Molten salt is hardly the sort of thing you would want to take a bath in, but from a health and safety point of view it is far from the worst thing you could find in power generation.

By Jason Deign

The CSP industry’s growing dependence on molten salt for heat transfer and thermal energy storage should not pose a health and safety threat, experts claim.

Even though it has an operating temperature of up to around 565ºC, the relatively un-reactive nature of the fluid means it is less hazardous than many other materials found in power generation or industrial settings.

“Dealing with molten salts requires the regular precautions that are used when working with any fluid substance at high temperature,” says Giuseppe Casubolo, sales and marketing director for solar salts at SQM Europe, a thermo-solar salt provider based in Chile.

“The molten salts do not burn and the risk associated to them is that in case of contact severe skin burns can happen.

“However, thanks to the fact the salts solidify very quickly in case of spillage, the possibility to get in direct contact with molten salts is very improbable if basic, commonsense practices are used.”

Compared to other industrial processes, working with molten salt is nothing to write home about, says Bill Gould, chief technology officer at SolarReserve. “In the world of industrial safety, working with molten salt even at 1000ºF [538ºC] is actually pretty tame,” he comments.

“Consider that oil refineries and chemical plants have routinely dealt with fluids and gases at even higher temperatures. And even the 15 or 20 trough plants which use a heat transfer fluid at 700ºF [371ºC] have dealt with similar high temperature concerns for years.”

In a CSP plant, the potential for workers to come into direct contact with molten salt is reduced as a result of the closed nature of heat transfer and thermal storage systems.

Non-toxic and non-flammable

And salt has the added advantage of being non-toxic and non-flammable, and remaining in liquid form at the normal operating temperatures required for CSP. “There is virtually no gaseous phase,” Gould says.

“As a result there is no risk of inhalation of poisonous gases such as H2S or ammonia, which we commonly find in refineries, nor even oxygen displacing nitrogen. Because salt is hot, there is no risk of aspiration nor ingestion.”

Furthermore, because molten salt does not burn there is no explosion nor combustion risk as you would find with flammable petroleum products or with the synthetic heat transfer fluids used in trough plants, according to Gould.

“Industrial safety procedures have been established to deal with the common yet far more dangerous hot fluids in refinery and chemical plants,” he notes. “The most common risk in dealing with molten salt is simply protecting personnel from burns.”

The relative safety of using molten salt is underscored by what happened in the decommissioning of the 10MW Solar Two demonstration plant in the Mojave Desert, in 1999. There the molten salt was cooled until it solidified, then prilled and sold to farmers as fertiliser.

“You would never pour Therminol nor many other heat transfer fluids on food crops, but that is exactly what was done for that plant,” Gould points out.

Naturally, none of this obviates the need for due precautions when handling molten salts. These precautions begin at the design stage of the plant, when engineers take care to select welded piping joints, ring-joint piping flanges and valves with extended bonnets, to minimise leakage.

During operation, plant managers apply industry guidelines, such as those offered by the Occupational Safety and Health Administration in the US, to make sure pipes and equipment are fully insulated so workers do not come into contact with hot surfaces.

Higher temperatures

“In designing and operating plants with molten salt, industry builds upon the successful practices and procedures in other industries which face working with fluids at even higher temperatures,” Gould states.

Another obvious precaution is to give equipment plenty of time to cool down before maintenance crews are allowed to service it.

Here power towers have an advantage over parabolic trough plants because the salt is drained out of the system and into a cold tank every night, so piping and equipment cools and can be serviced on a daily basis.

Casubolo adds that even in the event of a major spillage, such as a breach in the storage tank, the molten salt would be relatively easy to deal with because it would solidify at about 220ºC, essentially on contact with the outside environment.

“In case of large leakage the storage areas of the plants are contained by a concrete base which collects the spillage,” he says.

Gould remembers that applying standard health and safety precautions allowed Solar Two to operate for almost half a decade without a single burn or injury related to molten salt.

And since molten salt is pretty much the most dangerous element in a CSP plant, that bodes well for health and safety across the industry in general—although that is no cause for complacency, of course.

To respond to this article, please write to Jason Deign.