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Comment by marcosdumay | Hacker News Reader

It's not that.

The 95% is the limit. You can't extract solar power with a higher efficiency. (Funny thing that you can extract energy from Earth, radiating it away, with ~99% efficiency.)

86.8% and and 40% are theoretical limits to some current technologies. The others are people debating what is the best available on the market (with varying definitions of "available").

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Could you explain why 95% is "the limit"? Are you saying 95.0% efficient solar panels are possible, but 95.1% efficient solar panels are impossible? What's the limiting mechanism here and why is it such a nice round integer?

> why is it such a nice round integer?

Significant Figures [1]

[1] http://www.bbc.co.uk/schools/gcsebitesize/maths/number/round...

This is a good question - And this is a partial answer (from a link above): https://news.ycombinator.com/item?id=13939399

Not sure why you were downvoted. Seems like this community is showing signs of academic elitism

Those ultimate efficiencies come from thermodynamics[1], since both the Sun light and Earth heat are thermal.

Both are rounded.

1 - https://en.wikipedia.org/wiki/Carnot_cycle

And yet, you can take a solar cell out to Pluto where the mean temperature is 40K rather than 300K and theoretically get far higher efficiency than 95%. So 95% isn't really a fundamental limit.

Yep, I'm assuming the solar cells are on Earth. Seemed like a good assumption to make.

If you just send them into space with good enough radiators, you'll be able to get 3 9's of efficiency (mid way to 4). But you don't get to reuse the heat waste.

You may be interested on that:

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

I suppose it's similar to why you cannot have a 100% efficient wind farm?

Wind is not thermal energy, so energy extraction from it can achieve 100% efficiency in theory.

Turbines do have some theoretical limitation, but it's a technology limitation, not an ultimate theoretical one.

I thought it was more fundamental than that; to capture energy, there needs to be energy, if your wind turbine transforms all of it, there's no current anymore and the system will change phase.

Yes, Betz's law [0]:

"Consider that if all of the energy coming from wind movement through a turbine was extracted as useful energy the wind speed afterwards would drop to zero. If the wind stopped moving at the exit of the turbine, then no more fresh wind could get in - it would be blocked."

[0] https://en.m.wikipedia.org/wiki/Betz's_law

Thanks, I didn't know the name.

This is a fundamental limitation of turbines. Thus, a technological limitation for wind-power. Other technologies could have other limits.

Besides, that's a limit on collecting efficiency (how much you can collect / how much is there), not usage efficiency (how much you can use / how much you collect). You don't "destroy" the wind when your turbines don't collect it, it's still there.

I see, I considered the turbines as the only mean. Are there other transformer types for fluids ?

A solar cell could never achieve the Carnot limit, so the 95% number is not relevant. In order to extract the maximum amount of power from a solar cell, must operate it at it's max power point. When you do this, the cell is in forward bias and must emit light (because of the physics of absorption/emission). Emitting light is a loss mechanism and thus people take this into account when calculating the theoretical efficiency for solar cells. It has nothing to do with currently available device on the market and is just accounting for how solar cells work.

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