Enhancing PCMs thermal conductivity: A comparison among porous metal foams, nanoparticles and finned surfaces in triplex tube heat exchangers

Increasing latent heat thermal energy storage system thermal conductivity is of primary importance to take advantage of their capability of storing large amount of thermal energy. For this task, various solutions have been proposed through the years and a throughout comparison depending on the final application is still lacking. In this paper, the melting process of PCMs embedded in a Triplex-Tube Heat Exchanger (TTHX) is investigated numerically by considering three different methods that include separately or together metal foams, nanoparticles addition and finned surfaces. Organic PCMs with different melting points are used as PCMs in the middle shell of the 3D (TTHX) to maximize latent heat depending on local temperatures. Water across inner and outer tubes is considered too as the heat transfer fluid. Results are presented in terms of liquid fraction, temperature evolution as well as charging the energy storage rate. The results show that a composite of PCM/Metal Foam with porosities that vary from 0.98 to 0.92 engenders shorter melting comparing to pure PCM. By inserting metallic foam with different porosities and nanoparticles with 5% volume fraction in the TTHX (Case A), the melting time decrease can achieve a 69.52% when compared with Pure-PCM. Regarding the melting process in pure Multilayer-PCM (Case B), for all metal foam porosities the foam/nano-PCM device shows a shorter melting time even if nanoparticles have minor impact compared to metal foams, reaching a 83.48% in terms of reduction if nanoparticles and metal foams are employed. Finally, for the Case C, melting times are smaller when comparisons are done with Cases A and B for pure PCM. Furthermore, in the finned surfaces of TTHX (Case C), the inclusion of nanoparticles with foam reduced the melting durations by 53.17% compared to the TTHX (Case A) with pure PCM.