Flux density measurement for industrial-scale solar power towers

Kurzfassung

For separate acceptance tests of a solar power tower’s heliostat field and receiver, it is necessary to determine the solar flux density distribution over the whole absorber surface. Integrating the flux density delivers the receiver input power, which is required for calculating the energy conversion efficiencies of both heliostat field and receiver. Furthermore, flux density measurement is valuable for supervision and control during operation of a power tower. Flux density at small-scale prototype receivers has mostly been measured by using a camera and a moving bar so far. The moving bar is a white diffusely reflecting target which is moved quickly through the radiation’s focus in front of the receiver surface. At the same time, a digital camera cap-tures the radiation reflected off the moving bar, which allows determining the incident flux density. At industrial-scale receivers though, the installation of a moving bar is hardly feasible due to difficult construction and high costs. Therefore, the development of a measurement method without any mov-ing parts is aspired. For this purpose, the radiation reflected off the absorber itself can be measured in order to calculate the incident flux density [1]. Preliminary work on this method is still immature and has not yet lead to a reliable and satisfying measuring accuracy under all conditions [2]; achieving this is a main aim of the presented thesis. The central challenge with measuring flux density by reflection off the absorber is the absorber’s non-diffusive reflectivity, which depends especially on the direction of the incident radiation as well as on the observation angle [1]. Hence, detailed understanding of reflection at the structured surface of open volumetric receivers as well as tube receivers and following software-aided correction of these effects are essential for reducing the measurement uncertainty. The improvements will be imple-mented and tested at the Solar Tower Jülich. Finally, the improved flux density measurement system is planned to be used in a demonstrational acceptance testing at the Solar Tower Jülich, including a comparison of measurements and simulation results.