CSP Today

Two innovative technologies to cut the cost of energy storage

By Heba Hashem on May 30, 2014

Currently, adding energy storage to Concentrated Solar Power (CSP) costs about $30 per kilowatt-hour of capacity, contributing between 15 and 20% of the project cost. Can this be reduced?


By Susan Kraemer

CSP produces electricity like any thermal plant, but rather than burning coal or gas to heat fluids for steam-driven turbines; the sun provides that heat. Storing this thermal heat makes it dispatchable on demand, allowing solar generation after dark.

Typically, storage represents about one fifth of the total cost of a CSP project with storage.

But Minnesota-based Terrafore Technologies founder and CTO Anoop Mathur claims to have devised an ingenious way to reduce storage costs.

“The SunShot goal is about $15 by 2022 and we’ve now got it down to about $16,” says Mathur, an ARPA-E exhibitor and speaker with four decades of experience in advanced materials and solar energy research.

Phase Change thermal storage

Mathur has devised a single-tank thermal storage system using encapsulated phase-changing inorganic salt mixtures able to store 50% more energy per unit volume than the current two-tank molten salt thermal storage system.

His company broke through the technology barriers to making high-temperature thermal storage cost effective and practical, by leveraging phase change materials within tiny porous capsules containing various inorganic salt mixtures with different melting points in a single tank.

By layering these pill-sized capsules, with the ones containing the lowest-melting salts at the bottom of the tank, they can cascade phase-change thermal storage from top to bottom.

As the hot fluid from the tower flows down the tank, it transfers its heat and cools. Capsules with high melting points at the top transfer the hottest heat. So as it goes down, the fluid cools. But residual heat can still be extracted by the low melting point salt at the bottom.

While Mathur could stack salts with many different melting points, he found three was best.

“Using three salts, the energy density increased by 52% but with any larger numbers of salts, the maximum we got was 55%,” he explains. “So, economically, three was the trade-off between the benefit of increasing energy density versus the complexity of using more salts.”

Recovering the heat, the flow direction is reversed and pumped up the same tank, so this is an efficient way to both transfer and store the heat – in a single tank.

Cutting-edge technology

Terrafore’s technology was among cutting-edge, cost-reducing technologies showcased at the Peer Review Technology Forum at this month’s SunShot Summit in Anaheim.

Mathur says many researchers have tried and are still trying to increase the energy density of storage by using the phase change in salts. His solution is encapsulation.

But encapsulating salts that can melt at high temperatures is not easy, and they expand when they’re hot. Terrafore’s solution is to encase the salt in capsules with a shell made of a special form of clay, and make a space inside to allow expansion.

“Then a metal coating is deposited to seal off pores in the clay and make the capsules robust,” Mathur explains.

The metal chosen was nickel, because it can be applied “cool” – using low temperature chemical vapour deposition – yet go on to withstand the very high temperatures it is exposed to during use.

Making capsules with voids is key

“We invented this new ‘sacrificial’ method to make these capsules,” he explains. “First we coat – easily sourced – commercial salt prills with a polymer – selected because it decomposes to gases at high temperatures – then add the shell casing of clay.”

“Exposed to heat, the polymer now escapes through the pores in the shell, leaving a void space inside, so the salt can expand and contract as it is heated and cooled.

“Creating this void space is the key”, Mathur points out, as it enables the single tank. On the other hand, encasing the salts in small capsules with room to expand and contract increases the heat transfer area, because the surface area per unit of salt is raised.

“I’ve worked with phase change storage since back in the ‘70s,” says Mathur, a former SunShot awardee. “Many companies have tried using phase change but they encountered the problem with heat exchangers and heat transfer.”

The problem was that when the stored heat was extracted, salts would freeze on the heat exchanger tubes. Researchers tried scrapers, jets and bubbling heat transfer fluid through the molten salt.

Mathur’s solution of sequestering the salts in separate tiny “cages” makes it possible to extract the heat during each cycle without the salts freezing on the heat exchanger.

His capsules have weathered over 5,000 cycles so far, and can withstand over 10,000, but it will take about $3 million to demonstrate the process at scale.

His firm can make billions of these 10mm capsules using the off-the-shelf encapsulation technology used by the pharmaceutical industry, so it requires no novel manufacturing equipment.

Thermal storage in concrete

Another innovation comes from NEST AS, formed in 2011 and about to showcase a 1 MW-hour pilot project with Masdar.

While Terrafore sees power tower technology using molten salts as their ideal CSP market, NEST is banking on a system designed for thermal oil as well as direct steam technology.

“We think we can do thermal storage at lower cost for parabolic trough system projects; the most bankable CSP,” says NEST AS Sales VP, Jarl Pedersen.

“Just for that technology, we have a cost advantage over molten salt because there’s no need to introduce a new heat transfer fluid. We use the heat transfer fluid that’s already in the parabolic trough system, a big advantage for operation and maintenance.”

Domestic content advantages

Using a thermal energy storage system based on integrating the heat exchanger tubes in simple concrete aggregates, the company also creates a cost advantage in that most of the stony material can be sourced locally, which is important for the domestic content rules in many countries.

On a per-gigawatt-hour-of-storage basis, their modular storage system would take up about the same space as the two-tank molten salt storage system, and comprises modular and scaleable boxy units that can be scaled up or down based on storage needs.

“We can build down to maybe a half megawatt hour of storage,” says Pedersen. “We’re really focused though on a megawatt hour and up.”

They have completed prototyping and testing of their columns; for temperature cycling and structural integrity, in Oslo, Norway at the laboratories of Det Norske Veritas (DNV). And now they are ready to do future commercial pilot projects like the Masdar project.

Piggybacking on fossil energy to reduce GHGs

While NEST AS is aiming initially at the traditional CSP trough market, and Terrafore Technology is designed for use with newer power tower technology – both firms see a future in non-CSP markets for their thermal storage.

Thermal storage can be piggybacked onto other thermal energy sources, geothermal, coal, oil or nuclear plants to store waste heat from any industrial process, like cement or paper manufacturing, so it can help reduce the greenhouse gas emissions per kWh of energy supplied by fossil generation.

Also, the electricity transmission grid has a need for ancillary services – large amounts of standby power for several hours, depending on weather, load and geography.

Thermal energy storage can hold this surplus potential energy, to supply the grid when really needed with ancillary services for energy protection and load control on demand.

“CSP developers should be looking at this market,” says Mathur. “A 100MW CSP with maximum possible storage, perhaps 1200MWh electrical equivalent with a 400 MW turbine, and a bid to provide up to 350MW of ancillary or regulation service will be more profitable than just energy storage itself.”

To comment on this article, please contact the author,Susan Kraemer.