Using molten salt as a heat transfer fluid can deliver significant advantages over alternatives such as mineral oil. A pilot facility for trough plants is due to come online this year, however, a number of challenges remain.

By Rajesh Chhabara

Molten salts can handle temperatures of up to 550ºC – much higher than their organic oil counterparts. The medium has already proven to be a preferred choice for thermal energy storage for concentrating solar plants.

Now, molten salt is about to be given a test drive as a heat transfer fluid (HTF) in a trough plant, with the first pilot using molten salt as HTF due to come online in 2010.

The benefit of using molten salts as the HTF in the solar field is that is potentially eliminates the need for expensive heat exchangers. With molten salt as the HTF, the solar field can be operated at higher temperatures - up to 550ºC - than other heat transfer fluids such as mineral or synthetic oil.

Operation at high temperatures also makes it possible to use more efficient, standardised steam turbines. The molten salt medium is also much cheaper, more environment-friendly, non-toxic and non-flammable compared with oil.

A study carried out by US-based Kearney & Associates for Sun Lab in 2002 projected the potential reduction in levelized electricity cost by switching from VP-1synthetic oil to a molten salt HTF at 450ºC in a trough plant with 6 hours storage would be slightly over one cent/kWh, a very significant gain for a trough power plant.

The study said the cost reduction could be more than 1.5 cents/kWh if a higher temperature at 500ºC can be maintained.

High risk

But the main challenge is that molten salts freeze at relatively high temperatures in the range of 120ºC to 220ºC.

This presents a real risk of the salt freezing in the solar field pipeline during the night and affecting the plant operation. At very high temperatures of around 600ºC, the salt can decompose with a potential risk of corrosion in the system.

“Maintaining the whole solar field of a trough plant above 260ºC during the night is expensive and risky,” says Santiago Arias, Chief Infrastructure Officer at Torresol Energy.

Torresol is using molten salt as HTF in the Gemasolar project, a concentrating solar tower power plant. “We are not considering passing the molten salts through the troughs.”

Arias says that apart from the risk of freezing, huge thermal loss during the night is another concern in using molten salts in troughs.

There are other issues as well. “Pumping of molten salts is not an easy task. Packing or mechanical seals available are not suitable for this purpose.”

He says that using molten salt in a tower plant has the same problem, but the risk is limited to a short area. He adds that valves, piping, instruments, gaskets and tracing systems also currently are not suitable for using molten salts as HTF in trough fields.

Due to these properties of molten salts, freeze protection, solar field preheating, collector loop maintenance and the selection of appropriate materials for piping and fittings are important issues that need to be addressed.

Pilot ready

Recent developments in technology that tackle these challenges are encouraging. The Italian National Agency for New Technologies, Energy and the Environment (ENEA), a research organisation, has developed a concentrating solar power trough system that allows use of molten salt as HTF. The system uses salt mixture of 60 percent NANO3 and 40 percent KNO3.

Archimede Solar Energy, an Italian company which partnered with ENEA in the testing phase of the technology, has begun producing an advanced solar receiver tube for thermodynamic solar thermal plants with molten salt as HTF under license from ENEA.

According to Archimede, its patented process involves a coating on the tubes that can withstand temperatures of up to 600C and a vacuumed casing that ensures maximum sunlight yield.

The surface coating on the tube is made of a thin film multilayer structure that includes an inferior layer of metal, reflecting in the infrared, and a superior layer of antireflective ceramic material. An anti-reflective coating on the external glass is capable of a solar transmittance higher or equal to 96.5 percent.

The company says that the solar receiver tubes have an absorbance equal or higher than 95 percent, an emissivity lower than 10 percent at 400ºC and 14 percent at 580ºC. The material used to make the tubes is corrosion-resistant.

Archimede is also building a demonstration CSP trough plant that will use molten salt as HTF as well as for heat storage.

Scientists are also trying to develop salt mixtures with lower freezing point. For example, the US Department of Energy’s Sandia National Laboratories are trying to develop new salt mixtures with freezing point below 100ºC. A low freezing point will make it more manageable if used as an HTF in a trough solar field.

In spite of the promising developments in technology, the scepticism will remain until the first commercial scale trough plant is commissioned to successfully demonstrate the use of molten salts effectively.

To respond to this article, please write to:

Rajesh Chhabara: rajesh.chhabara@csrworks.com

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Rikki Stancich: rstancich@gmail.com