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STE/CSP Technologies > Central Receiver

Central Receivers - Crescent Dunes

Central Receivers - Crescent Dunes

Photo Credit: ©SolarReserve

Central receiver (or solar tower) systems use a field of distributed mirrors – heliostats – that individually track the sun and focus the sunlight on a receiver at the top of a tower. By concentrating the sunlight 600–1000 times, they achieve temperatures from 800°C to well over 1000°C. The solar energy is absorbed by a working fluid and then used to generate steam to power a conventional turbine. In over 15 years of experiments worldwide, solar tower plants have proven to be technically feasible in projects using different heat transfer media (steam, air and molten salts) in the thermal cycle and with different heliostat designs.

The high temperatures available in solar towers can be used not only to drive steam cycles, but also for gas turbines and combined cycle systems. Such systems can achieve up to 35% peak and 25% annual solar electric efficiency when coupled to a combined cycle power plant.

The efficiency of these plants is usually better than parabolic trough plants, because fluid temperatures are higher. This leads to better thermodynamic performance and it also facilitates storage: smaller volumes are possible because of the higher temperature difference between the cold and the hot tanks.

With the technology proven, there are now several landmark projects currently operating in the world. Three commercial size power plants of this type are located in Spain, notability the Solucar Solar Complex, with the operating PS-10 solar tower of 11 MW; the PS-20 with a 20 MW capacity and 0.5 hours of steam storage; and the 20  MW  Gemasolar  with  a  molten salt heat storage. In the United States, a few larger projects are currently operating or under construction in California and Nevada. The largest one is Ivanpah Solar Electric Generating System, which began commercial operation in 2013. At 392 MW, it is by far the biggest solar power plant in the world. A 110 MW Crescent Dunes Solar Energy plant located in Nevada is another solar thermal power plant using solar tower technology, with molten salt thermal energy storage. The plant currently under construction features advanced molten salt power tower energy storage capabilities. The project is expected by early next year to deliver enough firm, reliable electricity from solar energy to power 75,000 homes in Nevada during peak demand periods, day and night, whether or not the sun is shining.

Early test plants were built in the 1980s and 1990s in Europe and USA. These included SOLGATE which heated pressurised air; Solar II in California that used molten salt as heat transfer fluid and as the thermal storage medium for night time operation; and the GAST project in Spain that used metallic and ceramic tube panels. The concept of a volumetric receiver was developed in the 1990s within the PHOEBUS project, using a wire mesh directly exposed to the incident radiation and cooled by air flow. This receiver achieved 800°C and was used to operate a 1 MW steam cycle.

Summary

Central receiver plants collect the solar energy concentrated by the heliostat field in a central receiver mounted at the top of the tower. There have been several alternative heat transfer fluids which have been successfully tested until now: saturated steam, superheated steam, molten salt and pressurized atmospheric air.

The first commercial central receiver plant was commissioned in June 2007 in Spain (11 MW) and two other ones are now operating (50MW in total). The typical size for a tower plant so far varies between 10 to 150 MW.

  • Concentrates solar radiation on a point receiver at the top of a tower
  • Enables operation at high temperature level (550ºC over heated steam and 565ºC for molten salts; working with pressurized air at temperatures of up to  1000ºC are under study)  and provides heat storage capabilities
  • Has high net solar to electrical efficiency and is a commercially proven technology
  • Size: 10 to 150 MW
  • Demo plants built in the 80’s
  • Three commercial plants in operation in Spain (11 ,20 MW and 17 MW with 15h of storage) and 1 more under construction (17 MW + 15h storage)
  • Solar energy to electric power performance: Design point 20%. Annual 17%.
  • Larger projects under construction in the USA.
  • Prices of PPA that will make projects feasible: between 14 and 18 c€/kWh depending on the level of solar radiation, size of the plant, capacity factor, financial conditions, etc.
Case Study

The The Solucar Complex in Seville is home to the world’s first solar towers, PS10 and PS20. PS10 is an 11 MW plant with a central receiver. Its solar field is comprised of 624 Solucar heliostats, covering an area of 75,000 m2. Each heliostat tracks the sun on two axes and concentrates the radiation onto a receiver located on tower that is 115 m tall. The receiver converts 92% of received solar energy into steam. The plant generates enough electricity to power 5,500 households.

The PS20 is twice as big and was constructed after PS10 began operating. PS20 works in the same way as its predecessor, with a solar field of 1,255 heliostats and a tower of 160 meters. The plant can power 12,000 homes with the electricity it produces.

Both plants have thermal storage that allows for 30 minutes of full production even after the sun goes down. Thermal storage in this case is used to boost power production under low radiation conditions. Additionally, the PS plants can use natural gas for 12% -15% of their electrical production.

Source: Abengoa

Gemasolar is the first commercial scale plant in the world to combine central tower receiver and molten salt heat storage technology. The plant has been operational since May 2011. The importance of this plant lies in its technological uniqueness, as it has paved the way for other plants of this type, such as the recently commissioned Tonopah plant and the upcoming new Noor 3 pant in Morocco.

Gemasolar, with its 20 MW installed capacity, can supply 110 GWh per year. It can produce electricity about 6,400 hours per year – a capacity factor of 75%. The plant provides clean, safe power to 25,000 homes and reduces CO2  emissions by more than 30,000 tons a year. The molten salt storage tank permits independent electrical generation for up to 15 hours without any solar feed. In the summer of 2013, the plant achieved continuous production, operating 24 hours per day for 36 consecutive days, a result which no other solar plant has attained so far.

Gemasolar’s power tower has a height of 140 meters. The receiver on top of the tower is like a radiator that is heated to a temperature of about 565oC by the sunlight reflected by 2,650 heliostats with a total reflective surface of about 300,000 m2.

Source: SENER / TORRESOL Energy