*2.3. Thermoelectric Function Testing of the Novel TEP*

The performance test of the novel TEP was mainly divided into two parts. The first was to discuss the thermoelectric performance test of the novel TEP using the titanium dioxide nanofluid as the electrolyte under different filling amounts and pressures. Afterwards, the thermoelectric function test of four different electrolytes, including nanofluid, seawater, deionized water, and polymeric nanofluids, composed of nanoparticles added to sodium hydroxide filled into the novel TEP at different temperatures, was investigated. The experimental process of thermoelectric performance is shown in Figure 3.

**Figure 3.** Thermoelectric function testing process of the novel TEP.

The present research mainly exploits the one-dimensional thermal resistance model [22] to evaluate the heat dissipation characteristics of the novel TEP, and the thermal resistance was used to judge the heat dissipation capacity. The larger the thermal resistance value was, the worse the heat dissipation effect was. The thermal resistance of the interface between the heat source and TEP was controlled between 0.05 and 0.09 ◦C/W. In addition, the thermal conductivity of the novel TEP was derived through the one-dimensional

Fourier heat conduction equation. Thermoelectric performance experiments were carried out in different filling ratios of the novel TEP electrolyte, tube pressures, and electrolyte compositions, and thermocouples were employed to detect the heat transfer of the novel TEP under different temperatures. The temperature difference between the bottom and the top of the novel TEP was adopted to calculate the thermal resistance of the TEP, and then the thermal conductivity of the novel TEP was derived. The thermocouple measurement temperature points and thermal resistance analysis are shown in Figure 4. TA is the top surface temperature of TEP, TB is the side surface temperature of TEP, TC is the bottom surface temperature of TEP, and TD is the heating source temperature.

**Figure 4.** Thermal resistance network of the novel TEP.

The effects of filling ratio and vacuum pressure for the novel TEP are major parameters on the thermal performance. In the low-vacuum glove operation box, the oil-free pump was used to evacuate the pressure to the experimental test. The electrical property experiments of the novel TEP with four different solutions involving seawater, deionized water, nanofluid, and polymeric nanofluid are the output current density and power density between 25 and 100 ◦C. The filling electrolyte processes of the novel TEP are as follows: firstly, fix the TEP in the low-vacuum glove operation box and open the tube cover. When the vacuum pressure reaches the experimental pressure via vacuum pump, close the suction valve. Use the operating gloves to tighten the tube and the cap of the novel TEP. Open the suction valve to make the box pressure reach normal pressure. Then, fix the TEP on the heating platform of the low-vacuum heating system with a downward force of 14.7 N through the baffle plate. The filling ratio experiment was inspected at 400 torr vacuum pressure, at 7 mL (100%), 5.6 mL (80 %), 4.2 mL (60%), 2.8 mL (40%), and 0 mL (0%), and the vacuum pressure tests were 400, 500, 600, and 760 torr, respectively. The experimental processes of the thermoelectric performance were as follows. First, fix the TEP tube on the heating platform, and apply heat dissipation paste on the bottom to make the heat transfer uniform. Connect to the anode of the aluminum tube and cathode of the carbon rod respectively, and connect a desktop electric meter. Power on the operating platform and set the heat source temperature, and start the heating device and set the heating time to 40 min. When the temperature, current, and voltage of the heat source stabilize and reach the experimental temperature, record the temperatures, output current, and voltage. Finally, adjust the heat source to the experimental temperature of 25 to 100 ◦C, and reset the heating time. Repeat the above the steps and measure temperature, current, and voltage of the novel TEP for different vacuum pressures and filling ratios under four different electrolytes.
