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Research Papers

Model Development and Performance Studies of a Concentrating Direct Absorption Solar Collector

[+] Author and Article Information
Ramsatish Kaluri

Siemens Technology and Services Pvt. Ltd.,
Research and Technology Center India,
No. 84, Keonics Electronics City,
Bangalore 560100, India
e-mail: ramsatish.k@siemens.com

Sanjay Vijayaraghavan, S. Ganapathisubbu

Siemens Technology and Services Pvt. Ltd.,
Research and Technology Center India,
No. 84, Keonics Electronics City,
Bangalore 560100, India

1Present address: GE Global Research, No. 122, EPIP Zone, Whitefield, Bangalore 560066, India.

2Present address: CSIRO Energy Center, 10 Murray Dwyer Circuit, Mayfield West, NSW 2304, Australia.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received November 13, 2013; final manuscript received August 19, 2014; published online September 30, 2014. Assoc. Editor: Prof. Nesrin Ozalp.

J. Sol. Energy Eng 137(2), 021005 (Sep 30, 2014) (12 pages) Paper No: SOL-13-1338; doi: 10.1115/1.4028399 History: Received November 13, 2013; Revised August 19, 2014

A detailed three-dimensional (3D) computational fluid dynamics (CFD) model of a direct absorption solar collector (DAC) is presented. Radiative transfer equation (RTE) is coupled with Navier–Stokes equations and solved numerically to predict the collector efficiency. The spectral properties of absorbing liquids are captured using a band-averaged absorption model. This numerical model is validated with experimental data for two different types of absorbing fluids viz., gray (graphite particles in water) and nongray (copper sulfate) fluids. The validated model is used for parametric studies to determine the right design choices for an improved collector. Impact of optical concentration ratio (CR), optical density of the fluid, mass flowrate, and thermal insulation on the collector efficiency were studied. Increase in collector efficiency of up to 28% is seen due to higher optical CRs, which is attributable to good absorption characteristics of the receiver and reduced area for losses. The collector efficiency does not improve with absorption coefficient of the fluid beyond a certain value for a given thickness of the fluid layer. The range of mass flow rates considered in the study was found to have no impact on collector efficiency. Thermal insulation is found to be very effective in minimizing the overall thermal losses and enhancing the collector efficiency. The numerical model presented here may be used to identify optimum CR, absorption coefficient of liquid for a direct absorption concentrating collector.

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References

Figures

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Fig. 1

Schematic of experimental setup for hybrid solar collector

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Fig. 2

Schematic of the absorber tube

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Fig. 3

Spectral variation of absorption coefficient (α) for graphite solution of various concentrations in water, measured using a spectrophotometer

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Fig. 4

Spectral variation of absorption coefficient (α) for copper sulfate solution for various concentrations in water, measured using a spectrophotometer

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Fig. 5

Profile of incident radiation along a line across the width of the receiver. The heat input and the CR for the profile above corresponds to 116.7 W and CR = 109, respectively. For this case, A′ = 100441 and σ2= 1.963 × 10−3.

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Fig. 6

Comparison of predicted collector efficiency with experimental data for different cases with 2 g/l graphite solution in water as absorption fluid at optical concentration ratio, CR = 109

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Fig. 7

Comparison of predicted collector efficiency with experimental data for different cases with 40 g/l copper sulfate in water as absorption fluid at optical concentration ratio, CR = 109

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Fig. 8

Contours of temperature (K) at top wall, y-mid, and bottom wall for graphite solution at test conditions of case 1, Table 2. The mass flow rate is 0.015 kg/s.

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Fig. 9

Contours of incident radiation (W/m2) at top wall, y-mid, and bottom wall for graphite solution at the test conditions of case 1 (see Table 2)

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Fig. 10

Extinction of incident radiation (W/m2) along the depth of the fluid layer for graphite solution at the test conditions of case 1 (see Table 2)

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Fig. 11

Contours of temperature (K) at top wall, y-mid, and bottom wall for copper sulfate solution at test conditions of case 1 (see Table 3)

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Fig. 12

Contours of incident radiation (W/m2) at top wall, y-mid, and bottom wall for copper sulfate solution in bands 1 and 2 at test conditions of case 1 (see Table 3)

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Fig. 13

Extinction of incident radiation (W/m2) in bands 1 and 2 along the depth of the fluid layer for copper sulfate solution at test conditions of case 1 (see Table 3)

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Fig. 14

Comparison of predicted collector efficiency for graphite and copper sulfate solutions as absorbing fluids, for conditions considered in the present study

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Fig. 15

Variation of CR with the diameter of spot size at the focus for lens A with aperture area of 0.215 m2

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Fig. 16

Contours of incident radiation (W/m2) at the top wall of the absorber tube for different CRs. Note that the input power is same in all the cases (150 W).

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Fig. 17

Collector efficiency at different CRs for graphite solution of concentration 2 g/l in water

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Fig. 18

Effect of absorption coefficient (α) on collector efficiency for CR of 27 and mass flow rate 0.015 kg/s and fluid inlet temperature of 30 °C.

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Fig. 19

Comparison of Reference G173 solar spectrum and solar-weighted absorption spectra of graphite solution of path length 20 mm at different fluid concentrations

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Fig. 20

Effect of mass flowrate on collector efficiency at different mass flow rate and CRs. (A1) CR = 27, m· = 0.0295 kg/s, (A2) CR = 27, m· = 0.015 kg/s, (B1): CR = 48, m· = 0.0295 kg/s, (B2): CR = 48, m· = 0.015 kg/s, (C1): CR = 683, m· = 0.0295 kg/s, (C2): CR = 683, m· = 0.015 kg/s.

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Fig. 21

Influence of thermal insulation on collector efficiency for graphite solution and copper sulfate solutions

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