Experimental and numerical investigation of red sand–iron composite for solar thermal energy storage applications

This study experimentally and numerically evaluates dry red sand and a red sand–iron composite as sensible heat-storage media under natural solar exposure in Baghdad, Iraq. The composite was prepared using a nominal 1:1 bulk-volume batching ratio of as-received dry construction sand and iron filings; this ratio was not verified gravimetrically. Temperatures were monitored over three consecutive days, and the median daily profiles were compared with predictions from a transient two-dimensional computational fluid dynamics (CFD) model. In the numerical model, the composite was represented as a homogeneous medium with an idealized iron volume fraction of 0.50, and its effective thermophysical properties were calculated using mixture relationships. Approximate normalized indicators derived from the median temperature profiles showed that the red sand–iron composite reached a maximum temperature of 46.5 °C, compared with 39.7 °C for red sand. Its estimated volumetric storage capacity was 135.3 MJ m⁻³ , representing an increase of approximately 23.9% over the 109.2 MJ m⁻³ obtained for red sand. The heat-retention efficiency at 19:00 increased from 32.6% for red sand to 38.4% for the composite. In contrast, the estimated gravimetric storage capacity decreased from 41.2 to 25.9 kJ kg⁻¹ because the iron-rich composite had a lower mass-specific heat capacity. Statistical comparison of the experimental and numerical temperature profiles produced RMSE values of 0.37 °C for red sand and 0.25 °C for the composite, with corresponding R² values of 0.9989 and 0.9997. These results indicate that iron addition primarily improves heat-transfer rate, thermal response, and volumetric storage performance rather than gravimetric storage capacity. Because the energy indicators were estimated using idealized properties and the study did not include measured specimen masses, long-term cycling, or pilot-scale testing, the findings are limited to the present small-scale configuration. Further work should determine the optimum iron loading using measured packed-bed properties, repeated thermal cycling, and pilot-scale validation.