The top surface of the ocean is heating up. This increases the stratification, with the warmer, lighter water
on top and the cooler, more dense water on the bottom. This prevents the vertical circulation of water. The
layer that contains the stratification is called the thermocline. Nutrients such as nitrate and phosphate
ions and iron are in abundance below the thermocline, but the absence of vertical circulation makes them
unavailable to the phytoplankton living in the sunlit layer where photosynthesis takes place. Large areas
of the tropical ocean are becoming deficient in phytoplankton activity. This can be corrected by upwelling
nutrient-rich cold water from below the thermocline with wave-powered pumps.
The wave-powered upwelling pump being developed and tested by Philip Kithil's company, Atmocean, has been analyzed.
From information on their website, www.atmocean.com, it is a vertical tube made of thin plastic film with a heavy
check valve plate at the bottom. The tube is suspended from a float and a wire rope runs through the tube to connect
the float to the check valve plate. A slack mooring keeps the float in somewhat of a fixed location while the waves
lift it up and down. When the float is in a wave trough, the check valves open and the weight of the valve plate
pulls the tube down to engulf a small volume of water. As the water surface rises, the check valves close and the
float attempts to pull up the entire mass of water in the tube. But the buoyancy pull-up force is limited to
avoid excessive bulging at the lower end of the tube. The float is therefore submerged until the next wave trough.
While submerged, the sack of water is moved up slightly. The process is repeated with each wave cycle, so there
is a slow, intermittent upward transport of water.
The wire rope relieves the vertical stress in the plastic film, but when the closed check valve plate is pulled up it creates a pressure at the bottom of the tube which tends to make it bulge out. The float buoyancy is limited to a value that avoids excessive circumferential (hoop) stress in the film. The upward pumping rate is proportional to the diameter of the tube and inversely proportional to its length. However, the tube diameter must be about the same as the diameter of the valve plate. Otherwise the tube could not easily move down to engulf another quantity of water. Also, the open check valves should create a minimum of drag as the plate drops down. I have the impression that the check valve plate and tube diameter should be limited to one meter to allow storage, handling, and deployment.
We are free to string any number of tubes with their respective check valve plates on a single wire rope.
Breaking a single upwelling tube into N equal lengths multiplies the pumping rate by N because the float
buoyancy can be multiplied by N and applied to N check valve plates without exceeding the allowable
circumferential stress in any tube. The upper end of each tube might be joined to the valve plate above it
to form a closed system, or each stage might simply discharge in the vicinity of the intake above it.
Furthermore, we can upwell from any level to some higher level, not necessarily the ocean surface. There are many different ocean situations that require custom design of remediation and enhancement systems. The upwelling pump described above is also an important component of a wave-powered aeration system to be introduced and described in the next two sections.
Dead zones are deficient in dissolved oxygen and are also called anoxic zones. A troublesome one has been
appearing off the coast of Oregon. The California Current runs from north to south along the coast. The
Coriolis force diverts the fast-moving surface water in the current to the right, out to sea. The sea level
is higher on the right side of the current than it is on the shore side. This creates a return flow toward
the shore underneath the current. This returned water upwells toward shore, bringing nutrients which are
good for phytoplankton. But this deeper water has been depleted of oxygen by the respiration of ocean
creatures and the decay of organic matter. What used to be a productive fishery is being destroyed. A
wave-powered aeration system might help.
Aeration and oxygenation systems are commonly used in lakes and inland waterways. Aeration systems pump air through perforated pipes so that small bubbles are produced. The increased surface-to-volume ratio of the small bubbles helps them to dissolve more readily as they rise up through the water. Liquid oxygen is also used, perhaps because oxygen is the desired species and having to include four times as much nitrogen creates an excessive disturbance of the bottom sediment. Oxygen is used in lakes in heavily industrialized parts of northern Italy. Large compressors pump air through perforated diffuser pipes in Lake Elsinore (about 40 miles east of Los Angeles). Somebody was telling me about an aeration system near the Brooklyn Queens Expressway. I think it was in Wallabout Channel between Williamsburg and the Brooklyn Navy Yard. Electric power is available for these inland locations. This is not the case when we require remediation in the open ocean.
Wave motion is a possible source of power to aerate ocean water. We might rig up a float tugging on a pump against a mooring anchored to the bottom. Perhaps several anchors spread out in a 2-d pattern with slanted mooring lines would define a reference point in the ocean to react against the float and pump. However, I have difficulty figuring out how to compensate for changes in the ocean level due to tides. Perhaps a reader may know how this can be accomplished. However, I think I see how the float and pump can work against the inertia of the column of water trapped in the upwelling pump during its upstroke. This is described in the next section.
The air pump for this system is a vertical cylinder with intake and exhaust valves at the bottom end. The push
rod for the piston comes out of the top end of the cylinder. The cylinder can be built into the float.
To compress air, the push rod must be pulled down by at least a pair of wire ropes that are guided past the
cylinder and attach to the wire rope that runs through a single upwelling pump or a stack of upwelling tubes
and their respective check valve plates.
The orientation of the pump (piston on top and valves on the bottom) is chosen to facilitate clearing out water that might get into the pump. The air intake is via a snorkel whose open end should be above the water surface. A spring pushes the piston outward to draw air into the cylinder. This happens when the float is in the trough of the wave. There is always tension in the wire rope due to the gravitational pull on the check valve plate(s) of the upwelling pump(s). The spring must overcome this tension plus the water pressing on the outside of the piston. There is a complicated balancing act between all the parameters of the system (spring law, air piston area, check valve weight, spectral distribution of wave heights and periods, upwelling pump tube diameter and length, etc.) and I haven't got it all figured out yet. But I'm pretty sure everything will go together and we can see how well it works.
When the water surface rises and the upwelling check valves close, the air intake valve closes, the air pump starts to compress the air, the upwelling pumps are pulled upward, the float submerges, and the snorkel tube must protrude above the surface while the wave peak goes by. This can be accomplished by mounting the snorkel on its own float and connecting it to the air pump intake port by means of a flexible tube of sufficient length. There will be times when we get some water into the air intake due to wind and wave action, which is why I think we want the piston on top and the valves on the bottom.
The air pump outlet pipe extends down to the perforated diffuser at the chosen discharge depth. Suppose we choose to discharge at 10-meter depth. Water pressure increases by approximately one atmosphere (atm) for every 10 meters (32.8 ft) of depth so we must compress the water to an absolute pressure of 2 atm, 1 atm for the overlying air pressure, and 1 atm for the water depth. The air is taken in at 1 atm, so we must squeeze the volume in half, if the compression is isothermal. Some trial calculations suggest a cylinder a few inches in diameter, and a stroke of 20 inches. If the wave period is 10 seconds, the process is probably isothermal.
Just as in the case of the upwelling pump, the wave height does not appear explicitly in the analysis. Pushing the piston into the cylinder lengthens the distance between the float and the upwelling pump, so the float surfaces sooner than if the air pump were not in the system. The exact relation between wave height and air pumping rate will probably require more analytical and programming skills than I have. Maybe I can get some help. Let's look at the broader picture, which gets even murkier.
We have seen that both Nature and our upwelling pumps can bring up water with both low dissolved oxygen (O2)
and high carbon dioxide (CO2). We may be able to help the O2 deficiency by aeration, depending on how the
performance numbers turn out. But the possibility of upwelling high CO2 and having it outgas to the atmosphere
is bothersome, although this out-gassing has probably been going on all along in natural upwelling and nobody
worried about it. What can we do about it?
It's a dirty trick, but we can bring up one scarey thing to make another seem not so bad. We know that excess dissolved CO2 in the ocean is interfering with the formation of calcium carbonate (CaCO3) shells and skeletons due to the decrease in alkalinity of the ocean. The problem is expected to worsen, so the possibility of out-gassing CO2 from the ocean would help maintain its alkalinity. But this is just passing the buck between the ocean and the atmosphere. We want to decrease both the atmospheric and oceanic CO2. How?
Sequestration in geologic formations is being considered, but does not seem to have progressed very far, and appears to target concentrated CO2 sources like smokestack emissions before they are disbursed into the atmosphere. Removing CO2 that is already distributed throughout the atmosphere and concentrating it for sequestration adds another layer of difficulty. Studies are showing that biofuels produced by destructive farming practices (corn is a big offender) increase CO2. Not much help there.
CO2 and water are the raw materials required in bulk for terrestrial and oceanic photosynthesis. The ocean requires no irrigation and has the other ingredients, such as nitrate and phosphate ions, iron, and vitamin B12. They just have to be redistributed to where they can be most useful. In most of the tropical ocean, they are trapped below the thermocline, where stable stratification prevents them from circulating up into the euphotic zone, where the sunlight is sufficient to enable photosynthesis. Since the ocean food supply is declining, it would seem logical to upwell the nutrients needed to restore and enhance the ocean productivity. Then, on a full stomach, we can examine whether the oceanic or atmospheric CO2 is increasing or decreasing due to the upwelling. My guess is that more CO2 will be tied up in the carbon cycle of the ocean food chain.
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