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H O R I Z O N 2 0 2 0 P R O J E C T S : P O R TA L

I S S U E S E V E N

191

P R O F I L E

S O C I E TA L C H A L L E N G E S : E N E R G Y

time, or they may be coupled with a wind turbine or a photovoltaic

panel installation in order to buffer the power output. The latter

application requires higher storage capacities, associated with a

larger size of the battery, as the amount of liquid is directly

proportional to the energy capacity of the battery. Indeed, the

limited energy density of RFBs results in large volume systems

when substantial amounts of energy need to be stored. Such

large volumes are not always practical and may increase the cost

of the system.

Parallel circuits

The present project was motivated by this need to improve the

energy density of RFBs. The concept is based on the observation

that the charged liquids of the battery could be discharged via a

secondary chemical reaction to generate hydrogen, instead of

being discharged conventionally to generate electricity (Fig. 1). The

discharged liquids can then return into the RFB for further

charging. The hydrogen gas produced can be compressed and

stored for different uses, such as fuel for fuel cell vehicles, or be

converted again into electricity in a stationary fuel cell.While one

half of the battery is being chemically discharged to produce

hydrogen, the other half can also be chemically discharged, but

this reaction is more dependent on the chemistry of the RFB.

The unique aspect of this approach is the incorporation of a

parallel circuit, which permits the production of secondary

products. For hydrogen production, this secondary circuit is

composed of a bed of inexpensive molybdenum-based catalyst.

In the dual-circuit configuration, the reaction rate for hydrogen

evolution is not required to be particularly fast, as in a

conventional electrolyser. In this way, the amount of energy stored

by this modified RFB is no longer limited by its size, and surplus

renewable energy can continue to be stored.

Demonstration

Currently, in order to validate this concept a demonstrator based

on a modified 10kW (40kWh) all-vanadium RFB is operating in

Martigny, Switzerland (Fig. 2). A commercial RFB was acquired

and equipped with the secondary circuit for hydrogen generation

on demand. Typical profiles of energy production by wind

turbines and a district electrical consumption can be applied to

this modified RFB, and the secondary circuit can be used to

generate hydrogen when the surplus energy exceeds the capacity

of the battery.

The efficiency of the RFB is about 80%, depending on the power

of charge and discharge. However, when the system is discharged

through the production of hydrogen, its energy efficiency is

lowered – the efficiency depends on the RFB chemistry and the

chemical discharge reactions, which is not considered a limiting

aspect of this system as the source of energy is renewable, and

thus cheap or even free. To date, this concept has been tested on

two specific chemistries of RFBs: all-vanadium and cerium-

vanadium redox chemistries. However, it may be extended to other

RFBs. The long term objective is to be able to apply it to any

commercial RFB in the form of a plug-in system that can be

added to increase their energy capacity.

Future projects for the demonstration site of Martigny are currently

being developed. For instance, the installation of a hydrogen

refuelling station based on the production of hydrogen by an

electrolyser and using an RFB for the buffering of surplus energy is

expected to start by the end of 2015.

Professor Hubert Girault

Laboratory of Physical and

Analytical Electrochemistry

Energypolis

EPFL

te l :

+41 21 69 33151

hubert.girault@epfl.ch http://lepa.epfl.ch/

Fig. 2 Demonstration project for the direct generation of hydrogen by a redox flow battery in Martigny, Switzerland