Improved Performance of Ceramic Solar Absorber Particles Coated with Black Oxide Pigment Deposited by Resonant Acoustic Mixing and Reaction Sintering

Abstract

Spherical particles based on bauxite-type raw materials, commonly referred to as proppants, are state-of-the-art for particle receivers of concentrated solar power plants. Particles are heated in a fluidized reactor by focused sunlight and are transported to a heat-exchanger or a storage tank. Therefore, key properties for absorber particles are high solar absorptance and mechanical endurance. Due to their relatively poor content of color-giving transition-metal cations, bauxite-derived proppants show limited solar absorptance, which is even deteriorating by long-term heat exposure. A deep-black Cu, Mn, Fe- pigment with a spinel structure was employed to coat standard proppants in order to improve solar absorptance. The coating process was performed by high-energy, high-speed mixing of proppants and small quantities of spinel powders in a resonant acoustic mixer. A continuous powder coating is achieved by electrostatic attraction between the proppant surface and the coating particles. Consolidation and strong attachment of the coating is achieved by the subsequent sintering beyond the spinel melting temperature. Chemical reaction and bonding between spinel coating and proppant lead to the incorporation of Al, Mg and Ti into the spinel structure. Coated bauxite proppants exhibit a significantly improved, long-term stable solar absorption accompanied by a promising abrasion resistance. The presented coating methodology is considered to be scalable to industrial production.

Using an Al-Incorporated Deep Black Pigment Coating to Enhance the Solar Absorptance of Iron Oxide-Rich Particles

“…To begin with, these new particles exhibit negligible absorptance loss when compared with bauxite proppants, which had lost almost 15% of as-received solar absorptance as previously reported. [13,14]. Beyond highly promising optical performance of as-received iron oxide particles, spectrometric analysis given in Figure 5 proved their further enhancement through the 'Al-deep black coating from 0.89 and 0.92.…”

“…Subsequently, a heat treatment at 1200 • C was applied for 2 h to ensure the bonding of pigment to the particle surface. Details of the coating method can be found in a previous publication [14]. A muffle furnace was used to apply thermal aging to the particles at 1000 • C for 1 week to estimate their long-term optical endurance.…”

“…In order to assess the potential of increased solar absorptance by additional surface modification, iron oxide-rich particles were coated with deep black pigment using the same coating and sintering parameters for bauxite proppants, as reported in our previous study [14]. Figure 3 depicts the visual appearance and SEM/EDS mapping of iron oxide particle cross sections at the surface after deep black pigment coating.…”

“…study [14]. Figure 3 depicts the visual appearance and SEM/EDS mapping of iron oxide particle cross sections at the surface after deep black pigment coating.…”

“…In the subsequent sintering stage at a temperature slightly above the pigment's melting point, uniform coverage of the proppant surface was ensured. Moreover, owing to the chemical reaction between deep black and Al, Mg and Ti components of proppants, an Alrich spinel was formed with higher temperature stability, which is promising for longterm use [14].…”