Download Energy Harvesting and Wireless Sensor Networks

Energy Harvesting and Wireless
Sensor Networks
Adam Skelton
Purpose of Energy Harvesting
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Increase the field lifetime of the nodes.
Energy harvesting allows on-site charging of rechargeable
batteries, which can be cycled hundreds of times before
their performance degrades.
With proper hardware and energy management, the
lifetime can be extended almost indefinitely. For example,
a NiMH battery will decrease to 80% of its rated capacity
after about 500 full cycles. However, if it is cycled daily at
only 10% of its capacity, the lifetime will increase to 5000
cycles, or about 13 years.1
Sources of Ambient Energy
Design Objectives of a Solar Harvesting
System
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Transfer energy from the solar panels to the batteries as
efficiently as possible
Mimic the ideal charging characteristics of the batteries
for maximum lifetime
Prevent battery overcharge and undercharge
Reduce or eliminate backwards discharge current when
the solar cells are not providing power
Provide stable, low-noise power to the mote
Provide data to the mote for use by energy-aware
programming
Simplest Design
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Attach solar cells directly to the batteries through a
diode.
With proper matching of battery and solar cell voltages,
this should be possible without the use of a DC-DC
converter.
Advantages:
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Simple
Cheap
Disadvantages:
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No overcharge protection
No monitoring or control of charge current
Another Design
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Use a DC-DC Converter to regulate
the solar cell voltage and establish a
constant output voltage
Advantages
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Easier to maintain the ideal operating
point of the solar panel
Fixed output voltage
DC-DC converters have a current limit
which should help avoid overcharging
Disadvantages
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All DC-DC conversion involves energy
loss
High-frequency switching introduces
noise into the system
Heliomote, an Existing Design
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The Networked and Embedded Systems Lab (NESL) at the
University of California, Los Angeles has already developed
solar harvesting hardware for the mica motes.
Their design is freely available, so we could either use it
directly or modify it for our own purposes.
Advantages:
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Already tested
Existing nesC interface
Battery overcharge and undercharge protection
Provides a steady 3V to the mote
Provides solar cell voltage and battery charge current data to the
mote
Disadvantages:
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More expensive
Requires PCB fabrication
To Do
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Purchase equipment
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Solar panels (Solar World 4-4.0-100), available for $27 each
More samples of the MAX859 DC-DC Converter
Build and test the two preliminary designs: attaching a solar
panel directly to the battery, and using a DC-DC converter.
Decide if we want any Heliomotes. The PCB plans are freely
available, so it should be easy to have them fabricated.
There is a also a spin-off company from previous UCLA
students called Atla Labs that makes a proprietary version of
the Heliomote. I have emailed them for pricing.
Get working hardware into the field and establish a test-bed
for energy-aware algorithms.
References
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Mpower Solutions Inc.
(http://www.mpoweruk.com/life.htm)
Energy Scavenging for Mobile and Wireless Electronics p. 26