(612e) Through-the-Glass Thermal Imaging of Solar Cavity Receivers

Solar cavity receivers lie at the heart of many high-temperature solar thermal processes. Measurement of the temperature distribution inside the cavity is of critical important to determining the efficiency and successful operation of the receiver. Traditional contact thermometry approaches, e.g. using point-measurement thermocouples, can be costly and challenging at high temperatures, and cannot provide a high-resolution map of the temperature distribution inside a solar cavity receiver. Non-contact approaches, e.g. infrared thermography, allow for convenient spatial mapping without the need to physically touch the high temperature process. However, the application of infrared thermography to solar cavity receivers has been severely limited by the fact that many receivers utilize a glass front window which is opaque to thermal radiation, rendering infrared thermography of the cavity’s inner surfaces ineffective.

In this work, we present the design, construction and successful demonstration of a thermal camera capable of seeing through glass. This "through-the-glass" thermal camera utilizes a high-resolution grayscale CMOS optical sensor equipped with a narrow bandpass filter with a center wavelength of 772 nm (which is near the transmission peak of many optical glasses), to effectively probe the monochromatic spectral radiance of any high temperature surface. Being sensitive to short-wave photons, our device can effectively see through glass and similar optical glazings, allowing direct contactless measurement of the inner surfaces of a cavity receiver. By utilizing a standard optical CMOS sensor, the device achieves much higher resolution at a much lower cost than can traditional thermal cameras operating in the mid-infrared. An additional advantage of the short-wave approach is that the device is also impervious to any radiatively participating media present inside the cavity, making it a very flexible measurement technique.

Using a calibrated photometry sphere with a known spectral radiance, we experimentally determined the device sensitivity (grayscale value per photon per second), which allows the spectral radiance and ultimately temperature to be determined from a simple grayscale image. A thermal blackbody operating at above 1000 °C was used as a secondary calibration and a thermal validation of the photometric calibration. We then successfully used our device to map the temperature distribution inside a solar cavity receiver operating at temperatures well in excess of 1000 °C. The results serve as an experimental proof-of-concept for through-the-glass contactless thermal measurement of solar cavity receivers. We believe this low-cost and flexible technique will become a staple in characterization of high-temperature solar receivers, and may well find utility in many other high temperature industrial applications.