Improving the Performance of Unglazed Solar Air Heating Walls Using Mesh Packing and Nano-Enhanced Absorber Coating: An Energy–Exergy and Enviro-Economic Assessment

Abstract

This study aims to upgrade the effectiveness of unglazed solar air heating walls (SWs) using mesh packing and nano-enhanced black paint. In this regard, two SW cases with 10 cm and 15 cm plenum thicknesses have been fabricated and tested simultaneously with different modifications. In other words, six different SW configurations have been designed and empirically investigated in this research. Unmodified SWs with two plenum thicknesses have been tested in the first experiment. Iron meshes have been utilized in both SWs in the second test. In the third experiment, the impact of the combined usage of mesh packing and Fe (iron) nanoparticle-enhanced black paint (absorber coating) at 2% w/w concentration on the performance has been evaluated. Experimental results exhibited that the combined usage of mesh packing and nano-doped paint in the SWs with 10 cm and 15 cm plenum thicknesses improved the average effective efficiency value by 29.54% and 31.20%, respectively, compared to the unmodified cases. Also, the average exergy efficiencies of the six tested SW configurations were attained in the range of 6.24–12.29%. Moreover, the findings of this study showed that reducing the plenum thickness and applying the combination of meshes and nano-coating improved the annual carbon dioxide savings by 44.72%.

1. Introduction

The energy demand of the world is increasing day by day due to the growth in the population [1,2]. Therefore, governments and researchers are looking for alternatives for a secure, clean, and continuous energy supply. It should be noted that a large percentage of energy usage is covered by buildings. Therefore, sustainable solutions should be developed to meet both the electrical and thermal energy demands of buildings. Solar energy is one of the efficient and environmentally friendly alternatives that can be utilized in building systems [3,4], and it can be used effectively in many applications such as electricity generation [5,6], space heating [7,8] and domestic hot water production [9,10,11]. It is also possible to simultaneously generate thermal and electrical energies using hybrid systems [12].

Space heating is an important requirement in buildings and should be met in an efficient and environmentally friendly way [13]. Solar air heating systems (SAHSs) can be used for this purpose [14]. SAHSs can be designed as solar air heating walls (SWs) for the integration of these systems to the buildings such as façades. There are also some configurations of SWs in the form of Trombe walls (TWs) and hybrid photovoltaic–thermal (HPVT) systems. Singh et al. [15] developed combined SAHSs with a TW. The developed systems have lower payback periods and environmental influence in comparison to conventional systems. Paraschiv et al. [16] performed a techno-economic evaluation of SAHSs structured as SWs and showed that using SWs in buildings can reduce heat losses between 9.8 and 12.51%. Xu et al. [17] structured HPVTs as SWs for electricity generation and air–water heating purposes. They obtained electrical efficiency in winter and summer of 12.5% and 7.6%, respectively. Also, the water tank temperature reached 40 °C in summer. Qi et al. [18] numerically investigated the effect of wall structure on the effectiveness of a two-channeled SW with a porous surface. They found that the temperature of a porous wall is constantly greater than the temperature of the surrounding environment. Charvát et al. [19] developed vertically placed SAHSs with phase change materials. According to the findings, when the solar radiation intensity changes quickly, the outlet air temperature fluctuations may be lessened by the phase change material combined with the absorber plate. Hong et al. [20] developed a Venetian blind-integrated TW in their numerical study. They aimed to regulate the shading and flow of air in the TW in their work. Using Venetian blinds improved cooling energy savings in the range of 5.0–5.8%. In another work, Luo et al. [21] analyzed an HPVT SW system experimentally at various seasonal climatic conditions. The total yield of the SW was 63.4% in summer climatic conditions. Khanlari et al. [22] designed different glazed SW configurations in their numerical and experimental work. According to their results, the parallel-pass flow structure exhibited better performance results. Habib et al. [23] developed a vertical SAHS with black painted iron chips and a nanocomposite. Using a nanocomposite improved the stored thermal energy at forced and natural convection modes by 21.2% and 20.7%, respectively, in comparison to the pure paraffin. Gandjalikhan Nassab and Moein Addini [24] designed and tested an SAHS with flexible winglets for space heating purposes in their numerical work. Using winglets led to an improvement in natural airflow rate of 56%. Moghadasi et al. [25] experimentally analyzed a vertically placed octagonal SAHS to be utilized in buildings. The payback period of the system was found to be 5.17 years. Moreover, the daily thermal yield was found to be approximately 79%. Hatamleh et al. [26] used the phase change material in a building-integrated SAHS in four locations. Using the developed SAHS improved the yearly energy savings by more than 325 kWh. Abu-Hamdeh et al. [27] used a phase change material-integrated SAHS. Their approach decreased the energy consumption of the building by 5.6%.

Mesh packing applications can upgrade the heat transfer process in solar air heating applications [28]. There are many various models of packed bed solar air heating systems (SAHSs) available in the literature [29,30,31]. Singh et al. [32] analyzed a two-flow SAHS with mesh packing. The maximum thermohydraulic efficiency attained was 80% according to their experimental results. Nowzari et al. [33] developed an SAHS with a packed bed and pierced transparent cover. The average thermal efficiencies for single and double-flow SAHSs were attained in the ranges of 39.7–55.2% and 45.4–60.8%, respectively. Jasim Mahmood [34] analyzed an unglazed double-flow SAHS with a perforated absorber and mesh packing. The highest instantaneous temperature difference in the system was 38.6 °C at a 0.003 kg/s flow rate. Chouksey et al. [35] used black-painted wire meshes in an SAHS for performance improvement. The highest thermal performance enhancement value was 54% in comparison to the unmodified system. Additionally, mesh structures have been also utilized in the air channel in heating parts (SAHSs) of solar dryers and significantly improved both thermal and drying performances [36,37].

Nanoparticles are materials with advanced thermophysical properties and can be used for different purposes in thermal applications [38,39]. Nanofluids are the most common applications of nanoparticles [40]. Nanofluids gave superior results in comparison to the conventional working fluids in thermal energy systems [41]. In recent years, nanoparticles have been used in the absorber coating material of solar energy systems for increased thermal conductivity [42,43]. Darmaraj et al. [44] proposed an SAHS with baffles and graphene nano-doped black paint and increased mean thermal efficiency by 13.24%. Khanlari et al. [45] developed a vertically placed SAHS worked as an SW with holed baffles and nano-coating. The exergetic efficiency was improved in the range of 9.25–10.58%. Sivakuar et al. [46] used cupric oxide nanomaterials in the paint of an SAHS and used the system in a drying process. According to their experimentally obtained results, the drying period decreased by 6% using a nano-coating. Kumar et al. [47] used nano-coating in a triangle-shaped SAHS. Employing nano-coating with graphene, the highest enhancement in efficiency was attained as 48.23%. Al-Kayiem et al. [48] used alumina nanoparticles in the solar vortex generator. The temperature increased 28.5% compared to the conventional system because of using nanoparticles.

This research aims to improve the performance of solar air heating walls (SWs) using iron meshes and an iron nano-enhanced absorber coating. In this regard, two SW boxes have been manufactured with varying plenum thicknesses. In the first test, unglazed SWs with two plenum thicknesses have been examined. In the second test, the SWs have been modified with mesh packing material. In the third experiment, the effect of simultaneous utilization of mesh packing and nano-coating on the performance has been analyzed. Different from other works, the combined usage of mesh packing and a nano-enhanced absorber coating has been tested in unglazed SWs for the first time. The major steps of the present work are shown in Figure 1.

2. Materials and Methods

In this work, we aim to improve the performance of unglazed SWs using mesh packing and nano-doped black paint. In this context, two types of absorber coating materials have been utilized in the systems. The first coating material is conventional matt black paint. Iron (Fe) nanoparticles were mixed with matt black paint in the second coating material for enhanced thermal conductivity. Fe nanoparticles with 2% w/w concentration have been used to improve the performance of SWs. The mixture has been mechanically mixed and sonicated to attain a homogeneous nano-doped coating. The preparation steps of the nano-doped coating are presented in Figure 2 as a schematic illustration. It should be indicated that the average thermal conductivity was improved 7.13% (from 0.6392 to 0.6848 W/m·K) by adding Fe nanoparticles. It should be noted that the particle concentration value and preparation techniques were applied considering previous scientific studies [42,46,47,49,50].

In this work, two SW cases with 10 cm and 15 cm plenum thicknesses were developed and manufactured to evaluate the impact of plenum depth on the thermal and enviro-economic performance. The SW cases were manufactured using a 0.1 cm thick aluminum sheet. Axial suction fans with 40 W power were placed to each system. The SWs with 10 cm and 15 cm plenums were named SW1 and SW2, respectively. Also, guiding plates were placed on the air channels of the SWs to increase the residence time of air. Aluminum sheets with 0.1 cm thickness were used as absorber surface. In the first test (Exp. 1), developed SWs were tested, and this experiment has been used as a control group for a comparison of examining the impacts of the modifications in the further experiments (Exp. 2 and Exp. 3). In Exp. 2, iron meshes have been used to upgrade the heat transfer and turbulence intensity. Previously, the effectiveness of iron meshes in different solar–thermal systems has been numerically and experimentally analyzed [36,37]. Iron meshes are cost-effective extended heat transfer surfaces and can be used in various solar energy systems. However, iron meshes may have some disadvantages such as the possibility of corrosion in long-term use. Since the iron meshes are quite low cost and available in the worldwide market, they can be replaced easily for maintaining the SWs. It should be noted that 12 iron meshes have been used in each system. Iron mesh-integrated SWs with 10 cm and 15 cm plenum thicknesses were named as SW1/MP and SW2/MP, respectively. In Exp. 3, the developed nano-doped black paint was applied to the absorber surfaces of the mesh-integrated systems. The aim of Exp. 3 is to evaluate the impact of combined employment of mesh packing and nano-doped paint on the performance of SWs. SWs containing mesh packing and nano-paint with 10 cm and 15 cm plenums were named SW1/MP-NP and SW2/MP-NP, respectively. It should be indicated that SWs with 10 cm and 15 cm plenums have been tested simultaneously at the same environmental conditions. A schematic view and photographs of the experimental setup are presented in Figure 3 and Figure 4, respectively. The analyzed SWs in this study have been designed according to the previous studies of our research group [22,45]. In previous studies, the applicability of SWs with various modifications has been analyzed. Different from previous studies, in the current work, the influences of using two modifications on the thermal performance of unglazed SWs have been experimentally analyzed. The geometrical parameters have no significant impact on performance improvement of the nano-coating. However, as stated, adding mesh modification in different systems may have various effects. Therefore, it is necessary to investigate the effect of using iron mesh modification in different systems.

Heat transfer mechanisms in the SW are illustrated in Figure 5. The general perspective of heat transfer in the SW is shown in the figure. Incoming solar rays are absorbed by the metal plate of the system. The working fluid is heated up in the plenum of the SW by convection heat transfer from the absorber surface. In other words, a large portion of the heat transfer to the air occurs by convection heat transfer. Also, a part of the gained energy by the absorber surface is lost to the environment by radiation and convection. By adding guiding plates and iron meshes, the heat transfer surface area increases, and heat transfer to the air by convection improves. There are some systems available in the literature that integrated extended heat transfer surfaces to SWs. In the present work, iron meshes with guiding plates were integrated to SWs different from previous studies for enhancing the thermal performance.

The experiments have been performed in Burdur, Turkey, in September. The experimental process in this work was completed in three days. Tests were started and terminated at 09:00 and 17:00, respectively. The temperature was measured every 30 s, and other metrics have been recorded at one-hour intervals. Temperature values of the environment, inlet and outlet of SWs have been measured and saved. Temperature was measured using K-type thermocouples and saved using dataloggers. A solarimeter and an anemometer were used to measure solar radiation and air velocity, respectively. A multimer has been used to measure the electric current and voltage of the fan to determine the energy consumption of the integrated fans. Additionally, a differential manometer was used to measure pressure drop in the developed systems. All measured parameters were then transferred to a computer for further evaluation. Moreover, the test plan of this work is presented in

Table 1. Test plan.

Exp. #	Tested Systems	Plenum Thickness	Modification	Absorber Coating
1	SW1	10 cm	-	Conventional black paint
	SW2	15 cm	-	Conventional black paint
2	SW1/MP	10 cm	Iron mesh packing	Conventional black paint
	SW2/MP	15 cm	Iron mesh packing	Conventional black paint
3	SW1/MP-NP	10 cm	Iron mesh packing	Nano-enhanced black paint
	SW2/MP-NP	15 cm	Iron mesh packing	Nano-enhanced black paint

. As can be seen from the given table, SWs with two different plenum thicknesses have been simultaneously tested with various configurations. In the first test (Exp. 1), conventional black paint was selected as an absorber coating material, and only guiding plates were integrated into the system. In Exp. 2, iron meshes have been added to the air channel of the SWs. In Exp. 3, the effect of combined usage of nano-enhanced black paint and mesh packing on the thermal performance has been analyzed. Also, details of the measurement devices can be seen in Figure 3.

3. Theoretical Calculations

The rate of solar radiation receiving by the SW could be defined as below:

$${\dot{En}}_{sun}=A_{abs} I,$$

(1)

In Equation (1), $I$ and $A_{abs}$ represent the incident solar radiation (W/m²) and the area of the absorber surface (m²), respectively.

Useful thermal energy gained by the SW can be achieved using the following expression [22]:

$${\dot{En}}_{thr}=\dot{m}{c}_p\left(T_{out}-T_{in}\right),$$

(2)

In the above expression, $c_p$, $\dot{m}$, $T_{out}$ and $T_{in}$ denote the specific heat capacity of flowing air (kJ/kg·K), mass flow rate of air (kg/s), and outlet and inlet temperatures of air (K) in the system, respectively.

The specific heat capacity of working fluid (air) could be calculated as follows [51]:

$$c_p=1009.26-\left(0.0040403T\right)+\left(0.00061759T^2\right)-\left(0.0000004097T^3\right),$$

(3)

The air’s mass flow rate in the SW could be found utilizing the expression below:

$$\dot{m}=\rho \dot{Q},$$

(4)

In Equation (4), $\rho$ and $\dot{Q}$ show the density (kg/m³) and volumetric flow rate (m³/s) of air, respectively.

The density of air can be found as below [52]:

$$\rho =353.44/T_f,$$

(5)

In Equation (5), $T_f$ presents the temperature of fluid (air).

Moreover, the thermal (energy) efficiency (%) of the SW can be calculated using the equation below:

$${\eta }_{thr}=\frac{{\dot{En}}_{thr}}{{\dot{En}}_{sun}}=\frac{\dot{m}{c}_p\left(T_{out}-T_{in}\right)}{A_{abs}I},$$

(6)

Due to high energy loss during conversion and transmission, the effective efficiency is a key factor which determines the amount of energy that is gained when primary energy is converted to mechanical energy and could be found as [53]:

$${\eta }_{eff}=\frac{{\dot{En}}_{thr}-\left(P_m/C_f\right)}{I{A}_{abs}},$$

(7)

where $C_f$ is the conversion factor (−), and this value could be assumed as 0.18 as reported by Cortes and Piacentini [54].

Moreover, the mechanical power needed to blow the air in the SW can be found as below [54]:

$$P_m=\frac{\dot{m}∆P}{\rho },$$

(8)

where $∆P$ denotes the pressure drop (Pa).

In addition, the coefficient of performance (−) is a significant parameter in evaluating solar–thermal systems, and it can be calculated for an SW using the following expression [55]:

$$COP=\frac{{\dot{En}}_{thr}}{{\dot{W}}_f}=\frac{\dot{m}{c}_p\left(T_{out}-T_{in}\right)}{{\dot{W}}_f},$$

(9)

where ${\dot{W}}_f$ (W) is the energy usage of the used fan in the SW. In this work, axial fans with 40 W power have been utilized in each SW to provide airflow.

Exergy expression for the SW can be written as shown below [56]:

$$\sum {\dot{Exg}}_{in}-\sum {\dot{Exg}}_{out}=\sum {\dot{Exg}}_{dst},$$

(10)

where ${\dot{Exg}}_{in}$, ${\dot{Exg}}_{out}$ and ${\dot{Exg}}_{dst}$ depict the inflow, outflow, and destructed exergy rates (W), respectively.

Inflow exergy for SW can be found by summing the received exergy rate by SW from the incident solar rays and exergy rate of inlet air. It can be written as below:

$$\begin{gathered} {\dot{E}xg}_{in}=A_{abs}I\left[1-\frac{4}{3}\left(\frac{T_{amb}}{T_{sun}}\right)+\frac{1}{3}{\left(\frac{T_{amb}}{T_{sun}}\right)}^4\right]+{\dot{m}c}_p\left[\left(T_{in}-T_{amb}\right)- \\ {T}_{amb}ln\left(\frac{T_{in}}{T_{amb}}\right)\right], \end{gathered}$$

(11)

In Equation (11), $T_{sun}$ represents the solar temperature (6000 K).

The outflow exergy in the SW system can be given below [45]:

$${\dot{E}xg}_{out}=\dot{m}{c}_p\left[\left(T_{out}-T_{amb}\right)-T_{amb}ln\left(\frac{T_{out}}{T_{amb}}\right)\right],$$

(12)

The exergy efficiency of SW could be defined as [57]:

$${\eta }_{exg}=\frac{{\dot{E}xg}_{out}}{{\dot{E}xg}_{in}}=\frac{\dot{m}{c}_p\left[\left(T_{out}-T_{in}\right)-T_{amb}ln\left(\frac{T_{out}}{T_{in}}\right)\right]}{A_{abs}I\left[1-\frac{4}{3}\left(\frac{T_{amb}}{T_{sun}}\right)+\frac{1}{3}{\left(\frac{T_{amb}}{T_{sun}}\right)}^4\right]},$$

(13)

Additionally, the sustainability index (−) is an exergy-based performance parameter which could be used in solar–thermal systems [58]. It can be found using Equation (14) [59]:

$$SI=\frac{1}{1-{\eta }_{exg}},$$

(14)

In addition to the energetic and exergetic analyses, the overall economic and environmental feasibility of renewable energy systems should be investigated. The capital cost ($CC$) of the SW could be calculated by summing up the expenses of the employed components in the manufacturing process. The capital cost values of SW1, SW2, SW1/MP, SW2/MP, SW1/MP-NP and SW2/MP-NP are 86.4, 94.8, 90.4, 98.8, 97.6 and 106 USD, respectively.

Additionally, the capital recovery factor used in the analyses can be written as:

$$CRF=\frac{i{\left(1+i\right)}^n}{{\left(1+i\right)}^n-1},$$

(15)

In Equation (15), $i$ and $n$ depict the interest rate and lifespan of the SW. These values were considered as 10 percent and 20 years, respectively.

The annual cost of the SW can be found as:

$$AC=\frac{CC}{\frac{1}{i}\left[1-{\left(\frac{1}{1+i}\right)}^n\right]},$$

(16)

The levelized cost of heating (USD/kWh) is a crucial performance metric in the cost-based evaluation of SWs, and it represents the economic feasibility of the system. This parameter depicts the mean paid monetary value for each heating unit that the SW will generate during its lifespan. It can be calculated as follows [60]:

$$LCOH=\frac{CC+CRF+AMC}{{\dot{En}}_{thr,useful}},$$

(17)

where $AMC$ shows the yearly maintenance expenses (cost) (USD), and this value is assumed as 2% of the $CC$.

The payback period of the SW is used to estimate the cost performance and reliability of the system. This metric was investigated to determine the reliability, suitability, and cost-effectiveness of SWs. It shows the time that is necessary for cumulative savings to equal the total investment expenses. It can be found below [55]:

$$PBP=\frac{CC}{FDS+{\phi }_{{CO}_2}},$$

(18)

In Equation (18), $FDS$ presents fuel depletion savings, which could be calculated as shown below [55]:

$$FDS=\frac{{\dot{En}}_{thr,useful}}{{\eta }_{thr}},$$

(19)

Additionally, enviro-economic investigation is utilized to estimate carbon emission savings and support renewable energy usage. ${\phi }_{{CO}_2}$ is yearly savings in CO₂ (ton/year), and this metric can be calculated as follows:

$${\phi }_{{CO}_2}=\frac{{\psi }_{{CO}_2}{\dot{En}}_{thr,useful,total}}{1000},$$

(20)

In Equation (20), ${\psi }_{{CO}_2}$ illustrates the mean CO₂ emission for power production using coal, and it is assumed as 2.08 kgCO₂/kWh [61].

Overall, the experimental uncertainty equation can be given as [62]:

$$W_R=\sqrt{\left[{(\frac{\partial R}{\partial x_1}{w}_1)}^2+{(\frac{\partial R}{\partial x_2}{w}_2)}^2+\dots +{(\frac{\partial R}{\partial x_n}{w}_n)}^2\right]},$$

(21)

In Equation (21), $R$ is the function uncertainty, $x_1$, $x_2$, …, $x_n$ are independent metrics and $w_1$, $w_2$, …, $w_n$ are uncertainties in the independent metrics. The calculated mean uncertainties for velocity, temperature, solar radiation, and effective efficiency are ±0.40 m/s, ±0.52 °C, ±16.46 W/m² and ±1.62%, respectively. Achieved mean uncertainties are in good accordance with similar research on solar–thermal technologies [63,64,65,66].

4. Results and Discussion

As stated before, “4E (energy, exergy, economic and environmental)” analysis has been conducted for developed SWs within the scope of the current study. The Results section (Section 4) has two areas of analysis: thermodynamic (energy-exergy) performance findings and enviro-economic investigation results.

4.1. Thermodynamic Performance Results

As mentioned in the previous sections, three different SW configurations for two plenum thicknesses were examined in this empirical study. Figure 6 shows time-dependent variations in environmental conditions among the performance tests. Solar radiation values in Exp. 1, 2 and 3 were in the ranges of 590–846 W/m², 579–853 W/m² and 580–838 W/m², respectively. Average solar radiation values were obtained in Exp. 1, 2 and 3 as 724 W/m², 729 W/m² and 723 W/m², respectively. It must be noted that the solar radiation values were measured from the vertical face for reliable performance calculations. Moreover, the average ambient temperature values in Exp. 1, 2 and 3 were 23.66 °C, 23.83 °C and 23.14 °C, respectively. As can be seen, the mean ambient temperature and solar radiation values are very close to each other, which facilitates a reliable comparison between the proposed system configurations.

Figure 7 presents time-dependent changes in temperature differences in air and gained useful thermal energy. In Exp. 1, the average gained heat for SW1 (unmodified system, plenum thickness: 10 cm) and SW2 (unmodified system, plenum thickness: 15 cm) was 89.25 W and 80.99 W, respectively. Also, the mean temperature differences between the outlet and inlet air for SW1 and SW2 were found to be 8.40 °C and 7.62 °C, respectively. In Exp. 2, the average useful heat values for mesh packed systems including SW1/MP and SW2/MP were 103.72 W and 94.0 W, respectively. Moreover, the average temperature difference values for SW1/MP and SW2/MP were 9.77 °C and 8.90 °C, respectively. As it can be seen, placing iron meshes in the system improved the average gained heat by 16.21% and 16.06%, respectively, for plenum thicknesses of 10 and 15 cm. In Exp. 3, the effect of the simultaneous application of mesh packing and nano-doped paint on the performance has been analyzed for two plenum thicknesses. In this experiment, the average gained heat for SW1/MP-NP and SW2/MP-NP was 117.14 W and 107.8 W, respectively. Additionally, the obtained mean temperature differences for SW1/MP-NP and SW2/MP-NP are 11.00 °C and 10.1 °C, respectively. As can be seen, the simultaneous application of mesh packing and nano-doped absorber coating improved the average gained heat for the SWs with 10 cm and 15 cm plenum thicknesses as 31.24% and 33.10%, respectively, compared to the base (conventional) cases.

Figure 8 shows time-dependent alterations in effective efficiency values. Effective efficiency considers the pressure drop in the system, which is the biggest difference between thermal (energy) efficiency. In Exp. 1, the instantaneous effective efficiency values for SW1 and SW2 ranged between 36.66 and 60.85%, respectively. The obtained average effective efficiencies for SW1 and SW2 in Exp. 1 were 49.25 and 44.93%, respectively. The effective efficiencies for SW1/MP and SW2/MP in Exp. 2 were found between the ranges of 44.78–66.42% and 39.55–60.88%, respectively. The mean effective efficiencies in SW1/MP and SW2/MP were 55.96% and 50.46%, respectively. In Exp. 3, the effective efficiencies were achieved for SW1/MP-NP and SW2/MP-NP in the ranges of 50.03–74.48% and 46.42–68.35%, respectively. The average values of effective efficiency for SW1/MP-NP and SW2/MP-NP were calculated as 64.02% and 58.95%, respectively. Similarly, Singh et al. [32] used wire meshes in a double-flow SAHS, and the stated modification significantly improved the effective efficiency value.

The time-dependent alterations in exergetic efficiencies are represented in Figure 9. The highest instantaneous exergy yield value was obtained as 16.20% in SW1/MP-NP in Exp. 3. The average exergy efficiencies for SW1, SW1/MP and SW1/MP-NP were attained as 7.45%, 9.61% and 12.29%, respectively. The mean exergetic efficiency values for SW2, SW2/MP and SW/MP-NP were found to be 6.24%, 8.10% and 10.60%, respectively. It can be seen that reducing plenum thickness and applying two modifications combined improved the average exergy efficiency almost twice. There are some similar results obtained in the literature with the present work. In a research, a solar air heating system with an energy storage component has been analyzed, and the exergetic efficiency was found to be in the range of 5–20% [67]. Fudholi et al. [68] examined a grooved air heater and found this metric in the range of 12.89–13.36%. In another work, conical extended heat transfer surfaces were integrated into a solar air heating device, and the exergy yield was 9–19% [69].

Figure 10 presents a comparison of some performance indicators including thermal efficiency, COP, and SI values for the tested SW configurations. The average thermal efficiency values for SW1, SW1/MP and SW1/MP-NP were 50.55%, 58.50% and 66.62%, respectively. The mean thermal efficiencies for systems with 15 cm plenum thickness including SW2, SW2/MP and SW2/MP-NP were 45.92%, 52.90% and 61.30%, respectively. It can be said that reducing the plenum thickness (from 15 to 10 cm) and combined utilization of mech packing and Fe nano-doped absorber coating improved the mean thermal efficiency by 45.07% (SW2 vs. SW1/MP-NP). There are some works on solar air heating systems that found thermal efficiency values in the range of 14.40–66.16% [70,71,72]. Additionally, similar performance enhancement findings were obtained using packing materials in some research. In a study, Prasad et al. [73] applied wire screens as a packing material to an SAHS and significantly improved their thermal efficiency values. In a different research study, iron meshes were integrated to a double-flow system, and this modification improved the thermal efficiency from 70.60–72.15% to 78.06–80.39% [37].

Moreover, the average COP and SI values for all tested SW configurations were attained between the ranges of 2.02–2.93 and 1.0669–1.1921, respectively. Using mesh packing and nano-added black paint in the systems (Exp. 3) with 10 cm and 15 cm plenum thicknesses improved the average COP values by 31.39% and 33.66%, respectively, in comparison to the analyzed base cases (Exp. 1). The mean SI values for the SWs with 10 cm and 15 cm plenums were improved by 5.58% and 4.92%, respectively, by a combined usage of nano-paint and mesh packing. Reducing the plenum thickness from 15 to 10 cm and combining the utilization of nano-paint and iron meshed improved the mean SI and COP by 11.73% and 45.04%, respectively (SW2 vs. SW1/MP-NP).

4.2. Enviro-Economic Performance Results

LCOH values for SW1 and SW2 in Exp. 1 were determined as 0.0132 and 0.0160 USD/kWh, respectively. For Exp. 2, the LCOH values in SWs with mesh packing configurations including SW1/MP and SW2/MP were calculated as 0.0119 and 0.0144 USD/kWh, respectively. Applying a mesh packing and nano-added coating combination in SWs with 10 cm and 15 cm plenum thicknesses lead to achieving LCOH values of 0.0114 and 0.0135 USD/kWh, respectively. As obviously seen, using mesh packing and Fe nanoparticles in black paint reduced the LCOH values for the systems with 10 cm and 15 cm plenum by 13.63% and 15.62%, respectively. Moreover, reducing plenum thickness from 15 to 10 cm, applying meshes in a flow channel and Fe nanoparticles in black paint reduced the LCOH value by 28.75% (SW2 vs. SW1/MP-NP). Also, payback times for the SW configurations with 10 cm and 15 cm plenum thicknesses were in the ranges of 0.414–0.477 and 0.488–0.579 years, respectively. It can be stated that using nano-coating and meshes in the systems with 10 cm (SW1/MP-NP) and 15 cm (SW2/MP-NP) plenum thicknesses decreased payback times by 13.27% and 15.75%, respectively, in comparison to the conventional cases (SW1 and SW2). Moreover, reducing the plenum thickness and applying the two modifications to the system reduced the payback time of the SW by 28.57% (SW1/MP-NP vs. SW2).

Annual carbon dioxide saving is a critical parameter in evaluating renewable energy systems environmentally. Yearly savings in carbon dioxide for SW1 and SW2 were attained as 0.179 and 0.161 ton/year, respectively. Also, this parameter was attained for SW1/MP and SW2/MP (Exp. 2) as 0.207 and 0.187 ton/year, respectively. The systems modified with both nano-coating and iron meshes in the system with 10 cm (SW1/MP-NP) and 15 cm (SW2/MP-NP) plenums gave 0.233 and 0.215 ton/year yearly carbon dioxide saving results, respectively. As can be seen, decreasing the air channel (plenum) thickness and applying the combination of meshes and nano-coating improved the yearly carbon dioxide savings by 44.72%. Similar carbon dioxide saving values with this work were obtained in some previous works [55,74,75]. In this study, the enviro-economic analysis has been performed to determine the effect of using a sustainable solar air heating wall in an air heating process instead of utilizing conventional energy sources. It is known that nanoparticles have high embodied energy, and it might increase the total embodied energy of the SW. In further works, life cycle analysis is planned to be performed to evaluate the overall environmental impacts of the developed systems.

5. Conclusions

In the present work, six different SW configurations have been designed and empirically analyzed. Experimental results showed that using nano-doped coating and iron meshes improved the mean thermal efficiencies of SW1 and SW2 by 31.79% and 33.49%, respectively. Also, mean COP values were attained in the range of 2.02–2.93. The payback time significantly reduced using the combination of mesh packing and nano-doped black paint configurations. In addition, levelized cost of heating values were attained in the range of 0.0114–0.0160 USD/kWh for the six tested SW configurations. According to the results of this work, reducing the plenum thickness from 15 to 10 cm and applying the combination of nano-coating and mesh packing gave the best energy–exergy and enviro-economic performance. In further works, different nanoparticles concentrations can be examined to specify the optimum SW configuration. It must be noted that prototype SWs have been designed, manufactured, and tested in this work. In future studies, SWs with larger scales can be developed to analyze their potential usage in building systems considering thermal loads. In other words, the utilized modifications in this study could be applied to available commercial applications of SWs. Moreover, the long-term durability observations of the presented modifications should be made.

The general outcomes of this work demonstrated the improvement in the performance of SWs using cost-effective and easy-to-apply modifications. The proposed SW prototypes in this experimental research are expected to be used in large-scale applications in the future considering suitable design, manufacturing, and installation procedures in order to contribute sustainable development goals and lower greenhouse gas emissions originated from space heating applications using conventional energy sources.