Modeling and Simulation for Determination of Parabolic Trough Receiver Heat Losses Using Transient Thermography

Kurzfassung

Solar thermal power plants become more and more important to cover environmentally friendly the upward tending power requirement with short running fossil primary energy sources at the same time. At the moment, many solar thermal systems are planned or already under construction. In this type of plants, concentrated solar radiation is converted into heat, which in turn can be used for the conversion into electric energy by means of conventional turbine processes. The farthest developed technology is characterized by parabolic troughs with mirrors that reflect and concentrate solar radiation on receiver tubes in which a synthetic oil flows. An important factor for the profitability of the whole plant is thermal heat loss of the receiver tubes. Thus, aged tubes have to be replaced if necessary. However, a measuring method to quantify the condition of already installed receivers with low uncertainty does not exist so far. Hence, the aptitude of transient thermography compared to an already existing, qualitative steady state measuring method is evaluated in this thesis. For this purpose, a receiver model, discretized in cylindrical coordinates, is created using the commercial software Dymola. By means of this model, different receiver conditions, possible transient excitation functions as well as all occurring environmental influences in a solar field like solar radiation, convective heat loss due to wind or radiative heat loss towards sky and earth surface can be simulated. After the development of transient measuring methods, the steady state method and the transient methods are compared according to objective criteria such as required accuracy in measurement or effort of realization of possible necessary boundary conditions for the measurement. Afterwards, the advantages of the transient methods are shown by means of an uncertainty analysis. Finally, a parameter study is carried out to analyze the most promising transient method under a broad variation of receiver and environmental conditions. A lookup table is created that allows the determination of receiver performance by means of measured data. Furthermore, with the help of the lookup table, a statement can be made about the receiver aging mechanism.