October 2014
15
E
MBEDDED
W
IRELESS
Successful energy harvesting
for wireless sensor nodes
By Cristian Toma,
Microchip
This article presents simple
energy-harvesting techniques
which can be used to implement
maintenance-free wireless sensors for
cost-effective networking
in applications such as smart
homes, building automation and
M2M communication.
Using energy harvesting to power wireless
sensor nodes need not to be complicated, or
costly. Careful consideration of the appro-
priate communication protocol and data
rate, combined with the power-saving fea-
tures offered by the latest RF devices, can all
be used to reduce power consumption. The
result is a wireless sensor node which can
operate almost indefinitely and with little or
no human intervention. Whilst the cost of a
wireless network may start with the hardware,
there are other factors which can add to the
cost. These may include additional hardware
and software costs, certification to standards
such as ZigBee and Bluetooth and also roy-
alty costs. The amount of energy that can be
harvested by a wireless sensor is typically
limited by cost and size. It is therefore criti-
cal to ensure that the amount of energy drawn
by the sensor and the wireless transmitter is
lower than the amount which can be supplied
by energy harvesting.
There are a number of sources from which
energy can be harvested, of which the most
common is solar power. Solar panels come in
sizes which range from large panels integrat-
ing a high number of solar cells, to the small
cells which are used to power products such as
calculators and toys. Other sources of power
include radio waves, which are received by an
antenna and converted into electrical energy,
and the electro-mechanical energy which is
harvested from a magnet moving near to an
inductor coil. Temperature gradients can also
be used to provide thermo-electrical energy
for harvesting using the Seebeck effect.
Common wireless protocols such as ZigBee
or Bluetooth may be the first which spring to
mind when considering which wireless tech-
nology to use for an energy-harvesting design.
However, not all designs will need to incur
the cost or the complexity of using an indus-
try-standard wireless protocol. The choice
will be based on the degree of compatibility
required by each design. For example, a wire-
less headset for a mobile phone will almost
certainly need to offer wide compatibility
whereas a simple RF remote control may not
need or be able to support the cost of compat-
ibility with products of other manufacturers.
Any design which includes a wireless sensor
will need FCC or CE certification so this cost
is common across all designs. The cost of
gaining certification to a specialised standard,
however, will typically be higher than the cost
for FCC or CE certification and can often be
underestimated.
The overall cost of implementing a wireless
standard goes much further than basic hard-
ware and software costs. Before going through
the compliance process the design will prob-
ably need to undergo pre-compliance test-
ing. Hiring the specialist equipment for these
pre-compliance tests can cost around $750 per
month. Then there is the cost of the actual cer-
tification process to a wireless standard which
will typically include costs for compliance
testing, profile testing and the appropriate
hardware sniffer tools. Altogether these tests
bring the typical cost for certification to the
ZigBee standard to around $3000. Then there
may also be an annual membership fee as well
as per-chip royalties which will need to be
included. The overall impact of certification
on the cost-per-unit will depend on the pro-
duction volume. Take, for example, a design
with hardware costs of around $1 to $1.5 per
unit over a production quantity of 10,000
units. At an estimated cost of around $10,000,
certification to FCC would effectively double
the cost per unit of the product. Then the cost
of certification to an RF standard, including
pre-compliance testing, the certification pro-
cess and RF test equipment, could easily push
the cost over $10,000.
Effective management of power consumption
is critical for a wireless sensor node powered
by energy harvesting and will have an impact
on every design decision. The configuration of
the RF transmission deserves special consid-
eration to enable the system to eliminate all
unnecessary power consumption. Parameters
