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Solar Direct Steam Generation - Water/Steam as Heat Transfer Medium in the Solar Field

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:

• Increased efficiency of the power generation (required solar field aperture is reduced)
• Cost-efficient access to water (compared to other fluids, also in arid areas)
• Thermal stability at high temperatures
• Water is neither inflammable nor harmful to the environment
• No additional medium is required throughout the whole process
• No heat exchangers are needed between the solar field and the steam turbine process, reducing the energy losses and costs

The disadvantages compared to plants with synthetic oil in the solar field are:

• Higher pressure in the solar field (ca. 20 bar in oil solar fields and up to 150 bar in DSG solar fields)
• Due to that, thicker and more expensive piping is required
• A two-phase mixture of water and steam in the solar field
• Demanding process control
• So far, no cost-effective solution for thermal energy storage is commercially available

	Previous Research Work
	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 leads to the successful technology transfer to the industry. (more)
	Demonstration of the Development
	The successful operation of the direct steam generation could be verified under real solar conditions in a test facility at the Plataforma Solar de Almería (PSA), owned by the Spanish cooperation partner CIEMAT. The facility offers a unique infrastructure for the technology’s research and is operated by the Spanish CIEMAT (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas). (more)
	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
	The ways to achieve a fundamental understanding of the two-phase flow inside the absorber tube with measuring technique procedure are limited. One interesting way is to observe the flow with a wire-mesh sensor. The sensor, which is situated at the cross-section of the absorber tube, delivers high definition data of the distribution of steam and water in time and space. (more)