Energy harvesting has been around for centuries in the form of windmills, watermills and passive solar power systems.
In recent decades, technologies such as wind turbines, hydroelectric generators and solar panels have turned harvesting into a small but growing contributor to the world’s energy needs. This technology offers two significant advantages over battery-powered solutions: virtually inexhaustible sources and little or no adverse environmental effects.
Macro-scale harvesting technologies differ in many ways but have one thing in common: they ‘feed the grid,’ typically adding kilowatts or megawatts to the power distribution system. As such, they are not game changers for electronic designers whose mission in life is to snip the wires – including power cords and even battery-powered systems where the perpetual device is the ultimate design goal.
For this second goal, micro-energy harvesting systems are the answer. Energy harvesting’s new frontier is an array of micro-scale technologies that scavenge milliwatts from solar, vibrational, thermal and biological sources. A few years ago, micro-harvesting could have been called a scientific curiosity. But the design community’s long march to ultra-low-power (ULP) technology has had the unexpected result of pushing micro-scale energy harvesting out of the lab and onto the designer’s bench.
Now, designers are sizing up ULP not just from the consumption side but from the production perspective as well. Understanding ULP from the sourcing side will be every bit as challenging as it was from the consumption side not that many years ago. The primary reason is that harvested power is derived from ambient sources so it tends to be unregulated, intermittent and small.
ULP sets targets
While there is no widely accepted definition of ULP, it is helpful to consider it in the context of batteries because 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 examples 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 than battery life, which suggests button cells. Typical power dissipation of 1 mW and 1500 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, radios and Bluetooth headsets. There are also applications that consume on the order of milliwatts as well, but have not yet been adapted to micro-harvesting. The most promising application is remote sensors.
Harvester technologies
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 availability is at least an order of magnitude less than that of the first three. Estimates vary, but Table 2 shows the approximate amount of energy per unit available from four micro-harvesting sources.
Micro-harvesting provides power at the same order of magnitude that carefully designed ULP circuits typically consume. The three most promising technologies – based on light, motion and thermal scavenging – have different characteristics:
* Large solar panels have made photovoltaic harvesting a well characterised technology. Approximately 1 mW of average power can be harvested from each 100 mm² photovoltaic cell. Typical efficiency is roughly 10% and the capacity factor of photovoltaic sources (the ratio of average power produced to power that would be produced if the sun was always shining) is about 15 to 20%.
* Commercially available kinetic energy systems also produce power in the milliwatt range. Energy is most likely to be generated by an oscillating mass (vibration). But electrostatic energy harvested by piezoelectric cells or flexible elastomers is also classified as kinetic energy. Vibrational energy is available from structures such as bridges and in many industrial and automotive scenarios. Basic kinetic harvester technologies include (1) a mass on a spring; (2) devices that convert linear to rotary motion; and (3) piezoelectric cells. An advantage of (1) and (2) is that voltage is not determined by the source itself but by the conversion design. Electrostatic conversion produces voltages as high as 1000 V or even more.
* Thermoelectric harvesters exploit the Seebeck effect, which states that voltage is created in the presence of a temperature difference between two different metals or semiconductors. A thermoelectric generator (TEG) consists of thermopiles connected thermally in parallel and electrically in series. The latest TEGs are characterised by an output voltage of 0,7 V at matched load, which is a familiar voltage for engineers designing ultra-low-power applications. Generated power depends on the size of the TEG, the ambient temperature and, in the case of harvesting heat energy from humans, the level of metabolic activity. According to the Belgian-based research corporation IMEC, at 22°C a wrist-watch type TEG delivers useful power of 0,2 to 0,3 mW on average for normal activity. Typically, a TEG continuously charges a battery or super-capacitor and requires advanced power management to optimise efficiency.
Despite their differences, photovoltaic, motion and thermal harvesting also have a few things in common. They generate erratic voltages instead of the steady 3,3 V or 1,8 V design engineers sometimes take for granted; they also provide intermittent power and sometimes no power at all.
Conversion technology is only part of the solution. Since micro-harvesters tend to generate intermittent power, the most common system architectures are called hybrids because they include energy storage in thin-film batteries. A typical energy harvesting system includes conversion, temporary storage and a heavy dose of sophisticated power management circuits, analog converters and ULP MCUs. To take ULP to the next level, it is highly desirable to integrate as many of these circuits as possible on a single chip.
Best practices
Energy harvesting does not exactly rewrite the rules of realising the best power efficiency in circuit design, but some of its best practices may be counter-intuitive to many engineers.
The key design goal is to match the power circuits to the application circuits for the best overall performance. Applications can then be developed knowing that technology will support that product.
Several techniques that solve the unfamiliar power management problems that micro-harvesters present include:
* Optimising switched power supplies because they can boost very small source voltages. By chopping the input signal, switchers allow designers to control its magnitude and frequency. Switching topologies also dissipate very little power but introduce unwanted frequencies that must be normalised.
* Since tasks such as charging the gate capacitance of a MOSFET could consume a large percentage of the harvested energy, a current-source gate charge rather than a voltage-source gate charge will often make sense.
* Another technique is to use more than one power converter circuit. The first circuit could be unregulated but capable of charging a capacitor. Once sufficient energy is stored in the capacitor, it can be discharged and the signal conditioned by a more sophisticated power converter circuit.
Silicon implications
What are the implications of micro-harvesting for ICs? The most common system component would be an MCU. Because MCUs have more to do than just be stingy with energy, other criteria have to weigh in, including wake-up time (which also has an impact on energy consumption), operating voltage, frequency and code density. Texas Instruments’ platform of MSP430 microcontrollers offers an example of a controller well suited to energy harvesting applications. The most recent generation of MSP430 MCUs, the MSP430F5xx generation, allows designers to tap into peak execution performance of up to 25 MHz while consuming as little as 160 μA/MHz.
A system powered by a small battery – or micro-energy harvesting device – will need to communicate information to another system or to a central collection point. Supplying power to a network of sensor-transmitters has traditionally required expensive wiring installation or routine battery changes. Gathering data from difficult or dangerous-to-reach locations using wired sensors may be impossible or may compromise the safety of personnel installing wiring and replacing batteries.
Low power, wireless communication for energy-harvesting applications is the clear alternative. Low-power RF transceivers are designed to operate in the ISM bands (315, 433, 868 and 915 MHz) but can be programmed for frequencies in the 300–348, 387–464 and 779–928 MHz bands. These transceivers also offer noise immunity, selectivity and blocking to ensure reliable communications even in noisy environments.
With TI’s CC430 platform, the combination of the power-efficient, highly functional MSP430F5xx MCU and a TI low-power RF transceiver on a single chip offers a unique low-power/performance mix and high integration to help break down barriers to RF implementation such as stringent power, performance, size and cost requirements, helping bring wireless connectivity to new applications.
Integration adds value
From an IC architecture perspective, integration is almost always a good thing. With ULP designs, the benefits are particularly helpful. Of the five important components of a harvesting system – conversion, temporary storage, power management circuits, analog converters and ULP MCUs – the last four are the most likely first step toward high integration.
In addition to delivering low-power advantages, integration also reduces package size and cost, which are high priorities for small sensor applications that might be deployed, for example, in a mesh network or some other ‘network of things’ – for example, innovative RF sensor networks that report data to a central collection to analyse and regulate information such as smoke in the atmosphere to detect forest fires, moisture or pesticide information in crop fields or even humidity levels in a winery.
According to the ARC Advisory Group, the worldwide market for industrial applications of wireless technology will grow at a 32% compound annual growth rate (CAGR) over the next five years, and is projected to exceed $1 billion in 2010. A major stumbling block to date in deploying wireless technology in industrial settings has always been powering the radio and the application circuits. Both installation and periodic maintenance have been identified as being problematic. If line power has to be extended to the application, the value of wireless communication is reduced. If batteries are used, they have to be replaced from time to time.
The combination of ULP and microharvesting is likely to be the most cost-sensitive, sensible approach to addressing these challenges; the CC430 technology platform is an ideal solution to provide the micro-harvesting and ULP needed for these applications. Sensing applications are limitless, power supplies are not; solutions like the CC430 platform that combine low power and high functionality with the know-how to take the mystery out of RF design, help bridge this gap to usher in a new age of energy solutions.
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