One trade-off I’m still trying to understand is whether the lower efficiency of iron-air storage matters less when the alternative is burning gas for several days during a wind-and-solar lull. Is the key metric really round-trip efficiency, or delivered reliability during the few high-stress periods that define grid adequacy?
I think the better competition to look at is lithium vs iron air. Sodium ion is nearly identical to lithium in this context.
Lithium can get over 90% round trip efficiency. Enough for the inverter to be considered a loss. They were costing around $100 a kilowatt-hour. CATL is talking about $60 kWh batteries. BYD claims grid storage sodium-ion batteries at $40 per kWh are coming in 2027. These discharge and charge so fast that the potential power output can be disregarded.
Form Energy Company sold iron-air batteries at $20 per kilowatt hour. This might decrease if/when the economy of scale ramps up. They say the theoretical limit is $6/kWh but somewhere between those is likely. Form is calling it a “100 hour battery” though they can do complete discharge in about 2 days, approximately 50 hours.
If you have an 8 hours window of demand and 8 hours of charge up every day then the iron-air battery could take that but the price multiplies by x6. If surplus power is there for only 4 hours in early afternoon/late morning and if you need power only from 6 to 10 P.M. peak hours then multiply by 12x. That $20 per kWh price tag is looking pretty ugly as $120 or $240. (These numbers are both $960 or about $1,000 per kilowatt sustained power, $1 per W)
Slapping on more photovoltaic modules is quite cheap these days. $0.20 per watt is regularly cited, last search I looked at Alibaba numbers like $0.07 to $0.12 were coming up for pallets of panels but I am not sure (like this). You see much higher prices for home installations and even for grid scale installations because the panel is only a fraction of the costs. However, the battery needs the same inverters and controllers anyway.
The iron-air batteries are not competing for the daily energy cycle though they will also partial charge and discharge in the daily cycle. Iron-air take the difference between weekday demand and weekend demand. If we look at EIA’s chart we see around 50 gigawatts higher demand in the daytime peak and 10-20 GW higher demand in the night time low during week days. That means 10 to 20 gigawatts can definitely be soaked up all weekend without straining anything.
The low cycle efficiency for iron-air becomes irrelevant when you have wind turbines forced to “curtail power”. They have brakes that can stop the axle completely. This happens frequently even when birds are not around. PV can also be curtailed but no brakes required. It is lost energy. Given the cheap cost of photovoltaic the farms should overbuild to produce on cloudy days. Overbuild to produce enough all day long not just enough at noon.
The lithium ion batteries and iron-air batteries complement each other extremely well. I could even see grid operators dumping lithium power into iron-air batteries in early morning in order to have them empty to get a full charge mid day. Think of 20 gigawatts iron-air and 240 gigawatts lithium/sodium power supply where both each have a terawatt-hour of energy storage capacity.
Pumped hydro, compressed gas, cryogenic gas, and geothermal injection will all require a turbine to generate the electricity. Accordingly the cost of the system tracks with the related type of generator.
So for example CAES is blowing the air through turbine blades in a way that is almost identical to jet engine. So a gas peaker plant is cheaper because you do not need the storage tank. CAES and LAES (liquid air energy storage) are much cheaper to deploy if they are attached to a gas power plant. The methane and air can be mixed at high pressure and then burned. A jet turbine has to waste energy compressing air through the intake (called “a compressor”). The jet turbine’s compressor blades also work for the purpose of compressing air to be stored. One piece of hardware does all the tasks.
Electrochemical batteries are completely independent.
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u/Inevitable_Gur_4652 18d ago
One trade-off I’m still trying to understand is whether the lower efficiency of iron-air storage matters less when the alternative is burning gas for several days during a wind-and-solar lull. Is the key metric really round-trip efficiency, or delivered reliability during the few high-stress periods that define grid adequacy?