[throwbadubadu]

More renewable is good.. but there is always one fear in my crazy mind about these: This isn't actually renewable but taking energy out of a huge reservoir (Same for solar if pea counting, but solar is really endless until the end of the solar system).. and if we'd scale it massively the result could be another catastrophe. (Same btw. with geothermal?).

(Btw.. I maybe got tidal wrong, so if it is just taking energy from waves via wind this counts as endless solar. Still, consider systems which would really break the tides and take energy out of the earth<=>moon system).

The answer to this fear is that this reservoirs are so massive they are quasi endless in regard to what we ever could take out? It is hard to find numbers for these crazy thoughts :)

[card_zero]

Some numbers here: https://en.wikipedia.org/wiki/Tidal_power#Principle (last paragraph). Tidal energy is wasted by natural coastlines all the time, being converted into heat and erosion. If this effect is constant, it looks like the day gets longer by 7 minutes 12 seconds every 36.5 million years. (I calculated that because it's a 1% reduction in rotational energy.) Human extraction of tidal energy is much smaller than the energy wasted by tides acting on all the world's coastlines, I assume. To some extent it takes energy away from coastal erosion, which also seems fairly benign.

[evv555]

>Tidal energy is wasted by natural coastlines all the time

Erosion is a major part of Earth's carbon cycle[1]. It's not a given that you can scale this technology without impact.

1. https://carbonclaire.com/the-carbon-cycle/#:~:text=The%20geo....

[ralfd]

Sanity check with quick googling:

> Geological evidence ... indicate that 620 million years ago the day was 21 hours, says Mardling.

[whatisyour]

Erosion is not always wastage.

[bbor]

For a detailed discussion of “what is the environment, and what does preserving it look like?”, see the Red Mars trilogy. Very interesting questions about nature as aesthetic vs nature-in-its-own-right

[mft_]

Could you expand? I'm interested in what you're getting at.

[wongarsu]

Erosion makes gravel and sand, which are quite useful. It also makes minerals and nutrients accessible to life.

But we spend a lot of time and money managing erosion, and at the same time (often accidentally) create lots of erosion in other places. I doubt extracting tidal energy will have effects on a global scale, though it might cause some local changes

[Eddy_Viscosity2]

I think he's referring to nature making less florida.

[actionfromafar]

OTOH there's a case to be made for keeping florida diluted.

[idontknowifican]

erosion actually creates nests for various sea birds

[piaste]

It's good that you are aware it's a crazy thought :) Nothing wrong with looking into it though. Wikipedia offers:

> 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

[lloeki]

> 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.

I think you're touching on the problem very nicely here: the problem is not "how much raw energy is there" (because there's an absolute fuckton of it) but "when does the impact of capturing this energy - thus removing it from a complex ecosystem whose stability may depend on it - and turning it into something else - thus adding to a complex ecosystem in other ways - becomes a problem", which may or may not come well before the theoretical raw energy cap.

[piaste]

I'm not sure those considerations are applicable. There is no "ecosystem" that depends on the rate of slowdown of Earth's rotation - we do need to leave some rotation to keep flora and fauna alive, but that's a "reservoir" problem, to use the OP's formulation, not an intensity one.

As for the addition of energy (heat) to the planet, sure, but there's nothing specific about tidal energy here. All energy sources except solar (and wind/hydro which are direct solar derivatives) add heat to the planet that would otherwise have stayed sequestered. But it's many orders of magnitude less than the heating from solar energy, which is why increasing Earth's absorption factor by a few points is an infinitely bigger problem than all the energy we are directly producing or can hope to directly produce in the next few centuries.

[zaroth]

Also worth noting that trying to extrapolate the effects of current technology at this timescale is pointless.

There will be tide changes in human’s technological capabilities long before there could be any reckoning for over-extraction of the Earth’s rotational energy.

For example, in our ability to bring mass into orbit. Add a couple orders of magnitude in that capability, and humans can start directly tuning the Earth’s total solar irradiance by shooting lunar dust into a Lagrange point, reducing the amount of sunlight hitting the Earth by fractions of a percentage.

The Lagrange point is not perfectly stable, so the dust naturally dissipates over a 10-20 year timescale. If you don’t keep sending more dust, the “planetary sun-shade” naturally dissipates, so there’s zero risk of overdoing it.

Basically, within the next ~50 years, I predict we will gain the ability to turn down the planet’s thermostat in a very safe and predictable fashion, without having to pollute our own stratosphere. Overheating the planet becomes a total non-issue.

[ComputerGuru]

How much dust would you need to shoot up to affect a statistically significant change in the amount of sunlight getting through?

[zaroth]

A fuckton.

Roughly 100 million tons annually of lunar regolith launched into L1 by railgun.

https://journals.plos.org/climate/article?id=10.1371/journal...

[SoftTalker]

> within the next ~50 years

Very optimistic. We haven't even been to the moon again in 50 years.

500 years, maybe.

[bbor]

With all seriousness, I’ll throw in “the unexpected breakthroughs in intuitive AI will aid this effort.” I sincerely think that LLMs will at least make R&D cheaper.

Beyond that, “we haven’t been to the moon” isn’t a fair summary of our tech imo - a HUGE portion of that is political in origin, and private companies have invented reusable rockets which is pretty damn important

[lloeki]

> There is no "ecosystem" that depends on the rate of slowdown of Earth's rotation

I mean, raise acidic/temperature levels only so slightly and it kills off coral reef, which in turn destroys fauna that depends on it, leaving huge areas barren.

What I'm getting at is that small changes can have dramatic domino effects. Mass-scale tide dampening could have unforeseen effects.

Not saying any of the solutions are going to be a problem, merely that it's better to ask crazy questions about what happens when they ramp up at scale than handwave things away with uniform spherical cows.

[Karellen]

> at this rate the Earth will stop rotating in 50 billion years,

That's not what the link you posted says. It states that that's when the Earth-Moon system will be tidally locked, so that the Moon orbits the Earth at the same rate that the Earth rotates around its axis.

Kind of if the earth slowed so that one "day" took the equivalent of 28 days now. The earth would be locked with one side always facing the moon, so there would be no tides, but it would still be rotating. Except it will end up be a longer "day" than that, because as energy is transferred to the Moon its orbit is raised, which slows it down.

[piaste]

You're right, my mistake.

Ignoring for a second both the human factor and the Sun having gone red giant much earlier... I wonder what kind of ecosystem that would create. If the Earth were tidally locked to the Sun (which is what I was picturing in my confusion), it would clearly end up as two deserts with a tiny strip of life along the twilight circle. But how would complex life evolve to adapt to a 28*24h day/night cycle? We do have decently-sized ecosystems in caves so long nights aren't insurmountable, but being baked to really high temperatures for a month and then cooling down to freezing for another month sounds like quite a challenge.

[dylan604]

This is nice and all, but in 50 billion years, the moon's distance from the Earth will much greater which means it's gravitational effect on Earth will be much smaller.

[Karellen]

OK. Do you have any reason to suspect that the people making their calculation didn't take that into account? Of all the things that will vary over time, including the distance between the Earth and Moon, do you really think they'd have missed the fact that the Earth's gravity varies with distance?

If you've got what you think is a more accurate number, and can show your working, I'm sure the wikipedia editors would be willing to take a look at it.

[zakary]

You’re right that nothing is unlimited. Luckily in this case, the energy of waves and tides is given to the oceans mostly by the gravity of the moon pulling the water as it passes overhead. Also a little bit by the wind which is fed by solar heat energy. The moon is slowly moving away from earth and eventually, in hundred of millions of years, it will impart significantly less energy into the tides.

The energy that the moon gives the tides is essentially the same as how the sun gives energy to the ground with light. That is to say: If we don’t collect it, it just gets turned into another kind of energy that is absorbed by the environment. For tides and waves that would be mostly heat, and a little sound. And most of that heat would eventually be radiated back out into space. So suffice it to say, while there is a finite pool of energy stored in the tides, it is so massive we could never make a difference, and it gets recharged everyday by the moon.

[LM358]

"Tidal Energy Is Not Renewable": https://cs.stanford.edu/people/zjl/pdf/tide.pdf

Previous discussion on HN: https://news.ycombinator.com/item?id=37383283

[dogtimeimmortal]

Thanks for this! Something has always bugged me when I hear about big plans for tidal energy.

TLDR, the 1000 year estimate is is derived from the geometric series proof(i needed a refresher). Wolfram explains it pretty well: https://mathworld.wolfram.com/GeometricSeries.html

...then i needed a refresher on exponents and logarithms: https://wou.edu/mathcenter/files/2015/09/Exponents-and-Logar...

It's good to do the math and think about the big picture sometimes. ;)

p.s. i tend to feel somewhat skeptical about a lot of renewable projects i read about in the news. And even if the math all works out and it really IS The Best Solution i cant help feeling we are ignoring the elephant in the room(population growth). Maybe human beings should start preparing themselves to live with less(and be happy with that).

[ComputerGuru]

Population growth is effectively over.

[Cthulhu_]

Even if that were true (and it won't be for another 5-6 decades using current predictions at least), energy consumption is anything but. And if there is a risk of electricity usage plateauing, they'll just find something new like training AI models or spinning GPU's to churn out digital money (where said churning is entirely artificial).

[dogtimeimmortal]

Little addition, couldn't resist sry:

After reading a bit of the comments thread above, i couldn't resist adding a bit more.

> The single most important assumption in this paper is that energy consumption will increase by 2% per year.

Quick reality check: 1.02^1000years = 398,264,652x increase! This could potentially be an unrealistic projection. Let's consider what we know from the article(and google).

Here's a few numbers pulled from the article for reference: ~1.73 x 10^20 J tidal energy lost per year naturally 5.67x10^20 J was the total global energy consumption in 2013(10^18 is 1%)

So 398,264,652 x 5.67x10^18 J = 2.258 x 10^27 J! That does seem like a lot. In comparison total sunlight absorbed on earth's surface each year is roughly 3.85 x 10^24 J.[1]

> It might also be that tidal energy extracted by humans, comes out of some fixed 'budget'

So lets compare that to what is already being consumed and think about what amount of energy generated might be realistic. Top google result tells me global energy consumption is about 5.80 x 10^20 J2 these days.[2]

I guess the next question is what would be a realistic upper limit for the amount of energy we could harvest globally from the tides? Referencing the Stanford article again, we have an estimated annual energy loss of 1.73 x 10^20 J total from natural friction of the tides globally. Is it realistic to think we can harvest an equivalent amount of energy for the grid? I wonder what sort of impact a turbine project that size would have on the earth? Is it even possible? For sake of being rational, let's say we think a network of turbines 1% of that size(10^18J) is potentially possible. How many turbines might you need to make for something that size?

So lets take the kite-turbines from the article at the top. Each one of the produces 1.2MW. Does that mean it produces 3.78×10^13 J annually? Or is it closer to half that(1.89×10^13J)? In a 10^13 J ballpark we would need to produce about 10^17 to approach our max limit 10^20 J harvested. Correspondingly a project 1% the size still needs 10^15(a quadrillion) of these turbines globally. That seems like a lot and it just sort of seems in my mind(maybe i'm wrong) that there would be some sort of environmental impact. Also, now that you have shrunk your project the total energy produced is now a fraction of 1% of the current total global annual energy consumption, and grows more insignificant every year consumption rises.

Not sure I agree that population growth is over, but isn't it true that energy consumption per capita is also increasing? Either way I think we still have the realistic expectation of continued 2% growth in global energy consumption for the next few decades at least, doubling every 35 years. Unless we can find ways to harvest larger quantities of tidal energy without breaking the ocean it just doesn't seem like a silver bullet to me as far as climate goes. Assuming we could harvest 25% the quantity of what occurs naturally, that only gives us 0.4325x10^20J annually, which is only 7% current global consumption - in 35 years that number will only equal 3.5% of the total consumption. I guess it's a question of how much you want to screw up the environment in order to save it.

TLDR, saving the planet with tidal energy seems kind of misrepresented imho. Build wooden megaliths instead. ;)

[1] https://www.ucdavis.edu/climate/definitions/how-is-solar-pow...

[2] https://www.theworldcounts.com/challenges/climate-change/ene...

[usrusr]

How would solar be the same? That "reservoir" is emptying wether we hold silicon into the photon flow or not. Tides and wind aren't quite as far from just taking what is already disappearing, but even there: the natural drag of land and sea floor shapes is not only so many more orders of magnitude larger than anything we could ever build, there's also the nonlinearity of drag, if we slow down the natural flow a tiny little but the effect of natural drag will decrease accordingly and the total difference to natural flow will be much smaller than it would be if drag was linear to speed.

[ben_w]

Tidal energy comes from the moon (and the sun) pulling the oceans as the Earth spins. Extracting energy from this effectively increases the drag between the ocean and the land, which slows down the rotation of the Earth. This effect even occurs without extracting tidal energy, and some of the dinosaurs (IIRC) had 19 hour days.

The amount of energy in the rotation of the Earth is so huge that this basically doesn't matter, 10 TW for 660 million years, which is into the realm of "will too much carbon be subducted for photosynthesis to continue?" timescales.

[Y-bar]

I propose two possible solutions:

A. Build some huge-ass rockets on the moon to accelerate it to offset any slowdown from changes in tidal energy.

B. Divert a few billion tonnes of asteroids to crash into the moon's aft, adding both to the moon's velocity and mass.

I believe that if we begin working on any of these solutions within a hundred thousand years or so from now we should be able to offset most of the projected long-term lengthening of the day.

[ben_w]

You'd have to put the rockets/target the asteroid impacts on the Earth, the effect on the Moon from this interaction is to push it into a higher orbit to conserve angular momentum of the combined Earth-Moon system.

[Mistletoe]

> "will too much carbon be subducted for photosynthesis to continue?"

Great, you gave me a new fear. :)

[jacquesm]

You're edging on the laws of thermodynamics there: on a long enough time scale all energy is non-renewable, every little bit of consumption gets you that much closer to the heat death of the universe. But given the vast reserve in that system for once humans won't be able to meaningfully affect it unless we start consuming far more than we do today. Interesting aside: that momentum is a giant energy reserve and conceivably you could rob some of it to create a means of escaping the gravity well even if there had not been any other.

[lm28469]

> (Same btw. with geothermal?).

Not a chance, we're barely scratching the surface

> More renewable is good.. but there is always one fear in my crazy mind about these

It depends a lot on the rate of maintenance, materials used, &c. For example small windmills are next to useless: https://solar.lowtechmagazine.com/2009/04/small-windmills-pu...

[lloeki]

> Same for solar if pea counting, but solar is really endless until the end of the solar system

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".

[IanCal]

I think you're assuming renewable means infinite.

> - nuclear

Nuclear isn't renewable, but there's a huge amount of it and it doens't have the same problems.

> (and so the sun) is not renewable:

The energy we get from it is. It's not a fixed resource that we're depleting. When we burn all the coal, it's gone. Tomorrow there will be more sunlight. We're not cutting chunks off of the sun to get energy for solar.

> - fossil fuel is renewable... on a geological timescale,

Only some of them, and only if we change the definition of renewable. Coal is there because there weren't fungi that could break down lignin at the time.

> 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.

We could probably benefit significantly with reducing the energy flux right now.

[lloeki]

> I think you're assuming renewable means infinite.

No, for the purpose of this dumb exercise of mine, I mean the resource we use for energy production is replenishable.

> It's not a fixed resource that we're depleting.

What I mean is that the Sun is going to exhaust its H supply. We just happen to harvest part of its output, but it's depleting itself whether we harvest it or not, so we might as well do it :shrug:.

So at our scale it looks like infinite but in the strictest sense it's a consumable that does not replenish.

Fundamentally that makes it not different from nuclear fuel (whether heavy fissile ones or hydrogen if we manage fusion someday) which as you mention are in ample supply, yet we consider them to not be renewable while the sun would be? The only difference is a) the Sun is (much) bigger thus will last (much) longer and b) we delegate fuel logistics and fusion reaction to the sun.

But then again, trees are not really renewable, they got to get energy from somewhere to grow... IOW they're essentially CHO-based solar batteries.

As I said, I was not trying to make any point, just being extremely pedantic about semantics for fun ;)

> the definition of renewable

> A renewable resource (also known as a flow resource) is a natural resource which will replenish to replace the portion depleted by usage and consumption, either through natural reproduction or other recurring processes in a finite amount of time in a human time scale. When the recovery rate of resources is unlikely to ever exceed a human time scale, these are called perpetual resources

https://en.wikipedia.org/wiki/Renewable_resource

Essentially the definition is focused on resource when it's really about time (esp. our extremely small time scale vs stellar scale). The resource in reality doesn't replenish, rather it's so huge that we can consider our usage of it well below rounding error. IOW from our very small "rounding error" human point of view it might as well be considered infinite.

[IanCal]

We could push it further and say there's no renewable source possible - at least until the multivac finally gets the answer.

> The resource in reality doesn't replenish

Sunlight does. Wind does. A water wheel in a river is renewable power. Solar panels producing power today don't make tomorrow darker.

[friend_and_foe]

Yes, you're right. What you're doing when harvesting tidal energy is introducing a friction like resistance on the moon as it revolves around the earth and on the rotation of the earth, and taking energy from it's energy of motion, that is, slowing it down. But I'd expect with the masses and velocities involved that this amount is miniscule and not even close to being measurable. That said, we didn't think we could impact the earth by burning fuel, and if something like this were adopted large scale the impact could surprise us and be a real problem.

Solar isn't as limitless as you'd think. It is limited to the surface of the earth, more precisely, the surface area of the 2 dimensional disk from the perspective of the sun. And on earth, that's what drives life, so harvesting solar energy on the surface is something that, if deployed beyond what we think we need right now, can have a negative impact on the earth. Harvesting it in space and beaming it down would heat the earyy up too, as the energy is used it produces waste heat which means on a large scale even using the energy on earth would have a negative impact.

[HPsquared]

There's also ocean thermal energy conversion, which exploits the temperature gradient in the ocean.

https://en.m.wikipedia.org/wiki/Ocean_thermal_energy_convers...

[dsign]

Alien archeologists from the future: "Earthlings caused a catastrophe by trying to suck every bit of tidal energy. Via tidal acceleration[^1], they made their natural satellite fall into their planet. The curious thing is that the kind of planetary technology level they deployed to extract energy from the tides was more than enough to build huge rotating habitats, which they even knew of and called O'Neill cylinders[^2]. But Earthlings called names anybody proposing the idea, and it never really took off before their moon fell on their heads. This is a textbook case of how a civilization's culture affects their development."

[^1]: https://en.wikipedia.org/wiki/Tidal_acceleration

[^2]: https://en.wikipedia.org/wiki/O%27Neill_cylinder

[jvanderbot]

Yeah I sometimes wonder about our use of energy.

Like two whole percent goes to cracking puzzles, and if you prove you crack those puzzles we let you buy things. Those puzzles? Guessing hashes for Blockchain. Not cancer or science or engineering, just automated planet wide gambling.

Then how much is used for ads? Like top to bottom, serving ads. Must be a ton. As a planetary anthropologist, seeing how much infra we built to watch people do silly dances or say things on TikTok... Like we're so addicted to ourselves that we built all this to watch ourselves do things.

[MRtecno98]

I get your point with blockchain(altough i'd like a source for 2% of the entire human energy production going into it) but ads are useful actually, they make people know about what other people do which drives economy, and economy is basically a fancy word for the system that distributes each human's limited time into the various tasks society needs(which do include large scale planetary development but also for example, your local supermarket which makes you eat).

As for tiktok and silly dances, humans don't need only food and water to work reliably, they also need mental health. So leisure, amenities, relationships etc.(yeah tiktok included) are an addiction in the same sense food water and oxygen are an addiction, we just need them to work.(obviously while "addiction" to leisures is in general not a real problem, addiction to a specific thing in this list is, just like addiction to a specific food is a problem. I'm not saying tiktok addiction is a good thing)

If you think humanity's purpose is to develop their knowledge of the universe and techonological grasp, then i reassure you that(almost) everything we do on this spinning ball is a cog in the machine that ultimately sustains that development

[smolder]

> economy is basically a fancy word for the system that distributes each human's limited time into the various tasks society needs

That's a bit too idealist a view for me. The economy isn't strictly or even mostly about fulfilling needs of society, but the aggregated demands of those with economic power, both wants and needs. A lot of what we spend effort on is not useful to society and a lot of needs go unmet. Certainly not every wasteful activity can be waved away as investing in our mental health. Some leisure is a net negative there, too. Some spending is destructive, with no upside. We humans are only somewhat-rational actors.

[opeon]

the tidal systems are insanely vast and powerful. the gravitational pull of the moon and the sun is so large that even if you deploy a massive amount of kits on a small surface, you're unlikely to have an impact on the tides. theoretically it's possible, but it'd require an implausible scale of deployment

[doubloon]

in the past twenty years the plains states like kansas and oklahoma began having earthquakes due to injection of petroleum wastewater into the ground. The government had to stop them because private industry literally does not care what gets destroyed

[amelius]

And most electricity eventually turns into heat. So there's a limit there too.