**5. Application demonstration**

Although a high output voltage and output power could be achieved by the fabricated micro-TEG, a high thermal source is needed. In turn to low-thermal sources, its output power is in small value, which cannot be used as a power source for electronic devices. To overcome this issue, a DC-DC converter is required, which amplifies the output voltage of the micro-TEG from an mV range to V range of the output of the DC-DC converter. Thus, this makes micro-TEG possible for powering electronic devices with low-power consumption. In this section, the micro-TEG for powering calculator and twist watch is demonstrated. A DC-DC converter is utilized to boost the output voltage of the micro-TEG up to sufficient levels to store in an energy-storable unit, which is subsequently supplied to electronic devices. The energy storable unit can be a capacitor, a supercapacitor, or a rechargeable battery. We have developed successfully micro-supercapacitors-based graphene nanowalls with PANI in liquid state [53] and solid state [54] and with MnO2 [55]. Although these microsupercapacitors show a high charge and discharge processes, their storable energy is lower than that of commercial rechargeable battery. In this section, a rechargeable battery from Enercera [56] is employed for the application demonstration. Two applications utilizing the micro-TEG are conducted, as follows.

#### **5.1 Micro-TEG for powering portable electronic devices**

**Figure 13(a)** illustrates the experimental setup for the micro-TEG as a power source for the calculator. It consists of Peltier (as a heat source), copper blocks, temperature sensors, the DC-DC converter, a rechargeable battery, and a calculator. *Micro-Thermoelectric Generators: Material Synthesis, Device Fabrication, and Application… DOI: http://dx.doi.org/10.5772/intechopen.102649*

**Figure 13.**

*(a) Experimental setup for powering portable electronic device. (b) DC-DC output as a function of temperature difference. (c) Battery charged up by the micro-TEG. (d) Micro-TEG as a power source for calculator.*

The harvester energy is accumulated and stored in the rechargeable battery via the DC-DC converter and then supplied to electronic devices. **Figure 13(b)** shows the output of DC-DC converter over the temperature difference across the micro-TEG. The experimental results indicated that output of DC-DC converter reaches 2.8 V at Δ T = 2°C and 4 V at T = 8°C. **Figure 13(c)** shows the rechargeable battery characteristic, which increases from 0 V to 1.8 V, taking approximately 8 minutes. **Figure 13(d)** shows the demonstration of using micro-TEG as an electrical power source for the calculator. The calculator can be powered on and used once the rechargeable battery gets over 1.5 V.

#### **5.2 Micro-TEG for powering wearable electronic devices**

**Figure 14(a)** illustrates the experimental setup for powering a twist watch. One side of the micro-TEG is in contact with human skin while another side is attached to the backside of the twist watch. α-Gel is pasted on both sides of the micro-TEG to enhance heat transfer between interfaces. The DC-DC converter and rechargeable battery are employed, which are similar to those mentioned in Section 5.1. The DC-DC converter, rechargeable battery, and micro-TEG are arranged on the twist watch, as shown in **Figure 14(b)**. **Figure 14(c)** shows the output of the micro-TEG and battery charge when twist watch is worn. It takes approximately 5 minutes for the rechargeable battery to reach 1.2 V. With this energy, the twist watch is powered on and runs.

Demonstrated results in this section indicate a high potential using the micro-TEG for powering not only portable electronic devices but also wearable electronic devices. Further integrated functions, including sensing (humidity, temperature, gases, etc.),

**Figure 14.**

*(a) Experimental setup for powering wearable electronic device. (b) The photo of the self-powered twist watch. (c) TEG output and battery charge-up.*

displaying (screen display), and transmitting (radio frequency, Bluetooth, etc.) functions, should be investigated to produce a smart system for using in wireless IoT sensing systems.

### **6. Conclusions**

In this work, not only basic knowledge about thermoelectric generators but also experiences on material synthesis, device fabrication, and application demonstration are reported. By investigating electrochemical deposition, high-performance thermoelectric materials have been achieved. Three kinds of high-performance thermoelectric materials, including thick bulk-like thermoelectric material, Pt nanoparticles embedded in a thermoelectric material, and Ni-doped thermoelectric material, are reported and discussed. Besides the material synthesis, novel fabrication methods can also help increase the output power and the power density of the micro-TEG significantly. Two fabrication processes, micro/nano fabrication technology and assembly technology, are investigated to produce high-performance micro-TEG. Moreover, the fabricated micro-TEG is successfully demonstrated for powering portable and wearable electronic devices. The contents of this paper are based on our experimental research. It is our hope that this review may be a useful reference for those working in the field of thermal-to-electric energy conversion, especially on the micro-TEG.

*Micro-Thermoelectric Generators: Material Synthesis, Device Fabrication, and Application… DOI: http://dx.doi.org/10.5772/intechopen.102649*
