Improved Solar Energy Storage Materials for Concentrating Solar Power

That’s the ability to store daytime heat energy from the sun for use when the sun is down. CSP can act as "partially dispatchable" power instead of the intermittent and daylight-only power which limits PV’s value and strikes fear in the heart of Independent System Operators (ISOs), who tend to favor more consistent and dispatchable power sources. Amory Lovins of RMI suggests that "baseload" is an outdated term and the new grid will be able to handle all varieties of asynchronous power. But Lovins has yet to assume a role of keeping the lights on — which is the job of the ISO.

CSP uses the sun’s energy, concentrated with mirrors, to heat a fluid and drive a turbine. There are "trough" and the more efficient "power tower" architectures.

Like Symyx, Intermolecular, or Wildcat Discovery, Halotechnics is in the business of high-throughput chemical and materials discovery. Halotechnics is specifically targeting new high-temperature storage materials in the molten salt and molten glass family for CSP storage applications.

The eight-employee startup has screened 20,000 unique materials in molten salt and molten glass and has filed seven patents. The firm is able to run 100 experiments every day in the search for an energy storage medium with optimum characteristics.

"There’s a bottleneck in CSP," according to Raade, "and that’s the efficiency of molten salt heat transfer."

The higher the temperature of the storage medium, the less material needs to be used and the lower the capex of the installation. And CSP needs every penny in savings in order to compete against PV. The chart below from GTM Research clearly shows the impact of storage on the levelized cost of energy (LCOE) of CSP.

Today, molten salt storage is widely used in solar thermal plants in Spain. The “solar salts” are composed of 60 percent sodium nitrate (NaNO3) and 40 percent potassium nitrate (KNO3) — both commonly available materials. Currently, molten salt is used at about 565 degrees C, but Halotechnics is looking at higher temperature salts at 700 degrees C and molten glass at even higher temperatures. The company’s business plan is now a licensing model, but the firm could pivot into becoming a supplier of the materials themselves or of the complete storage systems.

Halotechnics claims that molten glass at a temperature of 1,200 degrees C has the potential to reduce capex costs by a factor of ten when deployed at commercial scale. Raade sees molten glass as "exciting" because "it is dirt cheap." He adds that "everything is cheaper at high temperatures," noting that molten glass is handled by glass manufacturers all the time so the technology is already equipped to handle these temperature extremes.

CSP players and projects include the Gemasolar plant built by Torresol, which has demonstrated 24-hour power in a 19.9-megawatt system, as well as Solar Millennium’s Andasol project, Abengoa, SolarReserve, and the soon-to-be-public BrightSource Energy. SolarReserve has completed construction on the world’s tallest solar power tower, a 540-foot structure in Tonopah, Nevada at the Crescent Dunes Solar Energy Power Plant.

Almost 30,000 tons of molten salt are used in the Andasol plant in Spain, according to Raade. Halotechnics’ next step is a 300-kilogram demonstration system followed by a pilot test with ten tons of material and "more realistic pumps and volumes."

NREL’s Paul Denholm and Mark Mehos, in a November 2011 study (PDF) called "Enabling Greater Penetration of Solar Power via the Use of CSP with Thermal Energy Storage," say this about thermal energy storage (TES):

The use of thermal energy storage in concentrating solar power plants provides one option for increased grid flexibility in two primary ways. First, TES allows shifting of the solar resource to periods of reduced solar output with relatively high efficiency. Second is the inherent flexibility of CSP/TES plants, which offer higher ramp rates and ranges than large thermal plants currently used to meet a large fraction of electric demand. Given the high capacity value of CSP/TES, this technology could potentially replace a fraction of the conventional generator fleet and provide a more flexible generation mix. This could result in greater use of non-dispatchable solar PV and wind meaning CSP and PV may actually be complementary technologies, especially at higher penetrations.

Keely Wachs of BrightSource Energy notes, "The thermal storage sector is really starting to innovate and support higher-temperature steam operations, which translates to even greater cost advantages over other electrical storage technologies." Halotechnics is armed with almost $6 million in grants from ARPA-E, NREL, the NSF, and the DOE.