A common question when considering energy harvesting solutions to wireless powering of devices is: "...why can't we just use batteries? Surely with batteries lasting longer these days, being energy dense and being cheap these will do nicely."

Batteries, however, have three major limitations:

- Reliability
- Environmental impact
- Lifetime and cost of changing

Reliability
There are a number of areas where the use of batteries to power wireless devices, often sensors of one sort or another, is impractical because batteries do not provide a high enough level of reliability. A typical cause of this will be high environmental temperatures. In industrial sensing situations there are therefore applications where and energy harvesting solution provides the missing reliability. These will tend to be high value but often niche applications.

Environmental
The widespread use of batteries has created many environmental concerns, such as toxic metal pollution. Battery manufacture consumes resources and often involves hazardous chemicals. Used batteries also contribute to electronic waste. Any developments that lead to the proliferation of batteries is therefore likely to lead to enormous environmental concern.

Lifetime
Whilst many batteries are rated for a typically 3 year and sometimes up to 10 years lifetime, the reality is that in most real world applications they last significantly less, often months rather than years. Lifetime is dependent upon both the duty cycle of the tasks to be powered and on the environmental conditions under which this happens. Even if never taken out of the original package, primary batteries can lose 8 to 20 percent of their original charge every year at a temperature of about 20-30 degrees C by self discharge. Newer chemistry and modern lithium designs have reduced the self-discharge rate of rechargeable batteries to a relatively low level (but still poorer than for primary batteries). Start making the battery do some intermittent work and this lifetime reduces drastically making battery change a necessity in almost all applications. This is by far the biggest argument for energy harvesting.

This is an issue for those niche applications with wireless sensor nodes placed in hard-to-reach locations (e.g. inhospitable and remote or hazardous industrial environments or even medically implantable devices) making the changing of batteries regularly both costly and inconvenient.

However, it is also an issue on a massively larger scale for pretty mundane applications where the access to the batteries is perhaps not difficult due to remoteness but is nevertheless impractical or expensive. Where EH will come into its own is in powering all of those applications where battery change is just inconvenient due to a combination of cost and volume of such changes and the unpredictability of battery lifetime.

For example, EnOcean, a German energy harvesting company, already have energy harvesting powered solutions in >100,000 buildings. These include light switches, temperature controls, window contact sensors, lighting control - all elements of a smart building which enable them to produce an average annual energy savings of 60%. In just one example they have installed 4200 wireless and battery-less light switches, occupancy sensors and daylight sensors in a new building construction in Madrid. These are powered by energy harvesters and embedded in the building. This saved 40% of lighting energy costs by automatically controlling the lighting in the building, removed the need for 20 miles of cables, avoided the use and changing of 42,000 batteries (over 25 years) and saved 80% of the cost of retrofitting a wired solution.

Even more mundane is the example of US energy harvesting company that was approached by a company that runs the toilets in various public buildings e.g. airports. This company has a problem with the taps that use infrared sensors to detect hands and switch on water. Evidently they regularly stop working as they exhaust the 3 cell batteries that power them generally in around 3 months. Every time one of those taps goes down they get a call and have to send in a qualified plumber to scrabble under the sink and change the battery at a reported cost of $75 a time. There are undoubtedly countless other trivial examples that aren't about hazardous areas, deep sea, underground access, just inconvenient and expensive to change batteries due to who can or will change them. Another example would be the need that hotel chains have to periodically go around changing batteries on the key cars door locks on its rooms. There are current project investigating energy harvesting solutions based on piezoelectric materials and the pressure of inserting the card. Another simple but elegant solution to the cost of regularly having to go up and deal with door locks with dead batteries.

Texas instruments have developed EH test kits to complement EH suppliers devices and cite the applications in a wide range of areas including remote patient monitoring, efficient office energy control, surveillance and security, agricultural management, home automation, long range asset tracking, implantable sensors, structural monitoring and machinery/equipment monitoring.

So there are already millions of opportunities in currently battery-powered or direct wired devices where energy harvesting could save considerable sums of money in terms of labour. However, the even bigger opportunity is in the so-called 'Internet of Things', an ecosystem of wirelessly connected devices many of which have to be mobile and/or inaccessible for battery change. Cisco claim that already by 2010 the IoT contained 12.5bn connected devices, that by 2015 this will be 25bn and by 2020 it will be 50bn devices.

Cisco predict that as many as one trillion devices will become connected to the Internet. Such connections include sensors on electrical lines to conserve energy, RFID tags attached to shipping crates to track inventory, temperature sensors in lab refrigerators to preserve medical supplies, tiny humidity gauges scattered on a forest floor to assess fire danger, sensors drifting in the ocean to track pollution, or body worn sensors connecting your vital signs to a remote healthcare provider. For IoT to reach its full potential, sensors will need to be self-sustaining. Changing batteries in billions of devices deployed across the planet and even into space is not an option and furthermore the disposal implications of billions of waste batteries is huge. Energy harvesting technologies therefore need to step up with a solution that allows fit and forget powering of the device.

Energy harvesters enabling micro-power generation provide new levels of efficiency and automation in the built environment, process control, vehicles and healthcare. However, recently a large effort is being placed into developing the technology for integration into textiles. While on the surface this may seem niche, if we delve deeper we find there is a substantial opportunity for the technology here. Textiles are the most common human interface because 70% of surfaces touched each day are textiles, including clothes, bedding, wall covering, upholstery and flooring. Textiles are flexible, comfortable and consumer oriented. They are stretchable and conformable and they present large surface areas to work with. They are highly engineered and ordered structures produced by an established, efficient, global supply chain and a mass volume application platform.

Textile electronics is primarily used for sensing as in health monitoring, warming as in outdoor wear, display and lighting including illuminating t-shirts and party fashions and control as with sleeve controls for your iPod and woven rollable keyboards. Then there is logistics, notably RFID. That includes active RFID where there is a battery in the tag for real time location, mesh networking and so on. These batteries will have energy harvesting in future. Beyond clothing, the technology is being integrated into car and train seats. For example, EnOcean energy harvesters are used to monitor occupancy of seats in trains so that operators can judge when to add or remove carriages. The sensors are powered by the act of someone sitting on the seat.

Energy Harvesting and Sportswear

Adidas is moving forward with products such as the virtual trainer. Their subsidiary Textronics focuses exclusively on textile electronics, specifically Wearable Physiological Monitoring Systems. Here the vision is to easily measure body data to help the consumer manage health and wellness goals.

At Europe's largest event on Energy Harvesting & Storage (Munich, Germany June 21-22 http://www.IDTechEx.com/EHEurope), Decathlon (owned by Oxylane Group) will present on the opportunity for energy harvesting for electronics sports products. The presentation will cover energy requirements and user needs and requirements for energy harvesting products.

The Textile Becomes the Energy Harvester

Increasingly researchers are turning their attention to incorporating the energy harvesting elements into the textile itself. In the UK the University of Bolton has developed a novel technology that integrates piezoelectric polymer substrate and photovoltaic coating system to create a film or fibre structure that is capable of harvesting energy from nature, including sun, rain, wind, wave and tide.

The raw materials used are inexpensive starting with the piezoelectric material which is extruded and poled. Since the organic photovoltaic material system is made in a normal atmospheric environment, the cost associated with the whole structure is magnitudes less expensive than ceramic based photovoltaic. The resultant material system is flexible and can be incorporated in textiles for a wide variety of applications. The University will also be presenting at this event.

Researchers at the University of Southampton's School of Electronics and Computer Science (ECS) are developing technology that may enable people to power electronics through their clothes and the carpets they walk on. Dr Steve Beeby and his team aim to generate energy through people's movement, eliminating the need to change batteries on devices. In a project funded by the Engineering and Physical Sciences Research Council (EPSRC), the Southampton team will use rapid printing processes and active printed inks to create an energy harvesting film in textiles. This film can also be printed on carpets, enabling individuals to generate energy as they walk around the home or office.

In the US, Georgia Tech researchers led by materials-science Professor Zhong Lin Wang have made a flexible fiber coated with zinc oxide nanowires that can convert mechanical energy into electricity. The researchers say the fibers should be able to harvest any kind of vibration or motion for electric current. Gold-plated zinc oxide nanowires, each about 3.5 micrometers tall, are grown on a flexible polymer fiber and these nanowires brush against untreated nanowires, which flex and generate current. Yarn spun from the fibers could lead to fabrics that convert body movements into electric current.

Cetemmsa is another company overseeing research projects in the use of sensors in sportswear and accessories. The company hopes to develop a range of electronics that can be integrated into clothing which could appeal to athletes, including heart rate monitors, cooling technology and low-power lighting solutions. The company is also working on integrated power sources for added electronic functionality such as organic photovoltaics, as part of the EU-funded Dephotex project. Dephotex is a European collaborative research project, co-funded by the European Commission and will be carrying out research on Photovoltaic Textiles based on novel Fibres for 3 years from November 2008 to October 2011. Cetemmsa will be presenting on this at the IDTechEx Energy Harvesting & Storage event.

Solarprint, G24i and Konarka will also present on Photovoltaics, including covering new form factors of PV applicable to textiles. The IDTechEx "Energy Harvesting & Storage" event brings the whole topic together, exploring new leading edge work in sectors such as textiles. Register early to obtain the discount at http://www.IDTechEx.com/EHEurope.

Munich seems to be the place for energy harvesting events with the IDTechEx event Energy Harvesting & Storage Europe and Wireless Sensor Networks & RTLS 2010 being held there on the 26th & 27th May.

This was a successful event with just under 250 delegates. The plenary talks successfully addressed the challenges faced when combining energy harvesting and wireless sensors in different practical environments. A good example of this was the presentation by Thomas Becker from EADS who posed the question: do we need energy harvesting and wireless sensor networks in aeronautical applications? Thomas provided clear motivation for such systems, namely decreased maintenance effort and increased lifetime of aircraft, but also highlighted the challenging requirements particular to the application. For the wireless aspect, the network would require certification and need to be totally reliable. The environment was very challenging with temperatures ranging from -55 to +85 oC, plus humidity variations, icing, exposure to hydraulic fluids, structural aging effects and vibrations. The system would be expected to work for a lifetime of 30 years. EADS has evaluated various types of energy harvesting. Vibration energy harvesting in rotor craft has been demonstrated obtaining mW of power from 17 Hz vibrations. On fixed wing aircraft, thermoelectric harvesting appears more attractive but issues remain regarding their successful integration and no solution is expected within the next 5 years.

Haydn Thompson from Rolls Royce continued the theme with a talk on self powered wireless sensors. The environment within an aero-engine is clearly even more challenging than the standard aeronautical specification with temperature variations ranging from -55 to 1300 oC! The potential for energy harvesting is clear - batteries are okay for test be applications but are totally unsuitable for production systems where there is plenty of vibration and thermal energy available to be harvested. Both vibration and thermal energy harvesting had been demonstrated with 100 mW and 200 mW being demonstrated form each approach respectively. The size of the energy harvesters remains an issue and much work remains to be done to develop harvesters for this application.

Other interesting talks on energy harvesting included a presentation from Pavegen Systems Ltd who have developed an energy harvester than looks like a conventional paving slab but that is able to develop between 2 and 3.75 W from an average footfall. They are seeking to licence the technology to act as a localised power supply for low level lighting and operation of systems at bus stops for example.

After the plenaries there were two parallel streams on energy harvesting and wireless sensor networks and an exhibition from various organisations and companies involved in energy harvesting. These included presentations form Microstrain, Cranfield University, University of Southampton, US Space and Naval Warfare Systems Command (SPAWAR), EnOcean, IMEC Holst Centre, Arveni, Humdinger and Meggit Ferroperm. These talks focused on the more technical issues relating to energy harvesting including aspects of piezoelectric vibration energy harvesters, MEMS implementations, increasing vibration bandwidth and thermoelectric energy harvesting. The talk from Cranfield on modelling of piezoelectric energy harvesting was, for me at least, very interesting. They have successfully modelled piezoelectric harvesters using ANSYS and we at Southampton have benefited from this research which has improved the accuracy of our simulations. The talk from Humdinger was on the novel topic of energy harvesting from airflows by exploiting aeroelastic flutter. They have developed a small harvester able to harvest about 1% of the wind energy obtaining 200 micro Watts at 3.5 m/s airflows. The research into MEMS energy harvesters mentioned at this conference was technically interesting but, as is often the case, felt like investigating solutions without a problem. The laws of physics do not favour microscale vibration energy harvesting and applications for low power, high frequency, albeit small, harvesters are not yet obvious. As power requirements fall and MEMS technology and materials evolve I'm hopeful this won't remain the case.

The next two energy harvesting events organised by IDTechEx will be held on the 13-14 October in Hong Kong and 16-17 November in Boston. A discounted rate will be available to Network members and further details will be available on the website.

I attended the above-named conference in Munich last week (24th and 25th March) and I have outlined below a few top-level things I took away.

On the conference itself, this was a relatively small event with around 60 delegates and very much focused on the practical end of energy harvesting. This meant a predominantly industrial audience and a focus on the here and now of application. The small size also ensured a good networking opportunity. I found it very useful for gauging the state of development since a study we published in mid 2008.

The development of energy harvesting technology in Europe still appears to be being driven by small companies - the usual suspects e.g. EnOcean, Perpetuum, Micropelt etc. There does however appear to have been a growing interest and involvement by some larger players e.g. Texas Instruments.

It is still only building automation and condition monitoring that have established themselves as viable applications so far. EnOcean have built an impressive market in the former and Perpetuum are establishing sales in the latter. The rest of the industry is still at the stage of R&D or exploring applications and trying to find that combination of volume and value to enable a viable business model.

The aerospace industry has a very real need for energy harvesting but there are some significant technical challenges of operating in such a harsh environment including extremes of temperature, weight, size and concerns about reliability to be overcome as well as finding the right type of energy harvesting for the specific applications. Nevertheless, this sector is relatively price insensitive so does offer hope for a reasonably significant and high value market if the technical issues can be overcome. Work so far has established that there is no one best energy harvesting technology and certainly no multi purpose solution possible.

The impending legislation on tire pressure monitoring in the EU could offer a huge market but indications so far are that the motor industry and legislators do not put a particularly high value on safety, seeking energy harvesting solutions costing cents rather than Euros. This is going to severely restrict the possible solutions.

Efforts to establish standards have certainly moved along in the last year and a half since I paid it much attention. At the level of being able to compare devices and ensure standard interconnection for power sources to wireless sensing devices there appears to have been progress through ISA100. This is about to become more formalised but is certainly heading in the right direction and addresses a significant concern we have heard time and again from end users of the technology. At the level of communications protocols there are two current approaches. The EnOcean Alliance has set a very low power wireless standard designed specifically for the needs of the building automation market. The ZigBee standard has now, with energy harvesting in mind, also been working on a lower power standard. This appears to be much more of a time consuming compromise effort and as such is not as low power as the EnOcean alliance standard but does promise a wider compatibility with other wireless devices in the overall ZigBee family of standards. There were arguments for both approaches and some concern that the EnOcean Alliance standard might not be as open as the ZigBee one. Nevertheless it is hard to argue with well the rapid progess and over 100,000 buildings having EnOcean technology installed and a full supply chain already in existence. I expect it will continue to be a case of horses for courses with each finding its champions for some time to come.

There have certainly been a number of advances in power management over the last 18 months with a number of speakers touching on this topic. It appears to be one of the main factors in driving up the amounts of useful energy made available from energy harvesting devices.

A concern expressed at the conference was that with improvements in battery lifetime outpacing improvements in energy harvesting and its cost reduction the situation where the battery lasts longer than the product it is powering is not far off. In this scenario the case for energy harvesting needs to be more clearly made. This means that consumer goods may not necessarily be the ideal market. We need to be finding those application where the lifetime of the battery is still not enough e.g aerospace and building management where lifetimes of 25 years + are wanted or those applications where environmental conditions are such that batteries can just not be relied upon.

There was an impressive demonstration of the Infinite Power Solutions 'Thinergy' a thin film flexible micro-energy cell rechargeable battery. The size of a large postage stamp and apparently able to handle 20 years plus of use with more than 45,000 charge and full discharge cycles. As the energy storage part of an energy harvesting solution this looks to have enormous potential and some very interesting applications have been demonstrated over the last year or so. It will be interesting to see whether in time this becomes the complementary component that allows energy harvesting to overcome its limitations of intermittency. Definitely one to watch.

Anyway, there was plenty more at this conference so if you need pointing in the direction of particular expertise please get in touch.

If you are interested in energy harvesting generally and particularly in the development of new research challenges in this field I encourage you to join the Energy Harvesting Network at www.eh-network.org.

Next we look forward to 'Energy Harvesting & Storage Europe and Wireless Sensor Networks & RTLS 2010' on 26th & 27th May, again in Munich. We have negotiated a 60 percent discount on this for members of the Sensors & Instrumentation KTN and the Energy Harvesting Network so if you are interested please do get in touch.