Numerical investigation on the effect of fin design on the melting of phase change material in a horizontal shell and tube thermal energy storage

TLDR: In this paper, the thermal performance during the melting process for phase change material (RT-50) in a horizontal LHTS unit was studied numerically with a view to optimizing the fin configuration.

Abstract: It has previously been proven that fins can significantly enhance the thermal performance of latent heat thermal energy storage (LHTS) units. Nevertheless, the magnitude of improvement, especially in a horizontal LHTS, is still less than that required to address some of the existing challenges in solar energy applications. The tendency of the phase change material (PCM) at the bottom of the horizontal storage to remain solid because of the absence of convection currents to promote heat transfer must be tackled practically for this technology to be viable in different thermal applications. Thus, the fin configuration around the circumference of the horizontal storage must be optimised to enhance the melting rate and therefore improving efficiency. In the present paper, the thermal performance during the melting process for PCM (RT-50) in a horizontal LHTS unit was studied numerically with a view to optimizing the fin configuration. The baseline case of bare heat transfer fluid (HTF) tubes was compared with finned surfaces with four different fin angles ( θ = 72 o , 60 o , 45 o and 30 o ) with four different heights (0.2, 0.4, 0.6 and 0.8 of the hydraulic radius of the annulus (Rh)). The average temperature of the PCM, its liquid fraction, and velocity distribution during the melting process were investigated. The numerical results showed that increasing fin height (using a fixed fin configuration: θ = 72 o ) significantly improved the thermal performance of the horizontal LHTS. When the fin height was varied from 0 (bare HTF tube) to 0.8 of Rh, a shortening of the total melting time by approximately 50% was observed. For this fin height 0.8 Rh, it was shown that having a smaller angle between the fins, with all of them mounted below the horizontal axis of the LHTS unit, led to significant enhancement in the thermal performance of the storage. This is because the enhanced heat transfer surfaces are targeted to the regions of the LHTS unit where heat transfer is poorest in the bare tube configuration, as mentioned above. Thus, the total PCM melting time was reduced by 6.7%, 14.3%, 16.7% and 10.0% when the fin angle was changed respectively from 72o to 60o, from 60o to 45o, from 45o to 30o, and finally from 30o to 15o.