Due to its elusive volumetric effect, the transparent packed-bed solar receiver that comprises quartz glass spheres and ceramic balls is a promising new approach as a reliable and cost-effective high-temperature receiver. However, this solar receiver suffers high reflection loss because the reflectivity of the transparent surface relates to the reflection area and the incidence angle, which limits its thermal efficiency and outlet temperature. To reduce the reflection loss of the transparent packed-bed solar receiver exposed to the incident solar radiation, a modified quartz glass Rasching ring (a hollow short cylinder whose outer diameter is equal to its length) with cut bottom surfaces is proposed and developed. The modified quartz glass Rasching ring can diminish the reflection loss by decreasing the reflection area through cutting bottom surfaces. Four types of mixed-packed solar receivers, including single silicon nitride ceramic ball (PB), quartz glass ball?silicon nitride ceramic ball (BB), quartz glass Rasching ring?silicon carbide ceramic ball (RRB), and quartz glass Rasching ring with cut bottom surfaces?silicon carbide ceramic ball (CRRB) models and prototypes are adopted to investigate the reflection loss of the modified quartz glass Rasching ring.

The overall reflection loss of four different mixed-packed solar receivers is tested by spectrophotometer and estimated using a weighted average scheme. Then, a 3D ray-tracing model is employed to investigate the reflection loss of the mixed-packed solar receiver based on geometrical optics together with the particle scale model. The incident solar radiation impinging on the inlet aperture of the mixed-packed solar receiver is produced by a parabolic solar concentrator associated with a grid light source in TracePro software. The test results under the parallel light condition have verified the reliability of the numerical model and the corresponding numerical method.

Results show that CRRB receiver can significantly reduce the reflection loss of concentrated solar radiation compared to the other three receivers. The reflection loss of CRRB receiver is only 1.9% when the tube-to-particle diameter ratio (D/d) is 5, while that of PB, BB, and RRB receivers is 8.8%, 11.6%, and 16.4% (Fig. 10), respectively. When the layer number of the modified quartz glass Rasching ring is less than 5.0, the reflection loss of CRRB receiver reduces as the layer number increases. However, the reflection loss remains constant when the layer number exceeds 5.0 (Fig. 11). Similarly, when the cut angle of the modified quartz glass Rasching ring is less than 45°, the reflection loss of CRRB reduces as the cut angle decreases. In contrast, the reflection loss remains unchanged once the cut angle exceeds 45° (Fig. 12). The greatest overall benefit would be achieved with 3–5 quartz glass Rasching ring layers with a cutting angle of 20° to 40°.

The modified quartz glass Rasching ring with cut bottom surfaces can significantly reduce the reflection of the packed-bed solar receiver. This modified quartz glass particle can be applied in advanced high-temperature power cycles as it is cost-effective and efficient.