3.1 Thermoelectric energy harvesting circuit

A thermoelectric energy harvesting circuit to power an electronic load is shown in Figure 9, and is based around a Linear Technology LTC3108 step-up DC to DC converter. The thermoelectric module output voltage is in the mV range when the

Figure 8.

Simplified thermoelectric energy harvesting block diagram [14].

#### Figure 9.

Thermoelectric energy harvesting circuit [14].

## Thermoelectric Energy Harvesting DOI: http://dx.doi.org/10.5772/intechopen.85670

achieve relatively small levels of power generation unless a significant temperature

The thermoelectric output voltage generated by a standard thermoelectric module can be boosted to a useful and stable level by using a low power boost converter and DC to DC converter. If the electrical power output from the DC to DC converter is then accumulated and stored for future use in a supercapacitor, it is possible to increase the potential output current of the system, and hence the overall power output of the thermoelectric energy harvesting system. A simplified block diagram of a thermoelectric energy harvesting system is shown in Figure 8, highlighting the five main stages of the system. The energy stored in the supercapacitor can be accumulated over time and released to the load when required. In some applications, it may be advantageous to use a voltage regulator after the supercapacitor in order to maintain a stable output voltage to the load. In general, the duty cycle of the electrical load is a critical factor in determining the design of a thermoelectric energy harvesting system. As highlighted earlier, the output power of a single thermoelectric module is often too low to power other electrical and electronic components directly unless a significant temperature difference or gradient is available, or several thermoelectric modules are connected together electrically in series and thermally in parallel. The use of temporary electrical storage in supercapacitors leads to a focus on the duty cycle of the load, as it is necessary to ensure the capacitor can be recharged before the load becomes active

A thermoelectric energy harvesting circuit to power an electronic load is shown in Figure 9, and is based around a Linear Technology LTC3108 step-up DC to DC converter. The thermoelectric module output voltage is in the mV range when the

gradient could be achieved across the modules.

A Guide to Small-Scale Energy Harvesting Techniques

to ensure repeatable and reliable operation.

Figure 9.

26

Figure 8.

Thermoelectric energy harvesting circuit [14].

3.1 Thermoelectric energy harvesting circuit

Simplified thermoelectric energy harvesting block diagram [14].

module is subject to a small temperature difference, and is boosted to a useful level by the LTC3108 step-up converter. The LTC3108 uses a boost converter, in the form of an external step-up transformer, an internal MOSFET and associated circuitry within the DC to DC converter, to increase the voltage from the thermoelectric module. Within the converter, a MOSFET switch is used to form a resonant step-up oscillator using an external 1:100 turn transformer and a small coupling capacitor C3 of around 330 μF. The frequency of oscillation is determined by the inductance of the transformer secondary winding and is typically in the range of 10–100 kHz. The AC voltage that is developed on the secondary winding of the transformer is boosted and rectified using an external charge pump capacitor C1 of 1 nF and the internal rectifiers within the DC to DC converter. The DC to DC converter itself is powered via the internal VAUX circuitry, from the input voltage supplied by the thermoelectric module, and when the VAUX supply exceeds 2.5 V, the main output of the DC to DC converter Vout becomes operational and can be programmed by the user to one of four regulated voltages of; 2.35; 3.3; 4.1; and 5 V [13]. The converter operates at very low input voltages of 20 mV, which can be achieved when a 1 K or higher temperature difference exists between the 'hot' and 'cold'sides of the thermoelectric module. Dependent on the input power received from the thermoelectric module, the DC to DC converter output Vout will be charged over time up to its regulated voltage [13], and in this case, the 1F supercapacitor will charge up to 5 V at a maximum current of 4.5 mA.
