The aluminium-ion battery

 The lithium price has soared as EVs have gained market share and demand for stationary storage (to "firm" renewable) has grown.  So the search for alternatives is heating up.   BYD is rumoured to be putting sodium-ion batteries in their cheaper cars, starting in Q2 next year.

But aluminium-ion batteries, which may solve the lithium problem, are close to commercialisation.  They have several advantages:

• They don't require rare-earth metals
• Can exchange up to 3 electrons per ion, compared to lithium's 1, meaning a higher energy density and volumetric capacity (about 4 times lithium's)
• Much cheaper, especially at current lithium prices
• Safer, because it's less likely to catch fire
• 70 times faster charging rate---the limiting factor isn't how much power the battery can take, but the cable charging the battery
• the heating induced by fast-charging isn't an issue, which means we won't need complicated cooling and battery management systems. 
• Aluminium-ion performance is now as good as lithium-ion with 300 Wh (watt-hours)/kilogram achieved in the laboratory.
• Aluminium is the most abundant metal in the Earth's crust, far more abundant than lithium

Of course, all is not plain sailing.  But, as so often with technological advances, necessity will drive innovation. 

Here's a nice video from "Undecided" by Matt Ferrell explaining about this Ozzie battery advance, with a tag-on short section on similar research at MIT.

Sodium-ion even cheaper than I thought

I wrote a piece a month ago about CATL's new sodium-ion battery.  The video I link to provided more information, suggesting costs are even lower than I said.

The cost at cell level will be $19/kWh vs lithium-ion phosphate (LFP) of $55-$60/kWh.  CATL expects $10/kWh in a couple of years.  $45/kWh at pack level, less than half the cost of LFP.   Production can be carried out on existing assembly lines, so they don't have to rebuild the entire factory.  Any factory making LFP could pivot to sodium-ion at minimal cost and time.   

They will retain 85% after 3.6 million miles.  I said 80% in my earlier piece; so this is even better, meaning that after 50 years, 75% of the battery capacity will remain.  Their life will be 3-6 times longer than the best LFP packs.  Energy density has dramatically improved.  A year ago it was 120-140 Wh/kg, too heavy for EVs. The new energy density is 175 Wh/kg, better than BYD's current blade battery (160 Wh/kg).  They can be charged and used from -40 Celsius to +70 C.  And they use abundant materials: sodium, aluminium and carbon.  They are maintenance free.  They can be safely transported at zero charge, unlike lithium batteries.  CATL has also developed a pack made up of both sodium-ion and lithium-ion cells, combining the best qualities of both. 

Years ago, the rule of thumb was that if battery pack costs fell to $100/kWh, that would make EVs cost the same up-front as ICEVs (petrol vehicles).   (EVs are already much cheaper to run)  We have shot way past that point.  The introduction of sodium-ion batteries means that ICEVs will no longer be cost-effective, and production will cease.  

But this will also transform the grid.  The cost of storage has more than halved, and will halve again.   Solar is already the cheapest electricity for everywhere except high latitudes, and now it can be combined with enough dirt-cheap storage to provide base-load power.   That probably means 8 hours of storage, but storage will be so cheap that even 12 hours will be perfectly feasible and economic.  High latitudes will still need long-term storage, but when your EV dies, the batteries will still have another 50 years plus of life in them, and then they can be shipped to high latitudes to provide completely free long-term storage.

This spells the end of the fossil fuel economy.   Except for air transport and cement making, everything we now do with coal, oil or gas will be doable with cheap electricity from solar plus sodium-ion storage.

Even in the USA, even with 25% tariffs on imported batteries, the plunge in storage costs means that the EV and storage revolutions will continue.

CATL reveals sodium-ion battery with 3.6 million mile lifespan

•  CATL's Naxtra sodium-ion costs are around $19/kWh at the cell level. LFP (lithium iron phosphate) cells are currently $55-$60/kWh (when bought at massive scale), so roughly 65% cheaper at the moment.  CATL thinks these sodium-ion cells could fall further in cost, possibly down to $10/kWh in 2 or 3 years.
• They are roughly half the cost of the LFP packs at pack level
• The Naxtra battery is capable of around 3.6 million miles (6 million kilometres) of driving before the capacity drops to 85%, 3 to 6 times as long as what we get from the very best LFP batteries today.
• Energy density.  Sodium-ion historically has struggled here. A couple of years ago, sodium-ion packs had an energy density of 120-140 Wh/kg.  The Naxtra pack now has an energy density of 175 Wh/kg, which means the Naxtra has now overtaken BYD's current Blade battery's energy density of 160 to 165.
• It performs much better in winter and summer, in a range between -40C to +70C, and can be charged at full speed even at -20C.
• Doesn't catch alight if pierced or in an accident.
• The Naxtra materials are abundant, cheap and not strategic: sodium (salt), aluminium, carbon.
• Sodium cells can be built on current LFP assembly lines.

As I pointed out in previous pieces on CATL's sodium-ion battery, the combination of low price and very long life make battery storage very, very cheap.  

Solar, with an LCOE* at $42.60 /MWh (Our World in Data figures) is already the second-cheapest source of electricity globally, and the cheapest in the sunbelt, and sodium-ion batteries will allow 12 hours of storage at under $1/MWh.  That doesn't include the cost of charging the batteries, but right now, wherever solar is plentiful, midday output of solar is often curtailed or sold at zero cost, because of excess supply.   The cost of solar quoted above includes income losses from curtailment/zero wholesale prices, so solar will get cheaper as 12 hour storage becomes the norm, and curtailment is no longer necessary. 

What does nuclear cost, i.e., what is nuclear's LCOE?  $155/MWh, according to Our World in Data.  That's the average global price; it's much more expensive in Europe and the USA.  And coal?  Again, the average global LCOE according to OWID,  is $110/MWh.  So, you can have solar, for $40/MWh, which is falling by 10%-plus every year, or coal, which is three times as expensive and not getting any cheaper, or nuclear which is four times as expensive, and getting more expensive (except SMRs**.  Maybe.)  

Wind will also have a rôle in our future grids, because it is seasonally complementary to solar, and is 20% cheaper than solar in high latitudes.  

Sorry, guys, fossil fuels don't stand a chance, except perhaps, in high latitudes.   And even there, HVDC*** power lines can import electricity from solar farms in low latitudes, more cheaply than nuclear or coal.

*Levelised cost of electricity.

**Small modular reactors

***High voltage direct current

Two optimistic trends

 It's easy enough to be deeply pessimistic about climate change.  Emissions continue to rise, and while these days one only occasionally encounters unhinged denialists, the acceptance that something needs to be done hasn't yet translated into effective action.  There is lots of "committed to" but little real action.  (Buying scammy carbon offsets isn't real action.  Real action is actually cutting emissions.)  Lots of blah-blah-blah, as Thunberg says. 

But there are two trends which really are signs that things are shifting.

The first is that over the last 14 years, solar has risen from 1% of new generation capacity installed to 51%, wind from 10% to 25%.  Over the same period, coal fell from 46% to 4%.  And this is the gross addition to the stock of coal-powered generation, that is, it does not include shuttering old coal power stations.  The long-term trend lines in the graphic are obvious, with solar moving from an insignificant sliver to dominance, and fossil fuels falling from 2/3rds to 1/7th.

What this means is that emissions from electricity generation, which contributes 30% of CO2 emissions globally, have prolly peaked.  To avoid a rise of 1.5 degrees, electricity emissions would have to fall by 10% a year over the next 10 years, and that isn't going to happen.  Nevertheless, if it happens over the next 20 years (highly plausible), this alone will cut total emissions by ±1.8% per annum.  

The second trend is the exponential growth in EV sales.  Land transport (which however includes diesel-powered freight transport on road and by rail) contributes another ±20% to CO2 emissions.  EV sales are growing by 60% per annum.  That means they could rise 6.5 times between now and 2026, taking them close to 100% of total car and light truck sales.   If car and light trucks last 15 years, that means that from 2026 onwards, emissions from this sector will be falling by 6% a year.

What's more, the car and light truck fleet will help stabilise the grid, either passively, by charging only when grid supply is high, or actively, by contributing some power back to the grid when there's a shortage.

The surge in the price of lithium is a threat, but two new battery technologies are already making their way out of laboratories into factories:  sodium-ion and aluminium-ion.  Battery costs will resume their decline.

N.B. Log scale!

If the transition to a nearly 100% renewable grid and a 100% electric land transport fleet takes 20 years, emissions will fall by 3.5% a year.  And that is almost acceptable.  It's certainly far better than it's been.  We'll miss the target of limiting the global increase in temperatures to 1.5 degrees, but assuming temperatures continue to rise by 0.2 degrees per decade, but halving as emissions halve, we'll avoid 2 degrees.  

New cheap battery of aluminium, sulphur and salt

 From New Atlas

The three main ingredients in the new battery, from left: aluminum, sulfur and salt

Rebecca Miller

Engineers at MIT have developed a new battery design using common materials – aluminum, sulfur and salt. Not only is the battery low-cost, but it’s resistant to fire and failures, and can be charged very fast, which could make it useful for powering a home or charging electric vehicles.

Lithium-ion batteries have dominated the field for the last few decades, thanks to their reliability and high energy density. However, lithium is becoming scarcer and more expensive, and the cells can be hazardous, exploding or bursting into flames if damaged or improperly used. Cheaper, safer alternatives are needed, especially as the world transitions towards renewable energy and electric vehicles.

So the MIT team set out to design a new type of battery out of readily available, inexpensive materials. After a search and some trial and error, they settled on aluminum for one electrode and sulfur for the other, topped off with an electrolyte of molten chloro-aluminate salt. Not only are all of these ingredients cheap and common, but they’re not flammable, so there’s no risk of fire or explosion.

In tests, the team demonstrated that the new battery cells can withstand hundreds of charge cycles, and charge very quickly – in some experiments, less than a minute. The cells would cost just one sixth of the price of a similar-sized lithium-ion cell.

They can not only operate at high temperatures of up to 200 °C (392 °F) but they actually work better when hotter – at 110 °C (230 °F), the batteries charged 25 times faster than they did at 25 °C (77 °F). Importantly, the researchers say the battery doesn’t need any external energy to reach this elevated temperature – its usual cycle of charging and discharging is enough to keep it that warm.

Although the type of salt in the electrolyte was chosen because it has a low melting point, it coincidentally has another benefit – it naturally prevents the formation of dendrites. These metal tendrils, which gradually grow between the two electrodes until they cause a short circuit, are a major hurdle for batteries, particularly lithium-ion cells.

The team says that this battery design would be best suited to the scale of a few dozen kilowatt-hours, like powering an individual home from renewable sources. They could also be useful as charging stations for electric vehicles, thanks to their rapid charging. Other types of batteries, such as a recent design using molten salt electrolyte and aluminum and nickel electrodes, could work better at grid scale.

The patents for the aluminum-sulfur batteries have been licensed to a spinoff company called Avanti, co-founded by one of the authors of the study describing the design. The first order of business is to build it at scale, and run it through stress tests.




The research was published in the journal Nature.

Source: MIT

Aluminium, salt water and coffee produces ..... hydrogen?????

From Science Daily

MIT engineers have found that when the aluminum in soda cans is exposed in its pure form and mixed with seawater, the solution bubbles up and naturally produces hydrogen -- a gas that can be subsequently used to power an engine or fuel cell without generating carbon emissions. What's more, this simple reaction can be sped up by adding a common stimulant: caffeine.

In a study appearing today in the journal Cell Reports Physical Science, the researchers show they can produce hydrogen gas by dropping pretreated, pebble-sized aluminum pellets into a beaker of filtered seawater. The aluminum is pretreated with a rare-metal alloy that effectively scrubs aluminum into a pure form that can react with seawater to generate hydrogen. The salt ions in the seawater can in turn attract and recover the alloy, which can be reused to generate more hydrogen, in a sustainable cycle.

The team found that this reaction between aluminum and seawater successfully produces hydrogen gas, though slowly. On a lark, they tossed into the mix some coffee grounds and found, to their surprise, that the reaction picked up its pace.

In the end, the team discovered that a low concentration of imidazole -- an active ingredient in caffeine -- is enough to significantly speed up the reaction, producing the same amount of hydrogen in just five minutes, compared to two hours without the added stimulant.

The researchers are developing a small reactor that could run on a marine vessel or underwater vehicle. The vessel would hold a supply of aluminum pellets (recycled from old soda cans and other aluminum products), along with a small amount of gallium-indium and caffeine. These ingredients could be periodically funneled into the reactor, along with some of the surrounding seawater, to produce hydrogen on demand. The hydrogen could then fuel an onboard engine to drive a motor or generate electricity to power the ship.

"This is very interesting for maritime applications like boats or underwater vehicles because you wouldn't have to carry around seawater -- it's readily available," says study lead author Aly Kombargi, a PhD student in MIT's Department of Mechanical Engineering. "We also don't have to carry a tank of hydrogen. Instead, we would transport aluminum as the 'fuel,' and just add water to produce the hydrogen that we need."

The study's co-authors include Enoch Ellis, an undergraduate in chemical engineering; Peter Godart PhD '21, who has founded a company to recycle aluminum as a source of hydrogen fuel; and Douglas Hart, MIT professor of mechanical engineering.

The MIT team, led by Hart, is developing efficient and sustainable methods to produce hydrogen gas, which is seen as a "green" energy source that could power engines and fuel cells without generating climate-warming emissions.

One drawback to fueling vehicles with hydrogen is that some designs would require the gas to be carried onboard like traditional gasoline in a tank -- a risky setup, given hydrogen's volatile potential. Hart and his team have instead looked for ways to power vehicles with hydrogen without having to constantly transport the gas itself.

They found a possible workaround in aluminum -- a naturally abundant and stable material that, when in contact with water, undergoes a straightforward chemical reaction that generates hydrogen and heat.

The reaction, however, comes with a sort of Catch-22: While aluminum can generate hydrogen when it mixes with water, it can only do so in a pure, exposed state. The instant aluminum meets with oxygen, such as in air, the surface immediately forms a thin, shield-like layer of oxide that prevents further reactions. This barrier is the reason hydrogen doesn't immediately bubble up when you drop a soda can in water.

In previous work, using fresh water, the team found they could pierce aluminum's shield and keep the reaction with water going by pretreating the aluminum with a small amount of rare metal alloy made from a specific concentration of gallium and indium. The alloy serves as an "activator," scrubbing away any oxide buildup and creating a pure aluminum surface that is free to react with water. When they ran the reaction in fresh, de-ionized water, they found that one pretreated pellet of aluminum produced 400 milliliters of hydrogen in just five minutes. They estimate that just 1 gram of pellets would generate 1.3 liters of hydrogen in the same amount of time.

But to further scale up the system would require a significant supply of gallium indium, which is relatively expensive and rare.

"For this idea to be cost-effective and sustainable, we had to work on recovering this alloy postreaction," Kombargi says.

In the team's new work, they found they could retrieve and reuse gallium indium using a solution of ions. The ions -- atoms or molecules with an electrical charge -- protect the metal alloy from reacting with water and help it to precipitate into a form that can be scooped out and reused.

"Lucky for us, seawater is an ionic solution that is very cheap and available," says Kombargi, who tested the idea with seawater from a nearby beach. "I literally went to Revere Beach with a friend and we grabbed our bottles and filled them, and then I just filtered out algae and sand, added aluminum to it, and it worked with the same consistent results."

He found that hydrogen indeed bubbled up when he added aluminum to a beaker of filtered seawater. And he was able to scoop out the gallium indium afterward. But the reaction happened much more slowly than it did in fresh water. It turns out that the ions in seawater act to shield gallium indium, such that it can coalesce and be recovered after the reaction. But the ions have a similar effect on aluminum, building up a barrier that slows its reaction with water.

As they looked for ways to speed up the reaction in seawater, the researchers tried out various and unconventional ingredients.

"We were just playing around with things in the kitchen, and found that when we added coffee grounds into seawater and dropped aluminum pellets in, the reaction was quite fast compared to just seawater," Kombargi says.

To see what might explain the speedup, the team reached out to colleagues in MIT's chemistry department, who suggested they try imidazole -- an active ingredient in caffeine, which happens to have a molecular structure that can pierce through aluminum (allowing the material to continue reacting with water), while leaving gallium indium's ionic shield intact.

"That was our big win," Kombargi says. "We had everything we wanted: recovering the gallium indium, plus the fast and efficient reaction."

The researchers believe they have the essential ingredients to run a sustainable hydrogen reactor. They plan to test it first in marine and underwater vehicles. They've calculated that such a reactor, holding about 40 pounds of aluminum pellets, could power a small underwater glider for about 30 days by pumping in surrounding seawater and generating hydrogen to power a motor.

"We're showing a new way to produce hydrogen fuel, without carrying hydrogen but carrying aluminum as the 'fuel,'" Kombargi says. "The next part is to figure out how to use this for trucks, trains, and maybe airplanes. Perhaps, instead of having to carry water as well, we could extract water from the ambient humidity to produce hydrogen. That's down the line."

All very nice.  But the key question is this:  how much energy does it take to make the aluminium pebbles compared to what is released when the hydrogen in burned or used in a fuel cell?  In other words, what is its "round-trip efficiency"?  Producing hydrogen via electrolysis has a round-trip efficiency of 40%.  Lithium-ion batteries have a round-trip efficiency of 90% or higher.  Using old coke cans (where the aluminium has already been made) may mean it's better than 40%, in which case it's a goer.  

The article doesn't say what happens to the oxygen presumably produced as part of this price.  I'm presuming it combines with the aluminium?  If not, hydrogen and oxygen is a very explosive mix.

Clean disruption

This remarkable video by Tony Seba shows how dramatically the energy and transportation landscapes are going to shift over the next 5 to 10 years.  I don't usually post videos, because they take a lot of time to watch, more time than the equivalent text.  But I make an exception with this one.  Almost every minute of it is worth watching, and its conclusions are very significant for our society.  Watch the whole thing, then if you want to go back and see individual sections, refer to the notes below the video.

Some key projections and points, to whet your appetite, with my comments in square brackets:

• The decline in battery storage costs is accelerating [this was before Tesla announced that costs of the Powerwall had halved in one year]
• By 2020, one day's storage would cost $1 per day, less than a cup of coffee.
• Large scale storage will completely displace gas peaking power plants, soon.  Already in some locales they are cost effective even at $350/kW storage, but by 2020 storage will be $200/kW and by 2024 $100/kW.
• 32% of generating assets are used for just 6% of the time.  Batteries will be much cheaper [but we might need more because of weather related variability]
• The plunge in battery prices means that by 2017 an "average" EV will cost $35,000, by 2020 $30,000, by 2022 $22,000.
• EVs have only 18 moving parts compared to an ICE (internal combustion engine) vehicle with 2000+.  They are 5 times more efficient than an ICE and 10 times cheaper to run.
• By 2025 100% of all new vehicle sales will be electric
• By 2020 all EVs will have self-driving technology.  Already self-driving is feasible for 90% of the time.
• At some point most people won't own a car but will share in a self-driving, automated EV fleet [like those robo-taxis SF authors used to talk about] 
• Installed base of solar has doubled every 2 years since at least 1990.  We are just 7 doublings away from solar providing not just 100% of all electricity but of all energy.
• The cost of rooftop solar will soon fall below the cost of transmission which means no alternative power source will be economically efficient, not nuclear, not fusion, not hydro, not coal nor gas.
• But we will still need utility-scale power for factories, data centres, smelting metals [and blocks of flats and offices and EVs] 
• Solar at 5.5 cents/kWh is the equivalent of oil at $10 barrel and gas at $5/MMBtu [recent solar contracts are now 40% to 50% below 5.5 cents/kWh]

Notes:

At 2;35, how ATT which invented many of the key mobile phone technologies asked McKinsey and Co how many cell phones would there be in the US in 15 years (2000).  McKinsey's answer: 900,000.  Actual number 109 million.

At 4:10, how the internet was never going to be anything important.  "The internet will catastrophically collapse in 1996" (Robert Melcalfe 1995) "There is no reason anyone would want a computer in their home" (Ken Olsen, 1977)  "I do not believe the introduction of motor cars will ever affect the riding of horses" (Scott-Montague, 1903)  Seba points out that it is the experts or insiders who will deny that disruptive opportunities and risks.

At 4:44, why do smart people at smart organisations consistently fail to anticipate or lead market disruptions?

Exponential technologies.  At 6:56, Moore's Law: for the same dollar we get twice the  computing power every 2 years.  If you double every 2 years, over a decade that's a 1000 fold improvement, over 2 decades it's a 1 million fold improvement and over 3 decades a 1 billion fold improvement.  Kryder's Law: hard disk $ cost down 50% every 18 months.  Hendy's Law: digital imaging (pixels per $) down 59% every year.  Butter's Law of photonics (network capacity): the $ cost of transmitting one bit falls by 50% every 9 months,  The convergence of all 4 led to the smartphone and internet revolutions.

Key exponential technologies, at 8:56.  Sensors (9:52) market up 1000 times, costs down 1000 times, power down 1000 times, physical size down 1000 times in just 7 years.

Energy storage, at 11:38.  From 1995 to 2010, lithium-ion batteries fell by 14% per year.  Then 2 new industries came into lithium-ion: transportation and energy storage.  This accelerated the decline in the cost curve, from 2010 to 2014, cost declines accelerated to 16% per annum.  (Note that Seba's presentation was made in March 2016: since 2014 battery costs have halved)  At 16% by 2020 storage will cost $200/kW, by 2024 $100/kW.  Tesla's Powerwall 1 and Powerpack 1 battery costs were already (at the time of the presentation) below Seba's projected cost curve.  Now of course they are much lower than his curve.  Tesla got a billion dollars in orders on the announcement, and as a result the gigafactory, already planned to double world output of lithium-ion batteries, will be expanded in size by 48%.  at 15:58, Tesla isn't the only one doing this: BYD, Foxconn, LG Chem, Samsung SDI, TDK, Apple, Bosch, VW, etc.

Business model innovation, at 16:46.  Storage as a service 17:25.  By 2020, one day's storage (which by the way is more than we need) will cost about $1 (18:38) .  Half the cost of a coffee.  At 20:23, large scale storage likely to replace peaking power plants : billions of dollar in power plants used for just a few hours a year.  According to ConEd, 32% of generating assets used for just 6% of the time,

EVs, 22:10.  Tesla Model S: the best car ever made, not just the best EV.  The best selling large luxury car in America.   At 23:26, The EV is 5 times more efficient than the internal combustion engine (ICE), is 10 times cheaper to charge (24:08), has 2000+ moving parts compared to 18 in an EV (24:47), which makes EVs 10 to 100 times cheaper to maintain, and has much more torque (25:35).  When will EVs replace ICEs (26:56)?  $35,000 EV by 2017,  $30,000 by 2020, $22K by 2022.  By 2025 all new vehicles will be electric.  And note: it won't just be the incumbent companies who do it--Foxconn is to make an EV costing $15,000 (30:58)

Autonomous vehicles (32:52).  Tesla capable of self-driving 90% of the time (33:08).  What an autonomous car sees (34:02) Cost of LIDAR sensors has fallen  from $70,000 in 2012 to $10K in 2013 to $1K in 2014 to $250 in 2016 and a projected cost of $90 in a year or so (34:42)  World's first 1 teraflops computer in 2000 cost $46 million (36:13).  Would now cost $59 (36:43)  Is the market ready?  Yes, in Brazil, China, India, where traffic congestion is horrendous, self-driving cars are very desirable (37:25)  In San Francisco, 50% of Uber rides are car pools (38:43)  Cars are parked 96% of the time (39:09).  The convergence of EVs, self-driving, and shared cars will mean the end of individual car ownership (41:17)

The solar disruption (43:28) Installed base of solar has doubled every 2 years since at least 1990 (44:00)  7 more doublings, or 14 more years until solar can provide 100% of  total energy, not just electricity (44:25)  Grid Parity or God Parity (46:03)?  80% of global market will reach grid parity by 2017 (46:15)  Colour TV technology adoption curve (46:47) God Parity (48:18) where cost of rooftop solar is cheaper than costs of transmission, implying that no matter what alternative technology is used, even if it has zero cost, rooftop solar would be cheaper.  And that will happen by 2020 (49:16)  But we will still need utility scale solar, because blocks of flats, office blocks, aluminium/steel smelting (50:29)  Solar at 5.5 cents/kWh is the equivalent of oil at $10 barrel and gas at $5/MMBtu (51:00)

Rechargeable aluminium for seasonal storage

 A fascinating idea. 

From New Atlas

Aluminum has an energy density more than 50 times higher than lithium ion, if you treat it as an energy storage medium in a redox cycle battery. Swiss scientists are developing the technology as a renewable energy stash for the European winter.

The problem is simple enough: as countries worldwide plan their moves toward zero-emissions energy, they need to deal with the intermittent nature of cheap renewable energy. On a daily basis, solar harvests most of its energy in the middle of the day, and this necessitates some kind of short-term storage solution that can park that energy in some form of battery, then release it again in the evening when everyone gets home and starts running TVs and dishwashers. These kinds of big battery projects are already installed in many areas and proving their worth.

But intermittency is a much bigger issue on a seasonal level. The further you move from the equator, the less Sun you get in the winter months. Parts of Scandinavia famously get no Sun at all for months on end – resulting in some pretty epic springtime parties, I'm told – but a much broader area is going to find itself very short on solar, every year, right when everyone's starting to crank up their heaters. The zero-carbon world needs a way to store absolutely massive amounts of excess renewable energy generated in the warmer months, then release it through the long winters. And it'll need to be affordable, or else it's not going to happen.

Researchers from Switzerland's SPF Institute for Solar Technology have been studying aluminum redox cycles for many years now, and with funding from the EU's Horizon Europe program and the Swiss government, they've just kicked off a research project called Reveal, drawing in nine different partners from seven European countries, to develop what looks like a very promising idea.

As a 2020 report from the SPF team states, a single, one cubic meter (35.3 cu ft) block of aluminum can chemically store a remarkable amount of energy – some 23.5 megawatt-hours, more than 50 times what a good lithium-ion setup can do, or roughly enough to power the average US home for 2.2 years, on 2020 figures. That's by volume – going by weight, aluminum holds a specific energy of 8.7 kWh per kilogram, or about 33 times more than the batteries Tesla uses in its Model 3.

Big fat blocks like that aren't exactly practical to work with, though, so the Reveal team proposes using 1-mm (0.04 in)-diameter balls of aluminum instead. Naturally, you lose some volumetric density here, but you're still coming out over 15 MWh per cubic meter.

Aluminum holds a phenomenal amount of energy compared to batteries or hydrogen

SPF Institute for Solar Technology

Getting that energy in and out is, of course, a lot more involved. During the "charging process," excess renewable energy would be used to convert aluminum oxide, or aluminum hydroxide, into pure, elemental aluminum. This is an industrial electrolysis process, requiring temperatures around 800 °C (1,472 °F), as well as novel inert electrodes, if you want to avoid the carbon dioxide emissions that accompany today's conventional aluminum smelting processes.

The team estimates it'll be possible to "charge" an aluminum redox system like this at an efficiency around 65%. All the raw materials here are relatively cheap and abundant, some of them indeed being scrap, with the added benefits of being very simple to store and transport. Yes, aluminum oxidizes on contact with ambient air, but it's only a surface layer, less than half a nanometer thick, representing a chemical energy loss of "far less than 1%" when those tiny 1-mm balls are stored in air.

To discharge the aluminum, you simply convert it back again. This can be done at low temperatures, using aluminum-water reactions at less than 100 °C (212 °F), generating aluminum hydroxide, along with pure hydrogen, which can be run straight into a PEM fuel cell stack for conversion to electricity. The process and the fuel cell also generate heat, which can be recovered at temperatures relevant for space heating or domestic hot water.

Inputs and idealized outputs of the low-temperature aluminum-to-hydrogen energy release process, assuming a fuel cell efficiency of 50%

SPF Institute for Solar Technology

There's also a higher-temperature process, running at over 200 °C (392 °F), which reacts the aluminum with steam to generate aluminum oxide, hydrogen and much higher levels of heat, more relevant for industrial applications.

In the Reveal model, the charging process would be done at central smelting depots, and the "charged-up" aluminum would be trucked out in bulk to be "discharged" on-site at apartment buildings, industrial facilities, and even individual homes, since the equipment needed is relatively simple and low-maintenance – well, apart from the fact that the aluminum-to-hydrogen conversion system doesn't exactly exist yet at this point.

Once it's out of juice, the aluminum oxides and hydroxides would be sent back to the depot for "recharging." Ideally, the Reveal team says, this aluminum will be cycled back and forth in this process indefinitely, so there won't be any ongoing raw material costs for a given system.