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.

• 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• 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 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 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 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 Purchase equipment

1. Solar panels (Solar World 4-4.0-100), available for $27 each2. 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.

References1. Mpower Solutions Inc. (http://

www.mpoweruk.com/life.htm)2. Energy Scavenging for Mobile and Wireless E

lectronics p. 26