Using phase-change materials for thermal storage in CSP is not a commercial possibility yet. But one team remains determined to bring it to market.
By Jason Deign in Barcelona
Endesa’s coal-fired power station at Carboneras, in Almería, Spain, was once considered one of the biggest polluters in Spain. So it is perhaps fitting that in recent years it has also been the site of a project that could have important repercussions in getting clean energy from the sun.
In May 2010, Enel-owned Endesa began building the world’s largest phase-change material (PCM) pilot at Carboneras, in association with the German Aerospace Center (DLR), the Solar Millennium subsidiary Milenio Solar, Flagsol, SCHOTT, Senior Berghöfer GmbH and MAN.
The 700 kWh demonstration plant was intended to show whether PCM could become a viable alternative to molten salt as a thermal storage medium for CSP.
In theory, PCM, which is where a material stores and releases energy when changing between its solid, liquid or gaseous states, holds much promise.
According to a paper on the subject by the US National Renewable Energy Laboratory: “Systems using PCM are useful because of their ability to charge and discharge a large amount of heat from a small mass at constant temperature during a phase transformation.
“Because high-melting-point PCMs have large energy densities, their use can reduce energy storage equipment and containment costs by decreasing the size of the storage unit.”
So, in essence, PCMs could help reduce CSP-with-storage costs by doing away with the need for bulky tanks filled with masses of molten salt, which is the standard for present-day plants such as Gemasolar.
The two technologies are not strictly comparable, though, says Doerte Laing, head of the Thermal Process Technology Department at DLR. “It is a completely different type of storage,” she says.
“You can’t compare it with the molten salt storage because a phase-change storage you would only use when you have a two-phase heat transfer fluid. This kind of storage is used when you want to use it for direct steam generation, for example.”
Phase-change storage is particularly suited to direct steam generation because about 65% to 70% of the energy in the PCM is locked up in its condensation or evaporation phase, she notes.
Laing says PCM could be combined with molten salt for pre-heating or superheating but if you were to rely on the latter alone for direct steam generation you would need about five times the storage volume.
DLR tested the potential of PCM at Carboneras using 8.4m3 of sodium nitrate with a boiling point of 306°C, linked to a 300 kWh super-heater module with 22m3 of concrete.
Sources close to the project have confirmed it has now finished, but it appears to have been a success in resolving a major challenge for PCM. According to Laing: “With phase-change storage you use nitrate salts and they have very low conductivity.
“That means if you just have your heat exchange tube inside the material and you want to discharge the storage then the power drops tremendously during the discharge because an insulation layer forms around the pipe.”
At Carboneras, DLR was able to get around this problem by adding fins to the exchange tube, increasing the heat transfer surface area. “With these kinds of fins we got a very high performance and we could run at high power levels,” states Laing.
In the US, a company called Terrafore has taken a different tack in solving the problem. It is looking at giving heat exchangers a coating that discourages the adhesion of freezing salt, and encapsulating the sodium nitrate within tiny beads of salt to increase its surface area.
“The work on encapsulated PCM is going very well,” reports Anoop Mathur, Terrafore’s chief technology officer. “One of the main challenges with encapsulating PCM is to create a void in the capsule during its manufacture.
“After many experiments we were able to successfully create a void in the capsule. It is exciting.”
The company is using a standard industrial method for encapsulation, but is currently grappling with how to carry it out at high temperatures. “The next step is to test for robustness to thermal shock and to thousands of thermal cycles between 300°C to 600°C,” Mathur says.
“This will be followed by scale-up to make sizable quantities in a commercial machine. To do this, we need to ensure the recipe is robust to variations in chemical composition and process parameters.”
The work should take a few months, and then there will be “at least one year of hard engineering work to scale-up to very large quantity production,” says Mathur.
Meanwhile DLR is pushing ahead with a new PCM project, this time with a German partner, although there again the likely timescale before commercialisation is a number of years. “The next step is we continue this development with industry,” says Laing.
So it seems PCM could still change the face of CSP… but not just yet.
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