Research Papers

Analysis of Nanofluid-Based Parabolic Trough Collectors for Solar Thermal Applications

[+] Author and Article Information
Justin P. Freedman, Hao Wang

Energy Storage and Distributed Resources
Lawrence Berkeley National Laboratory,
1 Cyclotron Road,
Berkeley, CA 94720

Ravi S. Prasher

Energy Storage and Distributed Resources
Lawrence Berkeley National Laboratory,
1 Cyclotron Road,
Berkeley, CA 94720;
Department of Mechanical Engineering,
University of California,
Berkeley, CA 94720
e-mail: rsprasher@lbl.gov

1Corresponding author.

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 26, 2017; final manuscript received March 31, 2018; published online May 29, 2018. Assoc. Editor: Marc Röger.

J. Sol. Energy Eng 140(5), 051008 (May 29, 2018) (8 pages) Paper No: SOL-17-1467; doi: 10.1115/1.4039988 History: Received November 26, 2017; Revised March 31, 2018

Solar-to-thermal energy conversion technologies are an important and increasingly promising segment of our renewable energy technology future. Today, concentrated solar power (CSP) plants provide a method to efficiently store and distribute solar energy. Current industrial solar-to-thermal energy technologies employ selective solar absorber coatings to collect solar radiation, which suffer from low solar-to-thermal efficiencies at high temperatures due to increased thermal emission from selective absorbers. Solar absorbing nanofluids (a heat transfer fluid (HTF) seeded with nanoparticles), which can be volumetrically heated, are one method to improve solar-to-thermal energy conversion at high temperatures. To date, radiative analyses of nanofluids via the radiative transfer equation (RTE) have been conducted for low temperature applications and for flow conditions and geometries that are not representative of the technologies used in the field. In this work, we present the first comprehensive analysis of nanofluids for CSP plants in a parabolic trough configuration. This geometry was chosen because parabolic troughs are the most prevalent CSP technologies. We demonstrate that the solar-to-thermal energy conversion efficiency can be optimized by tuning the nanoparticle volume fraction, the temperature of the nanofluid, and the incident solar concentration. Moreover, we demonstrate that direct solar absorption receivers have a unique advantage over current surface-based solar coatings at large tube diameters. This is because of a nanofluid's tunability, which allows for high solar-to-thermal efficiencies across all tube diameters enabling small pressure drops to pump the HTF at large tube diameters.

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Grahic Jump Location
Fig. 1

On the left is a depiction of the real-world circular geometry of a PTCs and on the right is the approximated rectangular geometry used in this study to solve the RTE. For simplicity, we have assumed that the liquid is surrounded by a material with refractive index of, nTube = 1 (the same as vacuum). This does not affect any conclusions of the paper as the refractive index of the surrounding/enclosing medium only affects the critical angle and the reflectivity at the surface, which can be easily incorporated into a detailed model. On the sketch on the right side of the sketch, the incident solar radiation enters the fluid in a cone of concentrated sunlight with an angle between the cone and the tube equal to the critical angle based on Snell's law. The HTF is therminol VP-1 seeded with Ag nanoparticles and the inlet temperature is uniform across the y-axis. The reflectivity of the bottom of the receiver is gray, where ρL = 1. The receiver is encased in a vacuum that eliminates thermal convection losses.

Grahic Jump Location
Fig. 2

(a) Spectral refractive index of Ag and therminol VP-1. (b) Spectral radiative absorption coefficient for Ag nanoparticles seeded in therminol VP-1 under Rayleigh scattering conditions, where fv = 1 × 10−9: (a) optical properties of Ag and therminol and (b) Rayleigh scattering.

Grahic Jump Location
Fig. 3

Temperature and thermal conductivity profiles for therminol VP-1 seeded with Ag nanoparticles and fv = 9.5 × 10−5. The receiver tube has a diameter, Ly = 76 mm, the inlet temperature is Tin = 566 K, the mass flow rate of the therminol VP-1 is 12 kg/s, and the incident solar heat flux is 40 suns. (a) Temperature profiles of the HTF as a function of depth along the y-axis at various positions along the receiver tube. (b) The effective thermal conductivity due to mixing during turbulent flow conditions, kturb, as a function of position along the receiver. Due to the high values of kturb, the temperature profiles in (a) exhibit small temperature variations along the y-axis. (c) The mean/bulk temperature of the HTF as a function of position along the receiver tube.

Grahic Jump Location
Fig. 4

((a) and (b)) Solar-to-thermal conversion efficiency as a function of fv for four different incident solar concentrations with Ag nanoparticle-seeded therminol VP-1 in a tube with a receiver depth of Ly = 76 mm and (a) Tin = 566 K/(b) Tin = 300 K. ((c) and (d)) Spectral radiative heat flux at the surface of the receiver tube, x = y = 0, for an array of fv values. A positive heat flux indicates that heat is absorbed by the receiver, while a negative heat flux represents re-emitted thermal radiation losses. The thermal radiation losses are greater in (c) than (d) because the temperature of the HTF is far greater.

Grahic Jump Location
Fig. 5

(a) Solar-to-thermal conversion efficiency as a function of fv for five different receiver depths. (b) Solar-to-thermal conversion efficiency as a function of τ for five different receiver depths. For small values of optical depth all four curves from (a) collapse to one curve that is independent of Ly.

Grahic Jump Location
Fig. 6

(Left y-axis) Normalized solar-to-thermal efficiency for volumetric and surface-based receivers. (Right y-axis) Normalized pressure drop required to pump the HTF.



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