Molten salt: The magic ingredient

6 November 2009

A CSP plant's fundamentals can be vastly improved by using molten salt as both the heat transfer fluid and storage medium. But if trough technology is to benefit, changes will be needed.

By Toby Price

Molten salt has everything going for it- it is highly efficient as a heat transfer and storage medium; it reduces balance of plant costs; and it is environmentally friendly. While a few kinks have yet to be ironed out - namely molten salt’s high freezing temperature -innovations in the pipeline may well result in molten salt becoming the future heat transfer fluid and storage medium of choice.

In solar applications, molten salt is used for a number of practical reasons. "Molten salt is a heat storage medium that retains thermal energy very effectively over time and operates at temperatures greater than 565ºC, which matches well with the most efficient steam turbines. Second, it remains in a liquid state throughout the plant's operating regime, which will improve long-term reliability and reduce O&M costs. And third, it's totally 'green'. Molten salt is a non-toxic, readily available material," explains Terry Murphy, Chief Executive Officer of SolarReserve

An additional advantage offered by molten salt is that it is a fraction of the cost of traditional heat transfer fluids (HTF), such as synthetic oils, which can cost up to ten times more.  

Finally, unlike other media such as synthetic oils and steam, molten salt is not only an excellent HTF but is perfect for thermal storage, enabling CSP plant owners to simplify systems, raise efficiency, generate dispatchable electricity and, ultimately, reduced levelised energy costs, (the cost of an energy-generating system including all costs over its lifetime). 

Boosting thermodynamic efficiency 

While power towers have employed molten salt as an HTF for some time, it is still being tested for use in parabolic trough plants, with oil being the preferred medium to date.  

Molten salt offers a number of advantages over oil. Firstly, oil has a maximum temperature of about 400°C, which limits the conversion efficiency of the turbine cycle. Molten salt, on the other hand, can be heated to a higher point allowing high-energy steam to be generated at utility-standard temperatures. Consequently, high thermodynamic cycle efficiencies of approximately 40 percent can be reached in modern steam turbine systems.  

In addition, using oil requires an additional heat exchanger to transfer the energy from the working fluid to storage, which increases capital costs. “This causes a loss in cycle efficiency of up to 7 percent,” explains Julie Way, Development Director at SolarReserve. 

Ulf Herrmann, Director of Thermal Process Technology at the German Aerospace Centre (DLR), has found that using molten salt in a parabolic trough solar field improves overall system performance by 3-7 percent, although he warns: “Salt only makes sense if operating temperatures over 400°C are feasible”.  

Molten salt does have some drawbacks. More expensive materials are required to avoid salt corrosion resulting in higher systems maintenance costs. Furthermore, because salt has a higher freezing point than other media, additional energy -equivalent to approximately 4 percent of collected solar energy - is required to prevent it from freezing at night. 

However, after factoring in these costs Herrmann concludes that molten salt can still cut LEC by between 10–15 percent. Similarly, Nikolaus Hurt, spokesperson for German-based CSP technology developer Solar Millennium, considers molten salt to be “one of the most promising solutions for driving down costs in the near future”. 

The thermal storage advantage

In addition to its application as an HTF, molten salt can be used as a heat storage medium.  “Energy storage is a key issue for successful implementation of CSP technology,” notes Rainer Tamme, Head of Thermal Process Technology at DLR. It enables plants to generate dispatchable solar electricity, making CSP significantly more attractive than other more intermittent renewables.  

While constructing heat storage facilities may be costly, the business case for such an investment is indisputable. Tamme explains that 2 hours of storage can decrease LEC by 10 percent, while 12 hours can reduce LEC by over 20 percent.

Plant owners investing in this technology can also sell their electricity for up to twice the rate during peak hours as during non-peak times, which is a big incentive. In short, dispatchable power is worth much more on the market. 

Molten salt also offers logistical advantages. Julie Way explains: “A trough facility that can only achieve a hot working fluid temperature of 400ºC will require approximately 3 times the thermal storage volume to generate a given amount of electricity as an integrated thermal storage system which stores energy at 565°C.”  Molten salt provides this opportunity and David Kearney of Kearney & Associates has calculated that thermal storage costs can be cut by 65 percent if molten salt is employed. 

“LEC drops because use of the highly efficient (99 percent) and low cost (20 times less than batteries) energy storage system improves the economic utilisation of the plant capital equipment and O&M crew”, says Greg Kolb of Sandia’s Solar Thermelectric Technology Division. 

Finally, when molten salt is used for both thermal transfer and storage, direct storage is possible, avoiding the need for an extra heat exchanger. A single tank thermocline storage system can also be employed, resulting in a substantially lower cost storage system.  

As Doug Brosseau from Sandia National Laboratories put it: “Thermal storage is good; thermal storage with molten salt is good; direct thermal storage with molten salt is better; thermocline direct thermal storage is best”. 

Advantages for trough technology?

With these advantages in mind, power tower developers have been investing in molten salt thermal transfer and storage technology for some time. Now the focus is turning to how molten salt can be used in parabolic trough plants.  

The high freezing point of molten salt has historically restricted the use of molten salt in trough facilities and a significant amount of effort is now being made by research bodies such as Sandia National Laboratories to develop new salt mixtures with freeze points below 100°C, where the freeze problem is expected to be much more manageable. 

On the components side, receivers manufacturer, Schott, has begun developing a new generation of receivers able to handle molten salt’s corrosiveness and higher operating temperature.

Solar Millenium is also conducting research into this area. According to Hurt, the company’s research and development into applications for molten salt is "progressing as planned”. 

Dr Arnold Leitner, president of Skyfuel Inc. aims to make molten salt HTF the standard in parabolic trough plants through a hybrid parabolic trough/compact linear Fresnel collector configuration called a ‘linear power tower’. By increasing the diameter of the receiver and focusing sunlight 85:1, he hopes to allow the salt to stay molten in the solar field, enabling the use of direct molten salt storage with a variant of parabolic troughs.

However, while the Italian research laboratory, ENEA, has proven that using molten salt in a parabolic trough solar field is technically feasible, further work is required if it is to become the standard HTF in parabolic trough plants.