Energy Harvesting for Sensors

Energy harvesting is a relatively new term for describing the powering of low power electronic sensor systems so that they can be used truly wirelessly, without the need for batteries that will eventually need recharging.

There are several sources in the environment that can be “harvested”, with the most popular being vibration, strain, temperature, and light. A common method for harvesting vibration uses a cantilever beam, to which one or two layers of piezoelectric materials are attached, to generate an electrical charge/voltage. The frequency range can be adjusted by attaching a tip mass to alter the natural frequency of the harvester to match the operational frequency and thus maximise the energy output.

Another method to harvest power from vibration utilizes the electromagnetic effect of a moving magnet in a coil of wire (or vice versa), which again can be tuned for maximum output at resonance. There are devices on the market now that are optimized for use with piezoelectric accelerometers, for industrial condition based monitoring. Harvesting power from strain is also usually harvested from vibration. A piezo patch device is stuck to the surface of the vibrating structure, thus generating charge/voltage.

Whilst a piezo-electric or inductive device can generate 10's to 100's of Volts, a thermo-electric device or small solar panels will generate only micro amps and millivolts. The use of thermoelectric technology as a means of energy generation is dependent on a temperature gradient between the surface that energy is to be harvested from, and the ambient air temperature. The energy produced may be in the range of a few tens of microwatts to hundreds of milliwatts depending on the size and specification of the Peltier or thermopile device, and the temperature differential applied across it. Maintaining a temperature differential across the harvester is dependent on airflow, without which the 'hot side' and 'cold side' temperatures will eventually equalise due to thermal conduction and no energy will be generated.

The use of photovoltaic technology as a means of energy generation using outdoor light is common, as there is high energy content in sunlight. However, indoor fluorescent and incandescent lighting produces many orders of magnitude less power than that of typical outdoor light. The energy produced (typically 10's to 100's of microwatts) from indoor light is too low to instantaneously power a wireless sensor node. The electronic devices that work with these harvesters therefore need to have the ability to utilize these very low inputs.
The EH-Link wireless node is a self-powered sensor, harvesting energy from ambient energy sources, and is compatible with a wide range of generator types, including piezoelectric, electrodynamic, solar, and thermoelectric generators. In addition to multiple harvesting inputs, the EH-Link features an on-board triaxial accelerometer, relative humidity sensor, temperature sensor, and signal conditioning for a Wheatstone bridge which is compatible with strain gauges, load cells, torque sensors, and pressure transducers, all in a miniature package.

Article by Ian Ramage from Techni Measure


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