10 ms·

https://news.ycombinator.com/item?id=13939040 https://news.ycombinator.com/item?id=13939040 ~34% https://news.ycombinator.com/item?id=13938690 https://news.yco

https://news.ycombinator.com/item?id=13939040 https://news.ycombinator.com/item?id=13939040 ~34% https://news.ycombinator.com/item?id=13938690 https://news.ycombinator.com/item?id=13938690 ~40% https://news.ycombinator.com/item?id=13938953 https://news.ycombinator.com/item?id=13938953 46% https://news.ycombinator.com/item?id=13939181 https://news.ycombinator.com/item?id=13939181 ~86.8% https://news.ycombinator.com/item?id=13939399 https://news.ycombinator.com/item?id=13939399 95% One... two... three... four... five theoretical maximums in comments. Something tells me that there's a lot more research to be done (or knowledge to be learned, at the very least) available here than we realize.

marcosdumay 10y ago
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").
- 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.
- jartelt 10y ago
  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.
syntaxing 10y ago
Admittedly, there still a lot of room for solar panel research for the upcoming years which is pretty exciting! The discrepancies is often due to fact we're defining "solar panels" quite different for each application and also the definition of input energy is different. A great example is engines. The theoretical (Carnot) efficiency is limited by the heat difference between the source and the sink. You can burn fuel at a higher temperature and pressure which will give you great efficiency, but it's not realistically possibly due to physical constraints (usually the steel that houses the explosions). That's why steel engines are often said to have a maximum fuel efficiency of 37%. This same idea applies to solar panel but it's even more convoluted. What is input energy? Is it all the energy from the sun or is it only the energy within the energy band that the panel can "see"? What is efficiency? The efficiency for different energy band is different for each solar panel as well. Worst of all, this efficiency changes for different temperature and is effected by hysteresis! By the time you store the energy, you also lose efficiency depending on the internal resistance of the energy storage (That's why MPPT exist and fun fact of the day, from what I know, there is still no reliable analytical method to account for this. Most MPPT trackers are done empirically). Sorry for the long rant, but yeah, solar panels are confusing and there is a surprising about of research that still needs to be done!
- nojvek 10y ago
  I heard about carnot energy but didn't realize the calculation was this simple. Suppose we say we get a insulated material that is string enough to handle high temperatures. We can beat the 37% right?
  - pjc50 10y ago
    Yes; I believe this is why jet turbine engines have blades made of single crystals of exotic materials.
  - syntaxing 10y ago
    It's a partial yes. You have to worry about pressure as well. You can burn fuel at a higher temperature by igniting the gas at a higher pressure. But there are certain constraints due to the fuel. If the pressure is too high, the fuel will spontaneous combust. That's why there are different octane rating of fuels and why higher octane fuel gives you better fuel efficiency (given your engine has some sort of valve timing like VTEC). This is also why Diesel engines tend to have better mileage than gasoline engines. Even though gasoline engines will perform better at the same pressure as Diesel, there just no way to bring the gasoline to the same pressure that the Diesel can handle. When and how you combust the fuel can give you different fuel efficiencies as well. For instance, the Brayton cycle can achieve easily over 50% fuel efficiency. But in order to achieve this, you need a special housing (jet engines uses this cycle) to be able to compress the fuel to the specific pressure and also handle the pressure. Also, special fuel is required. These two things makes it not really practical for the standard consumer. On a tangent, Internal combustion engines is probably one of the funnest and hardest class I took in graduate school...
jartelt 10y ago
When talking about solar cells, the efficiency is defined as electrical power produced divided by the solar irradiance on the panel (~1000 W/m^s without concentration). In this situation, the theoretical limit for a single junction solar cell on Earth is ~34% as shown by Shockley-Queisser in their detailed balance calculations. Multijunction cells can be more efficient than single junction cells and can theoretically reach ~70-80% efficiency if >30 junctions are used and concentrated sunlight is used. In practice it is prohibitively difficult and expensive to make a device with more than 4 or 5 junctions, so you will not see one sun efficiency greater than 70% in real life. The 95% number is for a Carnot engine and doesn't apply to an operating solar cell. In order to extract power from a solar cell, you need to operate it in forward bias (typically at the maximum power point). When at the maximum power point, the solar cell emits radiation and therefore has irreversible losses. This is why the real limit for a single junction cell is 34% and not 95%.

marcosdumay 10y ago

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").

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.
jartelt 10y ago
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.

syntaxing 10y ago

Admittedly, there still a lot of room for solar panel research for the upcoming years which is pretty exciting! The discrepancies is often due to fact we're defining "solar panels" quite different for each application and also the definition of input energy is different. A great example is engines. The theoretical (Carnot) efficiency is limited by the heat difference between the source and the sink. You can burn fuel at a higher temperature and pressure which will give you great efficiency, but it's not realistically possibly due to physical constraints (usually the steel that houses the explosions). That's why steel engines are often said to have a maximum fuel efficiency of 37%. This same idea applies to solar panel but it's even more convoluted. What is input energy? Is it all the energy from the sun or is it only the energy within the energy band that the panel can "see"? What is efficiency? The efficiency for different energy band is different for each solar panel as well. Worst of all, this efficiency changes for different temperature and is effected by hysteresis! By the time you store the energy, you also lose efficiency depending on the internal resistance of the energy storage (That's why MPPT exist and fun fact of the day, from what I know, there is still no reliable analytical method to account for this. Most MPPT trackers are done empirically). Sorry for the long rant, but yeah, solar panels are confusing and there is a surprising about of research that still needs to be done!

nojvek 10y ago
I heard about carnot energy but didn't realize the calculation was this simple. Suppose we say we get a insulated material that is string enough to handle high temperatures. We can beat the 37% right?
- pjc50 10y ago
  Yes; I believe this is why jet turbine engines have blades made of single crystals of exotic materials.
- syntaxing 10y ago
  It's a partial yes. You have to worry about pressure as well. You can burn fuel at a higher temperature by igniting the gas at a higher pressure. But there are certain constraints due to the fuel. If the pressure is too high, the fuel will spontaneous combust. That's why there are different octane rating of fuels and why higher octane fuel gives you better fuel efficiency (given your engine has some sort of valve timing like VTEC). This is also why Diesel engines tend to have better mileage than gasoline engines. Even though gasoline engines will perform better at the same pressure as Diesel, there just no way to bring the gasoline to the same pressure that the Diesel can handle. When and how you combust the fuel can give you different fuel efficiencies as well. For instance, the Brayton cycle can achieve easily over 50% fuel efficiency. But in order to achieve this, you need a special housing (jet engines uses this cycle) to be able to compress the fuel to the specific pressure and also handle the pressure. Also, special fuel is required. These two things makes it not really practical for the standard consumer. On a tangent, Internal combustion engines is probably one of the funnest and hardest class I took in graduate school...

jartelt 10y ago

When talking about solar cells, the efficiency is defined as electrical power produced divided by the solar irradiance on the panel (~1000 W/m^s without concentration). In this situation, the theoretical limit for a single junction solar cell on Earth is ~34% as shown by Shockley-Queisser in their detailed balance calculations. Multijunction cells can be more efficient than single junction cells and can theoretically reach ~70-80% efficiency if >30 junctions are used and concentrated sunlight is used. In practice it is prohibitively difficult and expensive to make a device with more than 4 or 5 junctions, so you will not see one sun efficiency greater than 70% in real life. The 95% number is for a Carnot engine and doesn't apply to an operating solar cell. In order to extract power from a solar cell, you need to operate it in forward bias (typically at the maximum power point). When at the maximum power point, the solar cell emits radiation and therefore has irreversible losses. This is why the real limit for a single junction cell is 34% and not 95%.