Particle Heating Receivers (PHR) offer a range of advantages for concentrator solar power (CSP). PHRs can facilitate higher operating temperatures (>700°C), they can allow for inexpensive direct storage, and they can be integrated into cavity receiver designs for high collection efficiency. In operation, PHRs use solid particles that are irradiated and heated directly as they fall through a region exposed to concentrated sunlight. The heated particles can subsequently be stored in insulated bins, with the stored thermal energy reclaimed via heat exchanger to secondary working fluid for the power cycle in CSP. In this field Georgia Tech has over five years’ experience developing PHR technology through the support of the DOE SunShot program and similar research efforts. Georgia Tech has dealt with the crucial challenges in particle receiver technology: particulate flow behavior, particulate handling, and particulate heat transfer. In particular, Georgia Tech has specialized in innovative advances in the utilization and design of discrete structures in PHRs (DS-PHR) to prolong particulate residence time in the irradiated zone.
This paper describes the development and results of lab-scale testing for DS-PHRs especially in the Georgia Tech high flux solar simulator (GTHFSS). The GTHFSS is a bank of high intensity xenon lamps with elliptical reflectors designed to replicate a concentrated solar source. Two series of tests have been undertaken: batch and continuous operation. Initially the DS-PHR has been tested in a batch apparatus in which a substantial but still limited quantity of preheated particulate flows through from an elevated bin through the irradiated PHR into a weighing box collecting bin. The use of a weighing box is advantageous since the flow rate of particulate is otherwise especially hard to measure. Temperature rise measurements and mass flow rate measurements allow calculation of energy collection rates. Calorimetry measurements, also described in the paper, are used to verify the incident concentrated radiation allowing the calculation of the collection efficiency. This preliminary series of experiments have been completed using the batch apparatus, with the efficiencies of the lab-scale DS-PHR being determined for a range of temperatures. Efficiencies above 90% have been measured at low temperatures roughly corresponding to the so-called optical efficiency, which is the rate of energy collection at low temperature and minimal heat loss. Batch experiment data indicates a collection efficiency of approximately 81–85% at an average particle operating temperature of 500°C. Lab-scale batch results at 700°C in proved to be unstable, and as such a rework employing a continuous recirculation loop is underway.
While the batch apparatus is convenient for preliminary work, it is challenging to reach steady state operation in the mixing and measurement section below the DS-PHR, which limits this apparatus in higher temperature experiments. Consequently, the experiment is being reconfigured for continuous flow, in which the particulate will be heated and recirculated by a high temperature air conveyor. The advantage of the high temperature conveyor has already been proved by its successful integration as a heater and mixer in the hot bin of the batch apparatus. Such a compact device was also quite advantageous in the limited confines of a typical laboratory simulator such as the GTHFSS. While continuous flow prevents the exceedingly desirable use of an uninterrupted mass measurement device, highly accurate mass flow data is still expected based on the use of a perforated plate flow control station. This device relies on the Berverloo effect to maintain a constant flow of particulate through an array of orifices, for which the flow is largely independent of upstream conditions. A weighing box will be used to calibrate and verify the mass flow. This paper will report on efficiency measurements with the batch flow experiments and present the preliminary steps taken to conduct the recirculation experiment. The bulk the research reported in the paper is sponsored by and done in support of the DoE Sun Shot initiative.