Ten teams working to drive down the cost of long duration storage are competing in a way, using federal grant support to make enough progress to earn a follow-on grant for pilot-scale production. Projects include a sulfur flow battery for full-week backup capability, and a more efficient means of converting electricity to hydrogen and back again.

Each project aims toward a goal of 5 cents/kWh for storage that can last for days, under the DAYS program of the U.S. Department of Energy’s Advanced Research Projects Agency (ARPA-E).

Here are highlights of the ten projects, spanning corporate, university and hybrid teams.

Sulfur flow batteries

Flow batteries use electricity to produce an electrolyte, which may be stored separately from the battery. The electrolyte is later “flowed” through the battery to generate electricity. As a result, long-duration storage using flow batteries requires only a large storage capacity for electrolyte.

Form Energy aims to achieve “full-week backup capability” with a sulfur flow battery “at a factor of 10 or greater cheaper” than lithium-ion batteries, said company co-founder Marco Ferrara in a video posted by global utility Enel. Form Energy may ultimately pilot its battery technology in a joint project with Enel.

“Aqueous sulfur flow batteries represent the lowest chemical cost among rechargeable batteries,” says Form Energy’s grant award notice, but have low efficiency. To improve efficiency, the firm is working on anode and cathode formulations, membranes and physical system designs.

A United Technologies project is focused on sulfur and manganese flow batteries, and has three project partners: Lawrence Berkeley National Laboratory, MIT, and Pennsylvania State University. The project aims to “overcome challenges of system control and unwanted crossover of active materials through the membrane.”

Electricity to hydrogen

A team at the University of Tennessee, Knoxville aims to improve the efficiency of the round-trip process of converting electricity to hydrogen and back again. The current process uses electricity to power an electrolyzer to convert water to hydrogen and oxygen, and then uses the hydrogen and oxygen in a fuel cell to produce electricity and water.

“It has long been a goal to make a regenerative fuel cell, a single device that functions as both a fuel cell and an electrolyzer,” said lead researcher Dr. Thomas Zawodzinski, as quoted in a university press release. “However, such devices have previously suffered from poor overall efficiency. The new project uses an alternative approach by changing one of the chemical reactions in the cell and bypassing the efficiency bottleneck.”

The Tennessee team will develop a system in which a reversible fuel cell converts hydrogen and oxygen to liquid hydrogen peroxide instead of water. Later, electricity will be used to power the same reversible fuel cell, now operating as an electrolyzer, to convert the hydrogen peroxide to hydrogen and oxygen—which are also stored, ready to begin the cycle again. “The benefit of using peroxide instead of water is higher efficiency in both charging and discharging the system,” says the ARPA-E award notice.

The research comes as global investment in “green” hydrogen—which is produced using solar or wind power—is accelerating.

Zinc-bromine flow batteries

Primus Power already makes zinc bromide flow batteries, and has been awarded a $4 million grant from the California Energy Commission to increase manufacturing capacity of its 25 kW, 5-hour EnergyPod 2.

Under its ARPA-E grant, Primus Power will work with the Columbia Electrochemical Energy Center to “eliminate the need for a separator to keep the reactants apart when charged,” by “taking advantage of the way zinc and bromine behave in the cell.” The new configuration is expected to allow all the electrolyte to be stored in a single tank, instead of multiple cells, thus reducing balance-of-plant hardware, and system costs.

Thermovoltaics

Antora Energy will use electricity to power resistive heaters, to heat carbon blocks to over 2000°C. To generate electricity, the carbon blocks will be exposed to thermovoltaic panels. With its ARPA-E grant, Antora will develop a “thermovoltaic heat engine,” seeking to double panel efficiency through new materials and “smart system design.”

Electricity to magnesium manganese

A team at Michigan State University will develop a modular system that heats magnesium manganese oxide (Mg-Mn-O) particles to a high enough temperature that the particles release oxygen. To generate electricity, the system will pass air over the particles (now Mg-Mn), initiating a chemical reaction that releases heat to drive a gas turbine generator.

Electricity to heat

Three projects aim to increase the efficiency of storing electricity as heat, and then using the heat to drive a turbine-generator set.

The National Renewable Energy Laboratory will develop a system that uses electricity to power a high-performance heat exchanger, which will heat inexpensive solid particles to over 1100°C. The particles will be stored in insulated silos for up to several days. When electricity is desired, the hot particles will be fed through a fluidized bed heat exchanger, heating a working fluid to drive a Brayton combined-cycle turbine attached to a generator. The Colorado School of Mines is a project partner.

Brayton Energy will “develop a key component” to enable a cost-competitive thermal energy storage system with innovative turbomachinery. The innovation will relate to Brayton’s “reversing, counter-rotating turbine design, in which each turbomachinery stage is designed to act as both a compressor and turbine, alternating between charging and discharging cycles.” The approach is expected to both increase efficiency and reduce capital costs, by simplifying the system.

Echogen will use electricity to heat a low-cost material such as sand or concrete. Later, to generate electricity, the heat would be used to heat liquid carbon dioxide—previously brought to a supercritical pressure—and the heated, supercritical carbon dioxide would expand through a turbine to generate electricity.

Pressurizing water underground

Quidnet Energy is developing a process to pump water into “confined rock underground, creating high pressures.” Under its ARPA-E grant, Quidnet will demonstrate the ability to generate electricity from water stored at pressure underground, and find suitable means to make the process work across multiple types of geography within the U.S.

Next steps

Five of the projects will be completed in 2021, and the other five in 2022. Each project team must estimate costs for a full-scale system, which will be evaluated in comparison to ARPA-E’s 5 cents/kWh goal for long-duration storage.

ARPA-E envisions a second phase of the program, to fund construction of one or more prototype systems that are placed in field use.

The Department of Energy is also supporting research and development of lower-cost photovoltaics, in a complementary initiative.