Numerical Modeling and Performance Assessment of a Passive Rooftop Integrated Collector–Water Storage Tank (IC-WST) Solar Preheater: Case Study for Lebanon

Rooftop water storage tanks (≈1000 L) are ubiquitous in Lebanon and many Mediterranean, and Middle East and North Africa regions but remain thermally inactive despite high solar availability and rising electricity costs. While integrated collector–storage systems have been widely investigated, their application as low-cost retrofits for existing large-capacity rooftop tanks has received limited attention. This study proposes and numerically investigates a fully passive retrofit concept that converts an existing rooftop tank into an integrated collector–water storage tank operating upstream of a conventional electric water heater. The design incorporates a floating, partially immersed solar absorber beneath a single glass cover and a passive hydraulic configuration that preserves thermal stratification through bottom charging and flexible top extraction. A coupled transient three-block numerical framework (absorber/cover, multi-node stratified tank, downstream electric water heater) is developed and verified against published experimental datasets using ASHRAE Guideline 14 statistical indicators and regression/residual diagnostics. Annual simulations based on Beirut Typical Meteorological Year data and a representative domestic hot-water demand profile indicate a reduction in electricity consumed by the electric water heater from 2810 to 2152 kWh, corresponding to 23.4% savings (658 kWh/y). Monthly savings exceed 38–41% in summer and remain positive during winter, with additional sensitivity analysis demonstrating robustness against increased thermal losses. A preliminary capital expenditure estimate based on local market quotations yields a simple payback period of 0.77–1.78 years. The results demonstrate that repurposing existing rooftop tanks as passive solar preheaters constitutes a technically feasible, economically attractive, and scalable pathway for reducing residential electricity demand in Mediterranean, and Middle East and North Africa climates. • Passive IC-WST retrofit for rooftop tanks. • Solar preheating using a 1000-L tank; no pumps/controls. • Transient multi-node model verified. • 23.4% annual electricity saving (≈ 658 kWh/y). • Multi-GWh/y saving potential at city scale.