How to store renewable energy in a smart way

Redox flow and power-to-gas support the transition to renewable energies

It’s one of the biggest challenges we face in the transition to renewables: How can solar and wind energy be stored for times when the sun isn’t shining or the wind isn’t blowing? The sun and the wind don’t perform in line with demand, so to ensure sustainability of supply we need storage technologies that can keep the power grid in balance.

Around the world, existing technologies such as pumped storage power plants and compressed air energy storage systems currently offer a capacity of around 100 gigawatts. But demand is growing. As renewable energies expand further, the storage capacities available today will not be enough: According to a study by the Boston Consulting Group, global demand is set to rise to around 430 gigawatts by 2030.

thyssenkrupp is responding to this growing market in two ways: by further developing the so-called redox flow storage technology and by continuing research into an electrolysis process for the production of hydrogen as the basis for power-to-gas technology.

Power-to-gas: Storing wind and sun in natural gas

But what exactly is behind these two concepts? Using power-to-gas technology, excess green electricity can be converted into hydrogen and in a second step into synthetic natural gas and then fed into the existing natural gas network. As a result, large quantities of renewable electricity are available for longer periods in the form of chemical energy. Thereby, the fields of application of power-to-gas are very diverse.

All power-to-gas concepts are based on water electrolysis. Electrolysis works by passing electricity through water, breaking the water down into hydrogen and oxygen. The hydrogen can either be fed directly into the natural gas grid or converted into synthetic methane by adding CO₂.

In order to increase the efficiency of water electrolysis to up to 80 percent in the future and improve its cost effectiveness, thyssenkrupp’s researchers are pursuing their own solution, based on the established chlor-alkali electrolysis process developed in-house. Research and development work on this started in 2013. A first laboratory-scale unit has already been built at the Gersthofen technical center near Augsburg.

Redox flow: A battery with high potential?

Unlike power-to-gas technologies, which can store large quantities of electricity over long periods, so-called redox flow batteries can be used to cover demand peaks of a few hours in duration.

With redox flow batteries, electricity is stored as chemical energy in two large tanks, in which salts are dissolved in inorganic fluids. The bigger the tanks, the more energy they can hold. The two tanks are connected by pipes and pumps to electrochemical cells (“cell stacks”). When electricity is introduced, the valency of the ions in the electrolytes changes and the fluids absorb energy.

As in any other battery, electrical energy is converted into electrochemical energy. This way redox flow technology allows up to 80 percent of stored electricity fed into the system to be taken out again later – significantly more than many other storage technologies. What’s more, storage systems based on redox flow allow separate scalability of power and storage capacity, are modular in structure and are not dependent on geographical conditions. They can be used virtually everywhere.

Industrial storage facility planned with initial capacity of 200 megawatt hours

Industrial-scale redox flow battery technology is still under development. In 2012, experts from thyssenkrupp implemented a first laboratory-scale cell, which is being operated successfully at the technical center of Energie-Forschungszentrum Niedersachsen (Energy Research Center of Lower Saxony, EFZN). A modified cell with a larger active cell area will shortly start long-term trials at thyssenkrupp Industrial Solutions’ research and development center in Ennigerloh.

Looking to the future, thyssenkrupp is thinking big: The aim is to develop a first industrial storage device with an initial capacity of up to 200 megawatt hours. Thereby the technical limit is still not reached, when it comes to the scalability of power and storage capacity.