Nanofluid-enhanced ice thermal energy storage: experimental performance, parametric effects, and a predictive correlation

Ice thermal energy storage (ITES) systems offer an effective means of shifting peak cooling loads to off-peak periods, but their performance depends strongly on the thermophysical properties of the phase change material (PCM) and on key operating and geometric parameters. This study presents a systematic experimental investigation of the external icing and cold energy storage performance of Al 2 O 3 and multi-walled carbon nanotube (MWCNT) nanofluids as PCMs in a U-tube cold thermal energy storage unit. Nine primary conditions were examined across three coolant inlet temperatures (−5.5, −7.5, and −10 °C) and six nanofluid concentrations, supplemented by parametric studies on heat-transfer-fluid flow rate, initial PCM temperature, and tube outer diameter. Reducing the coolant inlet temperature from −5.5 to −10 °C increased total stored energy by 95%, identifying thermal driving force as the dominant performance lever. Al 2 O 3 nanofluids exhibited a monotonic concentration–performance relationship, with 0.5 vol.% yielding an 18% stored-energy gain, whereas MWCNT nanofluids showed a non-monotonic response, peaking at 0.01 vol.% due to agglomeration at higher loadings. A coefficient-of-performance-based storage-rate index confirmed that lower coolant temperatures remain energy-cost-effective despite reduced refrigeration efficiency. Supplementary studies showed that initial PCM temperature and tube diameter exert moderate but practically significant effects, while flow rate has limited influence once the ice layer thickens. A Buckingham π-based empirical correlation for ice-layer thickness, validated against independent literature data, achieved over 95% of predictions within ±15% deviation. These findings provide quantitative, design-oriented guidance for optimising nanofluid-enhanced ITES systems.