Energy storage systems (ESS)

Renewable energy sources have been increasingly integrated into the global energy mix as the initial investment costs for these sources decrease. The shift towards renewable energy aligns with the initiatives in place to replace fossil fuels due to the depleting reserves and the harmful health and environmental effects associated with their continued use. However, the highly variable and intermittent nature of renewable energy sources result in the mismatch between the energy supply and demand. To address this mismatch and maximize the renewable energy capacity, energy storage systems have been integrated to renewable energy sources.¹

Energy storage systems (ESS) are versatile solutions in modern energy management. They store energy produced at one time for later use, facilitating a bidirectional flow of energy between the device and the power grid during both charging and discharging phases. Aside from allowing the seamless integration of renewable energy sources, ESS also performs the following functions. 

• Balances grid supply and demand on various time scales, from seconds to hours, to maintain grid stability and power quality.
• Provides fast response (ramping) to offer ancillary services for grid operators, helping regulate electric grid frequency in real-time.
•  Enables peak electricity demand reduction and price arbitrage by storing energy during low-demand periods and discharging it during high-demand times.
• Defers expensive electricity infrastructure investments by managing growing demand more cost-effectively.
• Offers backup power during grid outages for on-grid electricity consumers.
• Reduces end-user electricity demand and associated demand charges, particularly in commercial and industrial settings.
• Integrated with microgrids to enhance reliability and resilience in localized power supply applications.²

Energy storage systems (ESS) can be broadly categorized into two main groups, based on their integration with power or thermal energy grids: electricity ESS and thermal ESS. Electricity ESS can be further classified into four primary types: electrochemical, electromagnetic, mechanical, and chemical energy storage. Meanwhile, thermal ESS focuses on storing heat or cooling energy in specialized storage tanks.³

Electrochemical energy storage devices are the most traditional EES. Electrochemical EES devices convert stored chemical energy into electrical energy. Unlike that of traditional thermodynamic processes in thermal EES, the efficiency of electrochemical processes is not Carnot-limited. Thermal EES rely on the heat transfer between reservoirs and are constrained by the Carnot limit, whereas electrochemical processes involve the movement of electrons and ions through a chemical medium. This different mechanism allows electrochemical cells to achieve high efficiency without being restricted by the Carnot framework.⁴

Battery energy storage systems (BESS) as the dominant electrochemical energy storage technology

Electrochemical ESS include primary (non-rechargeable) and secondary (rechargeable) batteries and fuel cells. Among the electrochemical energy storage devices, batteries continue to have a dominant position in the energy market due to their high efficiency and excellent reliability. Batteries can be seamlessly integrated into different applications while requiring minimal maintenance, which is well reflected in global statistics where batteries account for 40% of total global energy output. Projections even indicate a five-fold increase in the consumption over the next decade.¹ This growth is driven by their pivotal role in enabling the transition to renewable energy sources and the significant environmental benefits they offer, a trend supported by ongoing research and development efforts.

Batteries, as a form of electrochemical energy storage, are composed of one or more rechargeable cells and have played a transformative role in the transportation and energy sectors. They contribute significantly to reducing greenhouse gas emissions by optimizing production processes, responsibly sourcing raw materials, and advancing recycling and reusing practices for battery components. In microgrid energy storage applications, a variety of battery types are widely utilized, with capacities ranging from several kilowatts to thousands of kilowatts, reflecting their versatility and adaptability to diverse energy storage needs.

Batteries vs. fuel cells

As both are electrochemical devices in nature, the working principles of batteries and fuel cells are similar. The key difference is that fuel cells are open systems that convert chemically bound energy supplied from external fuels, such as hydrogen, methanol, and hydrazine, whereas batteries are closed systems that convert chemical energy integrated into their structure. Not considering material degradation, fuel cells will continue operating as long as fuels are being supplied. Batteries, on the other hand, must be recharged once the stored chemical energy within it has been converted, else, its useful life ends. This fundamental distinction in their energy sources influences their respective roles and applications in various energy and power generation systems.

Similar to fuel cells, batteries are composed of two electrodes and an electrolyte enclosed within a dedicated container and connected to an external source or load. The electrodes sandwich the electrolyte. This common structure is important because it shows that both batteries and fuel cells follow similar principles in how they work. Even though they operate differently, understanding this shared foundation helps us grasp their roles as devices for storing and converting energy.

Conventional Batteries vs. Flow Batteries

A flow battery, also known as redox flow battery, is a special type of electrochemical cells. They are batteries composed of two chemical components dissolved in appropriate solvents and stored in vessels separated using a membrane. The dissolved substances in the electrolytes (anolyte and catholyte) provide the chemical energy that is transformed to electrical energy. A pump is used to circulate the liquids in the system. Reactions occur at the current collectors sandwiching the ion-selective membranes. Ion exchange and electron transfer occur. The cell voltage of flow batteries typically range from 1 to 2.2 V. Redox flow batteries are also rechargeable. Once an external voltage is applied, the reactions are reversed, driving the regeneration of the fuels in the electrolytes. Aside from that, flow batteries can be instantly recharged simply by changing the liquid electrolyte.⁵

Various chemistries and electrolytes have been explored for flow batteries, with an increasing trend among battery manufacturers to adopt vanadium as a fundamental material for both small-scale and large-scale applications. In the production of batteries, vanadium salts in sulfuric acid form the electrolyte. The utilization of vanadium in redox-flow batteries (VFBs) is gaining prominence as energy companies seek advancements in energy storage solutions.

The difference between conventional and flow batteries is that the chemical energy converted in flow batteries is stored as the electrolyte, whereas in conventional batteries, it is stored as the electrode material. 

What makes fuel cells and flow batteries different? Flow batteries work in a similar way to fuel cells. In fact, they have been commonly referred to as regenerative fuel cells. The key difference is that fuel cells directly consume a fuel, such as hydrogen, while flow batteries store and release electrical energy through the redox reactions of liquid electrolytes. In terms of components, fuel cells often use expensive catalysts like platinum, while flow batteries rely on redox-active materials for electrolytes.

References

¹ Mohammad Ali Abdelkareem, Mohamad Ayoub, Siren Khuri, Abdul Hai Alami, Enas Taha Sayed, T D Deepa, A.G. Olabi, Environmental aspects of batteries, Sustainable Horizons, Volume 8, 2023, 100074, ISSN 2772-7378.

² Energy storage for electricity generation. (2023, August 28). U.S. Energy Information Administration (EIA). https://www.eia.gov/energyexplained/electricity/energy-storage-for-electricity-generation.php

³ Xu, Yan Wang, Yu Zhang, Cuo Li, Zhengmao. (2022). Coordination of Distributed Energy Resources in Microgrids - Optimisation, Control, and Hardware-in-the-Loop Validation - 1.2.2.1 Electrochemical Energy Storage. Institution of Engineering and Technology (The IET). 

⁴ Ter-Gazarian, Andrei G.. (2020). Energy Storage for Power Systems (3rd Edition) - 9.1 General Considerations. Institution of Engineering and Technology (The IET). 

⁵ Olsson, Gustaf. (2018). Clean Water Using Solar and Wind - Outside the Power Grid - 10.3.4 Flow Batteries. IWA Publishing.