This study investigates the thermodynamic enhancement and exergy analysis of a pyramid solar still desalination unit (PSSDU) using a floating cellulose-fiber thermal layer combined with controlled low-grade waste-heat input under Egyptian climatic conditions.The proposed configuration was evaluated through energy and exergy analyses to clarify the influence of glass cover inclination, floating layer material and thickness, basin water depth,and auxiliary heat flux on freshwater productivity and irreversibility reduction. Compared with the conventional pyramid solar still, increasing the glass inclination from 35° to 45°improved solar utilization and condensation behavior,raising the peak instantaneous exergy efficiency from 5.3% to 6.4% and the accumulated exergy efficiency from 3.4% to 4.1%.Cellulose fiber exhibited superior thermal localization capability owing to enhanced interfacial evaporation and reduced conductive heat dissipation into the bulk saline layer.Among the tested configurations, a 75mm cellulose layer achieved the best thermal response, producing peak instantaneous and cumulative exergy efficiencies of approximately 8.0% and 5.3%, respectively. Increasing the supplied heat flux to 600W/m² significantly enhanced vapor generation and useful exergy recovery, whereas further increases intensified thermal losses and entropy generation. Reducing the saline water depth to 25mm improved evaporation effectiveness by lowering thermal inertia.Under the optimum operating conditions of 45°glass inclination,75mm cellulose fiber thickness,25mm water depth,and 600W/m² auxiliary heat input,the modified unit achieved maximum instantaneous and cumulative exergy efficiencies of 8.6%and6.0%,respectively, while reducing the cost per liter from 0.0915 to 0.0422USD/L.The findings demonstrate that combining interfacial thermal management with controlled waste-heat recovery provides an effective strategy for improving pyramid solar still energy,exergy and economic performance.