Most of the effort in the energy harvesting industry so far has been on creating smaller, more efficient energy harvesters and low power electronics for wireless sensing. However, between those requires electronics that interface to the energy harvester and process the power, such as rectification, step up/down and store energy so that the power can be delivered as needed. Energy harvesters used today to power sensors and other electronics have such appropriate interfaces. However, more can be done, especially tackling the problem of creating energy harvesters that can adapt to the environment, unlike today where most products have energy harvesters "tuned" for that particular application. Dr Paul Mitcheson from Imperial College, London, recently presented on their work tackling the issue.
Electrostatic energy harvesters: dead end
Dr Mitcheson has spent some time looking at electrostatic energy harvesters. This energy harvester consists of three wafers forming a capacitor structure. A small charge is applied to the structure, and when the plates are moved relative to each other a high voltage spike is generated. This system can generate spikes of a few hundred volts. The challenge for the interfacing electronics is that these spikes need to be converted into a constant 3V DC rail. The team spent some time developing appropriate interfacing electronics but found that the combination of high voltages and low current together with the operating ranges of today's electronics produces only a very narrow window of opportunity for the energy harvester. The energy harvesters are poor at low frequencies and at large sizes (more than 10mm in length) and low acceleration. Dr Mitcheson pointed out the limits of this energy harvester only became apparent when looking at the whole system, and he thought short term prospects for this technology "are bleak".
Piezoelectric energy harvesters are currently being developed by many companies. They provide an AC output and therefore the interfacing electronics requires rectification, and step up/down electronics. However, a challenge of piezoelectrics is that while they are efficient at optimal resonance, only a slight variation away from the optimal resonance causes a significant reduction in energy generation - the bell curve is very steep. Others try and compensate for this by using magnets to try and limit the vibrations within the main resonance of the device, which adds bulk and cost and does not enable true broadband ability.
Dr Mitcheson described how electronically pre-biasing the piezo electric material can be used to tackle the problem. A charge put on the material acts as a damper and requires the material to do more work against it. As a result, they have shown a power gain up to 20 times compared to an optimal resistive load.
The group are also looking to apply adapatability to electromagnetic energy harvesters. By using reactive components the resonance of the system can be changed electronically.
For more information, Dr Mitcheson will be presenting this work at the IDTechEx Energy Harvesting & Storage conference in Munich June 21-22 www.IDTechEx.com/ehEurope.
Top image of Dr Paul Mitcheson source: Imperial College London
For more read : Energy Harvesting and Storage for Electronic Devices 2010-2020