The carbon footprint of electric car batteries

Tesla model S
Tesla Model S (Andrzej Otrębski, CC BY-SA 4.0)

According to a recent Swedish review by Mia Romare & Lisbeth Dahllöf (2017), the production of 1 kWh capacity of lithium-ion battery causes emissions between 150 and 200 kg CO2eq/kWh (CO2-equivalent per kWh battery capacity). Calculating with 175 kg, just the batteries of a 100 kWh capacity Tesla Model S account for 17.5 t CO2eq. The footprint of batteries increases almost linearly with capacity since most effort goes into the production of individual battery cells and not into their aggregation.

This is already quite a large amount for the battery alone. At 2.31 kg CO2eq per liter petrol/gasoline, the climate footprint of such an e-car battery is the equivalent of 7575 L fuel – enough to drive a conventional car for more than 100 000 km.

What about the car itself? Climate footprint estimates for conventional cars range from 6 (small car) to 35 t CO2eq (large car). (Aside: Yes, that does mean you should drive your old car as long as you can because the manufacturing footprint is larger than small incremental mileage efficiency gains!)

An older study by the Union of Concerned Scientists (2015) concludes that a) the general manufacturing footprint of a Tesla S – not accounting for batteries – is about the same as that of a conventional car; and b) for the 85 kWh battery of the Tesla S model studied there, “manufacturing emissions are approximately 68%, or 6 tons of CO2eq higher than a comparable conventional gasoline vehicle”. Now, 6 t for 85 kWh capacity results in 70.6 kg CO2eq/kWh battery capacity – less than half of the lower estimate of the Swedish study. But then other studies (cited in Romare & Dahllöf 2017) range even from 30 kg to 270 kg/kWh…

Given the large footprint impact of battery capacity, it is clear that life-cycle assessments strongly depend on the total expected range of a car. It is interesting to note, that the 2015 Union of Concerned Scientist study, which concludes that the life-cycle assessment compared with a conventional car is positive, assumes a lifetime range of 288 073 km (179 000 miles) for the Tesla S. This range is reasonable for commercially driven cars but how about a privately owned commute car? It is reasonable to assume that most users would like to have a reasonable reserve for unforeseen events as well as reduced battery capacity (wear, temperatures) of one-third and can only charge the batteries at home. Of the official range of 460 km (285 miles), a well matching commute would be at a distance of 150 km (300 km daily range). Such a car could reach the lifetime range assumed in the model in about 3.2 years (assuming shorter trips on weekends and one holiday/yr), confirming the life-cycle assumptions of the 2015 study as reasonable.

There is a catch, however. If the battery capacity and range of the car is a poor match, the assessment quickly becomes unfavorable. If, for example, the car is used only for a daily commute of 50 km and the longer range bought only for the holidays (estimated here at 3000 km/year), it would take about 17 years to reach the estimated lifetime range, more than the average life-expectancy of a car (e.g. 12 years).

Finally, some interesting details can be found in a summary about a study financed by the German Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety (2016, I noticed it from a post by Clemens Gleich). This study tries to assess the climate effect under conservative conditions: Using the actual German energy mix, including transportation and losses, renewable fuel effect, car component recycling, etc. The study concludes that already in 2015, an electric car has a 12% lower climate footprint than a state-of-the-art, low-emission conventional car. The advantage is expected to increase to 20% in 2020 because electricity generation uses more renewables. This is low but non-negligible. Good. But …

While not all assumptions of the 2016 study are transparent to me, one assumption stands out: Footnote 6 mentions that the battery charge range is the only 150 km, i.e. the study assumes a battery capacity somewhere between 25 and 32 kWh. And the conventional low climate footprint car is specified as a “VW Golf 1.6 TDI BlueMotion Trendline 81 kW with 120g CO2/km (ADAC)”, which typically has a driving range of 1500 km (unrealistic standard test emissions of 85 g CO2/km). A small and lightweight commuter car with 150 km range is compared with a medium sized universal car with a range of 1500 km between refueling …

What can we learn from this?

  1. Carbon footprints of complex manufacturing processes are notoriously difficult to calculate and the scientific estimates may legitimately differ.
  2. The manufacturing footprint of both conventional and electric cars is large compared to its usage footprint.
  3. The manufacturing footprint of electric car batteries is very significant and depends highly on the battery capacity.
  4. Battery capacity and resulting range for e-cars should not primarily be discussed in terms of a range-weight-price trade-off. It is a sustainability trade-off. A hypothetical Tesla S, with a 125 kWh battery capacity and ca. 676 km range would already have the carbon footprint of two conventional cars.
  5. Whether the added footprint of a battery pays off in an electric car depends on the relation between battery size and lifetime range.
  6. To achieve a positive ecological footprint, it is essential to buy an electric car with battery capacity matching the expected range as closely as possible.
  7. Which makes it difficult to use the car for a holiday trip….
  8. Which leads me to the conclusion, that electric cars need smarter solutions. They don’t simply replace the current wasteful all-purpose-everything-I-might-ever-need fossil fuel car.

Smart solutions are already available. Electric cars are a great component in an integrated mobility system that includes public transport, e-bikes, car sharing, and on-demand car rental (with options to use different models for different purposes like commuting, transporting your new furniture, or a long holiday trip).

References

Updates

Interesting additional information: The consumer Total Cost of Ownership (TCO) for a battery-electric vehicle may be less than that of conventional vehicles: Jens Hagman et al. 2016. The total cost of ownership and its potential implications for battery electric vehicle diffusion. http://www.sciencedirect.com/science/article/pii/S2210539516000043

Note: see also my rough and amateurish estimation of global electrical conversion and future battery capacities; and https://i2.wp.com/saabblog.net/wp-content/uploads/2015/09/klimawirkungen_herstellung.png


(© Gregor Hagedorn 2017, CC BY-SA 4.0, publ. 2017-08-17, updated 2018-02-04)

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