Page 29

EETE MAR 2014

as those commonly found in WSNs that necessitate the use of power conversion ICs, which deal in very low levels of power and current. These can be 10s of microwatts and nanoamps of current, respectively. An energy harvesting WSN There is plenty of ambient energy in the world around us and the conventional approach for energy harvesting has been through solar panels and wind generators. However, new harvesting tools allow us to produce electrical energy from a wide variety of ambient sources. Furthermore, it is not the energy conversion efficiency of the circuits that is important, but more the amount of “average harvested” energy that is available to power it. For instance, thermoelectric generators convert heat to electricity, Piezo elements convert mechanical vibration, photovoltaics convert sunlight (or any photon source) and galvanics converter energy from moisture. This makes it possible to power remote sensors, or to charge a storage device such as a capacitor or thin film battery, so that a microprocessor or transmitter can be powered from a remote location without a local power source. In general terms, the necessary IC performance characteristics needed for inclusion and use in the alternative energy market include low standby quiescent currents, typically less than 6μA and as low as 450nA. The chip should also have low startup voltages, as low as 20mV, a high input voltage capability up to 34V continuous and 40V transients. But it should also handle AC inputs, feature multiple output capability and autonomous system power management, and auto-polarity operation. WSNs 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 micro-controller, a sensor and a transceiver. When trying to implement 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, Table 1: Energy sources and the amount of energy they can produce. 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% 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. 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 CFast eMMC/eMCP/MCP USB DOM Half Slim SSD DRAM Module 2.5”SSD www.electronics-eetimes.com Electronic Engineering Times Europe March 2014 29


EETE MAR 2014
To see the actual publication please follow the link above