Literature review

1.1 Literature review

Many state-of-art technologies can scavenge or harvest energy from the environment, such as light exposure, thermal gradients, human power, air flow, acoustic noise, and vibration [9]. Energy harvesting is to convert ambient energy into electrical energy. The environment has an inexhaustible energy supply compared with the common storage devices like batteries and fuel cells. Various approaches to extract energy from the environment to drive low power electronics are studied and compared in this chapter. As a result, the performance of the energy devices is characterized by their power density, instead of energy density used for traditional storage devices.

1.1.1 Light exposure

A popular and mature method to scavenge energy is light exposure. Solar cells or photovoltaic cells are the leading technology to convert solar radiation directly to electricity with high conversion efficiency. It can provide power at low operating cost, and is virtually free of pollution. Photovoltaic cells function by the photovoltaic effect [10], the generation of voltage when a device is exposed to light. The photons of the incident solar radiation excite the electrons in the semiconductor, thus allowing the electrons to move freely and the electric current to run through the load. The operation schematic is shown in Fig. 1.1. The device has efficiencies ranging from 12% to 25%

for single crystal silicon. The thin film polysilicon and amorphous silicon cost less than single crystal silicon cells but have lower efficiency [11]. Overall, photovoltaic

energy conversion offers sufficient output power besides being a mature IC-compatible technology. Nevertheless, the output power of photovoltaic devices depends heavily on the environmental conditions. For instance, the photovoltaic cells offer adequate power density up to 15 mW/cm² if the device is outdoors and operated primarily during daytime. However, in normal indoor office lighting, the same photovoltaic cell will only produce about 10 μW/cm². Because of this characteristic, photovoltaic cells are restricted to specific applications.

Fig. 1.1 Photovoltaic energy conversion [10]

1.1.2 Thermoelectric effect

Temperature variations can serve as a power source. This phenomenon is called the Seebeck effect, or the thermoelectric effect [12]. When two differential metals are connected in a closed loop, temperature variation in the loop causes the electron movement and a potential is built up between the two metals or semiconductors junctions.

The developed voltage is proportional to the temperature different between the hot and cold ends, and to the Seebeck coefficients of the two materials. Large Seebeck coefficients and high electrical conductivity can improve conversion efficiency and minimize power losses. Materials typically used for thermoelectric energy conversion

Photovoltaic cell

include such as Sb2Te3, Bi2Te3, Bi-Sb, PbTe, Si-Ge, polysilicon, BiSbTeSe compounds, and InSbTe, which are not completely compatible to the IC process. In [13], different annealing conditions have strong influence on the electrical resistivity of Bi-Sb and consequently the thermoelectric generator performances. An output power density of 140 μW/cm³ for a 100 ˚C temperature gradient is obtained but the temperature difference of this level is not common in a micro system [14]. So the output power is limited without large thermal gradients. The thermocouple element is used to produce larger output voltage, as shown in Fig. 1.2. Thus, connecting several thermocouple elements in a series configuration can be beneficial. However, large series resistance increases ohmic power loss and thus reduce the overall power conversion efficiency.

Fig. 1.2 Thermoelectric energy converter [12]

1.1.3 Human body movement

Electronic productions like PDAs or notebook are often limited by the battery capacity and necessity for recharging. Therefore, it is feasible to alleviate these problems by harvesting energy from the human movement. In recent years, needs of wearable electronic devices [15, 16] have grown significantly. Electric energy has been extracted by scavenging waste power from human activities, such as breathing,

body heat, arm motion, and walking. More attention is given to walking since this process seems a more practical source of power for the wearable electronic device.

For example, a small women’s shoe has a footprint of approximately 116 cm², as shown in Fig. 1.3. The shoe contains a piezoelectric shoe insert and the deformation of the piezoelectric pile generates power when walking. In addition, the motion of heels might be converted to electrical energy through traditional generator like spring.

The energy stored in this compressed spring can be returned later in the gait to the user. Consequently, a maximum electrical power of 8.4 W could be generated by a 52 kg user at a brisk walking pace.

Fig. 1.3 Shoe generation system [15]

This energy could be used in a variety of low-power applications, such as pagers, health monitors, self-powered emergency receivers, and radio frequency identification tags. The application is limited by the piezoelectric and IC integration issues as well as power delivery issues. The piezoelectric shoe inserts offer a good solution for specific requirement such as RFID tags or other wireless devices worn on the foot.

Piezoelectric insert

Metal spring Generator system

1.1.4 Acoustic noise

Acoustic noise is usually considered as pollution but it can be seen as a power source. In [17], the power density of acoustic noise is 0.96 μW/cm³ at 100 dB. The energy level was lower than the power source as mentioned above. Moreover, sound volume of 100 dB closes to the airplane engine, which exceeds the tolerance of human ears. Thus, the power source from the noise is not practical. Recently research and development of this method has extracted limited power from noise with extremely high noise level. Therefore, it is not a feasible power source for common application.

1.1.5 Wind

Wind power is a renewable energy, which is the conversion of wind energy into a useful form like electricity by wind turbines or windmill. A simple eight-blade windmill that could generate electrical power from wind was developed, as shown in Fig. 1.4(a) [18]. The windmill consists of a 750 cm² base and a 70 cm long upright, which are used to support the structure. On one end of the axial part there was a pulley which drives the generator, as shown in Fig. 1.4 (b). The generator was designed to working of electromagnetism. A piece of plastic pipe had plastic circles glued to it to form a bobbin for the coil wire. Two magnets were attached on the axial part of plastic pipe. Therefore, the magnets are held in place by their own attraction but it should be glued them in place to promise that they rotate without hitting the sides.

In this case, the power from wind is related to the air velocity. With slow wind at 3 m/s velocity, the average power is about 80 μW /cm³. The maximum average power density of 1060 μW /cm³ at 12 m/s air velocity was produced from a strong wind. This indicates more usable power can be generated from the large wind.

Fig. 1.4 (a) Photo of the windmill, (b) close-up of the simple generator [18]

1.1.6 Ambient Vibration

Ambient vibration was present in many environments. Most sources of vibrations in the environment are at low frequencies between 60 and 200 Hz [9]. Low level mechanical vibration occurs in exterior windows, aircraft, automobile, industrial environments, and small household appliances. The maximum power is extracted at resonant with ambient vibration. Theory and experiments show that more than 300 μW/cm³ can be generated [19]. A more detailed discussion of this method is presented in Chapter 2.

1.1.7 Summary of power sources

Comparison of the power source and energy storage devices is shown in Table 1.1. The table presents power sources and the values are estimates taken from literature or analysis. Based on the above survey, vibration is chosen as the source of energy scavenging due to its ubiquity and sufficient power density.

(a)

(b) Generator Pulley

Stand

Blade Magnet

Table 1.1 Comparison of power sources

Power sources Power density Commercially

available?

Solar (outdoors) [10] 15, 000 μW/cm² Yes

Solar (indoors) [10] 10 μW/cm² Yes

Temperature gradient [13] 140 μW/cm³ at 100˚C gradient Soon

Human power [15, 16] 330 μW/cm² No

Acoustic noise [17] 0.96 μW/cm³ at 100dB No

Wind energy [18] 1060 μW/cm³ at 12 m/s velocity No

Vibration [19] More than 300μW/cm³ No