By Jason Deign

It has been a busy time for graphite storage. But unless you have been watching closely you would not know it. In 2011 CSP Today reported on a number of research and development initiatives aimed at incorporating graphite storage into CSP.

Since then all of them have made quiet progress, but public announcements have been practically non-existent.

Perhaps the best-known approach is the one being taken by the Spanish CSP developer SENER in the US, with Department of Energy (DoE) SunShot Initiative funding.

SENER did not respond to a request for information from CSP Today, but what is in the public domain is that the company is investigating the use of solid graphite blocks as an efficient and economical thermal energy storage medium.

If successful, the project should be able to store energy at temperatures of more than 800ºC and perhaps as high as 1,650ºC “in a lifespan of 30 years with no parasitic energy consumption,” according to SunShot’s web site.

The site also indicates that challenges for the project include improving the layout of the storage array to cut the amount of piping needed, which currently makes it uneconomical, as well as upping the thermal conductivity and reducing the cost of the graphite used.

A second approach, employed by Graphite Storage of Australia, appears to be closer to commercialisation, but here again details are scarce.

In the last year the company, which has a research facility and pilot plant at Lake Cargelligo, 550 km west of Sydney, has launched a product called G1-SSR, which it calls “a simple and robust solar super-heater and storage receiver.”

Graphite components

Nick Bain, the company’s chief executive, confirms graphite “is very much a component” of the G1-SSR, but adds: “We won’t be talking publicly about it. We only really disclose what’s going on with commercial parties.

“We are in commercial negotiations on a number of transactions and we don’t need to publicise what we’re doing simply because we can deal with the market on a bi-lateral basis. I’m not sure how much publicity we will seek.”

Finally, what of one other graphite-based storage approach reported on in 2011: the use of graphite nanoparticles to improve the specific heat capacity of molten salts? The project was one of two being carried out with DoE funding at Texas A&M University.

The University’s Dr Debjyoti Banerjee says it has led down a path that may ultimately make graphite redundant as a storage material. “In addition to graphites and carbon nanotubes we also looked at ceramic nanoparticles, which are substantially cheaper,” he says.

“We found that with ceramic nanoparticles you can get similar levels of enhancement in the thermal storage capacity of molten salts at approximately similar concentrations, somewhere between 0.1 to 1% concentration.”

In addition, following a Department of Energy suggestion to investigate ultra-high temperature storage media, Dr Banerjee’s team has investigated the use of chloride salts as an alternative to existing nitrate compounds used for CSP.

These could enable heat storage beyond 1,000ºC, particularly when combined with ceramic nanoparticles.

Current measuring devices cannot work at these temperatures, but the Texas A&M team was able to confirm that the specific heat capacity of chloride salts were further enhanced by the addition of ceramic nanoparticles at the 700ºC limit of its measurement systems.

Future designs

“That's a very promising development which means this technology could be developed not only for conventional solar thermal power plants that are out there right now, but also the future designs that are being explored,” Banerjee says.

Chloride salts are, however, highly corrosive, so their use in a commercial setting would be dependent on the development of resistant pipes and containers. Such casings would also need to withstand the very high temperatures being contemplated.

This points to a continuing role for graphite compounds within CSP thermal energy storage systems.

If developers go above 1000ºC with heat storage they will probably have to use some kind of graphitic material or fire bricks for pipes and fluid-handling devices, because these are among the only compounds that can be stable at that temperature, while providing corrosion resistance.

“You might see pipes being made out of graphite blocks if the DoE or the industry wants to get to that temperature range,” Banerjee says.

Within the salt itself, he adds, there could be some scope for using graphite nanoparticles for specific heat enhancement, although the substance would be more susceptible to oxidation than ceramic alternatives.

That does not mean graphite nanoparticles cannot be applied with existing salt mixtures, though.

Even at a 1% concentration, Banerjee notes, a 30,000 metric tonnage storage tank would require 300 tons of nanoparticles, making either graphite or ceramic compounds the best candidates from a cost-efficiency perspective.

All this means it could be worth staying tuned to more news about graphite. If you can find anyone who is broadcasting, of course.

To respond to this article, please write to Jason Deign

Or contact the editor, Jennifer Muirhead