I am sure most of us remember the Dire Straight’s song: “Money for nothing and …” but how many would have thought that it could one day be applied to energy harvesting! The concept crossover is analogous to something from nothing and your power for free. OK, some of you might think that’s a bit of a stretch but the fact remains that energy harvesting is about the re-use of energy as the byproduct of another action and using it to power an autonomous wireless sensor node (WSN). For those of you not familiar with WSNs, they are basically a self-contained system consisting of some kind of transducer to convert the ambient energy source into an electrical signal, usually followed by a DC/DC converter and manager to supply the downstream electronics with the right voltage level and current. The downstream electronics consist of a microcontroller, a sensor and a transceiver.
When trying to implement one or multiple WSNs, a good question to consider is: How much power do I need to operate it? Conceptually this would seem fairly straight forward; however, in reality it is a little more difficult due to a number of factors. For instance, how frequently does a reading need to be taken? Or, more importantly, how large will the data packet be and how far does it need to be transmitted? This is due to the transceiver consuming approximately 50 percent of the energy used by the system for a single sensor reading. Several factors affect the power consumption characteristics of an energy harvesting system of WSN. These are outlined in Table 1.
Table 1. Factors affecting the power consumption of a WSN
Of course, the energy provided by the energy harvesting source depends on how long the source is in operation. Therefore, the primary metric for comparison of scavenged sources is power density, not energy density. Energy harvesting is generally subject to low, variable and unpredictable levels of available power so a hybrid structure that interfaces to the harvester and a secondary power reservoir is often used. The harvester, because of its unlimited energy supply and deficiency in power, is the energy source of the system. The secondary power reservoir, either a battery or a capacitor, yields higher output power but stores less energy, supplying power when required but otherwise regularly receiving charge from the harvester. Thus, in situations when there is no ambient energy from which to harvest power, the secondary power reservoir must be used to power the WSN. Of course, from a system designer’s perspective, this adds a further degree of complexity since they must now take into consideration how much energy must be stored in the secondary reservoir to compensate for the lack of an ambient energy source. Just how much they will require will depend on several factors. These will include:
(1) The length of time the ambient energy source is absent
(2) The duty cycle of the WSN (that is the frequency with which a data reading and transmission has to be made)
(3) The size and type of a secondary reservoir (capacitor, supercapacitor or battery)
(4) Is enough ambient energy available to act as both the primary energy source and have sufficient energy left over to charge up a secondary reservoir when it is not available for some specified period?