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An grid-connected energy storage system using Elestor technology.

Cost of storage

Defining cost of storage

To determine whether Elestor’s mission - Reducing electricity storage costs to the absolute minimum - is indeed accomplished, it is important to have a common understanding of the definition of Cost of Storage. This obviously goes beyond simply considering the investment costs (Capex) for a particular storage system.

Cost of Storage is a very important concept because, in essence, the figure determines the economic value of a storage technology, and thus of its market adoption, and finally of its impact on the energy transition.

Over the years, Cost of Storage has been quantified in several ways. Today, and particularly with flow batteries coming to the fore, the terminology as well as the mathematical approach needs some adjustment.

Levelised Cost of Storage (LCoS)

To objectively compare different storage technologies from an economic point of view, the so-called Levelised Costs of Storage, or LCoS, has been introduced. The LCoS says potentially what the bottom line costs are for storing 1 MWh, thereby taking several system characteristics into account.

In other publications, the LCoS is presented in various forms, whereby the differences are mainly found in the number of parameters that are considered in the calculation. Another scientific approach, taking many detailed parameters into account, is given by Lazard.

In its most simplified and traditional form, the LCoS calculation only considers the Investment cost per MWh’ [€/MWh], the lifetime of the storage system [cycles], and its typical Roundtrip (dis)charge efficiency [%], as shown below. This approach has been a proper and workable way to objectively compare the storage cost per MWh of different technologies. And this was indeed true for a long time – until flow battery technology started gaining popularity.

While the investment cost per MWh is a fixed figure for traditional technologies, this is different for flow batteries. After all, flow batteries can be designed with virtually any combination of power [MW] and capacity [MWh].

The investment cost for such systems is not only a function of their capacity [MWh], but also of their power [MW]. Therefore, above LCoS calculation cannot  be applied in this form for flow batteries.

Elestor Slide5.jpg

For flow batteries, the investment costs per MWh is not a fixed number

If, for instance, doubling the storage capacity of a traditional battery is desired, then the power is also doubled, automatically. In fact, a second complete storage unit is added in this case, obviously also resulting in double the investment cost (Capex).

Since power [MW] and capacity [MWh] of traditional batteries have a fixed ratio, the investment cost can simply be expressed in a fixed number of € per MWh.
The Capex of such systems is then calculated by multiplying this constant by the desired storage capacity.

This is different for flow batteries. With flow batteries, doubling the storage capacity [MWh] simply means a doubling of the volumes of active materials. The system’s power remains unchanged and this then obviously does not result in double investment cost.

The additional costs for doubling the capacity of Elestor’s solution are only marginal, because the active materials (H2 and Br2) were deliberately selected for their low cost.

Investment costs per kWh.png

Confused? Here is a different approach.

A practical, straight forward approach is to calculate the ratio of the total investment cost for a system and the total amount of electric energy [MWh] that this system delivers during its lifetime:

This calculation fundamentally leads to a true cost figure per MWh, whereby the most dominant factors are considered. Furthermore, it allows an objective comparison of all kinds of storage technologies, traditional and contemporary.

The approach presented on this page is deliberately simplified in order to explain the dominant factors determining the LCoS. Elestor would be delighted to provide an accurate LCoS calculation for specific storage applications, so please get in touch if this is what you’re looking for.


Stack replacement gives flow batteries a full second life

Going into more detail, operational expenses (Opex) and residual value can also be considered. The numerator of above formula then becomes the sum of Capex and Opex, minus the residual value of the system at the end of its life. Especially for Elestor’s battery, this residual value is a number with great relevance: During its lifetime, the electrochemical storage process does not degrade and therefore none of the chemicals are consumed. As a result, a large part of the storage system keeps its original value. Because of the fact that the HBr battery is truly accessible for service, upgrades and refurbishment, cell stacks can easily be replaced at the end of their lives. This gives the flow battery a full second life.

The residual value of a hydrogen bromine flow battery system is therefore significant. Including the residual value in the cost calculation for Elestor’s flow batteries results in considerably lower figures for LCoS than the costs displayed in the graph.

3D explanation of Elestor's flow battery

A flow battery’s lifetime does not depend on depth of discharge

Last but not least, the figure for ‘Capacity [MWh]’ must be interpreted as the practically usable capacity, which is not necessarily the same as the purchased capacity.

Traditional storage technologies do generally not allow full charge/discharge between 0% and 100% without compromising the system’s lifetime. In other words, if a traditional battery was to be used between 0% and 100%, then its lifetime would be dramatically shortened.

In battery specifications, this phenomenon is referred to as depth of discharge or DoD [%], which describes the maximum allowed charge and discharge level. The following graph shows the relation between DoD and the lifetime of traditional storage systems.

For traditional batteries, the practical DoD is roughly in the range of 70%, as these batteries are commonly operated between around 15% and 85% state-of-charge. This is done in order to prevent a significant lifetime reduction, which would occur with a larger DoD.

Flow batteries, however, allow a DoD of virtually 100% without it affecting its lifetimes. DoD has a great impact on LCoS calculations, and is therefore a very relevant figure:

The in-practice usable storage capacity is equal to DoD* purchased capacity.

As such, a DoD of 70% immediately results in a 50% higher LCoS, because only 70% of the purchased capacity is practically used. Including the DoD effect further enlarges the difference in LCoS between traditional storage systems and flow batteries.


Learn more

The hydrogen-bromine flow battery for a large scale integration of variable renewable electricity: State-of-the-art review

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Y.A. Hugo, W. Kout, G. Dalessi

Elestor B.V., Utrechtseweg 310-H40, 6812 AR Arnhem, The Netherlands


This article presents a state-of-the-art review of the hydrogen-bromine battery technology. The review aims to elaborate on the following topics: (1) the hydrogen-bromine flow battery, (2) the current status of technical developments on short-term and long-term cycling, and (3) the future direction for technology development.

Performance mapping of cation exchange membranes for hydrogen-bromine flow batteries for energy storage

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Yohanes Antonius Hugo a, b, Wiebrand Kout b, Antoni Forner-Cuenca a, Zandrie Borneman a, c, Kitty Nijmeijer a, c, *
a Membrane Materials and Processes, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven, the Netherlands
b Elestor B.V., 6827 AV Arnhem, the Netherlands
c Dutch Institute for Fundamental Energy Research (DIFFER), P.O. Box 6336, 5600 HH Eindhoven, the Netherlands
⁎ Corresponding author. E-mail address: (K. Nijmeijer).


Electricity storage is essential for the transition to sustainable energy sources. Hydrogen-bromine flow batteries (HBFBs) are promising cost-effective energy storage systems. In HBFBs, proton exchange membranes are required to separate the two reactive materials, enabling proton transport for charge balancing. In this paper, we present a comprehensive overview of the key properties and an experimental performance map of cation exchange membranes for HBFBs. Our study shows that membrane water uptake is an important property due to its strong correlation with membrane resistance and bromide species crossover. Long chain perfluorosulfonic acid (LC PFSA) membranes are shown to have a better power density–crossover tradeoff and a higher stability than other types of functionalized membranes. The good power density-crossover tradeoff of LC PFSA membranes is the result of the high level of separation of hydrophobic and hydrophilic domains in the membrane, leading to well-connected ionic pathways for proton transport. Reinforcement of long chain LC PFSA membranes further improves their tradeoff because it mechanically constrains the swelling (lower water uptake), resulting in a lower crossover but a similar peak power density. Consequently, reinforced LC PFSA membranes are the most promising option for HBFBs.

Low-cost wire-electrospun sulfonated poly(ether ether ketone)/poly (vinylidene fluoride) blend membranes for hydrogen-bromine flow batteries

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Sanaz Abbasia, b, Antoni Forner-Cuencaa  Wiebrand Koutb, Kitty Nijmeijer a, c, Zandrie Borneman a, c, *
Membrane Materials and Processes, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven, the Netherlands
b Elestor B.V., 6827 AV Arnhem, the Netherlands
c Dutch Institute for Fundamental Energy Research (DIFFER), P.O. Box 6336, 5600 HH Eindhoven, the Netherlands
⁎ Corresponding author. Membrane Materials and Processes, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, the Netherlands. E-mail address: (Z. Borneman).


Cost-effective dense membranes were developed by blending proton-conductive sulfonated poly(ether ether ketone) (SPEEK) with inert, mechanically stable poly(vinylidene fluoride) (PVDF) for hydrogen-bromine flow batteries (HBFBs). Wire-electrospinning followed by hot-pressing was employed to prepare dense membranes. Their properties and performance were compared to solution-cast membranes of similar composition and thickness. Electrospinning improved the ionic conductivity and bromine diffusion properties by providing interconnected ion-conductive SPEEK nanofiber pathways through a PVDF matrix. Relatively thin (~50–60 μm) electrospun membranes with a SPEEK/PVDF ratio (wt%/wt%) of 90/10 and 80/20 showed comparable Br3 − diffusion rates as the relatively thick and commercially available perfluorosulfonic acid (PFSA) membrane (~100 μm) at a 35%–42% lower proton conductivity. The latter can be attributed to the poorer ion conductivity of SPEEK compared to PFSA and the presence of PVDF. The HBFB single cell featured the best polarization behavior and ohmic area resistance with the electrospun membrane containing 80/20 (wt%/wt%) SPEEK/PVDF. However, the low thickness and insufficient chemical/mechanical stability of the ES 80/20 causes a rapid decay in the HBFB cycling performance. This study promotes a life-time comparison study between the low-cost wire- electrospun SPEEK/PVDF blend membranes (~€100 m− 2) and the typically used PFSA membranes (~€400 m− 2) for a long-term HBFB performance.

State-of-the-art membranes for hydrogen-bromine flow batteries: The mechanisms for proton and bromide-species transports

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Yohanes Antonius Hugo1, Wiebrand Kout1, Kitty Nijmeijer2

1Elestor B.V., Arnhem 6812 AR, the Netherlands
2Membrane Science & Technology Group, Department of Chemical Engineering,
Eindhoven University of Technology, Eindhoven 5600 MB, the Netherlands


In a hydrogen-bromine flow battery (FB), the membrane characteristics determine the intrinsic resistance and crossover rate of bromide species and water. The crossover issue (from the bromine side to the hydrogen side) affects the performance and lifetime of hydrogen-bromine FBs because the bromide species deactivate the platinum catalyst and the liquid floods the electrode, which makes it inaccessible for the hydrogen [1].

The effect of cations on the proton transport of PFSA membranes used in hydrogen-bromine flow batteries: observations and mitigation solutions.

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Natalia Mazur1*, Yohanes Antonius Hugo1,, Wiebrand Kout1, Friso Sikkema1, Ran Elazari2, Ronny Costi2
1 Elestor B.V., Arnhem 6812 AR, The Netherlands
2 ICL Industrial Products R&D, Beer Sheva, Israel


In search for cheap, high capacity energy storage, hydrogen-bromine flow batteries (HBFBs) are emerging as strong contenders [1], however, the volatility of the electrolyte and the associated risks must be managed. Bromine complexing agents (BCAs) are used as additives to the electrolyte, which have the ability to capture Br-ions and Br2 thus decreasing the bromine vapour pressure [1]. The BCA-HBr-Br2 complex separates as a high density oily layer.


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