In Spain this June, a new 19.9-MW concentrated solar power (CSP) tower in Fuentes de Andalucía, Seville, reached the unprecedented milestone of storing thermal energy to its fullest capacity and supply power for an uninterrupted 24-hour period.

Torresol Energy’s Gemasolar solar tower, which started up in April, comprises 2,650 heliostats on 185 hectares (457 acres)—all facing the top of a 140-meter (459-foot) tower, concentrating solar radiation at a ratio of 1,000:1 (Figure 5).

5. Towering over the rest. Torresol Energy said in June that its new 19.9-MW Gemasolar concentrating solar power tower supplied electricity for an uninterrupted 24-hour period. The plant uniquely uses molten heat storage technology. Courtesy: Torresol Energy

The plant is unique in the CSP sector because it is the only commercial-scale solar tower to combine central tower receiver and molten salt heat storage technology—a mode of thermal storage used more prevalently in parabolic trough CSP technology. At Gemasolar, the tower receiver is able to absorb 95% of the radiation from the sun’s spectrum and transmit this energy to the molten salt compound that circulates within the receiver and heats steam to operate a power-generating turbine. The system is capable of 15 hours of electricity production without solar radiation, helping overcome energy fluctuations.

Torresol opted to put molten salt technology in the solar tower because it enables the tower to more easily reach a much higher operating temperature (of more than 500C/932F). “The molten salts… make it possible to generate hotter steam at higher pressures, which significantly boosts the plant’s efficiency,” the company said. In the last weeks of June, “the high performance of the installations coincided with several days of excellent solar radiation, which made it possible for the hot-salt storage tank to reach full capacity,” explained Diego Ramírez, Torresol Energy’s director of production.

The plant’s molten salt technology and engineering work was provided by Spanish engineering and technology firm SENER, which has installed its signature two-tank molten salt storage system at several CSP projects, including Gemasolar. The firm told POWER it is now using research gains made from the dual-tank system—which uses one tank to store cold salts and the other to store hot salts—and is developing a less-costly, more-efficient single-tank storage system with an insulation barrier that naturally moves up and down according to the quantity of salts in each state.

SENER plans to test the system prototype at Torresol’s Valle 2 commercial CSP plant in Cádiz, Spain. At the same time, the firm is looking to validate a graphite thermal storage system, which would be suitable for temperatures of up to 650C and which the company says offers both technical and economic advantages compared with current systems.

Meanwhile, interest continues to grow in solar tower technology—even though Gemasolar is one of a handful of so-called “power towers” that are operational. The first two plants to come online were Abengoa Solar’s 11-MW PS10 and 20-MW PS20, which were commissioned in Spain in 2007 and 2009, respectively. Those plants use water as the working fluid and have a 1-hour storage capacity. eSolar’s 5-MW Sierra SunTower, which began operation in July 2009 in Los Angeles County, Calif., has no storage and uses wet cooling.

A number of solar towers are under development—though most will continue to use water as a working fluid. Among them is Brightsource’s 392-MW Ivanpah solar tower, under construction in San Bernardino County, Calif.—a massive installation that will feature dry cooling, but which has no storage. SolarReserve plans to open its 150-MW Rice Solar Energy Project in Rice, Calif., by 2013. That project will use molten salt (with an expected thermal storage efficiency of 99%) and dry cooling.

—Sonal Patel is POWER’s senior writer.