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Centrifugal receiver under testing: the ceramic beads in the rotating receiver can be evenly heated up to 1,000 degrees Celsius.
© DLR (CC-BY 3.0)
Solar tower power plants

The hot ceramic particles are transferred from the centrifugal receiver into a thermally insulated container. By means of a heat exchanger, the heat stored in the particles can then be used in a steam turbine or for process heat applications.

Combined heat transfer and storage medium: roughly one-millimetre-large ceramic particles are directly irradiated in the receiver.
© DLR (CC-BY 3.0)

Rotating receiver heats up ceramic beads

Until now, tube receivers have been used in solar tower power plants to absorb the concentrated solar rays and convert them into heat. The German Aerospace Centre (DLR) is now pursuing a new approach. In a rotating receiver, ceramic particles are evenly heated up to 1,000 degrees Celsius. The advantage of this is that the beads also act as a storage medium. The centrifugal receiver at the Jülich solar tower is scheduled to enter operation from mid-2015.

In solar tower power plants, many mirrors, called heliostats, focus the sun’s rays directly onto a point at the top of the tower, the focal point. This is where the solar receiver is situated with an absorber that heats up to between 600 and 800 degrees Celsius. The heat transfer medium in the absorber, such as air or liquid salt, then transports the heat to a central steam turbine that converts the steam into electricity.
The German Aerospace Centre (DLR) is currently pursuing a different approach: in the DLR laboratory a drum spins round that is somewhat reminiscent of a washing machine. It contains ceramic beads that are only one millimetre in diameter (figure below left). They are directly irradiated by heliostats and store the sun’s rays. In the drum – the newly developed centrifugal receiver – the ceramic particles are held on the wall by centrifugal force and thereby heat up evenly. The particles then gradually fall out of the receiver, which is inclined obliquely downwards, into a thermally insulated container. “Via a heat exchanger, the heat stored in the particles is used to generate steam in steam power plants or for process heat applications, whereby the particles then cool again,” explains Dr. Reiner Buck, Head of the Point Focus Systems department at the DLR Institute of Solar Research.

Combined storage and heat transfer medium

“A particle steam generator enables the heat in the beads to be utilised. Here particles are passed through a tube bundle where water is evaporated under pressure and is heated further,” says Buck in describing the procedure. The solar thermal energy can then be used directly and coupled to produce electricity in a power plant process. The advantage: the heated ceramic particles can also be used as a storage medium by storing them in non-pressurised, insulated containers. Such stored thermal energy enables power plants to feed electricity into the grid even after sunset. According to Reiner Buck, another advantage of the CentRec concept is its considerable flexibility: “The rotational speed of the drum can be used to determine how long the particles remain in the receiver and thus determine their temperature when exiting the drum – in accordance with the intended process for the thermal energy.”
The rotating receiver’s greater efficiency compared with air enables it to absorb more solar heat. “With air, the heat being stored has to be transferred to another medium such as ceramic material, and from there it then has to be released to the air again. In each case this involves losses,” says Buck in explaining the difference.

Higher efficiency through higher process temperatures

In addition to its use in power plant processes, the thermal energy stored in the beads can also be deployed as high-temperature process heat for industrial use. For such applications the heated particles, which have a high energy density, can be transported easily to their place of operation. The ceramic beads can be heated up to 1,000 degrees Celsius. That is significantly hotter than the fluid salt previously used for transferring heat in power stations, which can only be heated to 565 degrees Celsius. “The ceramic particles enable power plant operators to work with higher process temperatures between 600 and 800 degrees Celsius and thus achieve greater efficiency in the power plant process,” explains Buck. The researchers also expect reduced electricity production costs as a result of the system’s relatively low component costs, in particular for the receiver and ceramic particles.

The developers have successfully tested the receiver prototype (top figure), which has a capacity of ten kilowatts, using the DLR’s high-flux solar simulator and solar furnace in Cologne. Next year, the DLR researchers are testing the CentRec concept at the DLR solar tower in Jülich with a larger centrifugal receiver, which will have a capacity of 500 kilowatts.



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