Solar Thermal Power Stations Are a Prime Application for Canned Motor Pumps

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Parabolic trough solar thermal power stations use a pipe (receiver) carrying a heat transfer fluid mounted along the focal line of a parabolic trough-shaped reflector. One axis of the receiver tracks the sun’s axis in such a way that the sunlight is always concentrated onto the heat transfer pipe. The concentrated solar radiation heats the heat transfer fluid, generally a synthetic heat transfer oil, to approximately 400 °C.

Individual troughs are connected to one another using distribution lines. These distribution lines carry the heat transfer oil to a heat exchanger, which vaporizes water. Just as in a conventional power plant, this steam drives a steam turbine, which in turn drives a generator to produce electricity.

If thermal storage is integrated into the system (typically in the form of tanks of molten salt), the plant can store surplus heat during the day and use this to carry on generating electricity after sundown.

The two plants mentioned above use Hermetic CNPK canned motor pumps (Fig. 2) to recirculate the 400 °C synthetic thermal oil. The pumps have variable-speed inverter drives to allow the plants to vary their output efficiently. Motor cooling is by external tube heat exchangers (Spain) and air-cooled heat exchangers (Egypt).

Direct vaporization

As an alternative to indirect heat transfer using a heat transfer oil, it is also possible to vaporize water directly in the heat-collecting pipes of a parabolic trough power station and use the resulting steam to drive the turbines. Without discussing the advantages of direct vaporization in detail, suffice it to say that research into the corresponding technical challenges has been taking place for some eight years now at the world’s largest direct vaporization plant, the DISS (Direct Solar System) Parabolic Trough Test Station at Plataforma Solar de Almeria (PSA) in Spain (Fig. 4). The two 500 m long collection lines at DISS are proof of the functional reliability of direct vaporization under real conditions. A hermetic CAMKT 30/6 (PN 100) high-pressure pump recirculates the water at a system pressure of 100 bar and a temperature of 400 °C (Fig. 5). This high-pressure pump is a multi-stage canned motor pump of barrel design, cooled by an external heat exchanger. Thanks to the barrel design the pump requires only a single static seal instead of the six that would be required for a conventional multi-stage pump.

Asia’s first parabolic trough power station, in Kanchanaburi province in central Thailand, is based on the direct vaporization principle and also uses a hermetic Type CAMKT 44/3 (PN 100) high pressure pump with an external heat exchanger. Based on experience gained in Almeria, this pump is also a barrel design.

Conclusion

Canned motor pumps for high-temperature applications are used in many technical and industrial processes requiring the supply of process heat. Solar thermal energy has opened up a new field of application in which, thanks to their special characteristics, canned motor units are the pumps of choice.

* The author is with Hermetic-Pumpen GmbH, Gundelfingen/Germany. Contact: +49 (0)7 61 / 58 30 - 0

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