**1. Introduction**

In the last two decades, the researches on waste heat recovery technologies have been increased to diminish the global energy crisis [1]. The thermoelectric generators are the evolving technology for the waste heat recovery due to its non-polluting and silent operational characteristics [2]. The thermoelectric generators convert the thermal energy into the electricity using the Seebeck effect of semiconductor materials [2]. The automobile vehicles are the heavy consumers of the fossil fuel and approximately 30%–40% of heat is lost as the vehicle exhaust [3]. The significant research works have been done on the applications of the thermoelectric generators in the automobile vehicle for waste heat recovery. However, the low conversion efficiency of the thermoelectric generators restricts their commercialization in the automobile field [3]. To improve the performance of the thermoelectric generators in the waste heat recovery of the automobile vehicles, the hot heat exchanger is provided with various internal structures in the form of fins, inserts and protrusions.

Wang et al. have proved that the dimpled surface hot heat exchanger enhances the power output of the thermoelectric generator by 173.60% and reduces the pressure drop by 20.57% compared to inserted fins hot heat exchanger [3]. Niu et al. have recommended that the baffles should be installed in front of the thermoelectric modules and the baffle angle should be increased in the flow direction to enhance the performance of the thermoelectric generator and to reduce the pressure drop [4]. Liu et al. and Quan et al. have proved that the chaos shaped internal structure in the heat exchanger improves the power output and thermal performance of the automotive thermoelectric generators [5,6]. Luo et al. have improved the performance of the thermoelectric generators by proposing the heat exchanger with hotter side converges towards the inward [7]. Nithyanandam et al. have proposed that the metal foam-based heat exchanger shows 5.7 to 7.8 times higher power output than that without the metal foam for automobile waste heat recovery [8]. Cao et al. have proved that the heat pipe with insertion depth of 60 mm and gas flow direction of 15<sup>o</sup> enhances the open circuit voltage, maximum power and maximum power density of the automotive thermoelectric generator system by 7.5, 10.17 and 15.49%, respectively [9]. He et al. have shown that the plate type heat exchanger shows the maximum conversion efficiency of 5% for the louvered fins and 4.5% for the smooth and offset strip fins, respectively [10]. Lu et al. have proved that the hot heat exchanger configurations with uniform winglet vortex and non-uniform winglet vortex show higher power output of the thermoelectric generator than the hot heat exchanger without fins [11]. Rana et al. have the generated maximum power of 79.02 W by designing the heat exchanger with 0.08 m length, 1 m height, 4 mm gap size and 50 thermoelectric modules [12]. Suter et al. have proposed 1 kW thermoelectric stack with the counterflow parallel plate heat exchanger and 127 pairs of thermoelectric modules to convert the geothermal reservoir heat to electricity using the optimized stack volume of 0.0021 m<sup>3</sup> and optimized the conversion efficiency of 4.2% [13]. Zhao et al. have showed that the application of intermediate fluid improves the maximum power output and generation efficiency of the automotive thermoelectric generator system [14,15]. Lu et al. have shown that 1-inlet 2-outlet heat exchanger has improved the performance characteristics compared to 2-inlet 2-outlet and empty cavity heat exchangers [16].

The conducted literature review concludes that the numerous experimental and numerical studies have been demonstrated on the thermoelectric generator system for waste heat recovery. The experimental study on the thermoelectric generator system for waste heat recovery shows that the energy imbalance results into the excessive loss within the system and improper insulation results in heat loss from the system to the environment. In addition, the thermocouples embedded into the thermoelectric generator system for waste heat recovery show measuring errors in the temperatures of various parts of the system, and manufacturing complexity arises due to non-uniform material properties. The numerical study on the thermoelectric generator system for waste heat recovery requires the powerful computational devices which involve higher computational time and higher computational cost. In the last few years, the artificial intelligence techniques have a secured position as the effective prediction tools to predict and optimize the performances of the various physical system. The artificial intelligence techniques are the most efficient tools to accurately predict the performance of the thermoelectric generator system for waste heat recovery and diminish the limitations of the corresponding experimental and numerical approaches. Dheenamma et al. have shown the artificial neural network models to predict the overall heat transfer coefficient, friction factors of hot and cold fluids, and effectiveness of the plate type heat exchanger by considering the Reynolds number of hot and cold fluids, Prandtl numbers of hot and cold fluids and the concentration of the cold fluid as the input conditions [17]. Angeline et al. have formulated the artificial neural network to predict the performance parameters of open circuit voltage, maximum power and matched load resistance of the thermoelectric generator for various hot side temperatures. The predicted results from the artificial neural network are found within 3% error with the corresponding experimental results [18,19].

The application of the artificial neural network and adaptive neuro-fuzzy interface system for the performance prediction of the thermoelectric generator system for waste heat recovery has not been investigated. Therefore, six ANN models with various combinations of training variants, transfer functions and the number of hidden neurons as well as seven ANFIS models with various combinations of types of membership functions and the number of membership functions are formulated. The developed ANN and ANFIS models predict the current, power and thermal efficiency of the thermoelectric generator system for waste heat recovery under the hot gas inlet temperatures and voltage load conditions. efficiency of the thermoelectric generator system for waste heat recovery under the hot gas inlet temperatures and voltage load conditions.

*Symmetry* **2020**, *12*, x FOR PEER REVIEW 3 of 30

### **2. Experimental Set-Up 2. Experimental Set-Up**

The experimental set-up of the thermoelectric generator system for waste heat recovery is shown in Figure 1. The thermoelectric generator system for waste heat recovery is designed with heat exchanger, four cold fluid channels and 12 thermoelectric modules. The hot gas passes through the heat exchanger and the cold-water flows through the cold fluid channels. The thermoelectric modules are arranged between the heat exchanger body and the cold fluid channel to utilize the temperature difference between the hot gas and cold water. The thermoelectric modules convert the temperature difference of the hot gas and cold water into power using the Seebeck effect [20]. The heat exchanger and the cold fluid channels are constructed with aluminum material, whereas the thermoelectric modules are constructed with the skutterudite material. The heat exchanger is comprised of the frame with straight fins and guide fins in the inlet and outlet diffuser sections to enable the uniform distribution of the hot gas. The cold fluid channels are provided with the internal fins structure to enable the uniformity of water. Four cold fluid channels with two at the top of the heat exchanger and two at the bottom of the heat exchanger are arranged with three modules between each channel and heat exchanger. The thermoelectric generator system for waste heat recovery is installed in the airtight chamber filled with the argon gas at the pressure of 1 × 10<sup>5</sup> Pa. The electric heater supplies the hot gas at the required temperature using the thermostat controller. The mass flow rate of the hot gas is measured by the mass flow indicator with an accuracy of ±0.5% installed near the thermostat controller. The airtight vacuum chamber provides the constant temperature and pressure controlled by the chamber pressure regulator. In addition, the chamber pressure regulator indicates the inlet and outlet temperatures of the hot gas. The constant temperature chiller supplies the cold water to the cold fluid channels at the required temperature and pressure. The mass flow rate of the water is measured by the mass flow indicator with an accuracy of ±0.5% installed on a tube which transfers the cold water from chiller to the cold fluid channels. The temperatures of the hot gas at the inlet and outlet of the heat exchanger, temperatures of the cold water at the inlet and outlet of the cold fluid channels, temperatures of the thermoelectric modules and the chamber are measured using nine K-type thermocouples with an accuracy ±0.1 ◦C. The thermocouples are connected to a KEYSIGHT 34970A data logger with an accuracy of ±0.1% for monitoring the temperatures continuously. The thermoelectric modules are connected to the KIKUSUI PLZ334L electronic loader to record the current, voltage and power data with time. The accuracy of the electronic loader is ±0.1%, ±0.2% and ±0.6% for the current, voltage and power measurements, respectively. The experimental set-up of the thermoelectric generator system for waste heat recovery is shown in Figure 1. The thermoelectric generator system for waste heat recovery is designed with heat exchanger, four cold fluid channels and 12 thermoelectric modules. The hot gas passes through the heat exchanger and the cold-water flows through the cold fluid channels. The thermoelectric modules are arranged between the heat exchanger body and the cold fluid channel to utilize the temperature difference between the hot gas and cold water. The thermoelectric modules convert the temperature difference of the hot gas and cold water into power using the Seebeck effect [20]. The heat exchanger and the cold fluid channels are constructed with aluminum material, whereas the thermoelectric modules are constructed with the skutterudite material. The heat exchanger is comprised of the frame with straight fins and guide fins in the inlet and outlet diffuser sections to enable the uniform distribution of the hot gas. The cold fluid channels are provided with the internal fins structure to enable the uniformity of water. Four cold fluid channels with two at the top of the heat exchanger and two at the bottom of the heat exchanger are arranged with three modules between each channel and heat exchanger. The thermoelectric generator system for waste heat recovery is installed in the airtight chamber filled with the argon gas at the pressure of 1 × 105 Pa. The electric heater supplies the hot gas at the required temperature using the thermostat controller. The mass flow rate of the hot gas is measured by the mass flow indicator with an accuracy of ±0.5% installed near the thermostat controller. The airtight vacuum chamber provides the constant temperature and pressure controlled by the chamber pressure regulator. In addition, the chamber pressure regulator indicates the inlet and outlet temperatures of the hot gas. The constant temperature chiller supplies the cold water to the cold fluid channels at the required temperature and pressure. The mass flow rate of the water is measured by the mass flow indicator with an accuracy of ±0.5% installed on a tube which transfers the cold water from chiller to the cold fluid channels. The temperatures of the hot gas at the inlet and outlet of the heat exchanger, temperatures of the cold water at the inlet and outlet of the cold fluid channels, temperatures of the thermoelectric modules and the chamber are measured using nine Ktype thermocouples with an accuracy ±0.1 °C. The thermocouples are connected to a KEYSIGHT 34970A data logger with an accuracy of ±0.1% for monitoring the temperatures continuously. The thermoelectric modules are connected to the KIKUSUI PLZ334L electronic loader to record the current, voltage and power data with time. The accuracy of the electronic loader is ±0.1%, ±0.2% and ±0.6% for the current, voltage and power measurements, respectively.

**Figure 1.** Experimental set-up of the thermoelectric generator system for waste heat recovery. **Figure 1.** Experimental set-up of the thermoelectric generator system for waste heat recovery.

The uncertainties of various measuring instruments and the measurement errors are considered in the experimental data. The temperature, mass flow rate, voltage, current, power and thermal efficiency are the experimentally predicted output data. Thus, uncertainties of the temperature, mass flow rate, voltage and current as the independent parameters are calculated with errors of the measuring instruments. The uncertainties of the power and thermal efficiency as the dependent parameters are calculated based on the linearized fraction approximation as shown by Equation (1) [21]. The uncertainties in the temperature, mass flow rate, voltage, current, power and thermal efficiency are showed as ±0.24%, ±0.76%, ±1.27%, ±1.19%, ±1.74% and ±1.91%, respectively:

$$w\_r = [(\frac{\partial \mathcal{R}}{\partial \mathbf{x}\_1} w\_1)^2 + (\frac{\partial \mathcal{R}}{\partial \mathbf{x}\_2} w\_2)^2 + \dots + (\frac{\partial \mathcal{R}}{\partial \mathbf{x}\_n} w\_n)^2]^{\frac{1}{2}} \tag{1}$$

Here, *R* is the dependent parameter, *w<sup>r</sup>* is the uncertainty in the dependent parameter, *x*1, *x*2, . . . *x<sup>n</sup>* are the independent parameters and *w*1, *w*2, . . . *w<sup>n</sup>* are the uncertainties in the independent parameters.
