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.