What is a solar thermal power plant?

It is a solar thermal power plant that uses concentrating optics to focus solar radiation on a focal point or focal line on an absorber which is converted into thermal energy. The biggest advantage of high-temperature solar power plants is that the heat energy generated can be stored for several hours and recalled at any time.

 

There are two different systems: 

  • Solar farm power plant
  • Solar tower power plant

Solar-farm power plants are usually made up of many individual parabolic trough collectors, where the heat energy generated is transferred to the steam turbines via a very long pipeline network. The concentration ratio here is C= 60-80 suns. The power generators are driven by steam turbines and in this way electric current is generated. It is the same process as in conventional coal-fired power plants.

 

 

Picture: Shutterstock
Picture: Shutterstock

In addition to the advantages mentioned above, parabolic trough power plants also have many disadvantages.

 

  • The mirror surfaces are very susceptible to soiling by dust and sand 
  • The wind load can lead to orientation errors in the optics.
  • The end result is losses at the absorber (reflection, radiation, convection)
  • The absorber tubes are cooled by powerful wind.
  • However, the highest losses of about 45% occur in the collector field. This is because heat energy is quickly lost in kilometer-long pipelines.
  • Related to 100% of the solar radiation, losses of approx. 37% occur during the steam-driven thermodynamic cycle process.

 

 The first power plant of this type was patented in 1912 and put into industrial operation in Egypt in 1913.

 

Another version of solar farm power plants is dish power plant. These paraboloid collectors have a Stirling engine at the focal point, which converts the thermal energy directly into mechanical energy that generates the electricity. Since the parabolic shape of the collectors can concentrate much more solar energy, the concentration ratio is C=1000 suns thanks to the biaxial tracking system.

Picture: Shutterstock
Picture: Shutterstock

Similar to parabolic troughs, wind, dust and sand cause significant optical losses in these power plants.

 

Solar tower plant. Unlike the two solar power plants mentioned above, the tower power plant consists of a field of several thousand mirrors, which focuses the concentrated solar radiation onto an absorber at the top of the tower.

Temperatures of around 1000 °C can be achieved. The advantage of this design is that many kilometres of piping can be saved. This also eliminates the heat losses that occur in the pipes.

Picture: Shutterstock
Picture: Shutterstock

Just like the other power plants, the mirrors are exposed to wind, dust and sand. The resulting optical losses are considerable. Since the countless heliostats of the sun have to be tracked in two axes, additional optical losses are caused by the wind load. The concentrated rays miss the absorber. 

According to a study (10), the heliostat field contributes 50% of the total costs of the power plant and causes 47% of the losses per year.

 

How many weeks or months would it take these workers in Ivanpah to clean all 347,000 heliostats by hand?

And what do these mirrors again look like after two weeks?

 

All these circumstances lead to the following conclusion:  If someone wants to build solar power plants in the desert areas, then considerable losses must be expected. However, the losses can be avoided with suitable technology.

 

Because one thing is certain: open mirrors do not belong outdoors in the desert.