Molten Salts Can Melt Down the Price of Concentrating Solar Power-Plus-Storage
An employee runs diagnoses on heliostats at a solar thermal facility in Nevada. Photo by Dennis Schroeder, NREL
Concentrating solar power (CSP) has long held promise as a renewable energy technology.
CSP uses mirrors, or heliostats, to harness the power of the sun by heating and storing
an inexpensive medium such as sand, rocks, or molten salt for on-demand energy dispatch.
To spur CSP industry advancement and achieve an energy cost goal of 5 cents per kWh,
the U.S. Department of Energy’s (DOE’s) Gen3 CSP program funds research to explore the potential of several heat transfer mediums.
National Renewable Energy Laboratory (NREL) researchers are contributing to this effort,
tackling several challenges related to the use of one potential medium—liquid-hot
molten salt—for energy transfer and storage.
Off to the Races: Three Potential Pathways to Cost-Effective CSP
Three years ago, the Gen3 program established three pathways to potentially reach
the CSP energy cost goal: a liquid pathway (exploring use of molten salt as a heat
transfer material, led by NREL), a particle pathway (using sand-like particles as
a heat transfer material, led by Sandia National Laboratories), and a third pathway
exploring the use of gas as heat transfer material (led by Brayton Energy).
In March of 2021, DOE down-selected among the three pathways to fund further research
into particle-based storage, but also created an opportunity for NREL to further develop
the liquid pathway over the next two years.
Molten salt thermal energy storage technology is an efficient, reliable, and cost-effective
way to store solar power at large scale. Photo by Julianne Boden, DOE
Liquid Pathway Research at NREL: Singling Out Salts
Craig Turchi leads thermal energy science and technologies research at NREL. He said that molten
salts are a desirable option for a heat transfer and storage material—liquids are
easy to work with as they can be pumped through pipes and heat exchangers to move
around a CSP system. Unfortunately, some practical challenges also remain, which are
the focus of current NREL research.
Challenge 1: Tackling Tank Design To Keep It Hot
The prototype molten chloride salt tank will be built on the mesa top above NREL’s
Golden, Colorado, campus. Photo by Dennis Schroeder, NREL
While easy to move around, salts are also corrosive to the tanks and pipes that hold
them. In fact, according to Turchi, “Everyone initially thought that salt corrosivity
would torpedo this effort. We actually solved that problem by and large. NREL and
partners did a lot of great science on the salt chemistry—how to purify it, how to
make it relatively noncorrosive if you control the chemistry, and we demonstrated
that in the lab.”
So, corrosivity is not the biggest problem with using molten salts. Instead, the challenge
lies in achieving very high temperatures needed for …….