Parabolic trough power plants are the most proven technology for solar-thermal power plants. The majority of plants use liquid synthetic thermal oil as heat transfer medium, which is heated from about 295 °C up to 395 °C. The heat is supplied to a standard steam generation and steam turbine cycle for power generation., a procedure almost identical to the conventional coal-fired power plants. Though, the maximum steam temperature is limited to about 385 °C which is significantly lower than for the fossil-fueled steam circuits (>550 °C).
The used steam turbines as well as the steam process are technically very well-developed. The biggest potential for improving the efficiency is by increasing the pressure and the temperature of the steam. Currently, research is being done on alternative heat transfer media such as molten salt or water/steam, which may be heated to higher temperatures than the synthetic oil.
The usage of water/steam as heat transfer medium in the collector field is in general referred to as direct solar steam generation (DSG). The water from the steam turbine process is led to the inlet of the solar field in which it is pre-heated, evaporated and superheated to the desired temperature. The generated steam is immediately led to the turbine and the circuit is closed. Thus, no water is lost. Since the upper temperature is no longer limited by the thermal oil, it can be optimally chosen by considering the concentration of collectors, the costs and further boundary conditions.
Pros and Cons of Direct Steam Generations
The direct steam generation has the following important advantages:
The disadvantages compared to plants with synthetic oil in the solar field are:
Previous Research Work
Demonstration of the Development
Current Research Work at DLR
In the research project DUKE, DLR investigates the possibility to replace the recirculation concept by the once-through concept in order to decrease costs and push commercialisation of power plants with direct steam generation. Further research activities include the development of suitable cost-efficient thermal storage concepts and the optimization of the steam parameters respectively their interaction. (more)
Investigation of Flow Patterns Using a Wire-Mesh Sensor