ICE vs BEV vs FC

Friends, forgive my titular obscurantism. This is an article about how to compare the efficiencies and carbon emissions of cars of different types:

• ICE = Internal Combustion Engine i.e. petrol and diesel cars
• BEV = Battery Electric Vehicles
• FC = Fuel Cell cars

A cloud over Teddington!

While relaxing at de Podesta Towers yesterday, a momentary cloud of worry passed through the otherwise blissfully blue sky of my retirement. I thought: how do you compare the efficiencies of these different classes of vehicles?

• ICEs specify miles per gallon or litres of fuel per 100 km.
• BEVs specify how many kilowatt hours (kWh) of energy are required to travel 100 km.
• FCs specify how many kilograms of hydrogen (kgH2) are required to travel 100 km.

Comparing CO2 emissions

One way to compare these disparate types of vehicles is to compare their carbon dioxide emissions.

For all vehicles there are both proximate emissions which occur as the vehicle is driven, and source emissions associated with the charging – in the general sense – of the vehicle.

This categorisation is sometimes succinctly summarised as

• well-to-tank emissions,
• tank-to-wheels emissions,

which together add up to make well-to-wheels emissions.

• For ICEs there are both well-to-tank emissions and tank-to-wheels emissions.
• For BEVs ‘well-to-tank’ emissions occur at the power stations used to generate the electricity that charged the battery. There are no proximate or tank-to-wheels emissions.
• For FCs, ‘well-to-tank’ emissions occur at the power stations used to generate the electricity used to separate and compress the hydrogen gas. There are no proximate or tank-to-wheels emissions.

For ICEs, the CO2 emissions per kilometre are a multiplier of the fuel efficiency, with different factors for petrol or diesel, plus a factor for the emissions associated with the preparation of the fuel.

• Factor for creation of the fuel
• Vehicle specification in Litres per 100 km
• Factor for fuel type: petrol or diesel

For BEVs the the CO2 emissions per kilometre are the product of the three factors:

• CO2 per kWh of the charging electricity – the so-called carbon intensity of the electricity.
• The efficiency of the charging.
• Vehicle efficiency specified as kWh per 100 km

For FCs the the CO2 emissions per kilometre are the product of the two factors:

• CO2 per kWh of the electricity used in the electrolysis and compression of the H2 gas.
• Vehicle efficiency specified as kgH2 used per 100 km

So lets look at some examples.

Toyota Mirai FC 

• Specifications suggest an average of 0.75 kgH2/100 km
• Wikipedia suggests Electrolysis at 80% efficiency and compression together take 65 kWh/kgH2
• Current UK carbon intensity is 200g/kWh which is expected to fall to 100g/kWh in 2030.
• Multiplying we find CO2/km = 0.75 [kgH2/100 km] x 65 [kWh/kgH2] = 49 kWh/100 km
• Multiplying by the carbon intensity x 200 [g/kWh] = 97 gCO2/km in 2021 falling to 49 gCO2/km in 2030

Perhaps you can see why my brow furrowed over trying to figure this out!

Tesla Model 3 BEV

• Specifications suggest 26 kWh/100 km
• Charging efficiency is typically 90% i.e. if we use 28.6 kW of grid electricity to charge the car, 90% of that (26 kWh) will be stored in the battery.
• Multiplying by the carbon intensity of 200 [g/kWh] = 57 gCO2/km in 2021 falling to 28 gCO2/km in 2030

It is hard to pick a single ICE car comparable with these rather premium BEVs and FCs. I have looked through the Mercedes A-class brochure which states that under the Worldwide Harmonized Light Vehicles Test Procedure (WLTP) the proximate CO2 emissions are in the range

• 130 to 150 gCO2/km
• This corresponds to about 6 litres/100 km or 47 miles per UK gallon
• The above figures are only tank-to-wheels emissions. The well-to-tank emissions amount to roughly 36 gCO2/km (link)

Click for a larger version. Summary of the amount of carbon dioxide emitted per km of travel for a fuel call car (Toyota Mirai), a battery electric vehicle (Tesla Model 3), or a premium internal combustion engined car (Mercedes A Class). In the UK grid electricity is expected to have a lower carbon intensity by 2030 which should reduce the emissions associated with BEV and FC cars. This chart ignores any embodied carbon dioxide in the construction of the vehicles.

So in terms of carbon dioxide emissions, a BEV is likely to be better than an FC car and both are much better than comparable ICE cars.

BEV vs FC

The reason that the FC car is so much poorer than the BEV is because of the complexities of first obtaining hydrogen, compressing it (link), and then converting the chemical energy back to electricity in the fuel cell.

The advantages of fuel cell cars over BEVs (faster re-filling and longer ranges) are slowly being eroded by advances in battery technology. And  given the paucity of hydrogen filling stations, I think FC cars will prove to be a historical dead end.

BEV vs ICE: Efficiency

The key to the success of ICEs has been the astonishing energy density of their fuels.

Click for a larger version. This graphic (modified Wikipedia graphic) shows the energy density (kWh/litre) versus specific energy (kWh/kg) for various fuels. Strikingly, batteries have very low energy density and specific energy than ICE fuels. Note also the very high specific energy density of hydrogen (per unit mass), but the the very low energy density (per unit volume).

As the chart above shows the energy density of either petrol or diesel – whether expressed per kilogram or per litre – is way more than that of lithium ion batteries used in BEVs.

If we average gasoline (12.9 kWh/kg, 9.5 kWh/l) and diesel (12.7 kWh/kg, 10,7 kWh/l) we get a rough figure for liquid fuels of 12.8 kWh/kg and 10.1 kWh/l.

In contrast the figures for a lithium ion battery are in the range 0.1 – 0.24 kWh/kg and 0.25–0.73 kWh/l, smaller than liquid fuel by factors of at least 53 for per unit mass and 14 per unit volume.

It might seem that there would be no way that batteries could ever compete. But in fact ICEs are profilgate with their energy use.

ICEs only extract about 35% of this chemical energy as mechanical energy and then frictional losses in the engine and drivetrain reduce this to about 20%. So that makes the advantage factors of roughly 10 per unit mass and 2.8 per unit volume.

In contrast, after taking account of regenerative braking, BEVs can convert about 90% of their stored energy to motive power.

This still leaves ICEs with an advantage and to compete BEVs end up with a heavy battery which takes a large volume. And it can’t quite be made big enough to challenge the range of ICEs.

But it seems that BEVs have become ‘good enough’.

ICEs are very highly evolved with more than 100 years of continuous development, but lithium ion batteries are still relatively new and are likely to continue to get incrementally better and cheaper. And as the carbon intensity of the grid reduces over the coming years the cars will become greener still.

Blue sky 

And thus the cloud passed, and the sky cleared, and life at de Podesta Towers returned to its previous untroubled pace.

 

Tags: BEV, FC, ICE