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Energy Harvesting and Wireless Sensor Networks. Adam Skelton. Purpose of Energy Harvesting. 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.

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purpose of energy harvesting
Purpose of Energy Harvesting
  • 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
design objectives of a solar harvesting system
Design Objectives of a Solar Harvesting System
  • 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
Simplest Design
  • 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:
    • Simple
    • Cheap
  • Disadvantages:
    • No overcharge protection
    • No monitoring or control of charge current
another design
Another Design
  • Use a DC-DC Converter to regulate the solar cell voltage and establish a constant output voltage
  • Advantages
    • 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
    • All DC-DC conversion involves energy loss
    • High-frequency switching introduces noise into the system
heliomote an existing design
Heliomote, an Existing Design
  • 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:
    • 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:
    • More expensive
    • Requires PCB fabrication
to do
To Do
  • Purchase equipment
    • 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
References
  • Mpower Solutions Inc. (http://www.mpoweruk.com/life.htm)
  • Energy Scavenging for Mobile and Wireless Electronics p. 26