Green Hydrogen=Green Flag

Overcoming the limits of batteries with hydrogen energy storage

In this article, we will delve into the exciting world of hydrogen as a potential solution for energy storage, aiming to overcome the current limitations of Lithium Ion Batteries (LIB).

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The views and opinions expressed are those of the author and do not necessarily reflect the official policy or position of YEE.


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The importance of effective storage systems in the transition to renewable energy

A future powered by renewables needs effective storage systems. Unlike fossil fuels, wind and sunlight, two low-carbon energy sources at the centre of the energy transition, have some great limitations: they are intermittent and cannot be stored to be converted into energy later on. A turbine spins only where and when the wind blows, and a solar panel works only under daylight. Learning to work around those limitations can help us abandon fossil fuels faster, which is crucial, given the short time we have left to meet the Paris carbon emission reduction targets. Storage systems can help us overcome these limitations, by offering alternative ways to even out energy supply to the grid and by allowing the electrification of sectors that are not connected to the grid altogether.

In this article, I want to look at one of those systems: hydrogen. More specifically, I want to explore how we can store energy using this material, and see in what ways it can help us overcome some of the limitations of the more commonly used Lithium Ion Batteries (LIB). To do so, I will provide you with an accessible explanation of how hydrogen energy storage works. I will also show that LIBs have three main downsides that hydrogen storage can help mitigate: high impact of raw materials, low gravimetric energy density and limited long-term and high-capacity storage capabilities.

My goal here is not to advocate for the complete abandoning of LIBs, rather, I want to show how in some cases having an alternative can help us achieve the decarbonization of our economy faster.

How can electricity be stored in hydrogen?

Let’s start with the basics. How do you generate electricity with hydrogen? It’s pretty simple. Hydrogen atoms flow through a “fuel cell”, which splits their electrons from their protons and nucleus. The electrons then leave the fuel cell and run through a circuit, powering whatever device they are connected to. The end of the circuit is connected back to the fuel cell, where the electron re-joins the proton and nucleus from which it was split. The hydrogen cell is thus re-formed and, reacting with oxygen in the air, it transforms into water vapour. Of course, this is an oversimplification, for a more accurate, but still very accessible, explanation of the process, I redirect you to this video from Alex Dainis, PhD. Just to be clear, this is not a nuclear reaction, as we are not splitting the nucleus itself.

The next question is where do we get hydrogen from? Due to its highly reactive nature, hydrogen is often bonded to other materials. Thus, we need to extract it from other molecules before we can convert it into energy. There are different ways you can do this. The key things to keep in mind are two. Firstly, these processes take a lot of energy. Secondly, the energy source you use determines the name we give to the final product, together with its carbon footprint. Some examples of carbon-intensive production methods are steam methane reforming and gasification. The output of these processes will be called grey or blue hydrogen (in the second case, a carbon capture mechanism is used to limit emissions).

Another method to produce hydrogen is electrolysis. With the same fuel cell we mentioned before, you can split water molecules into hydrogen and oxygen, through a process that is exactly symmetrical to what we have described two paragraphs above. To do this, you need electricity. If that is produced from renewable sources you have green hydrogen, which is low carbon.

Now, the round-trip efficiency of green hydrogen is less than one, meaning that the energy you get from it is less than the one you used to produce it. This means that, whenever possible, it is more efficient to use a renewable energy source directly, without hydrogen as an intermediary. Whenever that is not possible, however, one can produce hydrogen as a way to store and transport energy. First, clean electricity is used to perform electrolysis. Then the resulting hydrogen is reconverted back into water when and where electricity is needed. Through this process, hydrogen can be used as a stock of electricity that can be displaced in space and time to better match our energy demands.

Obstacles to decarbonizing the economy using batteries

As you know, batteries can also be used to store and transport energy. Some of their limitations, however, pose important obstacles to our ability to fully decarbonize our economy. Hydrogen can help us overcome those obstacles.

High impact of raw materials

Firstly, the raw materials required to manufacture LIBs pose environmental, social and geopolitical challenges that become more and more pressing as the scale of production of this technology increases. Lithium and cobalt are two materials used in LIBs. Lithium mining, on the one hand, has a water footprint of more than 2000 liters per kilogram extracted. The practice has also been linked with “declining vegetation, hotter daytime temperatures and increasing drought conditions in national reserve areas”. Cobalt mines, on the other hand, are notoriously infamous for the terrible working conditions of their workers. At the same time, both materials are to be found in only a couple of regions throughout the world. This creates perverse incentives to adopt hoarding strategies, which artificially push up the price of these resources. Such a high level of concentration also decreases the resilience of the supply chain to unforeseeable external shocks, decreasing the long-term reliability of the industry as a whole.

Hydrogen, like lithium, can be used to store energy, however, unlike lithium, it is not a rare material and can be extracted with carbon-neutral technologies. Consequently, replacing some of the current and future demand for batteries with hydrogen-based solutions can reduce our consumption of these materials, and with that the challenges that they come with. This can also diversify the energy storage supply chain, increasing its ability to withstand exogenous shocks. Hydrogen systems also do not use cobalt.

Of course, this is only a part of the solution, the issues I have highlighted above need to be addressed independently of the fact that we introduce hydrogen in the equation. Nonetheless, this technology can help us reduce the scale of the problem. With this, it should also be noted that, while hydrogen is not a rare material, iridium and platinum (two materials often used in fuel cells) are. These materials come with their own environmental problems, which further proves that technical diversification is only part of the solution. The social and environmental patterns of exploitation behind mining need to be addressed, regardless. That, however, is a broader conversation that pertains to our economy as a whole.

Low gravimetric energy density 

Secondly, the low energy density of LIB makes them unsuitable as an alternative to fossil fuels in some applications. The aviation industry is an example of this issue. The table below shows the energy density of different materials, i.e., the amount of Megajoules stored in one kilogram of material ( = gravimetric density) and the amount of Megajoules in one litre of material ( = volumetric density).

Material Gravimetric energy density* Volumetric energy density Energy efficiency
Jet A1 (kerosene)
43.3 MJ/Kg
142 MJ/Kg
0.5 MJ/Kg

As you can see, compared to Jet A-1 (a common aviation fuel), a LIB providing the same amount of energy as an airplane’s fuel tank would be 86 times as heavy. Emily Pickrell, Energy Scholar at the University of Houston estimates that “if a jumbo jet were to use today’s batteries, 1.2 million pounds of batteries would be required just to generate the power of the jet engine it would be replacing. This weight would effectively need an additional eight jet planes just to carry that weight!”.

Consequently, replacing jet fuel with an equivalently powerful battery would make the plane too heavy to fly. Hydrogen, on the other hand, is more energy-dense than both LIBs and Jet A-1. Thus, it can provide the same amount of energy at a much lower weight.

Hydrogen’s energy density makes it a much better match for the electrification of the aviation industry than batteries. There are, however, some limitations to the potential of this gas. If we look at its volumetric energy density, a hydrogen tank would take 4 times as much space as a Jet A-1 providing equivalent energy. And this is assuming we are able to keep the gas in its liquid form at -252.8°C. Together with this, to this day the round-trip efficiency of hydrogen systems is still much lower than that of batteries. Finally, hydrogen aircraft are still in the early stages of development, meaning that we still need to wait for the large-scale commercial adoption of these vehicles.

Limited long-term and high-capacity storage capabilities

Finally, LIBs are less efficient at storing higher quantities of energy for longer periods of time than hydrogen systems. In some applications, we need this longer-term storage capacity. One case is that of intermittent energy storage.

As I said before, renewables’ energy supply cannot be adjusted to the specific demands of consumers and producers at any given moment. To address this, storage devices allow us to stock up energy in moments of excess supply, in order to release it back into the grid in periods of excess demand. Intermittency, however, is a multidimensional phenomenon that has a short-run and long-run component: fluctuations in supply can be intraday or seasonal. Looking at solar energy makes it easier to understand both. As the sun shines only during the daytime, at night panels will not produce any electricity. That is intra-day intermittency. At the same time, during summer days are longer, and, in many climates, less cloudy. Thus, output will be higher during June, July and August than it will during winter (as shown by the table below). This is what we call seasonality.

LIBs are more effective at smoothing intraday fluctuations. Battery storage facilities are cheaper to install, but more expensive to scale up, making them more suited for smaller capacity applications. Their higher round trip efficiency (look at the table above) also means that less energy is wasted in the process. Due to their higher rate of self-discharge, however, they cannot store electricity for prolonged periods of time, making them useless when it comes to seasonal intermittency. At the same time, hydrogen is better suited to supply that higher capacity, long term storage facility needed to smooth out seasonal fluctuations. On the one hand, hydrogen deposits show increasing returns to scale. They can be more costly than batteries to set up, but doubling capacity less than doubles the cost. This makes the technology better suited for higher capacity stockage. On the other, hydrogen has a lower rate of self-discharge, meaning that it can store energy for longer. These two characteristics make this technology a useful tool to smooth out seasonality, even when we account for its lower round trip efficiency (being able to store something is better than being able to store nothing).

To conclude, we can see that hydrogen can help overcome three important limitations of LIBs: high impact of raw materials, low gravimetric energy density and limited long-term and high-capacity storage capabilities. Nonetheless, the analysis also shows that hydrogen technology is still in its earlier stages of development. Consequently, important challenges need to be overcome before this technology can be deployed at scale. If used together, batteries and hydrogen will have a central role in facilitating the energy transition.

I would like to thank Tuur Knevels, who provided some crucial support in the drafting of this article. He is a passionate young engineer who has been active in the hydrogen and automotive industry for the past 3 years and is currently completing his degree in Aerospace Engineering whilst working as a freelance fuel cell systems engineer. We met back in July during the in-person training we organised as part of the AmPower Project. Of course, any potential incoherence in this analysis is solely attributable to me.

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