Piezoelectric Energy Harvesting Devices: An Alternative Energy Source for Wireless Sensors
Table 6
Power densities of typical ambient energy sources.
Energy source
Characteristics
Efficiency
Power density
Comments/challenges
Light
Outdoor Indoor
10–25%
100 mW/cm2 100 μW/cm2
While solar energy is harvesting is an established technology, aiming for small-scale harvesters is difficult because power output directly linked to surface area. For design of embedded wireless sensor nodes to be deployed indoors or overcast areas such as buildings, and forestry terrains, where access to direct sunlight is often not available, solar energy source may not be a suitable choice.
Thermal
Human Industrial
0.1% 3%
60 μW/cm2 10 mW/cm2
Electric current is generated when there is a temperature difference between two junctions of a conducting material (called the Seebeck effect). Thermal energy harvesting uses temperature differences or gradients to generate electricity. Efficiency of conversion is limited by the Carnot efficiency. The efficiency of thermoelectric generators is typically less than 1% for temperature gradient less than 40°C and it is hard to find such temperature gradient in the normal ambient environment.
Vibration
Hz-human kHz-machines
25–50%
4 μW/cm2 800 μW/cm2
Energy from vibrations can be extracted using a suitable mechanical-to-electrical energy converter or generator. Generators proposed to date use electromagnetic, electrostatic, or piezoelectric principles. Vibration energy harvesting is highly dependent on excitation (power tends to be proportional to the driving frequency and the input displacement).
Radio frequency
GSM 900 MHz WiFi 2.4 GHz
50%
0.1 μW/cm2 0.001 μW/cm2
Without a dedicated radiating source, ambient levels are very low and are spread over a wide spectrum. There is a limit to the amount of power available for harvesting since the IEEE 802.11 standard prescribes the maximum allowable transmission power allowable (1000 mW in the USA, 100 mW in Europe, and 10 mW/MHz in Japan).
