Storing solar power with grid-scale molten hydroxide

Seaborg Technologies, a Danish manufacturer of molten salt nuclear reactors, is working with its sister company, Hyme Energy ApS, to develop a molten salt thermal energy storage technology that can store large amounts of intermittent wind and solar power.

The novel technology was originally conceived for use in Seaborg’s next-generation advanced nuclear reactor, the Compact Molten Salt Reactor. But it acquired a life of its own when the company's experts developed a chemistry control method that holds the corrosion by sodium hydroxide at bay.

“Normally no chemist in their right mind would look at anything as corrosive as sodium hydroxide,” Seaborg CEO, Troels Schönfeldt, told pv magazine. “However, in the development of our reactor, we needed to use sodium hydroxide and were forced to develop these methods.”

Sodium hydroxide can be produced at low cost from seawater as a byproduct from chlorine production. Seaborg said this is six times cheaper that standard salts used for storage.

“Hydroxides can contain more heat per salt unit, making it more efficient and reducing the amount of salt needed compared to current salt uses,” the company explained. “This reduces the cost of salt as a storage medium by approximately 90%”.

Sodium hydroxide is produced across the globe in bulk, as a byproduct part of the alkali chloride process. Normally, bulk price would be around $150 per ton, or around $0.75 per kWh as a volumetric storage price for the storage medium.

The proposed storage system uses renewable energy to heat the salt using electrical heaters. It is based on two-tank molten salt storage designs developed for concentrated solar power (CSP) plants. It has a scalable storage capacity from 250MWh to 5GWh. A 1GWh facility with sodium hydroxides is expected to be able to store heat to produce power and heat for around 100,000 households for 10 hours of discharge.

The two tanks are able to store electricity as heat at 700 C. The high temperature provides large flexibility for how energy can be extracted back out, including Rankine cycle combined heat-and-power production, Brayton cycle heat-to-power, heat storage for district heating, and industrial process heat.

The immersion heaters are used to store excess electricity as heat, while pumps control the flow for discharging through salt to steam heat exchangers for producing steam to drive steam turbines for co-generation in the simplest setup. Seaborg said this storage system may have a maximum heat loss down to 0.5% to 1% per day and its charge and discharge are freely scalable. Its costs should be between 30% and 50% lower than conventional molten salt storage. Hyme believes it could  halve the price of long-term and large-scale energy storage facilities regardless of the geographical location.