This month, we look at a field trial of an energy storage system for energy harvesting that has proven reliability in harsh environments - and what that should mean for wireless sensor network design.
Our smart grid panel from a couple months ago identified energy storage as one of the more challenging issues to address. In most energy harvesting situations, it’s not as easy as sticking a CR2032 coin cell in a device and trying to be as power-efficient as possible. It’s also not just about selecting a rechargeable battery technology that meets the mAh specifications.
The thing about solar power is it’s sitting out in the direct sunlight. That means the system is exposed to temperature extremes, plastic housings disintegrate pretty quickly from exposure to UV and IR, and there are other potential problems from rain, wind, and contaminants. I look at the solar lighting systems in my backyard in Phoenix after a couple of years, and although these are relatively low-cost systems for consumer use, the wear and tear is obvious.
The recent news that TPL’s EnerPak units (Figure 1) have completed two years of field trials at a New Mexico substation might have gone fairly unnoticed. After all, a battery is a battery in most situations. But consider the first set of batteries used in this application failed miserably after just a few weeks, and you’ll understand that maybe picking an energy harvesting solution for a wireless sensor network needs a bit more attention and the right kind of technology.
The EnerPak is unique in that it combines batteries, supercapacitors, charge pumps, interfaces for a variety of harvesting devices, and a Texas Instruments MSP430 MCU in a self-contained energy storage system. It’s not just charging a battery. It’s managing the harvested (in this case, from solar cells) power along with the stored power and providing the optimum combination, including pulse power from the supercaps when called for.
Eight TPL EnerPaks and the associated solar cells have been performing in a sulfur hexafluoride (SF6) monitoring system at a Public Service of New Mexico substation in Albuquerque for the past two years. They have endured thousands of charging cycles and temperatures ranging from 5 °F to 100 °F as well as rain, wind, dust, and snow – with continuous operation. SF6 is used as an insulator in many high-voltage applications and is safer for humans to be near than the oil-based PCBs it replaces. However, SF6 is a potent greenhouse gas and needs to be monitored to be sure it’s not escaping from a pressurized system. This is where the wireless sensor network and an energy harvesting system to power it come into the picture.
This EnerPak application is a good example of the types of needs smart grid providers are experiencing when adding intelligence to their systems for increased efficiency and safety. It also illustrates how important storage is to energy harvesting and wireless sensor network systems. It’s worth a closer look at their technology – see www.tplinc.com for more information.
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