28-ton, 1.2-megawatt tidal kite is now exporting power to the grid

> The turbine shaft turns the [onboard] generator which outputs electricity to the grid via a power cable in the tether and a seabed umbilical to the shore.

> Movement of tides causes a loss of mechanical energy in the Earth-Moon system: this results from pumping of water through natural restrictions around coastlines and consequent viscous dissipation at the seabed and in turbulence. This loss of energy has caused the rotation of the Earth to slow in the 4.5 billion years since its formation. During the last 620 million years the period of rotation of the Earth (length of a day) has increased from 21.9 hours to 24 hours;[10] in this period the Earth-Moon system has lost 17% of its rotational energy. While tidal power will take additional energy from the system, the effect is negligible and would not be noticeable in the foreseeable future.

and there is some simple math in the [Tidal acceleration] page [0] that you may want to read. Basically, the tides slow the rotation of the Earth, with some of the energy being transferred to the Moon but most of it going into friction. The natural friction is estimated at a (surprisingly low, IMO) 3.64 TW, and at this rate the Earth will stop rotating in 50 billion years, long after the Sun has blown up.

So if we want to keep the Earth spinning (albeit slowly) until the Sun goes red giant, we can afford to cut that time down to a tenth, meaning we can produce 36 TW (before conversion to electricity, i.e. ignoring efficiency).

If we are being reckless and just want to use power until we figure out nuclear fusion or something better, let's say a million years or so, we can make some 180 PW from tides. But since it would almost all turn to heat, and since that is more energy than the Earth gets from the Sun, that would probably be unwise unless we find a way to get it off-planet with very high efficiency.

(Global electrical energy production is currectly about 28 PWh per year, equivalent to a constant 3TW or so.)

[0] https://en.m.wikipedia.org/wiki/Tidal_acceleration

Imagine you wrap the Earth inside a sphere of perfectly efficient solar panels, that would be energetically troublesome! Every bit of solar panel we lay removes a bit of that overall energy flux; the question is then how much can we afford without detrimental impact.

Essentially we dampen tides with these devices. Windmills are slowing the wind. How much can we afford before it has an actual detrimental effect?

Also, pedantically, "renewable" is a fun word.

The question of "renewable" is whether the energy/matter somehow gets back in some way... Of course there's entropy and such.

Say you cut wood (CHO) and burn it. If you grow enough wood it'd recapture H2O+CO2 and you have a nice endless loop (plus entropy). That would presumably be renewable.

In that sense (again, pedantically):

- nuclear (and so the sun) is not renewable: reversing U/Pu/whatever fission is a teeny bit out of our league so someday the supply will dry out. Fusion as we do it (and the sun too, being main sequence) is H->He and we don't exactly know how we could reverse that so someday the supply will dry out.

- fossil fuel is renewable... on a geological timescale, which is not really practical; we can't exactly grow big enough forests and bury them for millions of years to get fuel back. So someday the supply of humans surviving will dry out.

- tides/wind is not renewable: we get mechanical energy but don't return it to the original place (or maybe extremely indirectly in the form of heat)

So it's all named backwards!

Or really there's no renewable... Essentially it all pans out because the Earth is not a closed system, there's loss in every transformation but it's balanced by the energy influx from our nearby star.

So I guess the "renewable" thing is not really, it's more like "capturing energy from an astoundingly immense and complex system in a way that doesn't throw it in a runaway catastrophe one way or another before the sun exits its main sequence".