Peter Potzel’s research while affiliated with University of Stuttgart and other places

This paper describes a transient thermography method to measure the heat loss of parabolic trough receivers and separate their heat loss mechanisms. This method is complementary to existing stationary techniques, which use either energy balances or glass envelope temperature measurements to derive overall heat losses. It is shown that the receiver heat loss can be calculated by applying a thermal excitation on the absorber tube and measuring both absorber tube and glass envelope temperature signals. Additionally, the emittance of the absorber selective coating and the vacuum quality of the annulus can be derived. The benefits and the limits of the transient method are presented and compared to the established stationary method based on glass envelope temperature measurements. Simulation studies and first validation experiments are described. A simulation based uncertainty analysis indicates that an uncertainty level of approximately 5% could be achieved on heat loss measurements for the transient method introduced in this paper, whereas for a conventional stationary field measurement technique, the uncertainty is estimated to 17-19%.

Vorgestellt wird eine auf transiente Thermografie basierende Feldmesstechnik zur Bestimmung der Wärmeverluste von Parabolrinnen-Receivern. Durch thermische Anregung des innen liegenden Absorberrohres und Messung von Absorberrohr-, Glashüllrohr- und Umgebungstemperaturen lässt sich unter Verwendung eines numerischen Receivermodells auf die Wärmeverluste schließen. Zusätzlich ableitbar sind Emissivität der selektiven Beschichtung und die Güte des Vakuums, so dass Aussagen über einen potenziellen Schadensmechanismus ermöglicht werden. Die transiente Methode ergänzt bestehende stationäre Messtechniken. Sie ist, verglichen mit einer reinen Messung der Glashüllrohrtemperatur, aufwändiger in der Durchführung, bietet aber eine kleinere Messunsicherheit und einen höheren Informationsgehalt.

This paper describes a new transient method to measure the heat loss of parabolic trough receiver tubes using transient thermography. The method complements already existing stationary measurement methods which use energy balances or the observed glass enve-lope temperature to deduce thermal losses. It is shown that by a thermal excitation of the absorber tube and by both measuring the absorber tube and the glass envelope temperature responses, the receiver heat loss can be calculated. Additionally, the emissivity of the se-lective coating and the quality of the annulus vacuum can be derived. The advantages, challenges and limits of the new method are presented and compared to the established sta-tionary methods. Simulation results and first measurements which validate the simulations and findings are described. An uncertainty analysis shows that used in the solar field, this method reduces the measurement uncertainty from about 15 to 20% (conventional station-ary field measurement technique) to about 6% (new transient method).

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.