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Comment by crazygringo | Hacker News Reader

Tons of comments here highlighting that 2% annual growth rate in energy consumption is ludicrous.

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.

Then how much of a rotational slowdown do we get after 1000 years?

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According to the paper, tidal energy has a total capacity of 10^29 Joules, and current global energy consumption is 10^21 Joules per year, so if we converted 100% of current energy consumption to tidal energy and didn't increase it at all, it would last 10^8 = 100 million years (rounded to the nearest power of 10).

Equivalently, after 1000 years, we'd see a rotational slowdown of around 0.001%.

Thanks so much. That feels much more in line with intuition.

So maybe not technically renewable, but many orders of magnitude more renewable than e.g. fossil fuels...

Any useful interpretation of renewable energy has to do with purely human timeframes as thermodynamics doesn't allow for any energy source to be endlessly renewable. "If I use this source today will more of it naturally be there tomorrow" type consideration, otherwise you end up with a definition which either includes things like fossil fuels as renewable because you can wait millions of years or excludes things like sunlight because it won't be there in 10 billion years.

Well, there is a more important sense in which solar is renewable but not tidal energy. The sun does not burn out appreciably more quickly if we use a larger percentage of the available solar energy, it's there whether we use it or not. This is not the case for tidal energy.

The sun doesn't burn more quickly if we use it, but also not slower.

The sun will start becoming a red giant and kill us all in about one billion years, long before this tidal lock will eventually happen.

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