I am sure mostof us remember the Dire Straight’s song: “Money for nothing and … ” but how many wouldhave thought that it could one day beapplied to energy harvesting! The conceptcrossover is analogous to something fromnothing and your power for free.
OK, some ofyou might think that’s a bit of a stretchbut the fact remains that energy harvestingis about the re-use of energy as thebyproduct of another action and using it topower an autonomous wireless sensor node(WSN).
For those of you not familiar withWSNs, they are basically a self-containedsystem consisting of some kind of transducerto convert the ambient energy source into anelectrical signal, usually followed by aDC/DC converter and manager to supply thedownstream electronics with the rightvoltage level and current. The downstreamelectronics consist of a microcontroller, asensor and a transceiver.
When trying to implement one or multipleWSNs, a good question to consider is: Howmuch power do I need to operate it?Conceptually this would seem fairly straightforward; however, in reality it is a littlemore difficult due to a number of factors.
For instance, how frequently does a readingneed to be taken? Or, more importantly, howlarge will the data packet be and how fardoes it need to be transmitted? This is dueto the transceiver consuming approximately50 percent of the energy used by the systemfor a single sensor reading. Several factorsaffect the power consumption characteristicsof an energy harvesting system of WSN. Theseare outlined in Table 1 .
Table 1. Factors affecting the powerconsumption of a WSN
The harvester, because of itsunlimited energy supply and deficiency inpower, is the energy source of the system.The secondary power reservoir, either abattery or a capacitor, yields higheroutput power but stores less energy,supplying power when required butotherwise regularly receiving charge fromthe harvester.
Thus, in situations whenthere is no ambient energy from which toharvest power, the secondary powerreservoir must be used to power the WSN.Of course, from a system designer’sperspective, this adds a further degree ofcomplexity since they must now take intoconsideration how much energy must bestored in the secondary reservoir tocompensate for the lack of an ambientenergy source. Just how much they willrequire will depend on several factors.These will include:
(1) The length of timethe ambient energy source is absent
(2) The duty cycle ofthe WSN (that is the frequency with whicha data reading and transmission has to bemade)
(3) The size and type ofa secondary reservoir (capacitor,supercapacitor or battery)
(4) Is enough ambientenergy available to act as both theprimary energy source and have sufficientenergy left over to charge up a secondaryreservoir when it is not available forsome specified period?
State-of-the-artand off-the-shelf energy harvestingtechnologies, for example in vibrationenergy harvesting and indoor photovoltaics,yield power levels in the order ofmilliwatts under typical operatingconditions.
While such power levels mayappear restrictive, the operation ofharvesting elements over a number of yearscan mean that the technologies are broadlycomparable to long-life primary batteries,both in terms of energy provision and thecost per energy unit provided. Furthermore,systems incorporating energy harvesting willtypically be capable of recharging afterdepletion, something that systems powered byprimary batteries cannot do.
Ambient energy sources include light, heatdifferentials, vibrating beams, transmittedRF signals, or just about any other sourcethat can produce an electrical chargethrough a transducer. Table 2 below illustrates the amount of energy thatcan be produced from different energysources.
Successfully designing a completelyself-contained wireless sensor systemrequires readily available power-savingmicrocontrollers and transducers thatconsume minimal electrical energy from lowenergy environments. Fortunately, low costand low power sensors and microcontrollershave been available for a couple of yearsor so; however, it is only recently thatultralow power transceivers have becomecommercially available.
With analog switchmode power supply designexpertise in short supply around theglobe, it has been difficult to design aneffective energy harvesting system asillustrated in Figure 1. The primaryhurdle being the power management aspectsassociated with remote wireless sensing.
Fortunately, however, companies likeLinear Technology have introduced a broadrange of energy harvesting ICs whichfacilitate the power conversion and systemmanagement aspects of a WSN design. Thesedevices can extract energy from almost anysource of light, heat or mechanicalvibration.
Furthermore, with theircomprehensive feature set and ease ofdesign, they greatly simplify thehard-to-do power conversion design aspectsof an energy harvesting chain. This isgood news for the designers of WSN becausetheir high integration, including powermanagement control and off-the-shelfexternal components, make them thesmallest, simplest and easy-to-usesolutions available.
In conclusion, although power is for freefrom numerous ambient energy sources,system designers and systems planners mustprioritize the specific requirements oftheir power management systems from theonset in order to ensure efficient designsand successful long term deployments.
Tony Armstrong is director of productmarketing, Power Products, LinearTechnology Corporation. He can be contacted at
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