Figure 1: a) WSN and b) EH-powered sensor node
Batteries can power those devices for a limited time. Another solution consists in using energy harvesting (EH), aimed at converting ambient energy into electricity. This green technology also gives a theoretically unlimited lifetime to sensor nodes, in contrast with batteries.
Unfortunately, the power output of micro energy harvesters (Eh) is generally limited to some tens or hundreds of microwatts and the power consumption of RF-emitters or microcontrollers can reach some tens of milliwatts, banning a continuous running mode and implying intermittent measuring and data sending. Therefore, in EH and autonomous WSN, it is more appropriate to look at energy consumption for one measure instead of power consumption.
Also, it should be noted that the value 500µJ is a key number for WSN. This value corresponds to the needed energy to get a piece of information from the environment (temperature, humidity, etc.), to convert it into numeric data with an analog-to-digital converter (ADC) and to send it using standard protocols such as Bluetooth Low Energy or Zigbee. This energy could be reduced to some tens of µJ in the near future.
Therefore, functioning mode of EH-powered WSN can be summed up as follows (Figure 2
): the energy harvesting device harvests power from its environment and stores it in a buffer (capacitor, battery) (1); µC, sensor and emitter are in standby and consume about 5µW. Measurement (2) and emission (3) are performed when enough energy is stored in the buffer. Buffer is emptied; system returns to standby, waiting for a new measurement cycle (4).
Figure 2: WSN measurement cycle
(Click on image to enlarge)
As this measurement chain uses microcontrollers and electronic devices, supply voltage must be controlled and equal to about 3V; an electrical-electrical converter at energy harvester (Eh) output is therefore essential since Eh output vary through time and is not necessarily equal to 3V. This converter is also aimed at maximizing power extraction from Eh (e.g. MPPT). As a consequence, EH-based supply source can be represented as follows (Figure 3):
Figure 3: EH-based supply source
Many ambient sources, including light and temperature gradients, and the way to turn them into electricity are currently under investigation; we focus here on vibration energy harvesting (VEH), particularly suitable for machines, motors, pipes etc.