A CSP Tower With Air Energy Storage  

REW has an article on energy storage for solar thermal power using air - 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.

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.

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.