Experimental and numerical study of annular PCM storage in the presence of natural convection

• A latent TES unit based on an annular heat exchanger design is studied.
• An experimental loop enables visualisation of the melting/solidification front.
• The impact of the HTF injection side on heat exchange is evaluated.
• Specific heat measurement is necessary for accurate numerical modelling.
• Natural convection plays an important role especially during charging mode.

Latent thermal energy storage (TES) systems have shown growing potential in matching the production to the demand in Concentrated Solar Power (CSP) systems. Indeed, the improved storage density and the constant temperature release of energy allow for more compact heat exchanger design and simplify system management. The shell and tubes heat exchanger is the most promising technology, owing to its low cost. However, phase change phenomena occurring during PCM melting (charge) and PCM solidification (discharge) need to be carefully controlled. To better understand the heat exchange involved in this exchanger type, an annular latent storage unit filled with paraffin RT35 is experimentally and numerically studied. A testing loop with visualisation is built to analyse the influence of the heat transfer fluid (HTF) injection side on the system. The experimental test section is modelled with CFD simulations to explain the charge and discharge results. The combined study proves that an injection side coupled with free convection heat transfer mechanism influences the evolution of the PCM melting front. To conclude, a top injection for charge and a bottom one for discharge are recommended.