3 ms·

In the particular case of the "miles driven", what's wrong with the average? Is it because the amount of power used by an individual with an EV differs from bei

by drdeca 4y ago

In the particular case of the "miles driven", what's wrong with the average? Is it because the amount of power used by an individual with an EV differs from being proportional to the distance they drive with it, in a way that matters when aggregated? Like, if the distance amount a randomly selected person drives has some distribution, then, if power use was proportional to distance traveled, then E[(energy use)] = E[k * (distance traveled)] = k * E[(distance traveled)], and, average energy use is total energy use divided by number of people, which is the same as the expectation of the energy use of a randomly selected person, so, total energy use would be (number of people) * k * E[(distance traveled)] . So, therefore, if that's not true, then one of the assumptions in the argument must not be true, so therefore, (energy use) must not be k * (distance traveled). Now, not being exactly the same, that's expected, yes. Like, there should be terms relating to e.g. "how long people are stopped at traffic lights", stuff associated with whether they use the car at all that week, etc. , but, nothing from that jumps out at me as something which obviously would make the shape of the distribution of "how much each person drives" beyond just "how much does a person drive on average", matter a whole lot. I'm not saying it doesn't. I'm saying, even considering it, nothing jumps out to me as a reason it should be obvious.

robomartin 4y ago
> I'm not saying it doesn't. I'm saying, even considering it, nothing jumps out to me as a reason it should be obvious. I'll ignore questions about the distribution and highlight what is perhaps the most egregious problem in EV estimates: Energy is the wrong metric. Using the average allows one to speak in terms of energy. Yet energy isn't what we need. We need power. Yes, of course, at the end of the day you are trying to store energy. However, what we need to know is what our power requirements might be. Here's a super-simple example. This isn't meant to be a full analytical model, it's just a simplified hypothetical used to illustrate the important difference between speaking in terms of energy and power. Ten EV's are driven the same number of average miles in a day. The number of miles does not matter. If those ten EV's are plugged-in for slow-charging all night, they receive a certain amount of energy. This energy is delivered slowly (at a low power level) over, say, twelve hours. On the other hand, if the same ten EV's where to simultaneously plug-in for rapid-charging in, say, one hour, the situation is much different. They would receive the same energy. However, this requires twelve times more power. The difference isn't trivial. It's the same energy, yet you need twelve times more power (and the ability to deliver that power) in the second case. Real-life hypothetical: Hurricane landing in Florida at some point in the future when EVs (from motorcycles to semi-trucks) have fully displaced internal combustion vehicles. A very large percentage of the population needs to rapid charge during a very narrow window of time. Everyone is on the road evacuating. The limiting factor is power generation capacity as well as power deliver capabilities (the "grid"). This is a highly simplified view into why using averages to broad-stroke estimates is in the realm of a fantasy. You don't have to look at natural disasters as the only drivers for this sort of peak demand. Holiday traffic and other events or local conditions alter the distribution. Regional differences have the same kind of influence. For example, where I live, most people have to drive 30 to 60 miles each way to go to work. That's hundreds of thousands of vehicles. Bad urban planning? No argument from me. Guess what? It isn't going to change. Anyone who thinks so has never had to pleasure of locking horns with the urban planning agencies where they live. Not going to happen. And it isn't just work. Entertainment, high-end restaurants, etc., are also 30 to 60 miles away. That's why using averages to calculate energy is only valid if we are OK with people not being able to charge their cars and being stranded when peak power demand exceed available generation and grid (transport) capacity. In CA we've had cases of people not being able to charge their EV's during evacuations and being asked to delay charging during periods of high heat. I have written simulation models to analyze this at a reasonably detailed level of granularity. My model predicted the need for additional (new, on top of what we currently have) power generation in the order of 900 GW to 1400 GW for the entire ground transportation fleet in the US going electric. I did this many years ago. I have since concluded that this range seems to be an accurate prediction. For a sense of proportion, this means we need to build a thousand nuclear power plants. No, solar and wind, individually or collectively, won't do it. I've done the math on that too.
- drdeca 4y ago
  Thanks! So, I had interpreted your previous comment as saying that using the average miles driven per day is a bad way to estimate the total energy (per day) needed. But I guess you instead meant, not that the problem from averaging came from averaging miles over people and using that to get energy use per day, but rather averaging energy use per day over both people and days, when max power is also important, and is perhaps currently the limiting factor?
  - robomartin 4y ago
    > max power is also important, and is perhaps currently the limiting factor? Precisely! That's what I learned after spending time creating a reasonably granular simulation. Everyone talks about energy. This allows for hand-wavy arguments that end-up being false. I am not claiming that people do this intentionally or maliciously, BTW. Until someone takes the time to truly understand the realities of widely deployed EV's, it is impossible for them to understand what the problem looks like at scale. The hand-wavy statement usually takes the form of "just charge at night/work/off-peak", etc. What is not immediately obvious in this way of thinking is that this isn't at all a solution. It's hard to think in terms of aggregate demand. No matter how you charge, slow, fast, distributed across the day/week, etc. At the end of the day, what you discover is that there's an unavoidable steady-state power generation and delivery requirement. Energy is power spread over time. Energy is great, but it does not address the needs this second, or the next. That's power. And that's the issue. Not sure I provided this link before. I ended-up confirming my estimate many years after I came to this conclusion when someone finally asked Elon Musk the question. Here's his answer: https://www.youtube.com/watch?v=TcI6FaaDp8g&t=3510s https://www.youtube.com/watch?v=TcI6FaaDp8g&t=3510s
  - robomartin 4y ago
    > I had interpreted your previous comment as saying that using the average miles driven per day is a bad way to estimate the total energy (per day) needed. I should address this separately. The average isn't a good metric because it applies what is often a bad indicator of central tendency to the entire population. In addition to this, what matters in the case of electric vehicles isn't how many miles someone drives per day but rather how they charge. Here's an extreme exaggeration to illustrate the point: Let's assume three groups, each with a million cars. Let's also assume they all drive exactly the same miles, whatever that might be. The final assumption is that you can fully recharge the energy used in a day by slow-charging in 12 hours. Let's define rapid charging as taking two hours. In other words, it's three times faster than slow charge. Here's charging behavior for each group. Every day: Group 1: All 1 million cars plug in to slow charge at 1/12 the rate between 6 PM and 6 AM Group 2: 25% of the cars plug in to rapid charge every hour, starting at 6 PM Group 3: All 1 million cars plug in to rapid-charge at 6 PM All three groups require the same energy to recharge their cars. They drive the same distance. We'll call that 1 million units of energy. Trying to keep it simple, if delivered in one hour, that's 1 million units of power. Let's call Group 1 our baseline. They use 1 million units of energy and require 1/12 the power to deliver that energy over 12 hours. We can service this million car cohort with a power system that delivers 1/12 the power for 12 hours. Group 2 is interesting. Here's the number of cars charging per hour over the four hour period. 6:00 PM 7:00 PM 8:00 PM 9:00 PM 10:00 PM 250K 250K 250K 250K 250K 250K 250K 250K -------------------------------------------- 250K 500K 500K 500K 250K The first subgroup plugs in 250K cars and is on there for two hours. An hour after that subgroup plugs-in, the next 250K cars plug in for two hours, and so on. The last line represents the number of total cars plugged in for rapid charging per hour. The first hour we only need 125K units of power, which is 1/8 of a million. More than the Group 1 case. However, due to the overlap, we can't supply this group with a power plant delivering 1/8 the power, we need 250K. The last case is one where 100% of the cars plug in to charge simultaneously and for two hours. The 1 million in energy is delivered in two hours, which means we need a power plant capable of delivering 500K units of power for two hours. Imagine these groups exist in the same city. We have to build a new power plant to support their charging needs. What metric do we use? Average miles driven? That's pretty much irrelevant. This might give us a (likely bad) idea of how long they will have to charge. It will not tell us how big of a power plant we need to build. How about we build it to Group 1 requirements? 1/12 power? Well, that means we'll have blackouts when the other groups plug in. That's not good. OK, Group 3 then? Same problem. Good for that group, yet, we can't supply the aggregate charging needs. Ah, the average of the power required by all three groups. Once again, the average is useless. The average of 1/12, 1/4 and 1/2 is only 28% of the power required to service all three groups. We need to build a plant that can deliver 833K units of power. I hope this oversimplified example illustrates why speaking of energy is pretty much pointless. The limiting factor is the ability to deliver power. And that power requirement, in the simplest of cases, is dominated the peak of the aggregated charging behavior. In fact, the real calculation is far more complex than that. It's what I call the "Florida/California Evacuation Scenario". How do we cover the case of everyone --and I do mean everyone-- needing to rapid-charge right now? A hurricane, flood, fire, etc. is coming. You charge right now, as fast as possible or tens of thousands of people could die. The answer is obvious: A widely distributed storage network "at the edge", much like gasoline stations at nearly every highway exit store gasoline. However, that answer can't be taken as hand-wavy magic that instantly solves the problem. Ignoring the fact that this infrastructure is nearly impossible to build today, that stored energy has to be delivered at a reasonable rate (power) over an equally reasonable period of time. It can't take three years to charge massive batteries at 200,000 charging stations nationwide. I'd say, maybe a few months, two to four? This is a power requirement over and above that required to service normal daily charging. This isn't a simple matter at all. Places like Los Angeles are going to have absolutely insane power deliver requirements. Small towns will be more manageable. The key point --and what I have been saying-- is that we need to stop lying to ourselves with false/useless metrics. It's about power. One can't just magically waive this requirement as Gavin Newsom has here in CA. No more new IC cars in 2030 could presumably mean two million new EV's per year sold after that date. That is not a trivial number of cars. Not to mention we probably going to have lots more EV's already by then. We have announced no new power generation and delivery projects at all. We have dumped (burned, really) over a hundred billion dollars into a high speed rail nobody is going to use (if it is ever completed). If we were serious about EV's we would have dumped that money into new power generation and distribution projects instead. The reason we are not is that the public discussions surrounding EV's are filled with fantasies and hand-wave-itis, which means the public isn't informed and there's no way to gain votes in support of new power generation projects. This is why I say we need to start to discuss the truth, rather than the fantasies we are being sold.

by drdeca 4y ago

robomartin 4y ago

> I'm not saying it doesn't. I'm saying, even considering it, nothing jumps out to me as a reason it should be obvious. I'll ignore questions about the distribution and highlight what is perhaps the most egregious problem in EV estimates: Energy is the wrong metric. Using the average allows one to speak in terms of energy. Yet energy isn't what we need. We need power. Yes, of course, at the end of the day you are trying to store energy. However, what we need to know is what our power requirements might be. Here's a super-simple example. This isn't meant to be a full analytical model, it's just a simplified hypothetical used to illustrate the important difference between speaking in terms of energy and power. Ten EV's are driven the same number of average miles in a day. The number of miles does not matter. If those ten EV's are plugged-in for slow-charging all night, they receive a certain amount of energy. This energy is delivered slowly (at a low power level) over, say, twelve hours. On the other hand, if the same ten EV's where to simultaneously plug-in for rapid-charging in, say, one hour, the situation is much different. They would receive the same energy. However, this requires twelve times more power. The difference isn't trivial. It's the same energy, yet you need twelve times more power (and the ability to deliver that power) in the second case. Real-life hypothetical: Hurricane landing in Florida at some point in the future when EVs (from motorcycles to semi-trucks) have fully displaced internal combustion vehicles. A very large percentage of the population needs to rapid charge during a very narrow window of time. Everyone is on the road evacuating. The limiting factor is power generation capacity as well as power deliver capabilities (the "grid"). This is a highly simplified view into why using averages to broad-stroke estimates is in the realm of a fantasy. You don't have to look at natural disasters as the only drivers for this sort of peak demand. Holiday traffic and other events or local conditions alter the distribution. Regional differences have the same kind of influence. For example, where I live, most people have to drive 30 to 60 miles each way to go to work. That's hundreds of thousands of vehicles. Bad urban planning? No argument from me. Guess what? It isn't going to change. Anyone who thinks so has never had to pleasure of locking horns with the urban planning agencies where they live. Not going to happen. And it isn't just work. Entertainment, high-end restaurants, etc., are also 30 to 60 miles away. That's why using averages to calculate energy is only valid if we are OK with people not being able to charge their cars and being stranded when peak power demand exceed available generation and grid (transport) capacity. In CA we've had cases of people not being able to charge their EV's during evacuations and being asked to delay charging during periods of high heat. I have written simulation models to analyze this at a reasonably detailed level of granularity. My model predicted the need for additional (new, on top of what we currently have) power generation in the order of 900 GW to 1400 GW for the entire ground transportation fleet in the US going electric. I did this many years ago. I have since concluded that this range seems to be an accurate prediction. For a sense of proportion, this means we need to build a thousand nuclear power plants. No, solar and wind, individually or collectively, won't do it. I've done the math on that too.

drdeca 4y ago
Thanks! So, I had interpreted your previous comment as saying that using the average miles driven per day is a bad way to estimate the total energy (per day) needed. But I guess you instead meant, not that the problem from averaging came from averaging miles over people and using that to get energy use per day, but rather averaging energy use per day over both people and days, when max power is also important, and is perhaps currently the limiting factor?
- robomartin 4y ago
  > max power is also important, and is perhaps currently the limiting factor? Precisely! That's what I learned after spending time creating a reasonably granular simulation. Everyone talks about energy. This allows for hand-wavy arguments that end-up being false. I am not claiming that people do this intentionally or maliciously, BTW. Until someone takes the time to truly understand the realities of widely deployed EV's, it is impossible for them to understand what the problem looks like at scale. The hand-wavy statement usually takes the form of "just charge at night/work/off-peak", etc. What is not immediately obvious in this way of thinking is that this isn't at all a solution. It's hard to think in terms of aggregate demand. No matter how you charge, slow, fast, distributed across the day/week, etc. At the end of the day, what you discover is that there's an unavoidable steady-state power generation and delivery requirement. Energy is power spread over time. Energy is great, but it does not address the needs this second, or the next. That's power. And that's the issue. Not sure I provided this link before. I ended-up confirming my estimate many years after I came to this conclusion when someone finally asked Elon Musk the question. Here's his answer: https://www.youtube.com/watch?v=TcI6FaaDp8g&t=3510s https://www.youtube.com/watch?v=TcI6FaaDp8g&t=3510s
- robomartin 4y ago
  > I had interpreted your previous comment as saying that using the average miles driven per day is a bad way to estimate the total energy (per day) needed. I should address this separately. The average isn't a good metric because it applies what is often a bad indicator of central tendency to the entire population. In addition to this, what matters in the case of electric vehicles isn't how many miles someone drives per day but rather how they charge. Here's an extreme exaggeration to illustrate the point: Let's assume three groups, each with a million cars. Let's also assume they all drive exactly the same miles, whatever that might be. The final assumption is that you can fully recharge the energy used in a day by slow-charging in 12 hours. Let's define rapid charging as taking two hours. In other words, it's three times faster than slow charge. Here's charging behavior for each group. Every day: Group 1: All 1 million cars plug in to slow charge at 1/12 the rate between 6 PM and 6 AM Group 2: 25% of the cars plug in to rapid charge every hour, starting at 6 PM Group 3: All 1 million cars plug in to rapid-charge at 6 PM All three groups require the same energy to recharge their cars. They drive the same distance. We'll call that 1 million units of energy. Trying to keep it simple, if delivered in one hour, that's 1 million units of power. Let's call Group 1 our baseline. They use 1 million units of energy and require 1/12 the power to deliver that energy over 12 hours. We can service this million car cohort with a power system that delivers 1/12 the power for 12 hours. Group 2 is interesting. Here's the number of cars charging per hour over the four hour period. 6:00 PM 7:00 PM 8:00 PM 9:00 PM 10:00 PM 250K 250K 250K 250K 250K 250K 250K 250K -------------------------------------------- 250K 500K 500K 500K 250K The first subgroup plugs in 250K cars and is on there for two hours. An hour after that subgroup plugs-in, the next 250K cars plug in for two hours, and so on. The last line represents the number of total cars plugged in for rapid charging per hour. The first hour we only need 125K units of power, which is 1/8 of a million. More than the Group 1 case. However, due to the overlap, we can't supply this group with a power plant delivering 1/8 the power, we need 250K. The last case is one where 100% of the cars plug in to charge simultaneously and for two hours. The 1 million in energy is delivered in two hours, which means we need a power plant capable of delivering 500K units of power for two hours. Imagine these groups exist in the same city. We have to build a new power plant to support their charging needs. What metric do we use? Average miles driven? That's pretty much irrelevant. This might give us a (likely bad) idea of how long they will have to charge. It will not tell us how big of a power plant we need to build. How about we build it to Group 1 requirements? 1/12 power? Well, that means we'll have blackouts when the other groups plug in. That's not good. OK, Group 3 then? Same problem. Good for that group, yet, we can't supply the aggregate charging needs. Ah, the average of the power required by all three groups. Once again, the average is useless. The average of 1/12, 1/4 and 1/2 is only 28% of the power required to service all three groups. We need to build a plant that can deliver 833K units of power. I hope this oversimplified example illustrates why speaking of energy is pretty much pointless. The limiting factor is the ability to deliver power. And that power requirement, in the simplest of cases, is dominated the peak of the aggregated charging behavior. In fact, the real calculation is far more complex than that. It's what I call the "Florida/California Evacuation Scenario". How do we cover the case of everyone --and I do mean everyone-- needing to rapid-charge right now? A hurricane, flood, fire, etc. is coming. You charge right now, as fast as possible or tens of thousands of people could die. The answer is obvious: A widely distributed storage network "at the edge", much like gasoline stations at nearly every highway exit store gasoline. However, that answer can't be taken as hand-wavy magic that instantly solves the problem. Ignoring the fact that this infrastructure is nearly impossible to build today, that stored energy has to be delivered at a reasonable rate (power) over an equally reasonable period of time. It can't take three years to charge massive batteries at 200,000 charging stations nationwide. I'd say, maybe a few months, two to four? This is a power requirement over and above that required to service normal daily charging. This isn't a simple matter at all. Places like Los Angeles are going to have absolutely insane power deliver requirements. Small towns will be more manageable. The key point --and what I have been saying-- is that we need to stop lying to ourselves with false/useless metrics. It's about power. One can't just magically waive this requirement as Gavin Newsom has here in CA. No more new IC cars in 2030 could presumably mean two million new EV's per year sold after that date. That is not a trivial number of cars. Not to mention we probably going to have lots more EV's already by then. We have announced no new power generation and delivery projects at all. We have dumped (burned, really) over a hundred billion dollars into a high speed rail nobody is going to use (if it is ever completed). If we were serious about EV's we would have dumped that money into new power generation and distribution projects instead. The reason we are not is that the public discussions surrounding EV's are filled with fantasies and hand-wave-itis, which means the public isn't informed and there's no way to gain votes in support of new power generation projects. This is why I say we need to start to discuss the truth, rather than the fantasies we are being sold.