Source: CSP Today Parabolic Trough Report 2012-13

By Jason Deign

They say too much salt is bad for you. Try telling the CSP industry, though. Right now, molten salt is probably one of the main reasons why interest in solar thermal remains high.

Molten salt, and specifically a mix of 60% sodium nitrate and 40% potassium nitrate called saltpetre, has become the medium of choice for thermal energy storage, without which CSP essentially becomes an intermittent power generation source, like wind or PV but more costly.

Storage, on the other hand, allows CSP to complement or compete head-to-head with traditional base-load generation sources such as coal, gas or nuclear.

A number of energy storage media have been developed and continue to be investigated, but to date none has had the success of molten salt.

In March 2009, for example, the Andasol parabolic trough plants in Spain became the first commercial solar plants worldwide to store and deliver energy after sundown using molten salt.

The following year, at the 4.9MW Archimede integrated solar combined cycle parabolic trough plant in Syracuse, Sicily, the Italian utility Enel proved molten salt could also work as a heat-transfer fluid, in place of synthetic oils.

And in July last year, Torresol’s 19.9MW Gemasolar power tower in Spain was the first CSP plant to deliver electricity uninterrupted for 24 hours, again thanks to a molten salt storage tank.

More recently a number of CSP plants in development and construction have indicated they will be using molten salts for storage, including those listed in Table 1.

Table 1: CSP plants under planning, development and construction which will use molten salt storage

Source: CSP Today Global Tracker, October 2012

In addition to the above, BrightSource has confirmed that two CSP plants in the very early stages of planning, the Siberia plant in the San Bernardino County and the Sonoran West plant in Riverside County will be using molten salt storage.In China, the Gansu and Qinhai projects (both 50 MW parabolic troughs) have also indicated they will be using molten salt storage.

There are multiple advantages in using molten salt as a storage medium, compared to exotic alternatives such as graphite. Its behaviour and performance in CSP plants is well understood, for example, and it is based on widely available ingredients.

Heat-transfer fluid

As a heat-transfer fluid, meanwhile, its 565ºC-plus upper temperature limit trumps the 425ºC or so ceiling on hydrocarbon and synthetic oils.

Figure 1: Oil and Molten Salt HTF Parabolic Trough Schematic 

Source: CSP Today Parabolic Trough Report 2012-13

There is no need for a storage heat exchanger, and because of its salt’s higher operating temperature, only about two-thirds of the volume of heat-transfer fluid is needed.

This equates to about 30% lower storage tank and heat-transfer fluid costs, combined with improved performance (because the operating temperature is closer to standard turbine conditions) and lower environmental risks (because the salt is non-flammable and non-toxic).

Table 2: CSP Plants in Planning, Development and Construction which will use Molten Salt as a HTF

Source: CSP Today Global Tracker, October 2012

There are a couple of drawbacks with molten salt in CSP, however. One is that it has a fairly high freezing temperature of 238ºC, meaning it could solidify when the plant is not in operation unless the HTF is kept at a constant temperature.For this reason, where molten salt is used as a heat transfer fluid, a heat tracing system should always be installed to ensure that the flow of the salts can be closely monitored and potential freezing prevented.

Freezing is not much of a concern in storage tanks, though, believes Jesus Fernández-Reche, of the Spanish Centre for Energy, Environmental and Technology Investigation’s Almeria Solar Platform.

In the plants with molten salt-based thermal energy storage used in Spain, for example, he says: “The tanks lose about 1ºC a day, giving you about 40 days to do something before the salt begins to freeze.” 

Where molten salt is present in the heat transfer system, however, Fernández accepts there could be a greater risk of freezing, which may lead to a need for affected pipes to be cut out and replaced.However, having a tracing system in place means that even if the salts freezes, the process can be reversed returning the salts to their molten status.

Of perhaps greater concern than molten salt’s physical properties, however, is its cost. The prime materials involved in its production are the same as those used for fertiliser, so are subject to the vagaries of developments in the agricultural sector.

Salt purity

At the same time, though, few other industries require salt of the purity needed for CSP, which means the product needs to be designed specifically for heat storage in CSP plants.   

The interplay of these factors has meant that historically the price of molten salt has fluctuated widely, going from around USD$1 to $10 a kilo, according to Fernández. In any event, molten salt can end up becoming an important and often unpredictable source of cost in CSP plants.To overcome this, contractual price agreements can be set up between

According to Giuseppe Casubolo, sales and marketing director for solar salts at Sociedad Química y Minera (SQM) de Chile, one of world’s leading saltpetre suppliers, greater stability in CSP demand worldwide is likely to lead to more stable molten salt pricing in the future. 

Furthermore, he says, suppliers are keen to make concessions in order to court what they see as a potentially important new market for molten salt. “We have already done some efforts to stabilise the market and the price conditions for some projects,” he says.

“What we have done for some projects is freezing the prices when there is a company able to sign a contract and define a commitment. We are able to fix the prices and take some risk internally.”

But suppliers could go further if they had a better idea of the medium and long-term global demand for molten salt. This is an issue the CSP sector could definitely influence, for instance by submitting orders and forecasts on an industry-wide rather than project-by-project basis.

That sort of consolidated long-term view might not resolve the issue of molten salt cost completely, but “it would help a lot, definitely,” Casubolo says.

To respond to this article, please write to Jason Deign

Or contact the editor, Jennifer Muirhead