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Generating Power by Unusual Means By Dr David Maddison, VK3DSM The Landesbergen biomass power plant in Germany; it generates power by burning scrap wood. Image source: Statkraft – www. flickr.com/photos/statkraft/49866093642 (CC-BY-NC-ND 2.0) Energy is all around us in one form or another, but often in small amounts. Energy harvesting, otherwise known as power harvesting or energy scavenging, is the process of obtaining small amounts of energy from the environment to supply low-power devices. W ith a few exceptions, the amounts of power available from energy harvesting are small, and the expense required to obtain that power makes these methods not competitive with grid power, where it is available. However, these tiny amounts of energy can be very useful for powering small devices away from the grid; modern efficient electronics can often run on minimal amounts of power. This article will cover methods of power generation other than the ones most people are familiar with, like coal and gas generators, nuclear power plants, hydroelectric plants, solar, wind and wave power or burning biomass or waste. With these alternative power-­ generating methods, the power available is often on the order of nanowatts to milliwatts. In some cases, it may 16 Silicon Chip be possible to generate several watts (or more). The main applications for energy-­ harvesting devices include powering IoT devices such as remote sensors, ‘wearable electronics’, powering biomedical devices (like pacemakers) or charging portable devices like mobile phones. Energy harvesting principles The basic principles and technologies that energy harvesting devices utilise include the following. We will describe their uses when we look at particular implementations. • Using chemistry, such as in an electrochemical cell. • Using biochemistry, including the generation of electricity using bacteria or plants. • Using biomechanical principles, Australia's electronics magazine such as utilising bodily movement. • Using an electret, a dielectric material that maintains electric polarisation after it has been subject to a strong electric field. It is the electrostatic equivalent of a permanent magnet. • Using electric field gradients, such as causing a fluorescent tube to glow near a power line. • Using electromagnetic induction to generate electricity by Faraday’s Law, the “production of an electromotive force (EMF) across an electrical conductor in a changing magnetic field”. • Capturing electromagnetic radiation from radio waves via an antenna or from light, such as in a solar cell via the photoelectric effect. • Using electrostatic power generation to produce high voltages at very siliconchip.com.au low currents. This frequently involves materials rubbing against each other (via the triboelectric effect). • Using metamaterials, artificial materials with repeating structures that can interact with and manipulate electromagnetic waves in various ways. • Converting motion to electricity using electromagnetic, electrostatic or piezoelectric effects. • Using changes in air pressure to expand or contract bellows. • Using a temperature gradient, such as with a thermoelectric device. • Using the movement of air, like in a wind turbine. • Using the movement of water, ie, hydroelectricity. Below we will cover what energy-­ harvesting devices and techniques that we have found: Fig.1: the first self-winding mechanical watch that harvested energy from the motion of the wearer’s arm. Source: Fratello Watches – siliconchip.au/link/abxy Watches Since watches are small, low-­ powered devices, there has been much interest in energy harvesting to power them. Self-winding automatic mechanical watches were common before the advent of electronic quartz watches. They had a pendulum activated by swinging one’s arm that wound the mainspring. The first credible report of a self-winding pocket watch dates to 1777. In 1922, the first self-winding wristwatch was invented by John Harwood, and he was awarded Swiss patent 106583 in 1924. The watch was released to the market in 1928 – see Fig.1. The first solar-powered clock was demonstrated by Patek Philippe at the Basel Fair in 1952! Four hours of light per day was enough to keep the clock running indefinitely. The solar cell drove a motor that wound the mainspring. Patek Philippe went on to make a range of solar clocks; see siliconchip.au/link/abxm The solar-powered watch was first patented by Timex in 1969, but the first solar watch, the Synchronar 2100, was invented by American Roger Riehl. He partnered with Palo Alto, California based electronics company Ness Time for the project. The watch (Fig.2) was shown at the RJA Fall trade fair in July 1973 and remained in production until 1983; you can see a TV ad for it at https:// youtu.be/mIwxNkGKXb4 siliconchip.com.au Fig.2: the Synchronar Sunwatch was the world’s first solar-powered watch, released in 1972. Source: https://solarmuseum.org/cells/ synchronar-2100/ In many modern solar watches, the dial is translucent and the solar cell(s) are hidden beneath it. Seiko pioneered the so-called automatic quartz watch concept that used a rotating pendulum inside the watch. Instead of winding a spring, it drove a highly-geared miniature generator at up to 100,000 RPM to charge a capacitor or rechargeable battery. Seiko unveiled the technology in 1986 and today sells them under the Kinetic brand. Seiko still maintains a web page for these watches (siliconchip.au/link/ abxn) but we have seen statements that they are being phased out (see siliconchip.au/link/abxo). About eight million have been sold to date. The generator mechanism of the Kinetic watch has been experimentally used to power a cardiac pacemaker in an animal (more on that later). The PowerWatch uses a Matrix thermoelectric device to power it, in addition to solar energy (www. powerwatch.com). A review of the Australia's electronics magazine Fig.3: the Atmos clock mechanism: 1. Expansion chamber 2. Brass cover 3. Balance spring (counterweight) 4. Small chain 5. Mainspring 6. Pulley 7. Return spring 8. Balance wheel 9. Elinvar wire 10. Escapement 11. Winding spring Original source: Watch Collecting Lifestyle – siliconchip.au/link/abxs PowerWatch Series 2 is at siliconchip. au/link/abxp Atmospheric & solar clocks The Atmos is a very expensive clock currently available from Jaeger-LeCoultre that obtains its energy from environmental temperature and pressure changes. Expansion and contraction of liquid and gaseous ethyl chloride in a bellows as the temperature or pressure rises and falls cause a spring to be wound to power the mechanism – see Fig.3. The Beverly Clock in New Zealand (https://w.wiki/AUgH) has been running since 1864 without winding. However, it did stop a few times, mainly when there was insufficient change in atmospheric pressure or temperature to keep the mechanism wound. The Long Now Clock (funded by Jeff Bezos; https://longnow.org/clock), being built in the USA, is designed to run for 10,000 years. It uses sunlight falling on a chamber of air to move September 2024  17 Fig.4: harvesting atmospheric electricity to run an electrostatic motor. This type is called a corona motor. Original source: Rimstar – siliconchip. au/link/abx7 Fig.5: conventional (a) and auxetic (b) piezoelectric bimorphs for energy harvesting. Original source: https://pubs. aip.org/aip/adv/ article/7/1/015104/­ 240312/ a cylinder, which provides enough winding force to keep the pendulum going. It is also used to synchronise the clock to solar noon. So, in a sense, it is solar powered, although it does not use a photovoltaic panel. Atmospheric electricity There is a substantial electric field gradient in the atmosphere, so an electrostatic motor can be made to turn by having one electrode high in the air with the other at a lower level (see Fig.4). The power is meagre; at most a current of a few microamps can be drawn. For more on this, see the panel in this article on Hermann Plauson (page 26), the video titled “How Powering with Atmospheric Electricity Works” at https://youtu.be/2rVdEhyMR6A and the web page at siliconchip.au/ link/abx7 Piezoelectric energy Piezoelectricity involves the production of electrical energy from mechanical strain. Examples of sources of strain include motion, sound and vibration. The power generated is typically minimal, milliwatts or less. Piezoelectric materials include ceramics like quartz crystals and, more recently, piezoelectric polymers like polyvinylidene fluoride (PVDF) – see Fig.6. An example of a piezoelectric energy harvester is shown in Fig.7. Some piezoelectric substances are also pyroelectric. These crystals are naturally electrically polarised and produce a voltage when heated or cooled. This could be used for energy harvesting over a day by taking advantage of the natural changes in ambient temperature. Auxetic materials are artificially-­ structured metamaterials that expand in width rather than contract when stretched. Conversely, when subject to compression, they reduce in width. It has been proposed that auxetic materials could increase the energy-­ harvesting efficiency of piezoelectric devices, as shown in Fig.5. In that figure, (a) shows a conventional piezoelectric bimorph, which can generate power mainly in the stretching direction, while (b) represents a bimorph of auxetic construction. This can generate power simultaneously in both the stretching and transverse directions, resulting in an expected power increase of 176%. That is because it has increased power output in the transverse direction, as it can generate more stress in that direction, and the power output is proportional to the applied stress. Clothing has been proposed that incorporates piezoelectric materials to generate power for powering or charging devices. Such fabric utilises nanofibres and is said to be stretchable and breathable. See siliconchip. au/link/abxe Thermoelectricity Thermoelectricity involves the production of an electric current due to a thermal gradient between two dissimilar electrical conductors. A typical example of a device that utilises this effect is a thermocouple, although it produces tiny amounts of power at very low voltages. Peltier devices (Fig.8) also utilise this effect but with many more thermoelectric junctions. When a current is applied, it can move heat towards or away from an object. Alternatively, when a temperature differential is applied, it can generate a voltage and current, and thus be used for energy harvesting. Fig.6 (left): polyvinylidene fluoride (PVDF), a piezoelectric material, with deposited electrodes from a commercial supplier. Source: www.he-shuai.com/pvdf-piezo-film Fig.7 (right): a commercial piezoelectric energy harvester, model S118-J1SS-1808YB (from https://piezo.com). It can produce up to 0.7mW. Source: Piezo S118-J1SS1808YB – siliconchip.au/link/abxv Australia's electronics magazine siliconchip.com.au Fig.8: a Peltier device. It uses a combination of p-type and n-type semiconductor materials to create thermoelectric junctions. They are connected electrically in series and thermally in parallel. Original source: https://w. wiki/AUjV Electricity can be generated from a campfire using thermoelectric principles. Fig.9 shows a Peltier device attached to a heatsink that can generate power from a fire. The CampStove 2 from BioLite can produce up to 3W to power or charge USB devices (see siliconchip.au/link/abxa). The MATRIX Prometheus Thermal Energy Harvesting Module produces power by exploiting small environmental temperature differences, using the thermoelectric effect. The most powerful Prometheus device, the PRMT02-34465, produces up to 14mA (www.matrixindustries. com/0234465). This technology is used to power the MATRIX Perceptive Health Monitor, their Proximity Sensor and the PowerWatch (www.powerwatch.com). Stirling engines A Stirling engine is a type of heat engine that can function with very small heat differences and thus can be used for energy harvesting from lowgrade heat sources – see Fig.10. The Stirling engine can be connected to a generator to produce electricity. Stirling engines have been proposed by NASA to produce power on a future mission to Mars (see page 24 Fig.9: a DIY thermoelectric generator using an off-theshelf Peltier device, heatsink and other components. Source: https://youtu.be/x9a2rB-xWkY of the July 2024 issue; siliconchip.au/ Article/14916). Energy from bacteria Some exotic bacteria exchange electrons with the environment (‘extracellular electron transfer’ [EET]), so theoretically, they could be used to produce electricity. Mechanisms from these exotic bacteria have been genetically engineered into common E. coli bacteria. Such an approach could be used to convert wastewater effluent streams into electricity. However, this is very early work and practical applications are a long way off. The work was published at siliconchip.au/link/abxd Also see the video titled “Scientist engineered bacteria to generate electricity from wastewater” at https:// youtu.be/beI_qlsmNQ8 Power from plants A common experiment for children is (or used to be) to use a lemon, potato or other fruit or vegetable to make a basic electrochemical cell (see Fig.11). Pieces of different metals, such as zinc and copper, are used as electrodes, while the juice of the fruit or vegetable acts as the electrolyte. One such cell might produce 0.9V at 1mA. Several lemons can be connected in series to power one LED. A fun experiment was once performed to see if a 1000-lemon battery could start a car. See the video titled “Can a battery made from 1000 lemons start a car?” at https://youtu.be/­ 4f2wsQkQ71o Light can be turned into electrical energy via the photosynthesis mechanism using bio-­photoelectrochemical cells (BPECs). This early work is described in the scientific publication at siliconchip.au/link/abxl Biomechanical energy from the human body Raziel Riemer and Amir Shapiro calculated the energy available from the Fig.11: a drawing of a three-lemoncell battery lighting one LED. Source: https://w. wiki/AUjy Fig.10: the operating cycle of a Stirling engine, which can run from relatively low temperature differentials and could be used as part of a generator. Original source: https://youtu.be/ hbfkbcdw_OM siliconchip.com.au Australia's electronics magazine September 2024  19 Fig.13: an image from the Author’s 1989 US Patent 4798206 for “Implanted medical system including a self-powered sensing system” showing an assembly of piezoelectric PVDF polymer as the sensing element (#14). Fig.12: a biomechanical energy-harvester that mounts on the knee. Original source: www.researchgate.net/publication/51078340 motion of an 80kg human body under various circumstances (siliconchip.au/ link/abx9) and found the following power available: • heel strike: 2-20W • ankle motion: 67W • knee motion: 36W (see Fig.12) • hip motion: 38W • movement of centre of mass: 20W • elbow motion: 2W • shoulder motion: 2W They point out that the typical human body consumes the equivalent of 800 AA cells (which would weigh 20kg) by burning just 200g of fat. Cardiac pacemakers A rough estimate for the energy consumption of an implantable cardiac pacemaker is around 10-100µW. Over 5-10 years, that amounts to about 0.52Ah. The low power level makes it an ideal target for energy harvesting. That would mean, instead of the pacemaker having to be replaced when the battery goes flat, it could be powered indefinitely. Fig.13 shows one of the Author’s US Patents from 1989 for a pacemaker “self-powered sensing system”. It generates electrical signals from the heart’s motion using a polyvinylidene fluoride (PVDF) piezoelectric film. A Seiko Kinetic watch mechanism was also demonstrated experimentally to generate power for a pacemaker; see siliconchip.au/link/abxt Another option for powering a pacemaker is an ‘inertia-driven triboelectric nanogenerator’ (I-TENG), as described at siliconchip.au/link/abxb Triboelectricity The triboelectric effect is electric charge transfer due to two objects rubbing together. For example, a shoe rubbing on a carpet can result in a static electricity shock to the wearer when they touch a grounded object. A ‘drinking bird’ toy can be turned into a ‘triboelectric hydrovoltaic generator’ using two effects. A temperature differential powers the bird, while triboelectricity is used to generate power. Experiments demonstrated such a generator powering items like liquid crystal displays, temperature sensors and calculators. For further details, see siliconchip.au/link/abxc A triboelectric nanogenerator (TENG) is an energy-harvesting device that generates an electric charge using the triboelectric effect involving a periodic contact or sliding motion – see Fig.16. Low currents are produced at high voltages. Electret power generators Fig.16: four modes of triboelectric generators. Original source: www. researchgate.net/publication/322251641 20 Silicon Chip Australia's electronics magazine An electret is the electrostatic equivalent of a permanent magnet, and a moving electret can be used to produce power similarly to a magnet. You would probably be familiar with electrets in electret microphones; they serve to bias on the FET within the microphone capsule in the absence of an external voltage source. An electret-based power generator has been demonstrated using siliconchip.com.au ► Fig.14: an energy-harvesting prototype that converts vibration into electricity using MEMS technology and the electret principle. Original source: www.mesl.t.u-tokyo.ac.jp/ en/research/electret.html Fig.15: the circuit of the simplest possible crystal radio using a diode, long wire antenna and highimpedance headphones. Lacking a tuned circuit, it will receive all stations at once, but in practice, the strongest station will probably drown out the rest. Original source: https://w.wiki/AUjt microelectromechanical (MEMS) principles as described at siliconchip.au/ link/abxi (see Fig.14). The prototype produced 6µW from an acceleration of 13.73m/s2 at 40Hz Power from radio waves Crystal radios were made by children back in the day and could obtain useful radio reception without a battery. They were powered by harvesting the energy of the radio wave itself – see Fig.15. RF energy can also be harvested for other purposes using a tuned antenna and a rectifier that works at the desired frequency. They must be close to a source of RF, such as a WiFi router. Commercial modules to harvest RF energy include the Powercast P2110B, which converts RF to DC. It is optimised to absorb energy in the 850-950MHz range and can provide a regulated output of up to 5.5V – see Fig.17. Some YouTube videos demonstrate harvesting small amounts of power from commercial radio stations. The author of the following video manages to light ten LEDs, although he is only 1.6km from the radio station: “Free Energy From Radio Waves (https:// youtu.be/_pm2tLN6KOQ). Fig.18 shows another RF-energy-harvesting circuit. Peter Parker VK3YE looks at whether you can harvest enough power to drive a speaker with a crystal set next to a commercial radio station transmitter in the video titled “Crystal siliconchip.com.au set under a 100kW radio station: How does it sound?” at https://youtu.be/ xglEsaNkPSA cell) is made by inserting two dissimilar metal electrodes in the ground. Zinc and copper are two metals that can be used as electrodes. The soil Würth Elektronik’s energy must be moist for the cell to work. harvesting evaluation kit Multiple cells can be connected to Würth Elektronik (www.we-online. make a battery. com/en) offers an energy-harvesting It is not “free” energy because, as evaluation kit with several energy-­ with any cell, one or both of the elecharvesting options – see siliconchip. trodes will eventually be consumed au/link/abxq or deteriorate. Also, the ions in the soil will eventually be depleted, Earth batteries and a new location will have to be An Earth battery (or, more correctly, selected. Fig.17: a P2110B energy harvester module on a Powercast evaluation board. The module needs a suitable antenna and capacitor to operate. Source: All About Circuits – siliconchip.au/ link/abxw Fig.18: an energyharvesting circuit for ambient radio waves, although the amount of energy collected is tiny. Original source: https://youtu.be/ XpLCK88nVgU Australia's electronics magazine September 2024  21 The first Earth battery was invented by Alexander Bain in 1841; he used zinc and copper electrodes. From an electrochemical point of view, there is nothing unusual about an Earth battery, apart from the medium being the ground rather than a more conventional container such as a battery case. Power harnessed from Earth batteries should not be confused with telluric currents. Still, telluric currents might contribute to the overall EMF of the cell if the electrodes are sufficiently far apart. Telluric currents Telluric currents are electrical currents within the Earth or sea induced by magnetic disturbances from various sources, both natural and artificial. That includes space weather, such as the solar wind, sunspots and their interaction with the ionosphere. They can be a problem for underground and undersea cables and buried pipelines. As they can be influenced by the sun, they vary during the daily solar cycle. In the 1800s, problems in telegraph operation were recognised to be related to telluric currents due to sunspots. In 1903, W. Finn reported in Scientific American that an EMF of 768V with a current up to 300mA was recorded over hundreds of miles/ kilometres of telegraph lines in 1891. Telluric currents can be utilised in mineral exploration, to help locate areas of changes in the electrical conductivity of rocks that may indicate mineral deposits. Gravity batteries A gravity battery is a type of electromechanical battery where a mass is raised and then lowered by gravity to generate electricity. It can be used as a type of energy storage, powering a motor to raise the mass when power is cheap (excess is available) and then lowering it to generate power when it is more expensive (when demand is higher). We discussed some of these ideas in our article on Grid-scale Energy Storage (April 2020; siliconchip.au/ Article/13801). A gravity-powered light called the GravityLight was developed for use in less developed countries (see Fig.19). It is ‘charged’ by raising a 10kg mass by 1m and provides light for five minutes by delivering 20mA continuously. Unfortunately, the project was not a success. Hydroelectricity for camping A portable hydroelectric generator was produced for bushwalkers or campers (Fig.20). You have to anticipate being in an area with reasonably fast-running water. That is not always possible in the Australian bush but is more realistic in parts of the USA, Europe or New Zealand. The device is a bit heavy for many bushwalkers, at 1.5kg, and appears to be no longer available. Electromagnetic fields around power lines It used to be a classic demonstration to hold a fluorescent tube under a high-voltage power line. An electrical Fig.23: the electric field around highvoltage power lines. The red region is a reading of >15kV/m. Source: Quora – siliconchip.com/au/link/abxu current is induced due to capacitive coupling, causing the gases in the tube to fluoresce (see Fig.21). The electric field around a high-voltage power line is shown in Fig.23. There must be a sufficient voltage differential between both ends of the tube for it to light. There is a sufficient electrical field gradient to cause the tube to glow if held vertically but not horizontally. There are many anecdotal accounts (but few documented cases) from the USA of farmers and others building large coils or fences beneath power lines running across their properties to harvest power via electromagnetic induction. It is theoretically possible, but power theft is still illegal even when done ‘over the air’. Very large structures would be required to obtain useful amounts of power (to do more than, say, power some LEDs). With the cost of copper these days, the cost of the wire would exceed any worthwhile savings in electricity, despite the high cost of power. It would be cheaper to buy some solar panels and batteries. Fig.19: the GravityLight provides 20mA to a small lamp for five minutes by slowly lowering a 10kg weight. Fig.20: the “WaterLily Turbine”, a portable hydroelectric generator for charging USB or 12V devices in a running stream. 22 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.24: Alfred Traeger demonstrating the pedal-powered radio he invented in 1928. Source: https://w.wiki/AUk2 There is an interesting video that explains how to use a coil and capacitor to make a resonant LC circuit to harvest enough power to light an LED from various sources. It is titled “Stealing Electricity (the safe way)” and is at https://youtu.be/CLS8pbDNHbk Also see the video titled “Fences sucking power from under HV transmission lines” at https://youtu.be/­ lDm00Ww6qE4 Human-powered generators While pedal-powered generators are less common today due to the low power consumption of LED lights and the advent of lithium-ion batteries, they used to be a common way to power bicycle headlights. They draw power from the rider’s pedalling (see Fig.22). They could be either wheel-mounted (‘bottle dynamos’) or hub-mounted. They can generate about 3W at 6V Fig.25: the Author’s collection of hand-cranked devices. The red hand-cranked torch is from the former Soviet Union and has an incandescent bulb, while the blue one is a modern Chinese torch with LEDs and a reserve battery. The item at upper right is a magneto from an old telephone. (500mA), with some delivering 6W at 12V (also 500mA). Modern hub dynamos such as those from SON can also be used to recharge batteries or mobile devices. In earlier times, electricity was not readily available in the Outback, so Alfred Traeger invented a pedal-­ powered radio that was used for the School of the Air and for calling the Royal Flying Doctor Service (see Fig.24). The pedal generator produced around 200V at 100mA (20W). Transceivers from the Traeger Transceivers company were sold to Nigeria in 1962 and Canada in 1970. For further information about Traeger Transceivers visit siliconchip.au/link/abxf A human on a stationary bicycle can drive a higher-power generator, such as to charge a laptop. Instructions to do this are at siliconchip.au/link/abx8 There is a large variety of hand- Electric shoes Experimental shoes have been designed to harvest energy for a variety of possible purposes; one example Fig.22: a modern bicycle hub dynamo by SON (https:// nabendynamo.de/en/): Source: https://w.wiki/AUjW Fig.21: a fluorescent tube glowing under a high-voltage power line due to capacitive coupling of the electric field. siliconchip.com.au cranked devices that generate electricity for lighting or other purposes, such as those shown in Fig.25. Many early telephones had a hand crank magneto that generated 50-100V AC to ring a bell at the called party’s end, or alert an operator. While current for talking was supplied by batteries, they did not have sufficient power to ring the bell. Dynamite plungers were similar, although they are now obsolete. They comprised a T-handle attached to a linear rack gear that engaged with a circular gear connected to a generator. When the handle was pressed down, they generated a brief electrical current to trigger a detonator. Australia's electronics magazine September 2024  23 is shown in Fig.26. That energy-­ harvesting combat boot produces elecGPS Transmitter trical power via compression of bulbs in the sole of the boot, which drive Power Management Module microturbines to produce electricity to power a GPS tracker. Turbine Enclosure Children’s shoes that light up genAir Bulbs (3x) erally have batteries and are not self-powered, as explained in the video titled “How Light Up Shoes Work – See What’s Inside Sketchers Kids Litebeams” at https://youtu.be/ IIlpRgVBDYo On the other hand, kids’ scooter Fig.26: an energy-harvesting combat wheels that light up do use a small boot that powers a GPS tracker. Source: www.researchgate.net/ generator built into the hub. Power from trains coming down mountains On page 79 of the April 1988 issue, we described how regenerative braking by heavy ore- and coal-laden trains descending the Blue Mountains in Sydney (from mines in places like Lithgow) generated a significant amount of power, which was used to power passenger and empty freight trains ascending into the mountains at the same time. If ore or other heavy material is mined from mountains and carried down to sea level by trains, which then ascend empty, you effectively have a publication/325211019 generator powered by the potential energy of that ore (see Fig.31). Fortescue is developing an iron ore freight train in Australia that will charge batteries as it coasts down hills, to provide power for the return journey uphill to get more ore. Power from roads Energy-harvesting experiments have been performed for roadways. Methodologies that have been tried, shown in Fig.27, include: • Harvesting thermal differentials in between pavement and lower levels underground. • Devices exploiting Faraday’s Law of Induction to harvest mechanical energy (a magnetic field will interact with an electric circuit to produce an electromotive force). • Piezoelectric devices to harvest mechanical energy. • Solar panels embedded in, around and above roadways. Electrodynamic tethers An electrodynamic tether is a long wire deployed from an Earth-orbiting spacecraft – see Fig.28. As it passes through the Earth’s magnetic field, a current flow develops, according to Faraday’s Law of Induction. It can be used as a power source, but it results in some drag on the spacecraft. In 1996, NASA deployed a long tether from the Space Shuttle Columbia, which generated a potential of 3500V. The tether was intended to be 20.7km long but an electric arc caused the tether to break after 19.7km had been spooled out. It works as follows – ionospheric electrons are collected from the positively-­ biased anode at the end of the uninsulated tether. They flow through the electrical load, then to the negatively-­biased cathode, where they are discharged into the space Fig.27: some concepts of energy harvesting from vehicles travelling on roads. Original source: www.mdpi.com/1996-1073/16/7/3016 24 Silicon Chip Australia's electronics magazine siliconchip.com.au plasma and complete the circuit. Electrostatic generation from lunar soil NASA has proposed harvesting the electrostatic charge from lunar soil. The charge builds up over long periods due to the solar wind. They propose to collect the charge using a moving capacitor array that’s ‘raked’ through the lunar soil (see siliconchip. au/link/abxj). NASA estimates that a 1/3m2 collecting array could produce a maximum theoretical power of 147W (700V <at> 0.21A) – see Figs.29 & 30. Tiny solar cells Inexpensive, tiny solar cells can be used to power IoT or sensor devices, with energy stored in a small battery or cell. Even photodiodes can be pressed into service to generate power; see Fig.32. People in the developed world might not appreciate it, but for people living in less developed countries, night-time lighting is not always available and it is highly beneficial if they can get it. Certain charities, such as SolarAid (https://solar-aid. org), produce solar lights for people in these countries, and donors can also Fig.31: the ARES rail car, which climbs a hill using electricity during off-peak hours, then is released downhill during peak hours to produce energy via regenerative braking. Source: ARES North America – aresnorthamerica.com buy them for their own use. Many small solar panels are available for bushwalkers and campers to recharge devices. Some can be affixed to backpacks, while others are set up when camped. However, panels that are small and light enough to be affixed to a backpack provide only small amounts of power. I find that you typically get to a campsite well after peak sun. In my experience, it is better to carry batteries. Micro hydroelectric schemes New Zealand YouTuber Marty T made a ‘microhydro’ installation on his wilderness property using the motor from a scrap Fisher & Paykel Fig.29: the circuit of a theoretical capacitive charge collector with a differential drain to harvest electrostatic charge from the negatively charged lunar soil (regolith). Original source: https://ntrs.nasa.gov/api/ citations/20100032922/downloads/20100032922.pdf Fig.30: a proposed charge collector with an array of electron capture blades that can be raked through lunar soil to harvest electrostatic charge. Original source: https://ntrs. nasa.gov/api/citations/20100032922/ downloads/20100032922.pdf Fig.28: an electrodynamic tether deployed from a spacecraft. Original source: https://w.wiki/AUjv siliconchip.com.au Australia's electronics magazine September 2024  25 Fig.32: a BPW34 PiN photodiode can be used as a solar cell, producing up to 47µA at 350mV. The coin diameter is 24.26mm. Source: Core Electronics PRT-09541 – siliconchip.au/link/abxz Fig.33 (below): a wind turbine that can be used at a campsite. Source: Tex Energy – siliconchip.au/link/abxx Energy harvesting is not new In 1925, Estonian inventor Hermann Plauson obtained US Patent 1540998 for “Conversion of atmospheric electric energy”. He proposed harvesting atmospheric electricity with a network of balloons. H. Gernsback earlier described this idea in “Science and Invention”, February 1922 (siliconchip.au/link/abxg). It is unlikely this would have been practical. However, it was claimed in the description that a single balloon at 274m altitude could provide 400V at 1.8A, which certainly would be useful if attained! We suspect that it was under unusual atmospheric conditions and could not be achieved regularly. SmartDrive washing machine (similar to how we used one as a generator on a wind turbine in the December 2004 to March 2005 issues; siliconchip.au/ Series/84). The motor has to be rewired to reduce the voltage and increase the current, to make it more suitable for charging a battery bank. Details of motor rewiring are at siliconchip.au/ link/abxk (or refer to our articles), but many other resources explain how to do it. Also see this series of videos: 1. https://youtu.be/LVoeaKCEd2o 2. https://youtu.be/lbuvTSWh50U 3. https://youtu.be/8SWq5Pskpug A US YouTuber decided to see if he could make a hydroelectric system powered by rainwater collected on a roof. He calculated that 2W could be generated from rain falling on a house roof and going down the downpipes, but on his first attempt, he only got 0.19W. On his second attempt, he generated over 0.61W and, on the third attempt, over 0.91W. Of course, it has to be raining for this to work. In Australia, such a system might work best in the tropics, such as Far North Queensland. See the videos for more details: 1. https://youtu.be/S6oNxckjEiE 2. https://youtu.be/YLb4enCgnP4 3. https://youtu.be/vify0k2sHlQ Portable wind generators A wind generator can be used for bushwalking, provided it is anticipated there will be reasonable wind at the campsite. A model such as the Infinite Air 5T can produce up to 5V at 2A and weighs 1.65kg (Fig.33 shows the larger 3.2kg Infinite Air 18 model). As with the portable hydroelectric generator, we feel the weight is too high for most potential use cases. MEPAP The energy harvesting idea of Hermann Plauson. Source: www.reddit.com/r/Air_Fountain/comments/1cc3dx6/ 26 Silicon Chip Australia's electronics magazine The MEPAP (“Multipurpose and source Electricity Generator with Air Purifier”) is something Heath Robinson or Rube Goldberg might have dreamt up. It harvests electricity using vibration (piezoelectric materials), electromagnetic radiation (metamaterials), electromagnetic induction (inductive coupling), wind energy (mini turbine with dynamo) and thermoelectric energy, all to operate an air purifier device. It is described at siliconchip.au/ link/abxh, but we don’t know how well it works. SC siliconchip.com.au