In depth: Molten salt-based central receiver technology
While a few dents still need to be knocked out, the overall model is sound and the consensus is that central receiver tower technology has a very big future. CSP Today's Andrew Williams explains why.
Power-tower installations harness sunlight reflected by a field of mirrors, known as heliostats, onto a central receiver. A key advantage of molten salt-based central receivers is that turbine operation is not immediately affected by clouds or high wind speeds.
Moreover, they can dramatically increase productivity by allowing CSP plant operators to store excess heat during sunny periods in molten salt storage tanks, and convert it to electricity at night.
“Due to the high operating temperature, up to 560ºC in the receiver outlet, each kilogram of salt can store three times more energy than in a parabolic trough plant,” explains Santiago Arias, Chief Infrastructure Officer at Torresol Energy.
The high thermal storage capacity of molten salt also results in several operational advantages, including the better management of turbine power and improved asset utilization. The lack of mobile piping systems, swivel joints and thermal oil also reduces the potential for fire or land contamination as a result of leaks and, because fluids are concentrated in a small area, they are also subject to lower levels of thermal loss and maintenance costs.
Further advantages include the fact that the same fluid can be used in the receiver and for heat storage, avoiding the need for a heat exchanger. Because molten salts reach such high temperatures they also enable operators to maximize steam-cycle thermodynamic efficiency.
According to Craig Turchi of the CSP Program at National Renewable Energy Laboratory (NREL), the ability of salt systems to store salt directly, coupled with the large temperature-differential across the receiver, also makes this approach the lowest cost option for thermal energy storage.
“The low vapour pressure of the salt allows the use of thin-walled piping. Nitrate salt is [also] relatively inexpensive, non-flammable and non-toxic,” he says.
New era for central receiver technology
Last month saw the commissioning of Gemasolar, the world’s first commercial scale molten salt-based central receiver - constructed by Torresol, a joint venture between Masdar and Spanish solar company Sener.
The Gemasolar plant features 2650 heliostats, which reflect the light of the sun, concentrating the irradiation of 300,000 sqm of mirrors onto the reduced surface of a receiver.
Molten salts are then pumped to cool the surface ‘hot-spot’ down and stored at a temperature of 565ºC. Once the hot molten salt tank has reached a pre-agreed minimum level, plant operators start pumping salts to the steam generator, at the rate required, to feed a steam-turbine connected to an electrical generator.
“In other words, once we’ve ‘canned’ the sun energy in the thermal storage system, our plant is no longer a solar plant, but rather operates with the reliability and manageability of a thermal power plant,” says Arias.
“The completion of Gemasolar is the biggest news in molten salt towers. This is essentially the same technology as Solar Two, but at larger-scale and designed for commercial use,” says Turchi.
A major technical challenge to be overcome in the further development of molten salt receiver technology is how to demonstrate the durability of the receiver in commercial plants – meaning that more attention must be paid to plant availability issues.
“Receiver efficiency was tested by CIEMAT and SENER and by Sandia in the similar Solar Two demonstration plant, but the efficiency of receiver[s] under commercial operation is also of interest over the next few years,” says Félix Tellez, Head of R&D on High Solar Concentration Technologies at CIEMAT-PSA.
The high freezing point of salt also remains a significant hurdle to future development, but Turchi argues that this could be mitigated if novel salt blends manage to reduce the freezing point while also maintaining low cost, low corrosivity, and good thermal stability.
“Unlike trough plants, the amount of piping in a power tower is not excessive, so one can provide heat-tracing at modest cost,” he says.
Looking ahead, prospects for the continued large-scale commercial and market development of molten salt-based central receiver technology look good. Although only equipped with a 19.9 MW turbine, the Gemasolar facility will produce more than 100 GWh/year due to the huge storage system – also enabling round-the-clock production.
“Gemasolar is a clear breakthrough for the thermo-solar industry,” says Arias. “We are already considering the implementation of a tower plant with a 50 MW turbine, or a cluster of four similar towers sharing some common elements,” he adds.
Outside Spain, Solar Reserve has recently been granted federal approval to begin construction on its US$5 million Crescent Dunes plant in Nye County, Nevada later this year.
“Molten salt power tower technology is probably the leading contender for the future of CSP,” says Turchi.
Although molten salt-based central receivers are still much less widely used than the more common parabolic trough designs, many observers consider the technology to be the future of CSP.
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Image credit: Torresol