Energy efficiency series: electric drives, hydrogen and e-fuels?

Did you know that riding an e-bike solely on solar power can be up to 1,000 times more energy efficient than a conventional bicycle without an auxiliary drive? This is the conclusion reached by Tim Berners-Lee in the second edition of his bestseller entitled ‘How Bad are Bananas?’ in which, as the subhead reads, he examines ‘The Carbon Footprint of Everything.’ The reason is that PV cells convert much more solar energy into electricity than plants transform to calories that can be metabolised by humans. And electric motors are much more efficient when generating kinetic energy than a person’s legs.

Although the above only applies under certain conditions and disregards a number of circumstances such as health, these findings demonstrate how energy efficient electric drives are. They convert about 90 percent of the electricity they consume to kinetic energy.

Of course, an electric SUV draws much more energy because it weighs about 100 times more than a bicycle. This is exacerbated by the energy used to produce and transport it to the point of sale. Nevertheless, even heavy EVs are energy misers.

But which numbers does this all translate into in a 100 percent renewable energy scenario?

Electric drives, hydrogen and e-fuels?

After transmission and several conversion processes, about 85 percent of the electric energy that is fed into the grid finds its way into a car battery. In turn, electric motors convert roughly 85 percent of that into kinetic energy. This results in a total efficiency of 73 percent. Winter temperatures reduce range to approximately 55 percent because they require both the passenger compartment and the battery itself to be heated.

But even this figure is high compared to the alternatives. Employing best available technologies, green hydrogen contains about 70 percent of the electricity consumed during electrolysis. Further energy losses occur during storage and transportation, as a result of which only half of the energy used winds up in the tank of a hydrogen vehicle. Upon reconversion via a fuel cell, only a quarter of the electricity originally generated reaches the electric motor. Taking account of its own efficiency, approximately 22 percent of the energy yield translates into propulsion. At least the fuel cell produces enough heat for both passengers and the battery.

Combustion engines need the most primary energy

Common knowledge has it that this also applies to internal combustion engines. However, producing sustainable petrol and diesel is more energy intensive than green hydrogen. After all, the last in the list is the starting material for producing emission-neutral e-fuels.

Afterwards, the hydrogen forms bonds with the carbon dioxide filtered out of the air or sequestered from industrial plants to create the desired hydrocarbons. Depending on the method, this synthetic fuel contains slightly more or less than 45 percent of the green electricity which has to be used to perform all these chemical reactions. With an average efficiency of 30 percent, internal combustion engines thus put a mere 14 percent of the electricity originally generated onto the road.

Even less primary energy reaches the axles of vehicles with internal combustion engines running on biofuels made of harvested biomass. This is because cultivating rapeseed and grain to produce biodiesel and bioethanol requires a lot of land.

This land could be used to produce usable energy much more efficiently using solar PV technology. According to a study by the IFEU Institute commissioned by Environmental Action Germany (DUH) (Link in German), a mid-sized car can run on the solar power from just 3 percent of the surface area required to produce enough biofuel to operate a passenger vehicle with an internal combustion engine of the same class. In other words, an EV operated using electricity from solar PV production converts 33 times more sunrays to kinetic energy than a biodiesel variant.

Compact cars and public transport

Battery electric vehicles (BEVs) thus fare 33 times better in terms of energy efficiency than the alternatives generally discussed. However, anyone who wants to get from A to B in a really energy efficient manner should also consider the size of their car. An electric van tested by ADAC, the German equivalent of AAA, used almost twice as much energy as one of the compact cars to which it was compared.

However, technology and aerodynamics also play a role. The thriftiest EVs in the ADAC test were three mid-sized cars, including an SUV.

However, there is a more efficient four-wheel alternative: microcars. They are extremely popular in China. Usually designed as two-seaters, they have little cargo space, but are intended for inner-city transport where they prove to be more practical than conventionally sized cars. Their electricity mileage is not necessarily lower than that of thrifty sedans. In fact, the latter are more efficient when full of passengers. But microcar production requires much less energy. The only personal mode of transportation that is much more energy efficient is an e-scooter or any other type of (fully) electric two-wheeler.

Cooperative behaviour and AI-assisted networking

An entirely different approach to increasing energy efficiency consists in increasing traffic flow. The less often cars are made to brake and accelerate, the less energy they use – while maintaining or actually increasing average speed. Slowly coming to a stop at a traffic light instead of maintaining cruising speed and braking sharply just before the stop line conserves energy.

Overall efficiency of road transport can be improved applying the same principle through cooperative behaviour. AI-assisted networking of road users can be helpful. In Project LUKAS, a research centre spearheaded by Bosch monitored a T-junction in Ulm to determine how driver assistance systems can contribute to reducing braking procedures by communicating with each other and their surroundings. “We demonstrated that road users coming from the side street can often turn onto the main road without braking if cars with the right of way slow down just a little,” says Vincent Wiering from the University of Duisburg-Essen, who worked on the project. This was made possible by in-vehicle electronics slowing down the car on the main road automatically using pre-processed sensor data.

Wiering claims that it will definitely take a while to equip a significant number of intersections and cars with the necessary devices, “But the experiment also showed that a similar result can be achieved if road users are informed of such situations, e.g. on their smartphone, and slow down manually.”

Further instalments of the energy efficiency series

Drive selection is but one of the ways of increasing energy efficiency in transportation. BEVs reach the limits of their range especially when transporting goods and on long hauls. The next episode of the en:former’s energy efficiency series will shed light on the most efficient options.