, finally, for optimization of HTLES by geometry modifications of the heat exchanger. Mathematical model A numerical model for simulation of high-temperature phase change materials based on the transient modified Navier-Stokes equations was used in this thesis. The following simplifications of natural phenomena were introduced: density change due to the temperature and phase changes was neglected; radiation heat transfer was ignored; liquid fraction was assumed to be a linear function of the temperature; solid PCM is homogeneous material with isotropic (although temperature dependent) properties and without porosity; influence of the crystal orientation is neglected. Equations were discretized using the finite volume approach and implemented in the open source computational fluid dynamics toolbox OpenFOAM. The flow is assumed to be incompressible while Boussinesq approximation is used to account for body forces from temperature-caused density variations. A widely accepted fixed-mesh enthalpy-porosity method proposed by Woller was implemented for phase-change modeling. Conjugate heat transfer is modeled implicitly. For all equations and for both solid (heat exchanger) and PCM the same mesh was used. Unmodified heat equation is solved for the whole domain whereas velocity is set to 0 during solution of the momentum equation (that is, during PIMPLE steps). The geometry of heat exchanger - PCM interface coincides with mesh faces. Harmonic interpolation is used for discretization of the heat equation on interface cells, while no slip boundary condition is implemented for the momentum equation. Experimental setup Following the idea of a simple geometry and numerical reproducibility, experimental setup is constructed as a simple rectangular cavity. The cavity is heated from the isothermal left side and cooled from the isothermal right side, while other sides are adiabatic. The identification of the melting front is achieved visually, using a commercial digital camera, therefore the front plate of experimental setup is transparent. The dimensions of the cavity are 300 x 300 x 110 mm. A relatively large height is chosen to allow high Grashof numbers and a possibility of turbulent flow. Sodium nitrate, a very popular high-temperature PCM, with well-known properties is chosen. PCM is enclosed in an inner case which is constructed from stainless steel with borosilicate glass at the front and copper heaters on left and right. Heaters are built by fixing a 1 kW electric heater plates on 15 mm thick copper plates. Thick copper plates are used to achieve a uniform temperature distribution and to avoid significant vertical temperature variations. Sealing the inner case proved to be very challenging as a very elastic gasket is necessary to avoid high forces acting on the glass and breaking it due to the temperature deformations of the case. Since >300 ◦C temperature range is too high for most elastic gaskets, 3mm graphite sheets with thin layer of high temperature silicone were used. Silicone is known to react with strong oxidizing materials (such as sodium nitrate) which results in PCM leakage after more than 10-15 hours of system operation (above melting point). To prevent leakage, the sealant was replaced after every experiment. Thermal insulation, 100 mm of rock wool, is placed on the outside of the outer case while empty space between inner and outer case is also filled with rock wool. The front of the outer case is a triple insulated glass with removable insulation baffle between the outer and the inner glass. The baffle is filled with rock wool and can be removed to take photos. On top of both the inner and outer case, a square orifice is created. The orifice provides sufficient light inside the inner case for taking photos. The square shape of the orifice results in the light sheet that highlights melting front form. Three different experiments were run, distinct by the hot wall temperature (Rayleigh number) and cavity orientation. In all experiments, the shape of the melting front is characterized by very fast propagation near the top (free surface), and insignificant front movement in the lower part of the domain. Validation and verification Accuracy of the finite volume numerical simulation depends on model accuracy, discretization methods, mesh quality and case parameters (boundary and initial conditions etc.). Enthalpy-porosity method is also characterized by the ”unphysical” constant C which defines the flow in mushy region and for which there are no consensual values in literature. For each experiment, a series of simulations with different meshes and parameters such as melting temperatures and C values was run. The model was successfully validated on all three cases and some interesting observations were made. Value of the heat transfer coefficient differs significantly from the literature which deals with pure sodium nitrate properties. A similar observation was already reported by some authors, therefore the corrected heat transfer coefficient was calculated using iterative approach and used in subsequent simulations. Contrary to some authors, it is shown that the value of constant C has significant effect on the melting process, especially for coarser meshes. Good choice of C value can lead to accurate simulations on coarser meshes, thus significantly reducing the run time for simulation which can be of huge importance in optimization studies. It is also shown that temperature data comparison can lead to false validation. Heat transfer rate improvement Different finned shell-and-tube configurations were numerically analyzed to asses influence of the number of fins, fin positioning and fin material on charging and discharging process. Both quantitative and qualitative differences between different fin materials and configurations are observed. Two and four finned configurations were explored. It can be concluded that melting (charging) time varies greatly with different heat exchanger geometries. Bad utilization of natural convection can prolong melting for more than 50%. Also it can be observed that the melting time is not the most appropriate characteristic for determining the performances of the storage, as it is shown that melting time for one case is 64% longer than in the other case, while the stored energy during the charging is almost the same (within 10%). This is very important to note since the melting time is frequently used for measuring storage performances. The effect of heat transfer improvement by utilizing natural convection with fin reorientation is nullified during the storage discharge. However, in some specific work regimes where the available charging time varies and the discharging is slow (CSP with inconsistent energy surplus during the day and low night consumption), those modifications can be advantageous. Last part of this thesis considers the application of genetic algorithm for geometry optimization of the LHTES heat exchanger. Optimization is performed on the finned vertical shell-and-tube storage. Geometry is described using eight variables (height and length of each of the four fins). Benchmark geometry with four equally spaced and equally long fins was defined as the starting point for the optimization. Overall stored energy during 50 minutes of simulation is used as fitness function, which is equal to 53.5 kJ or around 50% melting for the benchmark geometry. The optimized HTLES can store 75.1 kJ or 40.3% more energy than the referent HTLES in the same time. Conclusion and scientific contribution • Unique, front propagation experimental data is provided that can be used for validation of HTLES models in general. • By comparing both front propagation data and temperature measurements with experiments, it is proven that using only temperature data can lead to false validation of the model. Influence of the constant C on simulation results can be significant for coarser meshes. • Different finned HTLES shell-and-tube geometries were analyzed. It is shown that influence of fin orientation plays a crucial role during charging. Significant improvement of charging performance can be achieved by positioning fins with respect to maximizing natural convection. Influence of the fin material (thermal conductivity) is not only quantitative (i.e. higher conductivity - faster charging): changing fin orientations of steel fins plays an insignificant role compared to aluminum fins, where right orientation can reduce melting time significantly.