Thermal efficiency enhancement of nanofluid-based parabolic trough collectors

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Abstract

The use of nanofluids in parabolic trough collectors is one of the most promising techniques for enhancing their performance. The objective of this work is to investigate the use of various nanoparticles (Cu, CuO, Fe2O3, TiO2, Al2O3 and SiO2) dispersed in thermal oil (Syltherm 800). A detailed parametric analysis is performed for flow rates from 50 to 300 L min−1, for inlet temperatures from 300 K to 650 K and for nanoparticle concentrations up to 6%, while the impact of the solar irradiation level on the thermal efficiency enhancement is also investigated. Moreover, a new index for the working fluid evaluation in solar collectors is introduced. The analysis is conducted with a developed thermal model in Engineering Equation Solver. According to the final results, the most efficient nanoparticle is the Cu, with CuO, Fe2O3, TiO2, Al2O3 and SiO2 to follow, respectively. It is found that the higher enhancement is observed for lower flow rates, higher inlet temperatures and higher nanoparticle concentrations, while it is approximately constant for the different solar irradiation levels. For the typical operating conditions with 150 L min−1 flow rate and 600 K inlet temperature, the thermal efficiency enhancement is found 0.31, 0.54 and 0.74% for Cu concentrations 2, 4 and 6%, respectively.

Keywords

Solar energy Nanofluid Parabolic trough collector Thermal efficiency Enhancement 

List of symbols

A

Area, m2

C

Concentration ratio

cp

Specific heat capacity under constant pressure, J kg−1 K−1

D

Diameter, m

F

Focal length, m

Gb

Solar direct beam irradiation, W m−2

H

Heat transfer coefficient, W m−2 K−1

hout

Convection coefficient between cover and ambient, W m−2 K−1

k

Thermal conductivity, W m−1 K−1

K

Incident angle modifier

L

Tube length, m

m

Mass flow rate, kg s−1

Nu

Nusselt number

Pr

Prandtl number

Q

Heat flux, W

Re

Reynolds number

T

Temperature, K

Tsky

Sky temperature, K

Ur

Overall heat transfer coefficient, W m−2 K−1

V

Volumetric flow rate, L min−1

Vwind

Ambient air velocity, m s−1

W

Width, m

Greek symbols

α

Absorber absorbance

β

Ratio of the nanolayer thickness to the original particle radius

γ

Intercept factor

ε

Emittance

ηopt

Optical efficiency

ηth

Thermal efficiency

θ

Solar beam incident angle, o

μ

Dynamic viscosity, Pa s

ρ

Density, kg m−3

ρmir

Mirror reflectance

τ

Cover transmittance

φ

Nanoparticle, concentration %

Subscripts and superscripts

a

Aperture

am

Ambient

bf

Base fluid

c

Cover

ci

Inner cover

co

Outer cover

fm

Mean fluid

in

Inlet

loss

Thermal loss

nf

Nanofluids

np

Nanoparticle

out

Outlet

r

Receiver

ri

Inner receiver

ro

Outer receiver

s

Solar

u

Useful

0

Reference

Abbreviations

CFD

Computation fluid dynamic

EES

Engineering equation solver

PTC

Parabolic trough collector

Notes

Acknowledgements

Dr. Evangelos Bellos would like to thank “Bodossaki Foundation” for its financial support.

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Copyright information

© Akadémiai Kiadó, Budapest, Hungary 2018

Authors and Affiliations

  1. 1.Thermal Department, School of Mechanical EngineeringNational Technical University of AthensAthensGreece

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