A Salty Solution to Energy Storage

Salt crystals

Dec 06, 2017

The search for efficient energy storage has been the holy grail of the renewable energy industry since its inception; this is driven by the fact that peak demand rarely coincides with peak production. The wind don’t blow, the sun don’t shine, it sounds like a Motown hit, but, without a cost-effective method to store energy for when it’s needed wind and solar power may never reach their full potential.

Utilities across the globe are moving away from fossil fuels, creating a market for grid-scale energy storage that is expected to reach $19 billion by 2022.  Currently, the most visible commercially available system is the Tesla Powerpack, which relies totally on lithium-ion (Li-ion) battery technology. In fact, Tesla uses the same 2170 cell in its Powerpack that is found throughout all their product lines including their automotive products.

To its credit, Tesla has made significant advances in the efficiency of Li-ion technology. With projects like the Mira Loma battery storage facility in Southern California under its belt, they have set the stage for this growing industry. The Mira Loma plant provides 80 MWh of storage, utilizing millions of individual 2170 cells.  This project is a good first step for Tesla, however, when you consider that California can generate up to 8 GWh of power from renewables, it becomes clear that a more cost-effective large-scale solution is needed.

Enter molten salt. Up until now, Li-ion and solid electrolyte Li-ion have dominated the battery industry. Their low-temperature operation and years as the backbone technology of the industry have made them the go-to storage source for everything from iPods to electric cars. However, this success has not translated into truly economical grid-scale power storage.

Grid applications are mission critical; they must be robust, dependable, and inexpensive, this is where molten salt’s high energy and power density capacity come into play. The use of salt as a battery electrolyte has been around since the forties. Originally developed by the Germans during WWII, salt batteries, a.k.a. Sodium-Sulfur (NaS), were used for their stability and long-term storage capacity, which is ideal for bombs and rockets. In fact, the same technology is currently used in military applications such as the Tomahawk, Patriot, and Sidewinder missiles.

NaS was also used in 1966 by Ford Motor Company for an electric vehicle application.  Unfortunately, its high-temperature operation (290-390 °C) doomed the Ford project and precluded it from widespread adoption. It wasn’t until the 1980’s that molten salt battery technology began to make its way into grid-scale applications. 

In 1983, NGK began developing NaS technology for a grid application in Tokyo, which resulted in a 90 MW storage facility ten years later. NGK currently operates NaS battery facilities in 190 locations across the globe, providing 530 MW of capacity.

However, NaS battery technology isn’t the only method utilizing molten salt for energy storage. Two years ago, Google’s parent company Alphabet unleashed its X moonshot factory on the energy storage problem, coming up with a novel approach for the use of molten salt. The project is called Malta and is based on the work of Robert Laughlin, a Stanford physics professor, who won the Nobel Prize in physics in 1998. Laughlin’s work is based on BTUs rather than electrons, taking advantage of molten salt’s thermal characteristics rather than its ability to store electricity.

Simply put, electricity from solar and wind power is used to turn a heat pump, which converts the electricity into hot and cold BTUs. The heat goes into molten salt and the cold into a liquid coolant. The density of each allows them to hold their energy for many days. When called upon, the process works in reverse, powering a heat pump that converts the BTUs back into electricity. And unlike current storage technologies, Malta’s molten salt tanks could last up to 40 years without replenishment. That is four times as long as lithium ion batteries, and more than double that of current NaS technology.

The success of this project will ultimately rely on efficiency. Laughlin, who is also a consultant on the project, proved the math. It’s been the Malta team’s job to design hyper-efficient heat pumps, turbines, storage, and transfer systems to maximize the effect. Efficiency seems to be the mantra of this group, consisting of less than ten researchers; they claim to be moving briskly towards commercial viability. Plans are currently in place to build a megawatt-scale plant, as Malta seeks partners in the energy industry to help bring this idea out of the lab.

The ultimate success of this project will rely on trade-offs, the cost of efficiency vs. the cost of materials. If the scale can be balanced using materials that are inexpensive and easy to obtain, scalability will fall in line, as well as mass adoption.

 

Image credit: David-Palenik/Shutterstock.com