Nanofluid-Enhanced Thermoelectric Generator Coupled with a Vortex-Generating Heat Exchanger for Optimized Energy Conversion

Abstract

This study investigates the impact of nanofluids (TiO₂, Fe₃O₄, Al₂O₃, and graphene) on thermoelectric power generation within a rectangular heat exchanger equipped with internal winglets. The integration of internal winglets in heat exchangers, alongside the use of nanofluids, is a recent strategy aimed at enhancing convective heat transfer. This numerical research analyzes fluid dynamics and temperature variations on both the cold and hot sides of the thermoelectric generator (TEG). Three different heat exchanger models are evaluated: the first model features a pair of winglets in both ducts; the second model only has winglets in the hot duct; and the third model does not include any winglets. The performance of the nanofluids is systematically compared with that of distilled water. The results show that the Al₂O₃ nanofluid produces the highest power output at 7.8461 watts, which is 1.5% greater than that of TiO₂ and 1.22% higher than distilled water. Moreover, using Al₂O₃ in a heat exchanger with winglets in both ducts results in a 5% increase in power generation compared to a configuration without winglets and a 2% improvement over a model that has winglets only in the hot duct. This enhancement can be attributed to an increased heat transfer area and improved fluid mixing, which together facilitate more effective heat transfer to TEG.

1. Introduction

In recent years, there has been a growing interest in thermoelectric generators (TEGs), despite their effects having been understood for nearly two centuries [1]. This renewed interest stems from the need to reduce gas emissions and the increasing energy demand driven by economic growth. Consequently, TEGs play a significant role in addressing these challenges, as they convert heat into electricity when a temperature gradient is applied [2,3]. The efficiency of a TEG depends on several factors, including the materials used, the geometric design of the legs, and heat dissipation [4,5,6]. In a study conducted by Bhutta et al. [2], the application of Computational Fluid Dynamics (CFD) in heat exchangers was explored, demonstrating that CFD is an effective tool for predicting the behavior and performance of various heat exchanger designs. Similarly, research by D. Patil et al. used CFD to simulate different winglet designs within a crossflow heat exchanger, aiming to enhance heat transfer and generate energy using TEG modules [3]. In their experiment, carbon dioxide (CO₂) was used as the working fluid to mimic the exhaust gases of a car. At the same time, natural convection served as the cooling method for the thermoelectric components, which could influence TEG efficiency. Additionally, a study by Deng et al. [4] employed finite element analysis to optimize the geometry of radioisotope thermoelectric generators (RTGs). They placed the RTG in a cylindrical electric heater with a radiator featuring winglets for cooling. The results indicated a remarkable 26.5% increase in power output. On the other hand, various studies have explored alternative materials for thermoelectric module components. One such example is the review conducted by Qin-Xue Hu et al. [6], which considers Ag₂Se as a potential substitute for Bi₂Te₃. The authors highlight that Ag₂Se exhibits high stability and power density at a temperature difference of 50 K.

Numerous investigations into thermoelectric generators (TEGs) concentrate on their applications in the automotive, aerospace, and everyday life industries [7,8,9,10]. The review by Riffat, S. B. et al. [9] highlights the main applications and potential of thermoelectric modules, including current medical, military, and specialized space applications that utilize TEG modules to generate energy. Similarly, a study conducted by Xiaosong Su et al. [9] proposes enhancing the energy efficiency of buildings by heating and/or cooling them through a new envelope design integrated with a thermoelectric cooler (TEC) and a radiative sky cooler (RSC). According to Ewert, M. K. [7], another application is the recovery of residual heat in automobiles. This study demonstrates that placing a TEG module in a vehicle’s exhaust system to recover residual heat from expelled gases not only generates electricity, but also reduces combustion emissions by 5%. In another study by Cheng, G. et al. [8], simulation using the finite element method focuses on a generation system installed in vehicle exhausts, where two cooling methods are coupled to optimize conditions for design and application in vehicles.

On the other hand, several studies aim to improve the designs of winglet heat exchangers by utilizing hybrid systems and phase change materials to maximize the performance of thermoelectric generators [11,12,13,14,15]. For instance, the work by Lui Zhu et al. [11] presents an optimization of the geometric structures and operating conditions to enhance the performance of segmented thermoelectric generators (STEG), demonstrating a 25.4% increase in efficiency by reducing contact resistance with a temperature difference of 350 K. Similarly, the study by Shifa Fan et al. [12] employs the finite element method to examine the effects of leg shape, fill factor, and leg height by establishing a three-dimensional model of TEG modules encapsulated in polydimethylsiloxane (PDMS). The results indicate that performance can be improved by increasing the number of thermocouples in TEG.

One study conducted by Chung, Y.-C. et al. [13] focused on enhancing the designs of winglet heat exchangers. They used the COMSOL 5.5 software for simulations to demonstrate the generation of thermoelectric power. This study utilized a two-duct heat exchanger with a thermoelectric generator (TEG) positioned between the ducts, where seawater flowed at various temperatures. Winglets were placed on the hot side, altering the geometric shape and the contact angle with the fluid. The results highlighted how different fin configurations affected heat transfer by generating vortices, which resulted in changes in fluid speed and pressure variations, ultimately improving heat transfer efficiency. Similarly, Charilaou et al. [5] conducted a study where a thermoelectric generator system was installed near exhaust pipes emitting high-temperature gases. They implemented a cooling method based on forced flow, also using the COMSOL simulation software. Winglets were incorporated on both the hot and cold sides of the heat exchanger, demonstrating that the strategic placement of winglets on both sides significantly improved the output power of the TEG. These studies provide a foundation for the current research.

In another investigation, Maduabuchi et al. [14] aimed to enhance the performance of a conventional bismuth–tellurium-based solar thermoelectric generator. They focused on optical and thermal concentration methods without compromising the simplicity and cost of the system. Using MATLAB R2020a Simulink software, they developed a model based on the first and second laws of thermodynamics. Their findings indicated an efficiency increase of 6.5%.

Recent studies, such as the one conducted by Borhani et al. [15], have performed a numerical analysis to enhance the performance of thermoelectric generator (TEG) modules using phase change materials (PCMs) and porous media. In this setup, PCMs are placed on both sides of the TEG, with the cold side functioning as a heat dissipater and the hot side serving as a constant heat source. Their findings indicate that reducing the porosity of the PCM leads to a 5.36% increase in electricity generation.

For the production of electrical energy using thermoelectric generators, a temperature gradient is essential; this gradient is often present in industrial processes due to the presence of machines, devices, or residual heat. One effective way to harness this gradient is through the use of heat exchangers. A comprehensive review by Hussam Jouhara et al. [16] examines the operation and performance of technologies commonly employed in the steel, food, and ceramics industries. It also explores various emerging direct heat-to-energy conversion technologies, including thermoelectric, piezoelectric, thermionic, and thermophotovoltaic power generation techniques. The study concludes that implementing a waste heat recovery method can significantly optimize the energy efficiency of existing industrial processes.

Another investigation by H.B. Gao et al. [17] evaluates the generation of thermoelectric power by recovering heat from a biomass stove. This study focuses on the properties and cooling system of the thermoelectric module, as well as the economic viability of using TEGs compared to conventional stoves, solar panels, and batteries. The researchers conclude that generating energy through TEG modules is a promising method for producing electric power and reducing pollution. In the research conducted by Jiin-Yuh Jang et al. [18], a TEG is installed on a rectangular chimney plate that vents hot flue gases. They note that the power generated by the TEG is highly dependent on its spacing. Consequently, they optimize both the spacing of the TEG modules and the thickness of the diffuser used in a waste heat recovery system by employing the finite difference method in combination with a simplified conjugate gradient method, alongside experimental validation. Additionally, studies like those by Pfeiffelmann et al. [1] aim to enhance the efficiency and economic viability of TEGs by investigating two key aspects: the materials used in TEG modules and the system’s connectivity. They emphasize that system cooling plays a crucial role in maximizing net output power. In their experiments, they employed two cooling methods, natural convection with air and forced convection with water, demonstrating that forced convection yields a higher net output power.

Various investigations have focused on enhancing heat transfer in heat exchangers by incorporating fins with different dimensions and geometric shapes within the ducts, as well as adjusting the angle of contact with the fluid [19,20,21]. For instance, a study by Li-Ting Tian et al. [20] illustrates how the addition of winglets generates vortices in the fluid, which increases speed and decreases pressure, thereby enhancing heat transfer between the fluid and the internal walls of the duct. Similarly, Anupam Sinha et al. [21] simulated winglet-tube-type heat exchangers, where the winglets are half the height of the channel to generate vortices. Their results, obtained using ANSYS FLUENT 14.5, indicate that heat transfer performance improves as the angle of attack increases, up to a particular specific limit.

There has been a growing interest in investigating the effects of nanofluids, as numerous studies have demonstrated that they enhance heat transfer due to their unique properties [22,23,24]. For instance, in the study by Ramos Castañeda et al. [24], the performance of thermoelectric generators (TEGs) was analyzed using nanofluids such as TiO₂, graphene, and Al₂O₃ as cooling systems. They also examined different structural configurations for the legs of the thermoelectric generator, including hollow and filled designs. This analysis was conducted using numerical simulations in ANSYS Workbench software.

Similarly, the research conducted by Yi-Hsuan Hung et al. [22] focused on the performance of Al₂O₃ nanofluid as a cooling system in a heat exchanger. Their results revealed that the nanofluid improved heat transfer by 40% compared to distilled water.

Consequently, the study of thermoelectric generators has intensified over the past few years, with a focus on exploring various approaches to enhance power generation efficiency. Many studies emphasize improving heat transfer to the edges of the TEG. Recent research has indicated that turbulence in the fluid can be initiated by incorporating rectangular winglets inside the heat exchanger. Furthermore, other studies have investigated winglets with different geometrical shapes to analyze the variations in vortex generation and their effects on heat transfer. These studies also examine how changes in the angle of attack influence vortex formation and heat transfer efficiency. Additionally, some investigations focus on improving heat transfer through various nanofluids, examining different concentrations and their behavior within the exchangers. Notably, a concentration of just 0.1% for certain nanofluids can lead to significant increases in heat transfer.

Building on this existing literature, this study employs two distinct strategies to maximize thermoelectric power generation: the implementation of vortex generators and the use of nanofluids in both ducts to enhance convective heat transfer within the heat exchangers. The simulations are conducted using Autodesk CFD software Autodesk 2024 version, and the results include temperature measurements at the cold and hot edges of the TEG, fluid velocity magnitudes, vortex formations, and the generated thermoelectric power.

2. Materials and Methods

This research utilizes Autodesk CFD simulation software to conduct a comprehensive analysis of how various nanofluids—specifically titanium dioxide (TiO₂), ferric oxide (Fe₃O₄), aluminum oxide (Al₂O₃), and graphene—affect the efficiency of a thermoelectric generator (TEG). The TEG is integrated into a specially designed co-current heat exchanger composed of two rectangular ducts. An assembly comprising three types of fins was developed to enhance thermal performance. Based on these designs, three heat exchangers were created: the first featured a pair of fish-shaped winglets in each duct, the second a pair of rectangular winglets in each duct, and the third a pair of cam winglets in each duct. These setups aim to explore the intricate interactions between the nanofluids and the thermal dynamics within the different heat exchangers, ultimately assessing their influence on the TEG’s performance.

The study uses a segmented approach, focusing on the nanofluids TiO₂, Fe₃O₄, Al₂O₃, and graphene while conducting a comparative analysis with distilled water. Initially, fluid dynamics are examined by three heat exchangers. This phase aims to evaluate variations in vortex generation and flow velocity associated with the use of nanofluids. Subsequently, heat transfer behavior is investigated by comparing three heat exchangers against one heat exchanger without winglets. Temperature measurements taken at the edges of the TEG are meticulously recorded for analysis and interpretation. Finally, the maximum output power of the TEG is calculated for the three heat exchangers. The results are then presented graphically to determine which combination of nanofluid and heat exchanger exhibits the best performance.

In parallel, the best-performing nanofluid identified in the earlier analyses is selected to compare the maximum power generated by the TEG across each heat exchanger configuration.

2.1. Materials and Dimensions

The winglets inside the heat exchanger are designed based on the different models proposed [13,25,26]. The rectangular ducts have identical dimensions: each measures 56 mm in width, 20 mm in height, and 100 mm in length, with a thickness of 1 mm, as shown in Figure 1. Alumina is used as the material for this structure. Inside the ducts, different shapes of winglets are installed. The first features a pair of fish-shaped winglets in each duct, the second a pair of rectangular winglets in each duct, and the third a pair of CAM winglets in each duct. The winglet dimensions are 10 mm in width and 1 mm in thickness, with a height identical to the duct, except for the fish winglets. These dimensions are shown in Figure 2, Figure 3 and Figure 4, respectively (fish, rectangular, and CAM). Additionally, a thermoelectric generator (TEG) is positioned between the ducts, aligned with the height of the winglets. Figure 5 presents a general model of heat exchanger and the TEG placed between them. It also shows the inlets for both the warm and cold flow.

The properties and technical specifications of the TEG are derived from the previous literature [27], as illustrated in

Table 1. Specifications of the TEG [27].

SPECIFICATIONS
Parameter	Conditions/description	min	typ	max	units
Open circuit voltage	At Th = 300 °C, Tc = 30 °C		8.4		V
Matched load resistance	At Th = 300 °C, Tc = 30 °C		0.9		Ω
Matched load output voltage	At Th = 300 °C, Tc = 30 °C		4.2		V
Matched load output current	At Th = 300 °C, Tc = 30 °C		4.6		A
Matched load output power	At Th = 300 °C, Tc = 30 °C		19.3		W
Heat flow across module			306		W
Heat flow density			9.8		W/cm2
Ac resistance	At 27 °C, 1000 Hz	0.25		0.45	Ω
Solder melting temperature	Cold side: SnAgSn solder Hot side: not solder, aluminum bonding	217			°C
Hot side plate		−60		300	°C
Cold side plate		−60		180	°C
Assembly compression				1.4	MPa
Mounting torque per screw	Screw diameter # of screws: 5 mm/2		0.3		kg-m
RoHS	yes

.

In this research, we utilize specific nanofluids that have been thoroughly studied by Ramos Castañeda et al. [24]. The nanofluids selected for this study include graphene, which is renowned for its exceptional thermal conductivity; titanium dioxide (TiO₂), well-known for its photocatalytic properties; aluminum oxide (Al₂O₃), appreciated for its stability and heat transfer efficiency; and iron oxide (Fe₃O₄), recognized for its magnetic properties. Each nanofluid is prepared at a concentration of 0.1%, allowing for accurate and controlled analysis. For comparison, distilled water is used as a baseline to evaluate the performance of the nanofluids. The detailed properties of these materials, which highlight their unique characteristics and behaviors, are comprehensively presented in

Table 2. Physical properties of fluids.

	$\mathbf{\kappa }/\mathbf{W}\ast {\mathbf{m}}^{-1}\ast {\mathbf{K}}^{-1}$	$\mathbf{\nu }/{10}^{-7}{\mathbf{m}}^2\ast {\mathbf{s}}^{-1}$	$\mathbf{\rho }/\mathbf{k}\mathbf{g}\ast {\mathit{m}}^{-3}$	$\mathbf{C}\mathbf{p}/\mathbf{J}\ast \mathbf{k}{\mathbf{g}}^{-1}\ast {\mathbf{K}}^{-1}$
Distilled water	0.6	7.47797	997	4181.7
TiO2	0.607	9.2143	1048	3502
Fe3O4	0.6274	8.174939	1003.06	4178.485
Al2O3	0.661	10.07544	1007.4	4154.7
Graphene	0.752	9.97062	1123.3	3343.188

.

2.2. Governing Equations

The calculations that support the numerical simulation conducted with Autodesk CFD are based on several key assumptions that enhance the clarity and effectiveness of the analysis. First, we assume that the fluid within the exchanger operates under steady-state conditions. Additionally, the fluid is characterized by fully developed, incompressible flow, which allows us to apply a turbulent flow model effectively. We have chosen to omit the effects of radiation as well as the electrical and thermal resistances in the thermoelectric generator (TEG) materials to maintain a clear focus on the primary fluid interactions. The governing equations for the conservation of mass, energy, and momentum in this steady-state fluid context can be expressed as follows [28,29]:

$$\rho \nabla ·\overrightarrow{u}=0$$

(1)

The Navier–Stokes equation,

$$\rho \left(\overrightarrow{u}·\nabla \right)\overrightarrow{u}=-\nabla p+\mu {\nabla }^2\overrightarrow{u}$$

(2)

The energy conservation equation,

$$\rho C_p\overrightarrow{u}·\nabla T+\nabla ·\overrightarrow{q}=Q$$

(3)

The equations which dictate the thermoelectric behavior are [30,31],

$${\overrightarrow{q}}_{TE}=ST\overrightarrow{J}-k\nabla T$$

(4)

$$\overrightarrow{J}=\sigma \left(\overrightarrow{E}-S\nabla T\right)$$

(5)

where the electric field ($\overrightarrow{E}$) could be represented by the gradient of electric potential (∇V), the equations presented clarify the relationship between electric potential and temperature in thermoelectric materials. By applying a specific temperature or electric current, one can determine the resulting electric potential.

2.3. Boundary Conditions

The boundary conditions for the simulations in this study are clearly defined as follows:

• The heat exchanger inlet is set for fully developed flow, which is analyzed as turbulent flow.
• The effects of radiation are neglected, and the walls not in contact with the TEG are considered adiabatic.
• The inlet velocity ranges from 0.5 m/s to 3 m/s, and the temperature difference is maintained at approximately 75 K.
• At the outlet, the pressure is held constant at zero, which is essential for accurately representing the conditions as the fluid exits the system.

This configuration is crucial for effectively simulating heat transfer and fluid dynamics within the heat exchanger.

2.4. Evaluation Parameters

The evaluation parameters of the TEG are defined as follows [32,33,34]:

The internal resistance,

$$R_{TEG}=N\left(R_{pn}+2{\displaystyle \frac{{\rho }_c{L}_c}{A_c}}\right)$$

(6)

Open-circuit output voltage,

$$V_{oc}=N{\alpha }_{pn}\left(T_h-T_c\right)$$

(7)

In order to evaluate the output power of the TEG,

$$I={\displaystyle \frac{V_{oc}}{R_{TEG}+R_L}}$$

(8)

$$P=I^2{R}_L$$

(9)

The maximum output power is therefore obtained,

$$P_{max}={\displaystyle \frac{V_{oc}^2}{4R_{TEG}}}$$

(10)

2.5. Mesh Independence

The numerical simulation in this study was conducted using Autodesk CFD, which utilizes the finite element method. Initially, mesh tests were performed on the structures under investigation to evaluate mesh independence, see Figure 6. The results of these tests are summarized in

Table 3. Percentage of mesh independence.

Type	Number of Nodes	Number of Elements	Mesh Independence (%)
			P (Pa)	V (m/s)	T (K)
Cam Winglets	8309	28,140	99.98	100	93.07
	33,238	124,772	99.98	98.19	99.99
	48,932	189,537	100	99.98	100
Rectangular Winglets	17,335	62,317	98.00	97.54	100
	21,343	77,951	98.56	97.04	100
	27,409	104,113	100	100	100
Fish Winglets	28,056	113,091	81.28	99.33	100
	88,338	385,379	90.56	99.38	100
	131,520	562,919	99.05	99.48	100

, which displays the percentage of mesh independence based on the number of elements and nodes. These data were obtained and presented by the software.

3. Validation

For this study, the TEG SPG 193-56 was used. To validate the method, the voltage and power graphs for this TEG were reproduced, as shown in Figure 7 and Figure 8.

In addition, Figure 9 illustrates a detailed comparison of the velocity profiles for rectangular winglet configuration with an impact angle of 45° degrees.

The data collected on temperatures, voltage and thermoelectric power closely aligns with data sheet, showing a variation of about 1%. Therefore, we can conclude that the proposed method is appropriate for the study conducted.

4. Results and Discussion

4.1. Fluid Dynamics Analysis in Nanofluid-Based Heat Exchangers

This section aims to compare the behavior of different winglets to identify differences in vortex formation and velocity magnitude, based on previous validations. These configurations will enable us to analyze the underlying dynamics effectively. Figure 10, Figure 11, Figure 12, Figure 13, Figure 14 and Figure 15 show the vortexes and maximum velocity reached the in-side duct for each winglets configuration.

Figure 10 and Figure 11 show the fluid behavior within the heat exchanger equipped with fish winglets, where the vortex generation and the velocity magnitude can be seen. This highlights that the maximum velocity reached by the fluid is 4.88 m/s.

On the other hand, Figure 12 and Figure 13 show the fluid behavior within the heat exchanger equipped with CAM winglets, where the vortex generation and the velocity magnitude can be seen. This highlights that the maximum velocity reached by the fluid is 6.00 m/s.

Finally, Figure 14 and Figure 15 show the fluid behavior within the heat exchanger equipped with rectangular winglets, where the vortex generation and the velocity magnitude can be seen. This highlights that the maximum velocity reached by the fluid is 5.96 m/s.

4.2. Edge Temperature Analysis of TEG in Nanofluid-Based Heat Exchangers

In this section, we analyze the temperatures at the cold (Tc) and hot (Th) edges of the thermoelectric generator (TEG). We compare the results of three heat exchangers with three shapes of winglets (fish, rectangular, CAM). The temperature in the cold duct is set at 25 °C, while the temperature in the hot duct is set at 100 °C. The study evaluates various fluids, including distilled water and nanofluids such as TiO₂, Fe₃O₄, Al₂O₃, and graphene.

As observed in Figure 16, the Al₂O₃ nanofluid exhibits a temperature difference of 1 °C higher than the other nanofluids for the heat exchanger with fish winglets.

In the same way, Figure 17 shows that the Al₂O₃ nanofluid exhibits a temperature difference of 1 °C higher than the other nanofluids for the heat exchanger with CAM winglets.

Finally, Figure 18 shows that the Al₂O₃ nanofluid exhibits a temperature difference of 1 °C higher than the other nanofluids for the heat exchanger with fish winglets.

Similarly, the temperature distribution at the cold (Tc) and hot (Th) edges of the TEG for every heat exchanger can be observed in the following tables (

Table 4. Temperature distribution at the cold (Tc) and hot (Th) edges of the TEG in an exchanger with fish winglets.

Distilled Water		TiO2		Fe3O4		Al2O3		Graphene
Tc	Th	Tc	Th	Tc	Th	Tc	Th	Tc	Th
26.201	98.8087	26.23	98.7794	26.1789	98.8305	25.89	99.1167	26.0835	98.9251
26.0256	98.9851	26.0783	98.9321	26.018	98.9933	25.7268	99.2843	25.9643	99.0419
25.8925	99.1209	25.9494	99.0602	25.8907	99.1223	25.6113	99.3973	25.865	99.1434
25.795	99.2162	25.848	99.1616	25.7999	99.2166	25.5289	99.4777	25.7847	99.2266
25.7217	99.2884	25.7778	99.2404	25.7249	99.2898	25.4732	99.5343	25.7204	99.2913
25.6728	99.3393	25.7233	99.2986	25.6687	99.3392	25.433	99.5725	25.6679	99.3404

,

Table 5. Temperature distribution at the cold (Tc) and hot (Th) edges of the TEG in an exchanger with CAM winglets.

Distilled Water		TiO2		Fe3O4		Al2O3		Graphene
Tc	Th	Tc	Th	Tc	Th	Tc	Th	Tc	Th
26.3181	98.7691	26.4731	98.6299	26.3384	98.7514	25.8198	99.2031	26.4022	98.6899
25.8215	99.2134	25.9159	99.1298	25.8316	99.2054	25.5874	99.4239	25.8849	99.1571
25.6396	99.3765	25.7035	99.3201	25.6454	99.3741	25.4986	99.5104	25.6793	99.3419
25.5532	99.4613	25.5975	99.4196	25.5533	99.4606	25.4497	99.5577	25.5768	99.4375
25.4984	99.5126	25.5361	99.4777	25.5004	99.5108	25.4183	99.5884	25.5156	99.4961
25.4582	99.5505	25.4929	99.5182	25.4602	99.5487	25.3963	99.61	25.4766	99.533

and

Table 6. Temperature distribution at the cold (Tc) and hot (Th) edges of the TEG in an exchanger with rectangular winglets.

Distilled Water		TiO2		Fe3O4		Al2O3		Graphene
Tc	Th	Tc	Th	Tc	Th	Tc	Th	Tc	Th
26.396	98.7093	26.5437	98.5758	26.407	98.7	25.9124	99.1062	26.4672	98.6389
25.9103	99.1472	26.0091	99.0603	25.9181	99.1411	25.6443	99.3678	25.9678	99.0957
25.7136	99.3244	25.7872	99.2593	25.7195	99.3197	25.54	99.4705	25.7569	99.2856
25.6067	99.4217	25.6643	99.3701	25.6105	99.41	25.4848	99.5249	25.6396	99.3915
25.5398	99.4832	25.5871	99.4405	25.5428	99.4806	25.449	99.5598	25.5662	99.4586
25.4928	99.5265	25.5341	99.489	25.4954	99.5242	25.4233	99.5848	25.5159	99.5049

).

As observed in the three heat exchangers, the Al₂O₃ nanofluid exhibits a temperature difference of 1 °C higher than the other nanofluids. Therefore, this nanofluid is expected to achieve the highest thermoelectric power among those analyzed.

4.3. Evaluating Power Generation of TEG in Nanofluid-Based Heat Exchangers

This study examines the power generated with a temperature difference (ΔT) of 75 K between the warm and cold flows. Table 4, Table 5 and Table 6 displays the temperature recorded between the cold (Tc) and hot (Th) edges of the thermoelectric generator (TEG) in an exchanger equipped with different winglets on both ducts. According to the technical data sheet for this TEG, a temperature difference of this magnitude can produce an output power of 1.5 W. The velocity range studied was between 0.5 m/s and 3.0 m/s. The study evaluates the performance of various fluids used in both ducts, including distilled water and nanofluids such as TiO₂, Fe₃O₄, Al₂O₃, and graphene. The objective is to identify which nanofluid performs best for this application.

As can be seen in Figure 19, which shows the thermoelectric power generated using nanofluids in heat exchangers with fish winglets, the Al₂O₃ nanofluid has an increase in power of approximately 2.8%.

On the other hand, Figure 20 shows the thermoelectric power generated using nanofluids in heat exchangers with CAM winglets, where the Al₂O₃ nanofluid has an increase in power of approximately 3.5%.

For the heat exchanger with rectangular winglets, Figure 21 shows that the nanofluid Al₂O₃ has an increase in power of approximately 3%.

As shown in Figure 19, Figure 20 and Figure 21, the Al₂O₃ nanofluid generates the highest power output compared to the other fluids. To evaluate this, we will compare the maximum power produced by the thermoelectric generator (TEG) using four different heat exchangers. The first exchanger features a pair of fish winglets in both ducts, the second exchanger has rectangular winglets in both ducts, the third exchanger has CAM winglets in both ducts, while the fourth exchanger has no winglets at all. This configuration enables us to analyze the effect of the winglets on heat transfer within the exchanger, which is ultimately reflected in the maximum output power generated by the TEG.

Finally, a comparison was made among three heat exchangers with different winglet configurations (fish, CAM, and rectangular), all using Al₂O₃ nanofluid, against a heat exchanger without winglets and using distilled water.

As can be seen in Figure 22, which shows the thermoelectric power generated using Al₂O₃ in three heat exchangers whit winglets against one heat exchanger without winglets and using distilled water, the CAM winglets have an increase in power of approximately 10.68%.

As observed, using the Al₂O₃ nanofluid significantly boosts the power output due to two main factors. First, the increase in velocity magnitude, as discussed in Section 4.1, demonstrates the enhanced performance of this nanofluid. Second, the impressive heat transfer properties detailed in Section 4.2 indicate that the temperature at the edges of the thermoelectric generator (TEG) is positively affected. Furthermore, Figure 21 illustrates that the strategic placement of CAM winglets in both ducts results in an even higher power output. This improvement is due to enhanced heat transfer at two key points: from the hot fluid to the inner walls of the upper duct, and from the lower duct to the cold fluid. This approach effectively optimizes performance.

5. Conclusions

The physical properties of nanofluids do not significantly affect the generation of vortices within the heat exchanger. However, they enhance heat transfer, as illustrated in Section 4.2, which presents the temperature differences achieved with each nanofluid.

Each nanofluid exhibit changes in its velocity profile, particularly regarding the maximum velocity attained. This variation impacts heat transfer from the fluid to the internal walls of the duct, and is subsequently reflected in the temperatures recorded at both the cold and hot edges of the thermoelectric generator (TEG), as well as in the maximum power output for each fluid. Among the nanofluids tested, Al₂O₃ demonstrates the most pronounced variation in both the velocity profile and the temperatures at the cold and hot edges of the TEG. Therefore, it can be concluded that this nanofluid yields the best outcomes in this study, particularly in the calculations of maximum output power from the TEG, where it consistently shows superior performance.

Modifications to the heat exchanger’s design, such as the inclusion of winglets in both ducts, lead to enhanced heat transfer. This design change results in a greater temperature difference at the cold and hot edges of the TEG, which in turn improves the maximum power output. When comparing the results of a heat exchanger without winglets using distilled water as the fluid, the power generated by TEG improves by about 10.68% when a pair of CAM winglets is in both ducts, along with the use of the Al₂O₃ nanofluid.