Grid-level energy storage serves multiple purposes, including backup power, peak demand response, frequency regulation, and balancing the effects of locally-intermittent renewable sources such as wind and solar. Each application lends itself to a particular type of battery. (I’m ignoring pumped hydro and compressed air energy storage, since those are dependent on the local geography. Batteries, on the other hand, can be deployed anywhere.) So what type of battery has a large energy density, deep discharge capability, high charge acceptance, and excellent cycling performance? In short … none. That’s why the German government is funding the M5BAT (Modular Multi-Megawatt, Multitechnology Medium-Voltage Battery Storage System) project, a storage initiative that will combine multiple battery technologies into a single storage unit.
By combining multiple types of batteries into a single storage unit, complete with bidirectional inverters and a sophisticated control system, the M5BAT project hopes to create a single “hybrid battery” that gives the best of each. And the beauty of the design is that it’s transparent to the end user - the whole thing behaves like one battery that meets all storage requirements without compromising. The M5BAT project will design and test various battery configurations and control systems to determine the technical and economic performance of hybrid batteries.
The Battery Technology
Lithium-ion (Li-ion) batteries are seeing a lot of press these days, and with good reason. They provide high energy density, efficient charging and discharging, and a respectable number of cycles. On the other hand, Li-ion batteries don’t handle deep discharges very well; too many deep cycles will drastically reduce their lives. They’re well suited for frequency regulation and maintaining power quality, but not for backup power.
Sodium-nickel-chloride batteries (sometimes called ZEBRA batteries) handle deep discharges with little degradation in quality or lifespan. They aren’t susceptible to fluctuating ambient temperatures, and they enjoy a high charge acceptance (quick charging), making them ideal for integrating renewable energy sources onto the grid. ZEBRA batteries, however, require a high operating temperature (over 250C); maintaining that temperature consumes up to 14% of the battery’s energy every day. Since warm-up and cool-down take several days, ZEBRA batteries work best when they’re constantly operating.
Lead-acid batteries represent the most mature battery technology that continues to evolve. They’re inexpensive, easily recycled, and have relatively long lives. Their energy density is somewhat low and, like their Li-ion counterparts, they don’t like frequent deep discharges. Lead-acid batteries are well suited for peak demand response and voltage regulation on the grid.
The Control System
An advanced SCADA (Supervisory Control And Data Acquisition) system will regulate the flow of power to and from the hybrid battery and the individual batteries within it. It will continuously monitor each battery’s state of charge, temperature, and other parameters. The SCADA system will decide which batteries receive power when it comes into the unit and determine which batteries will be drawn from when power is needed.
The Project
Construction of M5BAT began in 2015; the facility should be completed sometime in 2016. At that point, the system will be connected to a medium voltage grid for two years to test and evaluate the batteries and control system.
M5BAT will consist of 5 MWh of storage capacity, capable of delivering up to 5 MW of power. The average German household consumes an average of about 500 Watts, so the M5BAT could serve up to 10,000 homes. The typical duration of a power failure in Germany is about 16 minutes; this unit could conceivably provide backup power to 10,000 homes for up to one hour.
For more details, see the M5BAT web site. (It’s in German, so I suggest using a browser that translates. Google Chrome worked for me.)