By Andrew Williams

The quest to achieve higher operating temperatures in order to improve power cycle efficiency has prompted the CSP sector to look beyond mineral oils in search of alternative heat transfer fluids.

In recent years, ionic liquids such as molten salts have emerged as a promising solution. However, although they are cheap, readily available and relatively non-corrosive, the fact that most molten salts generally freeze at relatively high temperatures (in the range of 120ºC to 220ºC) means that there is a risk of them freezing in pipelines overnight and negatively affecting plant performance, forcing operators to insulate and heat trace.

In an effort to overcome this limitation, and to improve performance more generally, a number of new ionic liquids are currently under development and testing across the world.  But on closer inspection do these experimental ionic liquids provide any real advantage, and if so, which CSP technologies are best suited to make use of them? 

The major advantage of the new breed of ion liquids is that they do not freeze at temperatures above 100°C, making them stable over the temperature range at which many CSP systems operate.  This large liquid range also means there is less chance of phase transition as a result of vapour formation or freezing, both of which make engineering the systems more difficult, and loss of efficiency due to the phase changes.

The negligible vapour pressure also makes laboratory testing of materials at high temperature considerably easier than other commonly used HTFs.  However, as a result of this, little information is currently available on their behaviour under high pressures.

 “We are only just beginning to get information on these properties,” says Dr Elise B. Fox, Senior Engineer at US-based Savannah River National Laboratory.

“Ionic liquids can easily be designed to be non-corrosive and there are potentially up to 10 to the power of 18 different cation and anion combinations, which allow the ionic liquids properties to be tailored to specific applications.  They also have more heat flux per volume,” she adds.

Cash and flow problems

At present, the largest disadvantage of many of the new ionic liquids is that, unlike molten salts, their cost is currently relatively high.  However, as availability has risen prices have tended to decrease.  While there has been a large growth in the study of these fluids in the past decade, commercial uses are only just coming to fruition.  

“Significant cost savings can be incurred by the removal of hydrogen getter systems that are currently needed with common HTFs,” says Fox.

A further disadvantage is that, depending on the fluid used, there can be considerable viscosity differences.

“We are currently studying the use of eutectic mixtures to lower the viscosities of the fluids, while maintaining desirable properties such as high thermal stability and good heat capacity,” says Fox.

“Corrosion is [also] an issue for mild steel components above 400°C and for stainless steels above 580°C,” says Nathan Siegel a senior member of the technical staff of the Solar Technologies Department at Sandia National Laboratories.

“It may be possible to stretch the maximum use temperature out to 700°C, but this will require new formulations and a controlled atmosphere in contact with the salt to inhibit thermal decomposition,” he adds.

A flexible solution?

In terms of their application in CSP technologies, Fox’s view is that ionic liquids would be used primarily in parabolic troughs and linear Fresnel systems, although they may also have the potential to be ‘usefully integrated’ in tower-based systems.

“Molten salts are the preferred fluid for central receivers (power towers), but may also be deployed in parabolic troughs,” adds Siegel.

In summary, although still at a relatively early stage of development, the new breed of heat transfer fluids outlined above demonstrate a great deal of potential for application in CSP plants.  However, a number of key technical and financial hurdles must still be overcome before they are broadly accepted as a realistic alternative to existing approaches.

As CSP plants continue to operate at ever-higher temperatures in an effort to increase cycle efficiency, it is perhaps likely that the need to transform this potential into workable large-scale solutions will become increasingly pressing. 

To respond to this article, please write to: 

Andrew Williams: awilliams@csptoday.com 

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Rikki Stancich: rstancich@csptoday.com