The Elestor electricity storage systems are based on the Hydrogen Bromide Flow Battery principle. Originally developed in 1961 by NASA, and already in the 1980’s proven to last for more than 10,000 charge-discharge cycles, Elestor has now matured this technology to enable scalable, cost-effective use in a large variety of applications.

Working principle

The heart of a Hydrogen Bromide Flow Battery is the cell stack. This cell stack is on one side connected to an electrolyte fluid circuit (a water-based solution of bromine and hydrogen bromide, Br2/HBr), and on the other side to a Hydrogen (H2) gas circuit. Both the Br2/HBr and H2 circulate through their side of the cell stack, each in a closed loop. In each cell, the Br2/HBr and H2 circuits are separated by a proton-conductive membrane.

Picture courtesy of Dr. Kyu Taek Cho (reproduced by permission of ECS)

Picture courtesy of Dr. Kyu Taek Cho (with permission of ECS)

The chemical formula, representing the charge and discharge processes, is:


  • While charging, HBr is converted into Br2 and H2. The pressure in the H2 tank rises.
  • While discharging, H2 is consumed, the pressure in the H2 tank decreases, and electrical energy is released. During this process, Br2 is converted again into HBr.

High scalability

GEN2 StackOne of the unique features of HBr flow batteries is that, unlike for example Li-ion batteries, the total stored energy and the power of the storage system are de-coupled : The number of cells in the stack determines the electrical power [kW] of the storage system, while the volumes of the electrolyte and H2 tanks determine the total amount of stored electrical energy [kWh].

In other words, HBr based storage systems can be designed while independently dimensioning the electrical power and the stored energy. This implies that HBr technology allows scalability of power and energy in a very cost-effective way. A large variety of systems can be designed, tailor-made for each particular application.

No self-discharge

HBr flow batteries typically show negligible losses due to self-discharge, which means that the stored energy always remains available for instantaneous use, even after long periods of storage.

Short response time

HBr flow battery based storage systems show a very short response time, reacting within milliseconds to variations in power load. Therefore, these storage systems react immediately to sudden changes in the demand or supply of electrical energy.

No degradation

The chemical reaction of HBr storage systems is fundamentally 100% reversible. They do, by principle, not suffer from an intrinsic degradation mechanisms, as is for instance the case with Li-ion technology (e.g. dendrite formation, which limits Li-ion battery lifetime).

Ultra-high Power Density

The power-density of an HBr flow cell is extremely high (>10kW/m2 membrane). Compared to other types of flow cell systems, considerably less cell surface area is required with HBr, which enables ultra-compact and cost-effective designs.

100% Self contained

The Hydrogen Bromide battery is 100% self-contained and closed, and literally nothing goes in or out – except electricity.

Sir William Robert GroveThe fundamental working principle of a Hydrogen Bromide flow battery bears a high resemblance to the Hydrogen Oxygen fuel cell, which was, already in 1842, invented by the British Sir William Robert Grove.

By using Bromide instead of the less reactive Oxygen, the charge and discharge efficiencies rise from 35% to 90% (or even higher).