ENERGY SCAVENGING FROM VIBRATIONS

ENERGY SCAVENGING FROM VIBRATIONS

Energy scavenging, also known as energy harvesting refers to the process which enables the capturing and storage of ambient energy. Mostly, this term is used when the energies to be captured are in small amounts. Energy can be scavenged from different sources which include;

  • Wind: energy harvesting from wind sources has been used to power sailboats e.g. in ancient Egypt.
  • Water: hydropower plants produce approximately 24 percent of the world’s electricity. Recent research focussing on energy extraction from ocean waves has added yet another source of green energy.
  • Solar: This energy can be harnessed using solar panels or other devices and used to provide green energy.
  • Temperature: This is most commonly in the case of heat pumps which transfer heat from one point to another e.g. outside to inside a house. Other applications include geothermal plants which rely on the stable heat of the earth to provide electricity and a thermoelectric generator, which exploits the Seebeck effect to transform the difference in temperature between the human body and the environment into electricity.
    • Vibration
    • Vibration is a word that comes from the Latin word vibrationem (‘shaking or brandishing’). It refers to a mechanical phenomenon where oscillations occur about a specific equilibrium point and may be periodic (e.g. a pendulums motion) or random (like a tire’s movement on the road).
      Vibration is in many cases are not desired and can be used to point to a fault e.g. in a machine. It causes unwanted sound and results in wastage of energy. However, in some cases, vibration is important with examples being a mobile phone, a tuning fork or a woodwind instrument.
      To demonstrate the idea of vibration, we use the mass spring system shown in figure 1. When the mass, m is pulled upwards and then releases, it does the back and forth movement making displacements, x. These displacements decrease gradually as the energy in the spring (with a proportionality constant, k) is dissipated (damping) and eventually the mass comes to a standstill.
Figure 1: Mass-spring system

This proves the existence of energy in vibrations where in this case, it is used to move the mass up and down.

Vibration Energy Harvesters (VEHs)

There are several conversion techniques used in vibration energy harvesting. These are based on the following operating principles

1. Electromagnetic/ Inductive

This principle is applied in the case where a coil moves through a magnetic field thereby causing a current in the wire. This principle is shown in figure 2.
Figure 2: Electromagnetism principle

Energy harvesting devices based on this method are suited to applications at low frequency and in medium devices to drive relatively low impedance loads.
They have a limitation in that they are expensive to integrate in microsystems since micro magnets are expensive to manufacture and also require large mass displacements.

2. Piezoelectric
This method, as demonstrated in figure 3, makes use of the unique property of piezo materials whereby an applied mechanical stress results in an electric charge.
Figure 3: Piezoelectric disk
 The frequency response of a piezoelectric sensor can be observed as shown in figure 4. It shows the usable region and an area of resonance. Resonance is basically the phenomenon whereby a vibrating system forces another system to oscillate at a greater amplitude at a specific frequency. It is in this area that we can harvest the energy that would otherwise be wasted. Let us not forget that in some cases, resonance can be destructive.

Figure 4: Frequency response: input voltage vs applied force

Energy harvesting devices based on the piezoelectric principle provide suitable output voltages and are therefore well placed for miniaturization. This renders them as suitable candidates in MEMS applications. They however face a limitation in that the electromechanical coupling coefficients for thin piezoelectric films are relatively small. This means that large load impedances are required in order for the device to reach an optimum working point.

3.  Thermoelectric
As shown in figure 5, the thermoelectric technique makes use of changes in temperature between two physical locations. This becomes a thermal source and can be converted into electrical energy.
Figure 5: Thermoelectric energy harnessing
4. Capacitive
This techniques involves a change in capacitance which causes a subsequent voltage or charge increase. A capacitor is simply a device that can store electrical charges. Variable capacitors are suitable for MEMS application but they have a limitation in that they have a low power density and need to be charged to a reference voltage by the use of an external electrical source such as a battery.

Advantages for vibration energy harvesting


  • Saves on cost
  • Have long lasting operability
  • Are safe
  • Offer flexibility


Limitations of VEHs

  •  Low output voltages for electromagnetic systems
  •  Issues of miniaturization
  •  Problems in versatility as well as adaptation to the variable vibration sources.
  •  Small maximum displacement as well as inertial mass especially at MEMS scale.
  •  Limited power density at the micro scale.
  •  Narrow bandwidth which limits that resonant frequency-tuned applications are limited.


Areas of application of VEHs

Energy harvesting can be an alternative for micro powering. Imagine having no need to replace batteries by harvesting vibration energy and incorporating a suitable generator and storage system to power electronic device e.g. Wireless sensors, consumer electronics, MEMS actuators and low power devices. This would help us to tackle the biggest challenge faced by Wireless Sensor Networks (WSN) which despite having numerous application possibilities including surveillance, medical remote sensing, structural monitoring, military applications and aerospace as well as environmental monitoring, continue to be hampered by inability to be self-powering.

Major sources of vibrations which can be harnessed include;

1. Footbridge vibrations.

In a busy town or city, a lot of people pass over the bridges every day and with every step, there are some vibrations dues to walking, marching or jumping. In a research done in Sao Carlos Engineering School, Brazil, measurements revealed that pedestrians walking over the footbridge caused a 2.1 Hz vibration and a 4.2 Hz harmonic frequency. This can actually cause structural fatigue if not well designed.

2. Busy highways: The vibrations caused by passing vehicles can be used to provide energy.

References

1. Singh, U. K., & Middleton, R. H. (2007). Piezoelectric power scavenging of mechanical vibration energy. In 2007 Australian Mining Technology Conference" Smart Technologies for Overcoming the Skills Shortage" (pp. 111-118).
2. Roundy, S., Wright, P. K., & Rabaey, J. M. (2003). Energy scavenging for wireless sensor networks (pp. 51-85). Norwell.
3. Cottone, F. (2011). Introduction to vibration energy harvesting. NiPS Energy Harvesting Summer School.


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