This study analyzes the thermal performance of solar thermal energy using Double-pass absorber plate in Sanandaj, Iran. To this end, a mathematical model was encoded according to the energy and exergy balance equations and solved by MATLAB software. Given the environmental conditions and radiation intensity of a winter day in Sanandaj, the effects of external parameters such as radiation intensity and internal parameters like canal’ s height, inlet mass flow rate, absorber length as well as some physical parameters on the efficiency of the system were analyzed and the energy and exergy output of the system was studied. To validate the proposed model, the results obtained from numerical simulation were compared with experimental data, which showed an acceptable compatibility. Finally, the ability of the system to supply the thermal load of the building in the given day was examined and the roles of various factors in the area under thermal coverage of this system were analyzed. The obtained findings indicated that a system with an area of 3 m2 and mass rate of 250 kg/h in radiation intensity of 619 W/m 2 and temperature of 3. 45° is capable of supplying the thermal load of a space with an approximate area of 14 m.

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.

In the present study, a new and efficient design of a Double-pass solar air heater with converged air ducts is proposed and simulated numerically. In the designed solar collector, the inclined position of the absorber plate regarding the glass cover and bottom plate provides converged shapes for the upper and lower air ducts, in which the accelerated flows and extra turbulence generation cause an increased rate of convection heat transfer. This concept is evaluated by numerical solution of the governing equations for air turbulent flow consisting of the conservations of mass, momentum, and energy by the finite element method using the COMSOL Multi-physics. Besides, the conduction equation is solved to compute the temperature distributions inside all solid parts. Numerical findings reveal that the increased velocity of airflow inside the converged duct causes significant convection enhancement, especially in the downstream side of airflow with a thick thermal boundary layer. This behavior leads to a considerable decrease in the absorber temperature due to a higher convection coefficient. The air outlet temperature for the studied test case with the inclined angle of   is found 3.5  more than that obtained for the base model and the percentage of efficiency increase is calculated about 10% due to using a converged duct.

Solar air heaters are the simplest heat exchangers for converting solar irradiation into air enthalpy, but their efficiencies are low because the low density and heat capacity of the working fluid. To improve the performance, a new configuration of jet technique is proposed for heat transfer augmentation. The air jet streaks the heated absorber, which leads to extra turbulence and more rate of convection heat transfer.  This technique is examined numerically here for the first time in a two-glazed Double pass solar air heater by the Computational fluid dynamics simulation using the COMSOL Multiphysics. The hydrodynamic and thermal behavior of the heater is obtained by numerical solution of the continuity, momentum, and energy equations for both forced and free convection turbulent air flow based on the k–ε turbulence model. Also, the Laplace equation is solved for obtaining the temperature fields in solid elements. The performance of the proposed solar heater is investigated under different incoming solar heat flux and air mass flow rates. A significant increase in thermal efficiency for Double pass solar air heaters with jet impinging technique is seen in comparison to conventional solar collectors. The maximum percentages of efficiency increase for the studied test cases are 10% and 6%, corresponding to 0.02 kg/s and 0.04 kg/s air mass flow rates, respectively.

The present study discusses the effects of apping exible vortex generators on two-dimensional turbulent free convection air ow in a Double-pass solar air heater. To this end, two thin elastic winglets used as vortex generators were attached to two absorber walls near the inlet section at an attack angle of 65, C. This novel concept was elaborated through transient numerical simulation of the ow , eld based on the , nite element method and uid-solid interaction. In addition, an extensive comparison of four di, erent con, gurations was made in this study. The absorber and outlet temperatures as well as the ow rate and velocity , eld were carefully calculated, numerical results of which con, rmed the considerable enhancement of thermal performance compared to that of conventional parallel Double pass heater. The improvement rate was up to 54% in the case of delta T-= Tout-Tin from 13 to 20oC while reducing the flow rate by 33. 6%. The present numerical results were validated based on the experimental and numerical data reported in the literature.

The steady state effect of vertical fins is assessed on a photovoltaic/thermal solar air heater having a Double pass configuration in which fins are placed in the lower channel perpendicular to the direction of air flow. Air passes through the upper channel of the air heater and before passing in the opposite direction through the lower channel. The effects of design, climatic and operating parameters are evaluated on temperatures, efficiencies and other parameters. For fixed operating conditions, fins are observed to increase heat transfer area and rate, reduce cell temperature about 16oC, and improve thermal and electrical efficiencies. Higher packing factors are advantageous as they increase electrical output per collector area and reduce cell temperature.