Image Processing Reference
In-Depth Information
stored charge, i.e., high currents, are possible. A major advantage over accumulators is
longer lifetime (
years) and significantly higher number of load cycles. The technol-
ogy is rapidly progressing and commercially available products provide energy densities
up to . Wh
>
cm
. Currently, they are often used in photovoltaic systems to bridge the
day and night cycle of solar power.
/
Wireless Vibration Monitoring Case Example
he main requirement when choosing battery technology for the wireless vibration sensor was a
high energy density (to comply with size/weight requirements). A primary battery based on a sin-
gle lithium-thionyl chloride cell was chosen due to its high energy density, high nominal voltage,
low self-discharge rate and overall good properties at both high and low temperatures.
However, this type of primary battery has two main drawbacks. Large batteries may pose safety
problems (subject to transportation restrictions) and voltage delay due to passivation.
Due to the size requirements, the chosen cell was small enough not to be subject to any trans-
portation restrictions.
Passivationontheotherhandcouldstillbeaproblemasitmightleadtovoltagedelay.Volt-
age delay is the time lag that occurs between the application of a load on the cell and the voltage
response. As we have an extremely low duty cycle in the wireless vibration application (each sensor
node should perform one vibration measurement per weeks), this behavior becomes particu-
larly pronounced (since the passivation layer grows thicker). As described earlier, a capacitor bank
was added in order to secure sufficient energy to operate the radio circuit despite a varying oper-
ating temperature (see Figure .). .).This buffer also provides a remedy for potential passivation
issues, as the initial energy will be drawn from the capacitor bank, thus eliminating any voltage
delays.
27.9.3.2 Energy Conversion
Harvesting and converting waste energy from the environment certainly appears as an appealing
solution, as the lifetime of the device would (in theory) be limited only by the component lifetime.
However, this requires that the waste energy is readily available with a constant (or predictable) level.
If this is not the case, large internal buffers might be required to guarantee reliable operation (which
increases the size/weight/cost of the device). his also means that the applications in which the device
can be deployed might be restricted, or that special considerations and/or measures need to be taken
to guarantee some minimal level of power inflow (which makes device installation more complex
and costly).
Energy conversions can be achieved in many ways, but the most common methods include:
•
Photovoltaic cells: A wide range of photovoltaic systems are available on the market today
based on diverse materials and technologies that have reached different maturity levels.
Cell types also differ with respect to cost, electrical characteristics, efficiency, topology,
thickness, shape, mechanical properties, and adaptation to various lightning conditions.
Regardless of the varying energy conversion efficiency (%-%), the lightning condi-
tionshavebyfarthegreatestimpactonthefeasibilityofphotovoltaiccells.Intypical
environments, the illumination density will vary within at least three orders of magni-
tude. For example, compare the illumination density of full sunlight
cm
(∼
mW
/
)
,
