The work in this thesis has been a part of the EU-project AMADEUS, which is investigating new phase change materials (PCMs) for latent heat thermal energy storage (LHTES) systems. The overall objective for the AMADEUS project is to design a new LHTES system with greater thermal energy storage potential at higher temperatures, than today's LHTES systems. In order to achieve this, alloys with high melting points and high enthalpies of fusion have been investigated as possible PCM candidates. Eutectic FeSiB alloy with 64.2 wt% Fe, 26.4 wt% Si and 9.4 wt% B has been investigated and established as the most promising PCM alloy for the intended application. The AMADEUS project is coming to an end in 2019, and the main objectives in this thesis have been: - To investigate different FeSi alloys as possible and alternative candidates to eutectic FeSiB alloy as a PCM alloy in graphite crucibles, due to the high price of boron. - To investigate if different pretreatments would affect the interaction between eutectic FeSiB alloy and graphite crucibles. - To produce a 5 kg prototype PCM alloy in a crucible that will be tested in the prototype LHTES system at our partner institute and AMADEUS project coordinator Instituto de Energía Solar - Universidad Politécnica de Madrid (IES-UPM). Three different FeSi alloys were selected to be investigated as possible PCM alloys, based on their high melting temperatures and enthalpies of fusion. The alloys were Fe-21.89Si, Fe-33.54Si and Fe-50.80Si, where the number x in Fe-xSi represent the wt% silicon in the alloy. Experiments simulating LHTES systems with temperature cycles over and under the alloys' melting points where conducted. Then, the microstructure of the alloys in the graphite crucibles were investigated using Electron Probe Microanalysis (EPMA), and Light Optical Microscopy (LOM) was used to investigate the silicon penetration in the graphite crucibles. The microstructure revealed that SiC particles had been produced and was found almost everywhere in all samples. Different phase compositions than predicted by the Fe-Si phase diagram were found in all FeSi alloy samples. Since SiC had been produced by the reaction between liquid silicon in the FeSi alloy and the carbon in the graphite crucible, all compositions of the three FeSi alloys had shifted to a more iron rich composition in the Fe-Si phase diagram. Fe-21.89Si was found to be the chemical most stable FeSi alloy out of the three, with least amount of SiC in the samples and closest to the predicted composition. The analysis from LOM revealed that almost all FeSi alloys had wet and penetrated the graphite crucible. The higher silicon content in the FeSi alloy, the more will the alloy wet and penetrated the graphite crucible. Hence, Fe-50.80Si was the alloy that wet and penetrated the graphite crucibles the most. It is believed that a SiC crucible or a SiC coated graphite crucible could prevent the alloys from interacting with the crucible. The Fe-33.54Si was determined to be the most promising FeSi alloy in the application as a PCM, due to its high enthalpy of fusion and melting temperature. Deviating observations regarding the interactions between eutectic FeSiB alloy and graphite crucibles were made by NTNU and our partner institute, the Foundry Research Institute (FRI) in Krakow. Investigations were conducted in order to investigate if different pretreatments could affect the results. Both preheating of the crucible and leaching of the alloy were conducted, prior to temperature cycle experiments. However, no correlation between different pretreatments and the amount of interactions could be found. Natural variation in the alloy's composition or in the graphite crucibles could be the reason for the deviating results. Undiscovered differences in operation during experiments could also be a possible reason. Researcher Sethulakshmy Jayakumari (NTNU) and the author produced prototype PCM alloys in crucibles that will be sent to the project coordinator, IES-UPM, for final testing in the prototype LHTES system. The crucible will function as the vessel unit in the finished LHTES system. The four prototypes produced in this thesis were: - Eutectic FeSiB alloy in a graphite crucible - Eutectic FeSiB alloy in a graphite crucible with an inner coating of SiC - Fe-33.54Si alloy in a graphite crucible - Fe-33.54Si alloy in a graphite crucible with an inner coating of SiC The production of the prototypes was considered successful, and the FeSiB alloy's microstructure was investigated with EPMA. The microstructure match the microstructure found in the small scale FeSiB alloy experiments conducted by three other researchers from NTNU. Eutectic FeSiB alloy with 64.2 wt% Fe, 26.4 wt% Si and 9.4 wt% B is the most promising candidate as a PCM for the application in the LHTES systems. Fe-33.54Si has also properties that make this alloy a possible candidate for this application. The interactions observed between the Fe-33.54Si alloy and the graphite crucible can possibly be reduced by changing from a graphite crucible to a SiC crucible, or a SiC coated graphite crucible.