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Non-nuclear energy

Introduction to Concentrated Solar Power

Fission and radiation protection

What is Concentrated Solar Power (CSP)?

Heliostat, a sun tracking mirror for the central power system (Courtesy of Plataforma Solar de Almeria, Spain)
Concentrated Solar Power systems focus direct solar radiation through optical devices onto an area where a receiver is located. The radiation is transformed into heat. The first recorded use of such a technique was in 212 BC when Archimedes is said to have used mirrors to burn Roman ships attacking Syracuse. Nowadays the only applications are peaceful and they provide either heat or electricity.

What are the main components of a CSP system?

  • The solar field
    This is the reflective area and is quoted in square metres. It is used to collect solar radiation. The surface area quoted is not the land use area.
  • The solar collector element
    This is the largest of the solar collecting components. It may consist either of a single item or an assembly of elements.
  • The solar receiver
    This is the component in which solar radiation is converted into heat.
  • The balance of plant
    This covers all the elements and installations necessary for a CSP system, but not those related to solar conversion.

What are the main concepts?

CSP systems need direct sunlight because they concentrate the solar radiation. Four concepts are being developed:

  • The parabolic trough

Diagram of a parabolic trough

Parabolic trough collectors producing directly superheated steam (Courtesy of DISS - 2)

In this concept the solar field is the assembly of the collector element and the receiver. In a standard electricity generation system the solar collector assembly has a total length of around 150m. It consists of several solar collectors connected to a single axis sun tracking system. The solar receiver is a black-coated, vacuum glass tube containing a heat transfer fluid. The coating has a high radiation absorption coefficient and a low reflectivity.

  • The parabolic dish

Diagram of a parabolic dish
Parabolic dish with a Stirling engine operating under solar conditions (Courtesy of Eurodish project)

The dish itself is the solar collector. The surface of the dish is covered by mirrors which reflect light into the receiver. This receiver can be an engine, vessel or box located at the focal point of the parabola. In the case of an engine fluid is heated to 750°C and electricity is generated.

  • The central tower

Diagram of a central tower
Central Tower installation (Courtesy of Plataforma Solar de Almera, Spain

In a central tower installation, an array of heliostats (sun tracking mirrors) acts as the solar collector, concentrating solar radiation onto a central receiver located at the top of a tower. More complex structures are used to heat air or transfer fluid which in turn is used for electricity generation. A solar reactor can also be used to produce hydrogen or other chemicals.

  • The solar furnace

Diagram of a solar furnace
1MWth solar furnace (Courtesy of Odeillo, France 

This concept works in a similar way to the central tower, with heliostats reflecting sunlight onto a concave mirror, which focuses the light onto the furnace situated at the focal point of the mirror. High temperature reactions and the heat resistance of materials can be tested.

What is the principle of concentrated solar power operation?

Solar radiation reaching the collector's reflecting surface is focused onto a receiver. Therefore only direct radiation onto the reflecting surface is useful for the process. The types of receiver are:

  • A tube receiver, where radiation is absorbed by the tube's surface. This heats a fluid (water or oil) inside. The heated fluid in turn can be used to produce steam which powers a turbine to generate electricity.
  • A Stirling Engine, which is attached to a helium-filled cavity receiver. Thermal energy is converted into mechanical energy which in turn is converted into electricity.
  • A central tower, where the radiation heats air or water contained in a volumetric receiver. The installation of a solar reactor enables high temperature (>800 degrees C) chemical processes to take place.
  • A solar furnace, which can be used for testing small scale solar reactors and the heat resistance of materials.

What are the current uses of CSP technology?

Solar chemical reactor under solar operation, light filtered to show the reactor
  • As an energy source for the 'post-fossil fuel era'.
  • Parabolic troughs have been built and operated since the mid-1980s. A total 354MWe of installed capacity is operating commercially in the Californian desert.
  • National and international investment in research, development, demonstration and dissemination has resulted in important technical improvements. Electricity generation costs using current technology are estimated at around 0.12€/kWh for future plants. This has opened up opportunities for pre-commercial demonstration projects world-wide.
  • Current technology is focused mainly on centralised electricity generation with module sizes of between 20 to 200MWe. However, current developments could result in opening market opportunities for smaller size modules. These could be used for decentralised generation in industrial heat process applications, and for chemical processes such as metal, hydrogen or cement production.