Designing an effective charging infrastructure.


During a recent event on electric mobility charging infrastructure one of the speakers, Manuel Fernandes (President Tritium Europe) demonstrated that for a given investment a Fast DC charging infrastructure (the faster, the better says the caption) serves more clients than one based on AC, showing this infographic:

(be mindful of a small mistake: the last row shows the total number of sessions executable every day on the infrastructure built with one million euro, and is the product of the two previous rows. The result on the 22kW column is obviously wrong, as it should read 16*140=2.240 instead of 340)

I agree in very general terms with this thesis, but the calculations include a few significant mistakes which ought to be corrected; this table is the spreadsheet which calculated the numbers on the slide, to which I added a couple of rows:

  • UNIT COST – dividing the available capital (€1M) by the number of stations, we obtain their unit cost; it is rather evident that such cost does NOT INCLUDE the installation; as an example, the meter for a 350kW station costs over €25.000 ! Therefore, we must add to the hardware costs an estimate for the installation costs.
  • OCCUPANCY – multiplying the duration of each session (rows “Time”) by the number of Clients served by each Station (row “Clients”) we get the total daily occupation time for each station; assuming we can’t ask our clients to set their alarm clock for 3AM to recharge the car, we estimated in 18 hours per day the maximum availability. Dividing occupation by availability we get the occupancy rate (which can never exceed 100%) and can also be construed as the probability that any client visiting a station finds it occupied: the higher this probability, the lower the perceived service quality, especially when you consider that – at least in Italy – the occupancy rate of gasoline station is very, very low; in my opinion, the occupancy rate should never exceed 33%.
  • CHARGING POWER – the first row of the spreadsheet shows the maximum rated power of each station: in a real world, however, this value does not always indicate the power rate at which charging occurs: for example, very few cars charge in AC over 11kW and none charges over 210 in DC (very expensive vehicles which do not and never will represent the average stock). More realistic values are 11kW AC and 100 kW DC.

I have therefore recalculated the spreadsheet including all the corrections: realistic cost inclusive of installation, reasonable occupancy rates and realistic charging power levels. I have also added an additional 100kW column, deriving this new version:

Summing it all up, the number of sessions peaks when the charging infrastructure rated power equals the average maximum input power of the car stock; increasing the infrastructure power beyond that level merely increases costs without further reducing charging time.

Must we conclude AC stations are useless? on the contrary: when charging at night (when the car would not move anyway) they represent the best and cheapest option, but outside that scenario they represent a sub-optimal solution.


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