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Taking advantage of ambient energy

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

Keywords:Ambient energy? energy harvesting? thermoelectric generator?

Employing non-traditional sources for powering electronic circuits can now be done as these products can already operate at microwatt power levels. This paved the way for the rise of energy harvesting, which provides the power to charge, supplement or replace batteries in systems where battery use is inconvenient, impractical, expensive or dangerous.

Energy harvesting can eliminate the need for wires to carry power or to transmit data. It can power smart wireless sensor networks to monitor and optimize complex industrial processes, remote field installations and building HVAC systems. And otherwise wasted energy from industrial processes, solar panels and internal combustion engines can be harvested for useful purposes.

Ambient energy sources include light, heat differentials, vibrating beams, transmitted RF signals and any source that can produce an electrical charge through a transducer. Such "free" energy sources can be converted into electrical energy by using a suitable transducer, such as thermoelectric generator (TEG) for heat, a piezoelectric element for vibration, a photovoltaic cell for sunlight (or indoor lighting) and even galvanic energy from moisture. These energy sources can be used to power electronic components and systems autonomously.

Despite their complexity, energy-harvesting systems have already been deployed in transportation infrastructure, wireless medical devices, tire pressure sensing and building automation. In building automation systems, elements such as occupancy sensors, thermostats and light switches can eliminate the power or control wiring normally associated with their installation and instead use localized energy harvesting. A wireless network using an energy-harvesting technique can link sensors in a building to reduce HVAC and lighting costs by turning off power to nonessential areas when the building is vacant. The cost of enabling energy-harvesting electronics is often lower than that for running supply wires, so there is an economic gain to be had by adopting a harvested power technique.

Many of the advantages of a wireless sensor network disappear if each node requires its own external power source. Through power management developments have enabled electronic circuits to operate longer for a given power supply, that approach has its limitations. Energy harvesting provides a complementary approach, powering wireless sensors nodes by converting ambient energy into usable electricity.

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Figure: Here are the four main blocks of a typical energy-harvesting system or wireless sensor node.

Typical node
A typical energy-harvesting configuration or wireless sensor node comprises four blocks (figure): an ambient energy source; a power conversion component to power the rest of the node; a sensing component, comprising a microprocessor or microcontroller that processes measurement data and stores the data in memory; and a communications component, consisting of a short-range radio for wireless communications with neighboring nodes and the outside world.

Examples of ambient energy sources include a thermoelectric generator (TEG) or thermopile attached to a heat-generating source (such a windowpane). In the former case, a transducer can convert small temperature differences into electrical energy; in the latter, the piezoelectric device can accomplish the same conversion with mechanical vibrations or strain.

Once the electrical energy has been produced, it can be converted by an energy-harvesting circuit and then modified into a suitable form to power the downstream electronics. Thus, a microprocessor can wake up a sensor to take a reading or measurement.

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