3. Thermoelectric power generation and energy harvesting system

Thermoelectric power generation systems have typically needed to have a very high temperature gradient across the thermoelectric module(s) in order to achieve a useful electrical power output. This limitation has been a barrier to the successful application of this technology for power generation, and limited the technologies use to mainly niche applications, for example, in deep-space spacecraft power. However, with parallel developments in the area of electrical energy storage in supercapacitors, and low power DC to DC converters and boost converters, it is possible to develop a thermoelectric energy harvesting system that will operate from very low temperature gradients of around 1 K and be able to output useful power levels. This was previously very difficult to achieve and would have required several thermoelectric modules to be connected electrically in series, and thermally in parallel, increasing the overall system weight, size, and cost, and would only

Table 1.

calculation.

Figure 5.

results).

Figure 6.

24

(theoretical results).

Summary of performance characteristics of a 127 couple thermoelectric module obtained by theoretical

A Guide to Small-Scale Energy Harvesting Techniques

Power generated at the load PL with a temperature difference ΔT between 0 and 100 Kelvin (theoretical

Voltage VL and Current IL generated at the load with a temperature difference ΔT between 0 and 100 Kelvin

achieve relatively small levels of power generation unless a significant temperature gradient could be achieved across the modules.

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.

Thermoelectric Energy Harvesting

DOI: http://dx.doi.org/10.5772/intechopen.85670

3.2 Evaluation of the thermoelectric energy harvesting system

maximum of 4 mA, reducing the capacitor charge time significantly.

electronic water pump.

27

For the purpose of evaluation, a Melcor CP1.4-127-045L thermoelectric module

If an electrical load is connected across the supercapacitor output terminals, the supercapacitor will discharge and supply electrical power to the load. For evaluation purposes, a number of different electrical loads have been tested, and successfully operate from the electrical power delivered from the supercapacitor during discharge including; a piezoelectric buzzer; light emitting diode (LED); humidity sensor; pressure sensor; low power microcontroller; DC motor; and a miniature

The piezoelectric buzzer, LED, humidity sensor, and pressure sensor can be powered for a considerable amount of time before the 1F capacitor becomes discharged. For example, a Maplin electronic KU58 piezoelectric buzzer operates between 3 and 12 V with a maximum input current of 2.38 mA at 5 V, and successfully operates above 3 V for 18 minutes, and is still audible for in excess of one and a half hours when connected to the 1F supercapacitor. The miniature electronic water pump, DC motor, and low power microcontroller operate successfully from the 1F

and HX8-202-FM heat sink is connected for power generation, with a hot side temperature TH of 323 K, and a temperature difference between both sides of the module ΔT of 9 K, the thermoelectric module's open-circuit voltage is 310 mV. An LTC3108EDE step-up DC to DC converter evaluation board, configured to output 5 V on the Vout pin, was connected to the thermoelectric module output terminals as shown in Figure 9. When the thermoelectric module is connected to the LTC3108 converter, the module's output voltage and current is 116 mV at 30 mA. The boost converter and DC to DC converter increase the thermoelectric output voltage and supply a stable voltage of 5 V at 0.4 mA to the 1F storage supercapacitor, charging the supercapacitor. The 1F supercapacitor reaches maximum charge after approximately 3 h [14]. Increasing the temperature difference across the thermoelectric module, and therefore increasing the thermoelectric module's output power, would have the effect of increasing the output current of the DC to DC converter, up to a

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 to ensure repeatable and reliable operation.
