3 ms·

Could you elaborate on your last sentence? My naïve assumption was that theoretically the closer to 100% you could get the better, though for some applications

Could you elaborate on your last sentence? My naïve assumption was that theoretically the closer to 100% you could get the better, though for some applications you might take a cheaper and lower efficiency panel if it could consistently provide more than the required energy needs. With a storage system, you can just keep adding more storage to soak up higher conversion efficiencies. What am I missing? Why does 40% make little sense?

Animats 3y ago
Because utility-scale batteries are around 86% round trip efficiency. Pumped storage is around 79%. 40% as a conversion efficiency alone isn't good. That's not a round-trip value; heat loss in storage has to be considered, too.
- deleted 3y ago
  [deleted]
- 20after4 3y ago
  In a heat-based system couldn't you use more cells to absorb remaining heat?
  - vlovich123 3y ago
    Adding more cells won’t help for two reason. Let’s say the extra cells would help absorb, they can only capture 40% which means you’ve got exponentially increasing costs chasing after all the heat you didn’t absorb (+ physical location of where to put the cells). The real reason though is physics, namely the 2nd law of thermodynamics. If you could keep adding cells to capture the heat other cells couldn’t, you’d basically get really really close to a perpetual motion machine which we know is impossible. That’s because a good chunk of the unabsorbed heat is either reflected from the cells or not absorbable from the source. TLDR: Adding more cells won’t help due to economics, geometry, and fundamental laws of physics.
    - nine_k 3y ago
      Technically, perpetual motion is possible: consider a sphere rotating in vacuum. A source of free energy, a "perpetual engine", is indeed impossible.
      - vlovich123 3y ago
        A sphere rotating in a vacuum should still experience black body radiation and gravitational drag. Damn kids and their pesky attempts to violate the 2nd law.
        nine_k 3y ago
        Cooling down would not slow down the sphere, if the radiation is uniform. Gravitational drag, maybe, but only if there is something to drag nearby, and we postulate a vacuum. A sphere is symmetric and thus does not emit gravitational waves. I wonder if an effect akin to black hole evaporation could play a role: if one of the two virtual particles gets accelerated towards the sphere, it may bring some momentum to it. But, assuming that the space is isotropic, statistically such momentums should cancel out.
        vlovich123 3y ago
        A vacuum doesn’t exist in reality. In addition to virtual particles, gravity travels forever afaik. Which means your rotating sphere will interact with matter no matter how far away it is. Unless you’re hypothesizing a universe that doesn’t exist. In which case, we can also hypothesize one where the 2nd law isn’t an issue. Also, I’m not so sure that black body radiation wouldn’t have an impact on rotation. At some point, the object will have radiated all its heat and have no energy left. If the particles have no energy, how is the sphere rotating which implies kinetic energy? That implies that even when radiating uniformly, black body radiation must take away angular velocity, no?
      - BobaFloutist 3y ago
        Solar power is also perpetual motion for our purposes. The sun's going to be around for a loooooong time.
  - Tade0 3y ago
    For the cells to work you need a large temperature difference between the source and sink, so the waste heat you're producing is usually just a lot of slightly warmer coolant. Sometimes you can sacrifice some efficiency by warming the coolant to e.g. 75°C and distributing that as district heating.

Animats 3y ago

Because utility-scale batteries are around 86% round trip efficiency. Pumped storage is around 79%. 40% as a conversion efficiency alone isn't good. That's not a round-trip value; heat loss in storage has to be considered, too.

deleted 3y ago
[deleted]
20after4 3y ago
In a heat-based system couldn't you use more cells to absorb remaining heat?
- vlovich123 3y ago
  Adding more cells won’t help for two reason. Let’s say the extra cells would help absorb, they can only capture 40% which means you’ve got exponentially increasing costs chasing after all the heat you didn’t absorb (+ physical location of where to put the cells). The real reason though is physics, namely the 2nd law of thermodynamics. If you could keep adding cells to capture the heat other cells couldn’t, you’d basically get really really close to a perpetual motion machine which we know is impossible. That’s because a good chunk of the unabsorbed heat is either reflected from the cells or not absorbable from the source. TLDR: Adding more cells won’t help due to economics, geometry, and fundamental laws of physics.
  - nine_k 3y ago
    Technically, perpetual motion is possible: consider a sphere rotating in vacuum. A source of free energy, a "perpetual engine", is indeed impossible.
    - vlovich123 3y ago
      A sphere rotating in a vacuum should still experience black body radiation and gravitational drag. Damn kids and their pesky attempts to violate the 2nd law.
      - nine_k 3y ago
        Cooling down would not slow down the sphere, if the radiation is uniform. Gravitational drag, maybe, but only if there is something to drag nearby, and we postulate a vacuum. A sphere is symmetric and thus does not emit gravitational waves. I wonder if an effect akin to black hole evaporation could play a role: if one of the two virtual particles gets accelerated towards the sphere, it may bring some momentum to it. But, assuming that the space is isotropic, statistically such momentums should cancel out.
        vlovich123 3y ago
        A vacuum doesn’t exist in reality. In addition to virtual particles, gravity travels forever afaik. Which means your rotating sphere will interact with matter no matter how far away it is. Unless you’re hypothesizing a universe that doesn’t exist. In which case, we can also hypothesize one where the 2nd law isn’t an issue. Also, I’m not so sure that black body radiation wouldn’t have an impact on rotation. At some point, the object will have radiated all its heat and have no energy left. If the particles have no energy, how is the sphere rotating which implies kinetic energy? That implies that even when radiating uniformly, black body radiation must take away angular velocity, no?
    - BobaFloutist 3y ago
      Solar power is also perpetual motion for our purposes. The sun's going to be around for a loooooong time.
- Tade0 3y ago
  For the cells to work you need a large temperature difference between the source and sink, so the waste heat you're producing is usually just a lot of slightly warmer coolant. Sometimes you can sacrifice some efficiency by warming the coolant to e.g. 75°C and distributing that as district heating.