One challenge with energy sources like solar and wind is that their output varies with local environmental conditions, and not necessarily in ways that correspond to energy demand.
Hence, having energy storage capacity makes them easier to integrate into the grid. There are many options: pumped hydroelectric storage, tidal storage, batteries, compressed air, molten salt, and potentially hydrogen.
It is also possible to balance output from different kinds of renewable stations, using biomass, solar, wind, tidal, and other forms of energy to cover one another’s fallow periods.
Pumping heat
A reversible heat-pump promises a cheap way to store renewable energy on the grid
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At the moment, grid managers match demand by generating just enough electricity at any given time. But power from renewable sources, particularly the wind and the sun, is intermittent. Nevertheless, as costs come down and efficiency increases, renewable energy is being used more widely. Solar power is already the cheapest form of electricity in most sunny climes, and in America the Department of Energy wants it to provide 27% of the country’s electricity by 2050, up from less than 1% today. But without efficient grid-scale storage, costly backup generation will be needed to keep the lights on. In many poor countries that means running a smoky diesel generator.
A number of technologies are being developed to store energy on the grid, such as flow batteries which can accumulate energy in liquids and discharge rapidly. Giant flywheels and supercapacitors are also being explored. But Mr Howes thinks his invention can beat them all.
His system is based on a heat pump—a device like an air-conditioner that transfers heat from one place to another. In his case, though, the device is reversible, and when the heat flows back it works like a heat engine, converting thermal energy to mechanical power like a car engine.
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And energy-densities in supercapacitors are improving rapidly, according to Franco Gonzalez, one of the authors of a newly published report from IDTechEx, a market-research firm in Cambridge, Britain. A new device, the hybrid supercapacitor, has recently reached 30-40% of a lithium-ion battery’s energy-density, and the use of electrode materials that have an even bigger ratio of surface area to volume than existing materials (carbon nanotubes and graphene, for example) is likely to increase energy-densities still further in the near future. On top of all this, new types of electrolyte (the liquid between the electrodes) will allow both higher energy- and power-densities. These ionic liquids, as they are known, have the further advantages of being neither toxic nor inflammable—which is not always true of what they are replacing.
Supercapacitors also last longer than batteries. Repeated cycles of charging and discharging degrade a battery’s chemical components. This means that after a few thousand such cycles most lithium-ion batteries are giving up the ghost. According to Mr Gonzalez, supercapacitors can keep going for 1m cycles. Lithium-ion batteries are improving too, of course—but not as fast. IDTechEx therefore expects supercapacitor sales, currently worth less than 3% of those of lithium-ion batteries, to grow to over 10% in the next decade, creating a $6 billion market.
Sisyphus’s train set
A novel idea for storing electricity
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The prototype proved the principle, and now the company has bigger plans. In March it received approval from America’s Bureau of Land Management to lease land to build a track near Pahrump, Nevada. This would run larger trains than those at Tehachapi, and these would carry their rocks in concrete boxes, rather than loose. Once at the top of the track, the boxes would be raised by jacks built into the wagons carrying them, rotated and then lowered back down onto supports on either side of the track, so that they straddled the track above the height of a train, like bridges. Freed of their burdens, the trains would then run back downhill to fetch more loads. When the time came to generate power, the process would be reversed.
The hill ARES has chosen has a gradient of about 8%. The track itself is just under 9km (about 5½ miles) long. The company estimates that its proposed system will be able to store 12.5 MWh of energy, and deliver it back to the grid at a rate of up to 50MW. That is still small compared with pumped storage (the Dinorwig facility in Britain, for example, has a capacity of 10.8GWh and a maximum output of 1.8GW), but ARES’s engineers think it is enough to make commercial sense, at least in principle. And if principle turns to practice, it can be enlarged.
The one-year-old project is in Toronto, Canada—or, rather, just offshore, at the bottom of Lake Ontario. It was designed and built by Hydrostor, a company founded by Cameron Lewis, who developed the technology after working in the oil industry. The plant is operated by Toronto Hydro, a local power utility.
In this case the working fluid is air rather than water. The air is compressed on land and pumped through 2.5km of pipes to a station on the lake bed 55 metres below the surface, a head of water that generates a pressure five atmospheres above normal atmospheric pressure. Here, the air is stored in six spherical bags, known as accumulators, made of a proprietary material. Each accumulator has a capacity of 100 cubic metres.
But adding oomph is the incipient demand for vanadium pentoxide, a compound that is used as an electrolyte in vanadium redox flow batteries (VRBs). These batteries are as big as shipping containers and may be better at storing large amounts of wind and solar energy than stacks of lithium-ion batteries. VRBs house the electrolyte in tanks separate from the battery cell and can be charged and discharged almost inexhaustibly over 20 years (indeed, this gives the electrolyte enough residual value that it can be leased). Some analysts reckon that could make them cost-competitive with their lithium equivalents, and safer and more scalable to boot.
They currently use only 1-2% of the global vanadium supply, but the potential growth is producing a halo effect on vanadium prices. “The market just thinks VRBs are sexy,” Mr Smith says. Although the flow batteries are too bulky for use in electric vehicles, they may be ideal for stationary storage. China’s National Development and Reform Commission, a state planner, has called for lots of 100 megawatt (MW) VRBs to be built to help manage the fluctuations of wind and solar energy. A 200MW one billed as the world’s most powerful battery is being built in northeastern China—it is twice the size of a lithium-ion one installed in Australia with much fanfare by Tesla in December.
Hunt and his collaborators have devised a novel system to complement lithium-ion battery use for energy storage over the long run: Mountain Gravity Energy Storage, or MGES for short. Similar to hydroelectric power, MGES involves storing material at elevation to produce gravitational energy. The energy is recovered when the stored material falls and turns turbines to generate electricity. The group describes its system in a paper published November 6 in Energy.
“Instead of building a dam, we propose building a big sand or gravel reservoir,” explains Hunt. The key to MGES lies in finding two mountaintop sites that have a suitable difference in elevation — 1,000 meters is ideal. “The greater the height difference, the cheaper the technology,” he says. The sites will look similar, with each comprised of a mine-like station to store the sand or gravel, and a filling station directly below it. Valves release the material into waiting vessels, which are then transported via cranes and motor-run cables to the upper site. There, the sand or gravel is stored — for weeks, months, or even years — until it’s ready to be used. When the material is moved back down the mountain, that stored gravitational energy is released and converted into electrical energy.
Supplying clean power is easier than storing it
Cutting emissions relies on energy-storage technology coming of age
It sounds simple: lift heavy blocks with a crane, then capture the power generated from dropping them. This is not an experiment designed by a ten-year-old, but the premise of Energy Vault, which has raised $110m from SoftBank, a big Japanese tech investor. The idea has competition. A cluster of billionaires including Bill Gates, Jack Ma, Ray Dalio and SoftBank’s Masayoshi Son are backing other schemes to capture power. A firm incubated at Alphabet, Google’s parent company, wants to store electricity in molten salt. Such plans hint at one of the power business’s hardest tasks. Generating clean power is now relatively straightforward. Storing it is far trickier.
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One answer is to store power in batteries, which promise to gather clean electricity when the sun and wind produce more than is required and dispatch it later, as it is needed. In 2018 some 3.5 gigawatts of storage was installed, about twice the amount in 2017, according to Bloombergnef, an energy data firm. Total investment in storage this year may reach $5.3bn, it estimates. As this grows it could drive an extraordinary expansion. However at present only about 1% of renewable energy is complemented by storage, reckons Morgan Stanley, a bank. There are still plenty of hurdles to clear.
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As greater demand led to greater manufacturing scale, the cost of batteries dropped by 85% from 2010 to 2018, according to Bloombergnef. That makes batteries cheap enough not only to propel mass-market electric cars but for use in the power system, too.
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Alternatives include flow batteries, that use electrolytes in tanks of chemical solution, as well as mechanical means such as Energy Vault’s falling blocks. Hydrogen can also be made using clean power and turned back into electricity in gas-fired power plants or fuel cells. In the future liquefied gases might provide a solution. Unlike solar panels, which have become standardised, different batteries are likely to serve different purposes on a grid. “All batteries are like humans, equally flawed in some specific way,” says Mateo Jaramillo, who led storage development at Tesla, an electric carmaker.
Energy storage capacity and cost
Role of Long-Duration Energy Storage in Variable Renewable Electricity Systems
Laws in several U.S. states now require the adoption of zero-carbon electricity systems based primarily on renewable technologies, such as wind and solar. Long-term, large-capacity energy storage may ease reliability and affordability challenges of systems based on these naturally variable generation resources. Long-duration storage technologies (10 h or greater) have very different cost structures compared with Li-ion battery storage. Using a multi-decadal weather dataset, our results reveal that long-duration storage can fill unique roles, like seasonal and even multi-year storage, making it valuable to least-cost electricity systems. Indeed, we find that variable renewable power systems are much more sensitive to reductions in long-duration storage costs than to equal reductions in battery costs. Long-term modeling horizons, typically not used by utilities and regulators, are necessary to capture the role and value of long-term storage, informing technology investments and policy.
Projecting the Competition between Energy-Storage Technologies in the Electricity Sector
Energy scenarios in line with the Paris Agreement suggest a rapid growth of renewable energy capacity and, by extension, the need for increasing flexibility in electricity systems. Storage systems are considered a key solution to that end. As many storage technologies are emerging, a clear understanding of cost-reduction dynamics in the field is required. To date, various technologies still compete for market shares in different stationary storage applications, some of them, such as lithium-ion batteries, profiting from innovation spillovers from the electric-mobility sector. In this context, we project technology competition for electricity-storage applications until 2030, derive cost benchmarks for new concepts, and discuss potential policy interventions. This novel methodology can also be applied for technology-cost projections more generally, adding to the literature on experience curves and technology assessment.
Electricity Storage and the Renewable Energy Transition
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