RFID-powered Sensors Can Play a Big Role in the Internet of Things

While energy-harvesting and battery-powered devices can be the right choice for certain applications, passive RFID sensor tags offer a number of advantages.
By Mikel Choperena

Natural Power Sources Available for Autonomous Sensors
Harvesting energy from the environment would allow these devices to operate on their own, virtually forever. The most-used natural power sources include photovoltaic, vibration, thermal and radio frequency (RF).

Photovoltaic energy: Typically known as photovoltaic or solar cells, this power source can be a feasible solution, since the energy level harvested can be quite high, depending on the amount of space available. However, these devices require continuous exposure to sunlight, greatly decreasing their performance as the intensity of the sunlight is reduced.

Vibration energy: There are multiple solutions for retrieving energy from environments in which continuous vibration is guaranteed—imagine a sensor device located in a train bogie (wheel assembly). Vibrational energy harvesting systems are designed for relatively stable conditions. For a selected frequency, the energy is correctly stored. However, for small frequency variations, efficiency dramatically decreases, thus reducing the amount of energy harvested over time.

Thermal energy: The concept of taking advantage of a temperature gradient is not new. A thermoelectric device creates a voltage when there is a temperature gradient between the two ends of it. It is a very efficient system when the temperature difference is significant (the exterior and interior temperatures of an airplane in flight, for example).

Radio frequency energy (RF): Signals from a variety of RF sources (television and radio transmissions, GSM signals from mobile phones and cell towers, Wi-Fi systems and so forth) can be captured by a sensor's antenna. The signals are then converted and conditioned to the desired output. RF is a constant energy source in every city of well-developed countries, but the amount of energy harvested is very limited. It requires long load times before reaching adequate energy storage levels for sensor nodes. Outside of cities, the RF source is generally very limited.

The conclusion is that no single energy source is reliable, unless environmental conditions are under control. Photovoltaic and thermal energy harvesting are very interesting in terms of the amount of energy that can be harvested, but the source must be very powerful in order for a sensor to draw a large level of energy—which is unlikely in most real-world applications. Because it takes a long time for an RF-harvesting device to store an adequate amount of energy to operate, RF energy can be suitable for sensors located near an RF source for which their activity is restricted to brief and very spaced communications.

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