Central receivers being installed in recent commercial CSP plants are liquid-cooled and power a steam turbine in a Rankine cycle. San Diego State University’s (SDSU) Combustion and Solar Energy Laboratory has built and is testing a lab-scale Small Particle Heat Exchange Receiver (SPHER). The SPHER is an air-cooled central receiver that is designed to power a gas turbine in a Brayton cycle. The SPHER uses carbon nanoparticles suspended in air as an absorption medium. The carbon nanoparticles should oxidize by the outlet of the SPHER, which is currently designed to operate at 5 bar absolute with an exit gas temperature above 1000°C.
Carbon particles are generated from hydrocarbon pyrolysis in the carbon particle generator (CPG). The particles are mixed at the outlet of the CPG with dilution air and the mixture is sent to the SPHER. As the gas-particle mixture flows through the SPHER, radiation entering the SPHER from the solar simulator is absorbed by the carbon particles, which transfer heat to the gas suspension and eventually oxidize, resulting in a clear gas stream at the outlet.
Particle mass loading is measured using a laser opacity measurement combined with a Mie calculation, while particle size distribution is determined by scanning electron microscopy and a diesel particulate scatterometer prior to entering the SPHER. In predicting the performance of the system, computer models have been set up in CHEMKIN-PRO for the CPG and in ANSYS Fluent for the SPHER, which is coupled with VeGaS ray trace code for the solar simulator.
Initial experimentation has resulted in temperatures above 850°C with around a 50K temperature difference when particles are present in the air flow. The CPG computer model has been used to estimate performance trends while the SPHER computer model has been run for conditions to match those expected from future experimentation.