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98 Internet of Things Strategic Research and Innovation Agenda
Fig. 2.33 Ambient sources’ power densities before conversion.
(Source: CEA-Leti).
2.10.2 Energy Harvesting
Four main ambient energy sources are present in our environment: mechanical
energy, thermal energy, radiant energy and chemical energy. These sources are
characterized by different power densities (Figure 2.33).
Energy harvesting (EH) must be chosen according to the local environment.
For outside or luminous indoor environments, solar energy harvesting
is the most appropriate solution. In a closed environment thermal or mechanical
energy may be a better alternative. It is mainly the primary energy source
power density in the considered environment that defines the electrical output
power that can be harvested and not the transducer itself. The figure also
shows that, excluding “sun-outside”, 10–100 μW is a fair order of magnitude
for 1 cm 2 or 1 cm 3 -EH output power [132].
Low power devices are expected to require 50 mW in transmission mode
and less in standby or sleep modes. EH devices cannot supply this amount of
energy in a continuous active mode, but instead intermittent operation mode
can be used in EH-powered devices.
The sensor node’s average power consumption corresponds to the total
amount of energy needed for one measurement cycle multiplied by the frequency
of the operation.
For example, harvesting 100 μW during 1 year corresponds to a total
amount of energy equivalent to1goflithium.