Global demand is rapidly increasing for effective energy harvesting and energy storage systems to perpetually power microcontrollers and radio ICs used in a variety of embedded microelectronic systems. These permanent power solutions are considered ìclean and greenî technologies because they last much longer than traditional battery-powered systems and donít require replacement and disposal of conventional batteries.
Typical mobile or remote electronic applications that can benefit from energy harvesting include embedded Wireless sensors, active RFID tags, asset-tracking tags, powered smart cards, implantable medical devices, and other smart objects. These applications are usually battery-powered and operate with a limited duty cycle, meaning that the active mode (often the transmit mode in the case of wireless sensors) is limited and the device may be in sleep mode most of the time to manage battery life. Active operation is initiated in response to an outside signal or triggered by an internal timer.
Batteries often limit product lifetime and require periodic replacement. In many remote applications, batteries may not be easily accessible to end users or service personnel. Inactive applications resulting from dead batteries can cost thousands of dollars in service calls and equipment downtime. Therefore, it is advantageous to incorporate energy harvesting solutions with more efficient energy storage devices to enable longer system life and achieve lower total cost of ownership.
Harvests are less than constant
Effective ambient energy harvesting systems capture energy from the environment close to the application and temporarily store this energy in an energy storage device for later use by the system electronics. Examples of the energy sources available for such energy harvesting systems include mechanical (vibration, flex, strain, and similar), RF, thermal, and solar.
Most if not all of these sources provide intermittent and highly variable energy levels. For example, the energy harvested from mechanical vibration using piezoelectric elements is typically intermittent, variable voltage impulses, depending on the source of vibrational energy. Irregular supply of the mechanical vibration results in inconsistent energy output by the piezoelectric element, ranging from zero to hundreds of volts. This requires an energy storage device to store the energy when available and then power the application when needed. In addition, most ambient energy harvesting systems produce very low current and variable voltage, so the energy storage device is again needed to provide the proper voltage and current required by the load.
In short, energy storage is essential to bridge the gaps of available ambient energy and provide peak current to the load when needed. Such peak current delivery is mostly unachievable with todayís energy harvesters and the currents required by common microprocessors and radio ICs. In addition, energy storage devices provide instant-on capability, rather than waiting to harvest sufficient energy before the system can operate. The energy storage deviceís state of charge will fluctuate between various partial levels of charge, which can be problematic for some battery chemistries. Finally, the application may experience long periods of inactivity with no available ambient energy.
Smoothing the available energy
As a result, more powerful, long-lasting, highly rechargeable, small form factor energy storage devices are required in conjunction with ambient energy harvesting circuits to effectively enable autonomously powered, deeply embedded electronic systems. However, supercapacitors and other rechargeable batteries such as Li-ion, Li-polymer, NiMH, and lithium coin cells are often not up to the task. These batteries typically exhibit poor cycle life (limited rechargeability), high self-discharge rates, limited operating temperature range, and low power capability. In addition, traditional primary and rechargeable batteries are too bulky and have insufficient service lifetimes to serve the intended application. This requires periodic maintenance and replacement, which is often cost-prohibitive.
To address the variable output from most ambient energy harvesting systems and overcome the other weaknesses of traditional batteries and supercapacitors, a better energy storage device is needed. An ideal energy storage device would seamlessly interface to ambient energy harvesting systems and provide near loss-less energy transfer and storage. Such a device would provide high cycle life, long service life (longer than the application requires) to eliminate the need for replacement, broad operating temperature range (not degraded by high temperature such as direct exposure to the sun), low self-discharge rate to avoid wasting the ambient energy that was stored, and sufficient peak current to power the application. In addition, it would be affordable, safe, and environmentally friendly.
New micro-energy storage technology
Infinite Power Solutions, Inc. has developed a new class of micro-energy storage devices optimized to store energy harvested from ambient environments and enable perpetually powered remote devices. THINERGY Micro Energy Cell (MEC) products (Figure 1) are ultra-thin, highly rechargeable (>10,000 full depth of discharge recharge cycles), solid-state energy storage cells with high-performance capabilities. Even with uncontrolled charging sequences from ambient energy harvesters, these devices provide hundreds of thousands of partial charge/discharge cycles, which is ~100x more cycles than what typical rechargeable batteries offer today.
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Figure 1: THINERGY MECs store energy harvested from ambient environments. (click graphic to zoom by 1.4x) |
THINERGY MECs are fabricated using a sophisticated vacuum deposition process to sputter deposit thin layers of inorganic battery materials onto a thin metal foil substrate. The cathode material is Lithium Cobalt Oxide (LiCoO2), and the anode material is metallic Lithium (Li); together they form a powerful and highly rechargeable 4 V cell. A solid-state electrolyte called Lithium Phosphorus Oxy-Nitride (LiPON) is used to provide high mobility for the Li-ions, translating into high discharge rate capabilities. In addition, LiPON acts as a highly effective barrier to electrons to ensure an extremely low self-discharge current (measured in nano-amps) that enables decades of shelf life with no need for recharge. The combined battery and flexible metal foil Packaging materials form a total cell thickness of only 170 micrometers, making these micro-energy cells far thinner than other thin-film batteries, coin cells, and supercapacitors.
Available in easy-to-use development kits (see Figure 2), THINERGY MECs can harvest and store all forms of ambient energy by self-regulating and storing the flow of energy, providing a safe, reusable, and clean energy source that delivers a lifetime of power to electronic devices and systems without requiring any intervention or maintenance. Operating from -40 ∞C to +85 ∞C, these micro-energy storage devices offer near loss-less energy storage and discharge rates exceeding 70 C-rate – truly unique in the world of micro batteries.
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Figure 2: The THINERGY Application Development Platform simplifies the design and development of autonomously powered micro systems using energy harvesting. (click graphic to zoom by 1.2x) |
Tim Bradow is VP of business development and technical marketing at Infinite Power Solutions (Littleton, Colorado). He has more than 20 years of experience in the semiconductor industry, with responsibilities in applications engineering, technical sales, channel development, and worldwide marketing. His experience includes positions at Xilinx, RocketChips, and Honeywell. Tim has a BSEE from North Dakota State University.
Infinite Power Solutions, Inc.
303-749-4754
