4 ms·

Charging the battery itself alone might be efficient. Take your 90% efficiency as true, and assume electric motor efficiency as 90% too, now the net efficiency

by mrjin 5y ago

Charging the battery itself alone might be efficient. Take your 90% efficiency as true, and assume electric motor efficiency as 90% too, now the net efficiency drops to 81%. To reach your home, there most likely at least one step-up transformer and 2 or more step-down transformers. Assuming all those transformers are super efficient with a efficiency of 90% too, then the efficiency now becomes 65.61% for two transformers scenarios, 59.049% for 3 transforms scenarios. Considering the cost of manufacturing the battery, the huge weigh of the battery(85kWh now weighs around 540kg), the overhead of the power grid as a result of staggeringly over provisioning needed: assuming charging a 85kWh battery from 0% to 100% in 1 hour, using 220V AC power, nominal current required is 85 * 1000(kW) / 220(V) = 386.36A. Assuming power factor 80%, then the required currency is 386.36/0.8=482.95A, how much is going to cost for that capacity? 1 hour might be too aggressive, let take a concession to 8 hours, then the currency required is 60.37A. Just take look at your home's fuse, how much is that rating? Let's take another concession to require only 50% charge, then the required currency is ~30.19A, which is still around 10 times normal off-peak usage or ~3 times peak hour usage. Are you seriously considering to increase the capacity of the whole power grid by 3 to 10 times just for charging the car at home to 50% juice in 8 hours?

leoedin 5y ago
You're making some low assumptions for efficiency there - 90% is an absolute low end efficiency for transformers under full and non-linear load. Transmission losses for the UK grid average 8% total - regardless of how many transformers are involved. https://publications.parliament.uk/pa/cm201415/cmselect/cmenergy/386/38607.html https://publications.parliament.uk/pa/cm201415/cmselect/cmen... You also forgot to take into account that most cars don't drive 300+ miles every day. The average car drives (depending on country) maybe 7000 - 10,000 miles a year. That means it will either be fully charged rarely, or topped up a little bit every night. We don't have to have a grid which can charge every car in the country from empty every night, we just need a grid which can charge a small fraction them. In fact, in the UK where the average car drives about 7000 miles a year, the overall average power requirement is something like 200 W. That's well within the capacity of our grid. We will have to take care to limit the surge effect of every car being plugged in at 6pm - but that just means delaying the nightly top up of most cars until the early hours (or whenever electricity is available).
- mrjin 5y ago
  10% loss was just meant for quick calc, and I did not even factor in the loss in power generation etc. And the assumption was only 50% of the capacity, so that would drop to ~150 miles. Again, it's claimed value, how much you can actually get out of it really varies. If you just want to get 30 miles per day, then you will need ~ 6A, which is within household circuit rating but will still double the load of the ordinary family peak. Please don't simply use Watts to calculate AC load, it will not give you the real current demand.
  - leoedin 5y ago
    Using 10% for a quick calc is fine, if you only use it once. But you then multiplied that error by 3. The resulting assumption - that the transmission losses with 3 transformers would be ~73% (0.9^3) is completely wrong, given that we know total grid transmission losses are in the order of <10%
    - LargoLasskhyfv 5y ago
      They aren't...
    - mrjin 5y ago
      Well, look at your calc again. Transmission loss per my calc is actually 1 - 0.9^3 = 27.1%. At grid level, high efficient part(mainly stable industry loads) will cover up low efficiency of household loads.
- LargoLasskhyfv 5y ago
  While not familar with how things are in the UK, I understand DNO, which is the layer of the distribution network. Which means the part from power-plant->substation->high voltage line to at least one other substation near a city, or so. That does not include middle voltage into the city, or further downstepping until it is fit for your wall socket or similar. If it claims to, it is a political fiction like so many other. In reality transmission losses from the one virtual plant powering the grid to your wall socket range from about 50% to 60% depending on the grid and many other factors. Anything else is wishful thinking.

by mrjin 5y ago

leoedin 5y ago

You're making some low assumptions for efficiency there - 90% is an absolute low end efficiency for transformers under full and non-linear load. Transmission losses for the UK grid average 8% total - regardless of how many transformers are involved. https://publications.parliament.uk/pa/cm201415/cmselect/cmenergy/386/38607.html https://publications.parliament.uk/pa/cm201415/cmselect/cmen... You also forgot to take into account that most cars don't drive 300+ miles every day. The average car drives (depending on country) maybe 7000 - 10,000 miles a year. That means it will either be fully charged rarely, or topped up a little bit every night. We don't have to have a grid which can charge every car in the country from empty every night, we just need a grid which can charge a small fraction them. In fact, in the UK where the average car drives about 7000 miles a year, the overall average power requirement is something like 200 W. That's well within the capacity of our grid. We will have to take care to limit the surge effect of every car being plugged in at 6pm - but that just means delaying the nightly top up of most cars until the early hours (or whenever electricity is available).

mrjin 5y ago
10% loss was just meant for quick calc, and I did not even factor in the loss in power generation etc. And the assumption was only 50% of the capacity, so that would drop to ~150 miles. Again, it's claimed value, how much you can actually get out of it really varies. If you just want to get 30 miles per day, then you will need ~ 6A, which is within household circuit rating but will still double the load of the ordinary family peak. Please don't simply use Watts to calculate AC load, it will not give you the real current demand.
- leoedin 5y ago
  Using 10% for a quick calc is fine, if you only use it once. But you then multiplied that error by 3. The resulting assumption - that the transmission losses with 3 transformers would be ~73% (0.9^3) is completely wrong, given that we know total grid transmission losses are in the order of <10%
  - LargoLasskhyfv 5y ago
    They aren't...
  - mrjin 5y ago
    Well, look at your calc again. Transmission loss per my calc is actually 1 - 0.9^3 = 27.1%. At grid level, high efficient part(mainly stable industry loads) will cover up low efficiency of household loads.
LargoLasskhyfv 5y ago
While not familar with how things are in the UK, I understand DNO, which is the layer of the distribution network. Which means the part from power-plant->substation->high voltage line to at least one other substation near a city, or so. That does not include middle voltage into the city, or further downstepping until it is fit for your wall socket or similar. If it claims to, it is a political fiction like so many other. In reality transmission losses from the one virtual plant powering the grid to your wall socket range from about 50% to 60% depending on the grid and many other factors. Anything else is wishful thinking.