Energy harvesting: advancing step by step

22 November, 2015

There is no lack of energy on this planet – the sun bombards us with more energy than we can use every day, excess heat from computers and even our own human bodies has to be dealt with by sophisticated cooling systems, and vibrational energy such as the rattling of trains, the shaking of the stairs as we go up just gets dispersed and lost, the list can go on. Energy harvesting addresses the question of how to cost-effectively tap into all these resources, mainly with a view to powering devices with no access to the grid.

The underlying science for energy harvesting technologies has been around for a long time and is generally well understood. There are piezoelectrics to convert mechanical energy into electricity (and we are already familiar with them in gas lighters, for example). There are solar cells which have been able to power handheld calculators in schools and offices for the longest time, there are thermal generators using temperature differences to generate electricity (the Seebeck effect), and there are also chemical fuel cells. One could also add micro-hydro and micro-wind generators to the list.  All these great technologies harvest energy from their immediate surroundings which would be wasted otherwise.

However, in order to be useful the harvested energy needs to be stored somewhere, so it can be released when it is needed and in sufficient quantities (i.e. providing sufficient power). So while the science of energy harvesting has been well understood for a long time, it is the recent advances in adjacent areas of technology which are making energy harvesting increasingly attractive for a wider range of applications. These adjacent areas of technology are: advances in battery and particularly supercapacitor technology, power management and also low power applications. Advances in batteries and supercapacitors make it possible to store energy collected over a longer time period and then release it over a short period.  Additionally low cost chips which are now commercially available can handle the necessary power management, such as handling the charging of batteries & capacitors. Finally, there are low power applications ready to be used in conjunction with energy harvesting devices.

While the science of energy harvesting has been well understood for a long time, it is the recent advances in adjacent areas of technology which are making energy harvesting increasingly attractive for a wider range of applications.

One of the main emerging applications for energy harvesting are wireless sensor networks. These could be security systems, temperature regulating systems for buildings, smoke and chemical detectors, sensors for industrial settings and many more.  Particularly in the context of the “internet of things” sensors are plentiful: your fridge telling you, or your home-delivery supermarket, that you need some more butter or a pot plant notifying you that it requires more water. You may think that you can easily spot the signs of a thirsty pot plant, but in an agricultural context, sensors which give indication of the need to water or add nutrients to the soil can hugely reduce the amount of valuable water used and save costs. Also for applications in environmental monitoring (e.g. sensors over large areas recording meteorological data or detecting forest fires) the advantage of not having to replace batteries is particularly significant.

Sensor networks also find applications on our own bodies (“body networks”): we all like to be fit and healthy and gadgets to measure how many steps we have taken, how many miles we have run and how many calories we have burned are in vogue. A little bit of the energy that we use to propel ourselves over lush green fields at high speeds in the latest running outfit could be used to power that gadget itself. It may help to make us feel good about ourselves, but more importantly, prevents frustration when the gadget battery runs out in the middle of a particularly long exercise session.

One obstacle to harvesting mechanical energy from the moving body in this context is where movement is largest – at the joints, which is where demands on the energy harvesting device in terms of stretchability of the material are also the greatest. Research is currently underway to overcome this challenge and some recent developments from research in the USA and Korea address this issue.

In the medical device sector there are exciting advances too, such as the ability to power pace makers by tiny fuel cells running on blood sugar – or indeed power brain-machine interfaces with blood sugar which will one day give you the ability to control your computer with your brain. You yourself may be happy with a computer mouse at the moment, but in the case of a patient who has lost limbs this ability might open up a lot of opportunities.

The coming years will see advances in multiple energy harvesting devices: a device may choose which energy source it wants to tap into and/or be able to collect from various energy sources simultaneously.

Progress will come in little steps. Just as energy harvesting seeks out little quantities of unused energy and converts them into useful energy, so the progression of applications in this area will come in small stages using developments in other fields as stepping stones to create useful new devices which will make our lives more comfortable.

Britta Kleinsorge

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