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).