Making low-voltage energy harvesting practical: Part 3The future of micro-power energy harvesting

Making low-voltage energy harvesting practical: Part 3The future of micro-power energy harvesting

As energy capture matures and power demands scale downward, EH benefits will be everywhere

BY ROBERT CHAO, CEO
Advanced Linear Devices
www.aldinc.com

This is the third of a three-part series about energy harvesting. You can find Part 1 at http://tinyurl.com/cte56d7, and Part 2 at http://tinyurl.com/7aezvlp

Since the beginning of the human race, people have strived to harness the planet’s free energy sources, such as wind, solar, and water. On a macro scale, the technology has matured and come to fruition. On a micro scale, the development of ultra-low-energy harvesting (EH) devices is maturing rapidly.

Development curve matures

Today, there are some energy sources, such as photovoltaic and piezoelectric that output enough power for the current crop of EH modules to capture and store. However, other sources such as RF and body heat have outputs that are so miniscule that they require a low-voltage booster module (LVBM). This module brings such low output levels up to the minimum threshold input level required by the EH module to capture and store energy.

The electronics of the future will require less power and draw more of that power from environmental sources. And, EH technology will continue to become more efficient and eventually will be tomorrow’s power source for many applications.

The present state of low-power harvesting technology

On a milli-scale, the present methods of capturing free and scavenged energy in the hundreds of mV and double-digit µW work well in certain specialized markets such as sensors. However, there are many micro and nano sources with intrinsic energy characteristics that produce very low output; a few dozen mV or a handful of µW, not enough power to drive the present generation of EH modules without cascading the outputs, or placing a LVBM at the input.

Energy harvesting’s golden child

Today, micro scale EH is generally used to charge batteries, super capacitors or other energy storage devices. Widespread deployment of EH technology is still in its infancy, but there is one notable exception, sensors.

EH technology has particular applicability to sensors that are located in remote, isolated or environmentally extreme locations without any type of primary power. Fortunately, at such installations, there is often an abundance of wind or solar energy that can be captured and used to power, say, a piezoelectric circuit. The circuit is connected to the input of the EH modules to support the power supply of the sensor. This is an effective scheme because modern sensor footprints are powered by very low currents and voltages that are “power smart.” This means that they use little or no power until there is a need for it, and very little power during actual operation. For example, the sensor may be programmed to wake up and take a reading periodically, or if a radical change occurs in the material being sensed. The radio is also asleep until the sensor awakens it. At that point, there is usually only a few milliseconds power cycle while the data from the sensor is transmitted. After that, the circuits go back to sleep until the next cycle. For this application, EH is the ideal solution.

It’s all about technology

The 21st century is seeing semiconductor technology develop at a frenzying pace. Evolving semiconductor processes (see Fig. 1 ) include single-nanometer gate geometries, zero-threshold and nano-power metal-oxide semiconductor field-effect transistors (MOSFETs). These developments are going to bring micro energy-harvesting products on line by enabling low power, miniaturized devices capable of harvesting ultra-low-power sources without backup energy-storage devices.

Fig. 1: Zero- and nano-power MOSFETs (courtesy Advanced Linear Devices)

The present generation of energy-harvesting modules on the market cannot capture energy below about 300mV and 20 µW. Therefore, they cannot take advantage of many of the ultra-low-power sources in their native configuration. A solution is to add an ultra-low-voltage booster module that will boost the sources’ power output to a level high enough to drive the modules.

Some sources, such as photovoltaic and piezoelectric, can be cascaded in various configurations to produce sufficient energy to power today’s modules. But, that up-sizes the footprint and makes them impractical for many size and weight constrained applications. As such, without a booster module, many of the ultra-low energy sources remain untapped.

The LV booster solution

There is now a stand-alone ultra-low-voltage booster module available capable of boosting ultra-low-power sources to a level compatible with the current crop of EH modules. This module is capable of utilizing ultra-low-power sources with outputs as low as 40 mV and 2–µW. This device enables many of the formerly aloof waste or scavenged sources such as single cell photovoltaic, piezoelectric, ambient radiation, thermoelectric generators (TEGs), biomechanical sources (human motion, breath), radio frequency (RF) and embedded systems (implantable radio frequency identification or RFID) to be utilized as feasible power sources.

EH sources and their future

Photovoltaic. This is a well-understood and mature method of generating electrical power on a large scale and is an ideal and virtually inexhaustible and environmentally friendly source of micro power. Now that technology is available to capture and utilize the energy from a single photovoltaic cell, its potential as a harvested energy source is unlimited. Piezoelectric. Piezoelectric energy is developed by the linear electromechanical interaction between the mechanical and the electrical state in crystalline material. It is also a mature and well-understood source of energy. Such strain can come from any number of sources – motion, low-frequency seismic vibrations, acoustic noise, vibration from engines or the impact as the heel of a shoe hits the ground. Because it is very scalable, it can be configured to provide energy for all levels of EH applications. In the future, very small piezoelectric generators (50 µW and lower) will be able to power new ultra-low-power modules.Ambient Radiation RF. In abundance in both natural and man-made environments, ambient RF from either natural sources or ubiquitous radio transmissions is one of the hotter potential EH sources. However, most ambient RF sources have very little salvageable energy available. One theory is to place a large surface area of collectors in close proximity to the radiating wireless energy source and scavenge power from the RF waves (see Fig. 2 ). Antenna farms, with special antennae, will be developed that can collect sufficient energy to produce useful power from stray radio waves or, theoretically, even electromagnetic (EM) sources made practical by ultra-low input EH modules.

Fig. 2: Example of ambient radiation harvesting. Thermoelectric generators. Thermoelectric generators (TEGs) consist of two dissimilar material junctions that create a thermal gradient. Voltage is typically 100 to 200 µV/K per junction. For present EH applications, suitable voltage outputs are obtained by connecting multiple junctions in series/parallel. With new single-component EH modules, the need to cascade large numbers of junctions is reduced. This makes practical TEG devices with much smaller footprints than currently available. This will also translate into TEG devices more appropriate for applications that are footprint-sensitive (micro sensors, biomedical). Biomechanical sources. Potential applications for biomechanical energy harvesters are creating a stir. The human body is capable of providing a wide platform of energy that can be harvested. Joint movement, body heat, breathing, moisture and impact (walking) are all potential energy generators.

There are and will be, for quite some time, applications that can work with bulkier circuits and can use higher-output sources (those that require 300 mV and up, stacked or native). But for the sources that cannot be concatenated, ultra-small and unobtrusive ultra-low-power harvesters are really the only solution for utilizing them.

Ultra-low-power energy harvesting has only begun to peel back the layers of its technology. The present challenge is to capture energy in the low teens of mV and single-digit microwatts. As it matures and power demands scale downward, the world will see its benefits in many areas – biomedical, RF, all types of sensors, and consumer devices. Smaller, super-integrated modules are more efficient, less invasive (especially in biometric applications), less prone to failure since the separate component interconnect is eliminated, and less expensive. Until then, the current two-module approach, the low-voltage booster module coupled with current offering of EH modules, is the current leading-edge technology. ■