Energy Harvesting (EH) works on the small scale but will make a big impression. According to analysts the technology to gather ambient energy will become increasingly important - the market was worth $605 million in 2010 but is predicted to reach $4.4 billion by the end of this decade.

Why is this important? Because EH scavenges energy that would otherwise be lost as heat, light, sound or vibration. It does this at the microscale and uses the captured energy to improve efficiency and drive new technologies. While this approach won’t solve the looming energy crisis, it will make everyday and industrial processes more efficient and open up new applications like wireless sensor networks.

Developing wireless sensor networks powered by harvested energy is the remit of EnOcean. This is an alliance of more than 70 companies to provide interoperable wireless sensors and controls that never need a battery. The need for such technology is obvious, as currently 38 per cent of energy is consumed in buildings.

Through its work EnOcean has installed 4,200 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 per cent of lighting energy costs by automatically controlling the lighting in the building, 20 miles in cables, 42,000 batteries (over 25 years) and most of the cost of retrofitting.

Challenges faced

Existing commercial EH products prove that EH technology is viable and cost-effective – the lack of wiring and battery-changes providing a significant cost saving. Yet to ensure EH technology fulfils the market potential, we have to address the lack of traceable measurement and internationally recognised standards. This is needed to ensure that EH systems can be fully characterised, and their efficiency measured, defined and compared. Only then can EH systems be further improved. Without the confidence standards inspire, businesses and investors are unequipped to make the right strategic decisions on what EH products to invest in. It is the role of the Metrology for Energy Harvesting project to address this. It consists of seven of Europe’s National Measurement Institutes (NMIs), and aims to help develop ‘traceable’ (traced back to national standards) measurement methods.

Developing these methods is the next vital step to widespread acceptance and use of EH. The number of potential applications is expanding at a furious rate because the technology has reached a tipping point. The convergence between more efficient, affordable and reliable EH materials with a lowering of the power requirements for many electronic devices makes all sort of applications possible, from networks monitoring the oceans to biosensors.

Current research

At present the project is focused on specific projects within thermoelectric and vibration energy harvesting, but the research is expected to grow with time.

Thermoelectric materials convert wasted heat into electrical energy. Thermoelectricity is regarded as one of the most promising technologies for increasing energy efficiency in industrial processes and automotive applications, which produce a large amount of waste heat. Piezoelectric materials convert electrical energy into a strain (or vice-versa). The best known use of piezoelectricity is for medical ultrasound. Current research in these fields includes:

– The Physikalisch-Technische Bundesanstalt (PTB) in Germany has installed a measuring system to determine the Seebeck coefficient of thermoelectric (TE) materials. PTB is currently testing the system and making improvements such as a calibrated thermometer to ensure that the reference temperature, which is close to the sample temperature, is traceable to National Standards. Once these improvements are complete, the group will start to investigate and characterise different TE materials as candidates for reference materials for Seebeck coefficients in the temperature range 300K-900K.

– The UK’s National Physical Laboratory (NPL) has designed and built a system to reliably characterise the efficiency of commercial thermoelectric modules (TEGs) between room temperature and 150°C.There are two challenges to determining the efficiency of a TEG: the measurement of the electrical output power and of the heat flow. This system uses an absolute method for the measurement of the heat flow, by measuring the power dissipation of an electrical heater. A shielded heater avoids any parasitic heat flow. Consistent results have shown NPL’s system to be reliable. TEGs measuring up to 40 x 40 mm can be characterised – this is the most common industry standard size – but smaller and slightly larger TEGs could also be analysed.

– France’s Laboratoire National d’Essais has designed first electrostatic energy harvesters based on comb-drive MEMS. The device architecture has been optimised to maximise the electrical power converted from mechanical vibrations in the frequency range 1 kHz to 4 kHz. VHDL (VHSIC hardware description language) simulations on these systems indicate that they are capable of harvesting electrical powers ranging from 6 µW to 60 µW. The harvesters are fabricated through an industrial SOI (silicon on insulator) wafer process and will be distributed to the project partners for full electromechanical characterisations.

– NPL has also undertaken work recently into piezoelectric energy harvesters. These are typically cantilevers with an inertial mass at the end. The base of the cantilever is exited by ambient vibrations and the inertial mass exerts a force on the cantilever which generates a stress in the piezoelectric (yellow). NPL has developed a model that predicts the output of the beam based on the force at the cantilever tip. In order to investigate the effect of the coverage of the beam with piezoelectric elements NPL have made a cantilever with 30 elements along its length, and measured the power output.

Industrial engagement

Achieving this will require expertise and input from all aspects of science and industry including from leading research institutions on energy capture and storage, material science, and systems engineering as well as metrology. Yet just as importantly will be engagement with European businesses. The technology characterised through the Metrology for Energy Harvesting project has to reflect what industry wants and this requires input from companies, to find out the issues they currently have, and how the project can address them.

Europe is a world leader in energy harvesting R&D, and this project will both keep it there and service the needs of industry across the continent.

—-By Prof Markys Cain, Knowledge Leader at NPL and an IOP member