Heat transfer and melting characteristics of the phase change material inside U-tube based evacuated tube solar collector

Three-dimensional computational fluid dynamics simulation has been performed to understand the performance of an evacuated tube solar collector integrated with phase change material. A copper U-tube based evacuated tube collector is filled with three different phase change materials, namely, lauric acid, paraffin wax, and stearic acid, to compare their heat transfer rates and melting characteristics for varying solar radiation during the day time 09:00 – 13:00. The effect of flow rate of heat transfer fluid i.e., water is also investigated for all three phase change materials. Given distinctive physical properties of these phase change materials, the evolution of natural convection current and vortices with time within the collector are found different. This affects the melting and heat transfer behaviors of the phase change materials— lauric acid liquefied first followed by paraffin wax and stearic acid for the same time duration. Lauric acid was melted ≈95 % while liquid fraction of stearic acid was only 57.4 % for the time period 09:00 – 13:00. It establishes that lauric acid should be preferred where availability of solar radiation is less. The mass flow rate of heat transfer fluid affects its average outlet temperature as well as melting process of the phase change materials. It is found that liquid fraction of the paraffin wax became ≈10–15 % less when the mass flow rate increases from 0.02 m/s to 0.06 m/s.