Transient Measurement Method for the Determination of Parabolic Trough Receiver Heat Losses under Field Conditions - Testing and Optimization

Kurzfassung

Parabolic trough collectors represent one of the current most popular technology among concentrating solar power plants. Parabolic shaped mirrors concentrate solar radiation on a line of receiver tubes. These receiver tubes consist of an absorber steel tube inserted in a concentrical glass envelope. To provide an accuratemeasurement technique for the determination of parabolic trough receiver heat losses under field conditions, a transient infrared thermography measurement for the determination of parabolic trough receiver heat losses is developed. The principle of this measurement method is to apply a thermal excitation to the absorber tube temperature and to investigate the glass envelope temperature response. Both temperature signals are measured with infrared thermography. The glass envelope temperature dynamic response is investigated with respect to the absorber temperature excitation to provide a diagnosis about the thermal properties of the investigated receiver. The experimental setup consists of a radiation shield, which ismounted around the glass envelope. It reduces thermal radiation heat losses from the glass envelope to the ambient to a minimum. Installed fans on one side of the radiation shield induces a steady air flow inside of the shield. The measurement method was tested in previous measurement campaigns under laboratory conditions. This report covers the first implementation and optimization of the transient heat loss measurement method under field conditions. During the first field measurements that ambient conditions such as wind speed, wind direction and ambient temperature gradients have a significant influence on the measurement quality. A reproducibility of results could not be proven. Corrective measures were investigated to reduce the influence of ambient conditions. The installation of a first wind screen on the radiation shield could decrease the influence of wind. A second wind screen was then designed and its impact on the experimental setup was tested under laboratory conditions. The measurements obtained with the original setup could be approached with the new wind screen by increasing the ventilation power of the radiation shield. Laboratory measurements showed a better reproducibility than first fieldmeasurements. The optimized experimental setup was tested during a second field measurement campaign. The influence of wind could be eliminated up to 6 m/s, while the influence of ambient temperature variations remained critical. During this measurement campaign, the reproducibility of field experiments could be improved, reducing the uncertainty of heat loss measurements to 18.7 W/m.