Industry Insight
CSP thermal storage: Increasing the options
12 March 2010
Cheaper, more efficient technologies and materials may soon eclipse the swift ascension of two-tank molten salt storage systems.
By Emma Clarke in London
CSP plants with thermal energy storage may currently be in short supply, but there is no shortage of experimentation into new forms of storage technology. The next challenge lies in getting these low-cost, high-efficiency technologies commercially deployed.
To date, the most advanced thermal storage technology is the two-tank system using molten salt as the storage medium.
The two-tank indirect system is being deployed in the Andasol 1-3 50MW parabolic trough plants in southern Spain, and is planned for Abengoa Solar’s 280MW Solana plant in Arizona.
In this system, molten salts are stored in two tanks; one hot, the other cold. Salts on the way to the hot tank for storage are heated through a heat exchanger. When it is necessary to recover the thermal energy, salts pass back through the same exchanger transferring the heat to the oil which reaches temperatures of just under 400 degrees Celsius.
A similar version is a two-tank direct system, in which molten salts are used for both the thermal transfer and storage. This means direct storage and higher temperatures are possible and costly heat exchangers can be eliminated.
Torresol is deploying a two-tank direct system for its 17MW Gemasolar central tower plant. This system, designed by SENER, is capable of reaching temperatures above 500 degrees Celsius.
Developer SolarReserve also plans to use a direct two-tank molten salt system from United Technologies in its proposed solar power tower plants.
Molten salt technology is technically-proven and well-understood. But its disadvantages are its high freezing points and high investment costs. Molten salt uses a nitrate salt, a commodity product, which is subject to price volatility.
Studies have suggested that the cost for molten salt storage is in the range of US$30-$40/kWh depending on storage size. However, existing figures for molten salt may not be a true representation of prices, cautions Tom Mancini, CSP Program Manager at Sandia National Laboratories.
Alternatives do exist, but these are still in the research phase or are seeking backing for commercial deployment.
Single tank
One option is a single-tank thermocline storage system; a tank with a hot zone at the top, a transition zone and a cool zone at the bottom.
Single-tank systems using molten salt can significantly lower the cost of storage by replacing some of the salt with a low-cost quartzite rock and sand filler. SunLab reports that thermocline with quartzite costs in the region of US$20/kWh.
According to Mancini, one of the many issues yet to be resolved is how to remove heat from the system without destroying it, thereby causing the thermocline zone to expand and occupy the entire tank.
Sandia National Laboratories (SNL) demonstrated a 2.5-MWhr, thermocline storage system with molten salt and quartzite rock and sand about ten years ago, says Mancini. The US National Renewable Energy Laboratory (NREL) and SNL are now conducting performance modelling and cost analysis of the thermocline system.
Concrete
Another alternative is to use solid materials for heat storage. German civil engineering company, Ed. Züblin and the German Aerospace Centre (DLR) have developed a thermal energy system for parabolic trough plants that uses concrete as the thermal energy storage medium.
Here, the heated transfer medium (oil, water or steam) passes through pipes embedded in the storage concrete to heat it. To discharge thermal energy, the cold transfer fluid flows through the concrete in reverse direction and is heated up. The system operates to temperatures of up to 400 degrees Celsius.
Züblin and DLR have successfully tested a 20m3 concrete test module in Stuttgart using thermal oil as the transfer medium. Another test facility, currently going into operation in Spain, will test direct steam generation.
The primary advantage is that concrete is much cheaper than salt. That said, all the other elements, including tubes, add cost. This means that for a plant the size of Andasol, the technology only comes out slightly cheaper, says Carsten Bahl, senior engineer at Züblin.
However, costs do come down significantly in smaller plants of around 10MW, given that the technology is scalable, whereas two-tank systems are not.
One potential issue with this technology is how to add and remove heat rapidly, says Mancini. The technology might work technically, he says, but getting it to work cost-effectively is another matter. “There is no silver bullet,” he concedes.
Bahl, however, remains confident that with further work, the design could be optimised to reduce costs.
Pebble and air
UK-based HelioDynamics is seeking partnerships to develop its designs for two thermal storage systems: one that provides a more cost-effective solution to molten salt, and the other more efficient.
The first design uses a large sand mass that is interlaced with thinned, low-cost tubes that transfer heat to the sand for storage. By keeping the oil within the steel pipes, the aim is to maintain longer-term stability of the oil.
“You might need twice as much sand as molten salt, but it is considerably cheaper, environmentally benign, and inherently safe since leaks can only be small,” says Graham Ford, CEO of HelioDynamics.
The challenge is making the connections at either end of the steel tubes. “But this is entirely doable,” he adds. HelioDynamics says prices of less than US$ 14/kWh could be achieved.
The second system is a thermocline system using basalt pebbles and argon gas as the transfer medium. This concept can store temperatures of around 550-600 degrees Celsius, which is beyond parabolic tough but suitable for solar power towers or HelioDynamic’s high-temperature linear-Fresnel HDX concept.
“If you really want to push the boat out, you can work at an even higher temperature of 1000 degrees Celsius and run a gas turbine from it. The market isn’t there yet, but this does present an interesting possibility,” he says.
Phase-change materials
A further option is phase change materials (PCMs), which absorb or release heat when they change from solid to liquid and vice versa.
“There is tremendous energy density benefit to phase-change materials,” says Mancini. But the problem, once again, is how to add and remove heat from PCMs at the rate you need.
In 2008, the US Department of Energy Solar Energy Technologies Program (SETP) announced funding for research into and demonstration of a range of technologies including phase-change materials, molten salt and concrete. The goal of this programme is to reduce the cost of thermal energy to less than $15/kW.
Technically, the solutions exist, but getting these solutions into full-scale demonstration or commercial deployment will not be easy.
“The industry is driven by a conservative financing community, so it is difficult to see how these projects will see the light of day unless they are backed by a well-funded government institute that is prepared to put 20 years worth of work into it,” says Ford.
As Mancini notes: “Governments have to provide a leadership role to help determine which technologies will make sense.”
Until that happens, and until market forces drive development of new low-cost thermal storage, molten salt will dominate. “Because of the direction the industry is heading, molten salt is probably the only thing out there at the moment,” says Mancini.
To respond to this article, please write to:
Emma Clarke: emma.jane.clarke@gmail.com
Or write to the editor:
Rikki Stancich: rstancich@gmail.com
