In Situ Thermal Conductivity of a Paraffinic Phase Change Material in a Cold Finger Configuration

Phase change materials (PCMs) such as paraffins are widely applied to store thermal energy. PCMs displaying high thermal conductivities are desirable because they crystallize or melt rapidly when the temperature in the environment changes, absorbing or releasing latent heat accordingly. In this work, we experimentally determined the thermal conductivity of PCM layers in situ. To this end, layers that mimic macro-encapsulated PCM devices have been grown in a cold finger experimental setup. The thermal conductivity was determined as the adjustable parameter of a detailed 1D mathematical model for the process of phase change. It has been shown that the thermal conductivity of paraffin increases by as much as 40% to a value of 0.30 W m –1 K –1 when the rate of crystal growth increases from 0.06 to 0.19 kg m –2 s –1 . Furthermore, evidence shows that paraffinic rotator phases cause an increase in the thermal conductivity during the crystallization of the PCM. With the aid of a 2D computational fluid dynamics mathematical model, it has been shown that the heat transfer by natural convection in the molten paraffin was responsible for the noncylindrical shape of the paraffin layer. It is concluded that the crystallization rate should be considered for designing PCM devices with a faster thermal response.