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Power/Alternative Energy??

Taking advantage of ambient energy

Posted: 01 Sep 2011 ?? ?Print Version ?Bookmark and Share

Keywords:Ambient energy? energy harvesting? thermoelectric generator?

The collected data can then be manipulated by an analog-to-digital converter for transmission via an ultra low-power wireless transceiver.

Of course, the energy provided by the energy-harvesting source depends on how long the source is in operation. Therefore, the primary metric for comparison of scavenged sources is power density, not energy density. Energy harvesting is generally subject to low, variable and unpredictable levels of available power, so a hybrid structure is

used that interfaces to the harvester and to a secondary power reservoir. The harvester, because of its unlimited energy supply and deficiency in power, is the energy source of the system. The power reservoir, either a battery or a capacity, yields higher output power but stores less energy, supplying power when required but otherwise receiving a charge from the harvester.

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Table 1: Different elements affect the power consumption characteristics of an energy-harvesting system.

Radio sensors for building automation systems are a key area of application for energy-harvesting systems.

Consider the breakout of energy usage in the United States. Buildings are the No. 1 user, accounting for 38 percent of total energy consumption, closely followed by the transportation and industrial segments, at 28 percent each.

Moreover, building energy use can be categorized into commercial and residential consumption, representing 17 and 21 percent, respectively. A further breakdown of the residential figure reveals that heating and cooling account for 76 percent of total energy consumption in that domain.

With energy usage forecast to double between 2003 and 2030, energy savings of up to 30 percent could be attained via building automation.

Ambient energy sources
State-of-the art and off-the-shelf energy-harvesting technologies, for example in vibration energy harvesting and indoor photovoltaics, yield power levels in the milliwatts under typical operating conditions. While such power levels may appear restrictive, the operation of harvesting elements over a number of years can render the technologies broadly comparable to long-life primary batteries in terms of both energy provision and the cost per energy unit provided.

Further, systems incorporating energy harvesting will typically be capable of recharging after depletion. The same cannot be said for systems powered by primary batteries.

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Table 2: Here are the energy sources and the amount of energy they can produce.

As already discussed, ambient energy sources include light, heat differentials, vibrating beams, transmitted RF signals and just about any other source that can produce an electrical charge through a transducer. Successfully designing a completely self-contained wireless sensor system requires readily available power-saving microcontrollers and transducers that consume minimal electrical energy from low-energy environments. Low-cost and low-power sensors and microcontrollers have been available for a couple of years, and ultra low-power transceivers have just recently become commercially available.

The laggard in this chain has been the energy harvester. Existing implementations of the energy-harvesting circuit typically consist of low-performing discrete configurations, usually comprising 3D components or more. Such designs have low conversion efficiency and high quiescent currents, compromising end-system performance.

The low conversion efficiency will increase the amount of time required to power up a system, which in turn increases the time interval between taking a sensor reading and transmitting the data. A high quiescent current limits how low the output of the energy-harvesting source can be, since it must overcome the current level needed for its own operation before it can supply power to the output.

Power management is the key aspect to enabling remote wireless sensing, but it must be implemented starting at the concept of the design. System designers and planners have to prioritise their power management needs from the onset in order to ensure efficient designs and successful long-term deployments.

Integrated solutions
Integrated solutions are available that can overcome the deficiencies of current discrete energy harvester solutions. For example, Linear Technology Corp.'s LTC3109 is a dc/dc that takes a "system level" approach to solving a complex problem. It converts the low-voltage source and manages the energy between multiple outputs.

The part can harvest and manage surplus energy from extremely low-input voltage sources such as thermoelectric generators, thermopiles and even small solar cells. It operates from input sources as low as 30 mV, regardless of polarity. The LTC3588-1 a complete energy-harvesting solution optimized for low-power energy sources, including piezoelectric transducers. Piezoelectric devices produce energy either by compression or by deflection of the device. These piezoelectric elements can produce hundreds of microwatts/cm? depending on their size and construction. The chip operates from an input-voltage range of 2.7 V to 20 V, suiting it for a wide array of piezoelectric transducers, as well as other high-output-impedance energy sources.

About the author
Tony Armstrong is the Director of Product Marketing for Power Products in Linear Technology Corp.

To download the PDF version of this article, click here.


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