Energy Considerations

Golden Gate Bridge

Full disclosures, up front: I think evaluations of 1) full life cycle costs, 2) carbon footprint, 3) disposition needs for the end of the technology life cycle, 4) impact on wildlife and environment, or 5) strategic materials needs are often incomplete for large-scale use of either solar or wind energy. Such evaluations are complicated and usually more difficult than you might think. And of course we still lack large-scale energy storage technologies for power grid applications. The main problem (as our friends in Texas recently learned the hard way) seems to be that politicians and bureaucrats rather than engineers and scientists guide US energy planning and decisions. Sad, but no surprise.

Remember our post about large river basin drainage areas? A technically-minded friend recently sent the “tidal power project” article linked below, and asked what I thought:

The article about tidal hydroelectricity is interesting, but I don’t think it would be easy to make such a system work. The problem is that most natural situations involving flowing water include a delicate balance between the kinetic energy of the water currents and the amount and sediment load (silt by suspension, sand by saltation along the bottom) carried by the water. If kinetic energy decreases, sediment settles out. Examples include river deltas, where sediment carried by the river settles out and eventually creates sand bars, mud flats, and ultimately a growing delta because the kinetic energy of the river current dissipates as it enters a lake, the upstream end of a reservoir, or the ocean.

We can find an example involving tides here in California, right outside the Golden Gate. The current sweeping in and out through the Golden Gate with every change of the tides is quite strong in each direction, but the net movement of its sediment load is out to sea rather than back into the Bay. And when the current carrying the sediment flows far enough outside the Golden Gate, the kinetic energy dissipates, and it drops its sediment load. This in turn creates an arc-shaped ring of sand bars some distance outside the Golden Gate. You can find maps of the sand bars online, and the Bar Pilots use detailed maps of those sand bars for navigating ships in and out of the Gate.

My grandfather (and other people, I suppose) once proposed anchoring barges on the “Muddy Mississippi” River to generate electric power. The idea was to use the river current to turn a propellor under the barge to generate electricity. I don’t think anyone ever tried it because removing energy from the flow would create a sand bar just downstream from the propellor as the sediment fell out of the flow, and the sand bar would eventually divert flow away from the propellor. 

Bottom line is that I expect a scheme to generate energy from tidal currents would cause the flow to drop whatever silt or sand it might be carrying, and it would clog up the system. Make sense, or am I missing something?

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