Fig. 15 Constant pressure concept; external depressurization,

Saturated Steam Fig. 16 Steam accumulator with integrated latent heat storage material.

Although steam accumulators exhibit only a small storage capacity, the availability of these buffer storage systems can contribute to reduce the investment costs for storage capacity if they are combined with storage systems intended for longer periods of discharge. By reducing the requirements regarding response time and discharge rate the specific costs for storage systems with several hours of heat capacity can be reduced.

Acknowledgement

Part of the work presented in this paper has been funded by the German Federal

Environment Ministry under the contract code PARASOL/WESPE and part by the European

Commission within the 5th Framework Programme on Research, Technological

Development and Demonstration under contract no. ENK5-CT-2001-00540.

The authors are responsible for the content of this publication.

References

[1] Tamme, R., Laing, D., Steinmann, W. D., Zunft, S., 2002, "Innovative Thermal Energy Storage Technology for Parabolic Trough Concentrating Solar Power Plants”, Proceedings EuroSun 2002, The 4th ISES Europe Solar Congress, Bologna, Italy

[2] Tamme, R., Steinmann, W. D., Laing, D., 2003, „High Temperature Thermal Energy Storage Technologies for Power Generation and Industrial Process Heat", Proceedings FUTURESTOCK 2003, 9th International Conference on Thermal Energy Storage, 1.-4. Sept. 2003, Warsaw, Poland.

[3] Tamme, R., Laing, D., Steinmann, W. D., 2004, „Advanced Thermal Energy Storage Technology for Parabolic Trough", ASME-J. of Solar Energy Engineering, Vol. 126, May 2004.

[4] Eck M., Zarza E., Eickhoff M., Rheinlander J., Valenzuela L.: Applied Research concerning the Direct Steam Generation in Parabolic Troughs, Solar Energy, Vol.

74 (2003) pp. 341-351

[5] Beckmann, G., Gilli, P. V. (1984): "Thermal Energy Storage", Springer Verlag

Hansjorg Lerchenmuller, Max Mertins, Gabriel Morin

Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg e-mail: hansjoerg. lerchenmueller@ise. fraunhofer. de

Dr. Andreas Haberle

PSE GmbH, Solar Info Center, 79072 Freiburg, Germany e-mail:ah@pse. de

02

Dr. Stefan Bockamp, Dr. Markus Ewert, Matthias Fruth, Thomas Griestop E. ON Energie AG, Brienner Str. 40, 80333 Munich, Germany e-mail: markus. ewert@eon-energie. com

Dr. Jurgen Dersch

German Aerospace Centre (DLR), 51147 Cologne, Germany e-mail: juergen. dersch@dlr. de

Over the last few years Fresnel-Collectors have attracted a lot of attention within the solar thermal power sector. The main reason is comparatively low investment costs through simple components. The Fraunhofer Institute for Solar Energy Systems,

E. ON Energie AG and German Aerospace Centre (DLR) have carried out a feasibility study in order to assess the technology with respect to technical, economical and ecological aspects.

The mid to long term strategy of solar thermal electricity generation must aim at technical solutions with high solar shares. Thermal storage is not yet technically proven for direct steam generating systems. Therefore special configurations of hybrid operation are an interesting option from a technical and economical point of view. Full load hours of the power plant increase and allow for more stable plant operation. Based on Fresnel-Collectors, two different types of power plant configurations with low or zero CO2-emission are analysed in this paper:

• Hybrid operation of a solar field and a biomass vessel

• From the starting point of a Solar Only power plant, natural gas hybrid operation will be considered and the trade off between high solar share and low cost electricity production will be analysed in detail.

Calculations for this study were carried out in three steps:

• Thermodynamic calculations of the water/steam cycle were done with the commercial process simulation tool Ebsilon [1].

• Thermal and electrical yields were calculated with ColSim [2] for different solar field sizes and different options of hybridization. The simulations are based on the efficiencies of the power cycles — depending on ambient temperature and load — and hourly meteorological data for a site with a DNI of 2’247 kWh/(m2a) [3].

• Based on economic assumptions and on the results of the previous steps, calculations of levelised electricity costs (LEC) and profitability were carried out.