Numerical Simulations of Thermal Storage Systems : Emphasis on Latent Energy Storage Using Phase Change Materials (PCM) PDF Download

Author: Pedro Andrés Galione Klot
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Languages : en
Pages : 188

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Book Description
The present thesis aims at studying the use of phase change materials (PCM) in thermal energy storage (TES) applications and to develop and implement numerical tools for their evaluation. Numerical analysis is nowadays an indispensable tool for the design, evaluation and optimization of thermal equipment, complementing the experimental techniques. Two levels of analysis are carried out, one in the field of Computational Fluid Dynamics, allowing the accurate simulation of the complex heat transfer and fluid dynamics phenomena present in solid-liquid phase change problems; and another one in which the governing equations are treated assuming several suitable simplifications and integrating empirical correlations, intended for the study of whole thermal storage systems throughout several charge/discharge cycles. Furthermore, the specific application of thermal storage in concentrated solar power (CSP) stations is studied. Different single-tank systems, making use of both sensible and latent energy capacities of the materials, are evaluated and compared against the two-tank molten-salt systems used in current CSP plants. Moreover, a new single-tank TES concept which combines the use of solid and PCM filler materials is proposed, with promising results for its utilization in CSP. In chapters 2 and 3, a numerical fixed-grid enthalpy model for the simulation of the solid-liquid phase change is developed. This technique is implemented using the Finite Volume Method in a collocated unstructured domain discretization and using explicit time integration schemes. Issues regarding the form of the energy equation, the treatment of the pressure equation as well as the momentum source-term coefficient introduced by the enthalpy-porosity method, are described in detail in the first chapter. In the second, the possibility of taking into account the variation of the different thermo-physical properties with the temperature is dealt with. Thermal expansion and contraction associated to the phase change are taken into account in the conservation equations and different strategies for the numerical treatment of the energy equation are discussed in detail. Furthermore, simulations of an interesting case of melting of an encapsulated PCM are carried out using two and three-dimensional meshes, and the results are compared against experimental results from the literature. In the next two chapters, the issue of numerically simulating whole single-tank TES systems is developed. These systems are composed of a single tank filled with solid and/or PCM materials, forming a packed bed through which a heat transfer fluid flows. Thermal stratification separates the fluid layers at different temperatures. The zone in which a steep temperature gradient is produced is called "thermocline", and it is desirable to maintain it as narrow as possible in order to keep a high stored exergy. Different designs of single-tank TES systems ¿classified according to the filler material/s used¿ are evaluated for CSP plants. The analysis is performed evaluating different aspects, as the energy effectively stored/released and the efficiency in the use of the theoretical capacity after several charge/discharge cycles, obtaining results independent of the initial thermal state. The operating time is not fixed, but depends on the temperature of the fluid coming out of the tank, limited by the restrictions of the receiving equipment (solar field and power block). Degradation of the stratification is observed to occur after several cycles, due to the temperature restrictions. In this context, a new concept of single-tank TES is presented, which consists of the combination of different layers of solid and PCM filler materials in a suitable manner, resulting in a lower degradation of the thermocline and increasing the use of the theoretical capacity. This concept, called Multi-Layered Solid PCM (MLSPCM), is demonstrated as a promising alternative for its use in CSP plants.