Energy Harvesting

April 2010

challenging as it was from the consumption side not that many years ago. Primary reason: harvested power

is derived from ambient sources so it tends to be unregulated, intermittent and small.

While there is no widely accepted definition of ULP, it’s helpful to consider it in the context of batteries be-

cause they are easily the most common energy source for ULP designs today.

Medical applications in which electronic devices are implanted in or attached to the body are good ex-

amples of ULP designs that run on batteries. Here are three examples.

• Implanted medical device. Size and battery life are primary considerations. Power dissipation of 10 μW

and battery life of 15,000 hours would be typical.

• In-ear device. Size becomes more important that life, which indicates button cell. Typical power dissipa-

tion of 1 mW and 1,500-hour life.

• Surface-of-skin device. Limiting factor is the ability of skin surface to dissipate heat. Typical power

dissipation of 10 mW and 150-hour life.

Medical applications that consume milliwatts set the energy-production bar for harvesting devices. The

good news is that milliwatts are the same order of magnitude that micro-harvesters can generate.

Some non-medical products already use micro-harvesting sources. These include calculators, watches,

but have not yet been adapted to micro-harvesting. The most promising application is being remote sensors.

The most promising micro-harvesting technologies extract energy from vibration, temperature differentials

and light. A fourth possibility – scavenging energy from RF emissions – is interesting, but the energy avail-

ability is at least an order of magnitude less than that of the first three. Estimates vary, but Figure 2 shows

the approximate amount of energy per unit available from four micro-harvesting sources.

Harvester

technologies

Figure 2. Energy harvesting estimates

Energy Source

Harvested Power

Vibration/Motion

Human

Industry

Temperature Difference

Human

Industry

Light

Indoor

Outdoor

RF

GSM

WiFi

Figure 1: Macro v. micro energy harvesting comparison

Macro vs.

Micro

Energy Source

Solutions

Ultimate Goal

Macro

Renewable energy (e.g., solar, wind)

Energy management solutions

Reduce oil dependency

Micro

Energy from the environment (e.g., vibration, body heat)

Ultra-low-power solutions

Perpetual devices

ULP sets targets