Heat Transfer Modelling within Graphite/pcm Composite Materials for High Temperature Energy Storage

Moderately compressed exfoliated graphite (CEG) leads to highly porous graphite “foams” or matrices which have numerous actual and potential applications. The graphite/salt composite materials considered here for high temperature energy storage applications, as due to its superior heat conduction and thermal storage characteristics, are manufactured by uniaxial compression of a mixture of salt powders and expanded natural graphite (ENG). Developing systems for thermal energy storage using these graphite/salt compounds implies understanding and simulation of heat transfer and phase change phenomena within such materials. As a result of the complexity of structure at the microscopic scale, modelling at this scale becomes essentially impossible. The difficulty arises from the need to write separate energy equations for the regions of graphite, salt melt, solid salt, and at every graphite/salt interface. Rather than attacking the problem in this way, the pore-scale information can be used to derive continuum macroscopic equations that are valid in all regions. This change of scale can be accomplished by the method of volume averaging [1]. However, information about the microscopic representative elementary volume is necessary to carry out the volume averaging procedure. The objective of this work is threefold: to identify space-time scales allowing representing the thermal behaviour of ENG/salts compounds by means of and effective homogeneous medium, to derive the governing energy equations, and to establish formal relationships between macroscopic physical properties of the ENG/salt materials and those of their components.