Thermal performance analysis of different multi layer phase change materials based thermocline thermal energy storage systems

Abstract

A thermocline is a thin transition layer between hot and cold fluid mixture which is generated by thermal difference of the two fluid layers. A thermocline-based thermal energy storage (TES) tank is one of the most important systems for storing thermal energy for concentrating solar power (CSP) plants. The tank usually consists of different types of rocks as filler materials and molten salts as heat transfer media. Hot and cold fluids are stored in the same tank and the fluids are separated because of their thermal stratification. Thus, a temperature gradient is created between the hot and cold fluid zone, which results in a thermocline. In recent era, burning of fossil fuels make us concerned about its harmful effect on environment and long-term impact on climate change. Solar energy, for its abundance in the form of renewable energy becomes attractive source to generate electricity. Researchers paid their attention to find different thermophysical properties of heat transfer fluids (HTF) like alkali fluorides and molten salts as well as thermophysical properties of phase change materials (PCM). High maximum operational temperature, high heat capacity, high thermal conductivity, good sustainability, and low vapor pressure are the desired properties of HTF. Specific heat, latent heat, and the combination of both are the different forms of stored energy in the thermal energy storage (TES) tank. In the present work, the goal is to analyze thermal performance of TES tank and latent heat thermal energy storage (LHTES) tank numerically for single layered and multi-layered phase change materials (MLPCMs). Molten FLiNaK salt, a eutectic mixture of 46.5% LiF, 11.5% NaF and 42% KF was used as heat transfer media for this scheme. Inorganic salts as well as their mixtures were used as phase change materials. A 2D model of thermocline TES tank was developed which was considered as an isotropic porous medium filled with distinct solid spherical particles. A numerical model was built that describes the way of travelling heat through the thermocline TES tank. Continuity and Momentum equations in porous medium was used for this system to get the flow pattern. The heat transfer module of this numerical model is developed by a special form of energy equation called Dispersion-Concentric (D-C) equation. To solve the problem, the given system was subdivided into smaller, simpler parts. This was achieved by a particular space discretization in the space dimensions, which was implemented by the construction of a mesh of the object. The Finite Element Method (FEM) formulation of a boundary value problem was used to solve these equations. For time dependent equations an implicit method, Backward differentiation formula (BDF) was used. The work was done both for charging and discharging cycles of the heat transfer fluid through the TES tank. The predicted results were compared with the numerical results from the published literature to validate the mathematical model and the numerical scheme. Moreover, two-dimensional graphical representation of velocity, pressure, isothermal contour, streamline etc. were shown and the relation between temperature with time and space were presented. As the research work has been conducted for both single layer and multiple layers TES tank, a comparative study among the different configurations has also been represented. Temperature change with time and space, heat flux in fluids, heat flux in solids are some important key points of comparison in this work. Effect of different phase change materials on temperature were demonstrated. In the charging cycle, PCM F2 has the highest the interstitial heat transfer coefficient (IHTC) and the value is 412.3 W/ (m2. K) however, in the discharging cycle the value is 324.07 W/ (m2. K) for PCM F2. PCM F2 has also the highest value of total enthalpy difference in the charging cycle which is 1676 kJ/kg but in the discharging cycle, PCM F1 has the highest value of total enthalpy difference and the value is 1727 kJ/kg. However, this work helps to uncover a new horizon for storing thermal energy which govern us to harness more energy from sunlight by using concentrated solar collector.