A unique Solar air heater with a transpired slatted glass sheet cover and transpired absorber was developed and tested. The idea was to suck the air through the air slots to recover part of the short wavelength radiation absorbed by the glass sheets. Furthermore, the long wavelength emission from the absorber could also be recovered thus minimizing the total heat losses. A mathematical model was developed to predict the effects of variations in the input parameters on the collector thermal efficiency. The theoretical results showed that the thermal performance of the collector was sensitive to air flow rate, ambient temperature, Solar irradiance and absorber emissivity variations. The collector was tested under a Solar simulator over a wide range of air flow rate. The experimental results were in good agreement with the theoretical ones. An absorbing efficiency as high as 86% could be obtained with a reasonably low air pressure drop across the slatted glass sheet cover The sensitivity analysis of the collector to variations in input parameters was also investigated.

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.

This paper presents a new design of a spiral Solar air heater with a 90 turning of the air inside the passageway assembled between the absorber and bottom plate for the purposes of mixing process and extending the surface of heat transfer that finally leads to higher performance. To enhance the thermal efficiency, an air gap is considered at the top of the Solar collector to reduce heat loss. The proposed Solar collector is simulated numerically by the Finite Element Method using the COMSOL software. The set of governing equations for both forced and free convection turbulent air flows are solved based on the RNG k–ε turbulence model. In the energy equation solution, the effect of surface-to-surface radiation as an important phenomenon in Solar collectors is considered. Numerical results reveal a high thermal efficiency of 75% for the test case with 100  Solar heat flux and air mass flow rate of 0.01 kg/s. Compared to the conventional smooth duct Solar air heater with 35% thermal efficiency, the designed Solar collector operates with higher performance, and a more than 100% increase in thermal efficiency is achieved due to the applied technique with the limitation of pressure drop which is increases about three times in spiral Solar air heater.

Outdoor experiments on a once-through single glazed Solar air heater with perforated metal absorber plate were conducted to determine the practical effect of absorber plate porosity as well as suction air flow rate on the collector thermal efficiency and its total pressure drop. Three aluminum absorber plates were made perforated by drilling circular holes with different diameter/pitch ratios in square layout. A fan was employed at the top of the collector to suck ambient air from the bottom side through absorber plate perforations. The flow channel was designed such that uniform air flow over the entire absorber plate area could be achieved. Five levels of air mass flow rates (0.0065 to 0.0321 kg m-2 s-1) were adopted. Pressure drop across the apparatus was measured. The inlet air was preheated by short wavelength radiation absorbed by the cover as well as the long wavelength emission by the absorber prior to catching the heat from transpired absorber plate. A maximum thermal efficiency of 84% could be achieved for the most part of the porous absorber plate at the highest air mass flow rate. The collector with minimum porosity showed a maximum pressure drop. In some experiments, the glass cover was removed to determine the outdoor effect of glazing. Comparing the performance of the collector with and without glazing showed that the unglazed collector was about 25% less efficient than the glazed one at the same overall operating conditions. This reduction can be attributed to high top radiative and convective heat losses for the unglazed collector at the outdoor conditions. The pressure drop for the uncovered collector showed a lower magnitude in comparison to the covered one. The inlet air passes and heats up (21-59oC above the ambient) through the Solar collector, therefore the fresh and clean hot air can be continuously supplied for many purposes such as Solar drying system.

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.

The performance of a flat plate single-pass Solar air heater (SAH) modified at the entrance region of the heater was experimentally investigated. The entrance region was covered with glass cover instead of steel cover. The aim of using glass at the entrance region is increasing the heating area exposed to Solar irradiation. In addition, replacing the steel cover with glass reduces the shading effect occurred due to steel which decreases the temperature of the surface of the absorber and hence the outlet temperature of the air. Also, guide blades were placed in the entry region to ensure well air distribution on the absorber surface and hence enhancing the thermal performance of SAH. The modified SAH was compared with another one without air blades and without glass cover at the entrance. The experiments were performed at four air flow rates ranged from 0. 013 kg/sec to 0. 04 kg/sec. The modifications led to good enhancement in both the air temperature difference and the efficiency. The glazed-bladed SAH showed a good improvement in the daily thermal efficiency by 6. 72 % to 10. 5 % over the conventional heater and by 2. 16 % to 3. 25 % over the glazed SAH.

Solar air heaters (SAHs) have an inherent drawback: the conventional mechanism is low inside these collectors’ types. Use of perforations is a simple technique to improve convection, and this investigation experimentally assesses a novel design SAH utilizing three inclined perforated absorber plates. Two scenarios are considered to assess the dynamics and efficiency of this perforated SAH, including mair = 0.012 kg/s and 0.024 kg/s. Numerous sensors monitored the dynamics of the perforated SAH and ambient factors for 12 hours in October 2022. The experimental outcomes illustrate that the perforation method remarkably enhances the thermal efficiency of the perforated SAH compared with standard smooth SAHs. The daily thermal efficiency of the perforated SAH reaches 73.30% and 82.65%, while the outlet air temperature experiences peak values of 39 oC and 42 oC at noon and keeps within 90% of its maximum value for 2 hours for the scenarios considered. Improving the convection mechanism causes the flowing air to extract the absorber’s thermal energy more effectively. Hence the SAH can produce an airstream near its maximum temperature for an extended duration. In conclusion, the perforation method is a robust, simple method to boost the thermal efficiency of SAHs.

A very common technique to improve the thermo hydraulic performance of Solar air heater is providing artificial roughness on absorber plate. Diagonally chamfered cuboids has been used to generate roughness and applied on the absorber surface. A numerical study is performed to study the enhancement of the thermo hydraulic performance of the heater for thoroughness parameters of relative roughness pitch (transverse and longitudinal) of 6 to 14, cross section of cuboids from 8 mm to 14 mm and relative roughness height of 0. 44 to 0. 088. The Reynolds number is kept in the range of 5000 to 22500 with a constant heat flux of 1000 W/m2 on the absorber plate. The numerical computation is conducted by using the ANSYS FLUENT CFD software. The standard k-ε turbulence model with enhanced wall treatment has been used to handle the flow turbulence. The Nusselt number and the average friction factor are determined for different values of relative roughness pitch (transverse and longitudinal), cross section of cuboids and relative roughness height. The values of Nusselt number and friction factor of roughened Solar air heater is compared with smooth duct at similar flow condition to find out the enhanced performance.

A new idea is presented in this paper for improving the performance of Solar air heater (SAH) designed for space heating by employing a thin flexible guide winglet. In addition to the role of winglet in pushing the convective airflow toward the heated surface, it behaves as a vortex generator (VG) due to its vibration by fluid-solid interaction (FSI) that causes flow mixing and breaking thermal boundary layer. In flow simulation, the finite element method (FEM) is employed with considering a two-way strongly-coupled FSI approach at transient condition. Numerical solution of the governing equations, including the continuity, momentum and energy for convective flow and the equation of motion for VG are obtained by COMSOL multi-physics. The well-known  model is employed for computation of turbulent stress and heat flux. The present numerical results are validated against the most recent relevant literature. To provide a clear and deep understanding of the proposed concept, extensive comparisons are made between different test cases. Results reveal considerable performance enhancement of SAH with elastic guide winglet compared with clean Solar air heater (CSAH), such that 56% increase in the natural airflow rate and 9% decrease in the average absorber temperature is seen because of the flapping winglet. But, the air outlet temperature decreases about 14% due to flapping VG.  This study aims to make the proposed SAH as an essential renewable thermal-Solar system more efficient and attractive so that this improvement pushes the industrial society toward more sustainable infrastructure.