Graphite thermal storage: Coming soon to a CSP project near you?

A handful of developers and research groups are looking into graphite as an alternative to traditional storage materials for CSP. Could they be on to something?

By Jason Deign in Barcelona

The quest for information about graphite storage gets off to a bad start with SENER.

Yes, confirms Mercedes Sierra, chief executive at SENER Engineering and Services, the company is working with graphite maker GrafTech International and the University of California at Berkeley on a high-efficiency thermal storage system for solar plants.

But she will not be talking publicly about it until the first phase of the project is complete in September this year.

This somewhat limits the amount of information available on graphite storage, given that Nick Bain, chief executive of Australian developer Graphite Energy that heads up the only other commercially-led graphite storage project, is also cagey about giving too much away.

“Whilst we can make public certain descriptions it’s very early days for the technology and the product is not ready for widespread publicity at the moment,” he cautions.

This does not bode well for a substance that Bain says his company, which in 2009 took over another graphite storage developer, Lloyd Energy Systems, has been investigating for a decade, with a focus on CSP in the last five to six years. However, the effort could yet be worth it.

Increased efficiency

The upper operating temperature limit for molten salt is around 565 to 570°C in practical terms, whereas, as Sierra explained to CSP Today last year, graphite can be heated to thousands of degrees, which could greatly increase the efficiency of CSP.

Furthermore, only a few suppliers can offer molten salt of the quality required by the industry, which has increased costs to the point where the storage material is a significant expense for operators. Graphite, on the other hand, is comparatively plentiful and cheap. 

Until the research teams open up, however, how it will be used in CSP remains unclear.

As of last July, the SENER team, backed by a USD$1.15 million grant from the US Department of Energy, was known to be looking at specially designed solid blocks of graphite located at the base of solar towers; how heat would be transferred to and from them had not been finalised.

Sierra says “it is not the same concept” as the one being worked on by Graphite Energy, which is planning a 3MW pilot with private funding in Australia. “How long after that we are likely to supply product, we are not quite sure. It’s early days on the testing,” says Bain.

“We see our market as selling the product into solar thermal power stations: linear Fresnel, trough and even tower projects. We think it’s got good potential for the highest performing steam turbines.

Steam quality

“At the moment the industry is somewhat hamstrung because of the quality of steam to the turbine.”

Bain makes no bones about the challenges graphite faces. “It will require a lot of fine tuning. The concept is simple, but the deployment and implementation of the product is complex. Saying otherwise is somewhat naïve.”

And even if Graphite Energy or SENER can come up with a marketable product, “the more interesting thing is finding people who will buy it,” Bain remarks.

Nathaniel Bullard, lead analyst for North American solar at Bloomberg New Energy Finance, agrees: “It’s true that there’s potential for a new storage medium but there’s a lot of time and energy invested in the incumbent, which is molten salt.

“The difficulty of integrating a new critical aspect of the heat cycle is it will need a lot of guarantees from vendors.”

In the event, though, there might be a way to improve CSP storage with graphite without having to completely throw out current heat exchange designs.

Graphite nanoparticles

Dr Debjyoti Banerjee, of Texas A&M University, has found that adding just 0.1% to 0.5% of nanoparticles—solid particles with a nominal size of between one and 100 nanometres—to molten salt will improve its specific heat capacity by up to 100%.

One of his students, Darren Malik, did a parametric estimate on adding nanoparticles to CSP storage fluids and found they could reduce the cost of solar thermal power by around 50%, making it comparable to coal at around $0.15 a kilowatt hour.

Graphite is just one of the materials that that nanoparticles could be made of, and might not turn out to be the best one to use with molten salt.

Banerjee estimates it could take several research teams, and a budget of several tens of millions of dollars, to find out which nanoparticles work best with particular storage materials. “We are at the Wright brothers stage right now and we need to get to the Boeing 747,” he says.

“But in CSP, storage is 60% to 70% of the total cost. If you can retrofit existing systems just by adding nanoparticles, if you can come up with a pixie dust you can just put in to improve performance, it’s pretty well justified.”

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