Technology

As developers strive to increase the upper working temperatures of CSP plants, several research projects are currently investigating the potential use of alternative heat transfer fluids (HTFs), including molten-salt and liquid metal-based materials.

So, what are the key objectives of the projects? What progress has been made so far - and what are the prospects for commercialisation?

By Andrew Williams

Molten Salt
Since the beginning of October, a University of Arizona led project has been studying the composition, properties and costs of new molten-salt-based HTFs. As project leader Peiwen 'Perry' Li, Associate Professor at the Energy and Fuel Cell Laboratory - Department of Aerospace and Mechanical Engineering at the University of Arizona explains, the CSP industry 'needs a low-cost HTF' that is stable in a large temperature range from 200-1200 Celsius.
"The currently available heat transfer fluid, [based on] a nitrate salts eutectic mixture, cannot work at a temperature above 550 Celsius. Therefore, we need to develop the next generation heat transfer fluid. The new salts we develop will have similar properties and cost below one dollar per kilogram," he says.

In rising to this challenge, the key objective of the project, funded by the U.S. DOE 'SunShot' initiative, is to develop a salt heat transfer fluid that can melt at a temperature below 150 Celsius and remain stable at temperatures as high as 1000 Celsius. The project team, which also includes researchers from Arizona State University and the Georgia Institute of Technology, will also 'modify and tune the thermal and transport properties of this fluid for CSP industrial application.'
"A new HTF is expected to be developed [with] all properties tuned to meet the requirements [of a] high performance heat transfer and thermal storage fluid. The salts developed will be commercialized through collaboration with industrial companies," says Li.

Liquid Metal

Also in the U.S., the University of California - Los Angeles is leading a team of researchers from Yale University and the University of California - Berkeley in another SunShot funded project to investigate liquid metals as potential heat transfer fluids with the ability to withstand higher temperatures. Meanwhile, another interesting project at the University of Stellenbosch in South Africa also aims to investigate the potential of using metallic phase change materials as a storage concept to enable high temperature storage, at lower cost and with better heat transfer properties.

As Johan Kotzé, PhD Student at the Solar Thermal Energy Research Group at the University of Stellenbosch explains, the material used is a eutectic Sodium and Potassium alloy (NaK), which has a melting point as low as -12.8 Celsius.

"This solves another very tricky issue of solar power - most of the high temperature heat transfer fluids either solidify at fairly high temperatures, or need to be pressurised at very high pressure. Here is a heat transfer solution that is always liquid, and with fairly low pressures can obtain temperatures exceeding 1200 Celsius," says Kotzé.

"This is obviously easier said than done. NaK is very reactive with water, but so are many other chemicals. To be honest, most of us are used to driving a car filled with highly combustible petrol. This argument cannot be used as proof that NaK is safe, but rather the argument can be made that we can learn how to use this material in a safe manner," he adds.

Experiments are still in progress and, at this stage, a key question remains the suitability of NaK as a metallic phase change storage material. As an alternative, the team are currently looking into the viability of eutectic aluminium-silicon alloy, since it is 'well known,' and it melts at a 'fairly moderate temperature.'

However, in terms of its use as a HTF, Kotzé points out that the results 'look good' thus far and he believes that there is 'no doubt' that NaK will work as a HTF, since it has been 'proven' in the nuclear industry many times. Ultimately, the main aim of the project is the reduction of the LCOE through an increase in thermal efficiency, and, although the team would 'very much like' to commercialise the concept, Kotzé thinks it would require some 'serious investment.'

"I simulated an entire plant in Flownex [simulation software], and the results show that the system is very much feasible. I think that there is still much work to be done on this in order to obtain its full potential, but I believe that we are at a place that it is very much possible to build a system that can at least demonstrate the system," he says.

"I am sure that there are still many teething pains that we will encounter, which will be necessary, but to do so, we will have to build a small scale prototype," he adds.

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