Other Attempts to Mix the Seas
We were not the first – by any means – to note that there is a vast reservoir of cool water less than 100 meters below the very warm tropical surface waters that lead to hurricane genesis and intensification. Others have pointed this out, and sought to utilize that with various approaches. Likewise, others have pointed out that within that cool water reservoir are the nutrients needed for phytoplankton nearer the surface.
These other approaches were often advanced by brilliant and innovative teams, from whom we learned – largely by analyzing why their approach was not utilized. That is not a statement of superiority, but rather one enjoying the advantage of learning from their experience.
Most of the serious attempts at sea mixing involve using downwelling tubes that stretch from the surface down through the thermocline, and pumping water into the top of the tube which then outflows at the bottom – below the thermocline. The approach assumes that the warm surface waters then displace the cool waters at the bottom of the tube, and force the cooler water towards the surface. However, these prior concepts run into problems, and it’s worth briefly reviewing those concerns, and contrasting them to our Tropical Cloud Generation (TCG) approach.
The inherent problems with all tubular sea mixing concepts
Broadly, we can separate this into six main categories:
- Flow rate
- Power
- Mixing rate
- Mobility
- Wall / valve integrity
- Temperature gradient, patch size, and time
1. Flow rate:
The flow rate of a tube is largely limited by the tube diameter, the power used in pumping the water, and the wall integrity of the tube. One of the first patents issued on sea mixing was in 2009. Barber et all received patent USP #7,536,967 for the design of a system for pumping sea surface waters deep below the surface through a large vertical metal pipe from a tank just below the surface moored to a ship. In this design, a wind turbine above the tank would provide power for the pumps. The patent contains no details or calculations, so we did a few. The largest available wind turbine (222-m dia.) can provide only ~5 MW when the mean wind speed is only 8 m/s (at 150 m altitude, 300 K). If the pipe is 8 m in diameter and the mixed layer depth (MLD) is ~70 m deep, the Barber concept (with highly optimized pumps and manifolding) might be able to pump surface waters to 140 m depth (causing deeper waters to well up) at the rate of ~2.4E5 kg/s. The total mass of water in the upper 70 m of a sea patch 60 km in diameter is ~2E14 kg. So, it would take ~107 such systems (ships, huge wind turbines, pumps, pipes…) to pump 1/3rd of the water above the MLD in this one patch down to well below the thermocline in a month (this would reduce the sea surface temperature (SST) from ~ 30 C to ~ 25 C).
In 2010, Kitamura was issued USP #7,832,657 for proposing to attach eight 0.5-m-diameter pipes to a submarine-like platform to pump deep water up to the surface. He proposed to pump 400 kg/s through each of the pipes from a depth of 100-400 m to near the surface. Simple calculations show it would take 600 times more such structures per 60-km patch than Barber’s concept, and each of those would require a submarine platform.
Our above analysis provides useful insights and a good starting point. Key to achieving good efficiency for Barber was our assumption of the pipe being 8 m in diameter, thereby making it possible to pump 240 tonnes/s with 5 MW – primarily because the water velocity in the pipe was under 5 m/s. With proper design, it doesn’t take too much more power to pump surface waters down to 300-400 m, as may be needed in some oceanic regions (though these regions are not the “low hanging fruit” to target first).
2. Power.
It is clear that the concept of mixing the ocean layers is going to take a lot of power. For the Barber concept above, it would take ~535 MW of installed floating wind turbines to hypothetically mix the volume of water in a single patch, while the Kitamura concept would require 320 GW! It becomes quickly obvious that using wind power for this concept can’t work – the cost becomes absurd, and as we’ll see below, the required mobility of floating wind power at that magnitude is not acceptable. But we also realized that the key difference between the two concepts is a much larger diameter of flow, and a much lower velocity.
Naturally, others realized this as well.
Bowers et al (a team that included Bill Gates) were issued USP #8,685,254 in 2014 for a wave-driven method of pumping warm surface waters through a long pipe to below the 20°C isotherm. The method includes a buoyant tube, waves reflecting off the ship to which it is moored, flap valves, a long pipe, and a power source for operating the valves. There were no useful calculations in any of the various patents from this group.
Steven Salter in 2009 proposed at the 8th European Wave and Tidal Energy Conference a clever 100-m diameter floating wave pump which consists of a 200-m-long plastic tube buoyed by a large array of foam-filled tires with a complex 17-m high valve wall surrounding the top of the 100-m tube, mostly below the surface. It allowed surface water intake from the swell of waves, and then closed behind the swell and then let gravity serve to downwell the volume of water in the tube. With 17-m high valves all around the top of the tube, they could take in swells from any direction. Here, we see perhaps the ultimate extreme of reducing power needs by reducing the velocity of downwelling. Salter estimates ~3.5E5 kg/s in total downwelling by means of one of these pumps. Assuming his numbers, that would result in a net downwelling velocity within the tube of only ~0.045 m/s! This allows for the concept to achieve slightly higher pumping rate than the Barber concept that used 5 MW of wind power. A 1.4-m high 100-m wave-front has the potential to yield over 1 MW of power, but averages 300-500 kW.
But there are disadvantages to very low velocities. The total mixing rate at the target depth is proportional to the kinetic power of the stream at that depth. The kinetic power in a stream is given simply by 0.5*G*v^2, where G is the mass flow rate (kg/s) and v is the velocity (m/s). The kinetic power in the downflowing stream in the Salter 100-m tube (after the inlet eddies have died out) is a mere 350 W. Power is also needed to overcome the buoyancy force (~2E7 N for a 100-m diameter column to 200-m depth). At the stream velocity, that amounts to ~850 kW.
3. Mixing rate.
The velocity of the downwelling will impact the degree of mixing that the waters will undergo. In the Bowers concept, the water issuing from the metal pipe will be flowing at a higher velocity, causing a larger volume of turbulence and mixing as it ejects from the pipe, resulting in a greater area around the circumference of the flow becoming mixed. While the 8 m/s estimate from the metal pipe used in the Bowers concept will result in turbulent mixing that spreads well away from the pipe, the extremely low velocity downwelling at the outflow of the wave-driven tube will not mix far from the tube.
This becomes a critical issue relatively quickly: when the tubular structure initially begins the downwelling, 100% of the surface water that is being pumped down the tube would be at the initial sea surface temperature (SST), while 100% of the deep end of the pipe would be at the mean isotherm temperature that the tube descends to. However, as soon as the first warm water reaches the bottom (which takes ~75 minutes), then some of that water will mix with cold water producing cool water that will very slowly rise, with small eddies pealing off as it slowly rises; but some of that water (initially maybe a 20-m-thick wall of warm water) will start coming up right around the tube and slowly accelerate. (Remember, it had almost zero kinetic energy density in the tube. It will have even less when it begins to rise.) Eddies will peal off as it rises, but some water will get clear back to the surface not much cooler than went it started, at least after the first day of operation, resulting in a waste of the pumping action. This loss of pumping efficiency will increase rapidly as the column of water surrounding the tube normalizes, reaching something close to the mean sea temperature over the depth of the tube, 200 m.
We haven’t yet done the needed simulation (though we expect to at some point in the future) but a reasonable guess is that what one would see before long would be an oblate semi-spheroid region of mixed waters of perhaps 250-m central depth and 400-m radius at sea level surrounding the wave pump, and little change in the temperature profile in the ocean much beyond that. If the pump rate is 3.5E2 m3/s, its effective pump rate can be expected to drop by a factor of two every 4 days as the SST at the intake (and outlet) to the pump tube steadily drops to something approaching the mean sea temperature in the region to a depth of 200 m – say, 20 C. At that point, the wave pump becomes nothing more than an ocean heater. (This is somewhat analogous to using an air conditioner that exhausts into the same space that you are attempting to cool.)
The longwave radiation from this small cool patch is reduced, so its radiative forcing is increased – perhaps by a mean amount of ~15 W/m2 if it’s 20 C at the center and 28 C at the edge.
Of course, this is only imagining a downwelling pipe or tube that is not moving in reference to the sea surface currents. In order to fuel weather changes, the cool patch needs to be ~100 times larger (10,000 times the surface area) compared to what the wave pump seems likely to produce.
This may be worth a more direct comparison to further illustrate the point:
In the case of a TCG Sea Mixer, the preliminary steady-state simulations showed the downward velocity at 200-m depth was ~1.5 m/s. Some of the downwelling flow continued to a depth beyond 450 m, and a large fraction of upwelling mixed waters extended to a diameter of >1100-m (and this was partially limited by the size of the domain in the simulation). The kinetic energy density in a 1.5 m/s stream is more than 1000 times greater than that in a 0.045 m/s stream, and the mass flow from the TCG sea mixer is 100-200 times greater at 200-m depth (depending on the ship’s cruising speed) than from the wave pump. That factor of more than 100,000 in kinetic power means the churning continues long after the ship passes, and the total mixing will be several times what one calculates from the simple ratio of water mass to propeller pumping rate. (At some point, when we have the resources, we’ll do the complete time-dependent simulation that is needed to be more precise.)
This brings us to our 4th concern regarding the use of tubes or pipes in terms of downwelling:
4. Mobility
The reduced efficiency of drifting wave pumps due to a high fraction of the water being recirculated after the first week depends on two factors: the rate at which the surface of the tube moves in relation to the surface water, and the rate at which the outlet of the tube moves in relation to the deeper water. For the outflow, this is not likely to be much of a problem. Many of the surface currents in the ocean are relatively shallow (~100 m). Therefore, setting a tubular pump adrift that has a depth not significantly greater than the depth of the surface currents will result in the surface currents moving the top at a slightly faster rate than the average current speed seen at the lower depth. However, the complete wave pump and the column of mixed water surrounding it will be drifting at approximately the same speed as the waters farther away from it. Therefore, if simply left unattended, the water surrounding the pump quickly (in a few weeks) normalizes to some mean temperature, and the effective pumping efficiency drops to zero. Then it must be moved.
However, movement is not something that one can easily do with a very large diameter 200-m long tube serving as a hydrodynamic drag. So these designs would require that the tube be withdrawn, retracted, or collapsed like a large accordion, so it can be transported, unfurled, and begin pumping anew. Clearly, any approach to sea-surface mixing must be continually moved with respect to the surface waters, or the mixing will quickly become ineffective. For the TCG Sea Mixer, that is easy – it’s a modified ship capable of moving at least 25 km/hr. For large tubes, and large pipes, this becomes a very significant complication, which will only serve to increase stress and failure rates. This leads us to the 5th concern:
5. Wall and Valve integrity.
Whether we’re talking about metal pipes with welded junctions, or plastic tubes with fusion welds, one of the more significant concerns of a large tube in ocean waters is longevity. Plastic that is constantly exposed to sea water and sun has a poor track record, and 17-m valves sound like the gates in canal locks, which are massive and expensive. Welded junctions of thin metal could see fast wear in the face of constantly being “furled” and “unfurled”. The difference in current speed at the top of the tube vs the bottom when “unfurled” would create constant strain on every joint and connection to its structure. The longevity of such a system is highly uncertain and represents a significant concern.
The last concern about this concept is likely the most compelling reason to favor the TCG Sea Mixer design compared to a big-tube downwelling concept:
6. Temperature gradient, patch size, and time
As pointed out in several other places on this website, sea-surface mixing by itself is counterproductive – a 1.5 C reduction in mean SST over the entire ocean increases global warming as much as a doubling of atmospheric CO2. A 4 C reduction in SST in the tropics increases radiative forcing by ~17 W/m2. Patchy cooling makes sense only if it generates clouds, as increasing low-level cloud fraction over tropical seas where it is currently 5-10% to 50-60% reduces radiative forcing (heating) over those areas by 100-120 W/m2. That’s major sea cooling, but the cool patches have to be big, they have to be created fairly quickly, and they need to stay cool for much longer than it takes to create them. That takes power, mobility, efficiency, deep mixing, and strategy.
On another page, under Sea Breezes, we explain that the cool patches need to be large enough to generate the type of weather fronts often seen on tropical islands (or Florida beaches in July through September) in the afternoon. This requires 40-80 km diameter cool patches. The 0.5-km cool patches from a wave pump (or scores of them from scores of wave pumps) don’t come close to being able to do the job. The only way to do what is needed in about a month is with a large nuclear-powered ship – a repurposed decommissioned aircraft carrier that can deliver 160 MW (or more) of efficient pumping power.