Thanks to the reduction of circuit sizes and progress in microelectronics, basic electronic functions are consuming less and less power, allowing us to use a new ecological and durable supply source for wireless sensor networks (WSN): ambient energy, including sun, temperature and vibration.
Wireless sensor networks: goals and needs
Today, one of the goals of researchers and R&D engineers is to develop sensor networks able to collect data from their surrounding environment. WSN ( Figure 1a ) are made of several sensor nodes ( Figure 1b ); each node is able to get information from its environment (temperature, vibrations, light, etc.), to turn it into numeric data and to send it to a base station. Many fields, such as transportation, industry and aeronautics, have a strong interest in developing and using WSN to increase their productivity (real-time monitoring), reduce their costs or limit machine downtimes (predictive maintenance).
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