Micro-Power Energy Harvesting

Since the beginning of the human race’s self-awareness, people have strived to harness the planet’s free energy sources; wind, solar, thermal and hydro. On a macro scale, man’s dreams have come to fruition. The next frontier is capturing energy on a micro scale.

On the macro front, there are massive corridors of energy powered by wind and solar developments, which generate megawatts of electricity all over the world. And looking to the future, this macro-scale implementation of free energy shows no signs of abatement. However, this end of the free energy spectrum at rapidly approaching the maturity knee with technology no longer making momentous leaps. For wind, solar, thermal and hydro, the technology curves have, or are close to plateaued, and most gains now are incremental and mostly in the efficiency of the devices.

Figure 1. 21st century large scale wind and solar technology.

On a micro-scale, however, the exact opposite is true. The development of harvested scavenging energy, either by-product or free, is just beginning to evolve on a practical scale. As it turns out, capturing energy on a grand scale is much easier to accomplish, technologically, than on a micro scale.

The reason is simple — economies of the power scale. Large power generation techniques and technologies simply do not scale to the micro level, and are much easier to implement. At the macro level, applications are stationary, require environmental hardening, deal with high voltages and currents, and involve power transmission technology. All of which is mature and well understood. At the micro level, a completely different approach is needed to capture free or scavenged energy. Elements such as form factor, weight, bio-safety, portability, transparency and all the other considerations that define light-weight, portable, human-friendly devices, are the driving factors.

The technological evolution to develop and deploy micropower devices that can capture, store and deliver micro or nano-scale scavenged energy, has only recently (the last five or so years), become practical. Thanks, in large part, to nanotechnology, microelectronics that can capture energy from such sources as the human breath and radio waves will soon be practical.

What’s In the Field Today
Today’s working field implementations of micro-scale EH generally use it to charge batteries, super capacitors or other energy storage devices. Widespread deployment of EH technology is still in its infancy with one notable exception – sensors. While the theory is up-scalable to using EH as a primary power source, the applications aren’t quite there yet. They need to scale down further in power demands before EH becomes a viable, primary power source.

Where today’s EH technology has been realized is with sensor applications in industries such as in the transportation infrastructure, medical devices, industrial sensing, building automation and asset tracking.

Delving a bit deeper within the sensor arena, EH technology has particular applicability to sensor that are located in remote, isolated or environmentally extreme locations without any type of primary power. Fortunately, as it turns out, there is often an abundance of solar or wind energy available that can be captured by EH modules to support the power supply for the sensor and in many cases, the radio that transmits the sensor data.

Modern sensor footprints have evolved to the point where they can be powered by very low currents and voltages. In addition, they are “power smart,” meaning the monitor or control circuits use negligible power until there is need to wake up the circuit. For example, the sensor may be programmed to wake up periodically and take a reading. Or a very low power monitor circuit may wake up the system if a radical change occurs in the material being sensed.

On the RF side, the radio is kept asleep until the sensor wakes it. At that point, there is usually only a few-millisecond power cycle while the sensor data is transmitted. After that, the circuits go back to sleep until the next cycle. For this type of application, EH is becoming a viable power technology.

It’s All About Technology
The 21st century is seeing semiconductor technology develop at a frenzying pace. Evolving semiconductor processes include single-nanometer gate geometries, zero- and nano-power metal-oxide semiconductor field-effect transistors (MOSFETs), three-dimensional complimentary metal-oxide semiconductor (CMOS) structures (also known as FinFETs). As well, there have been quantum leaps in edge-of-the-envelope semiconductor composites (high-k dielectrics, for example). These are the avant-garde developments that are going to bring micro energy harvesting products online by enabling low power, miniaturized devices capable of harvesting ultra-low power sources without backup energy storage devices.

Figure 2. Vertical or “3D” transistor (courtesy Intel) and zero-/nano-power MOSFETs (courtesy Advanced Linear Devices).

Present energy harvesting modules cannot capture energy below about 300 mV and 20 μW. Consequently, these EH modules cannot take advantage of many of the ultra-low source options, in their native configuration because their output is well below the input threshold these devices require. Some of them, such as solar 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, many of the ultra-low energy sources remain untapped.

However, due to new developments in technology, edge-of-the-envelope manufacturers have developed standalone, ultra-low voltage booster modules that are capable of boosting ultra-low power sources to a level compatible with the current crop of EH modules. Such modules are capable of utilizing ultra-low power sources with outputs as low as 40 mV and 2 μW. These devices allow for the capture of 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 – RFID). With this ultra-low voltage module, these sources can now be captured and utilized as feasible power sources.

EH Sources and Their Future

• Photovoltaic. Perhaps the most prolific of the ultra-low voltage sources is 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. It is also one, if not the ideal, source of ubiquitous free energy. 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. One notable futuristic application is an ultra-thin, invisible skin made up of single layer photovoltaic cells that can be overlaid on a cellular phone’s screen to supply the power to run it.
• Piezoelectric. Piezoelectric energy is developed by the linear electromechanical interaction between the mechanical and the electrical state in crystalline material. It is a mature and well understood source of energy. 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. Such strain can come from any number of sources – motion, low-frequency seismic vibrations, acoustic noise, vibration from engines or impact as the heel of a shoe hits the ground for example. One edge-of-the-envelope energy supply for EH systems are crystals in micro-scale devices, such as in a device harvesting micro-hydraulic energy. In this device, the flow of pressurized hydraulic fluid drives a reciprocating piston supported by three piezoelectric elements that convert the pressure fluctuations into an alternating current. This current can then be used to supply any number of devices.
• Ambient Radiation RF. Existing 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 and ultra-low threshold energy harvesting modules are needed. One theoretical solution 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. 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.
• 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, in series/parallel, multiple junctions. With new single-component EH modules, there is a need to concatenate junctions drops, making practical TEG devices with much smaller footprints than currently available. This translates into TEG devices that become more practical for applications that are footprint-sensitive (micro sensors, biomedical). On the horizon is the development of materials that are able to operate in higher temperature gradients, and which can conduct electricity well without also conducting heat, improving efficiency and applicability to heat-sensitive installations such as the human body.
• Biomechanical sources. The potential applications for biomechanical energy harvesters are creating a stir of excitement. 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. One experimental model straps around the knee and can generate about 2.5 watts of power. This is enough to power some cell phones. In other areas, new ultra-low voltage capture modules, using the human breath as the power source for a mini wind turbine, or using sound or voice box vibrations, are possibilities.
• Embedded systems. Finally, but hardly ultimately, embedded systems are becoming more and more integrated into every aspect of our lives. There are so many devices, from alarm clocks to computers to security systems that are controlled by programmable microchips that can be powered, in part, by EH systems rather than batteries or supercaps.

Of course, not all applications will, necessarily, require an ultra-low-voltage module. But what the latest line of products does is widen the present playing field. Many of the current EH sources can be designed to work with them.

Conclusion
While low-power energy harvesting has only begun to peel back the layers of technology, Moore’s law is hard at work. Ultra-low voltage energy harvesting stands at the threshold of the technology necessary to bring the next generation of energy capture online. Once the ability to capture energy in the low teens of voltage and single digit microwatts matures a bit, and scales downward, the world will see its benefits in many areas – biomedical, RF, all types of sensors, and consumer devices, just to mention a few.

Technology will continue to integrate components into smaller, lighter and more reliable packages. Once this has been achieved, it will have wide-spread implications because it changes the footprint game plan. Single, integrated modules will be much more efficient, less invasive (especially in biometric applications), less prone to failure since the separate component interconnect is eliminated and less expensive that the two-module approach. The only question…is when.