Phenomenological modelling of solid/liquid phase transitions in non-ideal PCM
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Thermal energy storage technologies through solid/liquid phase change materials (PCMs) use the thermodynamic principles of melting and solidification to absorb and release thermal energy. In the ideal case, this technology allows to charge and discharge relatively high amounts of thermal energy at a constant, unique temperature. However, for most commercially available technicalgrade solid/liquid PCMs melting and solidification cannot be assigned to a single, unique temperature. Instead, the phase transition takes place over a temperature range in which solid and liquid phases coexist. Moreover, supercooling sometimes causes hysteresis in the phase transitions depending on the applied heating and cooling rates. These phenomena cause non-ideal phase transition behaviour and generally reduce the applicability of the PCMs. PCM models which can reproduce this non-ideal behaviour are crucial for the numerical analysis of the charging and discharging operation of latent heat storages. This contribution presents a generic workflow for the identification of phase transition models for industrial-grade solid/liquid PCMs. Adopting a purely phenomenological approach models are directly identified from PCM heat capacity measurement data. Thus, if the data contains information on temperature ranges with coexisting phases and hysteresis in the temperature induced phase transitions these phenomena are directly accounted for. The identified transition models predict liquid mass phase fractions using PCM temperature as a model input. These models are then used to describe apparent (effective) PCM properties in the phase transition temperature range, i.e. specific heat, density and thermal conductivity. Applications of the workflow are presented for different commercial PCMs from Climator Sweden AB. The effects of non-ideal phase transition behaviour on absorption and release of heat in a latent thermal energy storage are discussed by simulation studies.
Is part ofEurotherm Seminar #112 - Advances in Thermal Energy Storage. Universitat de Lleida, 15-17th May 2019. Edicions de la Universitat de Lleida. ISBN: 978-84-9144-155-7. 10 pp
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