Sara Mesgari

Thermal stability of carbon nanotube-based nanofluids for solar thermal collectors

Abstract Carbon nanotube dispersions are promising candidates for use as working fluids in high-performance solar collectors. However, one major stumbling block in the way of their widespread application is the difficulty in achieving stable nanofluid suspensions at elevated temperatures. In this study, the stability of plasma- and acid-functionalised multi-walled carbon nanotube dispersions at temperatures up to 150°C was investigated. Therminol 55 and propylene glycol were used as the main solvents, while water was used as a reference solvent. The results of UV-VIS-NIR absorption spectroscopy showed that no agglomeration occurred in the plasma-functionalised multi-walled carbon nanotube nanofluids heated to 150°C. However, minor variations were observed in the absorbance of acid-functionalised multi-walled carbon nanotubes in propylene glycol and therminol 55 base fluids at high temperatures.

Desing and on-sun testing of a hybrid PVT prototype using a nanofluid-based selective absorption filter

Hybrid photovoltaic/thermal (PVT) solar collectors represent a promising approach to generate electrical and thermal energy from the same compact package. Moreover, beam splitting can be incorporated in these collectors to physical decouple the receivers to optimize their efficiency. In such systems, the PV cells are illuminated only with the region of the solar spectrum that matches well with their spectral response curves, while the separate thermal receiver can be operated to produce valuable heat (i.e. a high temperature output). In this work, the design of a 8x suns concentrated PVT prototype using a liquid (nanofluid) selective absorption filter is presented. Finally, actual transmittance curves of different nanofluids are presented which demonstrate that they are excellent candidates to be used as selective band-pass filters in PVT collectors using beam splitting. Preliminary experimental results of this research are also reported - in which the hybrid output is compared with the electrical output of the same PV cell array without spectral splitting.

Experimental investigation of a nanofluid absorber employed in a low-profile, concentrated solar thermal collector

Recent studies [1-3] have demonstrated that nanotechnology, in the form of nanoparticles suspended in water and organic liquids, can be employed to enhance solar collection via direct volumetric absorbers. However, current nanofluid solar collector experimental studies are either relevant to low-temperature flat plate solar collectors (<100 °C) [4] or higher temperature (>100 °C) indoor laboratory-scale concentrating solar collectors [1, 5]. Moreover, many of these studies involve in thermal properties of nanofluid (such as thermal conductivity) enhancement in solar collectors by using conventional selective coated steel/copper tube receivers [6], and no full-scale concentrating collector has been tested at outdoor condition by employing nanofluid absorber [2, 6]. Thus, there is a need of experimental researches to evaluate the exact performance of full-scale concentrating solar collector by employing nanofluids absorber at outdoor condition. As reported previously [7-9], a low profile (<10 cm height) solar thermal concentrating collector was designed and analysed which can potentially supply thermal energy in the 100-250 °C range (an application currently met by gas and electricity). The present study focuses on the design and experimental investigation of a nanofluid absorber employed in this newly designed collector. The nanofluid absorber consists of glass tubes used to contain chemically functionalized multi-walled carbon nanotubes (MWCNTs) dispersed in DI water. MWCNTs (average diameter of 6-13 nm and average length of 2.5-20 μm) were functionalized by potassium persulfate as an oxidant. The nanofluids were prepared with a MCWNT concentration of 50 ± 0.1 mg/L to form a balance between solar absorption depth and viscosity (e.g. pumping power). Moreover, experimentally comparison of the thermal efficiency between two receivers (a black chrome-coated copper tube versus a MWCNT nanofluid contained within a glass tubetube) is investigated. Thermal experimentation reveals that while the collector efficiency reduced from 73% to 54% when operating temperature increased from ambient to 80 °C by employing a MWCNT nanofluid receiver, the efficiency decreased from 85% to 68% with same operating temperature range by employing black chrome-coated copper tube receiver. This difference can mainly be explained by the reflection optical loss off and higher thermal emission heat loss the front surface of the glass tube, yielding a 90% of transmittance to the MWCNT fluid and a 0.9 emissivity of glass pipe. Overall, an experimental investigation of the performance of a low profile solar collector with a direct volumetric absorber and conventional surface absorber is presented. In order to bring nanotechnology into industrial and commercial heating applications,

In Situ Exsolution of Bimetallic Rh–Ni Nanoalloys: a Highly Efficient Catalyst for CO2 Methanation

Unique CO2 methanation catalysts comprising bimetallic Ni-Rh nanoalloy/3DOM LaAlO3 have been successfully prepared via a poly(methyl methacrylate) microsphere colloidal crystal-templating route, followed by the in situ growth of Ni nanoparticles (NPs). Here, we show that unlike traditional Ni particles deposited on a perovskite support, the exsolution of Ni occurs on both the external and internal surface of the porous perovskite substrate, leading to a strong metal-support interaction. Owing to the exsolution of Ni and the formation of Ni-Rh nanoalloys, a 52% enhancement in the methanation turnover frequency was obtained over the Ni-Rh/3DOM LaAlO3 [13.9 mol/(mol h)] compared to Rh/3DOM LaNi0.08Al0.92O3 [9.16 mol/(mol h)] before reduction treatment. The results show that the low-temperature reducibility, rich surface adsorbed oxygen species, and basic sites of the catalyst greatly improve its activity toward CO2 methanation. The hierarchically porous structure of the perovskite support provides a high dispersion of bimetallic Ni-Rh NPs.

Effects of Silane Treatment of Steel Fibres on Mechanical Properties and Durability of SFRC

Reinforcing concrete with steel fibres has been investigated with the intention of enhancing the tensile strength and durability of the concrete by resisting microcracking. However, in order to take full advantage of the reinforcing effect of steel fibres, not only to the cracking strength of the matrix, but residually, improvements can be made to (1) the physical bond between the steel fibres and concrete matrix and (2) methods in achieving uniform dispersion of fibres in concrete. Surface treatment of fibres with silane has been proposed as an effective method to improve dispersion of fibres in mortar. The improved dispersion has been attributed mainly to enhanced hydro-philicity of fibres after silane treatment. However, despite the reported promising improvements achieved in terms of dispersion of fibres, the effects of silane treatment of fibres on strength of bond between fibre and concrete as well as the mechanical properties of steel fibre reinforced concrete (SFRC) have not been investigated. This paper proposes a simplified steel fibre silane treatment technique suitable for application in practice. In addition, the effects of silane treatment on strength of bond between fibre and concrete as well as mechanical properties of reactive powder concrete (RPC) including compressive strength, modulus of elasticity, and flexural strength are investigated. Furthermore, the volume of permeable voids and bulk resistivity of concrete are monitored to investigate the effects of silane treatment on durability of concrete and dispersion of fibres in concrete, respectively. The results show a noticeable improvement in the mechanical properties and durability of SFRC after silane treatment of steel fibres.

Spectrum splitting using gold and silver nanofluids for photovoltaic/thermal collectors

The thermal yield of hybrid photovoltaic/ thermal (PV/T) collectors is presently limited to low temperatures to prevent degradation of PV efficiency and thermal damage to the cells. This work reports a nanofluid optical filter, which transmits only the portion of the solar spectrum which is most efficiently converted to electricity by the underlying solar cell. This is achieved by suspending both core-shell silver-silica nanodiscs (Ag-SiO2 NDs) and gold nanorods (AuNRs) in an aqueous base fluid to absorb visible light and infrared wavelengths, respectively. The transmission spectrum of each nanofluid can be tailored according to PV cell spectral response and to accommodate for electricity and gas price fluctuations.

Thermoset polyester-based superhydrophobic microchannels for nanofluid heat transfer applications

Both microchannels and nanofluids have shown promise to enhance convective heat transfer. However, the major drawback of these two technologies is their significant increase of pumping pressure due to increased frictional drag (for high surface area microchannels) or increased viscoelastic frictional drag (for nanofluids). It is possible to decrease frictional drag, and overcome this drawback, by implementing superhydrophobic surfaces to create slip with the channel wall. In this work, surface microstructures fabricated from the thermoset polyester (TPE) were used to create superhydrophobic surfaces which are capable of reducing the frictional drag in channel flow and thus, reduce the pumping pressure. Preliminary experimental results of superhydrophobic microchannels with rib-and-cavity microstructures aligned transversely and longitudinally to the flow direction were studied with both distilled water and water-based multi-walled carbon nanotube (MWCNT) nanofluid as the working fluids. While pressure drop reduction of superhydrophobic surfaces and heat transfer enhancement of nanofluids were shown, it was observed that heat transfer degradation occurred at higher flow rates with MWCNT nanofluid as the working fluid due to the precipitation of nanoparticles.