A High-performance Double-pass Solar Air-water Heater

In the present work, a double-pass solar air-water heater has been designed for the first time to provide both hot air and hot water for using in many engineering applications. The proposed solar collector can be a useful device for the purpose of heating spaces and providing domestic hot water, simultaneously. It is like a conventional solar air heater but five water tubes are installed under the absorber plate with a large common surface area with the heated surface. The two-dimensional numerical simulation with the assumption of quasi-steady condition was done on a rectangular-shaped collector with a length of 1 m and a height of 0.128 m including five water tubes by solving the continuity, momentum, and energy equations for the turbulent airflow by the COMSOL Multi-physics. The effect of forced convection water flow inside the parallel tubes installed below the absorber plate is employed as a convection boundary condition in solving the air energy equation. Also, the convection boundary condition was employed on the boundary surfaces of the glass cover and insulation layer adjacent to the surroundings. Numerical simulations are carried out for the air and water mass flow rates of 0.01 kg/s and 0.03 kg/s, respectively. While the solar heat flux lies in the range of 5001100 W/m2. In several test cases, high thermal efficiencies (69% to 74%) are computed for the proposed collector, whose value is much higher than a smooth duct solar air heater operating under the same condition. A comparison is also made between the performances of double-pass and single-pass solar air-water heaters and a percentage of efficiency increase about 6% was found. This value reached to 75% when this solar collector is compared with a conventional solar air heater.