Numerical and Experimental Study on the Effect of Enhancement Absorber Plates on Solar Collector Efficiency for Comfort Room

A solar chimney (SC) enhances the natural ventilation of a building by encouraging heat transfer through passive solar energy (SE) and convection of heated air. In this process, air is being heated using the SE, and later, lighter air is used in the formation of a draft towards the chimney. When the hot air gains momentum in the chimney, it is removed from the collector, and the cold air is pulled outside. The geometry and the difference in temperature of the SC influence its performance. Also, the thermal energy conversion into mechanical energy occurs at the chimney base [1, 2]. SC performance has been extensively investigated with respect to geometric configuration, heat transfer (HT) mechanisms, and energy storage integration, owing to its potential as a passive ventilation and power-generation system. Several researchers have demonstrated that SC performance is highly sensitive to collector and chimney geometry. Yue et al. [3] studied the effect of SC ventilation. Albaldawi et al. [4] examined the effect of collector inclination angle and reported a noticeable improvement in thermal efficiency at optimized angles. Nie et al. [5] confirmed that three-dimensional simulations provide superior accuracy compared with one-dimensional models when predicting compressible and buoyancy-driven flow inside SCs. Ren et al. [6] further optimized rectangular SCs for greenhouse applications by adjusting collector inclination to reduce local pressure losses and enhance airflow.

Chikere et al. [7] revised preceding investigations upon improving the SC performance and introduced a new approach that combines the solar process with losses of heat energy from flue gas. Dos Santos Bernardes et al. [8] investigated the SC performance and compared several approaches for calculating the collector's heat fluxes (HFs) as well as their effect upon the performance of the SC. Buğutekin [9] studied the SC performance. Findings showed that the ideal height was 0.05 m. Moreover, the system’s performance is enhanced by the thermal mass of the ground. During the day, solar radiation (SR) warms the ground, which accumulates heat. At night, this stored heat is released to the atmosphere of the collector, thereby enhancing the performance of the regime. The SR absorption rate of the floor under the SC collector was measured by using a constraint that influences the system’s performance efficiency. Ismaeel et al. [10] presented a hybrid thermal material for thermal energy storage (TES). This substance is simple as well as capable of absorbing and storing the SE, thus enhancing performance and extending the time of working. Khidhir and Atrooshi [11] included the overall variation in thermal radiation within the conversion region and the resulting increase in temperature in response to hourly changes in ambient temperature. Sadiq [12] and Sadiq et al. [13] proposed compressed air energy storage (CAES) systems, such as biomass, low temperature geothermal, solar and waste heat, to reduce CO₂ emissions. Osama and Shyaa [14] and Sirror [15] studied a review paper about the effect of windcatcher and SC in passive cooling.

Al-Azawiey et al. [16] investigated the influence of collector height on airflow temperature and mass flow rate, demonstrating that enhanced HT directly improves system performance. Al-Azawiey and Hassan [17] enhanced the collector energy transformation efficacy and used a heat storage medium to extend its operating time. They used 6 ground substances that are obtainable in Malaysia, studying their solar thermal conversion and thermal storage capabilities experimentally and numerically.  

The novelty of this research lies in integrating ribbed and dual aluminum rib configurations into SC collectors to enhance the HT and airflow performance. In contrast to conventional flat-plate designs, the incorporation of aluminum ribs and a secondary AP promotes turbulence, disrupts thermal boundary layers, and enlarges the effective heat exchange surface. These modifications significantly improve the rate of mass flow as well as the temperature of the outlet air. Moreover, three-dimensional simulation of computational fluid dynamics (CFD) and practical validation are employed in this study, which guarantees reliable findings and presents an expediency of natural ventilation systems development together with low cost to create a natural ventilation system to enhance indoor comfort.

CFD is used to study the performance of fluid systems prior to manufacturing. In this work, CFD is used to simulate the flow of air throughout a solar collector, thereby enabling the impact analysis of adding ribs through ANSYS Fluent, advanced software for the flow of fluid modeling HT in complex geometries. The finite volume technique was employed for solving the equations of continuity, momentum [20], and energy through a steady-state analysis. Air was used as the working fluid to simulate the real-life fluid flow, and it was modeled as a perfect gas. And the buoyancy model was used to enhance simulation accuracy, and gravity was set to (-g) in the (y) direction. A reference pressure of 1 standard atmosphere (atm) was used to ensure reliable findings [11].

3.1 Creating geometric models

SolidWorks was employed to generate the geometric model of the system (see Figure 2), which was subsequently discretized using a tetrahedral mesh. A tetrahedral mesh is a three-dimensional discretization technique in which each element takes the form of a tetrahedron, a polyhedron with four triangular faces. Such a mesh is especially beneficial in finite element analysis (FEA), CFD, and other numerical simulations due to the flexibility of dealing with very complex and irregular geometry. Tetrahedral meshes are also more capable of fitting irregularly shaped surfaces and curved boundaries with greater accuracy than organized meshes (which are bounded by regular structures), meaning that high-fidelity simulations in such areas are more likely to occur. In addition, tetrahedral elements facilitate automatic meshing in computer-aided design (CAD) and simulation programs, saving time spent on manual mesh generation. Tetrahedral elements are also favored in early design studies and design optimization investigations.

(a)

(b)

(c)

(d)

Figure 2. (a) Mesh of the SC model, (b) mesh of the ribs at the absorber plate (AP), (c) mesh of the insert AP, and (d) longitudinal ribs on the insert AP

Table 1. Mesh details

The meshing approach adopted in this study is particularly effective for the geometries and flow fields that require fine resolution in specific regions of the computational domain. A mesh tetrahedron type was employed to ensure higher accuracy and numerical stability. Table 1 summarizes the mesh characteristics, while the thermo-physical properties of the aluminum plate and air were obtained directly from the ANSYS material database [21].

The 3D, steady-state conservation equations of mass, momentum, and energy are now explicitly stated and referenced. Air is modeled as an ideal gas under the Boussinesq approximation, with buoyancy activated via gravity in the vertical direction.

The turbulence model is clearly specified by using the k-ε model consistent with the buoyancy correction. The experimental absorber coating included surface emissivity and absorptivity values.

The resulting mesh configurations for the four considered case studies are illustrated in Table 1. SolidWorks was utilized to develop geometric models for the four SC case studies. In Case 1, a flat aluminum AP represented the baseline reference. Case 2 featured 12 flat aluminum ribs attached to the AP to generate turbulence and disturb the thermal boundary layer. Case 3 added a secondary AP positioned at the mid-plane of the collector, dividing the airflow into two channels. In Case 4, longitudinal flat aluminum ribs were incorporated on the secondary plate to intensify localized mixing and improve heat absorption. This modeling approach can be seen in Figure 3, ensuring accurate CFD analysis and enabling consistent comparison of thermal and flow performance across the four configurations.

(a) Case 1

(b) Case 2

(c) Case 3

(d) Case 4

Figure 3. SolidWorks software program for solar chimney case studies

3.2 Governing equations

These equations for simulation and the assumptions for the 3-D steady-state model are presented here. And the continuity formula can be expressed as [22]:

$\frac{\partial}{\partial z}(\rho u)+\frac{1}{r} \frac{\partial}{\partial r}(\rho v)=0$          (1)

$\begin{aligned} \frac{\partial}{\partial \mathrm{z}}(\rho u u)+\frac{1}{r} \frac{\partial}{\partial \mathrm{r}} & (\rho u v)=\frac{\partial p}{\partial z}+\left(\rho-\rho_0\right) +2 \frac{\partial}{\partial \mathrm{z}}\left[\left(\mu+\mu_t\right) \frac{\partial \mathrm{u}}{\partial \mathrm{z}}\right] +\frac{1}{r} \frac{\partial}{\partial \mathrm{r}}\left(\mu+\mu_t\right) r\left(\frac{\partial u}{\partial z}+\frac{\partial v}{\partial r}\right)\end{aligned}$          (2)

$\begin{gathered}\frac{\partial}{\partial \mathrm{z}}(u T)+\frac{1}{r} \frac{\partial}{\partial \mathrm{r}}(r u T)=\frac{1}{\rho}+\frac{\partial}{\partial z}\left[\left(\frac{\mu}{P r}+\frac{\mu_t}{\sigma_t}\right) \frac{\partial \mathrm{T}}{\partial \mathrm{z}}\right] +\frac{1}{\rho r} \frac{\partial}{\partial \mathrm{r}}\left[\left(\frac{\mu}{P r}+\frac{\mu_t}{\sigma_t}\right) r \frac{\partial \mathrm{~T}}{\partial \mathrm{z}}\right]\end{gathered}$              (3)

The thermal expansion coefficient $(\beta)$ is calculated by Eq. (1), assuming small temperature changes.

$\beta=\frac{1}{\rho}\left(\frac{\partial \rho}{\partial T}\right)_P \approx \frac{1}{\rho}\left(\frac{\rho_o-\rho}{T_o-T}\right)$             (4)

The density of air $(\rho)$ into a flow can be mathematically calculated by this equation:

$\left(\rho-\rho_o\right) \approx \rho_o \beta\left(T-T_o\right)$          (5)

3.3 Boundary operating condition and setup setting

To ensure the mass and momentum are conserved across the domain, the natural buoyancy-driven inflow and outflow of air under solar heating were simulated. And the AP, which was produced from the aluminum, was thermally characterized with an emissivity of 0.95 and an absorptivity of 0.9, reflecting its high capability to absorb incident SR and re-radiate energy as heat. The black-painted aluminum surface used in the experimental setup realistically models radiative and convection heat transfer (CHT) interactions within the collector. And the thermo-physical properties of materials considered in the model, including air, glass, steel, wood, and insulation, are summarized in Table 2. The complete set of applied boundary conditions is illustrated in Figure 4, which provides a schematic illustration of the computational domain, comprising inlet and outlet zones, heated AP, and surrounding collectors and chimney structure. Together, these conditions ensure that the numerical model accurately reproduces the thermal as well as the fluid dynamic environment of the experimental SC system.

Table 2. Thermal-physical properties of the material

Figure 4. The boundary conditions of computational domain

5.1 Validation

The Case 1 (baseline flat AP without ribs) outcomes were confirmed via the comparison of the simulated thermal performance with the investigational results reported by Al-Azawiey et al. [16]. The validation was conducted over an eight-hour operating period (08:00–15:00), which encompassed the morning increase in SR, thereby representing the full daily variation in collector behavior. As shown in Figure 8, the predicted temperature of the outlet air and the rate of mass flow exhibited a close correlation with the reference results, validating the CFD model's reliability in capturing the diurnal performance of the SC. Both datasets displayed similar trends, with temperature and airflow gradually increasing in the morning as SR intensified, reaching peak values around midday, and subsequently decreasing in the afternoon. The alignment between the present simulation and the benchmark results demonstrates that the governing equations, boundary conditions, and material property assumptions employed in this study are appropriate.

Figure 8. Validation between this study and Al-Azawiey et al. [16]

5.2 Numerical result

The simulated temperature spreading within the solar collector for different design configurations is presented in Figure 8, clearly illustrating the influence of ribs on the HT process. Figure 9 represents the baseline case without any internal enhancements, where the gradient of temperature between the inlet and the outlet air is relatively small, indicating limited heat absorption from the absorb plate to the airflow.

Figure 9. Temperature contour of the AP

In contrast, Case 2 (Figure 10) illustrates the configurations of different ribs. This improvement is attributed to the ability of the ribs to induce localized turbulence, disrupting the thermal boundary layer, and allowing for more effective energy transfer from the plate to the airflow. Monitoring AP temperature profiles is therefore essential, as excessively high plate temperatures may signal inefficient energy extraction, while uniformly elevated outlet air temperatures confirm successful enhancement of HT. These observations underscore the structural modifications, such as the rib integration, can be an effective, low-cost strategy for improving the SC collector performance, especially under the NC conditions. Figure 11 illustrates the insertion of double AP to enhance the temperature difference and maximize the heat transfer to the air. The existence of the eddy plate ensured that the total amount of surface area used to exchange the heat was maximized and further spreading of air was facilitated, which augmented the heat absorption capacity of the system. Case 4 (Figure 12) was constructed because of this structure and gave flat aluminum ribs to the eddy AP surface. These ribs were flow disruptors that created regulated turbulence in the stream of air. The turbulence created a good opportunity to penetrate the thermal boundary-layer, which usually develops on the HT surfaces, enhancing the CHT between the moving fluid and plate. This process brought a faster and better HT between the absorbing and the airflow. Therefore, the ribbed eddy plate design recorded significantly higher air temperatures than the other arrangements, at the peak outshine conditions, especially at around 1:00 p.m., where the solar intensity was at its peak, and the flow rates due to buoyancy were at their highest rate.

Figure 10. Temperature contour of the AP

Figure 11. Temperature contour of the eddy AP temperature

The exhaustive numerical research in four case studies indicated that Cases 2 and 4 recorded the highest gain in HT between the AP and the solar collector airflow. In Case 2, the enhanced thermal performance was mostly explained by a simultaneous increase of the mass flow rate and the temperature difference, which suggested that the introduction of the flat aluminum ribs was efficient in breaking the thermal boundary layer, increasing stronger turbulence and enhancing CHT, keeping the high airflow rates. This two-fold enhancement not only aids in the increased transfer of energy in the warmed surface of the absorbent to the flowing air but also helps towards an additional even distribution of temperature in the flow way. On the other hand, in Case 4, the improvement that took place was mostly because of a significant rise in temperature difference as opposed to a relative rise in the rate of mass flow. The longitudinal rib arrangement here increased localized mixing and heat absorption that resulted in the increase in air temperature, but since the mass flow in the generated flow was overall constrained by the induced resistance to flow a little. These trends can be well illustrated by Figure 13, which shows the unique thermal and velocity field pattern between the two layouts and the trade-off existing between the flow rate and temperature gain in optimizing rib geometry as used in the SC applications. Gradual acceleration results in passing air through a solar collector due to the unremitting absorption of the air through the AP, leading to the heating of the air that lowers its density. This acceleration due to buoyancy is more visible as the air nears the chimney inlet, within which a degree of sudden increase of velocity is noticed in both Case 2 and Case 4 (Figure 14). The sudden increase at this point is explained by the narrowing and increased pressure difference between the bottom of chimney and the outlet of collector, causing the hot air to ascend with more velocity. The effect of ribs on the floor of the AP is to change dramatically this velocity behavior; the ribs introduce turbulence locally in the smooth laminar boundary layer and improve the ability to mix the airflow. In addition to disruptions in the otherwise stable velocity profile, this turbulence amplifies the CHT between the AP and the moving air. This causes the collector velocity distribution to be more non-uniform with jets of higher velocity being in the downstream of the ribs and eventually leads to better thermal performance than the smooth, rib-free design.

Figure 12. Contour of the eddy absorb plate temperature

(a) Case 2

(b) Case 4

Figure 13. Contour of the difference of temperature between the collector inlet and outlet of the (a) Case 2 and (b) Case 4

(a) Case 2

(b) Case 4

Figure 14. Contour of the air velocity in the collector with ribs (a) Case 2 and (b) Case 4

Figure 15. Behavior of the four case studies (mass flow rate ($\dot{m}$) × temperature difference (∆T))

Figure 15 represents a relative comparison of the numerically predicted value with the experimental value of the key performance parameters of the SC regime that are the temperature of AP, the temperature of airflow and air velocity in the inlet of the chimney, which have all been obtained under the same operating conditions to provide a fair comparison.

5.3 Experimental result

The approaches show strong consistency, with CFD simulations generally predicting slightly higher values than those recorded experimentally. Both the numerical results, the findings of Al-Azawiey et al. [16], and the experimental data confirm a clear linear relationship between the collector component temperatures and the measured solar Intensity (SI), demonstrating that an increase in SR directly leads to a proportional rise in temperature. Moreover, Figure 16 illustrates that the AP temperature increases with solar intensity for all the case studies, while Case 2 recorded comparatively lower temperatures due to its enhanced PI, reflecting improved HT from the plate to the airflow.

Figure 16. Experimental result for mass flow rate ($\dot{m}$) × temperature difference (∆T)

The observed differences can be attributed to inevitable factors such as minor experimental setup inaccuracies, sensor calibration limitations, and variations in outdoor environmental conditions. As shown in Figure 17, the structural modifications significantly influenced performance, with the results compared using the combined thermal performance parameter ($\dot{m}$ × ∆T), where $\dot{m}$ represents the mass flow rate and ∆T the temperature rise. The numerical simulations clearly highlight the positive effect of adding ribs to the AP in the SC system. Both experimental and numerical approaches reveal a consistent trend, confirming that ribbed configurations deliver superior performance compared to the baseline.

Figure 17. Comparisons between experimental and numerical results