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Convective heat loss coefficient due to wind (W/m 2 K) I T Incoming radiation in 1-h (kJ/m 2 ) K Thermal conductivity of the air at mean temperature (W/m K) K e Extinction coefficient (m −1 ) L Thickness of the air layer between absorber plate and upper glass (m) L g Thickness of glass (m) m Mass of the water in the tank (kg) n The day of the year Nu Nusselt number q 1 Heat flux from the absorber plate to the upper glass (W/m 2 ) q 2 Heat flux from the upper glass to the ambient (W/m 2 ) q T Total heat loss from the system (W/m 2 ) R 1 Thermal resistance between the absorber plate and the upper glass R 2 Thermal resistance between the upper glass and the ambient R T Total thermal resistance between the absorber plate and the ambient R a Rayleigh number T a Temperature of the ambient (°C) T p Temperature of the absorber plate (°C) T Glass2 Temperature of the upper glass (°C) T s Temperature of sky (°C) ΔT p−c Temperature difference between the absorber plate and the upper glass (°C) U T Total heat loss coefficient (W/m 2 K)Greek letters α Thermal diffusivity (m 2 /s) βTilt angle of the water heating system (°)Abstract Solar passive water heaters are potential candidates for enhanced heat transfer. Solar water heaters with an integrated water tank and with the low temperature energy resource are used as the simplest and cheapest recipient devices of the solar energy for heating and supplying hot water in the buildings. The solar thermal performances of one primitive absorber were determined by using both the experimental and the simulation model of it. All materials applied for absorber such as the cover glass, the black colored sands and the V shaped galvanized plate were submerged into the water. The water storage tank was manufactured from galvanized sheet of 0.0015 m in thickness and the effective area of the collector was 0.67 m 2 . The absorber was installed on a compact solar water heater. The constructed flat-plate collectors were tested outdoors. However the simulation results showed that the absorbers operated near to the gray materials and all experimental results showed that the thermal efficiencies of the collector are over than 70 %.