DLR has been part of and major contributor to the entire research of direct solar steam generation. Starting from the first idea and the basic technical issues, the development eventually lead to the successful technology transfer to the industry. One of the early main topics was the research on how two-phase flows in horizontal, heated pipes occur in parabolic troughs. For example, it was shown at which conditions the mixture of water and steam attain the desired flow pattern for a safe operation. This is the basis for a secure design of this kind of power plant.
Subsequently, a solar field concept guaranteeing very robust operation for all possible irradiation scenarios was conceived. This so-called recirculation concept divides the solar field into two parts (see figure below). The first part concerns the evaporation of water, where only about one third of the water is evaporated. The liquid water and steam are then separated in a steam drum. The liquid water is pumped back to the inlet of the solar evaporation field, while the saturated steam is heated to the desired outlet temperature in the second part of the solar field. This concept is currently implemented in all commercial DSG systems. It is analogue to conventional steam generators with forced circulation, which are commonly applied in small fossil power plants – though, without fossil fuel.
Furthermore, safe operation methods could be developed, not only for the nominal operating conditions, but also for the daily start-up of the plant at sunrise. The transient - which means varying with time - conditions can be illustrated in DLR’s simulation tools and may be used for further improvements. They can for instance show what happens when clouds pass over the solar field and shadow the collectors – and above all, how to react in order to ensure optimal operation.
In cooperation with industry, exemplary power plants have been designed and the costs have been compared with synthetic oil solar fields. With the commercial recirculation concept, savings in the range of 10 % of the initial electricity costs can be achieved for the different configurations and locations. In the short term, solar fields with direct steam generation may be particularly cost efficient in combination with (existing) fossil power plants by saving a large quantity of carbon dioxide (CO2) during daytime.