A Fond and Sorrowful Goodbye to Tidal Energy

Here’s a question I was asked earlier today.  If the average ocean depth is two miles, and 70% of the Earth’s surface is covered in water, that means that there are tens of millions of cubic miles of water bobbing up and down due to the tides. Doesn’t that represent far more energy than we need?

There are two issues here. 

• You misunderstand what’s going on when the tides change. You’re probably aware that the tides are caused by the shifts in the gravitation fields of the sun and the moon relative to that of the Earth. This has the effect of dragging a comparatively small amount of water around the surface of the oceans (not up and down). If you want to see this in action, go into your kitchen, take a glass baking dish, fill it part way with water, put it on a counter and let the water level come to equilibrium.  Then gently and gradually slide a penny under one side, and watch what happens to the water.  Note that most of it doesn’t move at all, and that a tiny bit moves laterally in search of the new equilibrium.  (On a relative scale, the movement caused by the penny is millions of times more pronounced than the movement of the ocean waters caused by the above-mentioned gravitational shifts.)

• Even given that, the total amount of kinetic energy of the ocean waters that could theoretically be converted into useful energy in the form of, say, electricity, is still fairly impressive. But note that it’s being released all over the globe. There is no way to concentrate it at a single point to facilitate the process of extracting it.

Thanks for asking, but I hope you can see that this is a fairly uninteresting subject—especially as solar and wind continue to scale and plummet in cost.

Now that I’ve done you the favor of answering your question, let me ask you one in return.  If you did the experiment that I suggested above and you live in California or some other drought-stricken area, take the water outside and dump it on a plant that needs it very badly.  Peace out.  🙂