Simulation of shortwave infrared ratio thermometers for the remote opto-thermal characterisation of central external receivers

Abstract

The accurate knowledge of the receiver surface temperature Tsurf is important for a safe, efficient and durable power plant operation. Its distribution is typically measured in real time using ground-based longwave infrared (LWIR) thermal cameras. Their calibration requires a priori knowledge of the receiver surface LWIR band emittance $\varepsilon$LWIR. This parameter can be measured with great effort, using portable reflectometers for on tower optical inspection during periodical power plant maintenance. This paper analyses a new measurement principle, based on passive shortwave infrared (SWIR) ratio thermography, for the simultaneous measurement of surface temperature Tsurf and band emittance $\varepsilon$SWIR. The first SWIR ratio thermometer combines two narrow bandpass filters centered on water vapor atmospheric absorption bands (1.4/1.9 µm). This thermometer is sensitive to water vapor to block solar radiation, however thermal radiation emitted by the receiver is also attenuated. The applicability of this thermometer is limited for remote opto-thermal characterization. Under favorable operating conditions, it can measure temperature leve[ls above 550 °C with a relative temperature error $\Delta$T/T less than 2 %. The second SWIR ratio thermometer combines two narrow bandpass filters centered on atmospheric windows (1.64/2.09 µm). This thermometer is insensitive to water vapor and suited for remote distances, however it can only operate off-sun when receiver surface temperature is still above 300 °C, for instance during the cool down phase, before molten salts drainage. The relative temperature error $\Delta$T/T is less than 0.5 % for Pyromark 2500 and oxidized Haynes 230, while the absolute band emittance error $\Delta\varepsilon$ is less than 2.5 percentage points.