Possible synergies among DAC, renewable energy production and hurricane control.

[Renaud de RICHTER]

I forward to you all a message from my friend Denis Bonnelle

>>> Le mer. 29 sept. 2021 à 14:31, Denis Bonnelle <dbon...@ipsl.fr> a écrit :

This email is to show some possible synergies among DAC, renewable energy production, and another geoengineering issue: hurricane control.
I am not claiming that DAC is a realistic solution to fight climate change, I am just assuming this, as a hypothesis, whose consequences I try to investigate.
If DAC is realistic, this means that it can withdraw several gigatons of CO2 per year, i.e. that it can process even more millions km³ of air, i.e. not far from 1 km³ of air per second (1 y = 10^7.5 s). With air velocity up to 10 m/s, this means a cross-section around 100 km². (The 22th of September, I participated in a seminar about NET ("NET-Rapido") with the think tank Climate Strategies, and such orders of magnitudes have been emphasized by a speaker).

Air can be pushed through this cross-section by fans, hopefully carbon-free powered, e.g. by wind energy. This advocates for a direct use of wind energy, i.e., in windy regions, letting the wind into the DAC devices, without any fans nor wind turbines dedicated to power them (some other energy would be needed for the physico-chemical reactions needed to separate the CO2 from the air). My friend Renaud de Richter tells me that this is Klaus Lackner's choice, maybe among others.

A 100 km² cross-section could be broken down to, e.g., 10,000 cantilevered structures, each one having a 100 m x 100 m cross-section, or even to 1,000 ones, each 300 m x 300 m. So, my hypothesis that DAC would be realistic, implies that building such cantilevered structures could be seriously considered.

A related project has recently been proposed, and illustrated by the following drawing:

 

(https://www.rechargenews.com/wind/futuristic-multirotor-design-could-make-floating-wind-competitive-as-soon-as-2022/2-1-1021312 - notice that for French people like me, the Eiffel tower is a convenient reference for measuring a 300 m height)

Of course, there are some differences with DAC: here, this structure bears relatively few wind turbines, not DAC devices ; and it is floating on the sea - by the way, I had never assumed that the 1,000 DAC structures would be built onshore.

Offshore wind energy develops strongly due to various reasons, among which social acceptance and the possibility to reach better winds (stronger and less time-dependent). Both reasons reinforce each other: there are two ways of getting better winds: being at a higher altitude, and being over the ocean. But taller onshore wind turbines face more opponents, so that going offshore makes twofold sense to make wind energy at scale possible, even if it is quite more expensive by the MW (but, due to these better winds, not that more expensive by the MWh; and with a larger economic value as it needs less power back-up, or as it enhances the capacity factor of conversion devices which would use this power, such as electrolysers).

Offshore wind energy can be either "near offshore", linked to the shore by electric cables, or "high sea offshore". The mainstream idea is that the latter would be useful only through on-board "power-to-liquid" transformation, i.e. water electrolysis and use of the hydrogen to synthesize ammonia, methanol (using CO2), or synthetic jet fuel (using CO2 through Fischer-Tropsch reaction). The latter two are included in the "U" of CCUS that you, as CDR and DAC specialists, know well, at least for CCS. Of course, such on-board chemical plants would be strongly characterized by economies of scale, which is another argument on behalf of very large scales such as the 300 m x 300 m cross-section of the above drawing.

But why wouldn't wind energy developers design wind turbines with, at least, a 150 m radius and a 200 m tall tower, just extrapolating the current trends, and benefitting from the fact that, offshore, you no longer face the same political oppositions which, onshore, prevent them from such bold extrapolation?

The answer to this question is, again, about scale economies. So far, larger and larger wind turbines have proved cheaper by the MWh, but this is only thanks to the reduction of the relative part of some costs such as development costs, maintenance, balance of power, etc. But the hard physics of the wind turbine per se shows the contrary of scale economies. To harvest the wind from a x4 cross-section, i.e. from a x2 radius, you might think that a blade with a x4 area would be enough, but this is not all. This blade must also be thicker just to keep an unchanged geometry, and it must be even more mechanically reinforced, as all of the forces it endures are converted to torques by being multiplied by a "r" coordinate which now varies up to a doubled maximum radius. All this multiplies the required materials quantity by, at least, a x8 factor, and probably even more.

Until now (i.e. the record ≈ 10 MW wind turbines, with their blades slightly longer than 100 m and their towers above 150 m), the cost of this material wasn't the main part of wind energy's costs, but if you'd aim at, say, 200 m or 250 m long blades and a ≈ 300 m tall tower, this could be no longer true, which is a first reason why such a structure with "small" wind turbines would make sense.

The other reason could be that small wind turbine factories would face a shortage of clients, while being fully depreciated from an accountancy point of view, so that they would be able to propose wind turbines at very attractive prices, overall if somebody offers to buy them by the hundred.

What is the relation with DAC?

First, it proves that such giant cantilevered structures can make sense, notably when it comes to facing strong winds. The same about floating on the sea.

(I had made some further comparisons with a classical wind turbine, whose tower undergoes a strong torque due to a force parallel to its shaft. Having two towers arranged in sort of a quite vertical triangle, would be cheaper, provided that this triangle could always be in a plane including the wind's direction. This is impossible onshore, as the wind's direction isn't constant. But a floating structure can be oriented so that it always quite faces the wind. Maybe the figure above is also derived from such a comparison.)

Forces parallel to the wind would also exist if some or all of the wind turbines were substituted by DAC devices. Then, you can choose between two possibilities: being strongly anchored to the undersea ground, of being pushed by the wind and slowed down by a hydrokinetic turbine under the hull, which could produce some power, maybe cheaper than wind energy, as the water is denser than the air so that smaller "blades" can be used.

When such structures are dedicated to power production for usual onshore needs, either case (anchored structure or hydrokinetic turbine) but even more the latter, imply on-board conversion of this power. I have already discussed power-to-liquid, through water electrolysis and synthesis reactions, but DAC could be an interesting use of such power, with liquid or solid CO2 as an output. Notably, it could be useful as a first "proof of concept" of the idea of producing offshore power from high sea winds, and using it onboard to generate dense chemicals, with no need of handling them too often to their final users.

When such mobile floating structures are pushed by the winds, a force appears, which means a momentum exchange. In the global momentum balance, this exchange is between the air and the water. This could be useful for hurricane control.

A basic idea for hurricane control is that tapping some wind energy from it reduces its kinetic energy, thus its devastating power, and this idea has been developed in, e.g., "Taming hurricanes with arrays of offshore wind turbines", a very interesting paper by Cristina Archer, Mark Jacobson and Willett Kempton, which compares the economic values of the power produced by these wind turbines throughout the year, and of the reduction of the hurricane's damage.

However, this paper only deals with near offshore wind turbines, built on shallow undersea ground off the US southern and eastern shores (so that no control of the hurricane farther from these coasts is possible), and it only deals with kinetic energy exchanges, not momentum ones.

Momentum exchanges are not interesting per se, but because they control a much more powerful lever about hurricanes: angular momentum exchanges.

Even if the physics of hurricanes is very complex, the idea of reducing their angular momentum exchanges to control them is emphasized by the fact that they can't appear too close to the Equator, which proves that angular momentum is vital for them, and this is logical: a very powerful hurricane needs a very low pressure in all its quite central air stormy cylinder, which must attract new air only at its bottom in order to harvest the ocean's latent heat; at all the other altitudes, there must be something to protect this low pressure cylinder from anarchic air inlets from the outside, and this something is the centrifugal force (and an increased Coriolis's force) which is generated by the rotation of the whole hurricane, proportional (and even squared) to its angular momentum. If it weakens, the whole thermal machine will be weaker even if the water temperature is still the same.
I'm quoting this temperature, as a hurricane relies on two positive feedbacks. The latent heat one is as follows:

"more latent heat --> more air buoyancy --> a deeper low pressure near the hurricane's center --> stronger attraction of the winds by the hurricane --> more heat exchanges due to friction at the ocean's surface --> more latent heat in the whole machine".

It is quite difficult to act on it with a powerful lever, but it might be less difficult to act against the other positive feedback which hurricanes desperately need:

"rotation --> strong centrifugal forces --> the inner low pressures being protected at quite all the altitudes against anarchic air inlets --> this inner low pressure cylinder strongly attracting air from far outside at the ocean level --> this radial inwards air undergoing Coriolis's force along a quite long way and turning tangential  --> this Coriolis effect reinforcing the strong rotation which was the first step of our positive feedback".

And if "acting" on it would mean having a large floating structure being drawn by the rotating winds so that (angular) momentum is transferred to the ocean, it would be interesting to look for synergies with the mere existence of such a floating structure being subjected to such winds and being designed to generate something else useful for the climate. You can't bypass the idea of something like "power-to-liquid" happening on-board, but this "liquid" (or dense) material being CO2 could be the technologically simplest idea to begin with.

Such a device could be used to control many hurricanes by rotating around them for a large part of the hurricanes seasons in both hemispheres; for the rest of the year, they would just capture CO2 on windy oceans, e.g. being anchored not far from the Patagonian coast.

Anyway, I hope that studying such synergies more thoroughly could be fruitful for all the approaches of such an idea: DAC; hurricane control through angular momentum; and the broader trend about wind energy being harvested over high seas and converted through power-to-liquid schemes.

Best regards,
Denis Bonnelle.

Denis.B...@ipsl.fr

[Andrew Lockley]

Slowing down winds by stirring the ocean would be equivalent to increasing surface roughness in a model. This would be fairly easy to test, I think.

I'm not sure whether the below idea has been suggested before, but using membrane polymers for DAC means that these materials could be adapted to make kites. 

DAC Polymers - https://doi.org/10.1016/j.clet.2021.100145

Wind energy kites - 

https://www.powerengineeringint.com/emissions-environment/the-energy-kite-innovation-to-harness-maximum-wind-power/

The temperature difference between the ocean surface and the high altitude winds may be enough for low temperature temperature-swing DAC to be viable, meaning that the desorb step could be passive (or nearly so). 

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[Brian Cady]

Since hurricanes tap energy differences, reducing energy differences weakens them. Ocean Thermal Energy Conversion(OTEC) plants/machines use the difference between cold, near-freezing ocean bottom waters, which underlie the world's oceans around the globe a kilometer or so beneath the ocean surface, and hot tropical surface waters to generate power, typically electricity.

In doing so they typically raise more-fertile bottom water to or near the surface. This fertility has been seen as a problem and an opportunity: Many OTEC designs carefully mix their outflow of heated fertile bottom water and cooled, more sterile surface water, and discharge these some distance below the surface, to avoid the light-lit photic zone where plankton thrive. Others calculate that the value of this fertility to mariculture is sixty-fold the value of the OTEC power produced, if the slightly-warmed fertile bottom water outflow is released into the photic zone.

In addition to the power and nutrients, in tropical locations, where OTECs are best situated, the coolness from the bottom water itself is a marketable byproduct. In addition, the nutrients brought to the photic zone would grow more plankton, temporarily removing dissolved carbon dioxide as biomass.

So OTEC plants near tropical coast cities next to deep ocean, perhaps like Jakarta or Singapore, could power, nourish and cool, while reducing tropical surface water's heat, thus lessening hurricanes, cyclones and typhoons, all while temporarily doing CDR.

Brian

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[Chris Vivian]

Brian,

 

Where does the “value of this fertility to mariculture is sixty-fold the value of the OTEC power produced” come from?

 

The deep oceanic waters also contain high levels of dissolved organic carbon that need to be taken into account in calculating the overall benefit of CO2 being removed by phytoplankton production.

 

Also,

 

Best wishes

 

Chris.

(https://www.rechargenews.com/wind/futuristic-multirotor-design-could-make-floating-wind-competitive-as-soon-as-2022/2-1-1021312 - notice that for French people like me, the Eiffel tower is a convenient reference for measuring a 300 m height)

To view this discussion on the web visit https://groups.google.com/d/msgid/CarbonDioxideRemoval/CAAoq7hL2_FNXDeLyoos_Rr65aR5DTwbRaULvTyWBYyh5jO1%2BUg%40mail.gmail.com.

[Brian Cady]

Hi Chris,

I've forgotten exactly; it was an appendix to a OTEC feasibility study done at least two decades ago. Sorry for the inexactitude.

I believe that there's a 100 kW OTEC operated experimentally on Hawai'i; the outflow of which nourishes a few aquaculture operations, including a spirulina (edible algae) production facility. I haven't found any confirmation of this OTEC-spirulina integration on the web now, however. Here are some links, the first about the Hawaiian OTEC test plant:

https://www.power-technology.com/projects/makais-ocean-thermal-energy-conversion-otec-power-plant-hawaii/

The second link explains OTEC:

https://www.explainthatstuff.com/how-otec-works.html

This third link demonstrates that deep ocean water helps nourish spirulina:

https://www.nutrex-hawaii.com/pages/about-us

Brian

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