6 ms·

This is where I'm tripping up on this idea, too. The 20x difference in mass of a Tesla - but still more efficient than a human pedaling a bike may in fact viola

This is where I'm tripping up on this idea, too. The 20x difference in mass of a Tesla - but still more efficient than a human pedaling a bike may in fact violate the Laws of Thermodynamics. Also, I'm not sure if this is truly taking into consideration of the resources needed to make a Tesla vs. a bicycle. Or I guess the big one: the # miles/year driven by a usual car owner vs. # miles/year cycled by someone using purely human power to move oneself.

Dylan16807 7y ago
The car can absorb energy when braking, so the minimum expenditure based on the laws of thermodynamics is minuscule and irrelevant.
- dahart 7y ago
  That’s an extremely tall claim, that minimum expenditure is minuscule and irrelevant, maybe taller than the original unlikely claim that a car might be more efficient than a bike rider. Regenerative braking does not invalidate the laws of thermodynamics, we just have to identify the regeneration efficiency. So how much of the energy expenditure does regenerative braking in a Model 3 recover? From what I’m reading, it may be in the single digit percentages for highway driving. Tesla’s own web page says the theoretical max is 64% [1], and that’s only in a perfect situation (accelerate & immediately stop), it’s not accounting for driving for a long time without braking or using the electric features inside the car, e.g. aerodynamic loss, rolling loss, & heating & a/c. Wikipedia says including the accessories, typical best-case efficiency is around 50% [2]. But if you accelerate and drive for a mile without braking, you’ve already lost most of your initial energy to air & tires & battery conversion loss, the brakes can only capture energy from what’s left. [1] https://www.tesla.com/blog/magic-tesla-roadster-regenerative-braking https://www.tesla.com/blog/magic-tesla-roadster-regenerative... [2] https://en.m.wikipedia.org/wiki/Regenerative_brake https://en.m.wikipedia.org/wiki/Regenerative_brake
  - Dylan16807 7y ago
    You're proving my point. That's a big list of all the ways the car wastes energy that have nothing to do with the thermodynamic minimum to move the mass from point A to B. Those factors absolutely dominate the energy use. Maybe an analogy is useful? There's a claim that you can cook 20 pounds of food in one type of bonfire setup with less fuel use than if you cooked 1 pound of food in a different type of bonfire. The amount of energy needed to actually perform the chemical changes on a pound of food, even multiplied by 20, is negligible compared to the vast quantities of energy that are going to other things.
    - dahart 7y ago
      Oh, you're right, I didn't understand your point before. You're saying that the thermodynamic minimum is a theoretical limit not approached in practice, right? It's not related to regenerative braking. The connecting "so" made me think you were saying that regenerative braking is the reason the thermodynamic minimum is much smaller.
      - Dylan16807 7y ago
        It's more of "With thermodynamics we care only about the thrust to get up to speed. If we add regeneration then we can even factor most of that out, getting us to energy expenditures so very tiny that they're irrelevant to the total." I could have phrased it better.
        dahart 7y ago
        Got it, and likewise I could have not made assumptions and jumped to the conclusion that I understood your comment. I got lost between theory and practice. I think I agree with all of what you said when we're talking about theoretical limits and theoretical near 100% efficient regeneration. It's just that in practice, regenerative braking on a Tesla is not that efficient, and it isn't something that recovers most of your energy spent accelerating, it only recovers a minority share normally, and often only a very small minority.

Dylan16807 7y ago

The car can absorb energy when braking, so the minimum expenditure based on the laws of thermodynamics is minuscule and irrelevant.

dahart 7y ago
That’s an extremely tall claim, that minimum expenditure is minuscule and irrelevant, maybe taller than the original unlikely claim that a car might be more efficient than a bike rider. Regenerative braking does not invalidate the laws of thermodynamics, we just have to identify the regeneration efficiency. So how much of the energy expenditure does regenerative braking in a Model 3 recover? From what I’m reading, it may be in the single digit percentages for highway driving. Tesla’s own web page says the theoretical max is 64% [1], and that’s only in a perfect situation (accelerate & immediately stop), it’s not accounting for driving for a long time without braking or using the electric features inside the car, e.g. aerodynamic loss, rolling loss, & heating & a/c. Wikipedia says including the accessories, typical best-case efficiency is around 50% [2]. But if you accelerate and drive for a mile without braking, you’ve already lost most of your initial energy to air & tires & battery conversion loss, the brakes can only capture energy from what’s left. [1] https://www.tesla.com/blog/magic-tesla-roadster-regenerative-braking https://www.tesla.com/blog/magic-tesla-roadster-regenerative... [2] https://en.m.wikipedia.org/wiki/Regenerative_brake https://en.m.wikipedia.org/wiki/Regenerative_brake
- Dylan16807 7y ago
  You're proving my point. That's a big list of all the ways the car wastes energy that have nothing to do with the thermodynamic minimum to move the mass from point A to B. Those factors absolutely dominate the energy use. Maybe an analogy is useful? There's a claim that you can cook 20 pounds of food in one type of bonfire setup with less fuel use than if you cooked 1 pound of food in a different type of bonfire. The amount of energy needed to actually perform the chemical changes on a pound of food, even multiplied by 20, is negligible compared to the vast quantities of energy that are going to other things.
  - dahart 7y ago
    Oh, you're right, I didn't understand your point before. You're saying that the thermodynamic minimum is a theoretical limit not approached in practice, right? It's not related to regenerative braking. The connecting "so" made me think you were saying that regenerative braking is the reason the thermodynamic minimum is much smaller.
    - Dylan16807 7y ago
      It's more of "With thermodynamics we care only about the thrust to get up to speed. If we add regeneration then we can even factor most of that out, getting us to energy expenditures so very tiny that they're irrelevant to the total." I could have phrased it better.
      - dahart 7y ago
        Got it, and likewise I could have not made assumptions and jumped to the conclusion that I understood your comment. I got lost between theory and practice. I think I agree with all of what you said when we're talking about theoretical limits and theoretical near 100% efficient regeneration. It's just that in practice, regenerative braking on a Tesla is not that efficient, and it isn't something that recovers most of your energy spent accelerating, it only recovers a minority share normally, and often only a very small minority.