## 4 Applied Current CP
### Temperature Difference Electricity Generation Power Supply Technology  
Temperature difference electricity generation (TEG) is a technology based on the Seebeck effect of thermoelectric materials to convert heat into electricity. The p-type semiconductor material containing holes and the n-type semiconductor material containing electrons are connected at one end by a copper conductor to form a pn junction. Then, the two ends of the pn junction are connected to a heat source and a cold source, respectively, creating a temperature gradient across the pn junction. At this time, if a load resistor is added to form a closed circuit, a current can be measured in the system [51]. The temperature difference and output current generated by a single pn junction are very low; therefore, the TEG battery used for power generation is usually composed of multiple pn junction connections.  
Generally speaking, the lower the temperature value, the more difficult it is to utilize the waste heat. However, TEG is able to utilize the waste heat with a temperature difference of several tens of degrees. The single group of TEG sheets made by [PERSON] et al. [52] had an output power of 9.42 mW and an output voltage of 198.85 mV at a temperature difference of 16 \({}^{\circ}\)C. This makes it possible to utilize low-grade thermal energy, such as seawater, geothermal heat, industrial and agricultural waste heat, etc., to generate electricity that can then be used for the applied current CP in the above environments.  
[PERSON] [53] used the greenhouse buried heating pipeline as the heat source and the soil as the cold source to verify the effectiveness of the TEG cell of bismuth telluride material for the CP of the buried pipeline. The pipeline potential was \(-\)0.51 V (v.s.CSE) when no CP was added and \(-\)0.933 V when CP was available, indicating that the CP device was effective and the pipeline was protected. In a 90d natural corrosion experiment, when the average corrosion rate of the pipeline was 0.0042 g\(\cdot\)m\({}^{-2}\cdot\)h\({}^{-1}\) and the protection potential provided was \(-\)1.1 V, the degree of protection reached 92.79%.  
[PERSON] et al. [54] found that the internal resistance of the TEG unit did not change significantly with temperature difference, and the voltage fluctuation of the CP system was less than 0.2 V, which indicated that the TEG unit had high stability and could provide a long-term, stable protection current. An allotment of 3.5 wt.% NaCl was added to the concrete simulating liquid to simulate the corrosion of reinforcing steel in concrete under a Cl-erosion environment. When \(\Delta\)T = 20 \({}^{\circ}\)C, and when two TEG units were in series (series voltage about 2.7 V), the protection potential of the reinforcement connected to it at 3d could reach about \(-\)0.77 V (v.s.SCE). Electrochemical noise analysis found that the average value of the noise potential of the reinforcement gradually shifted positively from \(-\)40 mV at the beginning to \(-\)24 mV. The noise resistance gradually became greater, indicating that the reinforcement entered a thermodynamic steady state from a corrosion state. It showed that the corrosion of reinforcing steel was inhibited after the CP current was applied, which provided an experimental reference for the use of temperature difference power generation for CP of reinforcing steel in offshore structures.  
The output current of TEG is direct current, and the system has a simple structure with no moving parts and no noise; however, there are disadvantages, such as low power generation efficiency and short service life of the temperature difference components [55].