In parabolic trough solar collectors, the incident solar radiation is concentrated onto an absorber tube that runs along the focal line of a parabolic mirror. The heat absorbed by the tube wall is transferred to the heat transfer medium, which is pumped through the absorber tube. Heat transfer oils are usually used here. Absorption at the tube wall results in the highest temperatures there, and this is associated with high thermal radiation from the tube surface. This 'indirect' method of heat transfer to the medium is also always associated with increased entropy production.
The aim of the research project is to improve the efficiency of thermal solar collectors by developing an innovative heat transfer medium. For this purpose, a nanofluid with special radiation properties is to be developed, which is heated by direct volumetric absorption of short-wave radiation. The long-wave self-emission is reduced by a selective coating of the transparent tube wall.
For this purpose, the absorption and scattering behavior of different combinations of base fluids and dissolved nanoparticles in the solar emission spectrum is investigated. Due to absorption in the volume, the highest temperatures occur inside the tube. With the help of internally coated tubes, which have a high reflectance in the infrared range, the radiation to the outside - and thus the heat losses - could be significantly reduced.
High flow velocities in the tubes to realize high convective heat transfer coefficients are not necessary for volume absorption; in fact, low heat transfer coefficients are desirable. Due to the - compared to conventional heat transfer media - higher viscosity of the nanofluids, the required pump power does increase. However, since lower flow velocities are required, this can be compensated.
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IfT, Leibniz Universität Hannover
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