The cost of generating wind and solar power has been sinking like a stone, but the cost of storing all that energy for a rainy day has remained stubbornly high. With that in mind let’s take a look at a new advanced energy storage development announced by our friends over at Pacific Northwest National Laboratory.
PNNL has been working on bringing down the cost of sodium-β batteries (that’s β for beta). Sodium-β batteries are widely perceived to be the key to advanced energy storage for utility scale wind and solar energy power, but their relatively high cost has been an obstacle to widespread adoption.
Sodium-β Batteries For Advanced Energy Storage
Sodium-β refers to a class of rechargeable metallic batteries, in which the two electrodes are separated by a ceramic membrane made of beta alumina. Initially used to construct industrial furnaces, by the 1960’s beta alumina was rediscovered as a conductive material with applications for advanced energy storage.
According to the Energy Department, there are two promising materials for the positive electrodes, sodium-sulfur or sodium-nickel-chloride (the later is the ZEBRA battery, for those of you familiar with the topic).
In terms of performance potential, sodium-β batteries could far outstrip lithium-ion batteries, the current gold standard. In addition to advanced energy storage for utility operations, sodium-β batteries could also play a role in mobile energy storage for electric vehicles.
The main problem is that under current technology, the molten state of the sodium-β electrode materials is maintained by a high operating temperature, up in the 350 C range. The high temperature is the main driver of expense for the batteries. It contributes to a relatively short lifespan, and it also requires the use of more expensive materials.
With that in mind, let’s take a look at PNNL’s solution for lowering the cost of sodium-β batteries, published online in Nature Communications.
A New Liquid Metal Alloy Electrode
PNNL’s solution is a new liquid metal alloy electrode. In addition to lowering the operating temperature of the battery, the new electrode is also expected to improve the useful life of the battery, reduce the risk of accidental fire, and contribute to lower manufacturing costs.
The new alloy addresses an unwanted side effect, which is the behavior of the molten sodium when it comes into contact with alumina at a lower temperature. Here’s how PNNL sums up the challenge:
Lowering the battery’s operating temperature creates several other technical challenges. Key among them is getting the negative sodium electrode to fully coat, or “wet,” the ceramic electrolyte. Molten sodium resists covering beta alumina’s surface when it’s below 400 degrees Celsius, causing sodium to curl up like a drop of oil in water and making the battery less efficient.
The PNNL workaround involved using a sodium alloy rather than pure sodium. After some experimentation the research team came up with a liquid sodium-cesium alloy (cesium is a soft, silvery metal).
The results seem pretty impressive. A battery tricked out with the sodium-cesium alloy was able to function effectively at 150 degrees C. Its power capacity was about the same as a conventional sodium-β battery, and it also retained its storage capacity, indicating the potential for lifecycle cost savings. Here’s the numbers according to PNNL:
After 100 charge and discharge cycles, a test battery with PNNL’s electrode maintained about 97 percent of its initial storage capacity, while a battery with the traditional, sodium-only electrode maintained 70 percent after 60 cycles.
As for the cost of materials, one of the pricey parts of a conventional sodium-β battery is its steel casing. The lower operating temperature enables the use of polymers, which are far cheaper.
Although cesium would raise costs, the use of polymers and other less expensive components, combined with a longer lifespan, could result in lower overall costs.
The next step for the team is to scale up their test battery to a more useful size.
Scaling Up For Wind And Solar Power
The new alloy is just one path that PNNL is exploring to develop a low cost, utility scale energy storage system based on sodium-β batteries. PNNL is also working to lower costs through another utility scale sodium-β battery research project that teams the lab with a company called EaglePicher Technologies, LLC. Though perhaps best known for robotics, EaglePicher is also an advanced energy storage specialist and manufacturer.
In this project, EaglePicher and PNNL are working to shift the basic design of sodium-β batteries from a tubular shape in the electrolyte to a stacked, planar configuration.
Like the new alloy, the stacked design is aimed at lowering the operating temperature. The planar configuration also has the potential to operate more efficiently, leading to an estimated 30 percent increase in energy density, while contributing to lower manufacturing costs.
According to PNNL, cost-effective, utility scale sodium-β battery technology could be the means by which wind and solar become baseload generators.
Considering all the other advances in utility scale energy storage and new smart grid technology, it really is only a matter of time before wind and solar become just as steady and reliable as any mainstream fossil fuel.