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| Fig. 1: Diagram of a Salter Sink floating above the waves. (Source: M. Scott) |
Hurricanes require warm water (> 80°F/26°C) to power their destructive force. Lowering the surface water temperature by even a single degree Celsius has the potential to dramatically reduce the power of a hurricane. [1] Most hurricanes take a similar path to land, an area of ocean dubbed hurricane alley. [1] It is therefore not necessary to lower the surface temperature of the entire ocean; just a slice surface temperature reduction of the common hurricanes path may help reduce intensity. For example, although the entire surface area of the Gulf of Mexico is 1.6 million km2, One idea to lower the surface temperature of ocean water, posed by Prof. Salter of the University of Edinburgh, is called the Salter Sink. [2] In essence, the Salter Sink is a floating plastic tube, 10 m in diameter and 200 m in length, with the top meter sticking out above the surface of the ocean. (See Fig. 1.) As waves crest over the top of the tube, water builds up inside, pushing down the water level in the tube and forcing the top layer of ocean water to mix with the cooler water 200 m down. (See Fig. 2.) The ocean is stratified into two layers that don't mix well; the depth which divides these two layers of water is called the thermocline. In the Gulf of Mexico, the thermocline is at about 200 m.
A slightly more complicated design includes a one-way valve in the top of the sink. [2] This design has the advantage of ensuring a steady flow of warm water in the place of relying on the overtopping of waves. An additional design could involve a curved lip to the sink to toss more water into the sink as the waves rise and fall.
How much impact could such a simple device have to lower the temperatures over a wide swath of ocean? A few simple equations demonstrate the startling amount of mixing produced by a Salter Sink. To start, we can estimate the thermal energy generated from this mixing with the equation:
| E | = | ρ C| dV |
dt ) Δ t |
|
where ρ is the mass density of seawater, Cv is the heat capacity per unit mass, ΔT is the temperature difference between the water surface and the thermocline, (dV/dt) is volume of water captured by the sink per unit time, and Δt is time over which the sink operates. The water captured by the sink per wave can be estimated by the height of the sink above the ocean surface alongside the height and steepness of the waves. Let's assume a Salter Sink with a 10 m diameter, deployed in the Gulf of Mexico from January to May, where a wave comes every 70 m with an average height of 1.5 m. (See Fig. 1.) If the top of the Salter Sink is 0.75 m above the water, and we have a cleverly positioned curved lip as in Fig. 2, we can (generously) estimate that 2 m3 total water will enter the sink per wave. If there is a wave every 10 seconds, this results in dV/dt = 0.2m3 sec-1 of ocean water. Assuming now that ΔT = 20 °C, Cv = 3990 J kg-1 °C-1, and ρ = 1022 kg/m3, and deploying the sink between January and May (Δt = 5 months = 5 × 2.6 × 106 sec = 1.3 × 107 sec), we have
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| Fig. 2: Diagram of a Salter Sink floating above the waves. Water fills the curved lip (A), which is then transferred into the mouth of the sink as it rises above the waves (B). As the wave crests, the other curve lip begins to fill with water (C) and the cycle repeats. (Source: M. Scott) |
The fundamental premise of the Salter Sink is that replacing the warm surface water with the cooler water one layer down will effectively cool the surface. There are a couple of issues with this idea. Ocean depths above the thermocline are well mixed by waves, and so it is unclear how increasing the mixing would change the surface temperature. What does determine the surface temperature of the Gulf? The key determinants of the surface temperature of the Gulf is the amount of sunlight hitting the ocean surface and the evaporation constant of water. An oft-forgotten fact is just how incredibly powerful the sun is. Lets take, as an example, a best-case hypothetical Salter Sink that replaces the top 1 m of water at the ocean's surface with water from 200 m below the surface. Ignoring currents and mixing, how long would it take for the sun to re-heat the new surface water from 10°C back to 30°C? For simplicity, let's assume no clouds, evaporation, or mixing. The time Δt it takes to heat the water is then
| Δt | = | 1022 kg m-3 × 3990 J kg-1 °C-1 × 20°C |
| × 1 m × π / (1.0 × 103 W m-2) | ||
| = | 2.56 × 105 sec | |
| = | 2.97 days |
This is not very long. It is therefore likely that the sun would quickly heat up out any additional cold water that is brought to the surface by a Salter Sink.
Although they might not reduce the power of hurricanes, Salter Sinks have other advantages for the planet. Typically, there is very little mixing of the warm and sunlit but nutrient-poor water at the surface with the nutrient-rich but dark and cold deeper waters. [3] The Salter Sink would mix the waters near or just below the thermocline with those on the surface, likely resulting in a dramatic increase in marine life, as more nutrients are brought to the surface. The Sink itself would act as an artificial reef, not dissimilar to an oil rig or a fish aggregating device. [4] Therefore, Salter Sinks have the potential to increase the biocapability of the Gulf.
Salter Sinks would likely not interfere with thermohaline circulation, as their impact would be limited to the top 200 m of Caribbean or gulf ocean water, which is normally mixed in the summer and fall months by hurricanes. Further, the total heat and energy of the system is conserved vertically. This is in contrast to Ocean Thermal Energy Conversion (OTEC) systems, where wave energy is converted into electricity and taken onshore. [5]
As discussed above, the Salter Sink is not without issues. Another key issue is shearing. [6] Rigid tubes extending 200 m into the ocean would be subject to considerable linear and shear forces as underwater currents push the tube. This can be partially offset by shaping the tube as an ellipse, maintaining the round mouth of the top. If allowed to freely rotate, the Salter Sink will turn to minimize the stress due to the current without sacrificing the amount of water entering the tube per unit time.
The Gulf is a high-traffic zone for boats, and 10 k Salter Sinks would likely not be popular with the shipping industry. However, hurricanes are unpopular with freighters as well. It would be important to make the Sinks a very bright color, as is done with buoys, and place them as outside of known shipping routes as possible.
The single largest concern is the degree of mixing produced by the Salter Sink. While the energy calculations above suggest order gigawatts of energy are being removed from the surface, it is possible the warmer water emerging from the bottom of the tube quickly returns to the surface or otherwise unpredictable currents interfere with the system. [6] One alternative design could involve using the energy generated by the Salter Sink to directly pump the cold water from 200 m up to the surface. This would cool the surface dramatically more than effectively try to skim off the top of the ocean surface. However, the single greatest upside of the Salter Sink is its simplicity. Including turbines or valves adds a layer of complexity that may preclude any deployment.
The feasibility of building, releasing, and tracking 10,000 Salter Sinks is a further issue. However, reducing the power of a hurricane is no small task, and expecting an easy solution is foolish. Hurricanes are so destructive and expensive that it is worthwhile to explore solutions that can be easily tested. It should not be out of the question to build and evaluate the water-cooling properties of 10 Salter Sinks in the Gulf as a pilot experiment.
© Madeleine Scott. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. C. Trepanier, "North Atlantic Hurricane Winds in Warmer than Normal Seas," Atmosphere 11, 293 (2020).
[2] S. Salter, "A 20 GW Thermal 300-Metre3/sec Wave-Energised, Surge-Mode Nutrient-Pump for Removing Atmospheric Carbon dioxide, Increasing Fish Stocks and Suppressing Hurricanes," Proc. 8th European Wave and Tidal Conference, 7 Sep 09.
[3] G. de Lanza Espino and J. C. Gómez Rojas, "Physical and Chemical Characteristics of the Gulf of Mexico," in Environmental Analysis of the Gulf of Mexico, ed. by K. Withers and M. Nipper (Harte Research Institute, 2004).
[4] S. H. Kramer et al., "Evaluating the Potential for Marine and Hydrokinetic Devices to Act as Artificial Reefs or Fish Aggregating Devices," H. T. Harvey and Associates, 12 May 15.
[5] S. Harrison, "Ocean Thermal Energy Conversion," Physics 240, Stanford University, Fall 2010.
[6] S. Salter, "Wave Energy: Nostalgic Ramblings, Future Hopes and Heretical Suggestions," J. Ocean Eng. Mar. Energy 2, 399 (2016).