Tidal energy is not renewable

The single most important assumption in this paper is that energy consumption will increase by 2% per year. This kind of exponential growth leads to outlandish estimates for the amount of tidal energy that society will demand.<p>Energy consumption has decoupled from population growth rates and economic growth.<p>How much energy will we consume in 1,000 years? Most projections of the population have it stabilizing at around 15 billion. But continuing at its current growth rate (an optimistic assumption I think), gets us to about 150 trillion humans in 1,000 years.<p>And at 2% growth rate, each of those humans will consume 20,000 times more energy than a circa 2023 human.<p>Now state of the art technology wastes about 80% of the energy consumed, so this is equivalent to 100,000 times more useful energy consumed per human.<p>So the physics in this page is a good examination of the surprisingly large compounding effects of unchecked exponential growth.

I&#x27;m not sure the author realized this, but they&#x27;re actually making a statement about how crazy exponential growth is. Not about the sustainability of tidal power. This becomes obvious once you look closer at what that 2% growth rate (as assumed in the post) implies.<p>Our global energy consumption in 2008 was estimated to be 474 exajoules. The total energy received by the earth from the sun during a year is about 5 million exajoules, a fraction of which reaches the surface. 5 million is much more than 474. But at a seemingly modest 2% per year growth rate (as it was between 1980 and 2006), our energy consumption will match those 5 million exajoules in less than 500 years!<p>Think about that: if energy consumption growth continues at the current pace, then in 500 years we&#x27;ll either be using ALL solar energy received by the earth (leaving none for the biosphere), or we&#x27;ll have figured out some magic technology to produce 5 million exajoules of energy per year. Assuming the magic technology, where are we going to get rid of all that extra heat? It would effectively be like having a second sun on earth, cooking us in place.<p>edit: I copied the numbers above from a post I wrote in 2010, so it may be a bit out of date. But Sabine Hossenfelder recently made a video where she talked about a similar timescale, i.e. boiling oceans in 400 years: <a href="https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=9vRtA7STvH4">https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=9vRtA7STvH4</a>

&gt; So, the 2% growth rate for world energy consumption should be a conservative assumption.<p>An important caveat: this article assumes that energy consumption will continue to increase exponentially to get the 1000 year timeline of draining the rotational energy of the Earth.

One thing I notice is the assumption of unlimited exponential growth by 2% per year. That&#x27;s a huge fallacy. Quick check, yeah, 1.02^1031 = 735,829,316. I&#x27;m pretty sure we will be using magnitudes more energy than today because we&#x27;re more people and hopefully with better living standards for everyone. But even then that&#x27;s a lot. And on that scale I&#x27;m not entirely sure where the whole energy should go to... Maybe produce mass&#x2F;objects out of it?<p>If we reach peaked our energy consumption in merely 250 years, that&#x27;s less than 150 times our current consumption. I didn&#x27;t do the math, but would date to suggest this gives us a few years more time on this planet.

But don&#x27;t the tides naturally dissipate a lot of that energy anyway? E.g. whenever you go to the beach and see waves crash upon the shore, that&#x27;s tidal energy being dissipated as heat. If you stick a turbine in the mix to extract useful work <i>before</i> it turns into heat, isn&#x27;t it still turned into heat regardless?

Hmm so if we pump water to amplify the tides we can speed up the Earth&#x27;s spin and get rid of the leap second.

This is the old my kid was two feet tall a year ago and three feet tall this year, so by the time she&#x27;s an adult she&#x27;s going to be as tall as a house!

&gt; Based on the average pace of world energy consumption over the last 50 years, if we were to extract the rotational energy just to supply 1% of the world&#x27;s energy consumption, the rotation of the Earth would lock to the Moon in about 1000 years.<p>This is bananas. I stopped right there and closed it. I see somewhere else in this thread that they tried to do that by extrapolating an exponential growth curve through an outlier (the industrial revolution!) for a thousand years. Maybe that explains it.<p>But... that&#x27;s not an error. That&#x27;s just bananas. Absolutely insane.<p>Some quick googling, FWIW, gives the earth&#x27;s rotational kinetic energy as a quite plausible 2.1e29 J (though a little of this will not be extractable tidally, as the earth will lock to the moon at a few percent of rotation speed), and the total world energy consumption as 22.8 TWh&#x2F;year. So the back of my envelope says that at current consumption we have a hair over... two trillion years.

Tons of comments here highlighting that 2% annual growth rate in energy consumption is ludicrous.<p>But so what if we made a more reasonable assumption that annual energy usage will stabilize at, say, 5X of what it is currently, and the (unreasonable) assumption that we get 100% of that energy from tides.<p>Then how much of a rotational slowdown do we get after 1000 years?

Relevant: <a href="https:&#x2F;&#x2F;physics.stackexchange.com&#x2F;questions&#x2F;6400&#x2F;are-tidal-power-plants-slowing-down-earths-rotation" rel="nofollow noreferrer">https:&#x2F;&#x2F;physics.stackexchange.com&#x2F;questions&#x2F;6400&#x2F;are-tidal-p...</a><p>This theory is very interesting, although the author presents it with too much confidence for such big claims.

Every energy source becomes renewable on the right time scale.

This is mostly based on the assumption of a 2% growth rate in overall energy consumption, with tidal power as an ever-growing 1% of that. The effect of covering 1% of our current energy usage would be minimal. And, likely, the ability to exploit tidal power will plateau at some (fairly small) amount, due to geographic constraints.

Issues with exponential growth set aside, does wind power suffer from the same problem? If I understand correctly, wind mostly caused by the coriolis force, which is a consequence of the earth&#x27;s rotation. Would building too many wind turbines slow down earth&#x27;s rotation?

The time limit of 1000 years is the worst part of this paper. If the energy consumption literally grows 2% every year, in 3013, the humanity will be consuming 3.98x10^8 times more energy than now. If it&#x27;s really the case, the dissipation heat from the use of such a large amount of energy alone will kill us all, long before the tidal lock b&#x2F;w the earth and the moon.

Then technically neither is wind, since it slows down surface air currents and widens the effective boundary layer of the atmosphere.<p>Technically-technically, no forms of energy &quot;generation&quot; (technically just conversion) are 100% efficient so something is always lost to heat. I guess the important question is, what is the net effect in changing that kind of energy into purely thermal energy?

Wait, isn&#x27;t most of that &quot;tidal power&quot; that we would be capturing going into grinding rocks into sand along the coast?

Let me nitpick: The headline alone is right: Tidal energy cannot be renewed. But that is true for all renewables like wind and solar, because &quot;renewable&quot; is a misnomer. Real renewal would violate thermodynamic&#x27;s 2nd law. What the author actually seems to doubt is the ABUNDANCE of tidal energy (but supports that with questionable assumptions (cf. other comments here))

If you can slow down the Earth by consuming the tidal energy, can you also speed it up by correctly timing pumping water to reservoirs?

I think out of embarrassment, the student also decided not to showcase his proof that in 1000 years, the mass of human beings will outweigh the Earth.<p>Eh, who am I to criticize? They say that your early grad school years are a time to publish large amounts of papers that you don&#x27;t think are likely to stick. This is a little bit out there even by this standard though.

The author makes the point that the slow-down of the day is non-linear, and then he uses a linear equation to estimate the time until the Earth is locked to the Moon!<p>His doomsday scenario of total tidal locking would never occur -- as energy is removed from the rotation of the Earth, the maximum power level that can be extracted would <i>decrease</i>. Also, the cost-efficiency would also drop.<p>There would be a point where the day is &quot;merely&quot; longer, the Earth is not yet tidally locked to the Moon, but extracting more tidal energy is no longer worth the trouble.<p>The original point however is still valid. Even if the rotation was slowed to just 1&#x2F;2 of what it is now, the Earth would have a 48-hour day and that would obviously cause absolute havoc with the environment.

I really love this kind of thinking and explanation - it may not be correct per se, but it&#x27;s the TYPE of thinking we need.<p>Rather than fluffy repetitions of the slogans of in-groups, it&#x27;s an attempt to explore long term impacts using fundamental scientific principles, none of which are individually too complex.<p>And if we don&#x27;t agree with the 2% annual energy growth rate for 1000 years then that&#x27;s fine, we can fiddle the numbers for what we believe, and the same analysis can be valuable even if it ends up supporting a different conclusion.<p>I&#x27;m interested by the language style as well, it reads like they put it through a filter to generate &quot;simple English&quot; as a deliberate choice.

The comment here focus on the exponential growth of energy needs assumed. However that is besides the point, unless I am missing something. If the increase doesn’t happen in 1000, may be 10000. The main point for me is that by using the tidal energy we are slowing down the earth rotation. That just sounds intuitively wrong to me. This energy is already released because of the rotation, how can converting that impact the earth’s rotation?

A related question I think about from time to time. If our energy consumption grows, how soon we will destabilize the Earth?<p>Even 100% clean energy, if infused into the system may increase temperature, or speed up circulation of currents, and other effects.<p>Of course, it&#x27;s not nesessary all energy goes into temperature. Some can be conserved, e.g. as chemicals.<p>But in general, the more energy we have in our disposal, the more potential for damage.<p>We will dig deeper, smelt more ore, etc, etc.

Regarding the &quot;How Are Tides Formed?&quot; section with a nice neat diagram and some math&quot;<p>But that is not really correct. it would be but there are large land masses interfering with the process. Ocean tides are better conceptualized as water sloshing in a bathtub.<p><a href="https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=PSJRymZ5bJs">https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=PSJRymZ5bJs</a>

Can someone with more experience in the field comment on whether this paper is well founded or off base?<p>The abstract sounds... lets go with &quot;not completely implausible&quot; but the assertion that extracting 1% of the Earth&#x27;s energy from the tidal sloshing would slow the earth and tidally lock it in 1000 years feels extreme. Of the extreme assertions require extreme evidence variety.

Is it not the case that the moon-earth system may become locked, irrespective? I mean if we&#x27;re worried about future decades.. As long as this happens before the sun expands to make the earth uninhabitably hot, it&#x27;s a risk for .. some intelligence?

Where is the angular momentum going?

It&#x27;s a neat thought experiment, but the underlying assumption:<p><pre><code> The world&#x27;s energy consumption was about 5.67x1020 Joules in 2013.[18] This number has increased by more than 2% per year on average in the last 50 years. The average world economic growth rate in the last 50 years is about 3%, which requires a corresponding increase in the energy supply. So, the 2% growth rate for world energy consumption should be a conservative assumption. </code></pre> ... is a bit naive. If we&#x27;re consuming (does some math) `1.02^1000 = 398264651` ...<p>Four billion times as much energy as we do today. I don&#x27;t think there&#x27;s much risk of us growing our population to that degree, nor of us being that power hungry if our population stabilizes. We&#x27;ll be either extinct or back to a sustainable agrarian population far before we reach that upper limit. Honestly, if we produced that much power, I suspect we&#x27;d have long since boiled the oceans, making the whole argument moot.<p>TL;DR: Don&#x27;t extrapolate FAR into the future based on a small (relatively) set of data points.

Is this satire? It seems like there&#x27;s too much math in this to be satire.

Short story time <a href="https:&#x2F;&#x2F;archive.org&#x2F;details&#x2F;ExhalationByTedChiang" rel="nofollow noreferrer">https:&#x2F;&#x2F;archive.org&#x2F;details&#x2F;ExhalationByTedChiang</a>

It&#x27;s probably much worse than this. Once the earth stops rotating IMHO it&#x27;s likely the magnetic core stops rotating. Then the solar wind strips our aosphere in only 1000 years ...

Everyone is commenting on the unreasonable exponential-growth assumption. But regardless of growth, the article is still technically correct that tidal energy is not renewable.

Solar energy is not renewable too in the long run (very long run).

More quality science coming out of Stanford as usual.

This is a CS professor doing some naive math and assuming that the energy usage patterns of the past 50 years (an increase of 2% usage each year on average) could or would keep going for the next 1000 years.

a few lines of css will massively improve mobile experience

Neither is fission.

Needs (1998).

Now do wind

All this doom and gloom, I say let&#x27;s go for it though, most of us would benefit from a few extra hours in the day right? We can switch off the tidal power once it gets to 26 hours or so ;-)

&gt; ... if we were to extract the rotational energy just to supply 1% of the world&#x27;s energy consumption, the rotation of the Earth would lock to the Moon in about 1000 years ... and most life on Earth could be wiped out.<p>Important safety tip.

So they postulate how the moon will in 1000 years become stationary in orbit due to the tidal energy extraction as to why tidal energy is not renewable. Sorry but I&#x27;d need a few more than one institution to be backing this. Otherwise, it seems a bit too reaching to be anything other than a great plot for some cheesy disaster movie that they have phases of doing.<p>[EDIT - Reversed AI&#x27;s rewrite of my humble English, raw best with all its flaws, and whilst AI version good, just had an air of sterility and not me.]