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Clouds and sun: Although the sun is not visible in the picture, it was there anyway. It is recognisable on the illuminated CentRec receiver. The mirrors of the solar array are aligned so that they reflect the available solar radiation onto the CentRec receiver halfway up the solar tower.
© DLR
Solar thermal power plants – tower power plants
28.06.2018

The infrared image of the CentRec receiver in the Jülich solar tower shows the temperature distribution on the receiver. It is hottest inside the rotating chamber, where the concentrated solar radiation hits the particles: maximum values exceeding 1,000 degrees Celsius have been measured here.
© DLR

Solar water heater for ceramic granules

The innovative CentRec radiation receiver has passed its first practical test at very high temperatures in the DLR’s solar tower in Jülich. The receiver uses ceramic particles as a heat storage medium. These are inexpensive, enable very high temperatures and can be handled with relatively little effort. The practical test at very high temperatures is an important step on the way to marketing this receiver technology.

Around 2,500 large moving mirrors, known as heliostats, track the course of the sun at the DLR’s research site near Jülich. They reflect the sunlight to a common focal point near the top of a tower that is about 70 metres high. There, a receiver absorbs the concentrated radiation and converts it into high-temperature heat for power plant processes. The solar thermal tower power plant is both a model and test facility for future commercial power plants, for example in southern Europe and North Africa.

The scientists have developed and successfully tested a new concept for the central component, the receiver. The CentRec receiver consists of a cylindrical chamber that rotates at the focal point of the mirrors. Roughly one millimetre-sized ceramic particles are fed into the chamber from above. The rotation causes them to press against the hot inner wall of the chamber where they are heated to more than 900 °C. Gravity causes the particles to migrate downwards until they fall from the receiver into heat-insulated containers. The chamber's speed of rotation determines how long the particles stay in the receiver, and thus how warm they become. The hot ceramic particles can be used directly and, for example, coupled into the power plant process for electricity production. Alternatively they can be used as a storage medium that provides heat for a variety of applications as needed and is relatively easy to transport and store. The particles, which are made of bauxite, can withstand temperatures of over 1,000 °C and are both inexpensive and environmentally safe.

Ceramic particles: Higher temperatures, easy storage

The solar test operation started in September 2017. The operating temperature was successfully increased incrementally within 22 test days with good solar radiation conditions. In the last test of the test series, the temperature sensor at the outlet of the rotating drum indicated the maximum value of 965 °C for the particles. The researchers even managed to exceed the temperature target specified in the project by 65 degrees. From the outlet the particles then trickle down into a downpipe where a stream of air cools them down. The particles fall into a collection container where a conveyor system receives them and transports them back up to the receiver. The focus of the test operation was on achieving the temperature target. The transport cycle for the particles was therefore kept simple. In commercial operation the particles will travel further: they will only be returned to the CentRec receiver once their heat has been released to the users.

“Proof of the high operating temperature is an essential prerequisite for the targeted marketing of our receiver concept,” explains Dr Reiner Buck, Head of Solar Tower Systems at the DLR Institute of Solar Research. The liquid salts previously used as the heat transfer medium only allow temperatures of about 550 °C. The hot ceramic particles enable power plant operators to operate at higher process temperatures, resulting in improved efficiencies and therefore lower power generation costs.

In the next step, the team led by Reiner Buck wants to scale up the CentRec technology to a higher performance level, which will further reduce the generation costs for electricity and solar process heat.

Solar power plants in system comparison

In addition to the tower power plant concept, parabolic trough power plants, dish-Stirling systems and Fresnel systems are being developed and used as solar thermal power plants. In Europe, concepts and components have been tested at the international Plataforma Solar de Almería test field in Spain, with whom Germany has been working very closely for more than 30 years. The “Solar thermal power plants” BINE-Themeninfo brochure provides an overview. The BINE-Projektinfo brochure entitled “Night-time electricity from solar thermal power plants” explains how parabolic trough power plants can be used for producing night-time electricity.

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