Experimental comparison of MPCM slurry and conductive particle suspensions for enhanced heat transport in solar thermal systems

Abstract

Solar thermal collectors require heat transfer fluids that combine high thermal transport and storage capacity with dispersion stability and acceptable hydraulic performance. In this study, microencapsulated phase change material slurry (MPCS), expanded graphite (EG) and copper (Cu) suspensions, and EG enhanced MPCS were experimentally compared in the same closed loop solar thermal system under equivalent operating conditions. Heat transport, convective heat transfer, dispersion stability, hydraulic performance, and costs were evaluated to distinguish latent heat storage from conductive enhancement. The settling velocities of MPCM, EG, and Cu were 0.007, 0.130, and 1.160 mm/s, respectively, demonstrating the superior dispersion stability of MPCM. The 30 vol% MPCS exhibited a latent heat capacity of 46.2 kJ/kg and a total specific thermal storage capacity of 133.7 kJ/kg over 25–50 °C, approximately 28.3% higher than that of water, with latent heat accounting for approximately 34.6% of the total specific storage capacity. Increasing the flow rate from 1 to 8 L/min increased the average Nusselt number from approximately 7.5 to 37 and the heat transport rate from 81.2 to 400.5 kJ/h. The hydraulic heat transport index (HHTI) was introduced to quantify the balance between useful heat transport and pumping demand, identifying 4–5 L/min as a practical operating range. EG exhibited better thermal hydraulic performance than Cu because of its lower settling tendency and hydraulic penalty. Adding 1.0 vol% EG to 30 vol% MPCS increased heat transport from 335.7 to 380.0 kJ/h and HHTI from 0.78 to 0.88. These results provide a common experimental basis for comparing latent storage and conductive enhancement and demonstrate the potential of combining both mechanisms while accounting for dispersion stability and hydraulic resistance.