Advanced Battery Technologies for Stationary Energy Storage Applications: Page 2 of 5

Advanced Battery Technologies

Here I look specifically at those alternatives to conventional lead-acid energy storage technologies that are commercialized in stationary grid applications or deployed in pilot projects. Though lead-acid batteries have been commercialized for nearly 160 years, they have ceded market share in recent decades to next-generation secondary (rechargeable) battery technologies. This is perhaps most apparent in portable tools and consumer electronics, but is equally true in automotive traction and stationary grid applications. Lead-acid batteries have a long history in off-grid applications, but are seldom the technology of choice in emerging grid-interactive energy storage applications because of limitations associated with round-trip efficiency, energy density, depth of discharge and cycle life.

LITHIUM-ION BATTERIES
According to the most recent edition of the US Energy Storage Monitor (see Resources), lithium-ion (Li-ion) batteries are by far the dominant energy storage technology in today’s grid-interactive applications. Continuing a trend that dates back to Q4 2014, Li-ion deployments accounted for 94.2% of the energy storage market in Q2 2017. The leading manufacturers in this space are generally subsidiaries or divisions of well-known multinationals or specialty battery vendors.

Lithium has several characteristics that make it ideal for use in batteries. For one, it is the third chemical element on the periodic table, after hydrogen and helium, making it the lightest of all elemental metals. Additionally, it has the highest electrochemical potential (-3.02 volts) of any metal and is highly reactive. Inherent advantages and disadvantages are associated with these chemical properties. On the one hand, Li-ion batteries provide excellent energy and power density; on the other, they present potential issues with chemical and thermal stability.

All Li-ion batteries have three basic parts: a positive electrode (cathode), a negative electrode (anode) and a chemical compound (electrolyte) that allows the movement of ions between the electrodes. The cathode material is often a metal oxide, and the anode is usually porous graphite. When a Li-ion battery is charging, lithium ions migrate from the cathode to the anode; on discharge, the anode loses electrons and the cathode gains electrodes.

Manufacturers package Li-ion cells individually, often in pouch or cylinder form, and integrate multiple cells into a battery module with a battery management system to keep each cell balanced. To scale systems up, companies integrate multiple battery modules into a battery rack. Container-scale solutions incorporate multiple battery racks. Li-ion energy storage systems at this scale have multiple layers of battery management and control—at module level, rack level and system level—as well as multiple layers of safety and protection such as fuses, software, containment, climate control and so forth.

Li-ion batteries in general have relatively high intrinsic cell voltage and low self-discharge rates, and respond quickly when charging or discharging. Important differences exist between specific Li-ion chemistries, however, as each has different performance characteristics and a unique value proposition. In most cases, the name given to various Li-ion technologies refers to the chemistry of the cathode. Common Li-ion cathode chemistries in grid applications are lithium iron phosphate (LFP) and lithium nickel manganese cobalt oxide (NMC).

LFP-type Li-ion. In 1996, a research team led by John Goodenough at the University of Texas first described lithium ferro phosphate (LiFePO4) as a cathode material for rechargeable Li-ion batteries. Superior chemical and thermal stability is one of the most compelling features of LFP-type Li-ion batteries. Phosphate-based cathode materials release little heat and oxygen gas when exposed to high-temperature or overvoltage conditions, which means they are not susceptible to thermal runaway. In addition to being chemically stable, LFP cells are not combustible, which further improves their safety relative to Li-ion batteries with metal oxide cathodes. LFP batteries also offer an extended cycle life and lifespan compared to competing technologies. The tradeoff is that while LFP batteries can support high load currents and retain their power capabilities at a low state of charge, they have a relatively lower energy density and a higher self-discharge rate.

Notable grid-interactive LFP battery vendors include BYD, a Chinese manufacturer of automobiles and rechargeable batteries, and Murata. LFP-type batteries are in use in a wide variety of stationary applications. At the grid scale, project developers or EPCs have deployed BYD’s LFP-based energy storage systems at multiple locations across the US and Canada in both microgrid and frequency regulation applications. For example, EDF Store & Forecast commissioned a 19.8 MW/7.9 MWh battery storage project outside Chicago in January 2016 that provides the grid operator, PJM, with ancillary services, including autonomous frequency regulation and dynamic power reserves. At the other end of the application spectrum, Blue Planet Energy uses Murata’s LFP-type batteries in its residential energy storage platform to support nanogrid, backup power and self-supply applications.

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