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Research Papers

# Experimental Evaluation of Particle Consumption in a Particle Seeded Solar Receiver

[+] Author and Article Information
Hanna Helena Klein

Department of Environmental Science and Energy Research,  Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israelh.h.klein@ezklein.org

Rachamim Rubin

Solar Research Center,  Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel

Jacob Karni

Department of Environmental Science and Energy Research,  Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel

J. Sol. Energy Eng 130(1), 011012 (Dec 28, 2007) (8 pages) doi:10.1115/1.2804631 History: Received September 25, 2006; Revised May 21, 2007; Published December 28, 2007

## Abstract

This experimental study shows the behavior of a directly irradiated, high temperature, solar receiver seeded with a low concentration of carbon black particles as the radiation absorbing media in the presence of air or nitrogen as the working fluid. Experiments were conducted in the presence of highly concentrated solar energy with an energy flux of up to $3MW∕m2$ at the aperture of the receiver. 99.9% of the particles had an equivalent diameter of $<5μm$, but the remaining larger agglomerates accounted for 51% of the overall projected surface area. The molar ratio of carbon to gas in the fluid entering the receiver was 0.004–0.008. The heat transfer from the solar radiation to the working gas was accomplished almost exclusively via the particles. The receiver behavior during steady-state operation was evaluated. The receiver gas exit temperatures achieved during the experiments were between 1000 and $1550°C$. When nitrogen was used as working gas, its exit temperature exceeded the average wall temperature, whereas when air was used, its exit temperature was lower than the average wall temperature. The air flow may have been heated to some extent by the receiver walls, whereas in the case of nitrogen, the particle-to-gas heat transfer was dominant throughout the receiver. When the gas exit temperature was above $1200°C$, the particle seeded nitrogen flow absorbed 12–20% more energy than particle seeded air flow under the same operating conditions (insolation, particle load, flow rate, close proximity in time). The air tests reached high exit temperatures despite the reduction of particle concentration due to combustion. This indicates that heat transfer mainly occurs over a relatively short time period after the particle seeded flow enters the cavity close to the receiver aperture, before significant particle burning takes place. The energy due to carbon combustion was 3–5% of total energy absorbed in the high temperature air experiments. The carbon particles’ oxidation rate in the presence of molecular oxygen was found to be significantly lower than values documented in the literature for high temperature carbon black combustion in air. The high solar flux, which promotes very high $radiation→particle→gas$ heat transfer rate, might account for this retardation.

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## Figures

Figure 1

A schematic of the optical system with a heliostat as the primary concentrator and a parabolic stationary mirror as the secondary concentrator

Figure 2

Figure 3

Particle size distribution and relative cumulated projected surface area as a function of particle equivalent diameter (the particle number is normalized by dividing by the total number of particles)

Figure 4

Gas exit temperatures and insolation as a function of time for two experiments with air and N2 (No. 12 in Table 1) with a flow rate for both gases of 125 SLPM and a particle concentration of 3.5g∕m3. Initially, air was used and when steady-state conditions were reached, the gas was switched to nitrogen.

Figure 5

Average dimensionless wall temperature for the different sections in the receiver. Results from No. 4 in Table 1 are represented by a square and No. 12 in Table 1 by a diamond.

Figure 6

Relative power absorbed in the receiver for air versus N2 as a function of maximum gas exit temperature (°C). Data for the experiments can be found in Table 1.

Figure 7

ΔT between average gas exit temperature and average wall temperature in the receiver as a function of average wall temperature (°C) and gas composition. Data for these experiments are listed in Table 2.

Figure 8

Difference between ΔT(Tgasexit−Twall) (°C) for pairs of N2 and air experiments performed under very similar conditions (particle load, time of day, insolation, mass flow rate). Data for these experiments are listed in Table 2.

Figure 9

Average wall temperature for different receiver sections as a function of total particle load in the receiver. The experiments were conducted in the short version of the receiver (Sections A, B, F, and G in use) with the solar radiation limited by the Venetian blind.

Figure 10

Relative wall and gas exit temperatures in the four sections of the short version of the solar receiver for different particle concentrations in the gas

Figure 11

Relative power absorbed (calculated according to Eq. 6) in the gas as a function of mass flow rate. Data scattering is mainly due to differences in energy inlet to the receiver.

Figure 12

The time to total consumption as a function of activation energy at different temperatures. Equation 7 was used with a specific particle surface area of 73m2∕g, as listed for Asbury No. 5358.

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