Concluding from the simulation results in section 2.2 and pressure drop measurements (not discussed above), the optimal sand grain size with respect to heat transfer and heat exchanger auxiliary energy demand is in the range 2-3 mm. With larger grain size the effectiveness of heat transfer will drop whereas for a lower sand grain size the increasing pressure drop will lead to substantial auxiliary energy demand or a high heat exchanger volume, if the air velocity is decreased.

For selection of sand grain size the fluid bed cooler as shown in Fig. 1 might be a limiting element. Larger grain sizes lead to increasing erosion on the heating surfaces of the fluid bed cooler, so that the sand grain size is presumably limited to 1 mm.

Larger sand grain sizes can be used when the storage concept is modified as shown in Fig. 7.

Differing from the sand storage concept shown in Fig. 1, this storage concept does not require modifications of the steam circuit and equals with the conventional packed bed storage concept using a waste heat boiler.

Elements are used from the sand storage concept including air-sand heat exchanger, hot and cold storage tank and the sand transport system. In view of the thermal stresses in case of high change of operation modes, two separate air-sand heat exchangers for storage charging and discharging could be advantageous. In contrast to the storage system shown in Fig. 2, for storage discharge the heat is transferred again back in the heat exchanger from sand to air.

Which of the two storage concepts using sand is the better one will be evaluated in a cost analysis when more precise data of heat exchanger effectiveness are available.

Acknowledgements

The project was supported by the German Ministry of Education and Research (BMBF) through its FH3 programme.

Nomenclature

References

[1] S. Warerkar, S. Schmitz, J. Gottsche, B. Hoffschmidt, Air-Sand Heat Exchanger for solar tower power stations, 2nd International Renewable Energy Storage Conference (IRES II), November 2007, Bonn, Germany

[2] N. Siegel, G. Kolb, J. Kim, V. Rangaswamy, S. Moujaes, Solid particle receiver flow characterization studies, Proceedings of Energy Sustainability 2007 conference, ASME, June 2007, Long Beach, California

[3] H. Fricker, Regenerative Thermal Storage in Atmospheric Air System Solar Power Plants (ed: A. Steinfeld), Proceedings of the 11th International Symposium on Concentrated Solar Power and Chemical Energy Technologies, 2002, Zurich, Switzerland