How many trees does it take to fuel a car?
This short article is moving away from electric vehicles and sticks with ordinary combustion engines, but ones that run on biofuel. I still assume 12,000 miles per year, but at 45 MPG.
Biofuel is a bit of a strange term, as fossil fuel is really biofuel, just that it is very old. Very, very old. I am reliably told that it is even older than James May, whose birthday it is next week. Having just looked this up, I see that his initialised full name, JDM, stands for Japanese Domestic Market, so happy birthday for next week, the card and presents are in the post, honest.
But enough of that frivolity, can we really run a normal car off something grown in the garden. Well let us see.
I have decided that the best thing to grow in the UK is trees. Currently we have nearly double the area covered by trees than all urbanisation. 13% wooded coverage, up from 5% in the 1920s, we are about half the European average.
Now there are many different trees that can be grown, and to be honest, I hardly recognise any of them, but for this purpose, I have picked Poplar trees. I think they are the tall, thin ones that are related to Willow. They grow well in our rainy countryside.
A poplar tree takes between 5 and 10 years to fully grow, so we can say that, if we start from a muddy patch of flood plain, it will take 7.5 years until we can start cutting them down.
The yield, which is just the amount of wood we cut down, is somewhere between 1.25 and 8.61 dry tonnes per acre, or, taking an average and converting to Roman Catholic, this works out at 1.2 kg of dry mass per square metre of land.
As we thankfully don’t run steam powered vehicles any more, even down in Cornwall where they were invented, we cannot put wood into the fuel tank, we have to convert it to something more manageable.
Picking ethanol as a fuel, mainly because we have experience of making, using and drinking it.
Those IndyCars, that pretend to be fast, churn out nearly 700 HP from a 3.5 litre motor. Not a bad performance.
There are two main ways to turn a log into ethanol:
Cellulolysis, which uses hydrolysis and enzymes to make sugar.
Gasification, uses heat to make carbon monoxide and hydrogen.
Both methods then use a combination of fermentation and distillation to get the alcohol out.
Once the process is done, there is about 140 litre per tonne of dry feedstock.
What this boils down to is that for every kilogram of wood pushed into the bioreactor, we can get 1.2 kWh of useful energy out. A litre of ethanol has 6.7 kWh of energy in it.
Just as a reality check, gasoline, the proper word for petrol, has 9.5 kWh in a litre, diesel is even better at 10.7 kWh. That is why they are more economical and emit less carbon dioxide.
This is a gross efficiency ratio of 0.12% based on 1.2 kWh of fuel for every 950 kWh of solar irradiance. A normal solar panel will easily give an overall efficiency 10% and often is 15%.
Now I found this quite disturbing and wanted to check a bit more into this low efficiency. So I went to a tree farm, Healeys Cider Farm to be precise, they grow trees and make alcohol.
This is the same Healey family where Donald came from. Donald Healey, was born at Woodbine Cottage, Perranporth, so may have known a bit about smoking, he could make tyres smoke a treat in his Big Healeys.
Anyway, they now make cider, and I am told the Rattler is very good. This has an ABV, or alcohol by volume, of 6%, they have two local farms of 20 acres each and crop 150 tonnes of apples from them.
This is enough to make around 100,000 litres of cider, or 6,100 litres of pure ethanol. This is 0.04 lt per square metre per year. That is 0.25 kWh. A lot worse than a tree.
That is not the end of the story though. Cornwall has the highest solar irradiance of all the places in England. Around 1,400 kWh per square metre per year. This means that the gross efficiency of cider alcohol is 0.02%. Much worse than a tree.
So rather than buy a new house, I could just buy a wood and start making my own fuel. If I evict Winnie-the-Pooh from 100 Acre Wood, I could fuel up 8 cars per year.