wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Ultralow-Power Wireless Communication 10-3
management and basic communication tasks on a second low-power microcontroller and activating
the DSP only for measurements does save energy.
10.2.2 Energy Harvesting
The power supply is a bottleneck for many ultralow-power scenarios. Battery size is the most limiting factor
in miniaturizing nodes as battery capacities double only every 10 years in contrast to circuits, which
need only 2 years due to Moore’s law. As a result, batteries need to be replaced regularly, which is laborious
if it is difficult to access nodes. Energy-harvesting approaches provide an important alternative to batteries
and allow extending the node lifetime to its hardware limits. The obtained energy is usually temporarily
available depending on various conditions. The following energy sources can be used for harvesting:
• Solar: Several aspects should be considered if solar cells are used. First, the energy density for
outdoor and indoor usage varies strongly as can be seen in Figure 10.3. Second, the sun as a power
source is not available during the night. This can be in principle compensated by adding matching
energy storages such as batteries or ultracapacitors, but this is not feasible in Nordic countries
with polar nights. Third, the device needs access to light and cannot be integrated in nontransparent
materials or placed in dark areas.
• Radio-Frequency Power: Scheible et al. [SDE07] demonstrated how a wireless network is supplied
by an electromagnetic field created around an industrial production cell. This commercially available
product works well for static medium-sized environments that are less frequented by humans.
• Thermalelectric: The temperature difference between two adjoining materials can be used as an
energy source through the Seebeck effect. A first low-power thermalelectric generator was demonstrated
by Stordeur and Stark [SS97]. Its commercial successor by Thermo Life produces 30.μW/cm 2
from 5°C temperature difference on a 5.cm 2 module.
• Vibration: Ambient motions or vibrations are promising energy sources especially in industrial,
automotive, and avionic domains where vibrations are common. Mitcheson presents in [MGY04]
a good introduction of the different approaches from electromagnetic, electrostatic, to piezoelectric
technologies. The potential of energy harvesting for wireless communication from vibrations
was demonstrated in [R03].
• Human Body: The human body is a steady energy source emitting heat, movement, and vibration.
Thus, it can be scavenged by thermalelectric or vibration generators. Paradiso and Starner outline
the current approaches in [PS05]. Practical examples are switches of the EnOceans technology
[EO08] that use only the energy created by the human body for actuating.
Energy density
(μW/cm 3 or μW/cm 2 )
100,000
10,000
1,000
100
10
Solar
(outdoor, sunny)
Solar
(outdoor, cloudy)
Solar
(indoor)
Vibration
Acoustic noise
(@ 100 dB)
Temperature
gradient
(@ 5°C)
Batteries
(non-rearcharg.Li)
Batteries
(recharg. lithium)
Gasoline
Fuel cell
(methanol)
1
Energy harvesting
Energy storage (1st year)
FIGURE 10.3
Comparison of the energy density for different energy sources (based on [R03]).
© 2011 by Taylor and Francis Group, LLC