There are at the moment three important ongoing research topics concerning the operation of direct steam generation:
The once-through concept is an alternative to the recirculation concept. The solar field is not split into separate sections, but every collector loop converts the water into steam at the desired temperature. The concept allows for the elimination of several components, such as steam drum, auxiliary pump and header piping. This reduces not only investment costs, but also enables an even more effective operation. The layout of the solar field is distinctly simplified and the operation may be flexibly adjusted to the optimal conditions of the steam turbine process. On the other hand, the control of the temperature gets more important.
In the research project DUKE, experiments at the DISS facility showed that the outlet temperature of the steam can be maintained stable also for the once-through concept. In this project, the main focus was the development of control concepts that are easy to implement in commercial projects. In the long term, even more precise and faster models shall be developed in order to better react on varying solar irradiation conditions. The costs for the provision of steam may in total be reduced by up to 25 % compared to the recirculation concept, which also leads to a significant reduction compared to the existing synthetic oil power plants.
Power plants with direct steam generation require special thermal storage concepts, which differ from the commercial molten salt storage in the thermal oil power plants. The development of such storage technologies, in particular phase change material storage, is pursued by the Institute of Technical Thermodynamics at DLR.
The direct solar steam generation may not only be used for electricity generation, but also for provision of steam at high temperatures. Food production and desalination of sea water are two examples for processes with significant heat demand. The optimal operation parameters are individually determined depending on the configuration of the system. Thus, the interaction between different parts of the system during transient procedures (passing clouds, start-up) is an important field of research.
DLR possesses unique knowledge and experience within the entire field of direct solar steam generation due to its worldwide research projects and large-scale facilities.