Heat flux in latent thermal energy storage systems: the influence of fins, thermal conductivity and driving temperature difference

Abstract

Phase change materials (PCM) can increase the energy densities in thermal energy storage systems. Heat transfer rates in PCMs are usually limiting, different improvement methods were used previously, such as fins or improved thermal conductivities. Here, the influence of fin geometries, PCM thermal conductivity and discharge temperature of the secondary fluid are investigated by modelling. The analysed outcomes are their influence on stored energy, heat flux and stored exergy. The two-dimensional time dependent energy equation was solved for a rectangular enclosure with a secondary fluid with constant temperature as boundary condition on one side. The modelled PCM data based on a paraffin melting at 44 °C. The different improvement methods increase the heat flux, but the increase is lower than expected according to idealized calculations. The basis for this investigation is the evaluation of a dimensionless heat flux number formed from the heat flux, thermal conductivity, temperature difference from the secondary fluid to the phase change temperature and a characteristic length of the system. The influence of the better thermal conductivity is found to be lower at higher fin volume fractions. The increase of the discharge temperature difference had the strongest impact on the heat flux, but the exergy loss also increased. The exergy loss was also related to the achieved reduction in the discharge time as an indicator for the heat flux. Here, it was seen that the exergy losses outweigh the advantage in discharge time from a higher discharge temperature difference.