Concentrated flux is measured to determine the total amount of energy incident on a receiver aperture, or to examine the detail of the spatial distribution of the focal region flux. The most common approaches to flux measurement (sometimes known as flux mapping) are: the use of radiometers, calorimeters and photographic analysis of focal region images on diffusely reflecting targets, also known as the camera-target method. Flux distributions can also be modelled through characterisation of the concentrator surface and ray tracing of solar radiation reflected from it.
Radiometers are instruments that have an electrical response that varies with the incident radiation, in this case the concentrated solar flux. In CST applications, they are installed directly in the focal region and provide a single spatial point of measurement. Such devices detect radiation using either thermocouples or photodiodes. The current produced by a photodiode is proportional to the incident flux of photons, enabling direct measurement of solar flux over the area of the detector. A thermocouple is a junction between two conductors that converts a temperature difference into a potential difference via the thermoelectric effect. In a radiometer application, one of the conductors is joined to a heat sink that is held at a constant temperature, while the other conductor receives radiation.
Gardon and Kendall radiometers are commercially available designs suitable for detection of concentrated solar flux. A comparison between the Gardon and Kendall radiometers and the SUNCATCH calorimeter (described below) determined that the Kendall radiometer was the most reliable of the three (Kaluza and Neumann, 2001). As such, the Kendall radiometer is sometimes used for calibration of other radiometers and calorimeters. Gardon radiometers have the advantages of a wider angle of acceptance, a small size, mechanical robustness, and a faster response time than the Kendall radiometer (Hernandez et al., 2006; Ballestrin et al., 2006), but their accuracy is typically ±3%.
