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Efficiency: let’s clear the air

Today, two different technologies are used to power electric car engines: lithium batteries and fuel cells. In the public debate on the energy transition, some try to frame these two propulsion modes as opposites by comparing their respective efficiencies. But is efficiency really the only factor to consider?

By the way, what is efficiency?

Efficiency is an indicator of performance. It concerns the optimization of the consumption of a quantity of given resources to obtain an output. The higher the output, the greater the efficiency. In the transportation industry, efficiency is generally understood as the ratio of the amount of energy consumed to the number of kilometers traveled.

With the emergence of electric motor vehicles, some consider efficiency in a different light, questioning the relationship between the energy produced from a renewable energy source and the useful energy. Those who follow the energy transition debate are likely to have seen diagrams comparing the efficiency of fuel cell powered electric motors coupled to hydrogen tanks, and those powered by electric batteries.

Diagrams such as this one tend to focus on a single component of an issue that is inherently more complex.

These diagrams, which are regularly shared on social media, break down the different stages between the production of energy and its release into the engine block. They are generally published along with comments that convey a simplistic message: the energy supply chain is more efficient for battery electric cars than it is for those with a fuel cell-powered engine.

User experience

Of course, a simple diagram cannot summarize the complexity of the energy transition issue, nor the specificities of the various models currently developed to enable its adoption. While efficiency is indeed one of the elements that distinguish battery and fuel cell technologies, the problem is infinitely broader.

One of the main issues in this equation is the user experience. The majority of cars currently on the road still run on gasoline. This is despite the fact that internal combustion engines are not only polluting, but are also less efficient than battery electric motors, and those powered by fuel cells. It is therefore essential, if we aim to encourage the transition of motorists towards non-polluting vehicles, that we adapt to the uses of the greatest number. It is by providing a close match to current practices, rather than the matter of the efficiency of propulsion, that mass adoption will be enabled.

From an industrial perspective, the main issue is not so much the efficiency between the energy source and its actual use, but the productivity of the invested assets.

© Austin Scherbarth

While the electric vehicles on the market at present are suitable for moderate urban driving, their technical limitations prevent other usages. The range of these vehicles stretches from 150km to 350km for most battery-powered cars, and up to 500km for the most luxurious model. Furthermore, the actual figure is likely to be lower, depending on the conditions of use, especially in cold weather. Battery recharging time, which can be as much as 10 hours, does not therefore facilitate long journeys or intensive usage. Similarly, the use of batteries is today very difficult to envisage for industrial heavy duty equipment. Battery efficiency in kWh/km decreases as the required range increases, and quite quickly becomes close to that of FCEVs. Finally, the impact of a general massification of electric vehicles based only on batteries, in terms of the associated infrastructure costs to keep pace with the growing power demand, is stirring up strong debate. A recent German study modeled both approaches and showed the limitations of a single technology deployment.

Conversely, vehicles with fuel cells have ranges that are close to those of internal combustion engine vehicles, and refueling takes no longer than conventional refueling (600km for a tank that can be filled in 3 to 5 minutes). They are therefore suitable for everyday urban use, and also for longer journeys, such as holidays or business trips, as well as for industrial equipment. It would be easy, by focusing solely on the criteria of refueling time or range, to produce diagrams as simplistic as the ones dedicated to the issue of efficiency, which would enable equally definitive conclusions to be drawn.

These calculations are based on a 90 kWh battery capacity for the battery-electric vehicle.

Energy vector versatility: local efficiency and overall efficiency

© Jason Blackeye

The energy transition also covers broader issues than the mere issue of mobility, which requires looking at the question from the perspective of the overall efficiency of the system and not only local efficiency. From an industrial perspective, the main issue is not so much the efficiency between the energy source and its actual use, but the productivity of the invested assets. The increasing use of renewable — and therefore intermittent — energies drastically reinforces the usefulness of technologies that transform all of the available energy.

To ensure a fast and powerful movement towards the energy transition, it is necessary to propose complementary models that address the varied uses.

At the end of October 2017, Germany generated a new record for energy production, leading to negative prices on the electricity market. While surplus energy production events are likely to increase in the years ahead, it is essential to use versatile technologies to convert, store and transport energy — even if there is room for improvement regarding their efficiency. The conversion to hydrogen provides this versatility. This gas not only enables more durable storage solutions than batteries, but it can also be used directly in industry, without the need to convert it into electricity. Furthermore, the distribution of hydrogen is facilitated by the possibility of injecting it directly into the gas network, along the lines of the power-to-gas model. It can also be converted into methane during this operation, by the addition of CO2.

Complementarity

To ensure a fast and powerful movement towards the energy transition, it is therefore necessary to propose complementary models that address the varied uses, making it possible to obtain an overall efficiency of the system, and not a segment-based approach that deals with efficiency without sufficiently taking into account the user’s perspective and the overall economy of the system. Thus, it is counterproductive to consider battery electric cars and fuel cell vehicles as two competing models. These two models can and must coexist.