Liquid Fuels and Carbon Capture
Concept
We have spent the last couple of hundred years developing – enabling the planet to support more and more people, in more and more comfort.
However, the vast majority of the energy to enable that change has been generated by converting carbon to carbon-dioxide. We now need to repair the damage that has done by converting the excess CO2 in the atmosphere back into a non-gaseous form of carbon. We also need to stop producing the CO2.
Sometimes we do need the energy density of a liquid fuel – for example flying or transporting heavy goods. If we were to create a fuel for transport from the CO2 in the atmosphere, then the total amount of CO2 in the atmosphere would not increase.
The oil companies, which have produced so much of the carbon we have all burnt when we have used energy, are all skilled at chemical processes. They also have the infrastructure to distribute liquid fuels. If we were to mandate that they helped to extract the CO2 from the atmosphere, but could sell a small proportion of it, as a liquid fuel, it might be a big enough incentive for them to help, rather than try to block the change to a fossil-fuel free society.
Potential Liquid fuels
There are several potential starting points for such liquid fuels. Further polymerisation to make things more dense for storage will almost certainly also be needed, but we have to start somewhere.
Starting from atmospheric CO2, we would need Direct Air Capture of CO2 (DACC). The additional ‘S’ in DACCS stands for “and Storage”.
CO2 also needs to be captured from any chemical process where it is produced – e.g. from cement manufacture, and most current steel production processes.
Note that any process that captures CO2 will consume energy – it is essentially the reverse of burning coal or oil. DACC is sometime called “artificial trees”, because like trees, it uses energy to extract CO2 from the atmosphere.
We could potentially convert that captured CO2 initially to:
These all also need Hydrogen in their synthesis, so that hydrogen also needs to be made without creating any CO2 during its production.
The DACCS processes can extract pure CO2 from the atmosphere.
Why Energy Density matters
Mobile phones are only practical because we have small lightweight batteries that can provide enough power to drive the device. A Tesla car typically has half a ton of batteries – making the car about 50% heavier, and that extra weight takes energy to push around. Batteries are big and heavy compared to liquid fuels, and thus …
For some transport it will always be beneficial to be able to use liquid fuels, because the energy density of carbon-based liquid fuels is far higher than that of batteries. If these Carbon-based liquid fuels were made from atmospheric CO2, rather than fossil fuels which had been pulled out of the ground, then their use would not create an over-all increase in the CO2 concentration in the atmosphere.
Some numbers:
The Specific energy of an Electric Car battery is about 100 times lower than a fossil fuel:
- 1kg petrol (approx 1.4litres) yields about 12kWh of energy
- A charged 1kg Electric Vehicle battery delivers about 150Wh.
- For comparison, an old fashioned/standard lead-acid car battery stores only 30-50Wh per kg
This coheres with every-day experience – the weight of a 1kWh (1000Wh) home PV storage battery is at least 10kg, and is usually much heavier and the weight of an AA battery, which typically holds about 3.9Wh is about 25g or 1/16 of a kg. So 150Wh for a 1kg charged battery is a believable number.
However, Electric motors are about 90% efficient whereas an internal combustion engine is only 25% efficient.
So from 1kg petrol you could expect 25% of 12kWh = 3kWh = 3000Wh of energy from the engine
From 1kg battery you could expect 90% of 150Wh = 135Wh from the engine.
So electric batteries are more appropriate for very light cars going short distances, but not sensible yet, for heavy trucks or aircraft which need to carry much more energy to travel long distances. A particular problem with aircraft is that clearly it is not possible to recharge mid-Atlantic. The problem with trucks is that typically they are travelling for hundreds of miles and, although generally drivers have to take overnight/daily breaks of 11 hours in any 24 hour period, for the other 13 hours they take a 45 minute break every 4.5 hours.
Let us do a quick Fermi-estimate for a heavy lorry:
For an electric truck to be useful and competitive it would need to be able to drive for 4.5 hours and recharge in 45 minutes. In the UK HGVs are supposed to be limited to 60mph, so 4.5 hours driving looks like 270 miles.
A typical HGV does about 7 miles per gallon or a little over 1.5 miles per litre of fuel, and thus needs about 175 litres of fuel to go 270 miles, though typically HGV fuel tanks can hold up to 1500 litres, so drivers currently do not need to fill up with fuel every time they stop for a break.
If we take our efficiency numbers from before 1.4 litres of fuel yields about 3kWh of engine energy.
175 litres of fuel would thus produce 3 * 175/1.4 kWh of energy = 375kWh of energy.
On current battery technology that would mean we would need a battery weighing:
(375000/135) kg = 2777.78 kg, or 2.777 Tonnes
If we wanted to charge that 375kWh battery in an hour, using a 240V feed, it would need to take a 375000/240 = 1562.5 Amp feed. To charge in 45 minutes it would need 4/3 of that = 2083 Amps.
For comparison a typical UK house is connected to the energy grid with a 60-100Amp feed. The local grid could not cope if all the houses on a street tried to draw that much current simultaneously, but here to charge one HGV needs the power of a street full of houses, and a dedicated super thick power cable.
Clearly this is not possible with a standard supply wire – the wire would melt apart from anything else. You would need either a much higher voltage feed, or a large number of simultaneous charging cables per truck. Any truck charging station /rest area would need a lot of infrastructure investment to cope with multiple trucks charging at once.
Charging the battery at a more plausible current, say 30Amps, it would take 375000/(240*30) = 52 hours, so even the over-night charge would not be enough to allow an electric HGV travel for 4.5 hours.
Tesla does now have a 250kW super charging ability for their model 3s – with the intention of giving the car a very quick boost of charge to give an extra 75 mile range in 5 minutes. The Tesla super chargers run at 480V DC to the vehicle and need over 500 Amps. On the grid side, the super charger has to be connected to a substation, so these chargers are not available at home. Their chargers are actively cooled, do have multiple very fat connectors in the plugs, and the rate at which they charge decreases sharply as the battery fills up and it gets more difficult to push the lithium ions back into the graphite structure at the anode.
If you could charge an HGV’s electric battery at that rate, then it would take 375/250 hours to charge the battery after the 4.5 hour driving stint which would be about 1.1 hours – so too long to fit in the rest breaks between the driving periods.
To cover the 3 daily driving stints would take 3.3 hours – so could potentially be done overnight, but the weight of the battery for the truck would then be 2.777 * 3 tonnes = 8.333 tonnes.
This is why we claim that heavy lorries and HGVs for long distance delivery, still need the energy density of a liquid, carbon-based fuel.
The numbers for air-flight and shipping are even more problematic.
References / external websites:
Energy density of various fuels
https://en.wikipedia.org/wiki/Energy_density
Electric vehicle battery overview
https://batteryuniversity.com/learn/article/electric_vehicle_ev
Trucks fuel usage in mpg
https://infogram.com/average-hgv-mpg-uk-1gl8m3jy4okep36
Truck driver’s rest break rules