Characterization
This section discusses various techniques used to characterize materials, including FTIR, Raman spectroscopy, TEM and Zeta potential and UV/VIS spectroscopy.
In the FTIR technique, compared to pure MWCNTs, the GAMWCNT sample shows a strong indication of the existence of hydroxyl (O–H) groups. The sharp and wide peaks at 3446–3750 cm−1 are linked with the O–H stretching vibrations at the primary structure of both MWCNTs and GAMWCNTs with various intensities due to the interaction between the MWCNTs and hydroxyl (O–H) groups of gallic acid (GA) and hydrogen peroxide (H2O2). The GA is effectively linked to the pure MWCNTs by the free-radical grafting process, as per the FTIR spectrum. Raman spectroscopy is a prominent method for determining the chemical functionalization of carbon-based materials. According to this method, both immaculate MWCNTs and GAMWCNTs feature D and G bands at wavenumbers of ~ 1350 and 1590 cm−1, respectively. A technique known as TEM was employed to verify the success of covalent functionalization on MWCNTs. According to TEM, the surface of MWCNTs has been successfully modified to meet the requirements, as shown in Fig. 4. Another technique, Zeta potential, is used for analyzing the stability of nanoparticles in the base fluid. As per the Zeta potential test, for a pH range of 2.70–9.56, the GAMWCNTs display strong minus values ranging from − 16 to − 52.4 mV, which are far from the point of isoelectric. The GAMWCNTs exhibit a significant electrostatic repulsion force in the pH range of 3.10–9.56, which inhibits the MWCNTs from aggregating due to noncovalent interactions. The stability of the nanofluid was also confirmed by UV/VIS spectroscopy. The absorbance reading will rise as the amount of dispersed GAMWCNTs increases, and the relative concentration of GAMWCNTs remains stable till 60 days38.
Thermophysical properties of GAMWCNTs aqueous suspensions
The Thermophysical properties of GAMWCNTs and values for various concentrations are presented in Table 2.
An approximately 5% accurate KD2 Pro (Decagon Geräte, Inc., USA) Thermal Properties analyzer was utilized to measure the thermal conductivity of the nanofluids synthesized in this study. KS-1 prob, with a diameter of 1.3 mm and length of 60 mm used as a needle sensor, and its working principle is based on the transient hot-wire method. With less than 1% uncertainty, the recorded thermal conductivity for base fluid (DW) displays good compatibility with NIST data50. Compared to deionized water (DW), GAMWCNT-H2O nanofluids have a significantly higher thermal conductivity, as shown in Table 2, and the temperature of the working fluid and concentration of nanoparticles increases the thermal conductivity. The Brownian motion of nanoparticles in a fluid is the principal factor underpinning the increased thermal conductivity of the GAMWCNT nano-fluid, which rises with the increase in temperature. With a rise in temperature, the random mobility of nanoparticles in fluid increases. Therefore, thermal energy is transported very rapidly through the fluid. Table 2 shows that the maximum increase in thermal conductivity is 22.83% at 323 K for 0.1% weight concentration.
In this investigation, the viscosity of nanofluids was measured using an Anton Paar rotating rheometer (Anton Paar GmbH, Physica MCR 301). Shear rates ranging from 20 to 200 1/s were used for testing at various temperatures. The viscosity of the GAMWCNTs nano-fluid is greater than that of Deionized water (DW), as seen in the Table 2, while the difference is not significant. Furthermore, the effective viscosity of the GAMWCNT reduces as the temperature of the working fluid rises, which is almost equivalent to that of Deionized water (DW). Weakened intermolecular forces between the particles of the nanofluid could be the cause of this occurrence51,52,53. It can be seen that the addition of low nano-particles GAMWCNT concentration results in a small increase in viscosity value, which is beneficial because higher viscosity values diminish the effects of increased thermal conductivity of fluid owing to enhanced pumping power of heat transfer systems54.
Another significant thermophysical property is the specific heat capacity. Differential scanning calorimetry (DSC-Q2000, TA Instruments) was used to measure the specific heat of nanofluid produced at various weight concentrations and temperatures. Table 2 displays the specific heat capacity values recorded at various weight concentrations of GAMWCNT nanofluids and fluid temperatures. The values of specific heat capacity for deionized water (DW) are also presented for comparison. The specific heat capacity of nano-fluid based on GAMWCNT reduces as the weight concentration of nano-particles increases; when compared to base fluid deionized water, the drop in value of Cp was 0.33–1.42%, which is just a little decline. On the other hand, specific heat capacity increases with the rise in temperature of the nano-fluid.
The density of the GAMWCNT nanofluid and deionized water (DW) at various fluid temperatures and nanoparticle concentrations was also evaluated, and the findings are given in Table 2. The density of nanofluids was measured using a density meter Mettler Toledo (DM40). Due to the thermal expansion of the liquid, the density of the GAMWCNT nano-fluid and DW reduce a little as the temperature rises. It is observed that when the temperature is raised from 293 to 323 K, the density of the GAMWCNTs reduces by 0.9% for a weight fraction of 0.1 wt%. Furthermore, a linear correlation between nanoparticle concentration and density is seen, i.e., density increases with nanoparticle loading.
Analysis of thermal efficiency
Figure 5 shows the variation in thermal efficiency of a flat plate solar collector for different mass flow rates and weight concentrations of GAMWCNTs nano-fluid. A drop in heat removal factor (FRUL) and rise in heat absorbed factor FR (τα) are seen for the rise in mass flow rate (\(\dot{m)}\). Table 3 lists the values of heat absorbed and heat removal factors for GAMWCNTs at various flow rates and weight concentrations, and these values are compared with deionized water. It can be observed that the value of FR (τα) goes up with mass flow rate and is greater for GAMWCNTs nanofluid than deionized water. Increasing heat absorbed values and thermal conductivity of GAMWCNTs nano-fluid contribute to enhanced convective coefficient (h) values. As a result, the efficiency of the solar collector is seen with the increase in mass flow rate from 0.010 to 0.0188 kg/s for each GAMWCNTs weight fraction. It can be observed that at 0.1 wt.% GAMWCNTs and 0.0188 kg/s mass flow rate in comparison to base fluid deionized water, the maximum enhancement in thermal efficiency of FPSC is 30.881%. Additionally, it has been found that an improvement in LFPSC efficiency is attained with increasing weight fractions of GAMWCNTs. This is mostly because the system can absorb more energy.
Figure 6 presents the relationship between the thermal efficiency of FPSC and the reduced temperature parameter \(\frac{\left({T}_{i}-{T}_{a}\right)}{{G}_{T}}\) for various mass flow rates of distilled water as base fluid and GAMWCNTs based nanofluid at different weight fractions of GAMWCNTs nanofluids. It can be noted that GAMWCNTs nanofluids have greater FR (τα) values than base fluid. The highest value was attained at 0.0188 kg/s flow rate and 0.1% wt. concentration. The heat transfer rate is improved with increasing values of heat absorbed factor due to thinner thermal boundary layer thickness.
The heat coefficient for convection (h), whose value is proportional to the thermal conductivity (K) of the fluid utilized, improves the thermal efficiency of FPSC. The substantial improvement in convective coefficient (h) is primarily attributable to developing of a thin thermal boundary layer at the riser tube walls due to the GAMWCNTs nanofluid’s increased thermal conductivity and reduction in the thermal resistance between the heat transfer fluid and riser tube inner wall surface. Furthermore, the thickness of the thermal boundary layer is reduced using carbon-based nanoparticles like GNP and MWCNTs. The improved heat transfer coefficient (h) and thermal efficiency of flat plate solar collectors are also attributed to the specific surface area (SSA) and Brownian motion of GAMWCNTs in distilled water.
Compared to base fluid deionized water, there is an increment in energy loss factors for GAMWCNTs nanofluids at various flow rates, as shown in Table 3. Furthermore, the energy absorbed factor values rise with an increase in mass flow rate, as seen in Table 3. It is noted that with increasing GAMWCNT weight fraction compared to deionized water, the augmentation in energy absorbed parameter is 16.99%, 23.70%, and 28.07% at 0.0188 kg/s mass flow rate. The energy loss parameter is 6.17%, 6.69%, and 7.03%.
Effect of outlet temperature on the thermal performance of the collector
Many factors affect the efficiency of a flat plate solar collector, and one of the important factor is temperature gradient (ΔT) of the working fluid inside the collector. There is an improvement in thermal performance of FPSC with the temperature gradient because thermal efficiency is directly propotional to the difference of temperature between outlet and inlet as presented in Eq. (2). Moreover, inlet temperature is fixed for specific test run and enhancement in outlet temperature is achieved by utilizing nanofluids in comparison with base fluid.This enhanced value of outlet temperature positively effects thermal efficiency of the FPSC. Figure 7a presents the variation in outlet temperature at different weight fraction for various mass flow rates of GAMWCNT nanofluid at constant GT and inlet temperature. It can be seen that at a particular weight concentration, the temperature at the outlet reduces with the rise in the flow rate of the operating fluid. The deionized water and 0.1% weight concentration of GAMWCNT nanofluid have a 0.8710% and 0.9292% reduction in outlet temperature, respectively. On the other hand, outlet temperature increases with the weight concentration of GAMWCNT nanofluid in the solar collector. Compared to base fluid, the value of outlet temperature was high for various concentrations of GAMWCNTs nanofluid. The enhancement in temperature was 0.6774%, 0.6489% and 0.6183% when base fluid deionized water was replaced by 0.1% weight concentration of GAMWCNT nanofluid as operating fluid at 0.010, 0.0144 and 0.0188 kg/s respectively. There was an improvement in the heat gain value and thermal performance of FPSC due to an increase in the weight concentration of nanofluid. Thus, thermal efficiency is enhanced considerably by utilizing GAMWCNT nanofluid instead of base fluid water. The variation of outlet temperature with inlet temperature by keeping heat flux and weight concentration of operating fluid constant is also investigated, and the results are presented in Fig. 7b. It is observed that an increment in the output temperature occurs when the inlet temperature increases at a specific flow rate. The enhancement in outlet temperature was 4.78% at 0.010 kg/s, 4.95% at 0.0144 kg/s and 5.02% at 0.0188 kg/s in comparison to the inlet temperature. Due to enhancement in outlet temperature, the larger temperature difference is apparent when utilizing GAMWCNT nanofluids compared to deionized water, even though the value of Cp for GAMWCNTs is less than the deionized water (base fluid), leading to the higher thermal performance of the solar collector55,56.
Exergy analysis
The values of entropy generation (Sgen)and exergy destruction (Edest) significantly impact the exergy efficiency of heat transfer systems. Minimizing Edest and Sgen improves energetic performance in these systems. The variation in Edest and Sgen values for 0.010, 0.0144, 0.0188 kg/s by keeping heat flux (GT) and temperature at the inlet constant is presented in Fig. 8. According to the results, there was an enhancement in the values of entropy generation (Sgen) and exergy destruction (Edest) with the rise in mass flow rate from 0.010 to 0.0188 kg/s for same weight fraction of working fluid. This increment in values of Edest and Sgen was due to heat gain increasing as the mass flow rate rises and the outlet temperature of heat transfer fluid falls rapidly. On the other hand, for an increase in GAMWCNT weight fraction at a fixed mass flow rate, there was an enhancement in the value of heat gain factor and outlet temperature with a cost of increased friction factor (Fr). Consequently, the values of exergy destruction and entropy generation are reduced. Due to its superior capacity for heat absorption, 0.1% GAMWCNT nanofluid yields the lowest values of exergy destruction and entropy generation.
Figure 9 displays the variation in exergy efficiency (ηe) for GAMWCNT-based nanofluid at 0.010, 0.0144 and 0.0188 kg/s mass flow rate. For a given weight fraction, it has been found that exergy efficiency falls as the flow rate increases. Increasing values of Sgen are the main cause of this. Furthermore, the exergy efficiency rises instantaneously with increasing concentration of working fluid at a fixed mass flow rate. Compared to base fluid, higher weight concentrations of GAMWCNTs demonstrated greater exergy efficiency values. At 0.025,0.065 and 0.1% concentration of GAMWCNTs for 0.0188 kg/s, the improvement in exergy efficiency is 2.57%,4.18% and 5.53%, respectively, in comparison to the base fluid. The increment in exergy efficiency is 2.38%, 3.45%, 4.16% at 0.0144 kg/s mass flow rate and 1.62%, 2.42%, 2.91% at 0.010 kg/s for 0.025%, 0.065% and 0.10% weight concentration respectively.
Friction factor and pumping power
Increased values of friction factor and pumping power adversely affect the thermal performance of solar thermal systems, so values of these parameters should be minimum. Figure 10a displays the theoretical friction factor computed from Petukhov and Blasius empirical models and the friction factor determined from experiments on base fluid deionized water at fixed inlet temperature, heat flux and varying Reynold No. (Re). Including some variance, the fair agreement is found between values of these two types of friction factors (theoretical and experimental). It is noticed that the discrepancy between the experimental friction value (f) and the Blasius model is 7.23%, while the difference between the observed friction value and the Petukhov model is 8.26%.
The variation in friction factor values of GAMWCNTs nanofluid at various Reynolds numbers is presented in Fig. 10b. The values obtained for various nanofluid concentrations are compared with base fluid. It is observed that friction factor values decrease with the increase in Reynolds number. This is because when the Reynolds number increases, the density gradient decreases, lowering the magnitude of frictional resistance. On the other hand, as the concentration of GAMWCNTs rises, there is a small increment in friction values compared to deionized water. When GAMWCNTs are dispersed in the base fluid, the nanofluid’s viscosity grows, causing pressure drop and, ultimately, friction factor. Compared to base fluid, for 0.025, 0.65 and 0.1% weight fraction of GAMWCNT, the highest rise in friction factor (f) is 2.29, 3.66 and 8.63%. The increased weight concentration of GAMWCNT promotes pressure drop and pumping power because frictional shear forces are induced at greater viscosity and working fluid velocities.
The relative pumping power of GAMWCNTs and base fluid (DW) is shown in Fig. 11. It is observed that there is a slight increase in relative pumping power as nanoparticles’ weight concentration increases. However, the pumping power of GAMWCNTs nanofluid and base fluid deionized water is very close.
Performance index (PI)
Performance index (PI) is a key parameter to assess the effectiveness of GAMWCNT-H2O nanofluid in heat transfer systems like flat plate solar collectors. It is essential to remember that nanofluid used in solar collectors must-have performance index values of more than one, as failure to do so will negate any potential benefits and this specific nanofluid is not an acceptable operating fluid32,46. Figure 12 displays the performance index values at different flow rates. It is observed that for all weight concentrations of GAMWCNT, performance index parameters of more than one are found because the rise in efficiency of the flat plate collector outweighs the increase in pressure drop value. Furthermore, the values of PI increase with the rise in the weight concentration of GAMWCNT. Hence, higher concentration GAMWCNT nanofluid with increased Performance index and efficiency can be a viable alternative operating fluid in FPSC.
Size reduction of flat plate solar collector
The primary objective of this investigation is to evaluate how much energy and material may be saved in the development of FPSC with GAMWCNT nanofluids as heat transfer fluids. Figure 13 shows the possible size reduction at a different weight concentration of GAMWCNT nanofluid in a flat plate collector. It has been found that there is an enhancement in size reduction of the collector with the rise in flow rate at the fixed concentration of GAMWCNT nanofluid. Moreover, at a constant flow rate, increasing GAMWCNT concentration enhanced the possibility for flat plate solar collector size reduction. It is recorded that when FPSC operated at 0.0188 kg/s and 0.1% GAMWCNT nanofluid concentration, the highest size reduction, 27.59%, was attained as compared to FPSC with water as heat transfer fluid. Thus, FPSC using GAMWCNT nanofluid is more cost-effective than FPSC using water.
Economic analysis
The computation of all the energy required to construct a product or object is known as embodied energy. The ongoing advancement of industrial technology is due to decreased embodied energy. Various studies demonstrate that using nanofluids reduces energy production costs compared to using water. Since more useful energy is produced using nanofluids, the collector’s energy production costs are reduced, and its thermal performance is improved57,58,59. Economic analysis heavily relies on assessing embodied energy in flat plate solar collectors. Effective evaluation of the economic implications of flat plate collectors was done using the life cycle assessment approach34,47,60,61. Because more than 70% of EE originated from the construction of FPSC, the methodology adopted only considers the embodied energy (EE) during the construction and operation phases of FPSC62,63. The present research considers how mass and embodied energy affect the lowering of the flat plate collector size. At various concentration of GAMWCNT nanofluid and base fluid, the economics and embodied energy analysis is presented in Table 4. Glass and copper are the two main components of the solar collector. The embodied energy indexes for glass and copper are 15.9 MJ/kg and 70.6 MJ/kg, respectively64,65. The present analysis considers the size reduction of FPSC as a function of mass and embodied energy. It was found that the size of FPSC was decreased when GAMWCNT nanofluid was used in place of base fluid water, saving 321.72 MJ of embodied energy.
Additionally, as the area of the flat plate collector is reduced, there is a decrease in the demand for electricity, which lowers system operating costs. The payback period was 1.897 years for FPSC with GAMWCNT nanofluids at 0.1 wt.%, which was 6.228% shorter than using water as a heat transfer fluid. Therefore, it is concluded that FP solar collector with GAMWCNT nanofluid as heat transfer fluid is more efficient and saves more energy than FPSC with water.
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