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Energy Harvesting for Wearables

Wearable applications including smart watches, sport watches, fitness bands and more are growing in popularity, and so, too, is the demand for intelligent sensors to use in them. If analyst predictions are correct, the market is going nowhere but up in the coming years.

So what are the keys to making wearables ubiquitous for daily life? Significant features and functionality are a given, but equally critical is battery management. The more plentiful the features, the more power the wearable technology needs. Let’s face it, nobody will be happy with wearables with short battery lives.

Wearables manufacturers have several options for increasing battery life. The first involves reducing the power consumption of each individual block to a minimum. Another is using higher capacitance batteries — although in most cases this will also lead to larger battery size.

But there’s a great third option for extending battery life for wearables: energy harvesting. What if simply wearing the device could also extend its battery life? The concept isn’t new. One of the earliest examples of an energy harvesting application is the well-known self-winding watch, which was invented in 1770.

Today’s energy harvesting technologies have progressed significantly and the number of developments around energy harvesting is growing quickly in many different market segments:

  • A key unique selling proposition for watchmaker Citizen is micro energy harvesting with solar energy. Most of the company’s watches are powered by light that is harnessed on the dial.
  • Seiko, one of the pioneers of digital watches, launched a solar-powered GPS watch last year.
  • Earlier this year, Tissot introduced T-Touch Expert Solar, which includes various sensors and micro energy harvesting via solar power.

In the future it will be impossible to imagine many applications not using some form of energy harvesting technology. There are various applications in industry and in consumer markets where energy harvesting will be validated in the not-too-distant future:

  • Ultra low-power wireless systems (WSN)
  • Body Area Networks (to monitor vital signs/medical conditions)
  • Industrial monitoring
  • Intelligent thermostats
  • Smart textiles/garments

Independent of the energy source, all energy harvesting applications have one common denominator: A dedicated part that transforms the energy from the source into electric power. To recover the energy for use in an application, a separate dedicated part must efficiently condition the power so it conforms to the requirements on the application side.

For the wearables market, no matter if using a solar cell or a thermoelectric generator, at the end, every wearable application needs a boost converter, which transmits the power from the low-input voltage side to the high-output voltage side (charging a battery or a supercap, for example). Depending on the application and the requirements, the boost converter must be designed and configured perfectly to use energy from the source part efficiently. Hence, using a standard converter, which should cover a wide area of end applications, will not lead us to a good result.

Instead, when harvesting energy for wearable applications, the converter must fulfill three major requirements:

  • It must be a very highly efficient booster.
  • Depending on the energy source and often on the end application’s associated space limitation, it has to be able to start up and work under very low-input voltage ranges.
  • It must be optimized perfectly to the used energy source part and its operating point. This is called “maximum power point tracking” and is defined as the capability to reach exactly this operating point, where the maximum power can be achieved from the energy source.

Due to these three important factors, for each type of wearable application and for every different kind of energy source a dedicated boost converter is required to achieve a highly efficient and satisfying system.

The wearables market and its growth will be exciting to watch. Manufacturers gearing up for a place at the table should be looking at the possibilities of energy harvesting. Keys to that will be creativity and a strong focus on IC solutions that address all three of the major criteria outlined above.

23 comments on “Energy Harvesting for Wearables

  1. eafpres
    July 16, 2014

    Hi Thomas–nice article that brings out a couple of key points.

    Regarding the need for custom solutions for every application, over on the digital size we have FPGAs and other options to make a configurable part instead of many custom ones.

    Do you see somewhere in the future the potential for a configurable boost converter that could take the place of, say a half dozen or more bespoke parts?

  2. samicksha
    July 16, 2014

    Good Blog @Thomas, most of the points mentioned in blog are known to us and as wearables evolve more and more the need to good battery life is coming strongly, taking referance of Google glass which contains a puny 570 mAh lithium-polymer battery, everything in this new glasses is impressive but battery life is poor which is draging this piece of invention away from from market.

  3. Netcrawl
    July 17, 2014

    @samicksha, There's a lot of things we heard about Google Android Wear, but there's one thing that bother us a lot, that would be the battery life. I think its a major issue in today's wearable tech, and there's no easy or quick fix.

    Several companies have tried to address this issue, this has plagued the entire wearable and smartwatch market, Samsung has already tried to address this issue but they come out with nothing.  

  4. Netcrawl
    July 17, 2014

    @samicksha rechargeable lithium-ion batteries are still an ideal choice or best bet for wearable devices because of their high gravimetric energy density, the battery allow devices to run for longer. Today's lithium-ion batteries have a energy density of approximately 500 watts-hour per liter. And can still be considered as a “best in class” for commercially available rechargeable batteries.  

  5. Davidled
    July 17, 2014

    If engineer design in the smart way, energy could be saved partially. In the power mode design, he/she could monitor the event that most power could be consumed via power tracking debugger. At the event, hardware response could be analyzed and then power efficient system could be built in the device. Also it could be effected by the used processor since each processor has a different configuration and features.

  6. samicksha
    July 18, 2014

    I totally agree to your point on battery challenge, although most of the new werables bear same kind of problem, using gadget for multitasking makes them more power hungry.

  7. samicksha
    July 18, 2014

    You are right @Netcrawl on lithium-ion battery, we always have scope of improvement as we did recently with a silicon anode rather than graphite anode which helped us to yield 20% higher capacity than those previously available.

  8. vasanjk
    July 18, 2014

    Thomas

    I always thought if power could be harvested from wirelessly transmitted radio frequency, it could solve most problems. Several years back, we used to make simple AM receiver which is powered from received AM frequency RF power. It used a single germanium diode and a ceramic cap connected to the headphone.

  9. samicksha
    July 18, 2014

    Good point @Daej, with same i guess little education to consumers about battery would also help, as we understand lithium-ion which is highly used performs well at elevated temperatures but prolonged exposure to heat reduces battery life.

  10. Victor Lorenzo
    July 18, 2014

    @versanlk, count me as one of those that also played with that kind of radio receivers. I started with a very very old (1940's??) high impedance (10kohms) headset, a long piece of wire (antenna) and a germanium diode. After that I continued experimentation with the tuned antenna circuit, the ferrite antenna, the super regenerative and some more circuits.

    It was a realy fun doing that!

  11. vasanjk
    July 18, 2014

    Victor

     

    I too went along the path. It was terrific! We had single transistor circuits too. The best one was a 7 transistor design with a ganged capacitor 500pF for tuning into stations. It was fun to listen into Radio Ceylon and BBC in my mother tongue – Tamil.

  12. geek
    July 21, 2014

    “Several companies have tried to address this issue, this has plagued the entire wearable and smartwatch market, Samsung has already tried to address this issue but they come out with nothing.  “

    @Netcrawl: I think the issue still remains unresolved because producing efficient batteries was never the aim of R&D for any major company. Over the years, companies were looking to invest in display technologies, processor chips and software but battery optimization was never a goal. It's only fairly recently that companies have begun to find it a critical selling point if the battery is weak or doesn't last very long. I think it should take some time and a technological breakthrough to come up with a solution.

  13. geek
    July 21, 2014

    “i guess little education to consumers about battery would also help, as we understand lithium-ion which is highly used performs well at elevated temperatures but prolonged exposure to heat reduces battery life.”

    @samicksha: I think the consumers should also be educated on how to optimally charge batteries. According to experts, one of the ways to make the battery life longer is to charge it to full and let it discharge fully. Frequent charging of the battery makes the life shorter pretty easily.

  14. Vishal Prajapati
    July 22, 2014

    Radio frequency energy harvesting is very good option and freely available.

     

    But imagin the antenna of the radio you mentioned. It was several feet long.

     

    Since then the frequency has been increased from KHz to GHz and consequently the size of the antenna has also been decreased. But on the contrary the transmit power has also decresed. So, to accomodate the antenna and power conversion circuit in side a wearable device is a challenge.

     

    I think that is the only reason why many big companies haven't put efforts in this area.

  15. vasanjk
    July 23, 2014

    Vishal

    I was only thinking aloud when I said that. The trend is moving towards higher frequencies with less power and it is going to be so for ever.

    Though wireless charging involves a similar technology, it is exactly not harvesting, as I see it.

  16. Sachin
    July 31, 2014

    The boost converter is undoubtedly essential for any energy harvesting device. I could not agree more with the 3 requirements that make a great boost converter as you clearly pointed them out. However, I strongly believe that there is a fourth salient feature that most designers take for granted but which deserves some thought. A good boost conductor must also fit within a specific output limit. It is worth pointing out that it is not always that the converter that produces the highest power output given the same levels of input as other converters is necessarily the best and the output limit should be tied to the specific power needs of the device in question.

  17. Sachin
    July 31, 2014

    For miniature devices with large power consumption, there is a rising trend to separate the actual device from its power harvesting system. The latter, which also includes the boost converter, may be designed separately as an independent device, or could be built into a larger device, such as a Smartphone (which you will probably always be carrying with you anyway), and be used to charge the miniature device or piece of wearable tech wirelessly. It may be a little more costly but it reduces the space limitations that come with building it into the actual device and therefore results in a much longer battery life.

  18. yalanand
    July 31, 2014

    Interesting read. Reminded me of one of the reasearches that involve developing a wearable fabric that would be able to harvest solar energy. One such prototype, called the Powerweave, is already made as I can reckon. Such technologies would surely make wearable articles far easier to maintain. This seems a start to something greater in the times to come.

  19. yalanand
    July 31, 2014

    Vibration or movement sensors that can harvest energy seems to be the optimum choice for wearables since we are literally always on the go. Piezoelectric vibration sensors for example, can be atached to soles of the footwear to harvest energy without any bother. Such convenient options, once brought in the market would usher in interest undoubtedly.

  20. yalanand
    July 31, 2014

    Meeting the power challenges seems the toughest job out there right now, for all the tech giants. The functionalities they provide are increasing, so is the processing speed which enables faster multitasking, only Power is the limiting agent for them. But the alternatives are catching grounds, as your article has pointed out in detail. With the ongoing research, a solution to all our battery life woes is not too far.

  21. Thomas Kail
    August 11, 2014

    @eafpres1: First of all, thanks for your comment. For sure, the goal of every IC manufacturer is the development of the all-in-one device that suits many purposes. I´m sure, that it would be possible to manage several applications with just one configurable part, but not without any drawbacks. I´d say, at first we need to  distinguish between the different sources of energy which are going to be harvested. Is it thermal energy, solar energy or energy of an electromagnetic field or maybe mechanical energy or something else? Then it is a very important point, at which voltage level the booster has to startup correctly? Is this anyway a battery driven application, which provides the needed minimum voltage to startup the booster or is this a complete autarkic system without any battery, where the booster has to manage itself just with the harvested energy and a startup with very low input voltage is mandatory? The development of the booster concepts depends on different criteria and it is for sure a big challenge. To cut a long story short, I personally see in the future  the potential for configurable boost converters that could address more than just one dedicated application. On the other hand, we have done this successfully for many years in  areas such as for Power Management Units, which suit  various applications while supporting an array of different  microcontrollers. 

  22. eafpres
    August 11, 2014

    @Thomas–thank you for the detailed answer.  Your points about the energy source for “sleep” mode and how to start up points to the complexity of all these harvestig applications.  Definitely some good food for thought.  It will be interesting to watch this evolution.

  23. Davidled
    August 12, 2014

    As one of application for vibration and movement, if customer has pet animal, whenever pet animal is moving, sensor attached to animal would generate the energy and store it to energy source.  This energy could be used for residential purpose or gadget.

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