Techno-Economic and Environmental Assessment of Nanofluid-Enhanced Solar Water Heating Systems for Residential Applications

Residential water heating constitutes a significant share of household energy consumption, yet the performance of conventional solar water heating (SWH) systems is limited by the low thermal conductivity of water. This study presents a techno-economic and environmental assessment of nanofluid-enhanced flat-plate SWH systems using 2 wt% Al2O3/water and a novel 2 wt% TiO2–chlorophyll bio-hybrid nanofluid. Experimental characterization showed thermal conductivity enhancements of 24.7% and 14.2% for Al2O3/water and TiO2–chlorophyll, respectively, resulting in collector efficiency improvements of 14.9% and 9.0% compared with the base fluid. Annual energy yields increased to 3520.6 kWh for Al2O3/water and 3339.2 kWh for TiO2–chlorophyll, compared with 2978.5 kWh for the conventional system. Economic analysis over a 20-year lifetime revealed payback periods of 2.24 and 2.28 years, with net present values of $7430 and $7028, respectively. Life-cycle assessment indicated that although Al2O3/water achieved the highest annual CO2 reduction (1697 kg CO2-eq/year), the TiO2–chlorophyll system exhibited the lowest life-cycle emissions (46.1 kg CO2-eq/m2) and shortest carbon payback period (4.1 months). The results demonstrate that while Al2O3/water maximizes thermal and economic performance, the TiO2–chlorophyll bio-hybrid provides the most balanced and sustainable solution for residential solar water heating applications.