Preparation and characterisation of eutectic nanofluids for heat transfer enhancement in flat plate solar collectors

Sumair, Faisal Ahmed (2018) Preparation and characterisation of eutectic nanofluids for heat transfer enhancement in flat plate solar collectors. PhD thesis, University of Nottingham.

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Use of thermal energy storage (TES) materials in solar collectors is known to be the most effective way of storing thermal energy. The most conventional and traditional heat storage element is water. However, due to low thermal conductivity (TC) in vapor state its applications as a heat storage medium are limited. An alternative option is to utilize organic and inorganic TES materials as they both operate at low and medium temperature ranges. Organic TES materials such as paraffins are non-corrosive and possess high latent heat capacity. On the contrary, inorganic TES materials possess high density and appreciable specific heat capacity (SHC). Due to rapid progress and advancement in nanotechnology, varieties of nanomaterials were dispersed in various base fluid(s) to enhance thermo-physical properties. Here the current status and future development trends of TES materials has been presented. Furthermore, an extensive research on enhancement of TC and SHC of various TES material doped with nanomaterials has been discussed.

Enhancement in heat capacity (HC) and thermal conductivity (TC) with dispersion of graphene (GE) nanoparticles in low temperature molten salt was investigated. Three different nanoparticle concentrations (0.01, 0.05 and 0.1 wt. %) were dispersed in molten salt composed of 5.76% NaNO3, 21.36% KNO3, 24.84% Ca(NO3)2, 41.08% CsNO3 and LiNO3 7.44% by weight. Doping of GE resulted in enhanced HC ranging from 5-13%, whereas, TC showed enhancement up to 2.44%, with respect to GE concentration. Various theoretical models were tested to predict TC and HC of GE doped molten salt. Maxwell and Hamilton-Crosser TC models show good agreement with experimental results with deviation of ±3% while Nan’s TC model over predicted TC value. Conventional HC equation fits well with the experimental data with deviation <14%. Thus, the results obtained show the potential of GE doped molten salt as thermal energy storage (TES) medium in various heat transfer applications.

This work also investigates the rheological and corrosion properties of graphene (GE) dispersed in eutectic salts. It was observed that doping of GE has significantly enhanced the viscosity of base salt. This enhancement in GE dispersed eutectic salt is mainly due to the presence of solid GE sheets and its coagulation in eutectic salts. Various concentrations of GE and temperature ranges were studied here. Eutectic salt dispersed GE behaved as non-Newtonian fluid at 70, 80 and 90 oC except at 200 oC, where it behaved as a Newtonian fluid. Viscosity of nanosuspension was predicted using Einstein’s equation. Furthermore, corrosion studies using ASTM D130 method were performed to analyse the effects of base fluid on copper and stainless steel (SS304) at elevated temperatures. Fourier transform infrared spectrometer (FTIR) result shows that the presence of all nitrate bonds in synthesized base salt and GE dispersed base salt. X-ray diffraction depicts that the doping of GE in eutectic salt does not alter the crystal structure of nitrate molecules. EDX results confirm that both Cu and SS304 material were corroded. SS304 exhibited constant corrosion with an increase in GE concentration dispersed in eutectic salt, whereas copper exhibited an increase in corrosion rate with an increase in GE concentration.

Kinetic studies have been carried out for molten salt and GE dispersed molten salt. It was concluded that dispersion of GE has not altered decomposition rate of nitrate of ions.

In application, we have fabricated and tested two types of solar collectors, i.e., solar thermal collector and photo-voltaic/thermal collector for TES material. Test was carried out to confirm the thermal performance of both solar collectors under water, base salt and GE dispersed base salt as TES. Results proved that in both types of solar collectors, all three concentrations of GE (0.01, 0.05 and 0.1 wt. %) dispersed base salt were having high thermal performance than water and base salt. Thus, it was concluded that GE dispersed molten salt can be successfully used as TES material in both solar collectors.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Siddiqui, Mohammad Khalid
Khoiroh, Ianatul
Chan, Andy
Keywords: nanofluids, heat capacity, thermal conductivity, graphene nanoparticles
Subjects: T Technology > TP Chemical technology
Faculties/Schools: UNMC Malaysia Campus > Faculty of Engineering > Department of Chemical and Environmental Engineering
Item ID: 48390
Depositing User: Sumair, Faisal Ahmed
Date Deposited: 27 Feb 2018 12:54
Last Modified: 28 Feb 2018 00:56

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