In the modern world of connected everything, there are still sensors and equipment that can’t connect to the electric grid. Remote weather stations, pipeline monitors, navigation lights, security systems, and a host of other applications are forced to rely on energy sources that may be intermittent, unreliable, or of poor quality.
Energy-harvesting techniques are available to remedy this problem. The actual techniques deployed will obviously vary according to the source, the form of energy to be harvested, and the load to be supplied. Some devices will be very small (like remote wireless sensors), while others will be much larger (to provide energy for motors, for example).
There are many technologies that can be used for energy harvesting:
- RF energy. This form of energy harvesting utilizes RF energy in the environment and converts this into energy to power a small device. Receiving antennas are used to pick up the RF signals, which are then rectified and used.
- Piezoelectric energy harvesting. When a piezoelectric crystal is distorted, a voltage appears across the crystal. In this way, vibration or mechanical stress can create power. These devices generally only provide small amounts of power.
- Thermoelectric energy harvesting. This form of energy harvesting uses the same principle as that used in thermocouple temperature sensors. When a junction of two dissimilar metals is heated, a voltage difference is created across the junction. This is known as the Seebeck effect. Engineered compounds such as bismuth telluride have much higher Seebeck voltages than common metals, but even so, the efficiency of thermoelectric devices is generally low. Nevertheless, it can be utilized in some applications.
- Wind generators. While huge wind turbines are widely used for the large-scale energy production, small micro-generators can also power small systems.
- Solar cells. Converting sunlight into electrical energy is one of the more common methods of energy harvesting. Solar panels are relatively inexpensive and are reasonably trouble-free. However, they can’t provide power at night, and their output is severely reduced in cloudy weather.
All the above energy-harvesting techniques generally require some form of energy storage to be useful. The most common storage medium is an electrochemical battery, such as lead-acid, lithium, or nickel-metal hydride. The main drawbacks to batteries are their limited lifetimes, susceptibility to damage from over- or under-charging, and limited operating temperature range.
Another more recent energy storage option is the electrical double-layer capacitor (EDLC), more commonly known as an ultracapacitor. In many respects, an ultracapacitor is the same as a standard capacitor, except that it has an enormously higher capacitance, measured in hundreds or thousands of farads, compared to the millifarads and microfarads of common capacitors. Also, ultracaps are limited to low voltages, generally 2.5V or 2.7V maximum. This means they must often be connected in series to supply the voltages needed for standard electronic equipment. The main disadvantages are the ultracapacitor’s limited volumetric energy storage capacity and cost.
However, in applications where short bursts of high power and reliable operation are needed, they can be an excellent choice. For example, remote weather stations are often designed to collect data with low-power sensors. Then, every few minutes (or tens of minutes), they send a short radio packet to the home base with the accumulated data. These stations typically use lead-acid batteries for energy storage, with all the problems that they entail.
Another application would be chemical injection pumps for petroleum pipelines. Long pipelines often have pumping stations along their lengths, which inject small amounts of drag-reducing chemicals to allow the oil to flow more easily, thus saving on pump energy. They typically use very small pumps that run for several seconds every few minutes, each powered by a small solar panel and a lead acid battery.
In a case like this, either an ultracapacitor bank alone, or in parallel with the battery, can make a more robust system than just a battery. When very cold, the battery's equivalent series resistance (ESR) increases, sometimes to the point where it’s unable to start the pump motor. If there’s an ultracapacitor bank in parallel with the pump, the battery will keep it charged, even during long periods when there’s insufficient output from the solar panel, and the ultracap's very low ESR will always enable it to run the pump as necessary.
A similar application is one for solar-powered electric gates in agricultural areas (see figure 1). The power needed to open and close a gate is moderate, perhaps 100W. But because it happens only a few times per day, the total energy needed per day is quite low. The advantages of using ultracapacitors (reliability and temperature extremes) make them an attractive option. Other common applications, similar to weather stations, are remote monitoring of tank levels, well outputs, security/intrusion systems, and process control.