Could molten salt be the next big thing in energy storage?

Jordan Smith
By Jordan Smith

As debates have intensified about the need to shift towards a clean energy economy, energy storage technology has received increased attention. However, this discussion tends to focus on short-term storage in lithium-ion batteries.

Several exciting new projects are trying to change the conversation by investigating alternative options for long-term energy storage, which would allow producers to hold energy in reserve for days or even weeks.

The U.S. Government’s Advanced Research Projects Agency-Energy (ARPA-E) is currently funding ten long-duration energy storage projects.

Although the projects take very different approaches, the problem they’re trying to resolve is the same: If renewable energy, particularly solar and wind power, is going to provide the majority of our energy in the years to come, we’ll need to store energy for extended periods of time.

While lithium-ion batteries have established their dominance in the short-term energy market, which refers to energy stored for four hours or less, alternatives are needed to deal with longer periods of reduced energy production, such as cloudy weather lasting several weeks.

Thermal storage

Several programs being funded by ARPA-E seek to overcome this problem with thermal storage. These systems work by heating up materials to store the energy until it’s needed.

One of the projects given $2 million by ARPA-E is working on a system to store energy in molten salt. Brayton Energy’s initiative will rely on reversible turbines both to compress the energy for storage and release it when it’s needed.

The National Renewable Energy Laboratory secured a $2.7 million grant for another thermal storage project. NREL’s goal is to develop a system that will heat solid particles to over 1,100 degrees Celsius during charging. Then, the particles will be fed through a heat exchanger to heat a fluid, which will drive turbines to produce energy.

California-based Antora Energy, which got a $3 million grant from ARPA-E, is taking a different approach. Its project will heat carbon blocks to over 2,000 degrees Celsius before exposing them to thermal photovoltaic panels, which operate like traditional solar panels but respond to heat rather than sunlight.

Flow batteries

The ARPA-E projects’ other main approach to long-duration storage is flow batteries. Unlike the lithium-ion battery, flow batteries store energy in liquid form. They operate with two separate tanks of fluids which pass through a membrane to exchange electrical current and ions.

One of the initiatives aims to reduce the complexity of flow batteries. Primus Power will use $3 million to further develop a zinc-bromine flow battery. Due to the way zinc and bromine behave, the company is hoping to remove the need for a second tank in the battery.

The other flow battery projects are researching how to make the technology more cost effective. Form Energy will explore the possibility of using sulphur in a water-based solution, while United Technologies Research Center is working on a new battery chemistry that includes low-cost sulphur manganese.

Future prospects for long-term storage

One or more of these projects could play a major role in accelerating the transition to a low-carbon economy.

Although prices for renewable technologies like solar and wind power have dropped dramatically in recent years, the problem of energy storage continues to be a significant hurdle to a more rapid expansion of clean energy use.

Dr. Graham Cooley, chief executive at ITM Power, believes that batteries can only provide part of the solution. ITM is involved in another long-duration storage option: hydrogen storage techniques, which store excess renewable energy in the gas grid.

“It is difficult to match the unscheduled intermittent supply from renewables with demand,” Cooley said. “This is why we need long-term, large scale energy storage solutions, and this is why hydrogen is an indispensable part of the solution.”

The wide variety of competing systems in the long-duration energy storage market is creating challenges for utilities trying to decide which technology to use, according to Jason Deign, who authored a 2017 report entitled “Beyond Four Hours: The Transition to a more Flexible, and Valuable Long-duration Energy Storage Asset.”

Deign said that utilities want to purchase these assets to improve grid reliability, avoid peak time shortages, and power microgrids.

“If you are looking to pick winners, it’s really difficult because you’re not just talking about different options within a given technology group, there are actually different technology groups,” Deign said. “It’s not like saying which kind of PV is best, it’s like saying which kind of renewable energy is best because the difference between say thermal [storage] and batteries is [like the difference] between solar and wind or small hydro.”

Hopefully, the ARPA-E funding will go some way to answering these questions.

Jordan Smith is a freelance journalist and translator covering issues related to energy, the environment, and politics. His work has appeared on the independent news site Opposing Views, and at the Canadian Labour Institute.