With renewables being in large part weather-dependent, the issue of energy storage is slowly moving to the center stage of the incipient net-zero economy. Lithium ion batteries are kinda good, but expensive, take up a lot of space per a single unit of stored energy, and rely on rare minerals. Hydrogen storage is gaining a lot of attention, but there are other cutting-edge storage technologies that claim to square this circle and provide reliable long-duration solutions. Just now, I've read about liquid air storage that sounds interesting in the words of the company that is in charge of this project but also sparks doubt about whether this moonshot technology is really going to do the trick. Is it really more competitive than hydrogen storage (including in terms of occupied physical volume, capital and operational costs, and net carbon footprint – after all, it's refrigerated under extreme temperatures)? And more generally, what are the most viable options for long-duration storage of large amounts of energy in the context of ongoing decarbonization? If there is some overview of such methods that either exist or are under implementation – like liquid air storage facilities – that you can share with me, it would be nice
With renewables being in large part weather-dependent, the issue of energy storage is slowly moving to the center stage of the incipient net-zero economy.
Lithium ion batteries are kinda good, but expensive, take up a lot of space per a single unit of stored energy, and rely on rare minerals.
Rare? Not necessarily. The only essential mineral in lithium ion batteries is lithium. Some chemistries like nickel cobalt aluminum or nickel manganese cobalt require cobalt, but some others like lithium iron phosphate don't. So the only material you absolutely need is lithium (apart from the trivial plentiful ones that vary based on the exact type of the lithium ion battery chemistry).
Lithium need is 1.6 kg / 10 kWh of battery energy amount. This means global lithium reserves of 14 million tons can store 87.5 TWh. If we assume world population stabilizes at 10 billion, it's 8.75 kWh per capita.
My personal electricity use outside of summer is 6.6 kWh per day (in summer the extra happens only during sunshine hours because it's air conditioning use). However, about half of that is during the day so to store electricity for use during night requires only 3.3 kWh if I happened to live in an area where solar energy is feasible both in summer and in winter. For that storage need, 8.75 kWh per capita is enough.
Also reserves mean "what can be extracted at current prices". Lithium resources (what can be extracted if the price was higher) are 73 million tonnes, so with these 45.6 kWh per capita storage is feasible.
Also I don't necessarily agree that lithium ion batteries are expensive. If one kWh costs 100 EUR and lasts for 25 years, even taking into account capital cost that's 6.4 euros per year per kWh. If you cycle a battery 365 times per year, that's 17.5 euros of battery cost per MWh of energy. That's cheaper than solar power for example! So to store solar power for use during night, more than half of the cost is the actual solar power production cost and less than half of the cost is the storage cost. Also do remember that half of energy is used during daytime, so the less-than-doubled cost is only a problem during nighttime. So that's not even 50% energy cost increase.
Also don't underestimate the intelligence of chemists to invent new battery chemistries. There are at least sodium ion, potassium ion, iron air and sodium sulfur batteries in development.
Hydrogen storage is gaining a lot of attention
Hydrogen is useful in areas where solar power is not available during the winter. Then wind power must be used, but calm periods can last for multiple weeks. The main problems of hydrogen are its terribly low energy efficiency and the high cost of the equipment to both produce it (electrolyzers), transport it (pipelines), store it (lined underground caves), and convert it back to electricity (combined cycle power plants). If you add all these costs together and also take into account the low 35-40% efficiency of electricity-to-hydrogen-back-to-electricity, you'll see that hydrogen makes sense only in cases where you need to achieve not a single day but several weeks or months of storage duration.
I presume that in areas with plentiful solar power, hydrogen is used only to allow fertilizer production (ammonia is needed for that, and hydrogen is needed for ammonia) and carbon-free steelmaking. Then the hydrogen is a way to use and store excess electricity as hydrogen, but the hydrogen will probably never be converted back to electricity but rather used in steel and fertilizer industries. The hydrogen production is timed so that it is produced only when electricity is very cheap.
Just now, I've read about liquid air storage that sounds interesting in the words of the company that is in charge of this project but also sparks doubt about whether this moonshot technology is really going to do the trick
Liquid air energy storage will lose the battle with hydrogen and batteries. While it has slightly better 60% round trip efficiency than hydrogen, for example the storage part costs around 50 euros per kWh (and that only includes the storage part, not the liquid air creation and consumption parts). It can't compete with lithium ion batteries because of the low 60% energy efficiency. Lithium ion is better for short-term storage, although it is slightly more expensive in storage, because it has better 99% efficiency. It also can't compete with hydrogen because hydrogen storage costs around 0.15 euros per kWh, far less than 50 euros per kWh of liquid air storage.
Is it really more competitive than hydrogen storage (including in terms of occupied physical volume, capital and operational costs, and net carbon footprint – after all, it's refrigerated under extreme temperatures)?
It's a poor solution. Too expensive storage costs for long-term energy storage. Too poor energy efficiency for short-term energy storage.
And more generally, what are the most viable options for long-duration storage of large amounts of energy in the context of ongoing decarbonization?
Several technologies excel there.
One is hydropower, both in natural and pumped forms. It's not feasible where the geography doesn't permit it, but if it's feasible it's one of the cheapest options.
Another is load-side management in hydrogen production. The hydrogen probably can't competitively be turned back to electricity, but it doesn't matter: if you eliminate 1 kilowatt of consumption, it's equivalent for the grid balance than adding 1 kilowatt of production. So hydrogen essentially allows good uses for lots of very cheap electricity that wouldn't otherwise have any uses. The hydrogen will be stored for later use in industry.
Third may perhaps be new battery chemistries like iron-air batteries developed by Form Energy. Not all of the specs like efficiency have been released, but apparently it's 2000 euros per kilowatt, 20 euros per kilowatt hour, 100 hours. The efficiency is a question mark, though. If it's below that of liquid air energy storage, it may be it can compete neither with hydrogen nor with lithium ion batteries. But based on the initial information we have, they seem very promising.
Fourth is biomass. This is mainly in areas where heating is needed during the winter. The bulk of heating will likely be produced using heat pumps, but if heat is needed at times when electricity is expensive, biomass burning is an attractive option. Biomass can only be "charged" with direct sunlight and not with electricity, so although you can convert solar power into biomass, you can't convert wind power into biomass.
Fifth is thermal energy storage. It's feasible in individual homes today (although only for very short durations), for example in areas where there are lots of nuclear electricity, electricity is cheaper during the night so it makes sense to cool the home to be very cool in morning, and then let the heat naturally rise. However in the future the economics will shift and electricity will be cheap during sunshine hours. That's perfect for air conditioning: it's needed most when the sun is shining, so it doesn't matter if you need to turn off the AC during cloudy periods. Also large-scale thermal energy storage is feasible in district heating and cooling networks where massive caves store hot or cold water, in which case long-term storage like seasonal energy storage is feasible. These allow reliable heating and cooling even when solar and wind power are not available.
Sixth is nuclear energy. It can be produced all the time, even at times where wind and solar power are not available. Nuclear energy can't be "charged" -- we have only a finite amount of uranium and thorium. However, that doesn't matter because if we use breeder reactors and extract the fuel from seawater, we have enough fuel for several billion years for current human race power usage -- in which case the power output of sun has changed so much life on this planet may not be feasible anymore.
Also we can continue using fossil fuels provided that the carbon dioxide produced is stored back underground. Coal is problematic because it's very dense and coal mines are not sealed, so you can't pump the carbon dioxide produced by combusting the coal back into the coal mine. Oil is slightly better because oil wells are sealed, but oil as a liquid takes less space than the carbon dioxide produced by burning the oil. However, natural gas is very ideal: one cubic meter of natural gas produces one cubic meter of carbon dioxide, and natural gas wells are guaranteed to be sealed because the natural gas was stored there for hundreds of thousands of years. Thus, we can use all natural gas resources provided that we store the carbon dioxide back there, where the natural gas was. If all adjustable peaking electricity production (production during times when solar power and wind power are not available) was natural gas and natural gas was never used for baseload, the known natural gas reserves would last probably at least hundred years if not two hundred years. Furthermore, converting the natural gas into carbon dioxide reduces environmental risks: if the natural gas leaks, we're in trouble because it's a very strong greenhouse gas. If the carbon dioxide leaks, that's less worrying because it's a far less potent greenhouse gas.
I didn't list lithium ion batteries because they are best for short-term energy storage and you specifically asked about long-duration storage of large amounts of energy.
Some technologies that will not succeed:
- Compressed air energy storage. The problem is that when you compress air, it becomes very hot. There are two strategies: store the air in thermally insulated container and then expand it back to ambient pressure and temperature when power is needed (but the thermally insulated container is very expensive), or use massive heat exchangers to make the process isothermal (but in that case the heat exchangers cost so much it alone makes the project unfeasible), or compress the air into a cave, letting it gradually lose its heat, and then when expanding it again provide additional heat using natural gas (but in this case the natural gas consumption is so high this should be thought more of a way to improve efficiency of natural gas by compressed air rather than using only compressed air to store energy).
- Other gravity-based energy storage methods than hydropower. The problem is that you need heavy blocks you can stack over each other. The blocks can't be made by fracturing rock into small pieces because the blocks must stack over each other, and fracturing rock doesn't produce uniform blocks, so only concrete is feasible as the material. Concrete production produces very high carbon dioxide emissions, and the concrete alone even without emission trading prices is so expensive that the energy storage cost of these gravity-based schemes will be so high that it's practically the same as the cost of battery energy storage, but with very expensive charging-discharging cranes and with much poorer efficiency. Not competitive.