Industrial turbines for saturated steam are not designed for daily start-up and shut­down, because this kind of operation would reduce its life time considerably and would require often revisions and costly maintenance. Manufacturers recommend to operate the turbine at low load conditions overnight by means of a fossil-fired back-up boiler or thermal energy storage. So, the use of saturated steam turbines in a solar plant demands the implementation of costly maintenance procedures to assure a good performance and durability. However, the extra maintenance cost required by a saturated steam turbine can not be quantified in general, because the service conditions imposed by every manufacturer are different. This can be done only on a case by case basis. What can be clearly stated is that the O&M cost of a saturated steam turbine is higher.

Concerning operation requirements, saturated steam turbines seems to be less flexible than superheated steam ones, because great changes in the steam parameters can have a more dramatic effect. Nevertheless, the water/steam separator at the interface between the solar field and steam turbine brings some benefits. If the volume of this separator is properly designed, it can also act as thermal energy storage if the turbine is operated with gliding pressure. The amount of saturated water and steam inside this vessel can feed the turbine with saturated steam at gliding pressure for a few minutes, thus overcoming short cloud transients. This is a very important advantage of the saturated steam option.

Summarizing, when O&M issues are considered, DSG solar power plants with saturated steam have advantages and disadvantages when compared to the superheated steam option. However, an economic assessment of these advantages and disadvantages is still unfeasible because of the lack of experience with commercial DSG power plants. Though a theoretical study could be performed, the wide range of uncertainties provoked by the lack of experience very much limit the accuracy of results obtained from such a study.

Conclusion

According to the investigation performed the saturated steam option has a 4 % higher net yearly electricity production of the power plant. On the other hand for the plant size analysed, the initial investment required by the saturated steam option is of about 5% higher than for the superheated steam option. Though its maintenance cost is higher, the saturated steam option requires a less complex solar field and the water/steam separator located at the power block inlet can act as a thermal energy storage, which can feed the steam turbine with saturated steam at gliding pressure for few minutes, thus overcoming short cloudy periods. Further advantages of the saturated steam option are the lower complexity of the solar field and the possibility to use simpler collector options that are able to operate with good efficiency at 260-300°C.

Due to the lack of experience with commercial DSG plants, a complete economic and technical assessment of both options including all the aspects (yearly performance, initial investment, operation and maintenance) is still unfeasible until the first DSG solar power plants are installed and deliver accurate information. Therefore the results presented have to be regarded as preliminary but nevertheless the saturated steam DSG plant seems to be an interesting option for near term application in the lower capacity range.

References

[1] Eck M., Zarza E., Eickhoff M., Rheinlander J., Valenzuela L.: Applied research concerning the direct steam generation in parabolic troughs’, Solar Energy, Vol. 74, No. 4 , April 2003 , pp. 341-351