Stresses in containment vessel for encapsulated phase change materials

The present work presents a design for a pressure vessel for the encapsulation of zinc as a phase change material to store thermal energy collected through concentrated solar power. The zinc will be formed into spherical balls and coated with nickel through the electroless deposition method. An axi-symmetric finite element model was used to simulate the expansion of the nickel shell as the zinc expands and melts inside of the shell. The effect of external forces and imperfections in the nickel shell that could affect the deformation were also modeled. The aim of the simulations performed was to establish a suitable thickness for the nickel coating such that the expansion of the zinc will not cause the shell to fail. It was concluded from the results that while the nickel shell can deform to the amount that the zinc will expand, the added effects from point loads caused by the weight of the surrounding balls and any imperfections in the shell could cause failure. The thickness needed to ensure that the shell would not fail was greater than the thicknesses that are usually obtained through the electroless coating process. Thus this approach of using electrolessly plated nickel may not be the best possible method for encapsulating the molten zinc for thermal energy storage.;It is envisioned that the crimped cylinder could be filled with zinc powder. The zinc in powder form will be contained in a shell made from a stainless steel cylinder that is crimped and welded at each end. A three-dimensional finite element model was created to find the stresses caused by the expansion of zinc as it melts inside of the shell. The aim of the simulations performed was to find the amount of void space that must be left inside of the vessel so that the expansion of the zinc and the increase air pressure inside of the vessel will not cause failure of the shell. Results indicate that the cylinder with crimped and welded ends could easily contain up to 86% of the initial volume full of zinc with only a very small amount of plastic deformation. The crimped cylinder would be filled with zinc powder because of the odd shape of the crimped cylinder. In the tests performed with the zinc powder, the powder was found to merely sinter and not melt; therefore, this approach of using powdered zinc was deemed unacceptable.;Capped cylinders are easy to manufacture and fill with solid zinc cylinders. This section presents a design for a pressure vessel to use in the encapsulation of zinc as a phase change material in order to store thermal energy collected through concentrated solar power. The zinc will in initially be in rod form and will be contained in a stainless steel cylinder that has caps welded on each end. A two-dimensional finite element model was created to determine the stresses in an infinite cylinder caused by the expansion of zinc as it melts inside of the shell; the effects of the point forces on the cylinder caused by the stacking of the cylinders were also analyzed. A three-dimensional model of the cylinder was created to find the stresses on the inside of the cylinder at the corner where the weld is located. The purpose of the simulations performed was to find the amount of void space that must be left inside of the vessel so that the expansion of the zinc and the air inside of the vessel will not cause unacceptable internal pressures. It was concluded from the results that cylinder with caps welded at each end could easily contain up to 86% of the initial volume full of zinc, corresponding to an internal pressure of 2.03 MPa, with only a very small amount of plastic deformation (less than 0.5% strain). Based on the results, the capped cylinder was deemed as a potential encapsulation geometry to contain the zinc during the phase change and thermal energy storage process.