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Salt-Free Solar: CSP Tower Using Air

In December 2008, a 1.5 MWe solar thermal central receiver system was declared operational by plant construction company Kraftanlagen Munchen. Although solar tower technology had been built as early as the 1970s and a second commercial tower is now close to completion (see REW magazine July/August 2008) the so-called Test and Demonstration Power Plant Julich, in Germany, is the world's first solar thermal power plant erected which uses air as the medium for heat transport.

In all previous plants liquid media such as molten salt or oil have been used for the obvious reason of their high specific heat capacity, which in turn results in low volume flow rates and low pumping losses.

The great disadvantage of these concepts is that the solar radiation concentrated by the heliostat field to fluxes of 500 to 1000 suns is in air and that to transfer the heat it has to pass through a wall. This results in exchanger surface temperatures substantially higher than the fluid temperatures within. And, as the absorber surface faces the ambient environment it suffers thermal losses due to convection and – increasingly important for high temperatures according to the Stefan-Boltzmann law – re-radiation.

In contrast, the Jülich power tower uses the so-called volumetric effect to increase efficiency. Ambient air is sucked through a blackened porous structure on which the solar radiation is focused. The air cools the outer parts of the receiver and is heated up gradually to the design temperature level at the inner surface. Under ideal conditions, the temperature of the radiating outer surface can even be below that of the working fluid. Air also has the additional obvious advantages of being both environmentally benign and free.

The hot air is then fed into a state-of-the-art heat recovery steam cycle, conventionally used for the exhaust heat of gas turbines in combined cycle plants.

Professor Bernhard Hoffschmidt, Head of the Solar-Institute Jülich (SIJ) – part of the Aachen University of Applied Sciences – and initiator of the project, points out this implies another potentially big advantage, saying: ‘Gas turbines driven by fossil or biogenous fuels are easily integrated into the solar system and can supply power at times of no solar radiation, allowing for 24 hour operation.’ The SIJ is currently investigating different modes of hybridisation for various power levels and environmental conditions.

Alternatively, heat storage may be used to align supply and demand and the Jülich plant features a storage system consisting of honeycomb-type ceramic blocks, through which air passes in one direction for charging, and in the other for discharging. As the discharged air has the same temperature as when charging, no energy is lost, making the system highly efficient.

While considering the solar resource Jülich is not the ideal location for a solar power facility, it is when regarding the scientific and industrial resource. Situated in the Rhineland it is close to the German Aerospace Center (DLR), and the University of Aachen, with its institutes of high expertise regarding conventional power plants, as well as the SIJ. For that reason those mentioned have founded the Virtual Institute of Central Receiver Power Plants (vICERP) in order to set up a detailed dynamic simulation model of the power plant, used for testing advanced control strategies. A first enterprise has been founded aiming at the promotion of the technology.

This spring will see the first solar runs, but even today the list of up-coming improvements and innovations is long and many ideas are currently under investigation at the SIJ. These include:

  • New heliostat concepts and new means of control.
  • The absorber structure has been mainly designed for the reliability required of a prototype plant. In commercial plants designs with higher efficiencies can applied.
  • At the edge of the receiver, radiation of relatively low concentration is lost (the so-called spillage). Specially designed edge modules are to make use of this energy.
  • Storage concepts based on sand have been studied with promising results.
  • Custom-made boilers for the integration of gas turbines will be developed.

The biggest rise in efficiency, however, will be due to scaling up the plants to power levels where steam cycles can operate more efficiently. With big industry and foreign governments standing in line for the construction of the next plant, the step in this direction seems to be at hand.

Mark Schmitz is head of the Regenerative Systems unit of the Solar-Institut Jülich.

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