Optical performance of segmented aperture windows for solar tower receivers

Abstract

Abstract. Segmented quartz windows are a concept to build larger windows for receivers that require a closed aperture. Reflection losses are a significant loss factor for such solar receivers. Without any additional measures, the reflection loss can reach about 12%. One important measure to improve transmission is the application of anti-reflective coatings, which is beneficial in any case. Another option is modifying the window geometry, especially the edge surfaces of the glass segments. A certain fraction of the reflection losses are caused by a light-guide effect in the glass body, for rays entering through the front surface. Changing the cut surfaces in a way reducing the light-guide effect can significantly improve transmission of a segmented window. Several possible configurations are evaluated and discussed. The results of raytracing simulations verify the improvement. The final selection of the window configuration depends on the optical properties and on mechanical strength, manufacturing and cost considerations. This has to be evaluated for any specific receiver design.

Effect of Quartz Aperture Covers on the Fluid Dynamics and Thermal Efficiency of Falling Particle Receivers

“…Based on results for full coverage cases with T 0 = 500 K, results for the half and full coverage cases with T 0 = 873 K were obtained using values of τ 0-2.5 µm = 0.95, 0.98, and 0.99. In ANSYS ® FLUENT, transmissivity in a volume is implemented via an absorption coefficient [19]. Transmissivities of 0.85, 0.90, 0.95, 0.98, and 0.99 are equivalent to absorption coefficients for the considered quartz of 65, 42, 20, 8, and 4 m -1 , respectively.…”

“…The mechanical properties of quartz were measured at elevated temperature, and dome-shaped quartz windows were successfully pressure tested at ambient temperature. Several quartz half-shell aperture cover configurations have been investigated numerically to reduce reflection losses [19]. Best performing configurations and practical considerations were identified.…”

Design Evaluation of a Next-Generation High-Temperature Particle Receiver for Concentrating Solar Thermal Applications

“…Quartz is an ideal material due to its high transmissivity in the solar spectrum (<2.5 microns) and low transmissivity in the near infrared spectra [69], enabling concentrated sunlight to pass through the aperture to the particles while trapping heat within the receiver. Yellowhair and Ho [70] performed ray-tracing analyses on the 1 MWth NSTTF FPR to evaluate different configurations of apertures covered with quartz half-shells (glass tubes cut in half lengthwise similar to other designs in the literature [71]). A shroud (or hood) was investigated as a means to support the half-shells at a desirable angle relative to the incoming flux while also potentially mitigating adverse impacts from wind.…”

Experimental Test Setup of an Airwall to Reduce the Convective Heat Loss in Solar Thermal Cavity Receivers

“…Different solutions have been proposed to prevent these losses: install a transparent window (e.g., made from quartz) [1], which potentially would entirely eliminate convection losses. Due to the challenge of producing a quartz window of the required size and durability, an array of quartz tube segments compiling the window has been proposed [2]. Alternatively, an airwall was proposed as a transparent shield for the cavity.…”