Perpetual motion = endless energy?

@deepthought said
I did a back of an envelope calculation using some figures from Wikipedia, the Earth's rotational energy is of the order of 9.6E37 Joules. In 2016 world wide power production was 2311 gigawatts. So at that rate of consumption the Earth's rotational energy would run out in about 3 trillion years. There would be no observable effects over mere geological timescales. Sadly I doubt there's a way of doing it.

You could get energy by having a variation of the space elevator.

You know the moon recedes from Earth about 2 cm a year.

So if you got a couple of space elevators say one on the Luna north pole and Earth's north pole and "just' launched a rope from elevator to elevator, Luna's retreat could stretch the rope which could be attached to a generator say on Earth's elevator and Luna would pull on the rope slowly unwinding a mechanism that goes to a shaft on which is mounted an alternator.
Not sure how much energy you would get but it would be 24/7, still generating power at midnight.

Well, unless the sun is really 50 miles high like flatasssers say😉

Hay!
Hasn't anyone here but me noticed that tidal power is using the Earth's rotational energy to generate useful energy for us?
So the answer to the original OP question of "...couldn’t we harvest that energy? " is simply yes and it IS practical and is even already being done; just think 'tidal power'.

@humy

Yeah, I hear you loud and clear. I was trying to figure out the general effect of mass scale tidal energy harvesting on the dynamics of the system ( ie what does it do to the rotation of the earth). It seems pretty complex.

@humy said
Hay!
Hasn't anyone here but me noticed that tidal power is using the Earth's rotational energy to generate useful energy for us?
So the answer to the original OP question of "...couldn’t we harvest that energy? " is simply yes and it IS practical and is even already being done; just think 'tidal power'.

After thinking a bit, I’ve come to the conclusion with tidal power we are capturing energy from the earth-moon orbital dynamics, not capturing the the energy from the rotation of the earth. The rotation of the earth could stop and we could still be able to capture the same tidal energy as far as I can tell.

@joe-shmo said
After thinking a bit, I’ve come to the conclusion with tidal power we are capturing energy from the earth-moon orbital dynamics, not capturing the the energy from the rotation of the earth. The rotation of the earth could stop and we could still be able to capture the same tidal energy as far as I can tell.

No because each tidal cycle is completed in about 24 hours (24 hours and 50 minutes to be more accurate) because of the rotation of the Earth and if the Earth didn't rotate then each tidal cycle would be completed in about 30 DAYS because of the moon orbiting the Earth; which means considerably less energy involved. So most of the energy from tidal power comes from the rotation of the Earth relative to the Moon rather than the other way around. If hypothetically the Earth was tidally locked with the Moon then there would be NO tidal power from the orbit of the Moon around the Earth and yet that orbital energy of the Moon relative to the Earth would still be there, just not available for us to use.

However, that said, if there was NO MOON then there wouldn't be any useful amount of tidal power (there would still be a tiny amount from gravitational interactions between the Earth and other planets and also between the Earth and the sun but far too little for practical use I think) so one way of thinking of this as the presence of the Moon orbit as merely a tool to allow us to access the rotational energy of the Earth rather than be part of that energy.

@humy

Ahh...Yeah I somehow missed that blinding fact. Thanks for the explanation.

There are high tides in Canada and other places around the world but getting that power to the grid is another and also expensive proposition. That is one of the big problems with solar power, at least in the US, where the power grids are mostly around the perifery of the US but the best solar sites are the vast deserts of New Mexico, Arizona and Nevada. There is not much power infrastructure there and that has been estimated to have a cost of one trillion dollars to build before a single solar cell is in place, at least in gigawatt levels.

There is also power to be had in estuary waters where fresh water meets salt water, electricity can be extracted from that also.

@joe-shmo

I saw huge tides on Andros Island when I lived there working for AUTEC, tides going into Fresh Creek, I lived on the north island and AUTEC is on the south island and had to commute by boat every day and the tides coming in are really fierce and would be a clear case for application of tidal energy.
Same thing in Newfoundland, HUGE tides there. On vacation there in Digby we took a ferry boat in, car and all, and saw the tides up close, churning water, clearly another place tides could produce energy.
The effect of tapping that energy would be minimal I think, it would be thousands of years of total tide tappers to see any effect on the spin rate of Earth, which is slowly going down anyway. I don't think anyone would bother with the fact, say proven in a hundred years of total tide tapping, Earth spin went down by a whole millisecond due to those tidal taps, don't think that would be a big deal in a world with much heavier problems like climate change.

@sonhouse said
There are high tides in Canada and other places around the world but getting that power to the grid is another and also expensive proposition. That is one of the big problems with solar power, at least in the US, where the power grids are mostly around the perifery of the US but the best solar sites are the vast deserts of New Mexico, Arizona and Nevada. There is not much po ...[text shortened]... d in estuary waters where fresh water meets salt water, electricity can be extracted from that also.

There are severe ecological downsides to tidal power generation, at least in estuaries. There are reasons why this has not been adopted beyond lobbying by the oil industry.

@DeepThought
Yes, Fresh Creek in Andros is an estuary but if you did a tide project there, what effect would that have on the ecosystem? I wouldn't think you would totally dam the entrance to Fresh Creek, I could see that having an impact but suppose we only took out say 10% of the energy available would that have much effect on the eco there?

For one thing, the entire population around Fresh Creek is MAYBE one thousand so at a need of 24 Kwhr/day for each person you are talking 24 Mwhr/day or about 9 gwhr per year for the whole area, so there you are not talking about a huge city, just a village or two.