Page 60 - Energize March 2022

TECHNICAL

Flow batteries fill a gap in the

electricity storage system market

Developments in the field of electricity generation and consumption are placing an increasing reliance on electricity
storage, and there is an increasing demand for medium- and large-scale bulk electricity systems. Conventional storage
batteries, in horizontally integrated systems, are being used to meet this need, but suffer from limitations on storage

duration and other technical problems. The flow battery is a mature technology which can fill this gap.

BY MIKE RYCROFT, ENERGIZE

rid and off-grid applications are requiring increased storage from even the largest batteries today. Future scenarios foresee the
capacity, i.e., the capacity to deliver stored energy at a need for capacity in the region of days or longer periods.
Ghigh-power level for longer periods. Typical applications To achieve this requires the ability to separate capacity and
are in hybrid power systems for mines and microgrids, and in power output. Separating the energy storage medium from
wind or solar farms. Conventional chemical storage consists of the active elements allows this. An example of how this can be
batteries with fixed amounts of active material and hence fixed achieved is pumped water storage (PWS), as the energy storage
storage capacity. Capacity is increased by increasing the number of medium (water) is stored separately from the power generation
batteries. Typical examples are the increasing number of very large plant, and the capacity will be determined by the size of the
grid battery installations, ranging up to 400 MW power output and storage reservoir dam or cavern, while the power output is
1 GWh capacity. These large multi-MW constructions consist of determined by the generating plant. A PWS system is very large and
hundreds or thousands of small batteries. could only be located in a suitable geographic area. But there is a
While these multi-battery installations will play a key role in need for medium sized systems which could be located anywhere.
grid stabilisation and balancing, longer duration grid-scale energy Flow batteries are the answer to this problem.
storage needs different battery. Large scale bulk storage using
conventional batteries has the drawback of capacity deterioration Redox flow batteries (RFB)
and limited battery life (≈10 y), compared to other technologies In the chemical storage sector, separation of power rating and
such as pumped water storage and compressed air storage, and a capacity has been achieved by what is known as redox flow
product with matching durability is required. batteries. In a flow battery, the active material is contained in a
fluid form, which can be stored separately from the active element
Capacity and power output or battery cell. Energy is stored or recovered by passing the active
Two parameters are used to specify electricity storage systems (ESS) material through the cell, hence the name flow battery.
• Power rating: The maximum power that a ESS can deliver in The flow battery is based on electron transfer reduction/
kW or MW oxidation reactions, hence the common name redox flow battery.
• Storage capacity: The energy that an ESS can store and deliver, in In this context oxidation means a loss of electrons and reduction a
kWh or MWh gain of electrons. The basic construction of a redox flow battery is
shown in Figure 1.
In most units the two are related. For example, a 20 MWh ESS can
deliver 20 MW for 1 h, but can also deliver 2 MW for 10 h, or 1 MW
for 20 h. Stored capacity is not the same as delivered energy, which
can depend on the power level at which delivery takes place. Most
chemical cells’ capacity is quoted at the 10 h rate. Delivered energy
at higher rates would be lower and would be higher at lower rates.
In a conventional battery both the power and the capacity
are determined by electrode plate size. The plate size determines
the power rating of the battery, but also determines the amount
of active material in the battery, which, in turn, determines the
storage capacity. The power rating and storage capacity are thus
mutually limited by the physical size of the battery.
The growing requirement, linked to the use of varying resources,
is for longer term storage in excess of the several hours available Figure 1: Flow battery basic structure (Fraunhofer ICT)

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