Introduction
We're frequently told that non-motorised bicycles are the most energy efficient transport in the world. So, there's no way a full EV (BEV) can compete with a bike for energy efficiency.
Or can it?
For example, it's easy to show that a bicycle at 20km/h requires about 75W of power; whereas a typical BEV might use 8.2kW at 50km/h. So, it's no contest.
Or is it?
The problem is that when people make these kinds of comparisons, they're comparing the energy required for the final product rather than the energy required at the source. For a non-motorised bike, that energy comes from the sunlight used to grow crops or farm animals + the energy used to process the food, but for a BEV that energy can come from a renewable energy source (such as Solar PV). Thus the right question to ask is whether replacing the crops making the additional food the cyclist needs, with Solar PV is more than the Solar PV needed for the BEV for the same distance.
With a bit of maths and some publicly available data, we can work it out!
Method
The basic concept is to make some simple (but not unreasonable assumptions) about how we power the cyclist, alongside some corresponding calculations for the BEV. The bike conversions we need are:
We'll see that when we do the calculations, available data is often in different units, so we'll have to do some unit conversions too. Also, we'll end up calculating backwards. For the EV, it's:
We can see already there are fewer obvious areas of loss, but that's because I've combined the motor for the BEV and the BEV into the same box (whereas for the Bike, the human is the motor).
Bike Calculations
This website provides us with a handy table for calculating power consumption for a given bicycle speed:
At 20km/h it gives 75W, which means that travelling 20km requires 75Wh of energy (3.75Wh/km). A Wh is 860 calories, so 75Wh is 860*75=64,500 calories, or 64.5kcal.
Humans, being biological systems burn fat (and other chemical energy), and emit CO₂, much like a combustion car burns petrol, except that it outputs CO₂ from fossil fuels, which adds to the atmosphere, increasing global temperatures. But the key thing is that the mechanism is similar, because chemicals are burned, so the efficiency is similar. In fact it's about 25%[1].
So, the amount of energy the human needs is 64.5/0.25=258 kcal to travel 20km. That's 258/20=12.9 kcal per km.
My big assumption is where the human gets that energy from. I assume they're getting it from a sandwich with no filling, i.e. from bread. And I approximate that with flour, because most of it will be flour and the other stuff, e.g. butter will be less energy efficient, because the energy conversion factor going from the Sun to butter also goes through crops and cows, and therefore can't be better than crops alone. The same reasoning applies to the filling from a sandwich, e.g. egg mayonnaise. The eggs have to go through a conversion factor involving crops and chickens, and therefore can't be better than crops.
It turns out that there are 3.58 calories per gramme of flour. So, 12.9 kcal requires 3.6g of flour and that's tiny. For white flour, we are interested in the energy content of the flour, which is 353kcal per 11.3g of protein[5]. From [4] we can see there's about 32kg of protein per Hectare, which means there's 32000/11.3*353kcal=999646 kcal per hectare. A hectare is 100m*100m so there's 999646/(100*100)=99.965 kcal per m². So, we need 12.9/99.965=0.129 m² of cropland per km.
BEV Calculations
This is a bit simpler, I'll take our Renault Zoe and our SolarPV as an example. Our Zoe achieves about 3.8miles per kWh, which is 3.8*1.609=6.114 km/kWh. So, 1 km uses 1/6.114=0.164kWh which is 164Wh.
We have 3.96kW of SolarPV on our roof, which usually provides 3200kWh per year. Current solar panels are rated at 450W and on average are 1.6m² which means that 0.45/3.96*3200/1.6=227.273kWh is generated per m².
Thus we need 0.164/227.273=0.000722m² of SolarPV per km with a Battery Electric Vehicle.
Results
It takes 0.129m² of cropland per km of cycling, but 0.000722m² of SolarPV per km in our BEV. Thus a BEV is 0.129/0.000722=178.67 x more efficient than a cyclist in terms of land area, a truly astounding result!
Conclusion
Cycling takes up land area to feed cyclists. We can estimate the land area based on the energy used by the cyclist; and the most optimal amount is if the cyclist eats only plant-based food. Thus every km of cycling (per year) corresponds to an area of crop land on a yearly basis. With some simplified, but reasonable assumptions, the value is 0.129m² of cropland per km, for a cyclist travelling at 20km/h.
A BEV only takes 0.000722m² of SolarPV per km per year, about 180x more efficient than a cyclist even though the BEV needs 164/3.75=43.733 more final energy per km.
There are two main reasons for this. Firstly people, being biological systems are inefficient, roughly as inefficient as a combustion car because they both burn chemicals for energy (though as said before, combustion cars add to atmospheric CO₂ and global temperatures). Secondly, and most importantly, crops are astoundingly inefficient compared to solar panels when it comes to energy conversion efficiency.
There are flaws with my method and conclusion, but not flaws amounting to 2.3 orders of magnitude. I only picked a single crop (wheat), but perhaps other crops have better energy yields per m² per year. I picked our Renault Zoe, which has a pretty good range/km: other BEVs, especially bigger ones can be worse (but some are better). On the other hand, practical food intake is far more than just flour, sandwich fillings will be more energy inefficient and I ignored food processing (though it would be a minor contribution). Also, I could have picked an e-bike: that same 75W would translate to 43.7x better efficiency!
But the upshot is clear: when we estimate land-use for transport, cycling requires over 200x the area of a decent Battery Electric Vehicle, yet no-one would consider the land-use requirements for cycling to be excessive.
Refs:
[1] “Thermal energy generated during the chemical reactions that power muscle contractions along with friction in joints and other tissues reduce the efficiency of humans to about 25 %.”
https://phys.libretexts.org/Bookshelves/Conceptual_Physics/Body_Physics_-_Motion_to_Metabolism_(Davis)/10%3A_Powering_the_Body/10.09%3A_Efficiency_of_the_Human_Body
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