*3.7. Thermoelectric Generators*

As previously mentioned, the main limitation for the use of TEG is its cost-efficiency ratio; therefore, this type of generator still does not compete with conventional mechanical thermodynamic systems or the electrochemical systems of Rankine, Stirling, Brayton, expansion devices or fuel cells. These types of conventional generation systems have a wide application ranging from heat utilization with a lower output power range to greater than 10 kW. For their part, TE materials are usually utilized for applications with an output power of less than 10 kW [51], which makes them suitable for specific power requirements.

Yazawa et al. [40], focused their research on the economic viability that TEGs can achieve with respect to other types of electricity generation systems. The authors found that TEG modules with a ZT of 0.8 can achieve a cost-performance ratio of 0.86 \$/W. This makes TEG systems competitive with other power generation systems. It was also found that TEG systems have commercial profitability if they have a cost-performance ratio of less than \$1/W [34].

In the manufacture of TEG systems, the main component is the TEG module, and the other components such as heat sinks, and supports help the system to perform better. Mori et al. [42] estimated that the TEG modules only represent 60% of the cost of whole systems and focused the design of the system on heat concentration structures that would help to double the efficiency of power generation and that could reduce the total cost in the system by half. Likewise, Hendricks et al. [43] warned that the cost of the heat exchanger, which is the element of the system that most frequently increases the total costs of TEG modules, should be further investigated to achieve profitability of the system.

Lately, the automotive industry has initiated investigations into TEG systems due to the considerable losses of caloric energy that arise from the combustion process, which could be used to power other vehicle systems. Indeed, the incorporation of these energy recovery systems, could significantly reduce CO<sup>2</sup> emissions into the atmosphere.

Arsie et al. [47] proposed the incorporation of a TEG system in the exhaust of a car using commercial 14 Hz TEG modules. In their research, the temperature gradient was guaranteed using refrigerant on the cold side of the module; the system was connected directly to the battery and alternator of a vehicle, and using a longitudinal model, it was possible to determine that the system displaces the energy of the alternator by between 15 and 20%, having an average saving of ~1 g/km of CO<sup>2</sup> in standard driving cycles.

Fernández et al. [34] used commercial Bi2Te3 TEG modules for heat recovery from light duty diesel engines as they produce ~386 W of recoverable power under common vehicle driving conditions. In their research they found that in commercial TEG modules, it is only possible to recover about 37.6 W, if no additional improvements, such as advanced heat exchangers, are applied. The authors also found that when a cooling system is able to maintain the cold side of the TEG module at 50 ◦C (less than the engine system coolant temperature), up to 75 W of power can be obtained.

Heber et al. [21] in 2020, manufactured a TEG system for natural gas heavy vehicles, in which they used 168 commercial modules based on SnTe. The cost of the TEG was EUR 1811, and a maximum power of 1507 W was achieved with a power density 50 W/kg, a reduction in CO<sup>2</sup> emissions of 4.9 (9.4) g (CO2)/km, and a cost-efficiency ratio of 1.2 EUR/W, which suggests that the system is profitable.

Likewise, the use of TEGs as complementary systems has been investigated to compensate the cost-efficiency ratio in different applications to reduce CO<sup>2</sup> emissions. Thus, Bellos et al. [74] investigated the efficiency of a solar energy-induced TEG using commercial Bi2Te<sup>3</sup> TEG modules, and carrying out a financial analysis, found that the cost of the investment would be 1 EUR/W, with a payback period of 4.55 years and a leveled cost of electricity of 0.0441 EUR/kWh, indicating that this system would be unprofitable.
