Researchers take the ups and downs out of renewable energy

Summary

Scientists from the Free University of Brussels are part of a team looking for ways to store surplus renewable energy and retrieve it as needed

Vector power

Renewable energy is plentiful, but fickle. When the sun shines or the wind blows, there may be more than enough solar and wind power to go around, but when it’s cloudy or calm there’s a risk that the lights will go out. This is why coal, gas and nuclear power stations are still needed to fill the gaps.

One solution to these fluctuations would be to store the surplus renewable energy during periods of high production and draw on these reserves when production drops. But this is easier said than done.

“Most people think immediately of batteries, and they are very useful, but only up to a point,” explains Francesco Contino, professor in the department of mechanical engineering at the Free University of Brussels (VUB). “Their energy density is limited. You can only put so much energy into batteries, and we need to go far beyond that if we think about what is needed for a 100% renewable-energy world.”

Contino is one of the leaders of a new research project that will explore options for storing renewable energy chemically, by creating simple fuels. In addition to VUB, the Free project (“Flexible energy vectors for the future”) involves researchers at Ghent University, the French-speaking Free University of Brussels (ULB), the Catholic University of Louvain and the University of Mons.

The fuels they have in mind are extremely simple. For example, excess electricity produced by a wind farm might be used to split water molecules to create hydrogen and oxygen. The hydrogen could then be stored as a gas or a liquid. The energy could be recovered later by reversing the process, allowing the hydrogen to recombine with oxygen from the air to form water.

Avoiding overload

“Although it’s very simple to produce, and not very expensive, you can’t store hydrogen very easily,” Contino says. “It is a bit better than batteries, but to get enough energy density you need to compress the hydrogen to very high pressure, or liquefy it and keep it at very low temperatures.” 

Alternative storage fuels include using surplus electricity to produce ammonia, methane or methanol. This last substance, the simplest of the alcohols, is particularly promising since it is a liquid rather than a gas under normal temperature and pressure.

Batteries are very useful, but only up to a point

- Francesco Contino

The idea of using these substances to store energy is not new, but more work is required on how that energy can be recovered efficiently. “We are doing research into small, decentralised units that will convert the energy stored in the fuels back into something we can use, such as electricity and heat,” Contino says.

This decentralisation is important, since renewable energy production tends to be scattered. Wind farms may be off-shore or in isolated parts of the countryside, while solar panels may be on agricultural land or even in the middle of towns and cities. But the electricity network that connects these energy sources would overload if you tried to move large amounts of power around for the purposes of storing it centrally. 

So the ideal would be to store renewable energy where it is produced and trickle it back into the network as required, or use it locally.

In a city near you

Another aspect of the research involves exploring how the creation of methane and methanol – both of which require carbon – could be used to reduce emissions of the greenhouse gas carbon dioxide. In this case renewable energy storage might run hand in hand with using a fossil fuel, such as natural gas, or a more sustainable carbon source, such as biogas from the fermentation of waste.

Carbon capture methods also need to be found to ensure that turning methane and methanol back into energy does not increase carbon dioxide emissions.

We want to move to a full-scale demonstrator, near a wind farm for example, or in a domestic setting

- Francesco Contino

Over the next four years the Free project will use lab facilities shared by the universities for proof-of-concept studies in these areas. “After that we want to move to a full-scale demonstrator, near a wind farm for example, or in a domestic setting where we have enough heat or power demand to demonstrate the application,” Contino says.

That domestic demonstration might involve a city neighbourhood, with houses, shops and a school. “They would all have their solar panels and maybe a small wind turbine,” he goes on. “Then you would store and retrieve the energy locally, and distribute the heat and the power to the people as they need it.”

The collaborating academics will also be looking at the bigger picture of energy storage. For example, hydrogen and methanol can also be used as transport fuels, while ammonia is a raw material in the chemicals industry.

Contino: “We are using this consortium of five major universities to have a forum on future energy carriers: how can we use smart energy carriers to exchange energy in the best way possible.”

Photo: Ingimage