*2.2. Structural Design of the Novel TEP and Experimental Apparatus*

Figure 1 demonstrates that the novel TEP adopts materials including polytetrafluoroethylene (PTFE), Viton rubber (Gasket), carbon, and aluminum. The carbon rod and the aluminum tube with good thermal conductivities are respectively used as the cathode and the anode. In the insight design, the aluminum and carbon are insulated with PTFE and gaskets to avoid short circuits. Afterwards, the ionic compounds and the nanofluids possessing good thermal conductivity were filled into the tube as the electrolytes, which had four types, involving titanium oxide nanofluid, deionized water, surface seawater, and polymeric nanofluids composed of nanoparticles added to sodium hydroxide (NaOH). When the tubular electrode (aluminum tube) and the core rod (carbon rod) electrode have a temperature difference, thermal energy can be directly converted into electric energy by the redox reaction of the electrolytes (four kinds), and the electrodes can generate electromotive force. In particular, the novel PET device may use the structural design between the tubular electrode and the core rod electrode to provide a greater contact area with a heat source, and may be directly immersed in a heat source.

**Figure 1.** The Novel TEP device. (**a**) Entity photo, (**b**) dimensions and materials.

The principal design of the TEP is based on the energy conservation theorem. Thermal energy increased the rate of the redox reaction, which affected the current density and transferred the energy through electrons to generate electrical energy. The novel TEP is currently devised as a cylinder with a diameter of 17 mm and a length of 80 mm, as shown in the Figure 1b. The operating principle is that the heat energy generated by the machine tool or heat source was transferred to the novel TEP and then the temperature of the nanofluid inside the tube was raised in order to increase the redox reaction rate of the novel TEP, thereby generating additional output power and transferring the power to the carrier for heat recycling.

The experimental framework of the present study is shown in Figure 2. The lowvacuum heating system used in the experiment was manufactured by the thermal-fluid illumination laboratory of National Taiwan Ocean University (NTOU) from Keelung in Taiwan, for which the operating temperature was between 25 and 120 ◦C and the operating pressure was 300 to 760 torr. The novel TEP was fixed on the heating platform and the operation control panel regulated the time and heating temperature. The low-vacuum glove operation box was employed to fill the electrolytes into the novel TEP, which was from Hoyu Technology Co., Taipei, Taiwan. The oil-free vacuum pump was utilized for low vacuum pressure with a voltage of 100 to 115 V, a motor power of 560 W, and an exhaust speed of 100 l/min. Experimental temperatures were measured by the T-type copper-nickel thermocouples with a wire diameter of 0.32 mm, occupying a measuring range between –200 and 350 ◦C and an error range of ±0.5 ◦C. The data logger of the GL-800 was made by Graphtec Co., Yokohama, Japan, which had 40 measuring items containing temperature, voltage, and humidity, etc., with a sampling time of 0.1 ms and a measurement error of ±1%. In the present experiment, the temperature data of the thermocouple can be captured

and recorded on the hard disk and output as a Microsoft Office Excel table. The digital electric meter of TM-8155 with the measurement accuracy of ± (0.05% reading + 5 digits) used in the experiment was produced by Twintex Instrument Co., New Taipei City, Taiwan, which had the functions of measuring voltage, current, resistance, capacitance, and so on. The generated power density of the novel TEP was determined by the TM-8155. Eventually, the maximum measuring error for the thermoelectric performances of the novel TEP device was thus within ± 3%.

**Figure 2.** Thermoelectric performance experiment framework of the TEP.
