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What if we all had electric cars…

May 18, 2017

A little tech detour. Electric cars are great – I tried out a Nissan Leaf for a while a few years ago, loved it for short hops but no desire to own one. Anyway one thing rarely mentioned is the impact to the electric grid.

Summary: To approach the filling rates of petrol (fill your vehicle in a few minutes), a single EV filling “pump” would need the electric service of a large office building.

Back-of-the-envolope numbers for electric service:

gasoline energy content = 2.7 kg/gal * 46 MJ/kg = 124 MJ/gal
fuel pump flow rate = 5 gal/min flow (typ US)=> 620 MJ/min = 10 MJ/sec

= 10 MW peak energy transfer rate, per petrol pumps in service today

Lets say 5x reduction due to inherent fuel efficiency of EV’s

= 2 MW peak energy transfer rate, for a single EV charger with same filling speed as today’s petrol.

Note that this assumes the same “tank size” for an EV as for a petrol car.

In that case, compare : this would be equiv to typ. US electric service for 200,000 sqft of office space (3.5 football fields of office, or space for 1300 people at US densities). That’s for a single “pump” capable of a typical fill (80% of your tank) in a few minutes. A large filling station (16 pumps) would need as much electric service as the Empire State building.

However, EV users would likely accept longer fill times (just as users of alt-fuels like LNG/CNG had to).

Next, Energy cost to charge a small electric car. This part is indeed a winner.

petrol vehicle typ fillup: 10 gallons = 1240 MJ = 1240 MW*s = 344 kWh

electric vehicle: 5x more fuel efficient = 69 kWh
=> $6.90 at $0.10/kWh  (current typ. US residential rates. the filling station wants to make money too.)

consider that in the long run, taxes may be added.

compare to existing petrol = 10 gal  x  $2.50/gal = $25

Lastly, charge/fill times at a private residence

petrol car typical energy content of one fillup = 10 gal * 124 MJ/gal = 1240 MJ
electric car, 5x more efficient = 248 MJ

private residence, total residential service in a well connected US single family home: ~36kVA (240V x 150A) ; spare capacity for vehicle ~= 12 kVA (240V x 50A)

=> time = 248 MJ / 12kVA = 10000 sec = 2.8 hours, to fill your electric vehicle at a home charging station that would be within reach of ordinary people in the near future.

So – if typical use pattern is overnight filling at home, and you want to take advantage of cheap solar energy, then energy storage is required. If you don’t have energy storage and you fill at night, you need some other fuel to generate the electricity. If you fill during theday, you are leaving work/school/kids, so we return to the issue above of how much electric power it would take to do it in minutes instead of hours.

Electric cars are still pretty awesome, but it would be cool to see these issues addressed.

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