Energy scavenging, also known by the more marketing-friendly term, “energy harvesting,” is the process of capturing energy from such things as a heartbeat, body motion, wave motion, radio waves and solar radiation. It is energy that is otherwise wasted, but which can be useful in some cases--particularly with modifications to a chip's architecture.
From an IC design standpoint, energy harvesting adds a couple of twists. One of them involves partitioning. A second is cost, because many of the applications that benefit most from energy harvesting are price-sensitive.
There are five key areas involved:
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.shoe
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.ambientrf
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.
There also is work under way in other areas. One involves biology. As early as 2008, a group of researchers from MIT published a paper in 2008 that looked at the sustained voltage difference between plants and their surrounding soil. MIT also is working on microengines on chips, and Darpa several years ago was experimenting with chip-sized nuclear reactors. What ultimately becomes commercially viable is unknown, but research is beginning to ramp as design teams working on ever-greater functionality in smaller and smaller devices search for ways to sidestep the limitations of conventional battery technology.