1. Introduction
Thermoelectric generation systems (TEG) have undergone important advances in recent years due to the development of semiconductor materials, and thermoelectric devices have even been incorporated into domestic accessories. Thermoelectric generation has attracted much more attention by decreasing primary energy reserve and taking advantage of wasted heat [
1,
2].
A thermoelectric generator (TEG) is usually composed of two thermoelements connected in series, an n-type and a p-type. Thermoelements are connected by a metal (copper) and sealed by an aluminum ceramic material on the upper and lower surfaces [
3]. Various studies on TEGs have studied an extensive field focused on applications, such as automotive, aerospace, industrial, and everyday life [
4,
5,
6]. Normally if a temperature gradient is applied in a TEG system, there will be an electric field in the opposite direction associated with the thermoelectric system. This phenomenon is called the thermoelectric effect [
7,
8]. Thus, one of the essential factors determining the efficiency of thermoelectric generation is the temperature difference (∆T) between the hot and cold sides of the thermocouple.
Several researchers have studied graphene for the improvement of its mechanical, thermal, and physical-chemical properties. It has high thermal conductivity, less erosion, and higher stability than other nanoparticles studied. The literature focuses on studying the stability and thermal properties of GNPS. Nanofluid characteristics are affected by heat and mass transfer, such as Reynolds number (Re), nanoparticle concentration, the synthesis method, surface tension and shape of the nanoparticles, the effect of magnetic force, and the size of the nanoparticles.
For example, Figueiredo, M. et al. [
9] studied nanofluids up to 1% wt in aerosols. Heat transfer improvement came about because the surface tension of the nanofluid declined and increased the wettability, which caused better transfer between the system and the film.
Sanches, M. et al. [
10] studied the mathematical and physical correlation of the thermal properties of materials with their structure and how this affected thermal performance by applying silver and alumina nanofluids by spraying. They showed that increasing the concentration of nanofluids affected the thermal conductivity and viscosity of the nanofluid and decreased the heat transfer coefficients.
Sandhya et al. [
11] observed how the GNPS improved thermal conductivity, the viscosity, and the heat transfer capacity of the fluid in which the nanoparticles were dispersed. Objects that GNANs can be applied to include car radiators, electronic refrigeration, and solar cells.
In recent studies [
12,
13,
14,
15,
16,
17], a numerical simulation model was analyzed to study the performance of thermal thermoelectric generators with solar concentration (Solar TEGs). Nuwayhid et al. [
15] installed a TEG on the side of a wood stove. The way the TEG was cooled was by natural air convection. It was designed to be inexpensive and to provide optimal power.
For nanoparticles, the thermal conductivity is a critical factor. The structure of graphene is two-dimensional, and the shape is hexagonal. Therefore, its thermal, mechanical, and electrical properties are parameters of a lot of interest. The thermal conductivity of graphene is approximately 3000–5000 W/mK [
17]. Another study [
18,
19] developed a conversion technique for converting solar energy into electrical energy based on the thermal concentration and Seebeck effect.
The efficiency of a TEG, considering conduction heat, radiation heat, convection heat, and using an Al
2O
3 (Alumina) compound as a substrate for the solar collector, is numerically investigated considering two geometric models. The material lead-tellurium (Pb-Te) was used by Schmitz et al. [
20] in a model of a system considering temperature-dependent properties.
Zhou, M. et al. [
21] analyzed the electrical power of a TEG system at different mass flow rates for the refrigerant and different inlet temperatures. Nanofluids in traditional thermal energy systems have demonstrated better thermophysical properties, such as viscosity, thermal diffusivity, and thermal conductivity in comparison with base fluids. This study showed that the addition of copper oxide nanoparticles improved the heat transfer compared to the water in the Solar-TEG cooling system. Furthermore, the nanofluid efficiency was enhanced by 14% in comparison with pure water.
Huang et al. [
22] found an optimal geometric structure for thermoelectric coolers through an inverse approach. To find the optimal geometry, they found the cooling rate on the cold side. They studied the optimization of the geometry of thermoelectric coolers using a simplified gradient method.
Bahiraei and Hangi [
23] studied the water-based magnetic ferritic nanofluid Mn-Zn in a countercurrent twin-tube heat exchanger using a two-phase Euler-Lagrange method with a quadrupole magnetic field. The nano-liquid flowed down the wall of the tube as a coolant, and the hot water flowed down the ring. They studied the influence of some characteristics, such as particle size, concentration of NPS, magnetic field amplitude, and Reynolds number.
Faizal et al. [
24] estimate the feasibility of designing a small solar collector to produce the same outlet temperature as the one intended. They used a nanofluid as a working fluid. Efficiency, size reduction, cost, and embodied energy savings were calculated using numerical methods for different nanofluids. It was estimated that 8857 kg, 8618 kg, 10,239 kg, and 8625 kg of total weight could be saved for 1000 solar collector units for TiO
2, Al
2O
3, CuO, and SiO
2 nanofluids, respectively.
Hajian et al. and Kooetal [
25,
26] investigated the importance of the size of the nanoparticles and the percentage used in the solution. A discussion of the side effects and side effects of highly concentrated nanofluids was presented. Distilled water heat pipe treatment was more stable than the nanofluid. The thermal resistance of the heat pipe and the response time were respectively reduced by 30% and 20% compared to distilled water. The actual thermal conductivity depended strongly on both the temperature of the particles and the liquid and the properties of the material. This fact is due to the longer action interval of the potential between the particles.
Peyghambarzadeh et al. and Naraki et al. [
27,
28] studied the heat transfer coefficient with water-dispersed nanofluids for a car radiator. Fe
2O
3 and CuO were added at 0.15, 0.4, and 0.65% wt considering the best pH for better stability. The Reynolds number on the fluid side varied between 50–1000. The use of nanofluids improved the heat transfer by 9%.
Vermahmoudi et al. [
29] conducted experiments to study the heat transfer coefficient of FeO nanofluids dissolved in water for air-cooled heat exchangers. It turned out that increasing the flow rate, Reynolds number, and concentration improves the heat transfer coefficient. However, increasing the inlet temperature of the nanofluid adversely affects the heat transfer coefficient.
Cuce et al. [
30] studied the impact of nanofluids in TECs on performance parameters and cooling power. They place a water-cooled block on the hot side, and a water-to-air exchanger was put in to cool the system. Al2O3, TiO2 and SiO2 are used at 0.1, 0.5, and 0.1% wt. They conclude that the best efficiency was obtained with Al
2O
3 at 30 °C with 26% for no-load conditions.
Xing et al. [
31] showed that graphene nanoplatelets could reduce nanofluid deposition and standardization. The simulation uses thermoelectric and liquid modules in an Ansys workbench environment. GNAN is used as a coolant on the TEG’s cold side to achieve a better heat transfer. The results show that the heat, efficiency, output power, and voltage absorbed by the hot surfaces of the TEG system were increased by the liquid nano coolant.
Mohammadian and Zhang [
32] studied the effect of alumina nanoparticles in water on the COP of a TEC. They concluded that COP could be improved by using nanofluids on the hot side with low Re.
Selimefendigil and Öztop [
33] evaluated the efficiency of the TEG module located between two channels, carbon/water nanotubes flow with combined effects of particle inclusion, and flow pulsations, and with a Re = 250–1000, mass fraction of 0.01 and 0.04% wt, Strouhal number = 0.01–0.1, and amplitude = 0.25–0.95. When nanoparticle mass fraction, Re, flow pulsation, and amplitude rose, the efficiency improved.
Wiriyasart et al. [
34] experimentally analyzed the performance of a TEC model compacted to a heat sink with nanofluid, ferrofluid, and water. TiO
2 and Fe
3O
4 were used at 0.005 and 0.015% wt. The results showed that Fe3O4 showed a heat transfer 12.57% higher than water and TiO
2.
The use of nanofluids as coolant reduces the temperature on the cold side of Solar-TEG and increases the temperature gradient. In addition, nanofluids as refrigerants in Solar-TEG configurations improve TEG operating conditions better than water or air.
Using different geometries and how it influences the performance of the Solar-TEG, PbTe as a semiconductor material, and nanofluids as a refrigerant to improve performance, and including solar concentration makes this work a complete study.
In this study, the materials and methodology used for the simulation, the governing equations, the TEG schemes with and without a cooling system, and the different equivalent geometries proposed are presented in
Section 2. In
Section 3, the results for the efficiency, output power, and current as a function of the concentration ratio are presented. The heat, efficiency, and output power as a function of the Re with various concentrations of graphene nanoplatelets are obtained. The geometry of the TEG and the use of nanofluids is mentioned. Our results are obtained considering the influence of the thermal concentration ratio, nanofluids cooling, and convection heat on each model’s efficiency and power output. We determine the spatial distribution of temperatures and the efficiency as a function of the thermal concentration ratio at different heat values of convection. Considering the convection coefficient and the solar concentration ratio, which is a relationship between the area of the substrate and the area of the thermocouple, we obtain the optimal geometric model for maximum performance in the TEG. The efficiency of the thermoelectric models is presented, along with the output power for different cases (rectangular and trapezoidal), and, finally, in
Section 4, conclusions are presented.
4. Conclusions
The performance of a thermally concentrated Solar-TEG was studied, employing FEM and performing simulations in an ANSYS workbench. The properties of semiconductors are temperature-dependent, based on commercial materials. Therefore, equivalent models were developed to facilitate simulation. Three-dimensional steady-state geometries were designed to simulate the heat transfer behavior and efficiency of Solar-TEG. To achieve better cooling, GNAN was used.
Five different Solar-TEG geometries (three rectangular and two trapezoidal models) were evaluated and compared. Pout of the Solar-TEG increased as the surface area increased with the increase in the heat concentration coefficient. Geometry B offered the best performance, and the largest was 5.65%, with a panel area of 110 × 110 mm2. If the length of the element is set, reducing the cross-section of the thermoelectric element is the best way to improve efficiency. If forced convection occurs below h = 500 W/m2 K, the cooling conditions have little effect on solar TEG behavior.
When the value of the convection coefficient was very low (10 W/m2K), the efficiency dropped significantly due to the low heat transfer generated with this coefficient. Our results of Solar-TEG systems with nanofluids have shown a significantly improvement in TEG efficiency. A clear increase in Pout and the voltage of Solar-TEG using GNAN refrigerants were seen with the addition of 0.025, 0.05, 0.075, and 0.1% wt. Conversion efficiency was increased by 7%, 10%, 12%, and 15% for 0.025, 0.05, 0.075, and 0.1% wt GNAN, respectively.