I forward to you all a message from my friend Denis Bonnelle
>>> Le mer. 29 sept. 2021 à 14:31, Denis Bonnelle <dbonne...@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.bonne...@ipsl.fr
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