Phase change material-sand mixtures for distributed latent heat thermal energy storage: interaction and performance analysis

In this study two phase change materials (PCMs) mixed with sand were evaluated for distributed latent heat thermal energy storage (LHTES) coupled with a novel Flat-Panel ground heat exchanger (GHE) for shallow geothermal applications. N-Octadecane and a commercial paraffin-based PCM were mixed (30% v/v) separately with sand, which is commonly used as backfilling material for GHE. Both two mixtures underwent 16 thermal cycles and specimen’s temperatures and their variation over time were analyzed to evaluate phase change stability and supercooling. Grain size laser diffraction and pore analysis were performed together with optical microscopy, environmental scanning electron microscopy coupled with X-Ray spectrometry (ESEM-EDS) and Fourier transform infrared spectroscopy (FTIR) analysis to evaluate PCMs-sand dynamic interaction over time and temperature. Results shown that sand addition halves n-Octadecane phase change time, although leading to a limited supercooling equal to 1 °C. Sand addition to commercial PCM leaded to a similar increasing in heat transfer, however in absence of supercooling phenomena. These performances were constant through 16 thermal cycles. Therefore, PCMs mixing in sand as mixture for GHEs backfilling material can be considered a strategy to enhance thermal storage of backfilling material, by increasing the underground thermal energy storage and then the exploitation carried out by shallow geothermal applications.