The effect of solid particle recirculation on the efficiency of a cavity-type solar receiver

Abstract

High temperature steam or gas is used as the working medium for operation in advanced power conversion units, such as gas turbines or combined cycle power plants. The use of Concentrated Solar Power (CSP) technologies for energy generation relies on the heating of fluids to high temperatures. Conventional tubular or porous absorber volumetric receivers usually suffer from a relatively low thermal efficiency, less than 70%, and from high thermal losses. The solid particle solar receiver, in which the incoming solar radiation is directly absorbed by the solid particles and inner walls of the cavity receiver, promises to achieve higher thermal efficiency due to the enhanced heat transfer to the working fluid and the reduction in thermal losses. The main challenge in designing a high temperature solar receiver is to maximize the solar energy absorption and its efficient transfer to the working fluid, e.g. air. The objective of this research project is to characterize the thermal behaviour of a proposed cavity-type solar receiver concept based on the application of recirculating solid particles to achieve high temperature outlet air. In this concept, solar energy is absorbed by recirculating solid metallic particles, which, at the same time, transfer the absorbed thermal energy to the surrounding air in a cavity-type receiver. The cavity-type solid particle receiver is a well-insulated enclosure, designed to capture the incident solar radiation effectively, which allows the incident radiation directly through an aperture. In this study, a well-insulated cavity-type enclosure was designed and constructed. The designed solar receiver consisted of an externally insulated cylindrical cavity with a circular aperture at the top covered by a quartz glass, a recirculating fan installed at the bottom of the chamber and with an inlet and an outlet installed on the chamber wall. A series of experiments were conducted to characterize the fluid dynamics and thermal behaviour of the solar receiver. The results showed that the direction of the total air flow velocity was vertically upward from the outer periphery of the fan and vertically downward through the centre of the receiver. The measured air flow velocity was normalized by against the particle terminal velocity. The results showed that the normalized flow velocity was 3 times higher than the particles terminal velocity and 45 times higher than the minimum fluidization velocity of particles for the same case. Hence the particle can be recirculated in the cavity. The particles’ concentration at different regions of the solar receiver was also measured using a laser-based light-scattering technique. Black SiC of 70micron and 200micron SiC particle were used during this experiment. The effect of particle size and fan RPM on the particle concentration at different regions of the solar receiver was optimized using the test results of the particle concentration measurements. At an RPM of 1250, the measured opacity of the particle cloud’s concentration was 0.95 for 70micron and 0.80 for 200micron particle at a specific freeboard height of the cavity. It is an indication that particles’ concentration and opacity at a different region is dependent on the particle size and air flow velocity. The results showed that the particle concentration and opacity increased exponentially with the RPM. The opacity is a measure of the irradiation penetration characteristics of the Xenon arc radiation beam into the cavity. The calculation of Stokes number and turbulence intensity from these measurements helped in understanding the slip velocity and velocity fluctuation of air and particles in the cavity. The maximum Stokes number was 0.50 for 70micron particle whereas it was 4 for 200 micron particle measured at 500RPM. Hence, when comparing the Stokes numbers it becomes clear that the slip velocity was higher for 200micron particles which denote more interaction with air. Turbulence intensity measurement showed that an average turbulence intensity of 38% was observed near the cavity wall and 30% at the centre of the cavity. Therefore, a better forced convection heat transfer was expected between the particle, air and cavity wall surface in the proposed receiver. Finally a thermal test was performed to assess the thermal performance of the designed solar receiver. Black SiC and brown alumina particles at two sizes (70 and 200microns) were used in these experiments. The results showed that the air temperature in the solar receiver, when radiated by a 5kW Xenon arc lamp, increased by up to 83K resulting in a thermal efficiency of 55% for the proposed solar receiver. Here the air flow rate was set to 0.0055kg/s at an inlet temperature of 295K with no particles or fan forced circulation. When particles were added to the receiver, a 0.034% volume fraction, and the installed fan induced forced recirculation in the receiver the air temperature at the outlet of the receiver increased by 123K and the thermal efficiency reached 89% for the same air flow rate as before. The results of normalized thermal and power generation efficiency indicate that the use of recirculating particles enhances the thermal efficiency by 49% and power generation efficiency by 90%, when compared with the air only cavity-type solar receiver. Moreover the proposed recirculating flow receiver exhibits a more uniform temperature distribution than a conventional cavity-type solar receiver due to better mixing of air and particles, along with more effective radiation and convection heat transfer between them. A stand-alone feature of this solar receiver concept is that the air with particles is directly exposed to concentrated solar radiation monotonously through the recirculating flow inside the receiver and results in an efficient irradiation absorption and convective heat transfer to the air. The increase of thermal and power generation efficiency of more than 50% proved that the developed concept has the potential to enhance heat transfer from metallic particles to air through maximizing the heat carrying capacity by the particles and air from the receiver.