news / 2015-10-02

Solar thermal power plants with direct steam generation

With the direct steam generation concept, water circulates as a heat transfer fluid in the receiver tube. It evaporates there, is superheated and can be directly used to drive a turbine. (Source: DLR (QFly); PSA)

Oils were previously circulated in the receiver pipes of parabolic trough power plants as the heat transfer medium. Scientists from the German Aerospace Center (DLR) are now testing a new concept at the research platform in Almeria, Spain. Here water vapour is directly used in the pipes as the heat transfer medium. This enables higher operating temperatures to be achieved and the absence of an intermediate heat transfer medium eliminates a source of losses. This direct steam generation improves the efficiency of parabolic trough power stations.

With the DUKE research concept, DLR scientists are developing and testing the through-flow concept for solar thermal power plants. The water used as a heat transfer medium evaporates on the way through the one-kilometre-long receiver loop and is further heated as water vapour. With a temperature of up to 550 °C and a pressure of 110 bar, this superheated steam therefore comes very close to the parameters customary in conventional power plant technology. The steam can be directly used to drive a turbine, which is why the concept is known as direct steam generation.

Direct steam generation instead of recirculation

Direct steam generation systems have previously relied on the recirculation concept. The water as the heat transfer medium flows through three areas: steam is generated in the evaporator, the steam is then separated from the liquid water in the steam drum and the steam temperature is then further increased in the last part of the solar field. The new DUKE concept is different. The water flows through a continuous receiver tube and is superheated as steam. This approach places higher demands on the regulation than the previous concept. Clouds drawing across require a rapid response with DUKE, whereas with the recirculation concept their effect was mitigated by the steam drum. With direct steam generation, irradiance measurements and measurements of the steam temperature are therefore being further developed in order to automatically adjust the control system to the current irradiation.

Advantages of the concept

In the DUKE concept, the receiver string is scalable since each part is identically constructed. For large power plants, this means that only a few receiver strings have to be installed in parallel to achieve the desired power. The test system with a one-kilometre length achieves about 3 MW of thermal power when there is good irradiation.

 

The water or steam heat transfer medium enables higher temperatures than the oils used to date. These reach their limit areas at approx. 400 °C. However, the amount of water in the collector string needs to be continuously adapted to the irradiation conditions to ensure, for example, that the temperature remains constant when clouds draw across and the steam turbine does not have to be switched off for safety reasons. The system enables new water to be fed in at three points so that the operating parameters and thus the temperature can be changed quickly.

 

A follow-up project is planned in order to garner the necessary long-term experience.

Projects currently being funded

Project partners

Industrial partner

Solarlite

Project location

Plataforma Solar de Almería (PSA)