In a smart city, it should be possible to supply the countless devices and sensors with electricity using energy from the surroundings. This is why mini power stations on roofs, in pavements or on machines harvest power from movements, differences in temperature and the wind, to name just a few examples.
The ideal device for a Smart City is small, can transfer information reliably and works independently with minimal maintenance requirements over its entire service life. But where should these devices get the energy from to perform their tasks? It is not possible to connect to power cables everywhere and, for the majority of the IoT devices required, this would also be too complex. Batteries only provide power for a certain period of time – what’s more changing the batteries for thousands of devices would be far too expensive and would be an absolute nightmare for maintenance technicians.
Energy harvesting provides the solution: advances in semiconductor technology have made it possible to have integrated circuits with extremely low power consumption which can harvest energy from the surroundings. This could be natural or even artificial light, heat, wind or vibrations. Although the energy yield is low, it is enough to power small, economical IoT devices, such as sensors.
Vibrations and steps create power
The New York-based company MicroGen, for example, has developed micro generators which create power from vibrations. They use the piezoelectric effect and are manufactured with the help of MEMS technology: the vibrations cause the elastic deformation of the piezo material. This creates electrical voltage. The resulting energy is stored in thin-film batteries or small ultra capacitors. The London-based company Pavegen also uses the same effect; however, here it is pedestrians’ steps which deform the piezo elements in the pavement surface and therefore generate power. The kinetic energy from one step is converted into five watts of electrical energy. The British start-up has already used its tiles in several projects worldwide: in Rio de Janeiro, for example, a football pitch in the middle of a favela has been equipped with the floor tiles. Since 2014, these, together with solar cells, have ensured that the pitch is brightly lit for up to ten hours. “Whenever someone takes a step on our tiles, it’s enough energy to light a bulb for a few seconds. This movement can change the future of energy generation within our Smart Cities – as well as energy usage within developing areas of the world,” asserts Laurence Kemball-Cook, the 30-year-old founder and CEO of Pavegen.
The American company Perpetua, on the other hand, relies on temperature differences to generate electrical energy. The thermoelectric generators, called Power Pucks, can be installed on pipes, pumps, fans or motors, for example. The difference in temperature between their surface and the surroundings is sufficient for the semiconductor devices in the pucks to generate usable electrical energy – and to keep doing so over the decade. The generators work in the same voltage range as conventional batteries.
Mini generators in water meters
Dr Peter Spies and his team from the Fraunhofer Institute for Integrated Circuits (IIS) are pursuing another approach: they are using the rotation of the gear wheels in oval-gear flow meters, which are used to measure volumetric flows in pipes, to generate power. “With a suitable arrangement of magnets on the gear wheels and fixed coils on the oval-gear flow meter, it is possible to obtain electrical energy from the rotation, which can be used to supply a wireless module and transfer the measurement data wirelessly,” explains Spies, highlighting the advantages of the technology. As a result, energy can be obtained from each flow measurement, for example, in water pipes.
Power is in the air
What’s more, American and Chinese researchers from the Georgia Institute of Technology and the Beijing Institute of Nanoenergy and Nanosystems respectively are using another effect: their hybrid generator is based on the triboelectric effect. This refers to charging two materials electrically through contact and subsequent separation – it’s something you see in everyday life when you take off a jumper and it crackles. The team developed a generator which is made up of a silicon-based solar cell and a triboelectric nanogenerator. It consists of thin layers of plastic and Teflon, which are separated from one another by the air. When the wind blows, the plastic film vibrates between the Teflon layers, therefore coming into contact with the Teflon before being separated from it again. This creates “triboelectricity” – around 26 milliwatts can be produced in this way (one milliwatt can light up 100 small LEDs). “This research presents a feasible approach to maximise solar and wind energies scavenging from the city environments with the aim to realise some self-powered functions in Smart Cities,” the researchers write in an abstract, which they published in the journal ACS Nano.