Carbon nanohorns-based nanofluids as direct
sunlight absorbers
E. Sani,1* S. Barison,2 C. Pagura,2 L. Mercatelli,1 P. Sansoni,1 D. Fontani,1
D. Jafrancesco1 and F. Francini1
1
CNR-INO National Institute of Optics, Largo E. Fermi, 6, 50125 Firenze, Italy
2
CNR – IENI, Corso Stati Uniti, 4, 35127 Padova, Italy
*
[email protected]
Abstract: 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 Optical Society of America
OCIS codes: (160.4236) Nanomaterials; (350.6050) Solar energy; (160.4760) Optical
properties; (220.1770) Concentrators.
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Received 15 Dec 2009; revised 8 Feb 2010; accepted 10 Feb 2010; published 25 Feb 2010
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1. Introduction
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 [1]. Among these, the use of black particles
suspended in gases [2] 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 [3], whereas the black liquid collectors
exceeded thermal performance levels of conventional collectors in the low and medium
temperature range [4]. 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 [5]. 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 [6], Copper [7] 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 [12]. 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
[1]. 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].
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The latest in the family of carbon-based nanostructured materials, the carbon nanohorns
[15] 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 [16]. 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 [19], where the
dahlia-flowerlike morphology has emerged as an intriguing material within the great family of
carbon nanotubes (CNTs) [20]. 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 [21] 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 [22].
In this paper the SWCNHs were prepared by a new patented process (Carbonium Srl) [23]
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 ([email protected], 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 goodquality 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.
Fig. 1. TEM micrograph showing an example of a dahlia-like SWCNH.
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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.
Fig. 2. Comparative photographs showing the aqueous suspensions containing 0.01 g/l (a) and
0.1 g/l (b) of SWCNHs after 3 weeks of settling time.
Preliminary thermal conductivity measurements of water and nanofluids were performed
using the multi-current transient hot-wire technique [24]: 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.
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Table 1. Samples under investigation
Label
SWCNH Concentration (g/l)
A1
A2
0.001
0.002
A3
A4
A5
A6
A7
0.004
0.006
0.010
0.020
0.050
Fig. 3. Transmittance spectra for SWCNHs in water for different nanoparticle concentration.
The pure water transmittance spectrum is also shown, for comparison (dotted black line).
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 [25], 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 [23] examined in the wavelength range 300-2300
nm, the Rayleigh scattering regime applies [26], 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 [26], 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 [2], 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 LambertBeer law:
T (λ ) = e −α ( λ )l (1)
where l is the propagation length within the medium, i.e. 10 mm in our case. The spectral
extinction coefficient, that phenomenologically includes the spectral absorption and the
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spectral scattering coefficients, is shown in 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 [9], 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.
Fig. 4. Spectral extinction coefficient for the various samples.
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:
λMAX
F ( x) = 1 −
∫
λ
I (λ ) ⋅ e −α ( λ ) x d λ
(2)
min
λMAX
∫
I (λ ) d λ
λmin
where I(λ) is the incident solar irradiance, λMAX=2300 nm and λmin=300 nm are the maximum
and minimum considered wavelengths of the solar spectrum.
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 [27], 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.
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Fig. 5. Stored energy fraction as a function of the penetration distance in the nanofluid for the
various samples.
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:
λMAX
∫
S ( x) =
I (λ ) ⋅ α (λ ) ⋅ eα ( λ ) x d λ
λmin
(3)
λMAX
∫
I (λ ) d λ
λmin
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.
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Fig. 6. Stored energy spatial distributions along the light propagation direction within the
nanofluid (relative values).
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.
Fig. 7. SWCHN transmission spectra. The spectra are taken up to 1400 nm only because of the
lacking of signal at higher wavelengths due to the base water absorption.
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
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hydrophilicity [23]. 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.
4. Conclusions
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.
Acknowledgments
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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