Experimental investigation of energy, exergy, and economic performance of a solar still with hollow cement fins and carbon-enhanced PCM for clean water production

• MSS-5 achieved highest yield of 5.1 L/m 2 with 3 wt% carbon-enhanced PCM. • Energy efficiency of MSS-5 improved by 98.7% over the conventional still. • MSS-5 recorded highest exergy efficiency of 4.2% among all configurations. • Hollow cylindrical cement fins enhanced heat transfer and evaporation. • Carbon particles increased PCM thermal conductivity, reducing heat losses. • Cost per liter in MSS-5 reduced by 41.1%, payback period of 124 days. Freshwater scarcity is a growing global concern, especially in remote and arid regions where access to clean water is limited. Solar stills offer a simple and sustainable desalination approach, but their low productivity restricts widespread application. To address this gap, the present study aims to enhance solar still performance by integrating hollow cylindrical cement fins and composite phase change materials (PCMs) enriched with carbon particles. Five modified solar stills (MSS) are evaluated: a solar still with empty fins (MSS-1); a solar still with fins filled with pure paraffin wax (MSS-2); and three solar stills with fins containing paraffin wax mixed with 1%, 2%, and 3% carbon particles, designated as MSS-3, MSS-4, and MSS-5, respectively. Their productivity and performance are compared with the conventional slope solar still (CSS). Experiments are conducted in Silchar, India (24.82°N, 92.79°E). The performance is assessed in terms of freshwater yield, energy and exergy efficiency, cost per liter, and payback period. MSS-5 shows the greatest improvement, achieving 5.1 L/m 2 /day productivity, 48.7% energy efficiency, 4.2% exergy efficiency, and the lowest cost per liter of $0.0112 with a 124-day payback period. Compared to CSS, freshwater productivity increases by 27.8%, 51.8%, 67.5%, 76.8%, and 88.6% for MSS-1 to MSS-5, respectively. The use of 3 wt% carbon particles enhances the thermal conductivity of the PCM, resulting in better heat storage and higher system performance. The findings indicate that combining cement fins with carbon-enhanced PCM effectively improves solar still performance, providing a viable and cost-efficient approach for freshwater production.