Modelling of high temperature storage systems for latent heat

Kurzfassung

There is a huge demand for heat storages for evaporation applications. Thermal storage systems are used to increase the efficiency of thermal systems by an improved adaption of energy availability and energy demand. In this paper a possible solution for modular storage systems from 200-600 °C and pressures up to 100 bar is presented. The application of steam as a working medium requires the availability of isothermal storage if charging/discharging should take place at almost constant pressure. The saturation temperature range is between 200°C and 320°C. Therefore nitrate salts are used as phase change material (PCM). The solution developed at DLR is characterized by a modular concept of tube register storages surrounded by both sensible and latent heat storage material. The focus in this paper is on modelling of the PCM storage. A model is introduced for melting and freezing of the PCM. To perform with an acceptable heat transfer rate inside the PCM, fins are used to increase the overall thermal conductivity. Instead introducing mean storage material parameters, like thermal conductivity or specific heat capacity, the geometry of the finned tube is modelled by using discrete elements. Therefore the model allows detailed studies on heat transfer during space and time. The fin design can be varied in order to find an optimal configuration. A set of partial differential equations is created and solved. When considering a stand alone system, that means tube, fin and PCM, without a connection to other components, investigation is quite effective. In case of the PCM storage there is the big advantage, compared with a sensible regenerator, that the almost constant fluid temperature, when evaporating or condensing, leads to a uniform temperature distribution in fluid flow direction. Therefore only a very rough discretisation in axial direction is needed, what even allows bonding with other components e.g. from the Modelica Fluid Library. Sensible storages as they are used for preheating and superheating have a characteristic temperature gradient in axial direction. To describe their thermal behaviour concentrated models, using dimensionless numbers, are used.