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Use of metallic nanoparticles to improve the thermophysical properties of organic heat transfer fluids used in concentrated solar power
Available online at www.sciencedirect.com
ScienceDirect Solar Energy 105 (2014) 468–478 www.elsevier.com/locate/solener
Use of metallic nanoparticles to improve the thermophysical properties of organic heat transfer fluids used in concentrated solar power Dileep Singh a,⇑, Elena V. Timofeeva b, Michael R. Moravek a, Sreeram Cingarapu a, Wenhua Yu b, Thomas Fischer c, Sanjay Mathur c a
Nuclear Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA b Energy Systems Division, Argonne National Laboratory, Argonne, IL 60439, USA c Institute of Inorganic Chemistry, University of Cologne, Greinstr. 6, 50923 Ko¨ln, Germany Received 17 June 2013; received in revised form 7 February 2014; accepted 20 February 2014
Communicated by: Associate Editor Ranga Pitchumani
Abstract One of the approaches to enhance the efficiency, and consequently, reduce costs to produce electricity from concentrated solar power (CSP) is by the development of advanced high temperature heat transfer fluids (HTFs). Incorporation of metallic nanoparticles into conventional heat transfer fluids could significantly improve the thermal transport properties of the HTFs. This study reports on the synthesis and investigation of copper nanoparticles synthesized in-house and dispersed in two synthetic HTFs Therminol 59 (TH59) and Therminol 66 (TH66). Liquid phase reduction of a copper salt was used to produce copper nanoparticles. Suspensions with various copper nanoparticle loadings (0.5–2 vol.%) were prepared. Characterizations such as the thermal conductivity, dynamic viscosity, mass specific heat capacity, and fluid stability were performed on the suspensions. Thermal conductivity enhancements over the base fluids were as high as approximately 20% at a 2 vol.% particle loading. These enhancements in the thermal conductivity are higher than the predictions based on the Effective Medium Theory (EMT). Dynamic viscosity measurements showed that if good dispersion of nanoparticles is achieved, the composite fluids behave in a Newtonian manner and the dynamic viscosity increases over the base fluid are minor at temperatures 125 °C and above. Stability of the suspensions with time was also investigated. Based on the measured properties of the suspensions, a figure of merit for heat transfer was calculated to evaluate the viability of the suspensions. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Copper nanoparticles; Nanofluids; Thermophysical properties; Heat transfer
1. Introduction Among various renewable energy sources, CSP appears to be an attractive option. However, current costs to produce electricity from CSP are significantly higher than those from traditional fossil and nuclear power plants. ⇑ Corresponding author. Tel.: +1 630 252 5009; fax: +1 630 252 5568.
E-mail address: [email protected] (D. Singh). http://dx.doi.org/10.1016/j.solener.2014.02.036 0038-092X/Ó 2014 Elsevier Ltd. All rights reserved.
To reduce the cost to produce electricity by CSP, one of the strategies that are being undertaken is to increase the overall plant efficiency. One approach to improve the plant efficiency is to enhance the thermophysical properties of the HTFs used in the solar field. Current high temperature heat transfer fluids, such as synthetic oils, have low thermal conductivities and poor heat transfer properties. Enhanced thermal properties of the HTFs will lead to better heat transfer in heat exchangers to produce steam at higher
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temperatures to run the turbines, thus, increasing the overall system Rankine efficiency. Recently, it has been shown that the addition of solid nanoparticles to various fluids (water, polar and nonpolar organic fluids) can increase the thermal conductivity, density, and heat transfer coefficient of the suspensions by tens of percent (Choi, 2009; Yu et al., 2008). For the most part, the enhancements in the thermal conductivity follow the Maxwell EMT (Maxwell, 1873; Buongiorno et al., 2009). However, some literature results show higher enhancements than predicted by the EMT. Mechanisms postulated to explain the observed abnormal enhancements in the thermal performance are Brownian motion (Jang and Choi, 2004), micro-convection (Prasher et al., 2006a), clustering or agglomeration (Prasher et al., 2006b), particle shapes (Chopkar et al., 2008; Timofeeva et al., 2009, 2010), and other exotic mechanisms. Thermophysical properties of nanofluids were shown dependent on the particle material, shape, size, concentration, the type of the base fluid, and other additives. The interface between the particles and the base fluid also plays an important role in the thermal conductivity enhancements (Timofeeva et al., 2010; Huxtable et al., 2003; Barrat and Chiaruttini, 2003). The operating mechanism of thermal conduction will be specific to a particular particle/fluid system due to multiple parameters in nanofluids. Nanofluids with metallic nanoparticles have demonstrated thermal conductivity increases well above the EMT predictions (Cho et al., 2005; Hong et al., 2005; Eastman et al., 2001; Chopkar et al., 2007; Kang et al., 2006; Li and Xuan, 2000; Patel et al., 2003; Venerus et al., 2006; Godson et al., 2010). Data from different research groups are quite scattered, possibly because of the difference in the preparation technique, particle size, particle material, base fluid, surfactant, and also uncertainty in the measurements of the particle concentration and the thermal conductivity (Buongiorno et al., 2009). It was suggested (Lee et al., 2010) that metallic nanoparticles possess geometrydependent localized plasmon resonances (collective oscillations of the metal’s free electrons upon optical or other excitations), which is one of the major reasons for the growing interest in nanofluids and composites with metallic nanoparticles. The significant enhancements in the thermal conductivity, shown by the majority of metal containing nanofluids, indicate a great potential for revolutionizing industries that are dependent on the performance of heat transfer fluids, such as CSP. Two high temperature heat transfer fluids TH59 and TH66 with different properties and compositions were selected for this study. TH59 has excellent low temperature pumping characteristics down to 45 °C and high temperature thermal stability up to 315 °C and is ideally suited for combination heating and cooling applications delivering excellent heat transfer rates even at low temperatures. TH66 is one of the most popular high temperature, liquid-phase heat transfer fluids, which is suitable for operation at temperatures up to 345 °C and can be pumped at
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temperatures down to 3 °C. TH66 is specifically engineered to resist solid formation and system fouling. Both TH59 and TH66 provide reliable operation and long fluid life, even when operating continuously at the recommended maximum temperatures. The addition of nanoparticles into the HTFs flowing at CSP’s heat exchangers can have additional benefits such as minimizing the interfacial thermal resistance from viscous sub-layers and hence minimizing the temperature drop in the HTFs from the pipe wall to the center. The size of the viscous sub-layers is dependent on the density, velocity, dynamic viscosity, surface roughness, and the diameter of the pipe. The convective heat transfer coefficient is proportional to the inverse of the thermal resistance. As an example, if the Dittus–Boelter correlation (Dittus and Boelter, 1930) is used, then the heat transfer coefficient is related to the density to the 0.8 power, the thermal conductivity to the 0.6 power, the mass specific heat capacity to the 0.4 power, and the dynamic viscosity to the 0.4 power. Thus, an increase in the density, thermal conductivity, or mass specific heat capacity would result in a lower thermal resistance, a higher heat transfer rate for the HTFs, and higher system efficiency. Higher concentrations of nanoparticles increase both the thermal conductivity and the dynamic viscosity, and increase in the HTF dynamic viscosity is detrimental to the heat transfer performance. Therefore, nanofluids that show high thermal conductivity increases and low dynamic viscosity increases at low particle concentrations represent research and development interest. In this study, to establish the effects of particles on the thermophysical properties of the HTFs, copper nanoparticles were synthesized using a wet chemistry approach and dispersed uniformly in two commercial organic HTFs with different physical properties. Characterizations such as the thermal conductivity, dynamic viscosity, mass specific heat capacity, and fluid stability were performed. Based on the property measurements and the figure of merit criterion for the suspensions, heat transfer performance of the nanofluids in CSP environment was assessed. 2. Materials and methods 2.1. Synthesis of Cu nanoparticles The reduction of copper (II) ions by hypophosphate was used in the preparation of Cu nanoparticles as per the following reaction (Zhu et al., 2004): 0 þ Cu2þ þ H2 PO 2 þ H2 O ) Cu þ H2 PO3 þ 2H
Reducing agent was used in excess to ensure the completeness of Cu2+ reduction to metallic Cu and to eliminate the possibility of oxide (CuO and Cu2O) formation. The reaction was conducted in a 1000-ml round bottom flask shown in Fig. 1 with nitrogen gas purge to provide an inert atmosphere and a continuous mixing by a magnetic stirrer. The solution of 23.5 g of reducing agent (NaH2PO2 H2O, Aldrich) in 500 ml of ethylene glycol (EG, Fisher)
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Fig. 1. Schematic of the test setup for synthesizing Cu nanoparticles in solution.
Table 1 Thermophysical properties of TH59 and TH66. Composition
TH59 Alkyl substituted aromatic
TH66 Modified terphenyls
Thermal conductivity @ 27 °C, W/mK Thermal conductivity @ 204 °C, W/mK Dynamic viscosity @ 27 °C, cP Dynamic viscosity @ 204 °C, cP Specific heat @ 27 °C, kJ/kgK Specific heat @ 204 °C, kJ/kgK Density @ 27 °C, kg/m3 Density @ 204 °C, kg/m3
0.121 0.104 5.56 0.461 1.70 2.29 971 836
0.117 0.105 70.8 0.939 1.58 2.21 1003 882
was heated to 140 °C and maintained at that temperature throughout the synthesis process. A solution of 14.1 g of copper sulfate (CuSO4 5H2O, Fisher) dissolved in 100 ml of EG was added to the reaction mixture using addition funnel at an average rate of 1.5 ml/min. It was critical for the purity of the product to maintain the reaction temperature during the addition of copper salt. The 140 °C temperature was maintained for 30 min past the addition of the copper salt solution. After the heating was terminated, the mixture was cooled to the room temperature with continued stirring and N2 purge. The resulting solid product was separated from the reaction mixture by centrifuging, followed by decanting and washing once with pure EG, three times with ethanol, and once with acetone. 2.2. Phase and microstructural analysis The X-ray diffraction (XRD) analysis of the as-synthesized Cu nanoparticles was conducted using a Bruker D8 X-ray diffractometer with Cu Ka radiation. The samples were scanned from 2 theta values of 20° to 80° at an increment of 1°/min. The morphology and elemental composition of the as-synthesized Cu nanoparticles were investigated using
an electron microscopy (SEM) (Model 4700–II, Hitachi). The SEM samples were prepared by placing a drop of the dilute ethanol suspension of nanoparticles onto a silicon wafer and allowed to dry. The particle size was measured by dynamic light scattering (DLS) technique and nominal size was confirmed by the SEM images. 2.3. Dispersion of Cu nanoparticles in Therminol heat transfer fluids The desired concentration of nanoparticles in nanofluids was achieved by mixing appropriate amount of nanoparticles and the base fluids pre-mixed with surfactants. The amount of nanoparticles and base fluids were calculated from desired volumetric fraction of nanoparticles and known densities of materials. A summary of TH59 and TH66 base fluid properties is provided in Table 1. The dispersion of metallic nanoparticles into nonpolar organic HTFs requires use of surfactants. Previously silicon oxide nanoparticles were successfully dispersed in TH66 using benzalkonium chloride (BAC, Acros Organics) as a surfactant (Timofeeva et al., 2011). However, use of BAC surfactant with Cu nanoparticles did not provide sufficient stability of suspension, most likely due to the lack of specific interaction between the nanoparticles and the surfactant molecules. For that reason the surfactant known to anchor well to metallic surfaces – octadecyl thiol (ODT) – was added to aid suspension stability. ODT does not have groups miscible with aromatic groups of the base fluids, and therefore combination of two surfactants was found to provide good dispersion of nanoparticles as illustrated on Fig. 2. Because of the different compositions and properties of the base fluids, different approaches were used for preparing the dispersions in TH59 and TH66. A combination of oleic acid and BAC (1 wt.% of each for 1 vol.% of nanoparticles) was used to disperse nanoparticles in TH66. The
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Fig. 2. Schematic representation of bi-layer arrangement of (a) BAC and oleic acid and (b) BAC and ODT surfactants on the Cu nanoparticle surfaces.
surfactant was first dispersed into the base fluid TH66, followed by the addition of Cu nanoparticles. The mixture was homogenized by sonication using a Branson Sonifier (Model S-450) at 80-W output power and 40% duty cycle. It is suggested that the bi-layer arrangement of surfactant molecules provides good adhesion to the nanoparticle surface and miscibility with the aromatic solvent, as depicted schematically in Fig. 2. To prepare stable dispersions of Cu nanoparticles in the lower-dynamic viscosity base fluid TH59, a mixture of ODT and BAC surfactants was used in combination with the “digestive ripening” procedure, which is described in details in the results section. 2.4. Property measurements of Cu/Therminol suspensions The effective thermal conductivities of the base fluids TH59 and TH66, the base fluids with surfactants, and the corresponding nanofluids with surfactants and various loadings (up to 2 vol.%) of Cu nanoparticles were measured using the transient hot wire technique (Model KD2 Pro, Decagon Devices, Inc.). The reported values represent the average of at least 20 measurements. The dynamic viscosities of the suspensions were measured at temperatures ranging from 15 to 130 °C using a
Brookfield DV-II+ rotational type viscometer (Brookfield Engineering Laboratories, Inc.) with the SC4-18 spindle (instrument error 2%). The mass specific heat capacity measurements were conducted only for the base fluid TH66 and the corresponding suspensions using a differential scanning calorimeter (DSC, Model Q-20, TA Instruments). The DSC instrument was calibrated using an indium standard and measurements were conducted under the flow of high-purity nitrogen (N2) gas. For mass specific heat capacity measurements, a custom temperature program that follows the standard DSC test method (ASTM-E1269) was created: first, the temperature was equilibrated at 50 °C for at least 4 min to evaporate any absorbed moisture, then ramped to 285 °C at 10 °C/ min, and held isothermally for another 4 min. Subsequently, cooling to room temperature was conducted at 10 °C/min. 3. Results and discussion 3.1. Copper nanoparticle characterization The as-prepared copper nanoparticles were phase pure copper as confirmed by the powder XRD analysis (Fig. 3). The XRD patterns of the as-synthesized copper nanoparticles exhibited prominent diffraction peaks
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characteristic for cubic copper phase as shown in Fig. 3. An additional peak around 2 theta angle of 36° indicates minor contamination with Cu2O. Fig. 4a shows the SEM image of the as-synthesized copper nanoparticles. The particles appear to be in a 50–75 nm size range and equiaxed in shape. The particle size distribution was also measured using DLS technique and the average particle size from DLS was 180 nm for the Cu/TH59 suspension and 160 nm for Cu/TH66 suspension. However, there were two distinct concentrations of particle sizes at around 50 nm and 200 nm in both the Cu/TH59 suspension and the Cu/TH66 suspension (Fig. 4b). The larger particle sizes measured with DLS are probably due to the copper nanoparticle agglomerates. 3.2. Preparation of stable suspensions of Cu nanoparticles in Therminol 59 The as-synthesized Cu nanoparticles appear to be connected into extended three-dimensional (3D) agglomerates as seen with SEM imaging (Fig. 5a). This is most likely due to the slow addition of CuSO4 salt into the reducing solution, resulting in preferential nucleation of Cu atoms onto the nanoparticles that were previously formed in the reaction. These 3D agglomerates do not break by conventional sonication with ultrasound gun. Suspensions of such powders have limited stability in TH59 due to the low dynamic viscosity of the base fluid and tend to precipitate within hours despite the added surfactants. To break the agglomerates and homogenize nanoparticle sizes we used digestive ripening approach first proposed by Lin et al. (Lin et al., 2000) as an efficient method that leads to monodispersed nanoparticles from polydispersed ones. The digestion occurs through transferring materials from large particles to small particles through the surfactant-activated interface. First, the suspension of the as-synthesized Cu
Fig. 3. The X-ray diffraction pattern of the as-synthesized Cu nanoparticles compared with the expected diffraction pattern for pure copper and Cu2O.
nanoparticles in TH59 with added surfactants was subjected to heating at 120 °C for 36 h while stirring with magnetic stirrer. SEM of the resulting suspension (Fig. 5b) shows that the temperature ripening procedure broke 3D Cu nanoparticle agglomerates into individual grains, with a significant increase in surface roughness of particles. However, despite a long treatment, some particle agglomerates still remained at pretty large sizes. Next we have explored effect of extended sonication ripening onto the suspension morphology. A sample of the original Cu/TH59 suspension with ODT and BAC surfactants was sonicated at 40% output power in a 5 min “on” 15 min “off” cycle for over 8 h (125 min of total sonication). SEM of the resulting suspension is presented in Fig. 5c. The impact of ultrasound ripening onto the nanoparticle morphology is different from the temperature ripening effect. During the temperature ripening, digestive effect of surfactant and temperature induced segregation of smaller particles resulted in rather slow evolution of particle sizes. The ripening by sonication is more disruptive and appears to favor the digestion by the surfactants, which results in non-equilibrium product with a mixture of extremely small and large particles. Based on these results we have combined temperature and ultrasonic ripening processes in one experiment to accelerate particle homogenization process. Fig. 5d illustrates that well defined single crystal nanoparticles with a size distribution between 50 and 100 nm were achieved in only 4 h of sonication at a 120 °C temperature. The resulting Cu/TH59 suspension is very stable with a high thermal conductivity enhancement and a low dynamic viscosity increase as will be demonstrated below. 3.3. Characterization of Cu/TH59 and Cu/TH66 suspensions 3.3.1. Thermal conductivity The thermal conductivities of the Cu/TH59 and Cu/ TH66 suspensions with various copper loadings were measured and compared to the values of the pure base fluids TH59 and TH66. The thermal conductivity values were recorded automatically every 15 min for 20 h. Fig. 6 shows the results for the base fluid with the surfactants, compared to the copper loaded suspensions. With the increasing copper content, the thermal conductivity increased with the highest value of 0.14 W/mK, i.e. 20% increase for a 2 vol.% Cu/TH66 suspension. Stable reading of the thermal conductivity over time (as shown in Fig. 6) is the indicative of the suspension stability. Over the testing period of 15–20 h, no settling of the nanoparticles was observed. Fig. 7 shows the relative enhancement of the thermal conductivity of the various suspensions over the base fluids TH59 and TH66. The enhancements increase with the copper particle loading and are as high as 20% at a 2 vol.% copper loading for the Cu/TH66 suspension. Fig. 7 also shows the predictions of the EMT for spherical particles in a dashed line. There is a significant deviation between
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Fig. 4. (a) Scanning electron micrograph of the as-synthesized Cu nanoparticles. (b) DLS of Cu-TH66 suspension.
the EMT predictions and the observed experimental results. Similar enhancements over the EMT has been observed for fluids with other metallic particles (Cho et al., 2005; Hong et al., 2005; Eastman et al., 2001; Chopkar et al., 2007; Kang et al., 2006; Li and Xuan, 2000; Patel et al., 2003; Venerus et al., 2006; Godson et al., 2010). However, it is not clear as to the exact mechanism that contributes to the increased enhancements in the Cu/TH59 and Cu/TH66 systems.
3.3.2. Dynamic viscosity The dynamic viscosities of pure TH59 and TH66 base fluids with surfactant and the suspensions with various loadings of copper nanoparticles were measured at temperatures up to 125 °C (Fig. 8). The suspension behavior was found to be
Newtonian, i.e., there was no shear thinning/thickening observed with changing the shear rate. With nanoparticle addition, dynamic viscosity of the suspensions increased as shown in Fig. 8. However, the relative increase in the dynamic viscosity decreases with increasing test temperature. The increase in the dynamic viscosity of the fluid with the additions of nanoparticles is important since this would translate into additional pumping power requirements. 3.3.3. Specific heat The effective mass specific heat capacity (Cp(nf)) and density (qnf) of the Cu/TH59 and Cu/TH66 suspensions at various temperatures were calculated using the mixture rules: C pðnf Þ ¼
ð1 /np ÞC pðf Þ qf þ /np C pðnpÞ qnp ð1 /np Þqf þ /np qnp
ð1Þ
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Fig. 5. SEM images of Cu nanoparticles at different stages of digestive ripening: (a) as-synthesised, (b) temperature ripening without sonication, (c) sonication ripening without temperature control, and (d) combination of temperature and sonication ripening.
0.16
0.16
(b) Thermal conductivity (W/mK)
Thermal conductivity (W/mK)
(a) 0.15
0.14
0.13
0.12
0.11
0.15
0.14
0.13
0.12
0.11
Base fluid 0.5 vol. % Cu 2 vol. % Cu
Base fluid 0.75 vol. % Cu 2 vol. % Cu
0.10
0.10 0
5
10
15
20
25
Time (h)
0
5
10
15
20
Time (h)
Fig. 6. Thermal conductivity measurements of (a) Cu/TH66 and (b) Cu/TH59 suspensions at various Cu nanoparticle concentrations.
qnf ¼ ð1 /np Þqf þ /np qnp
ð2Þ
where Cp is the mass specific heat capacity, q is the density, /np is the volume concentration of the Cu nanoparticles, and the subscripts nf, f, and np denote the nanofluids, base fluid, and nanoparticles, respectively. For the Cu/TH66 suspensions with 1 vol.% and 2 vol.% of nanoparticles, the mass specific heat capacity was measured as a function of the temperature (Fig. 9) and compared to the values calculated using Eqs. (1) and (2). The mass specific heat capacity and the density of the base fluid as a function of temperature was taken from the manufacturer specifications. The mass specific heat capac-
ity 0.385 J/gK of copper was taken from the reference (White and Collocott, 1984). The density of copper was assumed constant in the tested temperature range. The experimentally measured mass specific heat capacity values and the predictions are in good agreement (within 5%) over the temperature range in which the measurements were made. The role of the specific heat is important from the standpoint of the energy transported by the heat transfer fluid. Therefore, the specific heat comparison for the base fluid and its suspensions on a volumetric basis is more relevant. For example, using the density values for the TH66 and 2 vol.% Cu/TH66, the volumetric specific heats capacity
D. Singh et al. / Solar Energy 105 (2014) 468–478 1.8 Calculated (1 vol. %) Measured (1 vol. %) Calculated (2 vol. %) Measured (2 vol. %)
TH66 TH59 EMT 15
Specific heat (J/gK)
Thermal conductivity enhancement (%)
20
475
10
5
1.7
1.6
1.5
0 0.0
0.5
1.0
1.5
2.0
1.4 40
2.5
50
60
80
90
100
110
o
Particle concentration (vol. %)
Fig. 7. Thermal conductivity enhancements of the Cu/TH59 and Cu/ TH66 suspensions as a function of the nanoparticle concentration. Dashed line is the predictions based on the EMT.
at 50 °C are 1.65 J/cm3 K and 1.72 J/cm3 K, respectively. Similarly, at 100 °C, the values for the base fluid and the suspensions were 1.75 J/cm3 K and 1.86 J/cm3 K, respectively. Thus, because of the high density of copper, on a volumetric basis the specific heat of the Cu/TH66 suspensions is 4–6% higher as compared to the base fluid. Similar deductions can be made for the Cu/TH59 suspensions.
3.3.4. Long-term stability Viability of the Cu/TH59 and Cu/TH66 suspensions for practical applications will depend on the ability of the particles to remain in suspension for long periods of time under static conditions, such as in the case if the CSP plant is shut down for a period of time. In this regard, as a first cut, settling of the particles due to gravitational force was estimated. The terminal velocity of the particles was estimated by equating the buoyancy force on the particle with
Fig. 9. Mass specific heat capacities of the 1 and 2 vol.% Cu/TH66 suspensions measured as a function of temperature using DSC.
the gravitational and drag forces in the following manner (Ross and Morrison, 1988): Vg ¼
2ðqnp qnf Þgr2 9lnf
ð3Þ
where Vg is the particle settling velocity, r is the particle radius, g is the gravitational acceleration with a value of 9.81 m/s2, and l is the dynamic viscosity. The settling velocity as a function of the particle size was calculated for the Cu/TH59 and Cu/TH66 nanofluids at the ambient temperature and presented at Fig. 10. One can see that at a particle size of 50 nm, the settling rate under the ambient temperature is under 1 mm/month for the Cu/TH59 and Cu/TH66 nanofluids. This implies that, at these particle sizes, the suspensions in both base fluids will be quite stable. However, if a particle size of 200 nm (or agglomerate of equivalent weight) is considered, then the terminal velocity in the Cu/TH59 nanofluids will be more than
200
25
(a)
(b)
Base fluid 1 vol. % Cu 2 vol. % Cu
Dynamic viscosity (cP)
Dynamic viscosity (cP)
70
Temperature ( C)
150
100
Base fluid 0.75 vol. % Cu 2 vol. % Cu
20
15
10
50 5
0
0
20
40
60
80
100 o
Temperature ( C)
120
140
0
0
20
40
60
80
100
120
o
Temperature ( C)
Fig. 8. Dynamic viscosities of the (a) Cu/TH66 and (b) Cu/TH59 suspensions as a function of the nanoparticle concentration and the test temperature. The error bars are smaller than the symbols.
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Using the reported particle sizes and properties of the base fluids, settling rates according to Eq. (3), at various temperatures have been calculated and presented in Table 2. As viscosity decreases with the temperature, the stability of nanofluids decreases as well. However, when exposed to higher temperatures nanofluids are most likely to be flowing in the system, and the convective flow currents are expected to keep the nanoparticles suspended in the base fluids despite the gravity effects.
10
Setting rate (mm/month)
Cu/TH66 Cu/TH59 8
6
4
3.4. Figure of merit for the heat transfer efficiency of Cu/ TH59 and Cu/TH66 suspensions
2
0
0
50
100
150
200
250
Particle size (nm)
Fig. 10. Calculated settling rates of Cu nanoparticles in the base fluids TH59 and TH66 at the ambient temperature based on the Stokes law (Eq. (3)).
8 mm/month. At the same time because of the higher dynamic viscosity, the Cu/TH66 nanofluids still could be considered stable at a particle size of 200 nm. Thus, from a practical standpoint, it is important to keep the particles de-agglomerated in suspension. Further, it should be noted that the above calculations only account for gravitational and buoyant forces, but do not consider the role of surfactants in keeping the particles in suspension. Table 2 Settling rates calculated for Cu-therminol nanofluids.
Settling velocity @ 27 °C, mm/month Settling velocity @ 204 °C, mm/month
160 nm Cu NP in TH59
80 nm Cu NPs in TH66
5.3
0.1
64.6
7.9
To compare the heat transfer performance of the Cu/ TH59 and Cu/TH66 suspensions to their corresponding base fluids, the ratio of heat transfer coefficients is used as the figure of merit (Yu et al., 2010). For a turbulent flow with the heat transfer coefficient described by the Dittus– Boelter equation (Dittus and Boelter, 1930), the following figure of merit can be used (Yu et al., 2010): !0:8 !0:4 0:6 qnf C pðnf Þ 0:4 lnf hnf Monf k nf ¼ ¼ ð4Þ hf Mof qf cpf lf kf where h is the heat transfer coefficient, Mo is the Mouromtseff number (Mouromtseff, 1942; Simons, 2006), and k is the thermal conductivity. In obtaining the above equation, it is assumed that the flow velocities for the suspension and the base fluid are same. The addition of nanoparticles is considered beneficial when the ratio of heat transfer coefficients for the nanofluid over the base fluid is above unity and not beneficial otherwise. The density and mass-specific heat capacity for the Cu/TH66 and Cu/ TH59 suspensions were calculated from the density and mass specific heat capacity of pure components using Eqs. (1) and (2); while their dynamic viscosity and thermal conductivity were measured experimentally. It should be pointed out that, based on a comprehensive review (Yu 1.2
φ =0.01 np φ =0.02 np
Heat transfer coefficient ratio
Heat transfer coefficient ratio
1.2
1.1
(a) Cu in TH66 1.0
0.9
0.8
0.7
0
20
40
60
80
100 o
Temperature ( C)
120
140
φ =0.005 np φ =0.0075 np φ =0.02
1.1
np
(b) Cu in TH59 1.0
0.9
0.8
0.7
0
20
40
60
80
100
120
140
o
Temperature ( C)
Fig. 11. Heat transfer coefficient ratios for the (a) Cu/TH66 and (b) Cu/TH59 suspensions compared to the corresponding base fluids as a function of the temperature.
D. Singh et al. / Solar Energy 105 (2014) 468–478
et al., 2012), the above figure of merit calculated based on the experimental thermo-physical properties of nanofluids and their base fluids is a good indicator of the heat transfer coefficient enhancement of the nanofluids. The calculated ratios of the heat transfer coefficients for the Cu/TH59 and Cu/TH59 suspensions are shown in Fig. 11. It can be seen clearly that at low temperatures, the Cu/TH66 and Cu/TH59 suspensions do not improve the heat transfer performance over the base fluids, mainly due to the high dynamic viscosities of the nanofluids. However, with the increase in temperature, the dynamic viscosities of nanofluids becomes closer to that of the base fluid and the heat transfer performance of the nanofluids is better than that of the base fluid as shown for the Cu/TH66 suspensions (Fig. 11a). However, the Cu/TH59 suspensions are not as competitive with the base fluid TH59 (Fig. 11b) and the relative decrease in the dynamic viscosity with the temperature is barely sufficient to achieve performance just slightly better than the base fluid. 4. Conclusions Copper nanoparticles have been synthesized using a wet chemistry approach and dispersed in two commercial solar heat transfer fluids (TH59 and TH66) to form stable Cu nanoparticle suspensions. Combinations of surfactants and in case of the Cu/TH59 nanofluids the additional particle size ripening technique were used to achieve stable nanofluids. The thermal conductivity of the Cu/TH59 and Cu/TH66 nanofluids increases with the increasing copper loading. The enhancements are above the predictions of the EMT. In addition to the thermal conductivity, the dynamic viscosity and the mass specific heat capacity of the suspensions were measured and settling of the particles from suspensions was estimated. The conclusion can be drawn that the Cu/TH66 nanofluids appear to be more stable than the Cu/TH59 nanofluids, and stability improves as the particle size decreases. A figure of merit criterion was used to assess the heat transfer efficacy of the suspensions as compared to the base fluid. The benefits of adding Cu nanoparticles to the high temperature heat transfer fluids TH59 and TH66 are more significant in the case of the TH66 base fluid. Acknowledgments This work was supported by US Department of Energy’s EERE Solar Energy Technology Program – ARRA funding. Discussions with DOE project managers, Mr. Joe Stekli and Dr. Levi Irwin are much appreciated. The EMS was accomplished at the Electron Microscopy Center for Materials Research at Argonne National Laboratory, a US Department of Energy Office of Science Laboratory operated under Contract No. DE-AC0206CH11357 by UChicago-Argonne, LLC. Assistance from Dr. Y. Yusufoglu is appreciated.
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Timofeeva, E.V., Routbort, J.L., Singh, D., 2009. Particle shape effects on thermophysical properties of alumina nanofluids. J. Appl. Phys. 106, paper number 014304. Timofeeva, E.T., Smith, D.S., Yu, W., France, D.M., Singh, D., Routbort, J.L., 2010. The particle size and interfacial effects on thermo-physical and heat transfer characteristics of water based a-SiC nanofluids. Nanotechnology 21, paper number 215703. Timofeeva, E.T., Moravek, M.R., Singh, D., 2011. Improving the heat transfer efficiency of synthetic oil with silica nanoparticles. J. Colloid Interface Sci. 364, 71–79. Venerus, D.C., Kabadi, M.S., Lee, S., Perez-Luna, V., 2006. Study of thermal transport in nanoparticle suspensions using forced Rayleigh scattering. J. Appl. Phys. 100, paper number 094310. White, G.K., Collocott, S.J., 1984. Heat capacity of reference materials – Cu and W. J. Phys. Chem. Ref. Data 13, 1251–1257.
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ScienceDirect Solar Energy 105 (2014) 468–478 www.elsevier.com/locate/solener
Use of metallic nanoparticles to improve the thermophysical properties of organic heat transfer fluids used in concentrated solar power Dileep Singh a,⇑, Elena V. Timofeeva b, Michael R. Moravek a, Sreeram Cingarapu a, Wenhua Yu b, Thomas Fischer c, Sanjay Mathur c a
Nuclear Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA b Energy Systems Division, Argonne National Laboratory, Argonne, IL 60439, USA c Institute of Inorganic Chemistry, University of Cologne, Greinstr. 6, 50923 Ko¨ln, Germany Received 17 June 2013; received in revised form 7 February 2014; accepted 20 February 2014
Communicated by: Associate Editor Ranga Pitchumani
Abstract One of the approaches to enhance the efficiency, and consequently, reduce costs to produce electricity from concentrated solar power (CSP) is by the development of advanced high temperature heat transfer fluids (HTFs). Incorporation of metallic nanoparticles into conventional heat transfer fluids could significantly improve the thermal transport properties of the HTFs. This study reports on the synthesis and investigation of copper nanoparticles synthesized in-house and dispersed in two synthetic HTFs Therminol 59 (TH59) and Therminol 66 (TH66). Liquid phase reduction of a copper salt was used to produce copper nanoparticles. Suspensions with various copper nanoparticle loadings (0.5–2 vol.%) were prepared. Characterizations such as the thermal conductivity, dynamic viscosity, mass specific heat capacity, and fluid stability were performed on the suspensions. Thermal conductivity enhancements over the base fluids were as high as approximately 20% at a 2 vol.% particle loading. These enhancements in the thermal conductivity are higher than the predictions based on the Effective Medium Theory (EMT). Dynamic viscosity measurements showed that if good dispersion of nanoparticles is achieved, the composite fluids behave in a Newtonian manner and the dynamic viscosity increases over the base fluid are minor at temperatures 125 °C and above. Stability of the suspensions with time was also investigated. Based on the measured properties of the suspensions, a figure of merit for heat transfer was calculated to evaluate the viability of the suspensions. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Copper nanoparticles; Nanofluids; Thermophysical properties; Heat transfer
1. Introduction Among various renewable energy sources, CSP appears to be an attractive option. However, current costs to produce electricity from CSP are significantly higher than those from traditional fossil and nuclear power plants. ⇑ Corresponding author. Tel.: +1 630 252 5009; fax: +1 630 252 5568.
E-mail address: [email protected] (D. Singh). http://dx.doi.org/10.1016/j.solener.2014.02.036 0038-092X/Ó 2014 Elsevier Ltd. All rights reserved.
To reduce the cost to produce electricity by CSP, one of the strategies that are being undertaken is to increase the overall plant efficiency. One approach to improve the plant efficiency is to enhance the thermophysical properties of the HTFs used in the solar field. Current high temperature heat transfer fluids, such as synthetic oils, have low thermal conductivities and poor heat transfer properties. Enhanced thermal properties of the HTFs will lead to better heat transfer in heat exchangers to produce steam at higher
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temperatures to run the turbines, thus, increasing the overall system Rankine efficiency. Recently, it has been shown that the addition of solid nanoparticles to various fluids (water, polar and nonpolar organic fluids) can increase the thermal conductivity, density, and heat transfer coefficient of the suspensions by tens of percent (Choi, 2009; Yu et al., 2008). For the most part, the enhancements in the thermal conductivity follow the Maxwell EMT (Maxwell, 1873; Buongiorno et al., 2009). However, some literature results show higher enhancements than predicted by the EMT. Mechanisms postulated to explain the observed abnormal enhancements in the thermal performance are Brownian motion (Jang and Choi, 2004), micro-convection (Prasher et al., 2006a), clustering or agglomeration (Prasher et al., 2006b), particle shapes (Chopkar et al., 2008; Timofeeva et al., 2009, 2010), and other exotic mechanisms. Thermophysical properties of nanofluids were shown dependent on the particle material, shape, size, concentration, the type of the base fluid, and other additives. The interface between the particles and the base fluid also plays an important role in the thermal conductivity enhancements (Timofeeva et al., 2010; Huxtable et al., 2003; Barrat and Chiaruttini, 2003). The operating mechanism of thermal conduction will be specific to a particular particle/fluid system due to multiple parameters in nanofluids. Nanofluids with metallic nanoparticles have demonstrated thermal conductivity increases well above the EMT predictions (Cho et al., 2005; Hong et al., 2005; Eastman et al., 2001; Chopkar et al., 2007; Kang et al., 2006; Li and Xuan, 2000; Patel et al., 2003; Venerus et al., 2006; Godson et al., 2010). Data from different research groups are quite scattered, possibly because of the difference in the preparation technique, particle size, particle material, base fluid, surfactant, and also uncertainty in the measurements of the particle concentration and the thermal conductivity (Buongiorno et al., 2009). It was suggested (Lee et al., 2010) that metallic nanoparticles possess geometrydependent localized plasmon resonances (collective oscillations of the metal’s free electrons upon optical or other excitations), which is one of the major reasons for the growing interest in nanofluids and composites with metallic nanoparticles. The significant enhancements in the thermal conductivity, shown by the majority of metal containing nanofluids, indicate a great potential for revolutionizing industries that are dependent on the performance of heat transfer fluids, such as CSP. Two high temperature heat transfer fluids TH59 and TH66 with different properties and compositions were selected for this study. TH59 has excellent low temperature pumping characteristics down to 45 °C and high temperature thermal stability up to 315 °C and is ideally suited for combination heating and cooling applications delivering excellent heat transfer rates even at low temperatures. TH66 is one of the most popular high temperature, liquid-phase heat transfer fluids, which is suitable for operation at temperatures up to 345 °C and can be pumped at
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temperatures down to 3 °C. TH66 is specifically engineered to resist solid formation and system fouling. Both TH59 and TH66 provide reliable operation and long fluid life, even when operating continuously at the recommended maximum temperatures. The addition of nanoparticles into the HTFs flowing at CSP’s heat exchangers can have additional benefits such as minimizing the interfacial thermal resistance from viscous sub-layers and hence minimizing the temperature drop in the HTFs from the pipe wall to the center. The size of the viscous sub-layers is dependent on the density, velocity, dynamic viscosity, surface roughness, and the diameter of the pipe. The convective heat transfer coefficient is proportional to the inverse of the thermal resistance. As an example, if the Dittus–Boelter correlation (Dittus and Boelter, 1930) is used, then the heat transfer coefficient is related to the density to the 0.8 power, the thermal conductivity to the 0.6 power, the mass specific heat capacity to the 0.4 power, and the dynamic viscosity to the 0.4 power. Thus, an increase in the density, thermal conductivity, or mass specific heat capacity would result in a lower thermal resistance, a higher heat transfer rate for the HTFs, and higher system efficiency. Higher concentrations of nanoparticles increase both the thermal conductivity and the dynamic viscosity, and increase in the HTF dynamic viscosity is detrimental to the heat transfer performance. Therefore, nanofluids that show high thermal conductivity increases and low dynamic viscosity increases at low particle concentrations represent research and development interest. In this study, to establish the effects of particles on the thermophysical properties of the HTFs, copper nanoparticles were synthesized using a wet chemistry approach and dispersed uniformly in two commercial organic HTFs with different physical properties. Characterizations such as the thermal conductivity, dynamic viscosity, mass specific heat capacity, and fluid stability were performed. Based on the property measurements and the figure of merit criterion for the suspensions, heat transfer performance of the nanofluids in CSP environment was assessed. 2. Materials and methods 2.1. Synthesis of Cu nanoparticles The reduction of copper (II) ions by hypophosphate was used in the preparation of Cu nanoparticles as per the following reaction (Zhu et al., 2004): 0 þ Cu2þ þ H2 PO 2 þ H2 O ) Cu þ H2 PO3 þ 2H
Reducing agent was used in excess to ensure the completeness of Cu2+ reduction to metallic Cu and to eliminate the possibility of oxide (CuO and Cu2O) formation. The reaction was conducted in a 1000-ml round bottom flask shown in Fig. 1 with nitrogen gas purge to provide an inert atmosphere and a continuous mixing by a magnetic stirrer. The solution of 23.5 g of reducing agent (NaH2PO2 H2O, Aldrich) in 500 ml of ethylene glycol (EG, Fisher)
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Fig. 1. Schematic of the test setup for synthesizing Cu nanoparticles in solution.
Table 1 Thermophysical properties of TH59 and TH66. Composition
TH59 Alkyl substituted aromatic
TH66 Modified terphenyls
Thermal conductivity @ 27 °C, W/mK Thermal conductivity @ 204 °C, W/mK Dynamic viscosity @ 27 °C, cP Dynamic viscosity @ 204 °C, cP Specific heat @ 27 °C, kJ/kgK Specific heat @ 204 °C, kJ/kgK Density @ 27 °C, kg/m3 Density @ 204 °C, kg/m3
0.121 0.104 5.56 0.461 1.70 2.29 971 836
0.117 0.105 70.8 0.939 1.58 2.21 1003 882
was heated to 140 °C and maintained at that temperature throughout the synthesis process. A solution of 14.1 g of copper sulfate (CuSO4 5H2O, Fisher) dissolved in 100 ml of EG was added to the reaction mixture using addition funnel at an average rate of 1.5 ml/min. It was critical for the purity of the product to maintain the reaction temperature during the addition of copper salt. The 140 °C temperature was maintained for 30 min past the addition of the copper salt solution. After the heating was terminated, the mixture was cooled to the room temperature with continued stirring and N2 purge. The resulting solid product was separated from the reaction mixture by centrifuging, followed by decanting and washing once with pure EG, three times with ethanol, and once with acetone. 2.2. Phase and microstructural analysis The X-ray diffraction (XRD) analysis of the as-synthesized Cu nanoparticles was conducted using a Bruker D8 X-ray diffractometer with Cu Ka radiation. The samples were scanned from 2 theta values of 20° to 80° at an increment of 1°/min. The morphology and elemental composition of the as-synthesized Cu nanoparticles were investigated using
an electron microscopy (SEM) (Model 4700–II, Hitachi). The SEM samples were prepared by placing a drop of the dilute ethanol suspension of nanoparticles onto a silicon wafer and allowed to dry. The particle size was measured by dynamic light scattering (DLS) technique and nominal size was confirmed by the SEM images. 2.3. Dispersion of Cu nanoparticles in Therminol heat transfer fluids The desired concentration of nanoparticles in nanofluids was achieved by mixing appropriate amount of nanoparticles and the base fluids pre-mixed with surfactants. The amount of nanoparticles and base fluids were calculated from desired volumetric fraction of nanoparticles and known densities of materials. A summary of TH59 and TH66 base fluid properties is provided in Table 1. The dispersion of metallic nanoparticles into nonpolar organic HTFs requires use of surfactants. Previously silicon oxide nanoparticles were successfully dispersed in TH66 using benzalkonium chloride (BAC, Acros Organics) as a surfactant (Timofeeva et al., 2011). However, use of BAC surfactant with Cu nanoparticles did not provide sufficient stability of suspension, most likely due to the lack of specific interaction between the nanoparticles and the surfactant molecules. For that reason the surfactant known to anchor well to metallic surfaces – octadecyl thiol (ODT) – was added to aid suspension stability. ODT does not have groups miscible with aromatic groups of the base fluids, and therefore combination of two surfactants was found to provide good dispersion of nanoparticles as illustrated on Fig. 2. Because of the different compositions and properties of the base fluids, different approaches were used for preparing the dispersions in TH59 and TH66. A combination of oleic acid and BAC (1 wt.% of each for 1 vol.% of nanoparticles) was used to disperse nanoparticles in TH66. The
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Fig. 2. Schematic representation of bi-layer arrangement of (a) BAC and oleic acid and (b) BAC and ODT surfactants on the Cu nanoparticle surfaces.
surfactant was first dispersed into the base fluid TH66, followed by the addition of Cu nanoparticles. The mixture was homogenized by sonication using a Branson Sonifier (Model S-450) at 80-W output power and 40% duty cycle. It is suggested that the bi-layer arrangement of surfactant molecules provides good adhesion to the nanoparticle surface and miscibility with the aromatic solvent, as depicted schematically in Fig. 2. To prepare stable dispersions of Cu nanoparticles in the lower-dynamic viscosity base fluid TH59, a mixture of ODT and BAC surfactants was used in combination with the “digestive ripening” procedure, which is described in details in the results section. 2.4. Property measurements of Cu/Therminol suspensions The effective thermal conductivities of the base fluids TH59 and TH66, the base fluids with surfactants, and the corresponding nanofluids with surfactants and various loadings (up to 2 vol.%) of Cu nanoparticles were measured using the transient hot wire technique (Model KD2 Pro, Decagon Devices, Inc.). The reported values represent the average of at least 20 measurements. The dynamic viscosities of the suspensions were measured at temperatures ranging from 15 to 130 °C using a
Brookfield DV-II+ rotational type viscometer (Brookfield Engineering Laboratories, Inc.) with the SC4-18 spindle (instrument error 2%). The mass specific heat capacity measurements were conducted only for the base fluid TH66 and the corresponding suspensions using a differential scanning calorimeter (DSC, Model Q-20, TA Instruments). The DSC instrument was calibrated using an indium standard and measurements were conducted under the flow of high-purity nitrogen (N2) gas. For mass specific heat capacity measurements, a custom temperature program that follows the standard DSC test method (ASTM-E1269) was created: first, the temperature was equilibrated at 50 °C for at least 4 min to evaporate any absorbed moisture, then ramped to 285 °C at 10 °C/ min, and held isothermally for another 4 min. Subsequently, cooling to room temperature was conducted at 10 °C/min. 3. Results and discussion 3.1. Copper nanoparticle characterization The as-prepared copper nanoparticles were phase pure copper as confirmed by the powder XRD analysis (Fig. 3). The XRD patterns of the as-synthesized copper nanoparticles exhibited prominent diffraction peaks
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characteristic for cubic copper phase as shown in Fig. 3. An additional peak around 2 theta angle of 36° indicates minor contamination with Cu2O. Fig. 4a shows the SEM image of the as-synthesized copper nanoparticles. The particles appear to be in a 50–75 nm size range and equiaxed in shape. The particle size distribution was also measured using DLS technique and the average particle size from DLS was 180 nm for the Cu/TH59 suspension and 160 nm for Cu/TH66 suspension. However, there were two distinct concentrations of particle sizes at around 50 nm and 200 nm in both the Cu/TH59 suspension and the Cu/TH66 suspension (Fig. 4b). The larger particle sizes measured with DLS are probably due to the copper nanoparticle agglomerates. 3.2. Preparation of stable suspensions of Cu nanoparticles in Therminol 59 The as-synthesized Cu nanoparticles appear to be connected into extended three-dimensional (3D) agglomerates as seen with SEM imaging (Fig. 5a). This is most likely due to the slow addition of CuSO4 salt into the reducing solution, resulting in preferential nucleation of Cu atoms onto the nanoparticles that were previously formed in the reaction. These 3D agglomerates do not break by conventional sonication with ultrasound gun. Suspensions of such powders have limited stability in TH59 due to the low dynamic viscosity of the base fluid and tend to precipitate within hours despite the added surfactants. To break the agglomerates and homogenize nanoparticle sizes we used digestive ripening approach first proposed by Lin et al. (Lin et al., 2000) as an efficient method that leads to monodispersed nanoparticles from polydispersed ones. The digestion occurs through transferring materials from large particles to small particles through the surfactant-activated interface. First, the suspension of the as-synthesized Cu
Fig. 3. The X-ray diffraction pattern of the as-synthesized Cu nanoparticles compared with the expected diffraction pattern for pure copper and Cu2O.
nanoparticles in TH59 with added surfactants was subjected to heating at 120 °C for 36 h while stirring with magnetic stirrer. SEM of the resulting suspension (Fig. 5b) shows that the temperature ripening procedure broke 3D Cu nanoparticle agglomerates into individual grains, with a significant increase in surface roughness of particles. However, despite a long treatment, some particle agglomerates still remained at pretty large sizes. Next we have explored effect of extended sonication ripening onto the suspension morphology. A sample of the original Cu/TH59 suspension with ODT and BAC surfactants was sonicated at 40% output power in a 5 min “on” 15 min “off” cycle for over 8 h (125 min of total sonication). SEM of the resulting suspension is presented in Fig. 5c. The impact of ultrasound ripening onto the nanoparticle morphology is different from the temperature ripening effect. During the temperature ripening, digestive effect of surfactant and temperature induced segregation of smaller particles resulted in rather slow evolution of particle sizes. The ripening by sonication is more disruptive and appears to favor the digestion by the surfactants, which results in non-equilibrium product with a mixture of extremely small and large particles. Based on these results we have combined temperature and ultrasonic ripening processes in one experiment to accelerate particle homogenization process. Fig. 5d illustrates that well defined single crystal nanoparticles with a size distribution between 50 and 100 nm were achieved in only 4 h of sonication at a 120 °C temperature. The resulting Cu/TH59 suspension is very stable with a high thermal conductivity enhancement and a low dynamic viscosity increase as will be demonstrated below. 3.3. Characterization of Cu/TH59 and Cu/TH66 suspensions 3.3.1. Thermal conductivity The thermal conductivities of the Cu/TH59 and Cu/ TH66 suspensions with various copper loadings were measured and compared to the values of the pure base fluids TH59 and TH66. The thermal conductivity values were recorded automatically every 15 min for 20 h. Fig. 6 shows the results for the base fluid with the surfactants, compared to the copper loaded suspensions. With the increasing copper content, the thermal conductivity increased with the highest value of 0.14 W/mK, i.e. 20% increase for a 2 vol.% Cu/TH66 suspension. Stable reading of the thermal conductivity over time (as shown in Fig. 6) is the indicative of the suspension stability. Over the testing period of 15–20 h, no settling of the nanoparticles was observed. Fig. 7 shows the relative enhancement of the thermal conductivity of the various suspensions over the base fluids TH59 and TH66. The enhancements increase with the copper particle loading and are as high as 20% at a 2 vol.% copper loading for the Cu/TH66 suspension. Fig. 7 also shows the predictions of the EMT for spherical particles in a dashed line. There is a significant deviation between
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Fig. 4. (a) Scanning electron micrograph of the as-synthesized Cu nanoparticles. (b) DLS of Cu-TH66 suspension.
the EMT predictions and the observed experimental results. Similar enhancements over the EMT has been observed for fluids with other metallic particles (Cho et al., 2005; Hong et al., 2005; Eastman et al., 2001; Chopkar et al., 2007; Kang et al., 2006; Li and Xuan, 2000; Patel et al., 2003; Venerus et al., 2006; Godson et al., 2010). However, it is not clear as to the exact mechanism that contributes to the increased enhancements in the Cu/TH59 and Cu/TH66 systems.
3.3.2. Dynamic viscosity The dynamic viscosities of pure TH59 and TH66 base fluids with surfactant and the suspensions with various loadings of copper nanoparticles were measured at temperatures up to 125 °C (Fig. 8). The suspension behavior was found to be
Newtonian, i.e., there was no shear thinning/thickening observed with changing the shear rate. With nanoparticle addition, dynamic viscosity of the suspensions increased as shown in Fig. 8. However, the relative increase in the dynamic viscosity decreases with increasing test temperature. The increase in the dynamic viscosity of the fluid with the additions of nanoparticles is important since this would translate into additional pumping power requirements. 3.3.3. Specific heat The effective mass specific heat capacity (Cp(nf)) and density (qnf) of the Cu/TH59 and Cu/TH66 suspensions at various temperatures were calculated using the mixture rules: C pðnf Þ ¼
ð1 /np ÞC pðf Þ qf þ /np C pðnpÞ qnp ð1 /np Þqf þ /np qnp
ð1Þ
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Fig. 5. SEM images of Cu nanoparticles at different stages of digestive ripening: (a) as-synthesised, (b) temperature ripening without sonication, (c) sonication ripening without temperature control, and (d) combination of temperature and sonication ripening.
0.16
0.16
(b) Thermal conductivity (W/mK)
Thermal conductivity (W/mK)
(a) 0.15
0.14
0.13
0.12
0.11
0.15
0.14
0.13
0.12
0.11
Base fluid 0.5 vol. % Cu 2 vol. % Cu
Base fluid 0.75 vol. % Cu 2 vol. % Cu
0.10
0.10 0
5
10
15
20
25
Time (h)
0
5
10
15
20
Time (h)
Fig. 6. Thermal conductivity measurements of (a) Cu/TH66 and (b) Cu/TH59 suspensions at various Cu nanoparticle concentrations.
qnf ¼ ð1 /np Þqf þ /np qnp
ð2Þ
where Cp is the mass specific heat capacity, q is the density, /np is the volume concentration of the Cu nanoparticles, and the subscripts nf, f, and np denote the nanofluids, base fluid, and nanoparticles, respectively. For the Cu/TH66 suspensions with 1 vol.% and 2 vol.% of nanoparticles, the mass specific heat capacity was measured as a function of the temperature (Fig. 9) and compared to the values calculated using Eqs. (1) and (2). The mass specific heat capacity and the density of the base fluid as a function of temperature was taken from the manufacturer specifications. The mass specific heat capac-
ity 0.385 J/gK of copper was taken from the reference (White and Collocott, 1984). The density of copper was assumed constant in the tested temperature range. The experimentally measured mass specific heat capacity values and the predictions are in good agreement (within 5%) over the temperature range in which the measurements were made. The role of the specific heat is important from the standpoint of the energy transported by the heat transfer fluid. Therefore, the specific heat comparison for the base fluid and its suspensions on a volumetric basis is more relevant. For example, using the density values for the TH66 and 2 vol.% Cu/TH66, the volumetric specific heats capacity
D. Singh et al. / Solar Energy 105 (2014) 468–478 1.8 Calculated (1 vol. %) Measured (1 vol. %) Calculated (2 vol. %) Measured (2 vol. %)
TH66 TH59 EMT 15
Specific heat (J/gK)
Thermal conductivity enhancement (%)
20
475
10
5
1.7
1.6
1.5
0 0.0
0.5
1.0
1.5
2.0
1.4 40
2.5
50
60
80
90
100
110
o
Particle concentration (vol. %)
Fig. 7. Thermal conductivity enhancements of the Cu/TH59 and Cu/ TH66 suspensions as a function of the nanoparticle concentration. Dashed line is the predictions based on the EMT.
at 50 °C are 1.65 J/cm3 K and 1.72 J/cm3 K, respectively. Similarly, at 100 °C, the values for the base fluid and the suspensions were 1.75 J/cm3 K and 1.86 J/cm3 K, respectively. Thus, because of the high density of copper, on a volumetric basis the specific heat of the Cu/TH66 suspensions is 4–6% higher as compared to the base fluid. Similar deductions can be made for the Cu/TH59 suspensions.
3.3.4. Long-term stability Viability of the Cu/TH59 and Cu/TH66 suspensions for practical applications will depend on the ability of the particles to remain in suspension for long periods of time under static conditions, such as in the case if the CSP plant is shut down for a period of time. In this regard, as a first cut, settling of the particles due to gravitational force was estimated. The terminal velocity of the particles was estimated by equating the buoyancy force on the particle with
Fig. 9. Mass specific heat capacities of the 1 and 2 vol.% Cu/TH66 suspensions measured as a function of temperature using DSC.
the gravitational and drag forces in the following manner (Ross and Morrison, 1988): Vg ¼
2ðqnp qnf Þgr2 9lnf
ð3Þ
where Vg is the particle settling velocity, r is the particle radius, g is the gravitational acceleration with a value of 9.81 m/s2, and l is the dynamic viscosity. The settling velocity as a function of the particle size was calculated for the Cu/TH59 and Cu/TH66 nanofluids at the ambient temperature and presented at Fig. 10. One can see that at a particle size of 50 nm, the settling rate under the ambient temperature is under 1 mm/month for the Cu/TH59 and Cu/TH66 nanofluids. This implies that, at these particle sizes, the suspensions in both base fluids will be quite stable. However, if a particle size of 200 nm (or agglomerate of equivalent weight) is considered, then the terminal velocity in the Cu/TH59 nanofluids will be more than
200
25
(a)
(b)
Base fluid 1 vol. % Cu 2 vol. % Cu
Dynamic viscosity (cP)
Dynamic viscosity (cP)
70
Temperature ( C)
150
100
Base fluid 0.75 vol. % Cu 2 vol. % Cu
20
15
10
50 5
0
0
20
40
60
80
100 o
Temperature ( C)
120
140
0
0
20
40
60
80
100
120
o
Temperature ( C)
Fig. 8. Dynamic viscosities of the (a) Cu/TH66 and (b) Cu/TH59 suspensions as a function of the nanoparticle concentration and the test temperature. The error bars are smaller than the symbols.
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Using the reported particle sizes and properties of the base fluids, settling rates according to Eq. (3), at various temperatures have been calculated and presented in Table 2. As viscosity decreases with the temperature, the stability of nanofluids decreases as well. However, when exposed to higher temperatures nanofluids are most likely to be flowing in the system, and the convective flow currents are expected to keep the nanoparticles suspended in the base fluids despite the gravity effects.
10
Setting rate (mm/month)
Cu/TH66 Cu/TH59 8
6
4
3.4. Figure of merit for the heat transfer efficiency of Cu/ TH59 and Cu/TH66 suspensions
2
0
0
50
100
150
200
250
Particle size (nm)
Fig. 10. Calculated settling rates of Cu nanoparticles in the base fluids TH59 and TH66 at the ambient temperature based on the Stokes law (Eq. (3)).
8 mm/month. At the same time because of the higher dynamic viscosity, the Cu/TH66 nanofluids still could be considered stable at a particle size of 200 nm. Thus, from a practical standpoint, it is important to keep the particles de-agglomerated in suspension. Further, it should be noted that the above calculations only account for gravitational and buoyant forces, but do not consider the role of surfactants in keeping the particles in suspension. Table 2 Settling rates calculated for Cu-therminol nanofluids.
Settling velocity @ 27 °C, mm/month Settling velocity @ 204 °C, mm/month
160 nm Cu NP in TH59
80 nm Cu NPs in TH66
5.3
0.1
64.6
7.9
To compare the heat transfer performance of the Cu/ TH59 and Cu/TH66 suspensions to their corresponding base fluids, the ratio of heat transfer coefficients is used as the figure of merit (Yu et al., 2010). For a turbulent flow with the heat transfer coefficient described by the Dittus– Boelter equation (Dittus and Boelter, 1930), the following figure of merit can be used (Yu et al., 2010): !0:8 !0:4 0:6 qnf C pðnf Þ 0:4 lnf hnf Monf k nf ¼ ¼ ð4Þ hf Mof qf cpf lf kf where h is the heat transfer coefficient, Mo is the Mouromtseff number (Mouromtseff, 1942; Simons, 2006), and k is the thermal conductivity. In obtaining the above equation, it is assumed that the flow velocities for the suspension and the base fluid are same. The addition of nanoparticles is considered beneficial when the ratio of heat transfer coefficients for the nanofluid over the base fluid is above unity and not beneficial otherwise. The density and mass-specific heat capacity for the Cu/TH66 and Cu/ TH59 suspensions were calculated from the density and mass specific heat capacity of pure components using Eqs. (1) and (2); while their dynamic viscosity and thermal conductivity were measured experimentally. It should be pointed out that, based on a comprehensive review (Yu 1.2
φ =0.01 np φ =0.02 np
Heat transfer coefficient ratio
Heat transfer coefficient ratio
1.2
1.1
(a) Cu in TH66 1.0
0.9
0.8
0.7
0
20
40
60
80
100 o
Temperature ( C)
120
140
φ =0.005 np φ =0.0075 np φ =0.02
1.1
np
(b) Cu in TH59 1.0
0.9
0.8
0.7
0
20
40
60
80
100
120
140
o
Temperature ( C)
Fig. 11. Heat transfer coefficient ratios for the (a) Cu/TH66 and (b) Cu/TH59 suspensions compared to the corresponding base fluids as a function of the temperature.
D. Singh et al. / Solar Energy 105 (2014) 468–478
et al., 2012), the above figure of merit calculated based on the experimental thermo-physical properties of nanofluids and their base fluids is a good indicator of the heat transfer coefficient enhancement of the nanofluids. The calculated ratios of the heat transfer coefficients for the Cu/TH59 and Cu/TH59 suspensions are shown in Fig. 11. It can be seen clearly that at low temperatures, the Cu/TH66 and Cu/TH59 suspensions do not improve the heat transfer performance over the base fluids, mainly due to the high dynamic viscosities of the nanofluids. However, with the increase in temperature, the dynamic viscosities of nanofluids becomes closer to that of the base fluid and the heat transfer performance of the nanofluids is better than that of the base fluid as shown for the Cu/TH66 suspensions (Fig. 11a). However, the Cu/TH59 suspensions are not as competitive with the base fluid TH59 (Fig. 11b) and the relative decrease in the dynamic viscosity with the temperature is barely sufficient to achieve performance just slightly better than the base fluid. 4. Conclusions Copper nanoparticles have been synthesized using a wet chemistry approach and dispersed in two commercial solar heat transfer fluids (TH59 and TH66) to form stable Cu nanoparticle suspensions. Combinations of surfactants and in case of the Cu/TH59 nanofluids the additional particle size ripening technique were used to achieve stable nanofluids. The thermal conductivity of the Cu/TH59 and Cu/TH66 nanofluids increases with the increasing copper loading. The enhancements are above the predictions of the EMT. In addition to the thermal conductivity, the dynamic viscosity and the mass specific heat capacity of the suspensions were measured and settling of the particles from suspensions was estimated. The conclusion can be drawn that the Cu/TH66 nanofluids appear to be more stable than the Cu/TH59 nanofluids, and stability improves as the particle size decreases. A figure of merit criterion was used to assess the heat transfer efficacy of the suspensions as compared to the base fluid. The benefits of adding Cu nanoparticles to the high temperature heat transfer fluids TH59 and TH66 are more significant in the case of the TH66 base fluid. Acknowledgments This work was supported by US Department of Energy’s EERE Solar Energy Technology Program – ARRA funding. Discussions with DOE project managers, Mr. Joe Stekli and Dr. Levi Irwin are much appreciated. The EMS was accomplished at the Electron Microscopy Center for Materials Research at Argonne National Laboratory, a US Department of Energy Office of Science Laboratory operated under Contract No. DE-AC0206CH11357 by UChicago-Argonne, LLC. Assistance from Dr. Y. Yusufoglu is appreciated.
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