Abstract
Efficient phase separation is one of the most crucial factor when it comes to operation of a direct contact thermal storage (DCTS) concept. In DCTS, energy is stored as latent heat or cold using a phase change material (PCM), which is in direct contact with a fluid used to deliver this energy, namely a heat transfer fluid (HTF). Droplet flow of the HTF through the PCM provides excellent heat transfer, although it also produces an emulsion at the contact interface between the two fluids. Extensive emulsification of the PCM is recognized as a limiting factor for DCTS, as it leads to PCM being transported from the storage vessel to the rest of the system, where it may crystallize and cause blockages. Investigation of enhanced separation of emulsions consisting of de-ionized water and methyl palmitate PCM (oil phase) by addition of surfactants was performed in an experimental DCTS device. A selection of 8 different surfactants based on an initial small scale screening was evaluated in terms of their ability to enhance phase separation. A near two-fold increase in the coalescence rate was achieved compared to a reference system without surfactants. Investigation of the effects of different surfactants on the dynamic interfacial tension and viscoelastic properties of the interfaces between oil and water phases were performed to identify key parameters critical for the process of coalescence. The results showed that chemical destabilization of emulsions facilitated by surfactants in a DCTS system is an applicable method to address the issue of extensive emulsification. Emulsion cascade collapse in a DCTS, a phenomenon scarcely reported in water-in-oil emulsions, is also observed and presented in the current study.