Portable Devices: The Benefits From Energy Harvesting
Wireless sensor networks (WSNs) are becoming an enabling technology in a wide variety of applications. Ultra-low-power wireless powering solutions cover a broad range of products, from batteries to power management ICs, to new technologies like energy harvesting. Low-power devices are being deployed for wireless systems such as mesh networks, sensor and control systems, microelectro- mechanical systems (MEMS), radio frequency identification (RFID) devices, and so on.
The actual number of wireless sensor nodes to be powered is hard to project, since the networks themselves depend on the location, site or facility where they are used. A WSN in a large warehouse is quite different from the sensors used in automotive tire pressure sensing, for instance. Potential applications include HVAC control, lighting control, security, fire safety, industrial process, transportation, medical and military/aerospace. These markets are expected to account for approximately 192 million unit sales worldwide in 2007, growing to about 561 million units by 2012, a compound annual growth rate of 24.0%.
Although many of these applications are stationary, nearly all wireless sensor networks use some form of battery back-up—both primary and secondary. Energy harvesting is positioning itself as an alternative to batteries, although it could end up being a “supplement” to batteries. Battery maintenance and replacement are often cited as the biggest reason to use energy harvesting.
Rechargeable batteries are a secondary power source in wireless sensor networks, so another primary power source must be used to charge them. In applications where rechargeable batteries are used, an energy harvesting source on the node itself, such as a solar cell, could be used to recharge the battery. Fuel cells and ultracapacitors are also potentially attractive for wireless sensor nodes that require high power outputs for hours to days.
One of the biggest market drivers for energy harvesting could be portable, low-power communications and computer devices. Any company that makes products for portable devices is eligible to benefit from this market opportunity. Many semiconductor companies, for instance, are already making ICs that could fit this space, or they could modify an existing product line. Cell phones, laptops, PDAs and similar devices are evolving in a way that is changing existing power architectures, giving energy harvesting technologies an immediate payoff. These opportunities extend not only to power management ICs, but also to energy storage solutions such as thinfilm batteries, microgenerators, capacitors and other emerging technologies.
Figure 1 shows some energy harvesting technologies and their current status relative to portable applications. To address some of the challenges, power solutions that augment batteries, adapters and battery charging ICs are getting built into mobile phones and laptops.
European mobile phone firms are looking into techniques for designing mini solar panels into handsets to extend standby times. Specific, “unnamed” handset firms have projects in place looking at how to incorporate small, 50mm solar cells working in parallel with battery cells to extend the time between recharges (not unlike the tiny solar cells in calculators, but more as an adjunct to the battery and charger).
According to the director of dc-dc converters at Texas Instruments, “Handset firms in Europe have approached us about designs for solar cells. There is a lot of research going into this.” Adding an 11mA trickle charge could be used in parallel with the battery cells to extend standby time to up to a week. Designers in the wireless and portable audio fields are looking at how to apply solar technology to extend battery life, and handsets with mini solar cells could be on the market within two years, according to TI. In the meantime, the company introduced the TPS61200, which supports input voltages of 0.3V to 5.5V during normal operation, and will continue to manage power down to 0.0V if the under-voltage lockout pin is connected directly to the output voltage.
Figure 1: Energy Harvesting Technology Status in Portable Applications Source: “Energy for Portable Electronics: From Micro to Nano,” Energy Technologies, Motorola Labs
Nanoexa is a nano-technology-based clean energy company that produces lithium batteries for portable devices, including a portable charging system based on solar power. The company has developed a variety of synthesis routes to manufacture nanomaterials for a wide range of applications. The synthesis process has been scaled up to production volumes at a competitive cost, and is suited for lithium ion battery applications. Nanoexa can manufacture both lithium ion and lithium polymer batteries for a variety of applications, from 18650 cells to ultra small RFID polymer pouch cells. The company is focusing on lower volume, higher margin niche markets. The largest volume of their polymer batteries are for small IT applications, like Bluetooth headsets, PDA memory backup, MP3 players, and digital cameras.
As much as a quarter of all portable electronic devices, including mobile phone handsets, MP3 players and laptop computers as well as wireless networking devices, could employ some form of energy harvesting within the next few years. Energy harvesting companies see the portable device market as a “future opportunity,” with most of them currently focusing on wireless mesh networks in building automation, industrial, transportation and medical applications.
Depending on the application, energy harvesting technology will provide the primary power source or be used to supplement on-board batteries. Considering that the worldwide mobile phone market alone is projected to reach one billion units in 2007, this could become “the next big thing” in power.
Figure 2: Relationship Between Power Level and Unit Sales for Low Power Devices
Advanced Linear Devices (ALD) has introduced a series of energy harvesting modules that begin operating at 0.0V in order to harvest energy from sources that generate intermittent energy impulses. The modules utilize a unique CMOS-based device which, when used in combination with an inexpensive programmer, allows softwarebased analog circuit trimming in up to 100,000 discrete steps with resolution of as low as 0.1 millivolt.
Low dropout regulators (LDOs) are most commonly found in applications that require less than 1A. The lessthan- 1A segment represents the largest segment of the dcdc converter/regulator market, at 68% in 2006. Of that figure, sub-250mA converter/regulators make up more than half of the unit sales. Lower-power dc-dc converters/ regulators are primarily found in established applications such as MP3 players, mobile phone handsets, laptop computers, and so on (see Figure 2). Therefore, the use of energy harvesting in portable devices could drive growth in this segment of the LDO market.
Portable medical devices are another good opportunity for energy harvesting. The harvesting of thermal energy from small temperature gradients can deliver useful amounts of power. The exploitation of body heat as a thermal energy source makes thermoelectric generators, for example, attractive for attached medical devices and biomedical implants. Energy harvesting from body heat is not limited to devices applied to the skin, however. Active implantable devices can also be located under the skin for applications such as cardiac pacemakers, muscle stimulators, neurological stimulators or Cochlear implants.n
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