Experimental and Numerical Investigation of a Novel Low-Cost Solar Air Heater with Large-Scale V-Shaped Fins to Enhance Heat Transfer

Abstract

This study investigates the performance of a novel, low-cost solar air heater equipped with large V-shaped fins using experiments and numerical simulations. The solar air heater consists of an absorber plate, a glass cover and airflow ducts. Its performance is evaluated under varying fin configurations: finless and (a)symmetric V-shaped fins with four, six, and eight fins. Computational fluid dynamics simulations using the RNG k-epsilon and discrete ordinate models were validated by experimental findings, showing good agreement with minimal discrepancies between both. The experimental setup recorded a maximum air temperature of 55 °C, corresponding to a temperature rise of 33 °C from an inlet temperature of 22 °C, under an inlet air velocity of 2.7 m/s. Results demonstrate that increasing the number of fins significantly enhances heat transfer efficiency, with heat transfer rising from 134.35 W (finless) to 233.29 W (8 fins). The large-scale fins improved thermal performance significantly while still maintaining a low-pressure drop. Moreover, the fins are very low-cost to implement, in contrast to most heat transfer enhancements in solar air heaters, making this design a very budget-friendly solution. This study provides valuable insights into optimizing solar air heater systems, contributing to the advancement of solar heating solutions for a wide range of energy-efficient applications.

1. Introduction

Solar air collectors (SACs) are commonly employed devices because of their excellent efficiency, uncomplicated design, and budget-friendly price [1]. SACs are a more general terminology including solar water/air heaters, photovoltaic panels or other systems that absorb solar radiation for a variety of applications. A solar air heater (SAH) is a specific type of SAC, and is one of the solar devices that can be constructed inexpensively. It can be assembled using readily accessible materials, resulting in a remarkably low fabrication cost. SAHs can be installed virtually anywhere and are associated with minimal maintenance and operational expenses [2].

A great deal of research is being done to improve the performance of solar air heaters (SAHs) and increase the amount of solar energy absorbed. Artificial roughness and ribbed surfaces have been widely explored to intensify turbulence and enhance heat transfer. Lanjewar et al. [3] studied experimentally the thermal conduction and resistance characteristics of a solar heating channel featuring a W-shaped ribbed texture on the absorber plate. The results showed that the thermo-hydraulic efficiency increases with the flow’s angle of attack and relative roughness height, reaching its maximum at a 60° angle. Rajendran et al. [4] experimentally improved the performance of a SAH by introducing artificial roughness through a V-shaped perforated fin on the absorber. Their study showed that baffled absorbers achieved the highest thermal efficiency and energy gain at a given mass flow rate. Jin et al. [5] numerically examined the heat transfer and flow characteristics of a SAH enhanced by multiple V-shaped ribs. A maximum thermo-hydraulic performance factor (TPF) of 2.35 is obtained, with the rib configuration playing a critical role. Similarly, Suresh et al. [6] demonstrated that trapezoidal ribs on the absorber plate could enhance thermal performance. They achieved a 5.54 times higher Nusselt number compared to smooth configurations, with a TPF of 1.72. Sharma et al. [7] evaluated SAHs with sine wave baffles, achieving a thermal efficiency of 78% and effective efficiency of 70.8%. In addition, optimal geometric parameters are also identified for maximum performance. These findings confirm that ribbed geometries are highly effective in enhancing SAH efficiency.

Several studies have also focused on improving absorber materials and coatings. Gürbüz et al. [8] investigated the integration of mesh tubes and nano-enhanced absorber coatings, demonstrating significant performance improvements in unglazed solar air collectors. Kumar et al. [9] studied the influence of V-shaped surface roughness profiles on the performance of a SAH both experimentally and numerically. They concluded that the roughness of the V-fins has a greater effect on the performance than both relative roughness slope or Reynolds number variation. In conclusion, the above works highlight the importance of absorber surface modifications and advanced coatings in boosting thermal and exergetic performance.

Alternative geometrical designs and optimized arrangements have also been explored. Jiang et al. [10] evaluated triangular-shaped SAHs across different regions in China, showing that performance strongly depends on solar irradiance and ambient temperature conditions. Jalal et al. [11] investigated the use of wavy fins on the absorber plate, reporting thermal efficiency improvements compared to smooth absorbers of 67.4% and 41.1% for seven and three fins, respectively. Ikrame et al. [12] analyzed the influence of baffle length and arrangement using CFD, showing that periodic baffles improved turbulence and achieved a maximum TPF of 3.267. Chang et al. [13] used CFD to identify optimal baffle types and numbers, concluding that transverse baffles significantly improved heat transfer coefficients and efficiency, with N = 6 baffles performing best at higher flow rates. Together, these studies demonstrate that geometric optimization is essential for enhancing SAH efficiency.

Finally, advanced materials and porous structures have also been investigated. Bayrak et al. [14] assessed SAHs with porous baffles made of closed-cell aluminum foams of different thickness, using energy and exergy analyses. Their results showed that 6 mm porous baffles at higher mass flow rates achieved the best collector efficiency and outlet air temperature rise, whereas smooth absorbers performed the worst under the same conditions. This highlights the potential of porous baffles and advanced absorber materials in improving SAH performance while addressing flow resistance.

Despite these significant contributions, there are still challenges and research gaps that need to be addressed. Many of the reported studies have focused on single design modifications, such as adding artificial roughness, incorporating baffles, or using advanced coatings. However, few have considered the combined effects of multiple parameters on overall system performance. Moreover, while ribbed or baffled geometries enhance heat transfer, they often lead to an increase in pressure drop, which can reduce overall efficiency if not optimized carefully. Similarly, although coatings and advanced materials can improve absorption and reduce losses, their integration into large-scale, cost-effective SAHs for practical drying or heating applications remains limited. Furthermore, much of the existing literature is either solely experimental or numerical, with few works offering a comprehensive approach that combines both methods. These limitations highlight the need for further investigations that integrate experimental validation with numerical modeling to optimize the balance between heat transfer enhancement and flow resistance.

Past research has indicated that the thermal efficiency of a SAH is notably influenced by its geometrical characteristics. However, increased heat transfer often comes with highly complex or expensive solutions. Therefore, this study investigates a simple design using relatively inexpensive materials, while significantly enhancing heat transfer. The novelty of our SAH lies in its small size, the use of steel for the absorber plate, and the inclusion of specific large-scale fin designs in the collector. This combination ensures the SAH is extremely cost-effective, making it an attractive option for harnessing solar energy. This work contributes to a performance-based selection criterion for optimal fin design, enhancing practical applicability. Conventional SAHs often suffer from high maintenance requirements, material corrosion, and complex designs. This limits their deployment in cost-sensitive regions. By using locally available materials and incorporating large V-shaped fins, our design achieves a balance of affordability, durability, and enhanced heat transfer. This approach highlights the practical value of a “low cost and high heat transfer” SAH.

This study further distinguishes itself from prior work by systematically evaluating the thermo-hydraulic efficiency of multiple fin configurations while considering pressure drop penalties, a gap often overlooked. The results of this work not only demonstrate improved efficiency but also provide a performance-based design criterion that can guide future SAH development. After validation of the numerical model, an exploration of various configurations is conducted, including different fin locations, varying numbers of V-shaped fins (4, 6, and 8), as well as a SAH without fins. CFD simulations are employed to optimize thermal performance. The distributions of temperature, velocity, and static pressure are presented and discussed for each configuration, along with their influence on the collector efficiency.

2. Materials and Methods

2.1. Experimental Setup

The processes of fabrication, installation, and testing were conducted at the Mechanical Modeling, Energy, and Materials (M2EM) laboratory in Gabès, Tunisia. Figure 1 illustrates an experimental model comprising a SAH. The SAH was oriented toward the south with a tilt angle of 34° to ensure optimal solar energy capture, based on the geographic and solar characteristics of Gabès, Tunisia. The key elements of this prototype include a cover, wooden insulation, and a steel absorption plate. The absorber’s dimensions measure 700 × 400 × 1 mm³. The plate is equipped with four V-shaped fins. These fins act as airflow guides, channeling it in a specific manner to improve heat transfer and to create turbulence in order to facilitate better mixing. By directing air through the heater, the V-shaped fins ensure close proximity to the absorber plate, maximizing heat transfer from the plate to the air. Additionally, this design promotes an even airflow distribution across the absorber plate’s surface, ensuring consistent heating and enhancing the overall effectiveness of the SAH. The use of fins is well established in SAHs, but the novelty of this study is their relative large size compared to current designs and hence relatively small associated pressure drop. The experiments were conducted at a latitude of 33.892° N and a longitude of 9.561° E in February 2023. Prior to initiating the tests, the collector fan was operated for 15 min to establish stable flow conditions. The experimental procedure commenced at 9 a.m.

Figure 2 provides a schematic and complete engineering drawing of the solar air heater. This drawing specifies the collector length (700 mm), width (400 mm), and height (200 mm). It also details the placement and orientation of the V-shaped fins, thereby clarifying the internal airflow path. Each fin has an apex angle of 44°, a leg length of 170 mm, a height of 150 mm and a distance of 125 mm between the ends of the V. Airflow was generated using a salvaged 120 mm case fan mounted at the inlet of the SAH, operated with a 12 V DC supply. The top glass cover has a thickness of 4 mm and a solar transmittance of $\tau =0.92$, allowing sufficient solar radiation to reach the absorber plate.

Table 1. Specifications of the measurement equipment.

Tools	Number	Range	Accuracy
Solar power meter	1	2000 W/m2	±10 W/m2
Anemometer	2	0.4 to 30 m/s	5%
Thermocouples	3	−30 °C to 120 °C	±0.1 °C

presents the instrumentation utilized for measuring temperature, velocity, flow rate, humidity, and irradiation intensity, including specifications of the measurement equipment. The intensity of solar radiation and ambient temperature, crucial factors affecting the performance of the SAH, are monitored hourly using a solar power meter and thermocouple, respectively. Airflow within the SAH is maintained at a constant velocity of 2.7 m/s, facilitated by the constant voltage fan positioned at the inlet. Air speed is measured using a vane anemometer with an accuracy of ±5%. Additionally, air temperature at the inlet and outlet of the SAH is recorded every hour using 2 thermocouples with an accuracy of ±0.1 °C.

2.2. Mathematical and Numerical Model

Numerous correlations are utilized for computing the energy efficiency and dimensionless parameters of SAHs, and these are mostly derived from experimental data. These correlations are contingent on factors including the air temperature (both in- and outlet), the air mass flow rate, and the intensity of solar irradiance. In this work, a Computational Fluid Dynamics (CFD) model is developed using equations that express the conservation of mass, momentum, and energy. These equations are presented below, respectively [15]:

$$\nabla ·\left(\rho \mathbf{V}\right)=0,$$

(1)

$$\nabla ·\left(\rho \mathbf{VV}\right)=-\nabla P+\nabla ·\left(\mu (\nabla \mathbf{V}-{\displaystyle \frac{2}{3}}\nabla ·\mathbf{V}I)\right),$$

(2)

$$\nabla ·\left(\rho \mathbf{V}H\right)=\nabla ·(K_f\nabla T).$$

(3)

In those equations, $\mathbf{V}$ represents the velocity vector of the fluid, $\rho$ represents the density, $\mu$ represents the viscosity, H represents enthalpy, and T indicates the fluid temperature. Additionally, in the momentum conservation equations, the value of I is equivalent to the unity tensor. The input flow boundary condition is derived from free flow conditions, while the output boundary condition is determined from within the solution domain. Furthermore, for the numerical analysis of the momentum and energy equations, an upstream second-order discretisation scheme was employed. The assumptions that closely align with the numerical conditions are as follows [16]:

• It is assumed that the flow is incompressible.
• The flow is considered to be in a steady state.
• Numerical simulations are conducted for 3D turbulent flow with a Newtonian fluid.
• All fluid properties are taken as constant values.
• The density behaves similarly to that of an ideal gas.

The simulations were carried out using ANSYS Fluent 2023R2. In this section regarding the numerical simulations, the process begins by validating the simulations on a benchmark geometry with 4 fins. Subsequently, the standard geometry is optimized, and this optimization is extended to three distinct modified geometries (without, with 6 V-shaped and with 8 V-shaped fins). The objective behind these modifications is to enhance the collector’s performance.

2.3. Numerical Analysis

CFD serves as a robust tool for investigating intricate fluid flow and heat transfer phenomena across diverse scientific and engineering contexts. The level of numerical precision in the employed mathematical model is contingent upon the initial conditions. In this particular simulation, a velocity of 2.7 m/s is introduced at the inlet of the SAH, while atmospheric pressure is enforced at the outlet. The glass is semi-transparent. The absorber is constructed from steel, and the insulation is composed of wood, as it appears in Figure 3. The various characteristics of the components examined in the SAH are presented in

Table 2. Thermo-physical properties of different materials used in the SAH.

	Density (kg/m3)	Specific Heat (Jkg−1K−1)	Thermal Conductivity (Wm−1K−1)
Wood	700	2310	0.173
Steel	8030	502.48	16
Glass	2700	840	1.1

.

2.4. Turbulence and Radiation Models

Turbulence and radiation models are crucial components in CFD simulations, particularly when studying complex flow and heat transfer phenomena. Turbulence models are mathematical formulations used in Computational Fluid Dynamics (CFDs) simulations to model and predict the behavior of turbulent flows. Turbulence is an unsteady flow regime characterized by irregular fluctuations in flow quantities. The RNG k–$ϵ$ turbulence model was employed to simulate the airflow inside the solar collector. This model has been widely validated for internal flows in solar thermal applications, showing excellent agreement with experimental measurements [17,18,19]. The RNG k–$ϵ$ model provides a computationally efficient way to simulate turbulent flows by solving two additional transport equations for the turbulent kinetic energy (k) and its dissipation rate ($ϵ$) [20]. Using the formulation as given in the ANSYS Fluent Theory Guide [21], these equations are given as:

$${\displaystyle \frac{\partial \left(\rho k{u}_i\right)}{\partial x_i}}={\displaystyle \frac{\partial }{\partial x_j}}\left({\alpha }_k{\mu }_{\mathrm{eff}}{\displaystyle \frac{\partial k}{\partial x_j}}\right)+G_k+G_b-\rho ϵ-Y_M+S_k,$$

(4)

$${\displaystyle \frac{\partial \left(\rho ϵu_i\right)}{\partial x_i}}={\displaystyle \frac{\partial }{\partial x_j}}\left({\alpha }_{ϵ}{\mu }_{\mathrm{eff}}{\displaystyle \frac{\partial ϵ}{\partial x_j}}\right)+C_{1ϵ}{\displaystyle \frac{ϵ}{k}}\left(G_k+C_{3ϵ}{G}_b\right)-C_{2ϵ}\rho {\displaystyle \frac{{ϵ}^2}{k}}-R_{ϵ}+S_{ϵ},$$

(5)

where the turbulent kinetic energy (k) and its dissipation rate ($ϵ$) are functions of the Cartesian coordinates ($x_i$), where i varies from 1 to 3, corresponding to the three spatial directions. The velocity components in these directions are denoted by $u_i$. The inverse effective Prandtl numbers for k and $ϵ$ are denoted as ${\alpha }_k$ and ${\alpha }_{ϵ}$, respectively, and they affect the effective viscosity (${\mu }_{\mathrm{eff}}$) terms. The production of turbulent kinetic energy due to velocity gradients is represented by $G_k$, while $G_b$ accounts for production caused by buoyancy. The dissipation of turbulent kinetic energy is expressed as $ϵ$. The term $Y_M$ refers to the contribution of compressible turbulent fluctuating dilatation to the overall dissipation rate.

Radiation models are mathematical formulations used in Computational Fluid Dynamics (CFDs) simulations to account for radiative heat transfer in addition to conduction and convection. Radiative heat transfer is the transfer of thermal energy in the form of electromagnetic waves, such as infrared radiation, between surfaces and participating media. The DO radiation model is applied to analyze the radiative heat transfer within the SAH [22]. This model is capable of simulating the interactions of sunlight with surfaces, including processes such as absorption, emission, reflection, and transmission, as well as its interactions with intermediate media, encompassing absorption, emission, transmission, and diffusion. The simplified Radiative Transfer Equation (RTE) for radiative heat transfer in a participating medium is as follows [23]:

$$\begin{array}{cc}\hfill \nabla ·\left(I(\overrightarrow{r},\overrightarrow{s})\overrightarrow{s}\right)+(\alpha +{\sigma }_s)I(\overrightarrow{r},\overrightarrow{s})& =\alpha n^2\left({\displaystyle \frac{\sigma T^4}{\pi }}\right)+{\displaystyle \frac{{\sigma }_s}{4\pi }}{\int }_0^{4\pi }I(\overrightarrow{r},\overrightarrow{s})\varphi (\overrightarrow{s}·\overrightarrow{s^{\prime }})d{\Omega }^{\prime },\hfill \end{array}$$

(6)

where $I(\overrightarrow{r},\overrightarrow{s})$ is the solar irradiance or solar flux at position $\overrightarrow{r}$ and direction $\overrightarrow{s}$, $\alpha$ is the absorption coefficient, n the index of refraction, ${\sigma }_s$ is the scattering coefficient, $n^2\left({\displaystyle \frac{\sigma T^4}{\pi }}\right)$ represents radiation and ${\int }_0^{4\pi }I(\overrightarrow{r},\overrightarrow{s})\varphi (\overrightarrow{s},\overrightarrow{s^{\prime }})d{\Omega }^{\prime }$ represents an integral over all solid angles ${\Omega }^{\prime }$, where $\varphi (\overrightarrow{s}·\overrightarrow{s^{\prime }})$ is a function of directions $\overrightarrow{s}$ and $\overrightarrow{s^{\prime }}$. The semi-transparent glass cover is modeled as a participating medium with defined absorption and scattering coefficients, while the absorber plate is treated as an opaque surface with a selective coating characterized by high solar absorptance and low thermal emittance. The DO model discretizes the radiation field into multiple directions and solves the radiative transfer equation, allowing accurate calculation of radiation interactions within the glass and between the glass and absorber. This radiative analysis is fully coupled with the convective and conductive heat transfer of the air and solid components, ensuring a thermally consistent solution.

The efficiency of a SAH is characterized by how effectively it converts incident solar radiation into useful heat energy within a given time frame. This thermal efficiency ${\eta }_{th}$ can be expressed as [24]:

$${\eta }_{th}={\displaystyle \frac{\dot{m}{c}_p(T_0-T_i)}{I{A}_c}},$$

(7)

where $\dot{m}$ is the mass flow rate of the fluid, $c_p$ is the specific heat capacity at constant pressure, $T_i$ and $T_0$ are the in- and outlet temperatures of the fluid, respectively, and $A_c$ is the collector area, i.e., the surface area of the solar collector. In addition to the thermal efficiency, the thermo-hydraulic efficiency is considered to evaluate the overall performance of the solar air heater. The thermo-hydraulic efficiency accounts for both the thermal gains and the hydraulic losses due to the pressure drop in the system, providing a more comprehensive measure of the device’s effectiveness. It is defined as [25]:

$${\eta }_{\mathrm{th}-\mathrm{hyd}}={\displaystyle \frac{Q_{\mathrm{u}}-\left(\frac{P_{\mathrm{mech}}}{C_f}\right)}{I{A}_c}}.$$

(8)

The term $C_f$ represents the conversion factor, for which Cortes and Piacentini [26] suggest a value of 0.18. The mechanical power, $P_{\mathrm{mech}}$, needed to drive the air through the duct is also provided by Cortes and Piacentini [26],

$$P_{\mathrm{mech}}={\displaystyle \frac{\dot{m}\Delta P}{{\rho }_a}}.$$

(9)

In addition, $Q_{\mathrm{u}}$ represents the useful heat gain of the collector (W).

The Reynolds number, which is a significant parameter for analyzing flow behavior, can be calculated using the following equation [27],

$$\mathrm{Re}={\displaystyle \frac{\rho ·V·D_h}{\mu }},$$

(10)

where V is the average flow velocity in the collector and $D_h$ the hydraulic diameter. In this study, the hydraulic diameter is calculated as

$$D_h={\displaystyle \frac{4A}{P}},$$

where A is the cross-sectional area of the air flow and P is the wetted perimeter in contact with the fluid. For the current study, Re = 51,400. The Richardson number is a way to compare the importance of natural versus forced convection in a fluid flow and is given as

$$\mathrm{Ri}={\displaystyle \frac{g\beta \Delta TL}{V^2}},$$

(11)

where g is the acceleration due to gravity, $\beta$ is the coefficient of thermal expansion, $\Delta T$ is the temperature difference, L is a characteristic length scale, i.e., the height of the collector. For all cases studied in this work, Ri was in the order of 0.05, indicating the flow in the SAH is convection dominated.

2.5. Modified Geometries

Figure 4 shows the benchmark geometry (4 V-shaped fins) and the modified geometries (without, with 6 V-shaped and with eight V-shaped fins), where all other dimensions of the SAH are kept constant.

Table 3. Fin details.

Number of Fins	Thickness	Material	Height	Inter-Space
4 V-Shaped	1 mm	steel	150 mm	300 mm
6 V-Shaped	1 mm	steel	150 mm	210 mm
8 V-Shaped	1 mm	steel	150 mm	210 mm

presents the details of the fins, such as thickness, material, number, and inter-spaces for the standards geometry and the modified geometries. They are made of steel and coated with a black selective paint, with an absorptivity of approximately $\alpha$ = 0.95 and an emissivity of about $ϵ$ = 0.15. The V-shaped structures are welded directly to the absorber surface. The selection of four, six, and eight fins for the solar air heater (SAH) was based on the dimensions of the collector and the need to evaluate its thermal performance under different conditions. The number of fins was chosen to ensure adequate coverage of the heat transfer surface while maintaining proper spacing, aligned with the collector’s geometry to ensure optimal airflow and prevent blockage. These configurations aimed to progressively enhance the heat transfer surface area. The stepwise increase in fin numbers (4, 6, and 8) enables a systematic comparative analysis of the effects on turbulence, thermal efficiency, and pressure drop.

3. Results and Discussion

This section presents and thoroughly discusses the key findings derived from both experimental and numerical analyses. Figure 5 presents the XZ plane, located at a height of 135 mm above the absorber plate along the y-axis, used to clarify all the numerical results (velocity and temperature contours). The choice of this plane corresponds to the best way that we can clarify the different results.

3.1. Mesh Independence Study

In CFD simulations, the computational domain is discretized into small cells or elements that form a computational mesh. The accuracy and reliability of the simulation results depend on how well the underlying equations are approximated on this mesh. The state of mesh independency is reached when the solution remains largely unchanged even after further refinement. This indicates that the results are stable and less sensitive to mesh resolution. Mesh independency is a fundamental aspect of numerical simulations and plays an important role in achieving accurate and reliable results. In this study, an unstructured tetrahedral mesh was used, and five meshes are evaluated to confirm the mesh independency of the solution, as demonstrated in

Table 4. Mesh parameters.

Mesh	Number of Nodes	Number of Elements
Mesh 1	1,072,901	547,756
Mesh 2	4,768,092	1,150,533
Mesh 3	6,813,046	2,260,757
Mesh 4	8,917,520	5,113,411
Mesh 5	10,625,487	7,894,012

.

To determine the optimal number of mesh elements, the pressure drop, velocities and temperatures at various locations within the SAH were used in a convergence analysis. The locations of the monitoring points within the SAH are shown in Figure 6a. Mesh convergence stability is attained for the pressure drop (pressure difference between in- and outlet) beginning with mesh 3, as shown in Figure 6b. This is confirmed by the air temperatures, as a slight difference can be seen in Figure 7a between mesh 1 and mesh 2 when compared to the other monitoring points. The histogram of velocities at the designated points, shown in Figure 7b, indicates that velocity values converge and remain consistent from mesh 3 onwards. Therefore, mesh 3 is chosen as the optimal option because it yields similar results while requiring a smaller storage size and shorter calculation time compared to meshes 4 and 5.

The average Y+ values on all relevant surfaces (absorber, glass, and collector walls) range from 3.8 to 4.55, with local maximum values around 12, which is appropriate for the RNG k–$ϵ$ turbulence model with enhanced near-wall treatment, ensuring proper resolution of the boundary layer. The key metrics are summarized in

Table 5. Mesh quality metrics.

Metric	Value
Average Orthogonal Quality	0.54
Average Aspect Ratio	2.43
Average Skewness	0.45
Average Y+ Values	3.8–4.55

. The corresponding Y+ distribution is also shown in Figure 8, illustrating the adequacy of the near-wall mesh.

3.2. Experimental Results and Validation of the Numerical Model

Figure 9 illustrates the fluctuations in ambient temperature and solar radiation during specific measurement days (21–24 February 2023). On the first testing day, solar radiation reaches its highest intensity at 12 p.m., registering at a peak value of 1145 W/m². Meanwhile, on the second day of testing, this peak occurs at 1 p.m., reaching a slightly lower value of 1070 W/m². On the third day of testing, this peak occurs at 12 p.m., reaching to 1043 W/m², and on the last day of testing, this peak occurs at 12 p.m., reaching to 1086 W/m². Consequently, the highest ambient temperature is recorded at 1 p.m. on the first testing day, reaching 24 °C. On the second day, this temperature peak occurs at approximately 3 p.m., with a maximum value of 24 °C. On the third day, the temperature reaches to 24 °C at 2 p.m. and on the last day, the temperature reaches to 26 °C at 2 p.m. Air temperatures at the SAH outlet were continuously measured and documented every hour over a span of four days. The observed variability in peak solar radiation time (12:00 p.m.–2:00 p.m.) over four consecutive days aligns with findings reported in the literature. Omojaro and Aldabbagh [28] observed that the efficiency of a solar air heater reached its highest value between 12:00 p.m. and 1:00 p.m., after which it began to decline. This peak corresponds to the period when solar radiation is typically at its maximum, influenced by atmospheric conditions and the sun’s position. This consistency with previous research supports the validity of our observations. Figure 10 displays the SAH outlet air temperature values for each hour of these testing days. Notably, as the ambient temperature and solar radiation intensity rise, there is a corresponding increase in the outlet air temperature within the SAH. As the temperatures and incoming solar fluxes show a very similar profile, this phenomenon is primarily attributed to the substantial solar energy being absorbed by the collector, which is facilitated by its high absorptivity (resulting from its black color). Consequently, the rise in air temperature results from the heat emitted by the absorber through forced convection. The peak air temperatures observed at the SAH outlet on the test days typically occur around midday, specifically at 12 p.m., 2 p.m., 1 p.m., and 12 p.m. The temperatures reached 53 °C on the first day, 51.9 °C on the following day, 55.1 °C on the third day and 53.4 °C on the last day.

The numerical model’s accuracy is confirmed by comparing the simulation results with experimental measurements of outlet-air temperature. Note that in the numerical simulations, the boundary conditions for incoming flux and inlet temperature were taken at a specific time during the day from the experiments and steady state CFD simulations were performed. As the transient heat transfer phenomena have relatively large timescales, this approach significantly reduces the computational effort, yet maintaining a good accuracy. The comparison reveals excellent agreement between the experimental and numerical outcomes across the four days, as depicted in Figure 10. The relative error between experimental and numerical results was quantified as

$$\mathrm{Error}(\%)={\displaystyle \frac{T_{\exp }-T_{\mathrm{num}}}{T_{\exp }}}\times 100,$$

(12)

where $T_{\exp }$ and $T_{\mathrm{num}}$ denote the experimental and numerical outlet-air temperatures, respectively. The comparison revealed excellent agreement across the four test days, with an average error not exceeding 3%. Figure 10 presents small discrepancies, which are mainly attributed to experimental uncertainties (±0.1 °C for thermocouples, ±10 W/m² for the solar power meter, and ±5% for the anemometer, see Table 1), environmental fluctuations such as variable solar radiation and wind conditions, and modeling simplifications including constant material properties and idealized fin geometry. Despite these factors, the close alignment between experimental and numerical results validates the robustness and predictive capability of the developed model.

3.3. Asymmetric Fin Configuration

It is well-known from literature that fins enhance heat transfer in SAHs [13,29]. However, the size of the fins used in this study is much larger compared to what is common in current literature, and the number of fins (maximum 8) is also lower to allow low-cost production. This allows the flexibility to test different fin locations. In this work, a configuration was numerically evaluated where the fins are placed slightly asymmetric in the SAH, where one-half of them is placed in the shear layer of the incoming air. This will affect the flow in the SAH and hence also the heat transfer. Figure 11 shows the temperature contours for a SAH equipped with eight V-shaped fins, comparing the performance of both symmetric and asymmetric configurations (refers to the unequal spacing between the two lines of fins in the solar air heater. The space between the fins is not the same on the left and right sides). The asymmetric V-shaped fins result in a slightly more uniform temperature distribution near the outlet and a higher outlet temperature compared to the symmetric configuration. Near the fins, the asymmetry between left and right shows the influence of putting one-half of the fins in the shear layer of the incoming flow. The increased flow disturbance improves the average convective heat transfer coefficient by 3.5%, while the pressure drop increase was less than 1%. Consequently, this leads to a more balanced heat distribution across the system, enhancing the efficiency of the SAH. Additionally, the asymmetry reduces localized hot spots, further contributing to the collector’s improved thermal performance and operational stability. Therefore, in the remainder of the paper, the asymmetric fin configuration is adopted. Figure 12 presents the velocity contours for the solar collector with symmetric and asymmetric fins. The analysis of the velocity contour plot reveals that the symmetric configuration allows for better air flow from the inlet, as the inlet is positioned exactly in between the first set of fins, thereby avoiding any significant obstruction. This facilitates a smoother entry of air into the collector. In contrast, the asymmetric configuration obstructs the airflow due to the positioning of the first fins in the shear layer of the incoming flow. The right half of fins significantly obstructs the airflow from the inlet, leading to a reduced average velocity, causing the air to spend more time inside the collector. This increased residence time of the air within the collector promotes better heat absorption and transfer due to prolonged interaction with the heated surfaces. Consequently, the asymmetric configuration results in an improved thermal performance and a more uniform temperature distribution.

3.4. Temperature Profiles Inside the SAH

The temperature contours within the four SAH configurations are depicted in Figure 13. This figure provides a representation of how the air absorbs thermal energy in the different prototypes. It is evident that in the SAHs with V-shaped fins, the temperature distribution is much more uniform compared to the system without fins. Note that even in the geometry without fins, the flow is slightly asymmetric downstream due to the Coanda effect, which can create an asymmetric flow in a symmetric geometry [30,31]. To compare the different configurations, the temperature variation between the inlet and the maximum temperature in the SAH is evaluated for each setup. These temperature differences were calculated as 24 °C, 37.16 °C, 53 °C, and 62.42 °C, respectively. The air temperature undergoes a gradual rise as it moves through the SAH and the temperature differences between in- and outlet were found to be 21 °C, 28 °C, 34 °C, and 36.4 °C for the same configurations. The more fins, the higher the outlet temperature of the air, meaning a better thermal performance.

Table 6. Thermal performance analysis of SAHs with V-shaped fins.

Geometry	Surface Area of Absorber (m2)	Outlet Temp (°C)	Heat Transfer (W)	Pressure Drop $\Delta \mathit{P}$ (Pa)
Without fins	0.28	38.4	134.35	3.73
4 V-shaped fins	0.68	45.2	179.27	4.29
6 V-shaped fins	0.89	51.6	217.91	4.43
8 V-shaped fins	1.09	53.4	233.29	4.52

shows that this increased heat transfer is both due to an increased contact surface area between the collector and the surrounding air, as well as an increased average heat transfer coefficient. However, the influence of the latter becomes less dominant for an increased number of fins. As usual, the increased heat transfer comes with an increase in pressure drop. However, this pressure drop is not proportional to the heat transfer increase as a comparison between four and eight fins shows that the heat transfer coefficient is increased by 46%, while the pressure drop increase was only 5%. A key aspect herein is the size of the fins, which significantly increase the heat transfer yet with a small pressure drop increase. Therefore, eight fins is the maximum number in this study as having more fins would reduce their size to fit the SAH, going more towards conventional SAH designs.

3.5. Velocity Contours

Figure 14 presents a visual of the air velocity distribution within the reference plane of the different SAHs. The modification of fins has the effect of decreasing the air velocity. Consequently, this reduction in air speed led to an extension of the average time during which air remains within the collector’s channels. This prolonged residence time enables the air to accumulate a significantly greater amount of energy within the collector. As a result, the modification of different numbers of fins improves the efficiency of the SAHs to facilitate heat transfer from the absorber to the surrounding air. The velocity distribution within the computational domain indicates how hot air flows over the absorber, ranging from 0 to 5.14 m/s in the geometry without fins and from 0 to 3.61 m/s for the standard geometry, from 0 to 3.54 m/s for the six V-shaped and from 0 to 3.44 m/s for the eight V-shaped geometry (note that the velocity range of the colorbar in Figure 14 is not full scale to make comparison between the different configurations more clear).

3.6. Turbulent Kinetic Energy (TKE)

The irregular movement of air allows for a more effective interaction between the fluid and the heated surfaces, which lead to higher heat transfer rates. Specifically, the V-shaped fins contribute to recirculation zones and vortex formation, further intensifying the turbulence. The level of turbulence in Figure 15, as well as the resulting heat transfer enhancement, tends to increase with the number of V-shaped fins. Although turbulence increases the pressure drop, the trade-off often results in improved thermal performance for the SAH system, since the pressure drop increase is rather small (see Table 6). This dynamic interplay between smooth flow and turbulent mixing is a key factor in determining the overall efficiency of each design.

3.7. Thermo-Hydraulic Performance

The thermo-hydraulic performance factor (TPF) was used to assess the net benefit of adding V-shaped fins to the solar air heater. The TPF was calculated using the relation [32]:

$$\mathrm{TPF}={\displaystyle \frac{{\displaystyle \frac{\mathrm{Nu}}{{\mathrm{Nu}}_0}}}{{\left({\displaystyle \frac{f}{f_0}}\right)}^{1/3}}},$$

(13)

where:

• Nu = Nusselt number for the geometries with fins
• ${\mathrm{Nu}}_0$ = Nusselt number for the geometry without fins
• f = Friction factor for the geometries with fins, calculated using the relation:

  $$f={\displaystyle \frac{2\Delta P{D}_h}{\rho v^{2}L}}$$

  (14)
• $f_0$ = Friction factor for the geometry without fins

This metric provides a direct comparison of the heat transfer enhancement relative to the pressure drop penalty. Results showed that the 4 V-shaped, 6 V-shaped, and 8 V-shaped configurations achieved TPF values of 1.57, 1.97, and 2.25, respectively, indicating a substantial overall performance enhancement compared to the baseline geometry without fins. The results show a consistent improvement in TPF with increased fin count, confirming that the enhancement in heat transfer outweighs the associated increase in pressure drop. This confirms the effectiveness of the fins in improving the overall energy efficiency of the system. The 8 V-shaped fins provided the highest TPF, confirming it as the best-performing geometry in terms of overall thermal efficiency. A comparison of the TPF for the proposed configurations in this work with other roughness geometries is presented in

Table 7. Comparison of the TPF for different roughness geometries.

Reference	Roughness Geometry	TPF
Alam et al. [33]	Perforated V-type obstruction	1.30
Promvonge et al. [34]	Multiple V-shaped fins	1.87
Singh et al. [35]	V-down rib with gap	1.93
This study	V-shaped fins (4, 6, and 8 fins)	1.57, 1.97, 2.25 respectively

. The results indicate that the highest TPF value achieved in this study is 2.25. In contrast, previous studies reported a TPF of 1.80 for solar air heaters (SAHs) with V-shaped ribs and 1.93 for configurations using discrete V-down ribs. Therefore, the use of 8 V-shaped fins in the present work demonstrates superior thermal–hydraulic performance, highlighting its effectiveness over other rib geometries.

3.8. Performance Evaluation of SAH Configurations: Thermal and Thermo-Hydraulic Efficiency

The thermal and thermo-hydraulic efficiencies for the various solar air heater (SAH) configurations are summarized in Figure 16. It is clear that both efficiencies improve as the number of V-shaped fins increases. The configuration without fins shows the lowest efficiency values (41.86%), whereas the introduction of four, six, and eight V-shaped fins progressively enhances the performance. Notably, the 6 V and 8 V-shaped fin arrangements exhibit substantially higher efficiencies, reaching up to approximately 72.56% in thermal efficiency and 72.55% in thermo-hydraulic efficiency for the 8 V-shaped design. This improvement is attributed to the increased heat transfer surface area and the augmented convective heat transfer facilitated by the fins, which promote better air mixing and turbulence. Despite the potential rise in pressure drop due to additional fins, the thermo-hydraulic efficiency closely follows the thermal efficiency trend, indicating that the heat transfer benefits outweigh the hydraulic penalties. Consequently, the 8 V-shaped fin configuration offers the best balance between enhanced heat transfer and acceptable flow resistance, making it the most efficient design among the tested configurations.

Table 8. Comparison of different solar air heater configurations and their performance.

Authors	Type of Solar Air Heater	Mass Flow Rate (kg/s)	Tilt Angle (°)	Thermal Efficiency (%)
Mutar et al. [36]	Single pass SAH with high porosity metal foams	0.045	45	33–62
Suman et al. [37]	Single-pass SAH with wavy absorber plate	0.0039–0.0118	30–60	26–38
Ben Amara et al. [15]	SAH with different heights of spiral-shaped fins	0.015–0.038	34	32.5–59
Chabane et al. [38]	Single-pass SAH with longitudinal fins	0.016	45	34.92–43.94
Chang et al. [13]	Absorber featuring 6 transverse fins	0.078	-	64.38
Chabane et al. [39]	Absorber equipped with 18 fins angled at 135°	0.032	-	The highest was 73
Rajendran et al. [40]	Multiple shape arrangements combined with fins	0.01, 0.02, 0.03, and 0.04	-	77.4
Qamar et al. [41]	Corrugated fins	0.01	-	66
This study	Single-pass SAH: no fins, 4, 6, 8 V-shaped fins	0.0065	34	42, 55, 67, and 73

summarizes the thermal efficiencies of various solar air heater configurations found in the literature. Notably, the configuration proposed in this study achieves a significant efficiency, despite its relatively compact size.

To quantify the cost-efficient design,

Table 9. Estimated cost of SAH components.

Description	Quantity/Area	Cost (USD)
Steel absorber plate (black-painted)	0.68 m2 (5 kg)	8.50
Glass cover (single transparent layer)	0.28 m2	8.40
Wood frame/insulation	0.44 m2	6.60
steel support	10 kg	22.00
Black paint (absorber coating)	1 unit	3.84
Clou/fasteners/hardware	1 packet	5.00
Welding consumables (E6013 electrodes)	2 kg	4.00
Glue (Epoxy)	0.5 kg	11.00
Fan (1 used from 3-pack)	1 unit	5.30
Total Estimated Cost	–	74.64
Cost per m2	–	117.7 USD/m2

presents a quantitative cost analysis of the large V-shaped fin designs developed in this study. The total cost is approximately USD 74.64 (≈TND 215), corresponding to 117.7 USD/m² (≈341 TND/m²). In contrast, the V-trough solar air heater reported by Rayess et al. [42] had a total cost of USD 388.58, which corresponds to 305.97 USD/m². The low cost of the design in this study is primarily obtained due to the use of locally available materials (black-painted steel, wood, epoxy glue, and welding consumables). This analysis quantitatively demonstrates the cost advantage of the proposed fin configuration.

Table 10. Cost comparison of different SAH configurations.

Configuration	Steel Absorber Area (m2)	Total Cost (USD)
Without fins	0.28	54.0
4 V-shaped fins	0.68	74.64
6 V-shaped fins	0.89	84.20
8 V-shaped fins	1.09	94.50

presents a detailed cost comparison of different SAH configurations, including no fins, and four, six, and eight V-shaped fins. It can be seen that increasing the number of V-shaped fins increases the total cost only slightly.

4. Conclusions

This study investigates the thermal efficiency of a solar air heater (SAH) equipped with large V-shaped fins through a comprehensive combination of experimental and numerical analyses. The validity and accuracy of the numerical model were confirmed by comparing the predicted outlet air temperatures to measurements obtained during experiments, ensuring a reliable foundation for further analysis. The numerical investigations were systematically conducted to assess the effects of various fin configurations on the thermal performance of the SAH while maintaining a constant airflow velocity of 2.7 m/s. During the experimental test days, the SAH demonstrated a peak outlet air temperature of 55 °C, underscoring its effective heat transfer capabilities. A thorough analysis of the velocity distribution and air temperature within the SAH highlighted the significant influence of different fin designs on the internal flow dynamics. The introduction of V-shaped fins increased turbulence levels, facilitating improved fluid mixing and promoting a more uniform temperature distribution throughout the collector. This enhanced energy absorption was crucial for optimizing the overall thermal performance of the system. Among the various configurations tested, the design featuring eight V-shaped fins emerged as the optimal choice, achieving an overall thermal efficiency of 73%. This configuration not only maximized energy capture but also maintained a modest pressure drop increase. Economically, the prototype has a total cost of approximately USD 74.6 (∼215 TND), confirming its affordability and practicality for real-world applications. Overall, the findings demonstrate that the proposed V-shaped fin configurations effectively balance a high thermal efficiency, acceptable pressure drop, and low fabrication cost, making them viable solutions for solar air heating applications. Future research could further explore different materials, geometric modifications, and operational conditions to refine the design, as well as integrating drying chambers, phase change materials, thermal storage, or hybrid heating systems to enhance performance under varying solar conditions.