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Central Receiver

Central receiver (or power tower) systems use a field of distributed mirrors – heliostats – that individually track the sun and focus the sunlight on 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, power 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 – around 550°C. 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.

Crescent-Dunes

Now with the technology proven, there are now some landmark operational projects running in the world. At present, there are only three commercial size power plants of this type in Spain, notability the Sanlucar Solar Park, with the operating PS-10 solar tower of 11 MW and the PS-20 of a 20 MW capacity; and the 20 MW Gemasolar with a molten salt heat storage. In the United States, a few larger projects are currently operating and under construction in California and Nevada. The largest one is Ivanpah Solar Electric Generating System (ISEGS), with a gross total of 392 MW of solar power, which began commercial operation in 2013. 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 CSP plants using solar tower technology, with molten salt thermal energy storage. The plant currently in commissioning phrase is the first utility-scale facility in the world to feature advanced molten salt power tower energy storage capabilities. The project will 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 80s and 90s 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.

Case Study: PS10 and 20 – worlds first commercial solar towers

Case Study: PS10 and 20 – worlds first commercial solar towers
The last Greenpeace CSP report detailed the project PS-10 which was an 11 MW solar tower installation with a central receiver. This plant is now in full operation and the developers, Abengoa, have progressed to building PS-20 which is twice as big. Both plants have thermal storage that allows full production for 30 minutes even after the sun goes down. Thermal storage is this case is used to boost power production under low radiation conditions. Additionally, the PS10 can use natural gas for 12-15% of its electrical production. The PS10 generates 24.3 GWh per year of clean energy, which is enough to supply 5,500 households. The PS10 solar field is composed of 624 Sanlúcar heliostats; the entire field has 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 PS-20 is built is the same location, the Plataforma Solar de Sanlucar la Mayor in Southern Spain. Working in the same way, the PS-20 will add electricity supply for another 12,000 homes to the operations. The PS-20 solar field has 1,255 heliostats and tower of 160 m.

Source: Abengoa Website

PS-10
PS20

Case Study: Invanpah – the biggest CSP power in the world

A bright prospect for tower technology lies with BrightSource Energy, partnered with NRG, google, Bechtel and Department of Energy of the U.S., has developed a high-temperature, high-efficiency decentralized tower technology. This CSP plant with a total of 392MW of electric generating capacity in Ivanpah, Nevada using its Distributed Power Tower (DPT) technology at a cost of approximately $4500/kW. The company has set up Luz II, a wholly-owned subsidiary of BrightSource Energy, who were responsible for the 1980s development of SEGS, for their technology development. The Ivanpah plant generates around 1079 GWh per year reliable solar electricity to serve more than 140,000 homes with clean energy and avoid 400,000 metric tons of CO2 per year in California. Located in Ivanpah Dry Lake, California, the three-unit power system is built on approx. 3,500 acres and created more than 2,600 jobs for construction workers and staff.

Source: Brightsource Energy

ivanpah