Future products will supplement these with energy-harvesting technologies such as solar cells or piezoelectric generators driven by vibration. In the longer term, “nuclear” batteries that use direct energy conversion of nuclear radiation may provide power in sufficient quantity, and for years at a time regardless of ambient temperature.
Several new power supply technologies are being investigated for future wireless sensors. Initially, these technologies will serve in a supplemental role to chemical batteries. These new sources provide smaller amounts of power on a continuous basis and will work together with a chemical battery that provides energy storage. These new technologies may eventually be able to take over the entire role of sensor power supply, bringing significant advantages over chemical batteries.
Here comes the sun
Solar cells—which perform direct energy conversion of solar radiation—is one technology that has been used experimentally and in field trials of wireless sensors. It is already common in supervisory control and data acquisition (SCADA), as well as in infrastructure applications, such as remote weather monitoring. For wireless field devices, the limitations of solar cells are that they may not harvest sufficient energy in all applications.
Piezoelectric power harvesting uses ambient mechanical vibrations to generate small amounts of electric power that is stored either in a capacitor or battery. Venture firms have experimented with these electric power supplies that can have form factors similar to batteries. Presently, these supplies are large but not unworkable for some industrial applications.
Nuclear direct energy conversion (DEC) devices sound frightening, but in fact, are very similar to solar cells. Instead of solar radiation, nuclear DECs use radiation provided by the decay of a radioisotope embedded within the battery. This radiation falls on a semiconductor device similar to a solar cell, and directly generates electric power.
The primary advantage of nuclear devices is that the source of primary energy is the radioisotope, which can have a very long life. Because the reaction is nuclear rather than chemical, it proceeds at the same rate regardless of ambient temperature—a significant advantage over chemical batteries in industrial applications. However, this isotope must also be chosen such that the radiation emitted has sufficient energy, is optimum for capture in a small space, and does not radiate outside the device. This limits the potential isotopes to those that emit alpha or beta particle radiation, which has very limited penetration capability. In addition, the radioactive decay must form a stable element that does not later emit harmful radiation through further decay. Two isotopes that meet these criteria are tritium, an isotope of hydrogen, and nickel 63, an isotope of nickel. The half-lives of these products are 12.3 and 100.2 years respectively. This means that a supply of the isotope will be emitting half its initial radiation 12 or 100 years after manufacture.
The challenge of nuclear batteries has been to increase the efficiency of conversion. In order to do this, researchers have developed microelectronic structures that are optimized for capture of the radiation. Current research focuses on building honeycomb structures for the semiconductor material. Researchers in this area envision tritium batteries the size of a common “D” cell battery that could provide 1 to 5 milliwatts of power, even after 12 years of service. This is far more than is required to drive most wireless sensors.
The industrial market is not large enough to promote the development of an entirely new battery technology. But fortunately, other market segments can also provide applications for these premium power supplies.