Decisive factors in experiments with concentrated solar radiation are the total power and the flux density in kilowatts per square meter (kW / m²) plus its distribution. The reliable optical measurement of these values is therefore a central task while conducting experiments. The working group of the large-scale plant SOF/HLS at the DLR site Cologne-Porz have been using special instruments and computer software for this purpose for more than 20 years.
The new measurement system FMAS (Flux Mapping Acquisition System) was developed by the working group SOF/HLS and has replaced the previous FATMES system since the beginning of 2016. FMAS is a portable measurement system that enables quick and easy determination of the flux density in both the Solar Furnace and the Solar Simulator as well as in decentralized applications.
Compared to the previous version, the new FMAS offers the user completely new possibilities for digital image processing using conventional and powerful state of the art hardware. The new system uses modern cameras and lenses with various filter combinations. FMAS also features new possibilities for evaluating the measurement data and is easier to use than the previous system.
The highly concentrated solar radiation is adjusted onto a diffuse reflective surface (target), which is detected by the camera. With a radiometer, FMAS converts the grey values of the image into values for the radiation flux density (see Figure 1). Regular monitoring of the sensors ensures measurement accuracy. The resulting projective error due to the camera’s viewing angle is corrected by a rectification algorithm using four pass points. The analysis also considers the influence of dark current and inhomogeneous shading of digital camera chips.
The user interface of FMAS is able to show the default monochrome camera image in a color representation (see Figure 2). All program control buttons are operable in a single window. Relevant image information such as the histogram or the maximum grey value are shown in real-time.
In the Solar Furnace the FMAS software additionally controls a double-filter-wheel for the camera in automatic or manual mode.
In the window presenting the measurement result the user can switch between a 3D representation and the side views of the flux density distribution (see Figure 3). Here both the overall performance (total power) as well as the performance on a free selectable surface (power on aperture) are displayed.
The red cross in the right illustration marks the center of gravity (CoG) of the flux density distribution. The flux density values between the graphics describe the isolines at different percentages of the maximum flux density (peak flux).