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Energy harvesting

Energy harvester produces power from local environment, eliminating batteries in wireless sensors

May 09, 2011 | Jim Drew | 222902645
Jim Drew of Linear Technology explains how remote sensors are able to operate without batteries by harvesting ambient energy from the surrounding environment.
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Recent advances in ultralow power microcontrollers have produced devices that offer unprecedented levels of integration for the amount of power they require to operate. These are systems on a chip with aggressive power saving schemes, such as shutting down power to idle functions. In fact, so little power is needed to run these devices that many sensors are going wireless, since they can readily run from batteries. Unfortunately, batteries must be regularly replaced, which is a costly and cumbersome maintenance project. A more effective wireless power solution may be to harvest ambient mechanical, thermal, or electro-magnetic energy in the sensors local environment.

The LTC3588-1 shown in Figure 1 is a complete energy harvesting solution optimized for high impedance sources such as piezoelectric transducers. It contains a low loss full wave bridge rectifier and a high efficiency synchronous buck converter, which transfer energy from an input storage device to an output at a regulated voltage capable of supporting loads up to 100mA. The LTC3588-1 is available in 10-lead MSE and 3mm 3mm DFN packages.

Ambient energy sources
Ambient energy sources include light, heat differentials, vibrating beams, transmitted RF signals or any other source that can produce an electrical charge through a transducer.

For instance,
Small solar panels have been powering handheld electronic devices for years and can produce 100s of mW/cm in direct sunlight and 100s of W/cm in indirect light.
Seebeck devices convert heat energy into electrical energy where a temperature gradient is present. Sources of heat energy vary from body heat, which can produce 10s of W/cm to a furnace exhaust stack where surface temperatures can produce 10s of mW/cm.
Piezoelectric devices produce energy by either compression or deflection of the device. Piezoelectric elements can produce 100s of W/cm depending on their size and construction.
RF energy harvesting is collected by an antenna and can produce 100s of pW/cm.

Successfully designing a completely self-contained wireless sensor system requires power-saving microcontrollers and transducers that consume minimal electrical energy from low energy environments. Now that both are readily available, the missing link is the high efficiency power conversion product capable of converting the transducer output to a usable voltage.

Figure 2 shows an energy harvesting power system that includes the energy source/transducer, an energy storage element and a means to convert this stored energy into a useful regulated voltage. There may also be a need for a voltage rectifier network between the energy transducer and the energy storage element to prevent energy from back-feeding into the transducer or to rectify an AC signal in the case of a piezoelectric device.

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