Is there a better solution than the battery for powering the growing number of both handheld, portable devices and stationary equipment located miles from the nearest electrical outlet?
The answer, as always, depends on the application. But energy harvesting -- extracting otherwise unused energy from the environment -- is becoming a serious contender in an increasing number of applications. To date, large-scale energy harvesting using wind and solar farms has been a small but growing segment of the world's energy diet. In 2007, the global photovoltaic market was estimated at $1.2 billion, with just fewer than half a million inverters shipped that year.
However, micro-harvesters, devices that produce a few milliwatts of energy from vibration, temperature differentials, light and other ambient sources are emerging from research labs and are finding commercial applications. A few milliwatts is not much, but it plays well with the ultra-low power technology initiatives many IC companies have been championing.
While harvesting will not replace every application's batteries in the near future, the advantages of this technology include sensors that last years without battery replacement or maintenance, as well as lower power consumption, with the ensuing impact on the environment as well as lower costs to end users in the long run.
Clearly the opportunity is enormous, but taking advantage of energy harvesting will require engineers to recalibrate their thinking from an energy-source perspective, particularly when it comes to power management design strategies. While it may be going too far to say that energy harvesting rewrites the rules of realizing the best power efficiency in circuit design, it is true that some of its best practices will be counter-intuitive to many engineers.
Fig. 1: Macro v. micro energy harvesting comparison
Click on image to enlarge.
From the broadest perspective, harvested energy may come from a variety of sources including kinetic (wind, waves, gravitational, vibration), electromagnetic (photovoltaic, antenna/rectanna), thermal (solar-thermal, geothermal, temperature gradients, combustion), atomic (nuclear, radioactive decay) or biological (biofuels, biomass).
There have been few attempts to estimate the size of the all-encompassing market because the technology is broad and diverse. It is also so new that many applications have yet to be identified. Estimates available for micro-harvesting today tend to be in niches where the technology is clearly a viable alternative to batteries.
According to the market research firm, The Darnell Group, more than 200 million harvesters and thin-film batteries will be in use by 2012. The market for automotive, home, industrial, medical, military and aerospace energy harvesting applications will grow from 13.5 million units in 2008 to 164.1 million units in 2013.
Wireless sensor networks that require remote nodes to run unattended for years are a primary target application. These sensor nodes may harvest energy from light, vibration, heat or another source, depending on their location. For example, watches, calculators and Bluetooth headsets are all potential candidates for photo-voltaic cell harvesting. Also, converting motion to electrical energy has been accomplished by Seiko in its Kinetic brand, and radios are also powered by vibration in products such as Freeplay's EyeMax wideband radio.
One of the most intriguing sources of harvested energy is body heat, which is the approach Seiko takes in its Thermic brand of watches. Next-generation of biometric sensors that measure vital statistics from simple pulse rates right up to ECG waves may even be powered by body heat.
Conversion technology is only part of the equation. A typical energy harvesting system includes conversion, temporary storage in a thin film battery and a heavy dose of sophisticated power management circuits, analog converters and ultra-low power microcontrollers (MCU). The critical design goal is to match power circuits to application circuits for the best overall performance. A designer can then develop an application knowing that technology will support that product.