Electrochemical Technology Dominates in Energy Storage Systems
Energy storage systems employ technological approaches that provide sustainable power for a utility’s customers. These systems must provide extended service during power outages, be easily replenished, be safe, reliable and cost effective. One approach for energy storage involves cross-disciplinary applications in chemistry and electrical engineering. In fact, much of the R&D for electrochemical energy storage is more chemical than electrical. There are several other non-chemical approaches for energy storage systems, but the electrochemical approach is now the dominant solution.
From an electrical standpoint, energy storage systems rectify the incoming ac and convert it to dc for storage in a battery. Then, the stored dc is applied to an inverter to produce an ac output that supplies utility power. These batteries also require some means for recharging them. All electrical functions depend on the characteristics of the batteries and their chemistry.
The chemical part of storage technology involves battery construction. One electrochemical battery is the redox flow battery (RFB) that refers to chemical reduction and oxidation reactions employed to store energy. They employ liquid electrolyte solutions that flow through a battery of cells during charge and discharge.
You can divide RFBs into two categories:
1. True redox flow batteries, where all of the chemical species active in storing energy are fully dissolved in solution at all times.
2. Hybrid redox flow batteries, where at least one chemical is plated as a solid in the electrochemical cells during charge.
True redox flow batteries have two chemical components dissolved in liquids contained within the system and separated by a membrane. Ion exchange through the membrane produces a flow of electric current while both liquids circulate in their own respective space. Cell voltage ranges, in practical applications, from 1.0 to 2.2 V. Generally, stacks of these batteries are used to store utility power.
An example of an RFB is ViZn Energy Systems Inc.‘s (Austin, Texas) Vanguard II battery stack that is immune to cycle-life degradation, providing more headroom to handle spikes in power requirements for demanding and unpredictable applications on both sides of the meter. All ViZn flow batteries incorporate the Vanguard II stack-control technology, which eliminates life-limiting issues such as dendrite growth, simplifies cell balancing, and removes thermal and electrolyte breakdown issues associated with high-frequency power switching.
Energy storage systems employ technological approaches that provide sustainable power for a utility’s customers. These systems must provide extended service during power outages, be easily replenished, be safe, reliable and cost effective. One approach for energy storage involves cross-disciplinary applications in chemistry and electrical engineering. In fact, much of the R&D for electrochemical energy storage is more chemical than electrical. There are several other non-chemical approaches for energy storage systems, but the electrochemical approach is now the dominant solution.
From an electrical standpoint, energy storage systems rectify the incoming ac and convert it to dc for storage in a battery. Then, the stored dc is applied to an inverter to produce an ac output that supplies utility power. These batteries also require some means for recharging them. All electrical functions depend on the characteristics of the batteries and their chemistry.
The chemical part of storage technology involves battery construction. One electrochemical battery is the redox flow battery (RFB) that refers to chemical reduction and oxidation reactions employed to store energy. They employ liquid electrolyte solutions that flow through a battery of cells during charge and discharge.
You can divide RFBs into two categories:
1. True redox flow batteries, where all of the chemical species active in storing energy are fully dissolved in solution at all times.
2. Hybrid redox flow batteries, where at least one chemical is plated as a solid in the electrochemical cells during charge.
True redox flow batteries have two chemical components dissolved in liquids contained within the system and separated by a membrane. Ion exchange through the membrane produces a flow of electric current while both liquids circulate in their own respective space. Cell voltage ranges, in practical applications, from 1.0 to 2.2 V. Generally, stacks of these batteries are used to store utility power.
An example of an RFB is ViZn Energy Systems Inc.‘s (Austin, Texas) Vanguard II battery stack that is immune to cycle-life degradation, providing more headroom to handle spikes in power requirements for demanding and unpredictable applications on both sides of the meter. All ViZn flow batteries incorporate the Vanguard II stack-control technology, which eliminates life-limiting issues such as dendrite growth, simplifies cell balancing, and removes thermal and electrolyte breakdown issues associated with high-frequency power switching.
1. Simplified flow-battery system employed by ViZn. Pumps on the right and left bottom keep the zinc-iron electrolyte flowing.
This unique multi-use capability is necessary for frequency regulation and other high-power applications while adding value to longer duration storage. All of ViZn’s systems are scalable, adding value for even the largest utility requirements. By interconnecting multiple units, both power and energy capabilities can be increased to offer utilities, as well as commercial and industrial customers, the optimal fit for any size project.
ViZn’s systems utilize an inherently safe, non-toxic, non-explosive zinc-iron electrolyte as shown in the simplified system in Fig. 1. On charging, the electrical energy supplied causes a chemical reduction reaction in one electrolyte and an oxidation reaction in the other. The ion-selective membrane between the half-cells prevents the electrolytes from mixing, but allows selected ions to pass through to complete the redox reaction. On discharge, the chemical energy contained in the electrolyte is released in the reverse reaction and electrical energy can be drawn from the electrodes. When in use, the electrolytes are continuously pumped in a circuit between reactor and storage tanks.
High-power flow batteries use multiple stacks of cells. The size and number of electrodes in the cell stacks is fixed and determines the system’s power rating. An advantage of this system is that it provides electrical storage capacity, limited by the capacity of the electrolyte storage reservoirs. Facilitating thermal management is use of the electrolytes as the thermal working fluids as they are pumped through the cells.
ViZn’s zinc-iron Redox storage technology provides large-scale energy storage. A modular unit is a 20- or 40-ft. shipping container that can be combined and scaled to provide storage solutions for projects ranging from 100 kW to 100 MW. This technology provides safety, simplicity, and use of abundant core non-toxic raw materials to achieve a 20-year expected life. Figure 2 shows its internal construction.
One example of a hybrid redox flow battery is the all-iron redox flow battery (IFB) developed by ESS (Portland, Ore.). The IFB is a durable, environmentally safe, long-duration storage solution. Its lifespan that exceeds 20,000 cycles, it has low maintenance requirements, and an energy capacity of over six hours. The IFB matches well with the 25-year life span of solar and wind projects, supporting those applications’ low levelized cost of energy (LCOE) requirements. In addition, the IFB’s inherent quick-response power electronics can perform ancillary services such as voltage and frequency support on microgrids and utility-scale applications.
The IFB’s all-iron redox flow battery technology uses iron dissolved in salt water as an electrolyte (Fig. 3). You can break down IFB performance to its plating electrode performance (negative electrode), redox electrode performance (positive electrode), and ohmic resistance loss. On the plating electrode, the ferrous (Fe2+) ion gains electrons and plates as solid iron on the substrates during charge, and the solid iron dissolves as ferrous ions and releases two electrons during discharge. The equilibrium potential for the iron plating reaction is -0.44V. On the redox electrode, the redox reaction between ferrous and ferric (Fe3+) ions occurs during charge and discharge. On the positive electrode, two Fe2+ ions lose two electrons to form Fe3+ ions during charge and two Fe3+ ions gain two electrons to form Fe2+ during discharge. The equilibrium potential between ferrous and ferric ions is +0.77V. Thus, the reaction in an IFB redox flow battery is reversible.
A recent report by IDTechEx Research, authored by technology analyst Dr. Lorenzo Grande covered Redox Flow Batteries 2017-2027: Markets, Trends, Applications. The report noted that RFBs retain most of their initial value thanks to the possibility to recycle their core components more easily than other battery chemistries. It also predicted that the RFB market will be worth $4B by 2027 and will include all stationary storage applications, from residential to commercial and industrial to grid-scale systems. It said RFBs can potentially make second-life li-ion batteries obsolete by offering a stable cycle life and reduced engineering and battery management system challenges. Recycling aspects will ensure that, once out of service, the raw materials will retain most of their value and will be used in brand new RFBs.
