Editor’s note: “Storing That Power” is a seven-part series detailing technologies capable of reserving power obtained from renewable sources. Read each week to learn more about pumped hydroelectric, industrial-scale batteries, flow batteries, flywheels, compressed air energy storage, gravel batteries and molten salt.
Batteries are the first thing most people think of when it comes to power storage, because that is what we are most familiar with in our day-to-day lives. Batteries are packages that store energy in chemical form until it is needed.
At the consumer level, some batteries are single-use, and cannot be easily recharged after they have been depleted due to the kinds of chemical reactions they rely on to provide power. Single-use batteries may be convenient for consumers, but they are not viable for significant power storage. At grid scale, batteries must be able to be charged when there is extra electrical production and discharged when there is extra demand.
One problem that most kinds of rechargeable batteries suffer over time is a loss of capacity. The repeated charging and discharging of the battery will lead to some of the chemicals crystallizing, and thereby losing the ability to store power, which leads to battery degradation over time.
Even if there is no demand on the battery, there will be a gradual discharge of the battery over time. Monitoring of the charge level of the battery can aid in keeping it kept fully charged until its power is needed. Typically, the stored power will be used within a few days and then the battery will be recharged again.
Unlike the batteries in your cell phone or laptop computer, the batteries for grid-level power storage use different kinds of chemical combinations for more efficient power storage. Chemical batteries such as sodium-sulfur batteries offer large-scale methods for storing power. Flow batteries are another specialized type of chemical battery that offer some unique features that can make them attractive in some cases. (We will take a look at flow batteries in a separate, forthcoming article.)
Sodium-sulfur batteries are one kind of liquid metal chemical battery that is used in large scale power storage. The properties of sodium-sulfur batteries make them unsuitable for most uses other than industrial level uses. Sodium metal is a hazardous material that will spontaneously burn if it comes into contact with any moisture, so it particularly needs to be kept in a contained environment.
Sodium-sulfur batteries also need to be kept hot, and have an operating temperature of 300-350 degrees C (572-662 degrees F). This makes them unsuited for more mobile applications, but larger scale installations can be thermally efficient and can work better than smaller sized batteries. The process of charging and discharging the battery generates a fair amount of heat, so that, once it is in operation, a sodium-sulfur battery does not usually require external heat to keep it at its working temperature.
The relatively low cost of materials needed for sodium-sulfur batteries makes them affordable, particularly in comparison with other types of batteries using rare and exotic materials that can be expensive to obtain. Sodium-sulfur batteries also have a good energy density, so a large amount of storage is able to be contained in a small space.
Sodium-sulfur batteries have an efficiency of around 90 percent, which makes them particularly effective for power storage. The largest sodium-sulfur battery installation is a 34 MW installation at Futamata wind farm in northern Japan.
Other kinds of battery grid-storage systems are also beginning to be used, as well. Manufacturers such as A123, which is a manufacturer of batteries for vehicles (BMW, Fisker, VIA Motors, etc.), also manufacture grid storage units with a number of installations throughout the world providing several megawatt-hours of power storage capacity. An installation at the Laurel Mountain wind farm in West Virginia is similar in size (32 MW) to the Futamata wind farm.
Other types of battery chemistries are also being explored for other cost-effective ways of storing electricity, though pumped hydro is still overwhelmingly the most common method for power storage. At the high tech end, some of the most advanced battery research is going on in the automotive industry, where battery manufacturers are looking to extend the range and performance of electric vehicles. Those developments are likely to find applications in other power storage systems, as well. At the other end, scientists are developing solutions for power storage using materials as basic as iron and air (essentially using the process of rust for power storage).