This simple link between P, W and f can be illustrated by figure 5
. By using log-log scales, with energy in abscissa and measurement frequency in ordinate, average power consumption is represented by straight lines of slope -1. Obviously, power sources can also be represented in this diagram. It allows to compare limited sources (batteries, lithium, wood…) and ambient energy sources (e.g. 100µW green line). For example, harvesting 100µW during 1 year corresponds to a total amount of energy equivalent to 1g of lithium.
Moreover, by taking this approach of looking at energy consumption for one measure instead of an average power consumption, it appears that:
Sending 100bits of data consumes 5µJ
Measuring acceleration consumes 50µJ
Making a complete measure: measure+conversion+emission consumes 250-500µJ.
Therefore, with 100µW harvested continuously, it is possible to perform a complete measure every 1-10 seconds (0.1-1Hz). This is in agreement with many industrial needs and especially with predictive maintenance topics.
Figure 5: Power, Energy and Frequency – PWf diagrams
EH Technological offers and actors
According to a study market performed by CEA-Leti, EH-powered aWSN is a field of growing interest. The technological offer is being improved and diversified. For the same reasons, the number of industrial actors increases. Figure 6
presents several industrial actors working on energy harvesting (except photovoltaics). Obviously, many research centers also work on EH (University Of Southampton (UK), MIT (USA), Peking University (CN), Holst Centre (D), Berkeley (US), INSA (FR), LETI (FR), Fraunhofer(D)…)
Figure 6: Industrial actors on EH (except photovoltaics)
Unfortunately, except for PV cells, EH is still perceived by industrialists as a non-mature technology that requires much improvements before being really interesting and widely used. Nevertheless, its advantages compared to wires or batteries are already well perceived. Table 1
presents some industrialists’ visions of EH (CEA-Leti study market). We compare these results with standard solutions, i.e. batteries and wires.
Table 1: Industrialists’ visions of EH