6 ms·

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

by ythn 10y ago

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?

Normal_gaussian 10y ago
> why is it such a nice round integer? Significant Figures [1] [1] http://www.bbc.co.uk/schools/gcsebitesize/maths/number/roundestimaterev3.shtml http://www.bbc.co.uk/schools/gcsebitesize/maths/number/round...
smdz 10y ago
This is a good question - And this is a partial answer (from a link above): https://news.ycombinator.com/item?id=13939399 https://news.ycombinator.com/item?id=13939399 Not sure why you were downvoted. Seems like this community is showing signs of academic elitism
marcosdumay 10y ago
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 https://en.wikipedia.org/wiki/Carnot_cycle
- Symmetry 10y ago
  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.
  - marcosdumay 10y ago
    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 https://en.wikipedia.org/wiki/Matrioshka_brain
    - deleted 10y ago
      [deleted]
agumonkey 10y ago
I suppose it's similar to why you cannot have a 100% efficient wind farm?
- marcosdumay 10y ago
  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.
  - agumonkey 10y ago
    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.
    - JLK_121416 10y ago
      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 https://en.m.wikipedia.org/wiki/Betz's_law
      - agumonkey 10y ago
        Thanks, I didn't know the name.
    - marcosdumay 10y ago
      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.

by ythn 10y ago

Normal_gaussian 10y ago

> why is it such a nice round integer? Significant Figures [1] [1] http://www.bbc.co.uk/schools/gcsebitesize/maths/number/roundestimaterev3.shtml http://www.bbc.co.uk/schools/gcsebitesize/maths/number/round...

smdz 10y ago

This is a good question - And this is a partial answer (from a link above): https://news.ycombinator.com/item?id=13939399 https://news.ycombinator.com/item?id=13939399 Not sure why you were downvoted. Seems like this community is showing signs of academic elitism

marcosdumay 10y ago

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

Symmetry 10y ago
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.
- marcosdumay 10y ago
  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 https://en.wikipedia.org/wiki/Matrioshka_brain
  - deleted 10y ago
    [deleted]

agumonkey 10y ago

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

marcosdumay 10y ago
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.
- agumonkey 10y ago
  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.
  - JLK_121416 10y ago
    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 https://en.m.wikipedia.org/wiki/Betz's_law
    - agumonkey 10y ago
      Thanks, I didn't know the name.
  - marcosdumay 10y ago
    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.