Reviewing Battery Energy Storage Technology Options

Distribution networks have undergone significant modifications as a result of the increased generation of solar and wind energy as well as the introduction of new types of loads like electric automobiles and heat pumps. Due to these factors, distribution system operators (DSOs) now confront additional technological difficulties, particularly because of the unpredictability of solar, wind, and EV charging stations.

Network reinforcements may not be as effective as energy storage systems (ESSs). Their advantages and economic viability are not entirely evident, though. Although there have been many advances in energy storage technologies, the emphasis of this article is on battery-based energy storage devices. Battery energy storage systems (BESSs) have garnered much attention because of their adaptability and predicted cost reductions.

Figure 1 depicts a typical grid-connected BESS circuit containing a battery bank, DC-AC converter, DC-AC filters, protective circuits, and a step-up transformer. This article examines the types of battery banks and their various features, such as efficiency, cost, and energy density, to name a few.

Figure 1.  Basic schematic of a grid-connected battery energy storage system. Image used courtesy of Stecca et al

 

Electrochemical Battery Technologies

Lithium-ion, sodium-sulfur, lead-acid, and redox flow batteries are the most common electrochemical technologies employed in grid applications. Figures 2 and 3 provide examples of grid-connected electrochemical storage, and the accompanying discussion characterizes these systems in terms of energy density, efficiency, lifetime, and prices.

 

Figure 2. Performances of different electrochemical battery technologies. Image used courtesy of Stecca et al

 

Figure 3. Performances of different li-ion battery technologies. Image used courtesy of Stecca et al

 

Lead-acid Batteries

When it comes to rechargeable batteries, lead-acid were the first to market. Today's lead-acid batteries have good efficiency (80-90%), a low cell cost (50-600 $/kWh), and are considered a mature technology. The biggest issue is their low energy density (20-30 Wh/kg) and short cycling life (up to 2500 cycles). Furthermore, a deep discharge shortens the lifespan of lead-acid batteries.

 

Sodium-sulfur Batteries

Sodium-sulfur batteries have a high energy density (between 150 and 240 Wh/kg). Reasonable efficiency (>80%), high working temperature (about 300 deg C), and long cycling life are all characteristics of NaS batteries (up to 4500 cycles). This technology has already been used as grid-connected energy storage to lessen the impact of generators based on renewable energy.

 

Redox Flow Batteries

The two chemical reactants of a redox flow (RF) battery are stored in two separate tanks, and the oxidation-reduction (redox) reaction takes place between the two electrodes, which are separated by a membrane. The electrodes and membrane system determine power flow in batteries, while energy capacity is determined by the number of reactants stored in the tanks.

Therefore, the design and operation are given more leeway when power and energy ratings are treated independently. Although their efficiency can reach 75%, redox flow batteries have a modest energy density (15-30 Wh/kg). Despite this, redox flow batteries are not constrained by the depth of discharge or the life cycle of the reactants. While redox flow batteries have their technological quirks, their economic performance has made them a serious contender as a grid-scale storage option. There have been numerous hypothesized chemical compositions for the reactants. However, vanadium-based and zinc-bromine-based are the most common.

 

Lithium-ion Batteries

The chemical composition of the cathode, which is often a lithium metal oxide, and of the anode, which is typically graphite, characterize the electrochemical properties of Li-ion batteries. This technology demonstrates high efficiency, which can reach over 90%. However, some commercial products offer a rated round trip efficiency of over 95%, high energy density (90-190 Wh/kg), and a long lifetime, reaching up to 10,000 cycles depending on the li-ion chemistry, as shown in Figure 3.

 In addition, this technology has a high energy density, reaching up to 190 Wh per kilogram. The cell's temperature, which is a significant contributor to the deterioration process, has an influence, despite this fact, on the lifetime. In recent years, lithium-ion batteries have emerged as the dominant EV technology, following their widespread use in electronic devices. Although it is still rather pricey, this technology is excellent for use in applications that link to the grid.

There are a variety of li-ion technologies available, including lithium cobalt oxide-based (LiCoO2), lithium manganese oxide-based (LiMn2O4), lithium nickel oxide-based (LiNiO2), lithium nickel cobalt aluminum oxide-based (LiNiCoAlO2), lithium nickel manganese oxide-based cobalt (LiNiMnCoO2), lithium titanate oxide-based (Li24Ti5O12), and lithium iron phosphate-based (LiFePO4). The performances of lithium iron phosphate, lithium nickel manganese cobalt, and lithium nickel aluminum cobalt are presented in Figure 3.

 

Figure 4. Costs of a lithium-ion battery cell and pack over the last years. Image used courtesy of Stecca et al

 

Of the electrochemical compositions that were considered, the technology based on lithium nickel manganese cobalt, NMC, offers the best performance. NMC has become the primary li-ion technology for stationary storage and electric vehicles in part due to its excellent performance, but other variables have played a role in this.

The cost of lithium-ion battery cells and packs may be seen in Figure 4. This cost has decreased steadily over the past few years. The remarkable cost decrease of over -75% in just six years, from 650 $/kWh in 2013 to 156 $/kWh in 2019, jumps out. Keeping in line with this pattern, additional cost-cutting is anticipated. Therefore, if energy storage systems had a reduced total cost of ownership, they would have a better chance of further establishing themselves in the electric power industry.

The primary performance indicators of the electrochemical battery technologies covered in the prior discussion are also illustrated in Figures 2 and 3. The performance of lithium-ion batteries is superior to that of the alternatives because they have a higher power and energy density, greater efficiency, and a lower daily rate of self-discharge. As a result, the industry has consistently selected them as the best option.

 

Key Takeaways of Battery Energy Storage Systems

Some of the takeaways follow.

• Increased solar and wind energy generation and the introduction of new forms of loads like electric vehicles and heat pumps have required substantial adjustments to distribution networks.
• Because of the variability of renewable energy sources and electric vehicle charging stations, DSOs have new technological challenges to deal with. The flexibility and expected cost reductions of BESSs have attracted much attention.
• Some of the most widespread examples of electrochemical technologies currently used in grid applications include lead-acid, sodium-sulfur, redox flow, and lithium-ion batteries.
• As a mature technology, modern lead-acid batteries are inexpensive to produce and offer high energy density. There is a major problem with these batteries' poor energy density and limited cycling life.
• Sodium-sulfur technology has already been employed as grid-connected energy storage to mitigate the effects of renewable energy sources.
• Although their efficiency can approach 75%, redox flow batteries only have a moderate energy density (15-30 Wh/kg). Regardless, redox flow batteries are not limited by the depth of discharge or the reactants' life cycle.
• Lithium-ion batteries have high energy density and efficiency exceeding 90%. Because of its superior performance, lithium nickel manganese cobalt, NMC, has supplanted all other Li-ion technologies as the standard for both stationary storage and electric cars.

This post is based on an IEEE Open Journal of the Industrial Electronics Society research article.

 

Featured image used courtesy of Adobe Stock