Preliminary Investigation on Greenhouse gas sequestration via Liquid Air Energy Storage

Global innovations in renewable energy give hope that humans will be able to curtail greenhouse gas emissions and slowly mitigate the impact of human caused climate change.  While innovations in renewable energy are significant the majority of innovations do nothing to offset warming that results from surplus greenhouse gases that are already in the atmosphere.  So called "negative emissions" technologies may serve as a way to reduce the volume of greenhouse gases dispersed in the atmosphere.  There are 6 major technologies that can be used to help remove surplus carbon dioxide they are reforrestation, bioenergy with carbon capture and storage, soil carbon, biochar, enhanced weathering, and direct air capture.  Each of these technologies should be considered as part of a long term solution to mitigating climate change, this paper will focus on means of enhancing Direct Air Capture technologies. 

Direct Air Capture is a very clean description of the technology, broadly speaking researchers are developing various means of blowing air across a capture medium, as the air flows past the capture medium grabs carbon-dioxide molecules.  Once the capture medium becomes saturated a second process will be used to extract that carbon dioxide where it can be used to help plants grow in a greenhouse, or to carbonate a beverage, or with an altruistic enough buyer that carbon dioxide can be buried underground.  As of the writing of this article estimates for Direct Air Capture put the cost of removing a metric ton of carbon dioxide from the atmosphere at somewhere between $250-$800 not a small cost.  To put this in perspective the United States produces something like 6.5 billion metric ton of carbon dioxide equivalent green house gases*, this means that to capture American greenhouse emissions you would need to spend between 1.625 and 5.2  trillion** dollars per year, to remove those emissions (for perspective the American economy in 2017 was about $19.5 trillion).  Part of the reason it costs so much to capture the carbon dioxide is a tremendous amount of energy is being used to do one thing, capture the carbon dioxide, of the publicly available information on direct air capture technologies, no firm indicates that they are doing anything but capture carbon from ambient air.  What these technologies need is a way to concentrate the air that they are extracting carbon dioxide from.

Liquid Air Energy Storage(LAES) is a relatively new entrant to the world of grid energy storage.  Using advanced refrigeration, companies like Highview Power are developing tools to use surplus grid energy to turn air into a liquid.  When electricity prices are low a refrigeration cycle is run on the air around the power storage facility.  This refrigeration process turns the air into a liquid that can be stored in massive cryo-tanks.  As the electric grid goes from a surplus to needing more power, that liquid air is cycled to expand back into its gaseous state, this expansion is used to run a turbine that provides electricity for the grid  (a more in depth explanation can be found here).  What is exciting about liquid air energy storage is that you are already moving the air for useful work, providing an opportunity to create symbiosis between the liquid air and direct air capture.  

There are at least two potential ways where Liquid Air Energy Storage can be used to augment Direct Air Capture.

1.  Add carbon scrubbers at either the beginning or end of the energy cycle.  During either the capture or release phase of the LAES lifecycle the air is run past a carbon capture materials.  This approach allows you to augment an already existing LAES system.  The tradeoff is that the capture medium is going to directly impact your intake/release systems ability to work efficiently.  On intake your intake fans will need to work that much harder to bring in the source air.  On the other end of the cycle where the "exhaust" of regasified air is being released the filter mediums will also be able to take advantage of exiting outflow, but the outflow will be slowed down by the obstruction (this is the same reason why you should clean filters on your computer and AC system)

2.  Capture the carbon dioxide during the refrigeration process.  The nitrogen, oxygen, and carbon dioxide in our atmosphere all of their own temperature for when they turn into a liquid (or solid in the case of carbon dioxide at 1 atmosphere of pressure).  This range of temperatures gives us an opportunity to integrate a way to scrub out the carbon dioxide by capturing it as it changes state from a gas to a solid.  Nitrogen becomes a liquid at −195.79 °C, oxygen liquifies at −182.96 °C, and carbon dioxide sublimates at −78.5 °C.  The relatively high temperature of carbon dioxide's freezing appears to give an opportunity to add a stage to the LAES lifecycle where solid carbon dioxide is scrubbed from the liquid air energy storage before being returned to the atmosphere.  The potential risks of this approach, would include, increased complexity of the refrigeration cycle, as the chamber/stage where the carbon dioxide is removed might not work with standard approaches to making cryo-liquids, and maintenance cost increases.  

At this time there is no way to cleanly model*** how Liquid Air Energy Storage would impact the cost of Direct Air Capture, that being said we can at least estimate how much energy the US would need to store to impact carbon dioxide levels.

So it turns out liquid air systems already filter out the carbon dioxide, this is just an inherrent part of the design, so basically I like this technology.

REGARDLESS we can still look into how much power you would need to store to filter out CO2

According to the wikipedia entry on liquid nitrogen engines, a kilogram of nitrogen stores about 100 watt hours of power.  To make life easier we are going to assume that all liquid air has about the same energy density.

From the table above we see that for every 1625 kilograms of air you liquify, about 1 kilogram of carbon dioxide would be extracted from the atmosphere

that indicates that for every kilogram of carbon dioxide you remove from the atmosphere you would be able to supply the grid with about

1625 kilograms of air - 1 kilogram of carbon dioxide =1624 kilograms of air for power

1624 kilograms *100 Wh/kg= 162,400 watt hours or 162.4 kWhrs

to extract a metric ton of carbon dioxide you would store enough energy to produce 162.4 megawatt hours of power

to extract all American produced carbon dioxide through LAES how many kilowatt hours of power would we need to store?

the US produces 6.5 billion metric tons of carbon dioxide equivalent green house gases of those 6.5 billion metric tons 82% are actuall carbon dioxide  (to make life easier the author is assuming that it isn't possible to capture the remaining 18% of green house gases through air liquification)

This means that the US currently produces 5.33 billion metric tons of CO2

the total energy storage requirement would therefore be

[162,400 kilowatt hours/metric ton of CO2 ]*5.33 billion metric tons of CO2

which works out to 865.6 trillion kilowatt hours of energy stored.  To put things in perspective the US currently consumes 3.7 trillion kilowatt hours of energy.  What is even crazier, the 865.6 trillion kilowatt hours is how much power is being stored, LAES is about 60-75% efficient which would mean the grid would need to produce closer to 1200 trillion kilowatt hours of electric power.

Obviously the United States can't afford to capture its carbon emissions in the way described above, but what if the country decided that we were going to go to 100% renewable energy and we, for some bizarre reason, decided that all power would be first used for carbon capture via liquid air before the energy went to the grid.  How much carbon dioxide could we capture?

3.7 trillion kilowatt hours of energy*(1 metric ton CO2/162400 kilowatt hours)= 22.783 million metric tons of Carbon dioxide

Thoughts and feelings

Honestly when I started this post I was hopeful that the math would indicate that there was a relatively
 affordable technology that was waiting in the wings to make our world more efficient.  The above
calculations are a great way to highlight how complicated the climate change challenge really is.
I still feel that liquid air energy storage should be considered, even if it doesn't filter the atmosphere 
as aggressively as I had hoped.  As of 2017 something like 28% of American green house gas emissions
are from electricity production, smart grid technologies can offset a tremendous amount of emissions.
Large scale grid storage also makes it that much easier for electric cars to be run in a cost effective 
fashion.  As a society we should challenge ourselves to look for solutiosn to the major problems facing 
us.  Fingers crossed that technologies like direct air capture and liquid air energy storage help to make 
our future more sustainable

*an equivalent ton of carbon dioxide is a way for scientists to communicate that while not all greenhouse gases have the same greenhouse effect, they can use this equivalency to communicate what the warming impact is.

** 250 dollars/metric ton *6.5 billion metric tons = $1.625 trillion

     800 dollars/metric ton *6.5 billion metric tons = $5.2 trillion 

(this math assumes all greenhouse gas emissions are carbon dioxide as I am lazy and it gets me within 25% of the actual answer and I wanted this to look clean)
*** for me, I mean I don't doubt if I could get some Direct Air Capture and Liquid Air experts in a room we could figur it out, but that's just not in the cards right now, sorry 'bout that