Energy sources such as sunlight, wave, wind and water, and ‘waste’ energy – radio waves, heat and movement – are virtually unlimited and essentially free. If they can be captured, that is. Energy harvesting (EH) technologies promise to do exactly that, explains Harry Xervos, an energy analyst with market research company IDTechEx; they will be crucial to decentralise power generation systems and develop distributed energy networks that will optimise energy efficiency worldwide, he says.
‘Perpetual availability of power is the vision of the future for EH,’ Xervos says. ‘By making use of ambient sources of energy such as light, heat, movement and vibration it will be possible to recharge devices while they are in use and “plugging in” will be consigned to the past.’
Windmills and waterwheels have harnessed the ‘free’ ambient energy in our environment for centuries. Now, this energy is increasingly being adapted to power autonomous devices such as the Kinetic watch that uses wrist movement to generate power for the quartz mechanism, and in the wind-up clockwork radio innovation that has transformed communication across Africa.
Discreet photovoltaic panels or miniature wind turbines are commonly used to charge traffic signs or parking machines to supplement or replace mains power. And the next step could be wearable textile solar panels to charge electronic devices such as mobile phones; oceanographic sensors powered by wave power; or even piezoelectric floor tiles that convert footsteps to electrical energy.
The challenge, however, is that EH produces very small quantities of useable electrical power, while consumer electronics are becoming increasingly power hungry. And while EH devices are great at capturing wind, sunlight or water power and converting it to usable power, they cannot store it.
‘Given the low power outputs of energy harvesters relative to some of the more prominent electronic devices, developments on ultra-low power electronics that reduce power usage will be paramount in order to meet EH technologies halfway,’ Xervos says, ‘thus making it easier for power outputs to reach levels that satisfy the needs of consumer electronic devices via optimising energy efficiency.’
To provide power over longer periods, EH technologies must be combined with batteries and capacitors – a concept exploited, for example, in low energy (LED) street lighting and signs using PV panels and battery storage to generate illumination through the night. In other applications, however, EH is set to replace batteries altogether, making devices lighter, less expensive and more convenient. Energy Independent Electric vehicles (EIVs) will be mainstream by 2026, according to IDTechEx, with the market projected to be then worth $500bn. EIVs don’t require a cumbersome and expensive battery and instead harness ambient energy via integrated solar panels. However, these so-named ‘lizard’ vehicles have the big disadvantage they cannot be used at night and, like cold-blooded animals, need to bask in the heat of the sun before becoming active.
In Taiwan, since 2013, the NanoWin Technology Company has manufactured light and flexible CIGS (copper indium gallium selenide) thin film solar modules, which are capable of driving a specially designed tourist minibus or ‘microbus’ at a speed of 5-12km h-1. Although the concept vehicle is a long way from commercialisation, it demonstrates the potential of five 96W solar roof panels in powering a light vehicle. Other possible applications for autonomous sunlight active vehicles include in agriculture, window cleaning, industrial inspections, and for data collection in hostile environments such as deserts – or even space.
Mars and beyond
EH could also play a major role in a future manned mission to Mars, regarded as the next big milestone for space exploration. Space missions generally need to reuse and recycle all of their resources, including energy, explains Khellil Sefiane, professor of thermophysical engineering at the University of Edinburgh, UK. Sefiane’s group is interested in harnessing solid carbon dioxide (dry ice) or other frozen solids such as methane, which can be found in abundance on the surface of planetary bodies. Recent reports suggest dry ice can be found on Mars.
‘We are concerned with using waste heat and dry ice to produce micro-generators by using the Leidenfrost effect,’ he explains. ‘The Leidenfrost effect can be seen when a cold drop of water is dropped onto a hot surface and the droplet moves and skitters across the surface. The difference in temperature means that there is a layer of vapour between the drop and the hot surface, promoting low friction movement.’
A sublimation heat engine converts temperature differences into mechanical work. Blocks of dry ice levitate on top of a hot surface of what look like spinning turbine blades, due to the Leidenfrost effect. The rate of rotation is controlled by the turbine geometry, temperature difference and solid material properties – dry ice, frozen methane or other cold media are equally suitable. The rotation of the dry-ice disc rotor is converted into electric power by coupling to a magnetic coil system to produce a microgenerator.
Sefiane’s group at Edinburgh recently constructed a simple magnetic coil generator based on a dry-ice Leidenfrost rotor to prove the concept.1 Both NASA and the European Space Agency have expressed an interest in developing this technology.
EH technologies also have an important role to play in transportation. Petrol engines convert around 20% of their fuel energy into motion, while the remaining 80% is lost mostly as waste heat. Electric vehicles are decidedly more efficient, converting around 60% of their battery energy to kinetic energy. In both cases there are opportunities to harvest waste energy, for example, by using regenerative brakes. Electric vehicles can simply use their motor in reverse as a generator to both brake and recharge the battery – thus improving their range. The system has to run in conjunction with conventional friction brakes to provide a complete stop but can achieve efficiency improvements of around 30%.2
For the petrol engine, the energy saving technology hails from Formula 1 motorsport. Since 2009 F1 cars have been using Energy Recovery Systems (ERS) that convert waste energy into useable horsepower. The ERS unit consists of a Motor Generator Unit – Kinetic (MGU-K), and Motor Generator Unit – Heat (MGU-H), plus an ‘energy store’ (ES) or battery and control electronics. The MGU-K is essentially a ‘flywheel’, similar to a friction powered toy car, which captures the kinetic energy of braking to generate electricity and can also act as a motor when the car is accelerating.
The MGU-H is a generator connected to the engine turbocharger and converts heat energy from exhaust gases into electrical energy, which can be stored or used to drive the car in conjunction with the MGU-K. An ERS turns an F1 car into a hybrid vehicle that can capture both braking and heat energy and repurpose it to electrical energy to drive the car with two engines and provide a significant boost in performance.
Waste energy takes many forms and currently there are a number of innovative projects addressing its recovery or harvesting. Freevolt technology, for example, launched in September 2015 by former UK science minister Lord Drayson, transforms ambient (waste) radio frequency (RF) waves into usable electricity to charge low power electronic devices. The device uses an idea first developed by NASA in the 1960s and consists of a multi-band antenna and rectifier, which capture RF signals, converting them to small amounts of electrical power. It promises to allow waste energy from 3G, 4G and WiFi networks to be harvested to charge low power Internet of things (IoT) devices: mobile phones, smart watches, RFID tags and electronic sensors that ‘talk’ to each other by BlueTooth or Near Field communications.
The CleanSpace Tag, for example, is one of the first users of Freevolt technology, Lord Drayson explained, and uses a carbon monoxide sensor to monitor personal exposure to air pollution and report data to your mobile phone, smart watch, computer or national pollution monitoring networks.. ‘The CleanSpace Tag has very low energy requirements of just 20 to 30 nanowatts, which is ideal for Freevolt to power sensors and beacons’, Drayson commented.
Africa and remote areas
EH is most important to those with no connection to a central energy source, or no electricity supply at all. In Africa, the problem is particularly acute. The World Bank reports that only 24% of the population of sub-Saharan Africa – excluding South Africa – has access to electricity and that the entire installed generation capacity of sub-Saharan Africa is only 28 Gigawatts, roughly equivalent to that of Argentina. Schoolchildren often cannot read after dusk, businesses cannot grow and clinics cannot refrigerate medicine or vaccines.
In November 2015, INTASAVE Energy, a global environmental non-profit organisation, launched a crowdfunding project to provide sustainable, scalable solar nanogrids (SONGS) to communities in Kenya – where 75% of people have no access to electricity.
‘Small SONGS are an ideal solution to giving off-grid communities access to energy,’ says Murray Simpson, ceo of INTASAVE. ‘Some parts of rural Africa will have to wait years to be on the national electricity grid. However, using scalable nanogrid technology and ionQube batteries, provides an immediate solution helping to lift many out of poverty.’
IonQube batteries are recycled from old laptop batteries and can be used in a modular fashion to store more energy as a community grows. SONGS (1-10kW) are small scale arrays of solar panels that use conventional crystalline silicon solar cells, which are the most widely used and cheapest technology available. Each INTASAVE installation is designed to serve a small village to provide access to electricity that will change lives.
‘Solar nanogrids were installed in 70 villages across Kenya in January and February 2016 with great success. Without the funds we raised through the UK governments Department for International Development (DID), research councils, our own investment and the ongoing crowdfunding campaign it would not have been possible.’
Recycling waste energy, meanwhile, is also of growing interest in construction of smart buildings. The PV roof panels on the recently renovated King’s Cross train station in London, for example, produce 175,000kWh of electricity/year, saving over 100t of CO₂ emissions. But busy train stations also have other potential. Laurence Kemball-Cook, a graduate of Loughborough University, has developed a system to convert footsteps into usable power. Now ceo of London based PaveGen, a company he set up to exploit the technology, Kemball-Cook’s PaveGen tiles have already been installed in 200 buildings across the globe. The tiles are manufactured from recycled polymer and truck tires and each footstep can generate around 7W, enough to power an LED lamp for 30s. ‘The technology is a secret, but it involves the piezoelectric effect, which is an ability of certain materials to generate an electric charge in response to compression, and induction by copper coils and magnets,’ says Kemball-Cook.
A 2014 pilot study at the central hub building of Macquarie University in Sydney, Australia, supported the idea of piezoelectric floor tiles.3 The results suggested that tiles covering 3.1% of the floor area with most foot traffic could generate an estimated 1.1MWh/year by piezoelectric energy harvesting, which is about 0.5% of the hub building’s annual energy needs. ‘The power generated by the tiles is ideal to power low energy lighting, information displays, or indeed any application where there is high footfall such as railway stations,’ says Kemball-Cook.
1 G.G. Wells et al, Nature Communications, 2015; doi:10.1038/ncomms7390
2 L. Jin, P. Chen and Y. Liu, Intern. J. Control and Automation, 2014, 7(12), 219.
3 X. Li and V. Strezov, Energy Conversion and Management, 2014, 85, 435.
Huw Kidwell is a freelance science writer based in Hitchin, North Hertfordshire, UK