Ocean currents may move at only 3 kts but represent one of the most energy-dense potentials on earth. Only fossil fuel deposits and geothermal hotspots have the potential for being denser.
A single current, such as the Gulf Stream, may represent the solar input of many thousands of square kilometers of open ocean- concentrated at the surface and transformed into motion. Currents vary in location and intensity, but to a lesser degree than wind and (except in desert areas) solar. They are far more constant than tides and waves.
I calculate that tapping a small part of the kinetic energy of contained in an area fifty-by-a hundred miles, and given a 3 kt average speed, would supply our entire planet' s energy needs. And that assumes a sparse distribution of the generators described below. To do the same thing with wind or solar would take up over a hundred thousand square miles of suitable territory.
Tapping ocean currents could be done with negligible effect on the currents themselves. How do I know this? Because the total energy available is extremely enormous. It's equivalent to somewhere in the vicinity of 1 it 1.5 million square kilometers of solar-collecting capacity. Any perturbation would be proportional to the percentage of the whole that is being tapped. Granted, this is something that would require further study. Even if extracted energy is minimal, a potentially nontrivial amount of local turbulence may be generated, which might confuse some of the less navigationally talented pelagic species. Fortunately the system described below would generate very little turbulence. Certainly no more than a bit of bad weather.
So, yes, the major problem isn't a lack of available energy. It's that you need large devices with broad cross-sections in order to tap remote current energy efficiently enough to make it pay off. If you build a bladed turbine, you'll be forced to work with a circular cross section. Because currents are fastest at the surface, and drop in speed with depth, it seems that ideal placement of a turbine would be close to half-in and half-out of the water- thereby allowing for its widest part to encounter the fastest speeds.
Unfortunately, moving blades in and out of the water would cause added drag at the air-water interface. An individual blade would only provide power half the time- unless the wind was right- and would be vulnerable to forces in directions other than the current itself- which would cause enormous stress. Furthermore, differences in air and water properties, as well as in speeds inherent to both, would dictate a design compromise away from optimum.
To build a bladed turbine large enough, one would have to build it incredibly strong. Strength equates with weight. Weight cuts into efficiency. Weight adds to cost.
A bladed turbine would need to be fully submerged to remain economical in its design. Being fully submerged, it would still encounter a speed gradient that would cause significant axial stress requiring a rather robust design. A bladed turbine would be inherrently inefficient in such a setting. It would have to be small and numerous. Again, too expensive, both in deployment and maintenance.
A bladed turbine is not the answer.
My solution doesn't use a turbine at all. It uses paired drogue chutes designed with high-aspect-ratio shapes optimized to put the bulk of the cross-section inside the fastest part of the current (right near the surface). Each chute would very large- thousands of square meters- and be attached to one end of a long semi-buoyant cable- potentially many miles long- that would, in turn, be connected at its center to an anchored barge containing a gearbox and generator (I mix units, that's right). While one chute is open, the second one would be closed- deflated so as to provide minimal drag as it is drawn opposite the direction of the active, open chute. If the two chutes were at the opposite ends 20-mile cable, and the current were traveling at 3 kt, and the chute was moving at 1.5 kts, it would take almost 6 hours to complete one down-current run.
When the open waterchute reaches the end of its run, it is closed- using a simple system that is nonetheless too complex to describe without the use of schematics- while its paired counterpart is opened. The closed chute is pulled back toward the barge while the open chute goes down the current in its place.
It's a process that would need to be actively controlled. As the chutes pass each other, for instance, they cannot occupy the same area of the ocean. The closed chute must be steered away, which is accomplished with a dynamically controlled rudder attached to the cable itself.
The chute and its cable is also a hazard to deep-drafted ships. Beacons and active navigation information would be in order.
The greatest obstacle isn't in the design and operation of the chute system, but rather in the anchoring of the barge itself.
I believe, however, that there will come a time when deep sea oil exploration rigs will outlive their primary use. The setting of anchors in the sea floor is a new potential market. As oil becames scarce, the need for alternative energy sources increases proportionally.
What impact would these waterchutes have on sea life? Well, for one thing, they are moving too slowly to trap anything. They can be equipped with deterrents that would protect larger lifeforms, and holes large enough for smaller ones to escape through.
Another obstacle is that such a system doesn't generate power constantly. The longer the cables, the higher the duty cycle. However, it can never reach 100%. Running multiple systems in tandem won't solve the problem completely either. Different units are bound to operate at different speeds, causing the down-cycle to arrive at different times. Sometimes it would average out into smooth power. Sometimes it would cause significant drops as multiple systems encounter simultaneous downtime. Such factors could be managed actively by scheduling reversals at less than full extenstion.
And then there is the huge problem of transmitting power from sea to land. Power lines could, conceivably, be run along the bottom of the ocean, or suspended at reasonably serviceable depths. Running power lines above the water is a special design problem but has the benefit of allowing the lines to be uninsulated. Either system may be more practical, depending on the individual circumstances.
Another obstacle is that ocean currents rarely exist close to where the power is needed. Fortunately, most of the world's population at least lives within a hundred miles of the ocean. Current farms could be built in many places around the world. Industries can also locate to where power is cheapest.
Such a system may well be the cheapest, greenest power source available on a global scale. It has an extremely high per unit cost due to the fact that it is only efficient at a large economy of scale. That cost translates, in turn, into a huge amount of power generation. Each one may be equivalent of an average-sized hydroelectric dam, or over a hundred wind turbines.
Will such a system ever be built? I doubt it. The obstacles are too great. The system would be too expensive to bring online. Only if we are desperate to be able to tap ocean currents would such a system be explored. Other sources of energy are bound to be far cheaper up front. It's an option though.
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