Assessment of Particle Candidates for Falling Particle Receiver Applications Through Irradiance and Thermal Cycling

Abstract

Falling particle receiver (FPR) systems are a rapidly developing technology for concentrating solar power applications. Solid particles are used as both the heat transfer fluid and system thermal energy storage media. Through the direct irradiation of the solid particles, flux and temperature limitations of tube-bundle receivers can be overcome, leading to higher operating temperatures and energy conversion efficiencies. Candidate particles for FPR systems must be resistant to changes in optical properties during long-term exposure to high temperatures and thermal cycling in highly concentrated solar irradiance. Seven candidate particles—two previously tested particles such as CARBOBEAD HSP 40/70 and CARBOBEAD CP 40/100 and the five novel particles such as CARBOBEAD MAX HD 35, CARBOBEAD Solar HSP, CARBOBEAD Solar CP, CARBOBEAD Solar MAX HD, and WanLi Diamond Black—were tested using simulated solar flux cycling and tube furnace thermal aging. Each particle candidate was exposed for 10,000 cycles (simulating the exposure of a 30-year lifetime of a concentrated solar power plant) using a shutter to attenuate the solar simulator flux. Feedback from a pyrometer temperature measurement of the irradiated particle surface was used to control the maximum temperatures of 775 °C and 975 °C. Particle solar-weighted absorptance and emittance were measured at 2000 cycle intervals. Particle thermal degradation was also studied by heating particles to 800 °C, 900 °C, and 1000 °C for 300 h in a tube furnace purged with bottled unpurified air. Here, particle absorptance and emittance were measured at 100-h intervals. Measurements taken after irradiance cycling and thermal aging were compared to measurements taken from as-received particles. WanLi Diamond Black particles had the highest initial value for solar-weighted absorptance, 96%, but degraded up to 4% in irradiance cycling and 6% in thermal aging. CARBOBEAD HSP 40/70 particles currently in use in the prototype FPR at the National Solar Thermal Test Facility had an initial value of 95% solar absorptance with up to a 1% drop after irradiance cycling and 4% drop after 1000 °C thermal aging.