Debjyoti Banerjee

Enhanced Specific Heat of Silica Nanofluid

Silica nanoparticles (1% by weight) were dispersed in a eutectic of lithium carbonate and potassium carbonate (62:38 ratio) to obtain high temperature nanofluids. A differential scanning calorimeter instrument was used to measure the specific heat of the neat molten salt eutectic and after addition of nanoparticles. The specific heat of the nanofluid was enhanced by 19–24%. The measurement uncertainty for the specific heat values in the experiments is estimated to be in the range of 1–5%. These experimental data contradict earlier experimental results reported in the literature. (Notably, the stability of the nanofluid samples was not verified in these studies.) In the present study, the dispersion and stability of the nanoparticles were confirmed by using scanning electron microscopy (SEM). Percolation networks were observed in the SEM image of the nanofluid. Furthermore, no agglomeration of the nanoparticles was observed, as confirmed by transmission electron microscopy. The observed enhancements are suggested to be due to the high specific surface energies that are associated with the high surface area of the nanoparticles per unit volume (or per unit mass).

Flow Loop Experiments Using Polyalphaolefin Nanofluids

Experiments were performed using a flow-loop apparatus to explore the performance of nanofluids in cooling applications. The experiments were performed using exfoliated graphite nanoparticle fibers suspended in polyalphaolefin at mass concentrations of 0.6 and 0.3 %. The experimental setup consisted of a test section containing a plain offset fin cooler apparatus (gap or nongap fin), which was connected to a flow loop consisting of a gear pump, a shell and tube heat exchanger (that was cooled or heated by a constant temperature bath chiller/heater), and a reservoir. Experiments were conducted using nanofluid and polyalphaolefin for two different fin strip layouts. Heat transfer data were obtained by parametrically varying the operating conditions (heat flux and flow rates). The heat transfer data for nanofluids were compared with the heat transfer data for neat polyalphaolefin fluid under similar conditions. The change in surface morphology of the fins was investigated using scanning electron micrography. The nanofluid properties were measured using rheometry for the viscosity, differential scanning calorimetry for the specific heat, and laser flash apparatus for the thermal diffusivity. It was observed that the viscosity was ∼10 times higher for nanofluids compared with polyalphaolefin and increased with temperature (in contrast, the viscosity of polyalphaolefin decreased with temperature). The specific heat of nanofluids was found to be 50% higher for nanofluids compared with polyalphaolefin and increased with temperature. The thermal diffusivity was found to be 4 times higher for nanofluids compared with polyalphaolefin and increased with temperature. It was found that, in general, the convective heat transfer was enhanced by ∼10% using nanofluids compared with using polyalphaolefin. Scanning electron micrography measurements show that the nanofluids deposit nanoparticles on the surface, which act as enhanced heat transfer surfaces (nanofins).

Pool Boiling Experiments on a Nano-Structured Surface

The effect of nano-structured surfaces on pool boiling was investigated. Saturated and subcooled pool boiling experiments were performed on a horizontal heater surface coated with vertically aligned multiwalled carbon nanotubes (MWCNTs). MWCNTs were synthesized using the chemical vapor deposition (CVD) process. In this paper, MWCNT forests of two distinctly different heights (Type A: 9-mum height, and Type B: 25-mu m height) were synthesized separately on silicon wafers. PF-5060 was used as the test liquid. The results show that Type-B MWCNTs yield distinctly higher heat fluxes under subcooled and saturated conditions for both nucleate and film boiling. Type-A MWCNTs provide similar enhancement in nucleate boiling (as Type-B) for both saturated and subcooled conditions. Type-B MWCNTs enhanced critical heat flux (CHF) by 40%. Increasing the height of the MWCNTs is also found to extend the wall super heat required for CHF. In contrast, Type-A MWCNTs provide only marginal enhancement in film boiling compared to bare silicon wafer, for both saturated and subcooled film boiling. Type-B MWCNTs enhanced the heat flux in the film boiling regime for the Leidenfrost point by 175% (compared to bare silicon wafer).

Subcooled Pool Boiling Experiments on Horizontal Heaters Coated With Carbon Nanotubes

Pool boiling experiments were conducted with three horizontal, flat, silicon surfaces, two of which were coated with vertically aligned multiwalled carbon nanotubes (MWCNTs). The two wafers were coated with MWCNT of two different thicknesses: 9 μm (Type-A) and 25 μm (Type-B). Experiments were conducted for the nucleate boiling and film boiling regimes for saturated and subcooled conditions with liquid subcooling of 0―30°C using a dielectric fluorocarbon liquid (PF-5060) as test fluid. The pool boiling heat flux data obtained from the bare silicon test surface were used as a base line for all heat transfer comparisons. Type-B MWCNT coatings enhanced the critical heat flux (CHF) in saturated nucleate boiling by 58%. The heat flux at the Leidenfrost point was enhanced by a maximum of ∼150% (i.e., 2.5 times) at 10°C subcooling. Type-A MWCNT enhanced the CHF in nucleate boiling by as much as 62%. Both Type-A MWCNT and bare silicon test surfaces showed similar heat transfer rates (within the bounds of experimental uncertainty) in film boiling. The Leidenfrost points on the boiling curve for Type-A MWCNT occurred at higher wall superheats. The percentage enhancements in the value of heat flux at the CHF condition decreased with an increase in liquid subcooling. However the enhancement in heat flux at the Leidenfrost points for the nanotube coated surfaces increased with liquid subcooling. Significantly higher bubble nucleation rates were observed for both nanotube coated surfaces.

Specific heat mechanism of molten salt nanofluids

Controversial results have been reported for specific heat of conventional nanofluids and molten salt nanofluids. Some water-based and organic-based nanofluids showed decreases in specific heat, while molten salt-based nanofluids showed highly enhanced specific heat. In this study, we propose a distinct heat storage mechanism to explain enhanced specific heat of molten salt nanofluids and compare with the specific heat mechanism of conventional nanofluids.

Enhanced Specific Heat Capacity of Molten Salt-Based Carbon Nanotubes Nanomaterials

This study aims to investigate the specific heat capacity of a carbonate salt eutectic-based multiwalled carbon nanomaterial (or high temperature nanofluids). The specific heat capacity of the nanomaterials was measured both in solid and liquid phase using a differential scanning calorimetry (DSC). The effect of the carbon nanotube (CNT) concentrations on the specific heat capacity was examined in this study. The carbonate molten salt eutectic with a high melting point around 490 °C, which consists of lithium carbonate of 62% and potassium carbonate of 38% by the molar ratio, was used as a base material. Multiwalled CNTs were dispersed in the carbonate salt eutectic. A surfactant, sodium dodecyl sulfate (SDS) was utilized to obtain homogeneous dispersion of CNT into the eutectic. Four different concentrations (0.1, 0.5, 1, and 5 wt.%) of CNT were employed to explore the specific heat capacity enhancement of the nanomaterials as the concentrations of the nanotubes varies. In result, it was observed that the specific heat capacity was enhanced by doping with the nanotubes in both solid and liquid phase. Additionally, the enhancements in the specific heat capacity were increased with increase of the CNT concentration. In order to check the uniformity of dispersion of the nanotubes in the salt, scanning electron microscopy (SEM) images were obtained for pre-DSC and post-DSC samples. Finally, the specific heat capacity results measured in present study were compared with the theoretical prediction.

Enhanced Sensible Heat Capacity of Molten Salt and Conventional Heat Transfer Fluid Based Nanofluid for Solar Thermal Energy Storage Application

In concentrating solar power (CSP) systems, the thermo-physical properties of the heat transfer fluid (HTF) are key parameters for enhancing the overall system efficiencies. Molten salts, such as alkali nitrates, chlorides or carbonates, and their eutectics, are considered as alternatives to conventional HTF (such as water or oil) to extend the operational capabilities of CSPS. However, the usage of the molten salt as the HTF is limited, since the heat capacity of the molten salt is relatively lower than that of conventional HTF. Nanofluid is a mixture of a fluid and nanoparticles. Well dispersed nanoparticles can be used to enhance the thermo-physical properties of HTF. In this study, silicon dioxide nanoparticles were dispersed into a molten salt and into a conventional HTF (Therminol VP-1, Solutia Inc). The specific heat enhancement of each nanofluid was studied and the applicability of such nanofluid materials for solar thermal storage applications was explored.Copyright

Investigation of High Temperature Nanofluids for Solar Thermal Power Conversion and Storage Applications

The aim of this study is to investigate the enhancement of thermal properties of various high temperature nanofluids for solar thermal energy storage application. In concentrating solar power (CSP) systems, the thermo-physical properties of the heat transfer fluids (HTF) and the thermal energy storage (TES) materials are key to enhancing the overall system efficiency. Molten salts, such as alkali nitrates, alkali carbonates, or eutectics are considered as alternatives to conventional HTF to extend the capabilities of CSP. However, there is limited usage of molten salt eutectics as the HTF material, since the heat capacity of the molten salts are lower than that of conventional HTF. Nanofluid is a mixture of a solvent and nanoparticles. Well dispersed nanoparticles can be used to enhance thermo-physical properties of HTF. In this study, silica (SiO2 ) and alumina (Al2 O3 ) nanoparticles as well as carbon nanotubes (CNT) were dispersed into a molten salt and a commercially available HTF. The specific heat capacity of the nanofluids were measured and applicability of such nanofluid materials for solar thermal storage applications were explored. Measurements performed using the carbonate eutectics and commercial HTF that are doped with inorganic and organic nano-particles show specific heat capacity enhancements exceeding 5–20% at concentrations of 0.05% to 2.0% by weight. Dimensional analyses and computer simulations were performed to predict the enhancement of thermal properties of the nanofluids. The computational studies were performed using Molecular Dynamics (MD) simulations.Copyright

Effect of Dispersion Homogeneity on Specific Heat Capacity Enhancement of Molten Salt Nanomaterials Using Carbon Nanotubes

The specific heat capacity of a carbonate salt eutectic-based carbon nanotube nanomaterial was measured in present study. Differential scanning calorimeter (DSC) was used to measure the specific heat capacity of the nanomaterials. The specific heat capacity value in liquid phase was compared with that of a pure eutectic. A carbonate salt eutectic was used as a base material, which consists of lithium carbonate and potassium carbonate by 62:38 molar ratio. Multiwalled carbon nanotubes (CNT) at 1% mass concentration were dispersed in the molten salt eutectic. In order to find an appropriate surfactant for synthesizing molten salt nanomaterials, three surfactants, sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), and gum arabic (GA), at 1% mass concentration with respect to the salt eutectic were added. In preparation of dehydrated nanomaterials, water was evaporated by heating vials on a hot plate. Three different temperature conditions (120, 140, and 160 °C) were employed to investigate the effect of dispersion homogeneity of the nanotubes in the base material on the specific heat capacity of the nanomaterials. It is expected that the amount of agglomerated nanotubes decreases with increase of evaporation temperature (shorter elapsed time for evaporation). The results showed that the specific heat capacity of the nanomaterials was enhanced up to 21% in liquid phase. Additionally, it was found that the specific heat capacity enhancement of the nanomaterials, which contained SDS, was more sensitive to the evaporation time. Also, it can be decided that GA is the most appropriate to disperse CNT into the aqueous salt solution. Finally, CNT dispersion was confirmed with scanning electron microscope (SEM) images for pre-DSC and post-DSC samples. Furthermore, theoretical predictions of the specific heat capacity were compared with the experimental results obtained in present study.

Experimental investigation of molten salt nanofluid for solar thermal energy application

The overall efficiency of a Concentrated Solar Power (CSP) system is critically dependent on the thermo-physical properties of the Thermal Energy Storage (TES) components and the Heat Transfer Fluid (HTF). Higher operating temperatures in CSP result in enhanced thermal efficiency of the thermodynamic cycles that are used in harnessing solar energy (e.g., using Rankine cycle or Stirling cycle). Particlularly, high specific heat capacity (Cp ) and high thermal conductivity (k) of the HTF and TES materials enable reduction in the size and overall cost of solar power systems. However, only a limited number of materials are compatible for the high operating temperature requirements (exceeding 400°C) envisioned for the next generation of CSP systems. Molten salts have a wide range of melting point (200°C∼500°C) and are thermally stable up to 700°C. However, thermal property values of the molten salts are typically quite low (Cp is typically less than ∼2J/g-K and k is typically less than ∼1 W/m-K). To obviate these issues the molten salts can be doped with nanoparticles — resulting in the synthesis / formation of nanomaterials (nanocomposites and nanofluids). Nanofluids are colloidal suspensions formed by doping with minute concentration of nanoparticles. Nanofluids were reported for anomalous enhancement in their thermal conductivity values. In this study, molten salt-based nanofluids were synthesized by liquid solution method. A differential scanning calorimeter (DSC) was used to measure the specific heat capacity values of the proposed nanofluids. The observed enhancement in specific heat is then compared with predictions from conventional thermodynamic models (e.g. thermal equilibrium model or “simple mixing rule”). Transmission Electron Microscopy (TEM) is used to verify that minimal aggregation of nanoparticles occurred before and after the thermocycling experiments. Thermocycling experiments were conducted for repeated measurements of the specific heat capacity by using multiple freeze-thaw cycles of the nanofluids/ nano-composites, respectively. This study demonstrates the feasibility for using novel nanomaterials as high temperature nanofluids for applications in enhancing the operational efficiencies as well as reducing the cost of electricity produced in solar thermal systems utilizing CSP in combination with TES.Copyright