How do solar thermal power plants generate electricity?

Solar thermal power plants, also referred to as concentrating solar power (CSP) plants, utilize mirrors to focus the sun's energy onto a receiver where a fluid is heated and used to drive a generator. There are three main types of CSP systems as described in the Video 1:

• Linear concentrator systems collect the sun's energy using long rectangular, curved mirrors which focus sunlight on tubes that run the length of the mirrors. This heats a fluid flowing through the tubes which is used to boil water in a conventional steam-turbine generator to produce electricity. (See Figure 1).
  
• Dish/engine systems use a mirrored dish similar to a very large satellite dish. The dish-shaped surface directs and concentrates sunlight onto a thermal receiver, which absorbs and collects the heat and transfers it to the engine generator. The most common type of heat engine used today in dish/engine systems is the Stirling engine. This system uses the fluid heated by the receiver to move pistons and create mechanical power. The mechanical power is then used to run a generator or alternator to produce electricity. (See Figure 2.)
  
• Power tower systems use a large field of flat, sun-tracking mirrors known as heliostats to focus and concentrate sunlight onto a receiver on the top of a centrally located tower. The receiver converts the solar energy to heat. The heat is used to boil water to create steam which is then used to power a conventional steam generator and generate electricity. A variation of the technology utilizes molten nitrate salt as the heating fluid because of its superior heat-transfer and energy-storage capabilities. The molton salt can be used to boil water and generate electricity immediately, or it can be stored for later use. The ability to store energy on a large scale allows the system to continue to generate electricity during cloudy weather or at night. It also allows the solar plant to be used as a dispatch resource for the electrical grid. Thus, when the grid operator must quickly ramp-up power to handle a problem, the solar plant can be used rather than a traditional peaker gas plant. See OWOE: How do solar thermal power plants generate electricity at night?.

The Ivanpah Solar Electric Generating System, located in California in the Mojave Desert, is the largest CSP plant in the US and second largest in the world. It consists of three fields of heliostats (for a total of 173,500 mirrors) and three 459-foot tall towers and can generate 392 MW of electricity utilizing water as the heat absorbing fluid. (See Video 2.) The 110 MW Crescent Dunes Solar Energy Project located near Tonopah, Nevada, was the first CSP plant built to incorporate a molten salt storage system and has the capacity to story up to 10 hours of electricity. (See Video 3.) The Noor Quarzazate power station in Morocco is the largest CSP plant in the world with a total of 510 MW capacity and also utilizes molten salt. It has the ability to store up to 8 hours of electricity.

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