Correction to grid long term storage, and carbon capture
By Ian Page – 2022.01.01
In yesterday's note I recorded that a recent paper Joule 5 2077-2011 Aug. 18, 2021, has proposed that the cheapest long duration storage for electricity would be HDV PEM salt which is heavy duty fuel cells for vehicles and storing hydrogen in salt caverns. This is around the same cost as natural gas combined cycle with full carbon capture and storage for periods up to at least 7 days and the cost increase with time is almost flat. This makes it the most significant since even natural gas with carbon capture has been shown to make a considerable contribution to global warming due to leakages of natural gas which is a very bad global warming gas.
Unfortunately, in the last 48 hours it has become apparent that Hyundai, which has over many years invested heavily in hydrogen fuel cell vehicles, has decided to close it down (put on ice, return to research?) due to intractable technical problems and lack of market potential. I read this as batteries are now good enough to eliminate the market.
I didn’t put this, and Toyota’s move away from fuel cells as well a few months ago, together with the joule paper at the time.
If there are not going to be any heavy-duty vehicle fuel cells, then the joule long term storage idea won't work.
There may, as I said, be a place for heavy lorries and ships' use of fuel cells, but this is a very different volume market which might not get the cost down fast enough.
My current understanding is that the primary difference between heavy duty and other fuel cells is that they are optimized for high power delivery rates, which means a shorter life. This is OK for long term storage.
My guess is that for lorries and ships the high-power periods of acceleration will be handled by a battery front end, allowing the fuel cells to run at their preferred power rating all the time without having to handle sudden power demands.
If this is so, then they would not be suitable for long term storage due to cost.
The next best non fossil based long term storage is hydrogen stored in salt caverns driving a combined cycle gas plant. This is about 40% more expensive per kwh due to the terrible round-trip efficiency of electricity to hydrogen to electricity, but its cost of storage is pretty flat
This would result in a long-term cost of storage of around $220 per MWh (cf 180 for NG CCS, or around $150 for unabated NG from pipelines.) This is expensive but for only about 4 weeks per year. Thus, the viability of the whole system depends on the wholesale cost of electricity for the other 48 weeks!
I also looked into the mad alternative I put forward for liquid CO2 storage- in particular what happens to the CO2 once it has expanded after generation.
The answer is that they store it in expanding plastic balloons.
While this works for a daily cycle where the balloons are not too large, for long term generation the balloon volume would be about 200 times the liquid CO2 volume. While balloons are cheap this might be a problem
The efficiency on a daily cycle basis is over 80% but this requires storing the heat of compression in rock and using it to expand the gas later. Again, this works on a daily cycle, and would work on a long-term cycle if the heat was stored underground where it cools slowly, but it's beginning to look like a serious engineering project rather than something off the shelf.
Liquid air storage doesn't suffer from the gas volume effect as it can simply be released to the atmosphere, however it also must get a lot of heat from somewhere to re-expand the liquid air. On a daily cycle this is the heat of liquefaction stored in something. On a weekly or longer cycle it would need serious engineering or a convenient source of low temperature heat such as hot pools.
I also mentioned direct air capture in a note and note that the efficiency depends obviously on the concentration of CO2. Typical flue gases only have about 15% CO2 the rest is nitrogen (Since 80% of air is inert nitrogen, it's still there after burning). However, if you use oxygen instead of air as the input, you get a hotter temperature (not having to heat up inert nitrogen) and an output that is only CO2 and water vapor. Cool it and you get quite a high proportion of CO2 which might be storable without fully drying or cleaning up.
Since oxygen is a byproduct of nitrogen production (about 1 volume of oxygen is produced for every 4 volumes of nitrogen), and of electrolysis hydrogen production (one volume of oxygen for every 2 volumes of hydrogen) it's a pretty cheap waste gas.
It would have to be produced within easy pipeline range of the unit burning gas, but by increasing the efficiency of the Combined Cycle Gas, as well as the capture of CO2, it might reduce the cost of Carbon capture and storage.
Additionally, I have come across a massive use of carbon capture that's been around for years,
A gas well typically produces a mixture of methane, higher -thanes, nitrogen, water, poisonous gases, and CO2- often quite a high proportion of CO2. Gas mains require dry sulphur free gas with very little nitrogen or CO2, partially to make it saleable, but also because in cold weather under pressure methane forms a crystalline clathrate with water vapor and clogs up pipes.
If the gas is to be transported as Liquefied natural gas, as it is chilled the CO2 liquefies and solidifies well before the methane and thus clogs up the pipes.
Thus, the natural gas industry spends a lot of money removing CO2 from its product. (It tends to use the CO2 to extract more oil from old wells but that's a different problem, or just release it to the air).
The conclusion is that CO2 removal from messy mixed gas streams is a well understood and mature technology and it's just a matter of cost, which isn't going to come down much without a dramatic invention of new technology so we shouldn't worry too much about technology, just getting a real cost added to the use of fossils.
On the other hand, direct air capture is working with incredibly dilute CO2 streams where huge amounts of air need to be moved through large areas of absorbent. The main approaches (alkaline hydroxide capture and liquid or adsorbed amines on beads, adsorbed quinones changing ionic state) have pretty well-defined costs and issues and without some completely new technology (e.g., some kind of hot electron plasma that ionizes the CO2 specifically so it can be attracted into a separate output pipe) are only going to get cheaper through mass production, or careful use of ambient wind, and low-cost electricity.
My conclusion stands - it's a lot cheaper not to put CO2 in the air than to take it out., but we are going to have to do some DAC just to get rid of the historical additions already, there
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