A solar central receiver absorbs concentrated sunlight and transfers its energy to a working medium (gas, liquid or solid particles), either in a thermal or a thermochemical process. Various attractive high-performance applications require the solar receiver to supply the working fluid at high temperature (900–1500°C) and high pressure (10–35 bar). As the inner receiver temperature may be well over 1000°C, sunlight concentration at its aperture must be high (4–8 MW/m2), to minimize aperture size and reradiation losses. The Directly Irradiated Annular Pressurized Receiver (DIAPR) is a volumetric (directly irradiated), windowed cavity receiver that operates at aperture flux of up to 10 MW/m2. It is capable of supplying hot gas at a pressure of 10–30 bar and exit temperature of up to 1300°C. The three main innovative components of this receiver are: • a Porcupine absorber, made of a high-temperature ceramic (e.g., alumina); • a Frustum-Like High-Pressure (FLHIP) window, made of fused silica; • a two-stage secondary concentrator followed by the KohinOr light extractor. This paper presents the design principles of the DIAPR, its structure and main components, and examples of experimental and computational results.

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A new Volumetric (Directly-Irradiated)

The Directly Irradiated Annular Pressurized Receiver (DIAPR) is a volumetric (directly-irradiated) windowed cavity receiver, designed for operation at a pressure of 10–30 bar, exit gas temperature of up to 1,300°C, and aperture radiation flux of up to 10 MW/m2. This paper presents test results obtained under various irradiation conditions and flow rates. Inlet aperture flux was up to 5 MW/m2; exit air temperatures of up to 1,200°C were obtained, while operating at pressures of 17–20 bar. Estimated receiver efficiency in these tests was in the range of 0.7–0.9. The absorber and window temperatures were 200-400°C below the permitted maximum, indicating that higher air exit temperatures are possible.

Methane reforming with carbon dioxide in a directly irradiated particle receiver seeded with carbon black is presented in this study. Carbon particles were entrained in the reacting gases and acted as heat transfer and reaction surface. The reactions were not catalyzed by a metal catalyst. The molar ratio between the entrained carbon particles and the working gases (Ar, CO2 and CH4) was 4–7 mmol carbon/mol gas. The temperature of the reforming experiments varied from 900°C to 1450°C with CO2/CH4 ratios of 2–6. Experimental results show that methane reacts at lower temperatures than expected for its thermal decomposition; this indicates that the decomposing reaction is enhanced by the presence of the carbon black particles. At 1170°C 90% of the methane reacted in the receiver during a residence time of 0.3 s. The reaction between carbon dioxide and carbon black is faster than is documented in the literature, but the reaction rate does not seem to change if only carbon dioxide and carbon black are present in the receiver, compared to experiments where methane is also part of the gas mixture. The experimental results indicate that a high solar flux, i.e., about 2500 kW/m2 or higher, significantly accelerates the reaction rate of methane decomposition. Total or partial blockage of the solar radiation reduced the yield by about 50%, compared to tests when the receiver was exposed to the full solar radiation flux, at the same operating temperature.