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by Dr David Maddison H ere we describe several large-scale energy storage technologies and some which work at smaller scales. By “large scale”, we mean applications that are larger than a domestic battery system that might be installed as part of an off-grid solar electric installation. This means backup power systems large enough for a hospital, factory, data centre or other large institution, all the way up to grid-scale energy storage. Grid-scale storage might be used to back up intermittent solar and wind production, or for load balancing or frequency control on the electricity grid. For grid-scale storage, pumped hydro is by far the most popular and cost-effective method. But it is often limited by the availability of suitable sites (ie, by geography) and by opposition to building dams – a particular problem in Australia. We published an in-depth article on Pumped Storage Hydroelectricity in the January 2017 issue (siliconchip. com.au/Article/10497). We won’t go back over that again. The purpose of this article is to investigate and describe the alternatives. The most obvious means of storing electricity is batteries. But batteries for large-scale energy storage are both costly and have a limited lifespan. Hence, much effort has been 12 Silicon Chip There are many reasons why large amounts of energy may need to be stored. The most significant these days is to store excess energy from intermittent renewable generators and release it at times of low generation. Pumped hydro is the most common (and oldest) storage method, but there are numerous alternatives either in active use or proposed. put into looking for other options (or alternative battery chemistries which are better suited to this task). These other options are: 1) “mechanical batteries” or flywheels 2) compressed air storage, either in tanks, cavities in the ground or underwater 3) liquid air (cryogenic) energy storage or high-temperature storage 4) gravity potential energy storage, using masses raised to a higher level to store potential energy whether by towers, underwater structures or trains No energy storage method is ever 100% efficient. The so-called “round-trip energy efficiency” needs to be considered. This is the proportion of the energy used to charge the system that is recovered on discharge. For comparison, pumped hydro is typically regarded as having a 70-80% round-trip energy efficiency. Storing large amounts of energy, no matter how it’s done, is very expensive and requires significant space and volume. This is just one of the reasons why adding large amounts of variable generation such as solar or wind power to a grid, in a cost-competitive manner, is so difficult. Australia’s electronics magazine siliconchip.com.au Fig.1: this shows how Ecoult’s UltraBattery hybrid technology works. One must either live with their intermittency, or factor the cost of the required energy storage into the generation costs. Energy storage objectives The main objectives for large-scale energy storage are: 1) For intermittent renewable generators, to take up excess energy produced under favourable conditions and then release this when the intermittent producers are producing little power or are offline (eg, no wind or sun). 2) To improve grid stability such as frequency or voltage stabilisation when huge swings occur in demand or due to intermittent production. 3) To make money for storage owners via “arbitrage”. In other words, they buy and store electricity when it is cheap and sell it later when it is more expensive. 4) To enable the building of smaller and more economic power stations than would by themselves be incapable of supplying peak demand. Supposing peak demand was 1500MW in a particular market, a cheaper 1000MW power station could be built, and stored power could be used to supply the extra 500MW for the peak period (eg, two hours a day). Objective #4 is only economical if the cost of the storage is lower than the cost of generation capacity. This is one of the purposes of pumped storage in the original Snowy Fig.2: a large-scale UltraBattery installation. These are DEKA brand batteries, made by East Penn Manufacturing in the USA, the parent company of Ecoult. Mountains Scheme. Note that in this article, many storage systems are described as having a kWh/MWh/GWh capacity as well as a kW/MW/GW rating. The former describes the total energy that can be stored while the latter indicates how quickly that energy can be delivered. So for example, a 1GWh system with a rating of 100MW could be expected to deliver 100MW for 10 hours or 50MW for 20 hours. Electrochemical (battery) storage For applications such as backup power supplies in small or medium-sized data centres, telecommunications hubs and some other facilities, traditional lead-acid batteries are still frequently used. They are an old technology (invented in 1859) but are of relatively low cost, and when managed correctly, reliable and predictable. They are also highly recyclable. Despite the relatively low cost of lead-acid batteries, there are reasons to use other battery chemistries. For example, lithium-ion types have a higher capacity for a given volume, have a greater permissible repeated depth of discharge and can have a better lifespan. As a result, lithium-based batteries are now used for grid-scale storage. As an example of a (small, designed to serve 1600 Fig.3 (and opposite): Australia’s “Big Battery”: the Hornsdale Power Reserve battery in South Australia. The wind turbine in the background is part of the associated wind farm whose energy goes into the battery. siliconchip.com.au Australia’s electronics magazine April 2020  13 Fig.5: a cross-section representation of a liquid metal battery. Fig.4: six 10kWh Redflow ZCell zinc-bromine flow batteries on the Bates family farm in Queensland, 2.7km from the nearest power lines. The batteries are charged from 72 260W Tindo solar panels, with an 18.7kW peak power capacity, plus two Victron Quattro 48/10000 inverters to supply mains power to the home people) grid-scale lead-acid battery, the King Island Advanced Hybrid Power Station in Bass Strait, as of 2014, employed a 3MW-capable, 1.5MWh advanced lead-acid battery as part of its storage system. The specific manufacturer or details of the battery are not mentioned on the owner’s website, Hydro Tasmania. At the time of installation, it was the largest battery in Australia and could supply the needs of King Island (in Bass Strait) for 45 minutes. The advanced lead-acid battery replaced an earlier failed 800kWh vanadium redox “flow” battery (initially installed in 2003). For a live dashboard of power generation at King Island, see siliconchip.com.au/link/aayr Australian company Ecoult (www.ecoult.com) was formed in 2007 but has been US-owned since 2010. It produces the UltraBattery (Figs.1 & 2), which was invented by the CSIRO. This hybrid battery technology combines elements of a lead-acid battery and a supercapacitor. Fig.6: these 800Ah/ 160W Ambri cells come in 216 x 137 x 254mm sealed stainless steel containers and weigh 25kg each. 14 Silicon Chip Compared to traditional lead-acid batteries, it can charge and discharge continuously and rapidly in a partial state of charge due to its ultracapacitor element, making it ideal for smoothing the output of intermittent energy sources like solar and wind farms. Its lead-acid component provides bulk storage of energy for times when the generator is providing little or no power. For more information, see the video “UltraBattery The Movie” at https://vimeo.com/208600432 South Australia’s 129MWh “Big Battery”, otherwise known as the Hornsdale Power Reserve (Fig.3), was manufactured by Tesla and can deliver 100MW. It is said to be the world’s largest lithium-ion battery. In November 2019, it was announced that its capacity and power would be increased by 50%. This is taxpayer-funded, with $15 million from the SA Government, $50 million in cheap loans from the Clean Energy Finance Corporation and $8 million from the Australian Renewable Energy Agency. Other battery chemistries are also becoming available for large scale storage, including next-generation lithium batteries like LMP (solid-state lithium metal polymer batteries) by Blue Solutions (www.blue-solutions.com/en/) and other solid-state lithium batteries such as those under development by Australia’s CSIRO (siliconchip.com.au/link/aays) and Deakin University (siliconchip.com.au/link/aayt). Fig.7: the electrochemistry of the Ambri cell. Alloying and de-alloying occur during the discharging and recharging process, with no long-term degradation of components. Australia’s electronics magazine siliconchip.com.au Fig.8: the Ambri battery system. Cells are aggregated into modular 10-foot shipping containers with a capacity of 1000kWh/250kW and an operating voltage of 500-1500V. The containers come ready to install and the contents require no maintenance. Flow batteries Flow batteries are also used for large-scale electrical storage. In a flow battery, the electrolyte is stored in tanks rather than within each battery cell (as with regular batteries). This confers several benefits, such as improved safety and less degradation with charge and discharge cycles. Disadvantages include lower energy density and lower charge and discharge rates than regular batteries. Pumps are needed, which require maintenance. Some flow batteries used in Australia are: • Monash University, Clayton, Vic has a 180kW, 900kWh vanadium flow redox battery as part of a hybrid battery to store energy in their Microgrid system • The University of NSW has a 30kW, 130kWh CellCube (www.cellcube.com/) vanadium flow redox FB 30-130 system for research, and to store electricity from a 150kW photovoltaic system • Base64 in Adelaide (www.base64.com.au/) has a 450kWh Redflow Energy bromine flow battery to back up a 73kW (peak) solar system Redflow (https://redflow.com/) is an Australian company that produces 10kWh zinc-bromine flow batteries (Fig.4) They are “designed for high cycle-rate, long time-base stationary energy storage applications in the residential, commercial & industrial and telecommunications sectors, and are scalable from a single battery installation through Fig.9: Beacon Power’s (https:// beaconpower.com/) flywheel system. The rotor assembly (hub, shaft and motorgenerator) is integrated into the carbon fibre “rim”. The rotor, which spins at 16,000rpm, is supported on a magnetic lift system and is in a vacuum chamber. The units are buried to contain any fragments ejected due to rotor failure. to grid-scale deployments”. The Redflow ZBM2 battery is intended for commercial use, while the Zcell flow battery is intended for residential or office use. Ambri (https://ambri.com/) is a US company that has developed a unique liquid metal battery system, comprising a liquid calcium-alloy anode, a molten salt electrolyte and a cathode made from antimony particles (Figs.5-8). This battery system was explicitly designed using cheap “commodity” materials (no rare exotic materials, or those with supply uncertainty due to location). It was also designed to be intrinsically safe, with no risk of fire (even if the container is breached) and no requirement for external equipment such as pumps or cooling systems. The system does not degrade with cycling, unlike other battery systems, and is cheaper than current or projected lithium-ion battery prices due to cheaper materials and simpler manufacturing methods. The nominal open-circuit voltage of an Ambri cell is 0.95V and capacity is 800Ah, with a maximum continuous power of 160W. Voltage cycling is in the range of 0.5V Fig.10: Beacon Power’s 20MW/5MWh FES installation in Hazle Township, Pennsylvania, USA; the world’s largest flywheel installation. Its 200 flywheels are used for grid frequency regulation. The tops of the flywheels are in blue, with the rotating masses buried — each flywheel assembly weighs 5t. The shipping containers contain control equipment. siliconchip.com.au Australia’s electronics magazine April 2020  15 The two major forms of energy loss in FES are in the bearings and frictional losses of the surface of the rotor against the atmosphere; therefore, the bearings used are usually zero-friction magnetic types and the rotor operates in a vacuum. Uses for flywheels in large-scale energy storage include: • • Fig.11: a schematic view of the Hitzinger DRUPS. “CB” stands for circuit breaker. The kinetic module is the flywheel assembly. to 1.25V while DC efficiency is over 80%. The cells operate at 500°C. They are self-heating when started and so require no external heating to reach operating temperature or to stay there. In September 2019, NEC announced they would use Ambri technology for an energy storage system. NEC has committed to purchase a minimum of 200MWh of storage that will be used in grid applications to provide energy for four hours or more, with full depth of discharge cycling. See the video titled “The Liquid Metal Battery: Innovation in stationary electricity storage” at siliconchip.com. au/link/aazq backup for intermittent wind and solar systems grid stability services such as for frequency and load balancing • uninterruptible power supplies with zero switching time for large organisations like hospitals, data centres or Australia’s King Island Renewable Energy Integration Project • the electromagnetic aircraft launch system (EMALS) as used by the US Navy (see our article on Rail Guns and Electromagnetic Launchers in the December 2017 issue: siliconchip.com.au/Article/10897). STORNETIC (https://stornetic.com/) is a German company that makes flywheel energy storage systems (Fig.14). They have installed a system in Munich, Germany, comprising of 28 flywheels that spin at 45,000rpm with a capacity of 100kWh, used for grid stabilisation. See the video titled “STORNETIC - The Energy Storage Company” at siliconchip.com.au/link/aazr One type of flywheel-based uninterruptible power supply (UPS) system is a diesel UPS or D-UPS, also known as a rotary UPS or diesel rotary UPS (DRUPS). A DRUPS Flywheel energy storage Flywheel energy storage (FES) involves storing energy with a rapidly spinning rotor in the form of rotational energy, also known as angular kinetic energy. The flywheel is typically connected to a motor-generator; it is sped up by the motor and when energy is to be extracted, generator mode is engaged, which reduces the rotor RPM as energy is extracted (Figs.9, 10 & 13). Flywheel storage systems have long lives and have a round trip efficiency of up to 90%. Fig.12: a Hitzinger rotary UPS as used in the King Island Renewable Energy Integration Project. 16 Silicon Chip Fig.13: NASA’s 525Wh/1kW G2 flywheel. This was an experimental energy storage system demonstrated in 2004 for possible use in spacecraft. Its rotational speed was 41,000rpm and it weighed 114kg. Australia’s electronics magazine siliconchip.com.au Flywheel and gravitational energy storage equations The energy of a spinning flywheel can be calculated from these two equations: Ef = 0.5 × I × ω² I = k × m × r² Here, Ef = flywheel kinetic energy, I = moment of inertia, ω = angular velocity (measured in radians/second and proportional to RPM), k = inertial constant (a value from 0 to 1 depending on flywheel shape), m = flywheel mass and r = flywheel radius. If we combine the above equations and create a new constant K, we get Ef = K × ω² × m × r². For comparison, assuming the flywheels to be compared are the same shape, we can see that flywheel energy storage goes up with the square of the angular velocity (or RPM) and the radius of the flywheel. Thus, if either the radius or RPM doubles, the energy storage quadruples. The amount of potential energy in a mass hoisted above the earth, assuming perfect efficiency, is: PE = m x g x h Here, m is the mass in kg, g is the acceleration due to gravity in metres per second squared (around 9.8 at the Earth’s surface) and h is the height. The result, PE, is in Joules. To convert Joules to MWh, divide by 3.6 x 109. Fig.14: multiple STORNETIC flywheel energy storage systems. consists of a diesel engine, an electromagnetic clutch, an alternator, a kinetic energy module (flywheel) and a choke (see Figs. 11 & 12). In normal operation, a DRUPS conditions the incoming mains supply, producing power at the correct voltage and frequency. Incoming power drives a synchronous alternator as a motor, to which is attached a flywheel or “kinetic module” for energy storage. Fig.15: a proposal from Apex CAES (www.apexcaes.com/) for Bethel Energy Center in Texas. It will be capable of generating 324MW for 48h. It uses natural gas to heat expanding air during power production. The cost is US$21/ kWh versus $285/kWh for a lithium-ion battery and will last 30 years, or three times longer than a lithium battery. siliconchip.com.au Power is conditioned both by the alternator, which stabilises the frequency and blocks higher-frequency harmonics and transients, and the choke which further blocks highfrequency harmonics. The alternator, with a special stator configuration, also blocks the upper harmonics of lower frequencies (such as the 3rd, 5th, 7th harmonics etc). In the event of a power failure, the flywheel continues to rotate, driving the alternator to generate power and losing speed as it does so. If the power failure exceeds a certain number of seconds, an electromagnetic clutch is engaged and the diesel motor starts. This drives the alternator (and brings the attached flywheel back up to speed) to produce power until mains power is restored. For more information, see the video “Hitzinger Rotary Diesel UPS” at siliconchip.com.au/link/aazs Fig.16: a surface view of A-CAES at the old Angas Zinc Mine near Strathalbyn, about 60km south-east of Adelaide. The water reservoir is full when the system is charged and empty when the system is discharged. Image courtesy ARENA. Australia’s electronics magazine April 2020  17 Why energy storage is essential for renewables Conventional coal, gas, hydroelectric and nuclear power plants are usually much larger and have a much higher “capacity factor” than wind or solar plants. The capacity factor represents the amount of power generated long-term compared to its “nameplate” capacity. Wikipedia states that Australia has a total nameplate capacity of 5,679MW in 94 wind “farms”, with an average 60MW nameplate capacity (and a total of 2,506 windmills). As the typical capacity factor of a wind farm in Australia is 30-35%, these farms on average can be expected to generate 1,703-1,988MW, an average output per farm of 18-21MW. Because the output of such generators is so variable, to keep the grid stable and meet energy demand, they are best combined with energy storage systems. With sufficient storage, the output of a renewable energy source can be considered “dispatchable”, ie, available on demand. This is not usually necessary with traditional power plants as their capacity factors are close to 100% and downtime for maintenance is normally planned in advance. Compressed air energy storage Energy can be stored by compressing air, which can then spin a turbine to recover the energy. In a large-scale system, the compressed air is held in an appropriate containment such as an unused mined-out cavity of a salt mine (Fig.15). As anyone who has pumped up a bicycle tyre or released the contents of an aerosol can knows, compressing gas heats it while expanding gas cools down. For maximum efficiency of compressed air storage, the heat from compression needs to be preserved and put back into the air when the air is discharged to produce power, as the heat contains a lot of the original energy. In some compressed air installations, the air is heated not only with the heat recovered from the original compression but by burning natural gas as well. The two largest compressed air energy storage plants are in Huntorf, Germany and McIntosh, Alabama, USA. The Huntorf plant was built in 1978, and it uses two empty mined-out salt domes which are typically charged for eight hours per day. Its rated capacity is 870MWh, typically providing for three hours of discharging at 290MW. It has a 42% overall efficiency. Fig.18: a rendering of Highview Power’s 250MWh/50MW CRYOBattery plant, to be built in the north of England. 18 Silicon Chip Fig.17: a Hydrostor system. Compressed air is stored in caverns and kept pressurised with water. The salt caverns are 600m deep and have a 310,000m3 total volume. They are at 100atm of pressure when fully charged. The plant in McIntosh was built in 1991, with a capacity of 2860MWh and it can discharge 110MW for 26 hours. It also utilises mined-out salt domes for storage. It burns natural gas in a “recuperator” to heat the expanding air and has an overall efficiency of 54%. Hydrostor (www.hydrostor.ca/) is developing Australia’s first Advanced Compressed Air Energy Storage (A-CAES) facility. The project is taxpayer-funded to the extent of $6 million from the Australian Renewable Energy Agency (ARENA) and $3 million from the Government of South Australia Renewable Technology Fund. It will use a disused zinc mine near Adelaide for compressed air storage, and will deliver 5MW with a 10MWh storage capacity (see Figs.16 & 17). Air will be compressed and the heat captured using a proprietary thermal storage system. The compressed air Fig.19: a schematic representation of cryogenic energy storage. Australia’s electronics magazine siliconchip.com.au Fig.20: Highview Power’s 5MW Pilsworth Grid Scale Demonstrator Plant. It began operation in April 2018 and is backed by UK taxpayer funding. See the video “World’s first grid-scale Cryogenic Energy Storage System launch” at siliconchip.com.au/link/aazt will be stored in underground caverns in the mine, filled with water to maintain pressure. During the charging process, water will be forced out of the caverns and up to a surface reservoir. Upon discharge of the air to produce electricity, water will return to the caverns to replace the air. The discharged air will also be heated with stored heat from the compression process. See the video “How Hydrostor A-CAES Technology Works (2018)” at siliconchip.com.au/link/aazu There are two different proposals for keeping compressed airbags at the bottom of the ocean. These are detailed in the videos titled “Underwater Energy Bags” at siliconchip. com.au/link/aazv (by Prof. Seamus Gravey) and “Underwater Energy Storage in Toronto” at siliconchip.com.au/ link/aazw (by Hydrostor). There is also a concept from the German Fraunhofer Institute for Wind Energy and Energy Systems Engineering for concrete energy storage spheres at the bottom of the ocean. See the following websites for more information: siliconchip.com.au/link/aayu siliconchip.com.au/link/aayv siliconchip.com.au/link/aayw Cryogenic energy storage Cryogenic energy storage is a type of compressed air storage where the air is compressed and cooled to a liquid form. UK company Highview Power (siliconchip.com.au/ link/aazx) has developed the CRYOBattery which is scalable from 20MW/80MWh to more than 200MW/1.2GWh (see Figs.18-20). It is claimed to be the cheapest form of grid-scale energy DIY Rubber band energy storage YouTuber J.L. Ibarra Avila built a simple device to use energy stored in rubber bands to turn a generator, producing a small amount of electricity to light an array of LEDs. See the video “Energy stored in rubber bands to generate electricity” at https://youtu.be/LT_nB07r-4g siliconchip.com.au Fig.21: the failed Crescent Dunes Solar Energy Project in Nevada, USA. One problem with such facilities is that they kill birds and insects that fly into its concentrated solar beam. Australia was to have one just like it. storage (£110 [around AU $206] per MWh for a 10-hour, 200MW/2GWh system). It has an efficiency of 60% in a standalone configuration or 70% when combined with the utilisation of waste heat and cold. In October 2019, Highview Power announced a 50MW/250MWh CRYOBattery project in the north of England with a five hour discharge time. See the videos “Highview Power – True Long-Duration Energy Storage” at siliconchip.com.au/link/aazy and “Liquid Air Energy Storage Animation 2018” at siliconchip. com.au/link/aazz Thermal energy storage Thermal (heat) energy can be stored when energy is plentiful or cheap and released later when it is needed. Heat energy is commonly stored in molten salt, and this was the subject of two commercial grid-scale projects as follows. There was a large $650 million, 135MW solar thermal power plant planned for South Australia, announced by the SA Premier on August 14, 2017. But despite extremely generous government backing of various kinds (including a $110 million loan), its cancellation was announced on April 5, 2019. The reason given was that it was not able to attract sufficient investor funding, perhaps because it was unlikely to ever make a profit, even with Australia’s very high electricity prices. The plant was to use a system of mirrors to heat molten salt in a tower during times of high solar radiation, and use the heat of the molten salt to drive a steam turbine to generate electricity including during cloudy periods and at night. So the heat stored in the molten salt could supposedly be used to generate power 24 hours per day. Could you run your home on compressed air storage? To store 3kWh of energy, you would need a compressed air cylinder of 2.5m in diameter and 13.7m long, charged to 750kPa or 7.4atm. Consider that the average Australian household consumes at least 10kWh per day. For more details, see the PDF at siliconchip.com.au/link/aayz Australia’s electronics magazine April 2020  19 Fig.22: the “Energy Vault” stores energy by lifting concrete blocks to form a tower. When later lowered to the ground, they drive a motor-generator to produce electricity. The proposed developer ran the only other such plant in the world based on the same technology, in Tonopah, Nevada, USA (see Fig.21). It was also dependent on government subsidies, failed to produce sufficient power and was shut down in April 2019. There is a working solar power tower in Ivanpah, California but its production has been disappointing, and it lacks thermal storage; the water used as the heat transfer medium has to be heated up every morning with natural gas. One ongoing problem with solar tower systems like these is that they tend to incinerate insects and birds; for example, see the video titled “Insects and birds affected by Ivanpah solar tower” at siliconchip.com.au/link/ab00 is not suitable for all locations. Bear in mind that gravitational potential energy storage has a relatively small energy density. For example, to store the energy of a single AA battery, you need to lift 100kg 10m. Or to store the equivalent of one litre of petrol, you need to lift about 30 tonnes 100m. So to store enough energy to be worthwhile, the mass or volume lifted must be very high. Besides pumped hydro, a few methods have been proposed for large-scale storage: 1) hoisting concrete blocks onto a tower using a crane, then lowering the blocks on the crane to drive a motorgenerator attached to the cable. 2) a similar method by which heavy weights on cables Gravitational potential energy storage Gravitational potential energy storage involves moving mass from a lower level to a higher level and then releasing it to liberate its potential energy. The most common form of large scale gravitational potential energy storage by far, also known as a gravity battery, is pumped hydroelectric power. Pumped hydro uses water as the mass medium as it is relatively dense and easy to move around using pumps and pipes. However, as mentioned above, pumped hydro Fig.23: a rendering of the SINKFLOATSOLUTIONS Heavy Underwater Gravity Energy Storage system, showing weights suspended from barges. 20 Silicon Chip Fig.24: the MGH gravitational potential energy storage system. A floating platform at sea lowers masses 1000m+ to the seafloor to release energy. Australia’s electronics magazine siliconchip.com.au Fig.26: the Gravitricity gravity storage system, with winches powered by motor-generators lowering masses down a specially-built shaft (up to 150m) or disused mineshaft (up to 500m). The masses are at least 500t each. Fig.25: a system outlined on the YouTube channel “McMillion Watts” to harvest ocean wave energy. are lowered into the ocean to a depth of 4km, or down a shaft in the ground, then later hoisted back up. 3) driving a train filled with rocks uphill and generating electricity when it later descends. 4) a (far-fetched) scheme where weights are hoisted and then lowered from a floating structure in the stratosphere. A simple and familiar example of gravitational energy storage at a small scale is the pendulum clock or a cuckoo clock, where weights are raised to “charge” the mechanism and released to power it. Energy Vault (https://energyvault.com/) proposes a gravity storage system whereby concrete blocks are raised with a crane powered by a motor-generator to charge the system, and lowered to produce power (see Fig.22). The company claims it costs half as much as pumped hydro with a 90% round-trip efficiency, a 30-year plus life and no cycle degradation. The system is modular and scalable and provides 20, 35 or 80MWh storage capacity and 4-8MW of continuous power for 8-16 hours. Each brick lifted weighs 35 tonnes. The system is said to be simple and inexpensive to build. A YouTuber by the name of Thunderf00t has critically analysed this proposal and disagrees with its claims of efficacy. One stated concern is the stability of the weights in high winds; see the video titled “Energy Vault -BUSTED!” at siliconchip.com.au/link/ab01 A French company called SINKFLOATSOLUTIONS (http://sinkfloatsolutions.com/) proposes to lower large concrete masses into the depths of the oceans (up to 4km deep) from barges. The system is called HUGES or Heavy Underwater Gravity Energy Storage (Fig.23). See the video titled “Underwater Energy Storage - How It Works” at http://siliconchip.com.au/link/ab02 MGH Energy Storage (siliconchip.com.au/ link/ab03) is another French company that proposes a maritime gravitational potential energy storage system (Fig.24). Offshore floating structures would be used to harvest wave energy. This energy is then used to raise weights up shafts dug deep into the ground onshore (up to 3000m deep). See the video “MGH Energy Storage – multi weight operation” at siliconchip.com.au/link/ab04 Note that most, if not all, schemes to harvest wave energy built so far have failed. See the video “WAVE AMPLIFICATION, WAVE POWER HARNESSING, SOLID MASS GRAVITATIONAL ENERGY STORAGE” at siliconchip.com. au/link/ab05 (see Fig.25) Gravitricity (www.gravitricity.com/) proposes a system of energy storage whereby weights of 500-5000t are raised in a deep shaft dug into the earth, or possibly using an abandoned mine shaft; see Fig.26. The company claims the following advantages on their website: • 50-year design life with no cycle limit or degradation • response time from zero to full power in less than one second • efficiency of 80-90% Fig.27: the ARES pilot installation with a 6t vehicle on a 9% rail grade near Tehachapi, California. A full-scale system would be much larger than this. Fig.28: an artist’s rendition of the proposed 12.5MWh/ 50MW ARES train in Pahrump, Nevada. The track length would be 9km with an elevation difference of 610m, a grade of 7-8%, a footprint of 19ha and total train mass of 8700t. It will be used for “ancillary services” such as frequency regulation to aid grid stability. siliconchip.com.au Australia’s electronics magazine April 2020  21 Fig.29: the StratoSolar concept of large helium or hydrogen-filled platforms floating 20km up with solar panels for electricity generation and masses on cables for gravitational potential energy storage for night-time energy production. • • • can run slowly at low power or fast at high power easy to construct near networks levelised cost well below lithium batteries Gravitricity says that each gravity storage unit can be configured to produce 1-20MW for between 15 minutes and eight hours. As with all gravity storage methods, the amount of energy stored is relatively modest. A 3000t weight lowered 1250m into a shaft will store about 10MWh. ARES or Advanced Rail Energy Storage (siliconchip.com. au/link/ab06) is a gravity potential energy storage system that uses masses raised on a rail system for energy storage (Figs.27 & 28). ARES proposes three levels of capacity, 20-50MW for ancillary services; 50-200MW with 4-8 hour duration for “renewables” integration; and grid-scale systems of 2003000MW with 4-16 hour duration. During charging, masses are picked up by the train in a lower storage yard and dropped off at an upper storage yard. After the masses are dropped off, the empty train returns to the lower yard to pick up more. The discharge process is the reverse. The process is automated and requires no new technology. All that is required is two storage locations with an appropriate height differential and an appropriate grade, and a path between them. ARES has developed a cabledrive system called “Ridgeline” for where the grade is too steep for conventional rail traction, allowing the use of sites with as little as 240m elevation change with grades from 20-50%. Fig.30: the internals of the GravityLight. The weight bag is not shown. See the videos titled “ARES-Technology” at: siliconchip. com.au/link/aaz0 and “A New Kind of Renewable Energy Storage” at siliconchip.com.au/link/ab09 MAPS (MAglev Power Storage) is a proposed system similar to ARES but using magnetically levitated “maglev” trains instead of traditional rails and wheels like ARES. It is claimed to be 90% efficient with a storage cost of US$0.020.03 per kWh. Studies and presentations appear to have been published around 2010 but nothing since. StratoSolar Inc. (www.stratosolar.com/) proposes energy generation and storage in the stratosphere! This company has planned buoyant platforms filled with helium or hydrogen 20km up with solar production by day and gravity potential energy storage at night (Fig.29). Multiple 1kg weights are to be suspended beneath the Using compressed air for off-grid energy storage The video “AMISH air POWER ~ OFF GRID” at siliconchip. com.au/link/ab07 shows how an Amish community in the USA uses compressed air to power their ceiling fans, sewing machines and other equipment (Figs.32&33). The compressed air is produced either with a petrol-powered compressor or by a windmill. The air is stored in tanks. A variety of machinery can be powered using air-powered motors, such as those available from Gast Manufacturing, Inc. (siliconchip.com.au/link/aayx) or DEPRAG SCHULZ GMBH u. CO. (siliconchip.com.au/link/aayy). 22 Silicon Chip Australia’s electronics magazine Fig.31: a GravityLight with weight bag. A DIY gravity phone charger YouTuber Tom Stanton converted a hand-cranked USB charger to a gravitypowered one (Fig.34). It was an interesting exercise, but clearly, not a practical one (as you will see if you watch his video). It demonstrates the low power density of gravity energy storage. See “Gravity Powered Phone Charger” at siliconchip.com.au/link/ab08 siliconchip.com.au Fig.34: modified hand-cranked USB charger components inside a 3D-printed case, converting it into a gravitypowered charger. Frame grab from Tom Stanton’s video. Fig.33: an example of a compressed-air powered air vane motor from Deprag. Inset shows the vane arrangement and off-centre rotor. Rotational speeds of 100-25,000rpm can be achieved. platforms, which will rise or fall the 20km between the ground and the platform to generate energy via a motorgenerator. Each kilogram mass will store about 54Wh of energy so 500 tonnes of masses will store 25MWh. This project seems to be inactive and we think it’s highly impractical. See the video “StratoSolar Introduction” at: siliconchip.com.au/link/ab0a Two other concepts of gravitational potential energy storage involving the use of large pistons and water were discussed in the SILICON CHIP article on Pumped Hydroelectric Storage in January 2017 (see link above). Storing energy in hydrogen gas Water can be electrolysed to produce hydrogen in a “power to gas” operation, to store excess energy for later use in an electrochemical fuel cell or via combustion. This concept is under investigation, but there appear to be severe economic and efficiency constraints. Japan has already committed to using hydrogen as a trans- Fig.32: a compressed air system powering various equipment in an Amish community, as shown in the linked video. The Amish have religious objections to using electricity. siliconchip.com.au port fuel, and there is a taxpayer-subsidised pilot project in Victoria to convert brown coal to liquid hydrogen for export to Japan for this purpose. The process was developed in the mid-nineteenth century for “producer gas”, and is a coal gasification method. Coal is reacted with oxygen and water at high pressure and temperature to produce, at the end of the reaction process, carbon dioxide and hydrogen. The hydrogen is then separated, liquefied and transported, and the CO2 is disposed of. Some general constraints of the use of hydrogen as a fuel are discussed in the video titled “The Truth about Hydrogen” at siliconchip.com.au/link/ab0b SC A gravity-powered light GravityLight (siliconchip.com.au/link/ab0c) is a gravitypowered LED lighting system design to replace dangerous and expensive kerosene lights in Africa and other undeveloped areas (see Figs.30 & 31). The user attaches the device to a sufficiently strong overhead support and fills a bag with up to 10kg of heavy objects such as rocks. As the bag descends about one metre, it turns a generator, powering one LED light. One raising of the weight bag provides 20 minutes of light, and two satellite lights can also be attached. The light output of the GL02 model is a modest 80mW/15 lumens for the primary light and 15 lumens combined for the two satellite lights. That is sufficient to see inside a typical African dwelling at night and also for reading. You can purchase this light if you want one. Another device intended to provide basic light in undeveloped countries is the solar-powered LuminAID. See the videos “What is GravityLight?” at siliconchip.com.au/link/ab0d and “Gravity Light Review” at siliconchip.com.au/link/ab0e Australia’s electronics magazine April 2020  23