The optimization of the poor heat transfer characteristics of fluids conventionally employed in solar devices are at present one of the main topics for system efficiency and compactness. In the present work we investigated the optical and thermal properties of nanofluids consisting in aqueous suspensions of single wall carbon nanohorns. The characteristics of these nanofluids were evaluated in view of their use as sunlight absorber fluids in a solar device. The observed nanoparticle-induced differences in optical properties appeared promising, leading to a considerably higher sunlight absorption. We found that the thermal conductivity of the nanofluids was higher than pure water. Both these effects, together with the possible chemical functionalization of carbon nanohorns, make this new kind of nanofluids very interesting for increasing the overall efficiency of the sunlight exploiting device.
© 2010 OSA
Solar thermal collectors are heat exchangers that transform solar radiation energy to internal energy of the transport medium. Typical solar collectors use a black-surface absorber to collect solar energy. Various limitations have been discovered with this configuration and alternative concepts have been addressed . Among these, the use of black particles suspended in gases  or in liquids as both solar radiation absorbers and heat transfer medium has found a significant development. In fact, carbon particles in gases have been demonstrated to be very efficient in the high temperature range , whereas the black liquid collectors exceeded thermal performance levels of conventional collectors in the low and medium temperature range . Usually, black liquids are ink-based fluids usually containing various organic compounds and inorganic particles. Organic inks, however, show serious drawbacks because they suffer light-induced degradation and thermal degradation at the high operating temperatures.
A recent development in solar thermal collectors is the use of nanofluids to directly absorb the light. “Nanofluid” is the name conceived to describe a fluid in which nanometer-sized particles are suspended. Nanofluids consisting of such particles suspended in liquids (typically conventional heat transfer liquids) have been shown to enhance the thermal conductivity and convective heat transfer performance of the base liquids . Heat transfer enhancement in sunlight exploiting devices is a critical point for the improvement of the overall system efficiency and compactness. The use of nanosized solid additives to base fluids is an innovative technique which is being studied to enhance the overall heat transfer. Different materials, such as Aluminium , Copper  and multiwall carbon nanotubes [8–11] have been added to different base fluids and characterized in terms of their performances for improving heat transfer efficiency. However, as to the optical properties of the nanofluids, they are highly dependent on the shape, size and optical properties of the particles, besides the properties of the fluid . Among the various absorber fluids which have been characterized in the literature as solar absorbers, water has been showed to be one of the most efficient ones . When nanoparticles are concerned, carbon nanotubes (CNTs), which are characterized themselves by a very high thermal conductivity [8,13,14], appear very promising in enhancing heat transfer when suspended in fluids as well [10,11].
The latest in the family of carbon-based nanostructured materials, the carbon nanohorns  are raising high expectations in a variety of applications, such as fuel cells as the electrode material, gas storage material making use of their high surface area, and carrier vehicles for delivering therapeutic drugs, genes or proteins thanks to their large surface area and large number of cavities . To the best of author’s knowledge, black absorber liquids based on carbon nanohorns (CNHs) have not been studied to date. A single-wall CNH (SWCNH) consists of a single layer of a graphene sheet wrapped into an irregular tubule with a variable diameter of generally 2-5 nm and a length of 30-50 nm; the tips of the nanohorns are cone-shaped with an average angle of about 20° [17,18]. The SWNHs assemble to form roughly spherical aggregates of mainly three types: dahlias, buds and seeds , where the dahlia-flowerlike morphology has emerged as an intriguing material within the great family of carbon nanotubes (CNTs) . The critical points that differentiate CNHs from CNTs are their high purity due to the absence of any metal nanoparticles (typically Fe and Co) used to catalyze nanotube growth during their production, the heterogeneous surface structure due to their highly strained conical ends, and finally the aggregation in spherical superstructures, typically ranging between 50 and 100 nm. Moreover, the rough surface structure of CNH aggregates with minimum Van der Waals interactions between the superstructures gives rise to better dispersion of CNHs in liquid media  and a much longer time stability of their suspensions. Moreover, a very important property in view of their potential use with respect to carbon nanotubes arises from the metal-free structure of nanohorns that makes their cytotoxicity negligible, as has been widely confirmed by experiments on mice and rats .
In this paper the SWCNHs were prepared by a new patented process (Carbonium Srl)  able to produce soot rich in SWCNHs. This method resulted in an excellent capability and, differently from other methods commonly used, could be easily scaled up for a really massive production. The investigated SWCNHs were suspended in water. Due to the inherent hydrophobic nature of SWCNHs, a new dispersion procedure has been developed to achieve a long-term dispersion stability. Then, we characterized the resulting nanofluids in terms of optical properties and thermal conductivity in the perspective to use it as absorber fluid in a sunlight collecting device. The measurement of the spectrally-resolved optical properties allowed us to evaluate the stored optical energy in the fluid and its spatial distribution, providing a very useful information for the collector and absorber design and for system optimization.
2. SWCNH and Nanofluid preparation and thermal characterization
The SWCNHs were kindly provided by Carbonium Srl (firstname.lastname@example.org, Padova, Italy) and prepared by a new patented process consisting in a gas phase-like method based on the graphite vaporization by induction heating. The soot analyses revealed the presence of good-quality carbon nanostructures, mainly SWCNHs, together with small amount of graphene foils and very little quantities of amorphous material, as confirmed by thermogravimetric analyses. The high resolution transmission electron micrograph (Fei Tecnai 12 transmission electron microscope operating at 100 keV) reported in Fig. 1 shows an example of a dahlia-like structure.
The SWCNH dispersions with concentrations varying from 0.002 to 0.1 g/l in water were prepared by the following procedure: the SWCNHs were mechanically dispersed in a water solution containing sodium dodecyl sulfate (SDS) as surfactant (SWCNH:SDS=1:1 by weight). A first homogenization was performed by an ultrasonic processor (VCX 130, Sonics & Materials) at 20 kHz and 70 W for 30 min. Therefore, a high pressure homogenizer was employed to optimize the dispersion. With this procedure long term stability was assured to the dispersions (no settling has been detected after a month). Moreover, a finer particle distribution was achieved. In fact, the mean particle size measured before and after the homogenization process by a laser particle size analyzer (N5 model, Beckam Coulter) revealed a decrease of the mean particle size from 67 ± 8 nm to 42 ± 5 nm after some processing time and the appearance of a peak indicating the presence of other particles having size at around 20 nm for longer processing time. The maximum measured diameters have been not larger than 90 nm. Figure 2 reports some comparative photographs showing some SWCNH dispersions.
Preliminary thermal conductivity measurements of water and nanofluids were performed using the multi-current transient hot-wire technique : the experiments were performed at ambient pressure and 30°C. An insulated platinum wire having a 25 μm diameter was used and the electrical current varied from 50 to 90 mA. At the investigated concentrations, the thermal conductivity of the nanofluids was found to increase with respect to the water up to around 10% for the 0.1 g/l dispersion, while higher concentrations showed higher values. However a further increase of thermal conductivities can be reached by decreasing the SWCNH mean size by means of a further homogenization process. Detailed investigations to assess optimal concentrations for thermal conductivity enhancement at the solar collector working temperatures are under way. Moreover, since a significant parameter to check for solar applications is the heat exchange of these nanofluids, an apparatus for the direct measurement of it is under development as well.
3. Optical characterization
Optical transmittance spectra at room temperature have been measured using a double-beam spectrophotometer (PerkinElmer Lambda900). The sample is held in quartz cuvettes, with 10 mm beam path length. To assess the optical characteristics of both bare SWCNHs and SWCNH aqueous suspension, transmission spectra were acquired with respect to the base fluid and air reference, respectively. Table I lists the investigated samples, which are labeled as A1…A7 for increasing SWCNH concentrations. Samples with concentrations higher than 0.050 g/l (like for example those described in the previous paragraph) were also prepared, but optical measurements on them were not possible in our setup because the optical transmittance at these concentrations was below the detection limit of our instrument. Anyway the optical properties we measured on more lightly concentrated samples can be easily scaled to high concentrations needed for the optimal thermal coefficient enhancement.
In Fig. 3 we show the overall transmittance spectra of the different samples with respect to the air, corrected for the reflectance term. They represent the spectral transmittance of the whole system built by nanoparticles and base fluid. From Fig. 3 it is evident the threefold advantage given by the inclusion of nanoparticles in the base fluid for solar absorber applications: in addition to the enhancement of thermal transmission properties described in Section 2 and of mass diffusivity reported in the literature , SWCNHs considerably reduce the resulting sample transmittance with respect to the pure fluid and boost the amount of captured light. This is due to two causes: from one side the direct absorption of photons by SWCNHs and on the other hand the scattering of light produced by the nanosized particles. This latter increases the length of the light path in the fluid and therefore further raises the absorption level. For nanosized particles  examined in the wavelength range 300-2300 nm, the Rayleigh scattering regime applies , where the spectral scattering coefficient varies with the sixth power of the particle size and with the inverse of the fourth power of wavelength. As the absorption coefficient is directly proportional to the particle size , for nanoscaled particles and given the characteristics of our experimental apparatus, the forward scattered light reaching the detector can be neglected, whereas for larger particles , forward scattering is significant and cannot be disregarded.
To better characterize the optical properties of the nanofluid samples, we calculated their extinction coefficient α(λ) from the spectral transmittance T(λ), according to the Lambert-Beer law:Fig. 4 for the different samples. The obtained extinction coefficient values are linear with the nanoparticle concentration, giving evidence of the absence of settling phenomena , at least for the typical times of our measurements (some days). This confirms the efficacy of the preparation method in improving the temporal stability of the nanofluid.
The evidenced spectral properties result extremely important to characterize the sunlight extinction behavior of the nanofluid and its energy storage capability. In fact, the fraction F of the incident power that after a path length x within the sample is no more available to be transmitted and it is stored in the material (either for direct absorption or scattering followed by absorption process), is given by the expression:
We calculated the fraction of the power stored in the sample using Eq. (2) from the CIE solar spectrum with air mass m=1.5 , for different SWCNH concentrations in the fluid. The result is shown in Fig. 5 . For the sample with the highest concentration level (A7), almost 100% of the incident energy is extinct in the first centimeter of penetration depth, whereas for the lighter one, 80% of energy is extinct after a 10 cm path within the nanofluid. On the other hand, pure water entails extinction of the incident sunlight as low as 39% after a 10 cm-long propagation path. This definitely demonstrates the helpful effect of the nanoparticle spectral features for efficient solar energy storage.
It has to be considered that nanoparticle concentration level also affects the spatial distribution S(λ) of the stored energy inside the nanofluid bulk. This distribution, for a cold fluid, no convective mixing and in absence of scattering is given by the expression:
Plots of the calculated stored power distributions along the light propagation direction are reported in Fig. 6 . Distributions refer to a single sided irradiated nanofluid as in the case of one generic radial direction in a transverse section of the absorber tube in a sunlight parabolic linear collector. The curves in Fig. 6 have been respectively normalized referring to the highest value of the distribution for the A7 sample and therefore have to be read as relative values. From these distributions, once scaled with the actual incident power on the tube external surface, the temperature profiles within the non-static fluid can be obtained using the energy balance equation and will be the subject of a next report. As we can see from Fig. 6, the energy is mainly absorbed in the first layers of fluid. For A7 sample, the energy is stored in a very short depth near the surface and inner layers are not directly heated by the light at all, resulting in a strong distribution gradient. As the nanoparticle concentration decreases, the stored energy distribution penetrates more deeply in the sample, producing, after the first steep gradient, a more uniform profile in the inner layers. As apparent from Fig. 6, the energy distribution for pure water is lower than those of the nanofluid samples, even for the lowest investigated concentration.
For a complete SWCNH optical characterization, we show in Fig. 7 the nanoparticle transmission spectra uncoupled from the base fluid contribution. This measurement has been performed using the spectrophotometer and holding a pure water sample in the path of the reference beam.
SWCNHs show a pronounced extinction peak at 260 nm (the dip in the transmission curves in Fig. 7), with a transmittance lacking other significant peaks and almost monotonically increasing towards the infrared. The comparison of the spectra in Fig. 7 with the sun spectrum suggests that a further improvement is possible for the use of SWCNHs in solar absorbers. In fact, the sun emission spectrum is peaked on the blue-green wavelength region and its spectral irradiance decreases towards the infrared. It is possible to exploit the large functionalization capability of carbon nanoparticles for a closer matching of their spectra to the incident sunlight spectrum. This represents another advantage of SWCNHs over different nanoparticles that have been investigated in the past (like metallic particles or different carbon nanoparticles). In fact for SWCNHs the functionalization has been demonstrated to remain possible even after surface treatments aimed to improve their hydrophilicity . Finally, as further developments of this work, investigations aimed to assess the optical characteristics of nanohorns with different base fluid and with different particle size distributions are under way. Moreover, the use of SWCNH-based nanofluids in the biomedical field is under study as well.
In this paper we characterized single wall carbon nanohorns (SWCNHs) in aqueous suspensions as new nanofluids for solar energy applications. Both optical and thermal properties have been investigated as a function of the nanoparticle concentration. The measured spectral transmission showed that SWCNHs play a significant role in improving the photonic properties of the fluid, leading to a significant increase of the light extinction level even at very low concentrations. Measured extinction coefficients allowed calculating the stored solar energy fraction, as a function of the penetration depth within the nanofluid for the different SWCNH concentrations, and the stored energy distribution within the cold and static fluid. We measured an increase in thermal conductivity up to 10% at the investigated concentrations. The knowledge of both optical and thermal properties of the nanofluid provides very useful information to the sunlight collector designer for the system dimensioning and heat transfer efficiency optimization. In conclusion, the use of SWCNH water nanofluid as absorber in solar devices seems a very promising step towards efficiency enhancement and more compact and integrated designs.
The work has been performed under the “Industria 2015” funding of the Italian Ministry of Economic Development. The authors thank Mr. Giuseppe Tognon (CNR-ITB) for kind help with TEM analyses and Dr. Mauro Schiavon (Carbonium srl) for kindly providing the SWCNH.
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