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Energy Scavenging And Storage Must Work Together

By John Blyler
Designing embedded systems in energy-sensitive environments requires both attention to power details and a system-level view of overall energy architectures. Successful designers must embrace both perspectives.

This isn’t easy. Most embedded hardware engineers are used to the fairly generous power offered by a wall socket or inexpensive traditional off-the-shelf batteries, where portability is a requirement.

But energy-sensitive embedded designs contain a time-dependency that many designers do not fully appreciate – one not associated directly the function of the battery. Instead, this time-dependency is related to the operation scenario of the application. More on this in a moment. First, let’s consider the power “pain points” that drive engineers to consider energy harvesting systems in the first place.

Pain points
A major pain point is the high cost associated with the use of wired power in any application for which traditional wall socket power is not readily available, such as wireless sensors in data acquisition devices for industrial processes, patient monitoring, remote data logging, agri-business and intelligent building energy controls.

In the past this problem was solved by using single-charge batteries, notes Steven C. Grady, vice president of marketing at Cymbet Corp. “However, the next pain point is having to deal with changing out batteries. The unknown time frame of battery failure and costs to change batteries has also been shown to be very expensive.”

This is where energy-harvesting techniques become attractive, because they can provide a relatively permanent source of energy. However, to qualify as a satisfactory solution, energy harvesting implementations must be on parity or even less cost to wired and battery solution. As Grady explains, the adage of ‘People go green, when it saves green ($)’ certainly applies to energy harvesting.

Other conditions make energy harvesting devices attractive. Sandip Kundu, professor at the University of Massachusetts in Amherst, Mass., says that in addition to being removed from typical power sources, energy scavenging is most attractive for portable devices that do not need large amounts of power and have low usage duty cycles. Low duty cycle of usage applications include food-tracking techniques that use smart labels that contain history of origin information, as well as routing and temperature data.

At the other end of the spectrum are energy scavenging devices used in the co-generation of power, such as with internal combustion engines and heat furnaces. Kundu notes that such cogeneration of power allows the design to combine techniques to improve overall efficiency, such as with the combination of a Sterling cycle engine and thermocouple-based electricity generator.

Servicing the Battery

The reason a designer would choose energy scavenging over a standard battery really boils down to cost and/or the constraints of serving a battery in the application, says Mark Buccini, microcontroller marketing director at Texas Instruments. He says that a simple solar calculator is a good example of the cost benefit of energy harvesters, because a solar cell is actually cheaper than a battery and can last for decades.

One of the best examples of the constraint imposed by servicing some battery-enabled applications is found in solar-powered satellites. Here’s where solar power scavenging really shines, Buccini says. “Solar power is abundant and the cost of replacing a battery in space is prohibitive.” Another more down-to-earth example of difficult-to-service battery applications would be implanted medical devices.

Even with multi-year battery life, most embedded applications eventually will need maintenance in the form of battery replacement. For example, low-cost systems such as underground water meters or tire pressure monitoring systems may require several hundreds of dollars in associated maintenance because they are hard to access.

Before jumping on the energy scavenging bandwagon, designers should heed the words of Jan Rabaey, Donald. O. Pederson Distinguished Professor at the University of California’s Berkeley Wireless Research Center. He says the size of the device dictates just how effectively it can scavenge energy. For example, most scavenging is a third-order factor of the volume of the node, although solar is a square of the node because it is a flat structure. “If you double the size of the device, you can roughly double the amount of energy you can scavenge,” he said.

Energy Storage is a Must

Almost all energy-harvesting scenarios require some sort of energy storage element or buffer. Even if the voltage and current requirements of an embedded application were so low as to be run directly on power captured or scavenged from the environment, such power would not flow in a constant way. The sun doesn’t shine all the time, or at least not on the same terrestrial spot. This means that some type of energy storage element is needed, if for no other reason than to provide a steady and predictable amount of power.

Storage elements or buffers are implemented in the form of a capacitor, standard rechargeable lithium battery, or a new technology like thin-film batteries (see Figure 1). What kind of energy storage is needed depends greatly on the application.

Some applications require power for only a very short period of time, as short as the RC time constant discharge rate of a capacitor. Other applications require relatively large amounts of power for an extended duration, which dictates the use of a traditional AA or a rechargeable lithium battery. Still other applications need the small footprint benefit of the capacitor and the low energy leakage advantage of a tradition battery. This is where the thin-film batteries are gaining acceptance, notes Adrian Valenzuela, product marketing engineer for ultra low-power MCUs at Texas Instruments.

Li-Ion Battery Thin Film Battery Super Cap
Recharge cycles Hundreds Thousands Millions
Self-discharge Moderate Negligible High
Charge Time Hours Minutes Sec-minutes
Physical Size Large Small Medium
Capacity 0.3-2500 mAHr 12-1000 μAHr 10-100 μAHr
Environmental Impact High Minimal Minimal

Figure 1: Characteristics of typical energy storage options (Courtesy of TI)

Understanding the Dependency

It should be apparent that the type of energy storage needed to complement an energy harvesting approach is dependent upon the embedded application. Designers must determine the energy capture profile and compare it to the energy storage profile, both of which are functions of the operational scenario of the embedded application. The operational scenario captures dynamic duty cycle or the (often) non-period timeline of the embedded devices usage model.

What tools are available to help the system architect or designer balance energy harvesting and storage cycle that are dependent on the operational scenario of the application? Not many.

There are very-low-power microcontroller kits that help the designer manage power and energy storage. But robust software tools that model the system-level duty cycle given a particular embedded energy input and load output are not yet available. They will be soon, judging by the growing interest in energy harvesting technology.

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Want to meet most of the experts quoted in this story? Then be sure to attend the Design Automation Conference (DAC) Pavilion Panel, Power Scavenging: Waste Not, Want Not

Everyone talks about low-power designs, long battery life and the environmental effects of so much power consumption. However, the consumption of power is an ever-increasing need that must be faced. Are there alternatives to generating “small” amounts of power for low-power gadgets from really unconventional methods? Let the experts tell you where some of the hidden power is available and how they are harnessing it for some of the most complex applications.

Panelists:

Sandip Kundu – Univ. of Massachusetts, Amherst, Mass.

Steve Grady – Cymbet Corp., Elk River, Minn.

Mark Buccini – Texas Instruments, Inc., Dallas, Texas

Moderator: John Blyler, Chip Design magazine

Organizer: Yatin Trivedi, Synopsys, Inc.

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