Preparation of high-conductivity, high-capacity phase change media capsules with enhanced thermos-chemical stability in thermal energy storage applications

Thermal energy storage using the effects of the latent heat in the solid-liquid phase transformation has found its way into every day applications. Reusable, organic phase change media stabilize low temperatures of parcels, and salt/water tanks act as buffer storage in residential heating/cooling and industrial processes. Emerging, renewable technologies for energy production and conversion profit from high operation temperatures. Alloys from abundant metals such as aluminum, silicon, and copper have suitable melting characteristics to preheat chemical conversion catalysts and process gas streams for heat-to-power applications. They typically absorb and release a large heat of fusion, facilitate better heat transport compared to salts, and are inert to thermal decomposition compared to organic compounds. Fundamental challenges emerge from the high reactivity of molten metals towards the preferred metal encapsulation materials. Stable encapsulations are necessary to protect the storage medium, and to preserve a constant heat exchange surface during the phase transition. This thesis explores the fabrication of stainless steel capsules for use with pure aluminum, as well as Al-12%Si and Al-26%Cu-5%Si alloys. Experimental capsules of cylindrical shape were prepared with or without interface modification, and tested by isothermal exposure up to 800 °C and in a newly developed test-bench with 100+ melting cycles and in-situ performance measurement. Iron aluminide formation on the interior capsule surface reduced performance. Interface modifications and corrosion layers were characterized by scanning electron microscopy, energy dispersive x-ray spectrometry, differential scanning calorimetry, and Vickers hardness micro-indentation. Results were integrated into numerical diffusion and heat transfer modelling, allowing predictions about the effects of atomic diffusivity, solubility, and thermal conductivity on heat storage performance. Ceramic diffusion barrier coatings for stabilized performance were deposited via modified reverse-dip-coating on steel encapsulation and silicon substrates, and characterized using the same methodology. Ceramic coatings and guidelines for conservative operation conditions were effective in stabilizing heat storage density and power. Boron nitride coating was demonstrated to prevent 90 % of the capacity loss occurring in the control group, in a scaled up pilot experiment with 200 MJ latent heat capacity in encapsulated Al-26%Cu-5%Si. Temperature scanning calorimetric data from the cycling test-bench validated numerical heat transfer simulations and performance predictions from diffusion modelling. Results from validated heat and diffusion modelling were used in the design of a 1.4 MJ lab-scale prototype storage, with automated charging and discharging of stacked encapsulated samples, using air as heat transfer fluid. Herein presented guidelines allow design and operation of robust steel encapsulations, which conserve high performance for 10¿000s of full-load-hours. Our experimental data and interpretative modelling focuses on a relevant material class, that combines low material cost with high conductivity, large capacity, and melting temperatures relevant to advanced power cycles. Experiments on lab-, prototype- and pilot-scale demonstrate how our findings contribute to practical implementation of novel highly performant metal latent heat thermal energy storage systems.