Thursday, August 7, 2014

Embodied energy in a car - update under way

John Biggins sent me a helpful email querying a number in my book's chapter on "Stuff".
I have a question about the embedded energy in a car, which you quote at 76000kWh. That seems awfully high to me. To a first approximation a car is a tonne of steel, with a raw material energy of 6000kWh: an order of magnitude less.The (admittedly biased) Society of Motor Manufacturers & Traders report quote an even lower figure of 2000kWh per car (page 17), which I suspect is probably meant to be simply the energy used per car by the car plant, neglecting materials.
The guardian also wrote about this in 2009 .
They asked a few manufacturers, and arrive at a figures in the ballpark of about 1-4 tonnes of C02 to produce a car, which we might reverse engineer guessing most of the CO2 comes from coal burning in either steel production or electricity generation, to get ballpark figures of probably no more than 10,000kwh per car.
Since these estimates actually differ from your figure by a magnitude, I thought I'd write and ask whether you particularly believe your 76,000kWh figure. Do you have any back-of-the-envelope type way to understand it?

This blog post is where I will record my working on this question. I will aim to justify or adjust my answer within a month, and will add to the book's Errata if necessary. If anyone wants to send me good references on embodied-car-energy to add to my own, please post a comment. Thanks! David


rks said...

Does this include the energy consumed by workers in the process of spending the money they are paid? In the similar EROI case I argue that we have something like an electric circuit, and increasing cost of energy production (e.g. more expensive oil extraction) acts in a similar way to adding resistance in the circuit to reduce output of the total system (~GDP).

Unknown said...

Robert: I think that's a separate question, but it's something that's bothered me also. Whilst the direct environmental impact of buying, say, a massage and a barrel of oil are wildly different, it's not at all clear to me that if the masseur buys a barrel of oil with the money you give them there's any real environmental benefit. And pretty much everyone indirectly buys barrels of oil.

Unknown said...

Hi David,

The CCC commissioned some work on lifecycle emissions of low-carbon technologies last year, which also looked at those for the conventional technologies they'd be displacing.

You can find it here: (the transport stuff starts on slide 131)

It includes a literature survey, so that might be helpful, but also projected forward the comparison of lifecycle emissions as economies decarbonise (e.g. where electricity is important in embodied the comparison can look very different now vs. 2030)

Everything in there is in g/km rather than absolute tCO2 per vehicle, but lifetime kms of around 150,000 seem fairly standard. We've got the spreadsheets if you want them.


Anonymous said...

The numbers discussed here seem in any case orders of magnitude smaller than the energy consumed by using the car. Even using "generous" assumptions of 15kWh/100km (standard number for electric cars, which are far more energy efficient than IC cars) and 100,000 lifetime km (modern cars easily achieve at least twice as much), the lifetime energy use amounts to 1,500,000 kWh, which is about 20 times as much as prof. MacKay's original figure for the embodied energy.

So, when it comes to lifetime energy cost of a car, manufacturing energy costs are an insignificant factor compared to the car's fuel efficiency, whether the number for the former is 76,000 or 24,000. Not that I think that we shouldn't try to get the number right, I just want to add some wider perspective to the discussion.

With regard to the discussion of energy consumption resulting from auto-makers spending their paychecks, that's obviously accounted for in the energy use calculation for the things they buy. Think for example of a situation where they use that money to buy a car: you'd end up counting the same energy twice if you tried to include worker's use of their wages in the energy cost of making a car.

Anonymous said...

Um, per previous comment:



100,000 lifetime km

=15,000 kWh


1,500,000 kWh which would be 15kWh / 1 km times 100,000km (this would obviously unreasonable - my car does absolute maximum about 77kW and 1km takes <1minute (at a fraction of that power) so 1km << 1kWh, consistent with original estimate of 0.15kWh/km for electric cars.

So even allowing for lower efficiency of IC cars and greater total mileage, perhaps not so much more than the embodied energy...

Unknown said...

Here's my calculation for lifecycle energy of an electric Golf (e-Golf ) vs diesel.

Lifecycle energy = Embodied energy (in production) + operational energy consumed by using the car

We’ll use 200,000 lifetime miles as reasonable expected life, although diesels have proven to run for longer. At average annual driving distance 12,000 miles (19,000 km), this equates to 16 years life, which should be expected for a Golf.

During this 200,000 miles, we can assume that the batteries in the e-Golf will need to be replaced at least once.

Production embodied energy

Using LCA software, Volkswagen has calculated around 24,000 kWh (85 GJ) for production of a standard Golf and 40,000 kWh (145 GJ) for production of an e-Golf.
pages 19-20

The difference in embodied energy between the e-Golf and TDI is the batteries = 40,000 – 24,000 kWh = 16,000 kWh

So embodied energy for e-Golf (assuming 200,000 miles life with one battery replacement) is actually = 40,000 kWh + 16,000 kWh = 56,000 kWh

Energy consumed by using the car.

The 2015 e-Golf has an official EPA rated combined fuel economy of 116 miles per gallon gasoline equivalent (MPGe) for an energy consumption of 29 kW-hrs/100 mi.

2015 Golf TDI (2.0) has an official EPA rated combined fuel economy of 36 miles per gallon equating to 93 kW-hrs/100 mi.

Operational energy, using 200,000 lifetime miles as expected life:

e-Golf = 200,000 (29/100) = 98,000 kWh
Golf TDI = 200,000 (93/100) = 186,000 kWh

Total lifecycle energy = Embodied energy (in production) + operational energy consumed by using the car

e-Golf = 56,000 kWh production + 98,000 kWh = 154,000 kWh
Golf TDI = 24,000 kWh production + 186,000 kWh = 210,000 kWh

The e-Golf wins with a 73% lifecycle energy consumption. However…. The above calculation assumes that you can actually get replacement batteries for the e-Golf after 10 years (minimum mandatory stocking period for any car manufacturer). If not, then your e-Golf is about as useless as a rechargeable drill without a charger and battery, which have a built-in obsolescence of about 3 years it seems. Given the rate at which battery technology is changing, I’m not sure it’s safe to assume that the same battery form will exist 10 to 16 years hence.

I’m sticking with the TDI…

Danny Bonnett said...

We should not forget that it is not just about energy - carbon is pretty important too (just having had the single highest monthly temperature rise over the accepted average - Feb 2016). Electricity from the grid can (and is being) steadily decarbonised. There is no realistic way to decarbonise diesel on a large scale. So, adoption of the e-Golf over the TDi will help to force that transition, and so the carbon benefit (of e-Golf over TDi) will increase over time.

Also, the exhaust pipe emissions are important to all those pedestrians, cyclists, other drivers, and those who live near busy roads. The e-Golf wins there too.

So let's go 'glass half full', assume the batteries will be available (actually cheaper, lighter, less embodied energy, and most importantly compatible), and go for the e-Golf.