https://orcid.org/0000-0002-6319-6421
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ABSTRACT
The thermal performance of a flat-plate solar collector (FPSC) using novel heat transfer fluids of aqueous colloidal dispersions of covalently functionalized multi-walled carbon nanotubes with β-Alanine (Ala-MWCNTs) has been studied. Multi-walled carbon nanotubes (MWCNTs) with outside diameters of (< 8 nm) and (20–30 nm) having specific surface areas (SSAs) of (500 m2/g) and (110 m2/g), respectively, were utilized. For each Ala-MWCNTs, water-based nanofluids were synthesized using weight concentrations of 0.025%, 0.05%, 0.075%, and 0.1%. A MATLAB code was built and a test rig was designed and developed. Heat flux intensities of 600, 800, and 1000 W/m2; mass flow rates of 0.6, 1.0, and 1.4 kg/min; and inlet fluid temperatures of 30, 40, and 50°C were used to perform the test runs. Using water and nanofluids, the efficiency of the FPSC was found to increase with the increase in heat flux intensity and flow rate, and decrease with the increase in inlet fluid temperature. When applying nanofluids in the FPSC and as weight concentration and SSA increased, a reduction in the values of absorber plate temperature (AP) and tube wall temperature (TW) was observed down to 2.86% and 3.03%, respectively, while the FPSC’s efficiency increased up to 9.55% for 0.1-wt% Ala-MWCNTs < 8 nm at 1.4 kg/min, compared with water. Good agreement was obtained between the experimental values and MATLAB code predictions for AP, TW, and efficiency with maximum differences of 3.02%, 3.19%, and 3.26% for water, and 4.24%, 3.94%, and 12.64% for nanofluids, respectively. Consequently, the MATLAB code was judged suitable for modeling the nanofluid-based FPSC with suitable precision. It was proved that the positive effects of using nanofluids in the FPSC were higher their negative effects on pressure drop because all the calculated values of performance index (PI) were more than 1. As weight concentration and SSA increased, PI increased up to 1.095 for 0.1-wt% Ala-MWCNTs < 8 nm. Therefore, it was concluded that the nanofluids considered in this research can usefully be employed as working fluids in FPSCs for improved thermal performance, and the 0.1-wt% water-based Ala-MWCNTs < 8 nm nanofluid was fairly the distinguished one.
Nomenclature
| A | = | Area (m2) |
| Al2O3 | = | Aluminum oxide |
| Ala-MWCNTs | = | Covalently functionalized MWCNTs with β-Alanine |
| AP | = | Surface temperature of the absorber plate (°C) |
| = | Bond conductance (W/m K) | |
| Cp | = | Specific heat (J/kg K) |
| CuO | = | Copper oxide |
| Cv | = | Valve flow coefficient |
| d | = | Diameter of tube (m) |
| DPT | = | Differential pressure transmitter |
| DW | = | Distilled water |
| EG | = | Ethylene glycol |
| FPSC | = | Flat-plate solar collector |
| FR | = | Solar collector heat removal factor |
| GNP | = | Graphene nanoplatelet |
| GT | = | Incident solar radiation (W/m2) |
| hi | = | Convective heat transfer coefficient inside riser tube (W/m2 K) |
| hl | = | Total head loss across the FPSC (m) |
| HW | = | Hottel-whillier |
| K | = | Thermal conductivity (W/m K) |
| KL | = | Minor loss factor |
| L | = | Length (m) |
| Qu | = | Useful energy of solar collector (W) |
| RTD | = | Resistance temperature detector |
| S | = | Absorbed solar radiation per unit area (W/m2) |
| SiO2 | = | Silicon dioxide |
| SSA | = | Specific surface area (m2/g) |
| T | = | Temperature (°C) |
| thk | = | Thickness (m) |
| TiO2 | = | Titanium dioxide |
| (Ti - Ta)/GT | = | Reduced temperature parameter (m2 K/W) |
| TW | = | Outside wall temperature of the riser tube (°C) |
| UL | = | Solar collector overall heat loss coefficient (W/m2 K) |
| V | = | Velocity of heat transfer fluid (m/s) |
| W | = | Width (m) |
| x | = | Length along fluid flow direction |
| x/d | = | Dimensionless axial distance |
| m | = | Mass flow rate of fluid (kg/s) |
| MWCNT | = | Multi-walled carbon nanotube |
| N | = | Number of rise tubes in the FPSC |
| P | = | Pressure (Pa) |
| PI | = | Performance index |
| Q | = | Heat transfer rate per unit length (W/m) |
| Subscripts | = | |
| A | = | Ambient air |
| ap | = | Absorber plate of the FPSC |
| B | = | Bottom |
| C | = | Collector |
| E | = | Edge |
| F | = | Fluid |
| G | = | Glass cover |
| I | = | Inner or inside |
| in | = | Inlet fluid |
| ins | = | Insulation |
| O | = | Outer or outside |
| out | = | Outlet fluid |
| rt | = | Riser tube of the FPSC |
| Greek letters | = | |
| α | = | Absorptance of solar energy |
| ηc | = | Energy efficiency of flat-plate solar collector |
| τ | = | Transmittance of solar energy |
| ω | = | Uncertainty |
Acknowledgments
The first author wishes to thank Ministry of Higher Education and Scientific Research, Iraq for funding his Ph.D. study through a scholarship. The authors gratefully acknowledge High Impact Research Grant UM.C/HIR/MOHE/ENG/45 and University of Malaya, Malaysia for support to conduct this research work.