Designing power systems to meet energy harvesting needs

by Patrick Chapman and Murugavel Raju , TechOnline India - October 22, 2008

Energy harvesting is emerging from the labs but designers need to adapt their ideas of power management system design to fully take advantage of its potential.

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.

The opportunity
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.

{pagebreak}Energy availability
Any analysis has to begin with energy availability estimates. Figure 2 (below) shows the approximate amount of energy per unit available from four ambient micro-harvesting sources.

The next step is to assess the amount of energy that can be harvested by a viable system. In the case of solar photovoltaic harvesting, that is a well-characterized technology because of its use in large solar panels. It will produce approximately 1 mW of average power from each 100mm2 photovoltaic cell. Typical efficiency is roughly 10 percent, and its capacity factor (the ratio of average power produced to power that would be produced if the sun was always shining) is about 15 to 20 percent.

Commercially available kinetic energy systems 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 1,000 V or more.

Fig. 2: Energy harvesting estimates by source

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 characterized by an output voltage of 0.7V 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 22C 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 optimize efficiency.

All three of the leading micro-energy harvesting sources described above have a few things in common. They produce erratic voltages instead of the steady 3.3V that is still the most widely used supply for electronic circuits. They also provide intermittent power and sometimes no power at all. It is up to the design engineer to address these problems with power converters and hybrid energy systems.

Power management
Here's where things get really interesting. An important boundary condition is that most micro-harvester energy technologies discussed so far deliver less than 0.5-V inputs. This output can make starting power converter circuits difficult. In addition, quadratic losses create real issues with conversion efficiency.

Using familiar linear regulator topology is ruled out in most (but not all) cases because linear regulators can only step down voltages. Switching regulators are a better fit. By chopping the input signal, switchers allow its magnitude and frequency to be controlled. Switching topologies also dissipate very little power. On the other hand, switchers reshape the signal spectrum and introduce unwanted frequencies. This issue becomes a cost issue because filtering is needed to manage the output.

Energy harvesting presents a very different design environment from the one to which most engineers are accustomed. In traditional power management applications, the most power efficient approach is to start with a high input voltage so conversion can be accomplished at low current and low power dissipation.

But the low input voltages typical of harvesting applications presents design engineers with exactly the opposite scenario. Low input voltage requires power management circuits to operate at higher current given a target output power. High current causes the size of the power converter to increase and makes it harder to make the system efficient.

There are several basic approaches to solving the problem of conditioning an erratic, low input voltage with cost-effective, energy-efficient filtering. Not surprisingly, they all require tradeoffs. Ohmic losses can be reduced with larger switches, for example, but larger switches require more gating energy, which may not be available. Switching frequency can be reduced to increase efficiency, but this requires larger filters.

The most important point for designers to remember is that with systems that generate only a few milliwatts of power, the overhead it takes to control that power can be equal to or greater than the amount of power being sourced. Normally simple tasks such as charging the gate capacitance of a MOSFET can consume a disproportionate amount of energy.

In these instances, consider using a current-source gate charge rather and a voltage-source gate charge. The tradeoff is that the circuit is more complex but overhead power and circuit leakage can be better controlled.

Also, consider using more than one power converter. A synchronous rectifier circuit such as the one in Figure 3 does not provide well conditioned power but it is a good solution for charging a capacitor that will periodically send a high-power burst to a second, more efficient, power converter. The second converter handles the signal conditioning needed by the application circuit.

Fig. 3: Synchronous rectifier circuit

In some applications, another gate charging operation -- this time with a voltage-source gate charge circuit -- can increase efficiency. The technique aligns several transistors ranging from small to large in a circuit (Figure 4).

The circuit, which was designed at the University of Illinois at Urbana, also automatically detects power draw and uses the appropriate size and number of transistors to keep efficiency high. Higher value transistors are used at high power. When the system is running at standby power levels, the smaller ones are used. The graph embedded in Figure 4 shows the advantage of this technique compared to not optimizing according to transistor size.

Fig. 4: Transistor width switching

The underlying lesson in these techniques is that contrary to the traditional approach to power conversion -- designing the most efficient converter possible with the assumption that it will deliver most energy -- is not always true for micro harvesting. Instead, the goal is to optimize energy output for the entire system. That sometimes means that the design does not target maximum efficiency for every part of the system.

{pagebreak}IC decision points
Designers also have to become keenly aware of the implications of their IC technology choices. Everyone is at least subliminally aware that advanced technology nodes produce more efficient semiconductor devices. These differences are often neglected in conventional circuit design because the cost of the submicron device is deemed to outweigh its efficiency advantages. Once again, micro-harvesting applications change the rules.

For example, a small power converter designed at the University of Illinois at Urbana for an early-stage energy-harvesting application achieved a 53 percent efficiency using ICs fabricated in a 1.5 ¼m process and an 8 ¼m inductor. In considering how to improve the converter, the design team calculated the efficiencies that could be achieved using different combinations of process technology and inductor size.

Figure 5 illustrates these results. The most advanced technology combination (0.25 ¼m process with copper interconnects and using a 25 ¼m inductor) was calculated to achieve 81 percent efficiency. Figure 5 also shows where the losses were eliminated.

Fig. 5: Dramatic efficiency gains of advanced ICs

Advanced-technology ICs are also prescribed for other parts of the application, including MCUs. Texas Instruments platform ultra-low powerMSP430 MCUs, which can consume as low as 160 micro-amps/MHz in active mode and lower than 500 nano-Amps in standby mode are a good example. TI also offers devices that combine their ultra-low power MCU with highly flexible radio frequency (RF) transceivers into a single-chip, compact design to implement ambient intelligence that can detect and report critical conditions in factories, automobiles, offices, homes and other environments, all without wiring or batteries. For example, battery free Joule-Thief technology from AdaptivEnergy combined with TI's MSP430 microcontrollers, RF and eZ430-RF2500 development kit provides wide-ranging ambient intelligence. Figure 6 below shows a block diagram of the Joule-Thief system.

Fig. 6: Joule-Thief block diagram

Click on image to enlarge.

Harvesting energy at a micro level and using that energy as efficiently as possible presents a new design paradigm and many challenges for engineers. The benefits of accepting these challenges are many, including taking the next step toward perpetual electronic devices, reducing system lifetime cost, and lowering the product's environmental impact. And the good news is that the tools are already in place to get started.

Patrick Chapman is a professor at the University of Illinois at Urbana-Champaign and focuses on power electronics for energy-harvesting applications. Murugavel Raju is a member of the MCU strategic marketing team at Texas Instruments.

References
S. Musunuri, P.L. Chapman, "Improvement of light-load efficiency using width-switching scheme for CMOS transistors," IEEE Power Electronics Letters, vol. 3, no. 3, pp. 105-110, Sept. 2005.

S. Musunuri, P.L. Chapman, "Optimization of high-width CMOS transistors for low-power dc-dc converter applications," 2005 IEEE Power Electronics Specialists Conference, June 2005, pp. 2151-2157. P. Niu, P.L. Chapman, L. DiBerardino, and E.T. Hsiao-Weckler, "Design and optimization of a biomechanical energy harvesting device," in 2008 IEEE Power Electronics Specialists Conference, Rhodes, Greece.

J. Penders, B. Gyselinckx, R. Vullers et. al. "Human++: From technology to emerging health monitoring concepts," White Paper published by IMEC vzw, April 2008, http://www2.imec.be/imec_sites/objects/7be71c7cb514f2be8c963ae242994a9a/human___white_paper_jp042008.pdf

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