Image Processing Reference
In-Depth Information
Also,aDC-DCconverterinthecontrolunitmightbenecessarytoadaptthevoltageand/orcurrent
level of the source to that of the system and to increase the total eiciency. his is especially important
at low available power levels. If, on the other hand, a battery is the main energy source, backup with
alternativepowercanbeusedtoextenditslifetimeandreducetheneedofservice.
27.9.3 Sources and Technology
The energy required to operate the WSN device can be supplied in three principal manners.
It can be stored in the device (energy storage), transmitted from a dedicated external source
(energy transmission), or it could be based on converting waste energy from the environment, the
monitored/controlled process or from some user action (energy conversions). The first category is
by far the most prevalent method employed in WSN devices, while most research is being conducted
in the energy conversion area. The approach of converting available energy, often labeled energy-
scavenging or energy harvesting, holds the promise of energy autonomous devices with lifetime
limited only by the component lifetime.
27.9.3.1 Energy Storage
If energy is stored in the device, great care needs to be taken when calculating the required energy
needed to operate the device for the targeted lifetime (as discussed in Section ..). Energy storages
can be divided into permanent storage and electrical buffer storage.
Permanent storages include:
•
Primary batteries: Primary batteries remain the most common means to power WSN
devices today. Although continuous improvement is being made to battery technology in
terms of battery capacity and cost, the energy density has not really improved that much
over the years. The practical energy density of primary batteries is around . Wh
cm
.
However, based on which technology you employ the energy density, open circuit voltage,
self-discharge rates, and properties at high and low temperatures can vary greatly.
•
Chemical storage/fuel cells: Combustible materials (chemical fuels) enable the storage of
energy with very high densities, which can be converted into electric power by converters
like a combination of a machine (motor, turbine) with a generator, or a fuel cell. Availabil-
ity of fuel cells is currently limited to hydrogen, methane, or methanol, with achievable
energy densities in the range of a little more than Wh
/
cm
.
•
Heat storage: Heat storage systems make the use of the latent heat involved in melting
or evaporation of so-called phase change materials (PCMs). Latent heat storages are for
example used for the heating and cooling of buildings. Commercially applicable PCMs
are available in a wide temperature range and with melting energies of up to . Wh
/
cm
.
/
Depending on the required energy and power densities, electrical buffer storages can be used in many
cases in order to buffer energy as described in Section ... Two types of electrical buffer storages
are commonly used:
•
Accumulators: Accumulators are based on similar chemical principles as primary bat-
teries, but they are designed for being recharged many times. Although performance
is slowly approaching that of primary batteries, lower energy densities, significantly
higher self-discharge rates, and the limited lifetime (around years) must be taken into
account.
•
Capacitors: Double layer capacitors, also named supercaps, ultracaps, or gold caps, are
special capacitors with very large capacitance and low resistance. As no chemical reac-
tions are involved with the charging and discharging processes, rapid changes of the
