With the will to increase the number of sensors around us and to respect several economic and environmental constraints, researchers and R&D engineers are looking for new green and unlimited energy sources that will allow to remove batteries or wires and to develop autonomous wireless sensor networks with theoretical unlimited lifetimes. These new sources are based on ambient energy.
Unfortunately, ambient energy is not very powerful -100µW/cmł is a good order of magnitude for energy harvesters- but is enough for many applications and especially in industry.
From thousand to million sensors in our environment “More and more sensors”, this is a general will to increase the amount of information collected from equipment, buildings, environments… allowing us to interact with our surroundings, to predict failures or to better understand some phenomena. Many fields are concerned: automotive, aerospace, industry, habitat. Some examples of sensors and fields are presented in figure 1.
Figure 1: Million sensors in our surroundings
We have chosen to focus our study on industry, which is one of the most economically attractive areas. In order to reduce machine downtimes, costs of maintenance and costs of broken parts replacements, more and more industrialists are interested in developing (wireless) sensor networks able to collect many information (pressures, vibrations, temperatures) from their equipment to implement predictive maintenance.
Unfortunately, it is difficult to deploy many more sensors with today’s solutions, for two main reasons: 1- Cables are becoming difficult and costly to be drawn (inside walls, on rotating parts) 2- Battery replacements in wireless sensor networks are a burden that may cost a lot in large factories (hundred or thousand sensor nodes).
As a consequence, industrialists, engineers and researchers are looking for developing autonomous wireless sensor networks able to work for years without any human intervention. One way to proceed consists in using a green and theoretically unlimited source: ambient energy.