Energy Harvesting and Wireless Sensor Networks

1. Energy Harvesting and Wireless Sensor Networks Adam Skelton

2. 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

3. Sources of Ambient Energy

4. 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

5. 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

6. 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

7. 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

8. 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.

9. References • Mpower Solutions Inc. (http://www.mpoweruk.com/life.htm) • Energy Scavenging for Mobile and Wireless Electronics p. 26