Energy harvesting getting bigger

Energy harvesting is increasingly used to power large systems too

By Morand Fachot

Interest in energy harvesting, the process associated with the collection of low-grade energy from sources such as ambient or waste heat, human power, solar, thermal and kinetic energy, and their conversion into electrical energy, is gaining momentum. Viewed initially mainly as a convenient way of powering sensors, small wireless electronic devices and low-power systems, it is also opening up opportunities for use in larger applications. The more so when it is used in connection with certain types of energy storage systems.

Pavegen tiles harvest energy from commuters' footsteps (Photo: Pavegen) Pavegen tiles harvest energy from commuters' footsteps (Photo: Pavegen)

Cutting the cord – less batteries please!

Tapping energy from low-grade sources is seen as an attractive solution for powering the growing number of electronic products and devices that operate independently from power networks or without batteries.

Energy harvesting, also known as energy scavenging, is already widely used for powering sensors and actuators, such as those found in certain types of MEMS (Micro-Electro-Mechanical Systems), which are increasingly deployed in sectors such as automotive and medical. International Standards for MEMS are prepared by IEC TC 47: Semiconductor devices, and they are tested by IECQ (IEC Quality Assessment System for Electronic Components) testing and certification.

Energy harvesting is useful for devices that do not require a lot of power and when changing batteries may present challenges, such as when they are installed in remote locations, or risks. This is the case in the medical field where energy-harvesting devices that can convert the movement of body parts such as the heart, lungs and diaphragm into energy could be used to power implantable devices – for instance, pacemakers. Research has been ongoing into these devices as well as into the piezoelectric materials that could be used in them. A self-powered cardiac pacemaker using a piezoelectric nanogenerator was demonstrated on a rat in June 2014.

Techniques to harvest energy for other medical devices are also being developed. One example is using jaw movements to power hearing aids, which avoids having to replace internal batteries. New energy harvesting processes, many of them highly ingenious, are being introduced all the time. Some have an entertainment value but may still lead to the development of useful applications. Many others pave the way for the development of more energy-efficient systems. Many IEC TCs (Technical Committees) develop standards applicable to energy harvesting applications.

The power of play

Some games or forms of entertainment that entail physical activity can include an element of energy harvesting potential, which can be used for various small applications and even pave the way to large-scale ones.

A small company, Uncharted Play, launched the production of two play devices that can convert kinetic energy to power a small LED light or to charge a mobile phone: Soccket, a football, and Pulse, a skipping rope. Both devices are designed for people who live in places without reliable access to electricity, or who have to rely on generators.

Nightclubs use a considerable quantity of energy for lighting, sound systems and more. Dutch company Energy Floors started producing the first piezoelectric energy-generating dance floor in 2008. The floor flexes slightly when stepped on, creating movement which is then transformed into electrical power by a small internal generator. The electricity produced can be used to power screens, sound systems, lights and more.

Energy-generating floors are now also commonly used by exhibitors and museums, allowing them to create interactive environments and experiences for the public and to convey their commercial or educational message.

International Standards for piezoelectric materials and devices are developed by IEC TC 49: Piezoelectric, dielectric and electrostatic devices and associated materials for frequency control, selection and detection.

Harnessing people's power

Moving on from entertainment and play, energy harvesting systems are also being implemented in larger schemes, in particular in places that see great numbers of people moving and walking through every day.

Energy harvesting pavements have been installed in some heavy pedestrian traffic locations, such as train stations or office buildings, for powering energy-efficient lights or other systems.

Other systems that harness kinetic energy are being installed or tested. They include a revolving door in one Dutch café. The door is equipped with a small generator that recovers the "muscle" energy of customers entering or leaving the place, converts it into electricity and stores it in supercapacitors. It is then used to power the café's LED lights and provides up to 4 600 kWh of energy savings in a year. IEC TC 40: Capacitors and resistors for electronic equipment, develop International Standards for supercapacitors.

A similar principle has led to the development of energy harvesting turnstiles.

A system to harness another form of human energy, body heat, has been installed in Stockholm's Central Station to collect the excess body heat of some 250 000 passengers who pass through the station every day. This heat is collected and used in heat exchangers to produce hot water, which in turn is pumped into the heating system of a nearby building, cutting its energy needs by some 25%.

Going up a gear for extra power

Energy harvesting is often perceived as being applicable mainly to small applications or to larger ones that rely on the collection and conversion of small amounts of mechanical or thermal energy from large numbers of players.

However, energy harvesting is increasingly finding new applications in demanding energy-intensive sectors such as transport, in particular when associated with innovative or improved storage systems.

A striking example of this was demonstrated by this year's gruelling 24-hour Le Mans car race in France. Three cars from different manufacturers, which included the winner, the runner-up, and a third car that was in second place before having to abandon the race shortly before its end, were all four-wheel drive hybrid cars that used energy-harvesting systems and different forms of energy storage.

The winning car had a regenerative braking system capability that recovered the moving car's kinetic energy under braking and stored it in a flywheel energy storage system on the front axle. The recovered energy was then used in acceleration phases to provide an additional boost.

The car that came second was fitted with a motor-generator boost system on the front axle. This recovered kinetic energy under deceleration and transferred it for storage in a bank of ultracapacitors. During acceleration, the stored energy delivered a power boost at each axle as required.

The third of the cars, the one forced to retire, also stored energy recovered during the deceleration phases. This one used a lithium-ion battery pack which provided additional boost during acceleration. IEC TC 21: Secondary cells and batteries, prepares International Standards for lithium-ion batteries.

The fact that regenerative charge braking can be used to convert kinetic energy under such punishing conditions, storing it in different systems – flywheel, ultracapacitors and li-ion batteries – shows that energy harvesting has a future way beyond small-scale applications, in more demanding energy-intensive operations.

Car racing is often a means of introducing technologies that eventually find their way into private vehicles, so these advances will not remain confined to the motor sports world. A leading car manufacturer has recently tested a flywheel system on the rear axle of a front-wheel drive passenger car to determine the potential for fuel savings. Initial results show a performance boost of 80 hp with improved fuel economy of up to 25%.

Flywheels are a form of mechanical storage system that contains components such as coils, motor and generators. IEC TC 2: Rotating machines, prepares International Standards for motors and generators. IEC TC 55: Winding wires, develops International Standards for wires used in coils.

The urban public transport sector in particular offers a great potential for energy harvesting. Regenerative charge braking and energy harvesting shock absorbers are being fitted to buses to charge batteries and supercapacitors for providing extra power. Data published by research company IDTechEx indicates that over 20 000 supercapacitor-based hybrid buses are in use worldwide. This is a huge global market that will make a major contribution to a more energy-efficient transport sector.

In most hybrid buses, even in existing hybrid Formula 1 cars and hybrid concept cars, supercapacitors with less energy storage can replace Li-ion batteries, improving performance, reliability and life, according to Dr Peter Harrop, chairman of research firm IDTechEx.

Pavegen tiles harvest energy from commuters' footsteps (Photo: Pavegen) Pavegen tiles harvest energy from commuters' footsteps (Photo: Pavegen)
Revolving door harvest energy from customers in Natuurcafé La porte in the Netherlands Revolving door harvest energy from customers in Natuurcafé La Porte in the Netherlands
Flybrid high-speed flywheel installed on motor vehicles (Photo: Flybrid Automotive Ltd) Flybrid high-speed flywheel installed on motor vehicles (Photo: Flybrid Automotive Ltd)