FORM - 2
THE PATENTS ACT, 1970 (39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION (See section 10 and rule 13)
Title: NANOFLUIDS FOR SOLAR THERMAL ENERGY
Applicant IITB MONASH RESEARCH ACADEMY
Inventors
1. Sushrut Sandeep Bhanushali
2. Murali Sastry
3. Prakash Chandra Ghosh
4. Anuradda Ganesh
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER, IN WHICH IT IS TO BE PERFORMED.

FIELD OF INVENTION
The present invention relates to solar collectors. In particular, the present invention relates to direct absorption solar collectors. More particularly, the present invention relates to direct absorption solar collectors with nanofluids as
working fluid therein.
BACKGROUND OF THE INVENTION
Solar energy is a major source of clean and alternative energy. Solar energy is typically harnessed for useful purposes as either Solar Photo-Voltaic or Solar Thermal systems. Conventional solar heating systems include flat plate solar absorbers, evacuated tube absorbers and concentrated parabolic absorbers. Solar thermal systems convert radiant solar energy into useful heat for industrial and domestic purposes.
Low and medium temperature solar thermal systems mostly rely on flat plate collectors or evacuated tube collectors, while high temperature solar collectors make use of optical elements such as mirrors, lenses and reflectors for converting radiant solar energy into heat. The current technologies make use of flat plates made of metals with high thermal conductivity, such as copper coated with absorbing coatings such as the black paints or selective coatings.
The flat plate solar absorbers rely on copper fins coated with selective coatings which heat under solar radiation. This heat is transferred to the fluid (water) flowing in the pipes welded to copper fins. Various thermal losses that occur here are: reflection at the fin surface, thermal resistance at fin junction and thermal resistance at fin-fluid interface.
The evacuated tube collectors address these thermal losses by sealing the heating element under high vacuum in order to minimize these losses. However, evacuated tube based setups are fragile as well as expensive to construct. Moreover, typically efficiency of evacuated tube collectors is in the range of 50-60 % at tropical temperatures.

Concentrated solar heaters rely on reflected solar radiation being concentrated by parabolic cylindrical reflectors on to a metallic tube through which the fluid such as water flows. Concentrated solar thermal systems use reflective elements such as parabolic reflectors to concentrate the radiant energy on a metallic pipe carrying the working fluid. Here, the metal pipe is illuminated only for 25-30 % of the angular area by the reflected radiation leading to uneven heating of the fluid and formation of steam layer at the heated interface thus leading to loss of efficiency.
All these technologies rely on radiative heating of the metal and then transferring the heat to a liquid by conduction. In each of the above systems, heat transfer occurs at the boundary, i.e. at the metal-fluid interface.
STATE OF THE ART
The article entitled- "Nanofluid optical property characterization: towards efficient direct absorption solar collectors" and written by Taylor R.A., Phelan P.E., Otanicar T.P., Adrian R., Prasher R. in volume 6, issue 1 of 2011 on page X1 of the (DOI- 10.1186/1556-276X-6-225) discusses that suspensions of nanoparticles (i.e., particles with diameters < 100 nm) in liquids, termed nanofluids, show remarkable thermal and optical property changes from the base liquid at low particle loadings. Recent studies also indicate that selected nanofluids may improve the efficiency of direct absorption solar thermal collectors.
The article entitled- "Applicability of nanofluids in high flux solar collectors" and written by Taylor R.A., Phelan P.E., Otanicar T.P., Walker C.A., Nguyen M., Trimble S., Prasher R. in volume 3, issue 2 of 2011 in the Journal of Renewable and Sustainable Energy (DOI-10.1063/1.3571565) discusses that according to recent papers, addition of nanoparticles to conventional working fluids (i.e., nanofluids) could improve heat transfer and solar collection [H. Tyagi, J. Sol. Energy Eng. 131, 4 (2009); P. E. Phelan, Annu. Rev. Heat Transfer 14 (2005)].
Another article entitled- "Carbon nanohorns-based nanofluids as direct sunlight absorbers" and written by Sani E., Barison S., Pagura C, Mercatelli L, Sansoni

P., Fontani D., Jafrancesco D., Francini F. in volume 18, issue 5 of 2010 on page 5179 of the journal Optics Express (DOI- 10.1364/OE.18.005179) discusses that
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. The optical and thermal properties of nanofluids consisting in aqueous suspensions of single wall carbon nanohorns were investigated and the characteristics of these nanofluids were evaluated in view of their use as sunlight absorber fluids in a solar device. It was found that the thermal conductivity of the nanofluids was higher than pure water.
The article entitled- "Optimization of nanofluid volumetric receivers for solar thermal energy conversion" and written by Lenert A., Wang E.N. in volume 86, issue 1 of 2012 on page 253 of the journal Solar Energy, (DOI-10.1016/j.solener.2011.09.029) discusses that improvements in solar-to-thermal energy conversion will accelerate the development of efficient concentrated solar power systems. Nanofluid volumetric receivers, where nanoparticles in a liquid medium directly absorb solar radiation, promise increased performance over surface receivers by minimizing temperature differences between the absorber and the fluid, which consequently reduces emissive losses.
The article entitled- 'Thermal properties of carbon black aqueous nanofluids for solar absorption" and written by Han D., Meng Z., Wu D., Zhang C, Zhu H. in volume 6 of 2011 on page 1 of the journal "Nanoscale Research Letters" {DOI-10.1186/1556-276X-6-457) discusses that carbon black nanofluids were prepared by dispersing the pretreated carbon black powder into distilled water and the size and morphology of the nanoparticles were explored. The photothermal properties, optical properties, rheological behaviors, and thermal conductivities of the nanofluids were also investigated. The nanofluids exhibited a shear thinning behavior. Carbon black nanofluids had good absorption ability of solar energy and can effectively enhance the solar absorption efficiency.
The article entitled- "Absorption and scattering properties of carbon nanohorn-based nanofluids for direct sunlight absorbers" and written by Mercatelli L, Sani E., Zaccanti G., Martelli F., Ninni P.D., Barison S., Pagura C, Agresti F., Jafrancesco D. in volume 6, issue 1 of 2011 on page X1 of the journal Nanoscale

Research Letters (DOI- 10.1186/1556-276X-6-282) investigated the scattering and spectrally resolved absorption properties of nanofluids consisting in aqueous and glycol suspensions of single-wall carbon nanohorns and 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 with respect to the pure base fluids.
The conference paper entitled- "Predicted efficiency of a nanoftuid-based direct absorption solar receiver" and written by Tyagi H., Phelan P., Prasher R. of 2007 on page 729 in the Proceedings of the Energy Sustainability Conference 2007 (DOI- 10.1115/ES2007-36139) discusses that solar energy is often used in applications such as electricity generation, thermal heating and chemical processing due to its renewable and non-polluting nature. It was observed that the presence of nanoparticles increases the absorption of incident radiation by more than 9 times over that of pure water.
The article entitled- "Evaluation of the effect of nanofluid-based absorbers on direct solar collector" and written by Saidur R., Meng T.C., Said Z., Hasanuzzaman M., Kamyar A. in volume 55, issue 21-22 of 2012 on page 5899 of the journal International Journal of Heat and Mass Transfer (DOI-10.1016/j.ijheatmasstransfer.2012.05.087) discusses that the effect of nanofluid were analyzed by using as working fluid for direct solar collector. The extinction coefficient of water based aluminum nanofluid has been investigated and evaluated by varying nanoparticle size and volume fraction. The particle size minimally influenced the optical properties of nanofluid and extinction coefficient was linearly proportional to the volume fraction. The improvement is promising within 1.0% of volume fraction and nanofluid is almost opaque to light wave.
The article entitled- "Scattering and absorption properties of carbon nanohorn-based nanofluids for solar energy applications" and written by Mercatelli L, Sani E., Fontani D., Zaccanti G., Martelli F., di Ninni P. in volume 6 on page 36 of the Journal of the European Optical Society (DOI- 10.2971/jeos.2011.11025) investigates the scattering and absorption properties of nanofluids consisting in aqueous suspensions of single wall carbon nanohorns and evaluates the

characteristics of the nanohorns of different morphologies and for suspensions prepared with different amounts of surfactant to be used as direct sunlight absorber fluids in solar devices,
The article entitled- "Radiative heat transfer analysis in plasmonic nanofluids for direct solar thermal absorption" and written by Lee B.J., Park K., Walsh T., Xu L. in volume 134, issue 2 of the Journal of Solar Energy Engineering, Transactions of the ASME (DOI-10.1115/1.4005756) reported a novel concept of a direct solar thermal collector that harnesses the localized surface plasmon of metallic nanoparticles suspended in water. At the plasmon resonance frequency, the absorption and scattering from the nanoparticle can be greatly enhanced via the coupling of the incident radiation with the collective motion of electrons in metal.
The article entitled- "Carbon nanotube glycol nanofluids: Photo-thermal properties, thermal conductivities and rheological behavior" and written by Meng Z., Wu D., Wang L, Zhu H., Li Q. in volume 10, issue 5 of 2012 in the journal Particuology (DOI- 10.1016/j.partic.2012.04.001) investigates the potential of using carbon nanotube (CNT)-glycol nanosuspension as such a medium, prepared by freeze drying-ultrasonic dispersing after oxidation treatment with HNO 3 and investigates the influences of the mass fraction of CNTs glycol nanofluids and temperatures on photo-thermal properties, thermal conductivities and rheological behavior.
The article entitled- "Radiative properties of nanofluids" and written by Said Z., Sajid M.H., Saidur R., Kamalisarvestani M., Rahim N.A. in volume 46 of 2013 on page 74 of the journal International Communications in Heat and Mass Transfer {DOI- 10.1016/j.icheatmasstransfer.2013.05.013) reported that the optical and radiative properties of nanofluids have received much less interest as compared to thermal conductivity and convection studies with nanofluids. The concept of using direct absorbing nanofluid (suspension formed by mixing nanoparticles and a liquid) is numerically and experimentally proved to be an efficient method for harvesting solar thermal energy.
The conference paper entitled- "Radiative properties of nanofluids and performance of a direct solar absorber using nanofluids" and written by Mu L.,

Zhu Q., Si L. in volume 1, issue 2 of 2010 on page 549 of the Proceedings of the ASME Micro/Nanoscate Heat and Mass Transfer International Conference 2009, MNHMT2009 (DOI- 10.1115/MNHMT2009-18402) reported that direct absorption solar absorbers are not common as compared with regular solar collectors. However, nanofluids can be used in direct solar absorbers to improve their performances and this paper evaluated the potential of using nanofluids in direct solar absorbers and experimentally evaluated the performance of a direct solar absorber built to use nanofluids as the working fluid. It was found that the radiative properties of nanofluids deviate significantly from that of the base fluid. The rate of temperature increase of the nanofluid is faster than that of water when the liquid stagnant in the absorber is illuminated by solar radiation.
The conference paper entitled- "Impact of size and scattering mode on the optimal solar absorbing nanofluid' and written by Otanicar T., Taylor R.A., Phelan P.E., Prasher R. in volume 1, issue 2 of 2009 on page 791 of the Proceedings of the ASME 3rd International Conference on Energy Sustainability 2009, ES2009 (DOI- 10.1115/ES2009-90066) reported that using a direct absorbing nanofluid, a liquid-nanoparticle suspension is numerically and experimentally an efficient method for harvesting solar thermal energy. The size and shape of the nanoparticles as well as the scattering mode (e.g. dependent, independent, and multiple) all impact the amount of energy absorbed and emitted by the nanofluid.
According to the prior art, it is well-established that metal nanostructures exhibit a high extinction coefficient in VIS and NIR regions of the solar spectrum due to large absorption and scattering cross-sections corresponding to the surface plasmon resonance thereof. Moreover, the plasmon resonance in nanoscale metallic structures has also shown its ability to concentrate electromagnetic energy into sub-wavelength volumes. Thus, they can be used as a preferred solar energy harvesting material. Further, nanofluids also increase the efficiency of the photothermal energy conversion process, particularly in nanofluid-based direct solar absorption collectors (DAC) or other volumetric heating systems. So, the nanoparticle additives in fluids make them cost-effective, save material and energy and also facilitate their application in direct as well as concentrated solar thermal systems.

DISADVANTAGES & LIMITATIONS WITH THE EXISTING KNOWLEDGE
The disadvantages with the various solar collector technologies and systems based on conventional fluids as well as those based on nanofluids or nanoparticles discussed above are given below:
A major drawback of the existing arrangements is the thermal losses by radiation (reflection, scattering), convection (due to air) and the thermal resistance between the metal and the working fluid.
Firstly, it should be noted that liquids have thermal conductivity of 3 to 4 order of magnitudes less than the metals (For example, thermal conductivity of copper and water is 393 W/mK and 0.607 W/mK respectively @ 300K). The thermal resistances increase with the surface corrosion of metal due to environmental factors.
Secondly, the fluctuation in metal cost also affects the pricing of a solar collector. Thus, minimizing the amount of metal required could also help in reducing the overall cost of the solar collectors.
The solar radiation is spread over the ultraviolet (UV), visible (VIS) and Near Infrared (NIR) part of the electromagnetic spectrum, typically in the range of 0.2 to 3 urn (200 to 3000 nm), with a peak between 500 and 550 nm. Out of the total solar radiant energy, only 7% is in typically present in the UV region, 44% in the Visible region and the remaining 49% in the Near Infrared region.
A pool of water left in the sun would show a rise in temperature. This phenomenon occurs because of water absorbing a part of the solar radiation, more specifically in the Near Infrared part of the electromagnetic spectrum by stretching and bending bond vibrations. This absorbed energy ultimately leads to the heating up of water.
The present invention uses this phenomenon in solar thermal collectors, known as direct absorption collectors.

OBJECTS OF THE INVENTION
Some of the objects of the present invention - satisfied by at least one embodiment of the present invention - are as follows:
An object of the present invention is to provide a direct absorption solar collector deploying nanofluids as working fluid therein.
Another object of the present invention is to provide a direct absorption solar collector deploying nanofluid additive as working fluid which can absorb solar radiation.
Still another object of the present invention is to provide a direct absorption solar collector deploying nanofluids additive as working fluid which has an absorption coefficient substantially higher than pure water.
Yet another object of the present invention is to provide a direct absorption solar collector deploying nanofluid additive, which can effectively convert the absorbed electromagnetic radiation into heat.
A further object of the present invention is to provide a direct absorption solar collector deploying nanofluid additive which can effectively dissipate the heat to the surrounding water medium (matrix) without radiative losses, such as fluorescence.
A still further object of the present invention is to provide a direct absorption solar collector deploying nanofluid additive, which is stable in solution as a stable dispersion.
A yet further object of the present invention is to provide a direct absorption solar collector deploying nanofluid additive, which is made of inexpensive materials having low cost of materials, energy and processing.
One more object of the present invention is to provide a direct absorption solar collector deploying nanofluid additive, which is effective for all above stated functions at lowest possible concentrations.

These and other objects and advantages of the present invention will become more apparent from the following description, when read with the accompanying figures of drawing, which are however not intended to limit the scope of the present invention.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a nanofluid-based direct absorption solar collector comprising a parabolic reflector with a glass pipe running parallel to the reflector axis and disposed at the focal point of the reflector for carrying fluid formulation therethrough to facilitate in harnessing the broadband absorption of substantially the entire range of solar radiations by using the the absorption profile based on plasmonic excitation of the fluid formulation.
Typically, the fluid formulation comprises a cocktail of different shaped metallic nanoparticle additives dispersed in fluids such as water or ethylene glycol or Glycol-water mixtures or high boiling fluids like oils and the plasmonic excitation peak thereof are adjusted by tuning the sizes and shapes of the nanoparticles. Typically, the cocktail of different shaped metallic nanoparticles in fluid comprises defined particle sizes and shapes, such as nanospheres, nanocubes, nanorods, nanowires, nanoprisms and nanoparticle clusters.
Typically, the fluid formulation comprises a cocktail of metallic nanoparticles of coinage metals such as Gold, Silver and Copper, preferably copper nanoparticles.
Typically, the concentration of said copper nanoparticles is of the order of 5 ppm to 500 ppm; more specifically 10 -100 ppm of copper or 10-100mg/litre of nanofluid.
Typically, the absorption profile of said cocktail is tuned by optimizing the particle shape, size and relative concentration of each particle shape for maximizing the efficiency of solar absorption.

In accordance with the present invention, there is also provided a method of harnessing solar thermal power by using a nanofluid formulation in a direct absorption solar collector manufactured according to the present invention, wherein nanoparticles additives of the nanofluid formulation are synthesized by a single-step method by a top down approach using wet chemistry techniques.
Typically, the nanofluid formulation comprises nanoparticles additives synthesized by a single-step method.
Typically, the nanofluid formulation comprises nanoparticles additives synthesized by a top down approach using wet chemistry techniques.
Typically, said method of synthesizing the nanoparticles additives includes the method steps of:
• synthesizing copper nanoparticles by reducing a copper amine complex in an aqueous medium at room temperature during a chemical reduction by using a strong reducing agent in presence of a capping agent;
• the capping agents are chosen from amongst, but not limited to octylamine, dodecylamine, teradecylamine, hexadecylamine, octadecylamine; ethylenediamine, cetyltrimethylarnmonium bromide, gelatin, chitosan and hydrolysed chitosan;
• selecting the reducing agents from amongst, but not limited to, hydrazine hydrate, sodium borohydride, Dextrose, citric acid and ascorbic acid.
• centrifuging and re-dispersing the synthesized nanoparticles in fresh deionized (Dl) water and repeating this process 2 to 4 times to eliminate any excess capping agent to obtain a nanoparticle dispersion;
• using a secondary dispersing agent from amongst but not limited to polyvinylpyrrolidone (PVP), Polyvinylalcohol (PVA), hydroxypropyl cellulose (HPC), polyamines, polyethylene glycols, gelatin, chitosan, gum arabic, xanthan gum and guar gum in the nanofluid formulation;
• diluting said copper nanoparticle solution with fresh distilled water for obtaining a final concentration of 1000 ppm of Cu in water;

• bubbling said copper nanoparticle solution with nitrogen to eliminate any dissolved oxygen for preventing the surface oxidation of copper;
• adding an anti-oxidant agent to the copper nanoparticle solution for obtaining enhanced surface oxidation resistance by scavenging oxygen;
• using said copper nanoparticle solution as the stock solution;
• centrifuging said stock solution part to obtain a solid part thereof; and
• drying said film at ambient temperature and conditions inside a desiccator.
DESCRIPTION OF THE INVENTION
Since the absorption coefficient of water in solar radiation spectral range is below 10-3 it does not absorb solar radiation effectively, and is largely transparent in the wavelength range from 0.1 to 1.6 urn.
However, if by adding additives to the heat exchange fluid, the solar radiation absorption could be increased in the aforesaid range, the fluid would absorb solar radiation. The additive are used as working fluid, which has an absorption coefficient 100 to 100,000 of times higher than pure water in the wavelength range of 0.1 micrometers to 2.0 micrometers; more specifically between 0.4 and 1.2 micrometers.
Here, the metallic nanoparticles of the coinage group (Gold, Silver, Copper) could play a vital role, as they exhibit plasmonic absorbance in the visible spectral range. This is because, nanoparticles exhibit a light matter interaction commonly referred to as surface plasmon resonance (SPR), which is the collective excitation of surface electrons or plasmon, i.e. plasmonic excitation. This plasmonic excitation decays by the route of plasmon-phonon coupling and thus leads to the heating of the matrix.
It is also known that the plasmonic excitation peaks can be manipulated by tuning the particle size and shape. For example, Copper nanoparticles of about 40 nm diameter show a plasmonic absorption band at 570 - 590 nm and Copper nanocubes of edge length 80-90 nm show a plasmon excitation at 680-700 nm.

On combining such particle shapes in a single fluid in the form of a cocktail of particle shapes and sizes, the absorption profile can be expanded to cover the entire solar radiation range.
Further, by tuning the relative concentration of various particle shapes, the absorption profile can also be matched to overlap with the solar radiation profile for maximizing the efficiency of absorption.
Accordingly, the present invention employs the formulations of nanofluids, which are cocktails of different particle shapes and sizes of metallic nanoparticies, for harvesting solar thermal energy.
In accordance with the present invention, nanofluids (stable dispersion of nanoparticies in base fluids) are employed as the working fluid for a direct absorption solar collector. More specifically, the additives tuned to absorb maximum part of the solar spectrum rather than a narrow wavelength range, are used in this working fluid.
The choice of materials is made on the specific properties of the nanoparticle materials. The metallic nanoparticies, more specifically, coinage metals (Gold, Silver and Copper) exhibit a unique light matter interaction property called Localized Surface Plasmon Resonance (LSPR).
The surface plasmonic resonance band of these metallic nanoparticies is in the visible part (400-800 nm) of the electromagnetic spectrum.
Furthermore, this absorption band can be tuned by tuning the size and shape of these nanoparticies. For example, Copper is about 100 times cheaper than Silver and 7000 times cheaper than gold. Therefore, Copper is selected as the nanoparticle material for the present invention.
The direct absorption solar collector deploys nanofluid additive and is made of inexpensive materials having low cost of materials, energy and processing (<10 INR / Litre of fluid formulation). This additive is typically effective at lowest possible concentrations, typically between 5 ppm and 500 ppm.

The nanoparticles show a Face Centered Cubic (FCC) crystal structure having a lattice parameter of 3.608A, which closely corresponds to the lattice parameter 3.615A of bulk copper.
Minor peaks corresponding to copper oxide (Cu2O) are observed, possibly due to surface oxidation of copper while making a solid film for XRD (X-ray Diffractometry).
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The present invention will be briefly described with reference to the accompanying drawings, wherein:
Figure 1 shows the spectrum of incident solar irradiance and the water absorption spectrum, which demonstrates that water absorption bands are well outside the solar radiation spectrum.
Figure 2 shows an absorption spectrum of water from Ultraviolet to Microwave region of electromagnetic spectrum.
Figure 3 shows the overlap of Solar radiation spectrum and the absorption profile of the metal nanoparticle nanofluids.
Figure 4a to 4c show the absorption profiles of Copper (Cu) nanomaterials of different shapes, e.g. Cu nanospheres, Cu nanocubes and a mixture of Cu nanospheres and nanocubes nanomaterials.
Figure 5a to 5c show the absorption profiles of Gold (Au) nanomaterials of different shapes, e.g. Au nanospheres, Au nanorods, Au nanoprisms respectively.
Figure 5d shows the absorption profiles of a cocktail of Gold (Au) nanomaterials of different shapes, e.g. Au nanospheres, Au nanorods, Au nanoprisms for broadband absorption of solar radiations.

Figure 6a to 6c show the absorption profiles of Gold (Au) nanomaterials of different shapes, e.g. Au nanospheres, Au nanorods and Au nanoprisms for broadband absorption of solar radiations.
Figure 7 shows the relative concentration of these different particle shapes for matching the absorption profile of solar radiation.
Figure 8a shows a stock solution of copper nanoparticles.
Figure 8b shows a UV-VIS spectrum of copper nanoparticles of Figure 3a.
Figure 9 shows an X-Ray Diffractogram (XRD) of the copper nanoparticles.
Figure 10 shows particle size distribution of as synthesized copper nanoparticles discussed above.
Figure 11a shows the micrograph generated using transmission electron microscopy of copper nanoparticles for a sample prepared by mounting nanoparticles powder on carbon coated grid support.
Figure 11b shows the micrograph generated using scanning electron microscopy of copper nanoparticles for a nanoparticle dilute dispersion drop dried on polished silicon wafer.
Figure 12a shows nanofluids recorded at various nanoparticles dispersions diluted with water for obtaining different dispersions for UV-VIS-NIR spectra thereof.
Figure 12b shows UV-VIS-NIR spectra of copper nanoparticle nanofluids at different concentrations for the range under consideration.
Figure 13a shows the nanofluid volume fixed at 100 ml and filled in a double walled Borosil glass tube in the reflected light.
Figure 13b shows the nanofluid volume fixed at 100 ml and filled in a double walled Borosil glass tube in the transmitted light.

Figure 14 shows a schematic arrangement of the experimental setup for solar heating of nanofluids kept in the inner tubes of the double walled Borosil glass tubes.
Figure 15a graphically represents the effect of concentration of copper nanoparticles on photo-thermal heating performance of nanofluids at various concentrations as against water (without any additive).
Figure 15b shows the temperature profiles of photo-thermal heating of nanofluids with respect to the concentration.
Figure 16a shows Zeta potential distribution of the copper nanoparticles at two different pH values.
Figure 16b shows the effect of pH on photo-thermal heating profiles of nanofluids, particularly copper nanofluids.
Figure 16c shows the effect of insulation on photo-thermal temperature profiles of nanofluids.
Figure 17a shows SEM images of copper nanospheres.
Figure 17b shows SEM images of copper nanocubes.
Figure 17c shows UV-VIS spectra of the mixture of copper nanospheres and copper nanocubes.
Figure 17d shows a schematic representation of copper nanospheres of different sizes with loose aggregation.
Figure 17e shows SEM images copper nanospheres with loose aggregation shown in Figure 17d.
Figure 17f shows a schematic representation of copper nanospheres, copper nanocubes and copper nanorods of different sizes.

Figure 17g shows SEM images of copper nanospheres, copper nanocubes and copper nanorods shown in Figure 17f.
Figure 17h shows a schematic representation of copper nanospheres, copper nanocubes and copper nanowires of different sizes.
Figure 17i shows a schematic representation of SEM images of copper nanospheres, copper nanocubes and copper nanowires shown in Figure 17h.
Figure 17j shows a SEM images of an optimized nanofluid formulation.
Figure 17k shows SEM images of another optimized nanofluid formulation.
Figure 18 shows Figure of Merit (FOM) v/s. Concentration: for spherical copper nanoparticles at varying concentrations, i.e. for cocktail of spherical and cubic copper nanoparticles at varying concentrations.
Figure 19 shows FOM of nanofluids plotted at different times during the heating cycle.
Figure 20 shows FOM for different Insolation.
Figure 21 shows a schematic representation of thermal resistances in conventional solar heating systems and direct absorption system.
Figure 22a shows a perspective view of the conventional parabolic trough solar concentrator collecting solar radiations.
Figure 22b shows side view of the conventional parabolic reflector of Fig. 22a.
Figure 22c shows an enlarged side view of the conventional parabolic reflector of Fig. 22b.
Figure 23a shows a side view of the Direct absorption nanofluid-based solar thermal collector.

Figure 23b shows an enlarged side view of the Direct absorption nanofluid-based collector of Figure 23a.
Figure 24 shows a typical commercial application of the invention, which employs a plurality of parabolic mirrors with glass pipes facing the same.
DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS
In the following, the Direct Absorption Solar Collector with nanofluids as working fluid and configured in accordance with the present invention will be described in more details with reference to the accompanying drawings without limiting the scope and ambit of the present invention in any way.
Figure 1 shows the spectrum of incident solar irradiance and the water absorption spectrum, which demonstrate that water absorption bands are well outside the solar radiation spectrum. Since, there is a little overlap, water absorbs very little solar radiation. It is apparent from this spectrum that out of the solar radiation received at the outer atmosphere, certain parts of the solar spectrum are attenuated by the Earth's atmosphere. The red curve plot in this graph represents the solar spectrum at sea level. The water absorption spectrum is plotted as a blue scatter plot, as seen here, water is almost transparent in the UV and Visible region of the spectrum, and has absorption bands with a low absorption coefficient (10-100 cm-1 range) at ~1475nm and ~1950nm. Thus, water Is largely transparent in the solar spectral region and is an inefficient absorber of solar radiation.
Figure 2 shows an absorption spectrum of water from Ultraviolet to Microwave region of electromagnetic spectrum. This plot is an absorption spectrum of water in a broad spectral range from 0.01 urn to 10000 urn and the absorption coefficient is plotted on a logarithmic scale, which is below 0.0001 in the solar spectrum range (300 nm to 1300 nm). Thus, it is apparent that water is largely transparent in the solar spectral range from 250 nm to 2500 nm with extinction coefficients lower than 1.

Figure 3 shows the overlap of Solar radiation spectrum and the absorption profile of the metal nanoparticle nanofluids.
Figure 4a to 4c show the absorption profiles of Copper (Cu) nanomaterials of different shapes, e.g. Cu nanospheres, Cu nanocubes and a mixture of Cu nanospheres and nanocubes nanomaterials.
Figure 5a to 5c show the absorption profiles of Gold (Au) nanomaterials of different shapes, e.g. Au nanospheres, Au nanorods, Au nanoprisms respectively.
Figure 5d shows the absorption profiles of a cocktail of Gold (Au) nanomaterials of different shapes, e.g. Au nanospheres, Au nanorods, Au nanoprisms for broadband absorption of solar radiations.
Figure 6a to 6c show the absorption profiles of Gold (Au) nanomaterials of different shapes, e.g. Au nanospheres, Au nanorods and Au nanoprisms for broadband absorption of solar radiations.
Figure 7 shows the relative concentration of these different particle shapes for matching the absorption profile of solar radiation.
Figure 8a shows a stock solution of copper nanoparticles. This stock solution is characterized by the following techniques:
1. UV-VIS-NIR spectroscopy: for LSPR absorption profile of the solution.
2. Dynamic Light Scattering: For mean particle size and particle size distribution.
3. Zeta Potential: To measure Zeta potential on particle surface for characterizing stability.

4. X-ray Diffractometry (XRD): For obtaining Crystal structure, Unit cell parameter and Phase purity of the nanoparticles.
5. Electron microscopy (TEM/SEM): For morphology, size distribution of nanoparticles.

6. EDAX: For composition of particles.
7. ICP-AES: For concentration of copper in solution.
Figure 8b shows a UV-VIS spectrum of copper nanoparticles of Figure 3a. The part of the stock solution was centrifuged and the solid was cast into a film. The film was dried at ambient temperature and conditions inside a desiccator. The X-ray Diffractogram was recorded.
Figure 9 shows an X-Ray Diffractogram (XRD) of copper nanoparticles. The nanoparticle solution was diluted and particle size distribution was measured in solution using Dynamic Light Scattering (DLS). The mean particle size was 20 nm.
Figure 10 shows particle size distribution of as synthesized copper nanoparticles discussed above.
Figure 11a shows the electron microscopy of copper nanoparticles, i.e. SEM micrographs of copper nanoparticles for a sample prepared by mounting nanoparticles powder on carbon support.
Figure 11b shows electron microscopy of copper nanoparticles, i.e. SEM micrographs of copper nanoparticles for a nanoparticle dilute dispersion drop dried on polished silicon wafer.
Figure 12a shows nanofluids at various dilutions, i.e. the nanoparticle dispersions diluted with water for obtaining dispersions in the range of 10 to 100 ppm., for which UV-VIS-NIR spectra of copper nanoparticle nanofluids were recorded, as discussed further.
Figure 12b shows UV-VIS-NIR spectra of copper nanoparticle nanofluids at different concentrations for the range under consideration. As seen in the spectra, the Surface Plasmon Resonance (SPR) band of copper appears in the range between 560 to 600 nm, which roughly coincides with the solar spectral maxima. The fluids were tested for solar radiation absorption by exposing a fixed

volume of fluids in the same geometry to sunlight and temperatures were monitored as a function of time.
Figure 13a shows the nanofluid volume fixed at 100 ml and filled in a double walled Borosil glass tube in the reflected light. The double walled glass tube had the inner tube dimensions of R=15mm and h=150mm. The fluid temperature was monitored by using 2-T type calibrated thermocouples placed inside the tube at 2 different locations to record the average temperature.
Figure 13b shows the nanofluid volume fixed at 100 ml and filled in a double walled Borosil glass tube in the transmitted light. The size of the glass tube was same and fluid temperature was also monitored in the same manner. The fluids were equilibrated at temperature T=30°C for keeping all fluids at same temperature at time t=0 s. The double walled glass tube of Fig. 8a and 8b were held facing south at an inclination angle of 750. (calculated as near optimum angle for a given latitude and day of year).
Figure 14 shows a schematic arrangement of the experimental setup for solar heating of nanofluids (fluid volume 100 ml, ) kept in the inner tubes of the double walled Borosil glass tubes (exposure area 45 cm2) DWGT, which are electrically connected to a data processor DP, a Data Acquisition System (DAS) and the thermocouples ThC, which supplied the temperature data to the data acquisition system (e.g. Nl Compact DAQ with NI9211 and NI9219, 4-channel data acquisition modules) and temperatures of the fluids in different tubes DWGT were recorded at 1 reading/sec.
The experimental results are shown in the different figures discussed below:
Figure 15a graphically represents the effect of concentration of copper nanoparticles on photo-thermal heating performance of nanofluids at various concentrations as against water (without any additive). As seen from the graph, the nanofluids attain higher temperature than pure water, when exposed to sunlight. While the water attains a maximum temperature of 45°C, the nanofluids attain a temperature in the range 58 to 68°C. Moreover, the heating rate of the nanofluid increases with the concentration of nanoparticles, but attains saturation

(i.e. no further temperature rise with increase in the particles concentration). On increasing the concentration further, the maximum temperature eventually attained, drops down due to the number of particles attenuating the incoming radiation is too high, such that the radiation is attenuated within a thin layer of fluid close to the glass wall, while the particles lying further down in the light path are not exposed to sufficient radiation, as it was attenuated in the earlier layers. Since the convective losses are highest near the glass walls, the highest temperature attained eventually falls down with the concentration after attaining a maximum. The concentrations were further fine-tuned to attain the highest temperature rise over the base fluid.
Figure 15b shows the temperature profiles of photo-thermal heating of nanofluids with respect to the concentration. Here, solar irradiance, i.e. Global irradiance and diffuse irradiance, was measured concurrently with the heating profiles using pyranometers.
Figure 16a shows Zeta potential distribution of copper nanoparticles at two different pH values. The fluids were tested for radiative heating performance with respect to pH playing a role in tuning the zeta potential. Zeta potential is the potential at the Helmholtz plane between the stern and diffuse layers on the nanoparticle-fluid boundary. It plays two major roles, first the dispersion stability, i.e. higher the zeta potential's magnitude (irrespective of its sign), higher is the dispersion stability; because Zeta potential dictates the electrostatic stabilization of nanoparticles by particle-particle repulsion, in addition to the stearic stabilization provided by the capping agent. The above phenomena prevent the coalescence of individual particles and formation of agglomerates/ aggregates, resulting in settling of the particles. Typically, Zeta potential above +/-30 mV is considered to be a stable dispersion. Secondly, Zeta potential is also crucial for particle-fluid coupling; i.e. higher Zeta potential means better particle-fluid coupling, which results in very low temperature jumps at the nanoparticle-liquid interface. Therefore, higher value of Zeta potential in a polar fluid, e.g. water leads to better thermal equilibration rather than the heat remaining localized on the particle surface. As seen here, Zeta potential of the copper nanoparticle could be tuned with the help of pH of solution. The solutions were found to be stable in basic pH range. The nanoparticle dispersion was destabilized and showed

aggregation when pH was lowered to neutral to acidic range. Zeta potential
affects the liquid layering at particle-fluid interface, the ordering of liquid
molecules leads to a quasi-solid like-liquid phase at the interface, which is
hypothesized to increase the thermal conductivity at the interface.
Figure 16b shows the effect of pH on photo-thermal heating profiles of nanofluids, particularly copper nanofluids.
Figure 16c shows the effect of insulation on photo-thermal temperature profiles of nanofluids. The insulation of the receiver tube was found significant for increasing the efficiency of the photo-thermal process. To demonstrate this, the fluids were filled in a single walled glass tube, in a double walled glass tube with air in between the two glass layers and in a double walled glass tube with vacuum in between the two glass layers. All three tubes were of identical dimension and geometry. It was observed that the vacuum prevents the heat losses due to air convection.
Figure 17a and 17b show SEM images of copper nanospheres (~25 nm) and copper nanocubes (~90 nm) respectively.
The particle shape is important to tune the optical properties of nanomaterials, which are well documented in the literature. In case of plasmonic nanomaterials, the plasmon band can be shifted or manipulated by controlling the particle size and shape, of which the latter.
Therefore, it is decided to use a mixture or cocktail of nanoparticles of various shapes for expanding the spectral attenuation range and effectively converting the radiant solar energy into heat, e.g. a combination of copper nanospheres and copper nanocubes. In accordance with the present invention, a mixture of particle shapes and sizes is used to expand the absorption in UV-VIS-NIR spectrum. The spherical particles show a single LSPR feature in the UV-VIS spectrum due to their isotropic character. The introduction of anisotropy in shape introduces multiple plasmonic modes.

For example, Nanorods of Au and Ag show at least two distinct LSPR bands, the first corresponds to out of plane plasmon resonance (transverse mode) while the second corresponds to the in plane plasmon resonance (longitudinal mode). Quadripular modes also exhibit a distinct absorption signature.
Figure 17c shows UV-VIS spectra of the mixture of copper nanocubes. As seen here, a cocktail of nanoparticles of same material but of different shapes or size is used for tuning the plasmon resonance band and to cover maximum area of the solar energy spectrum. Here, instead of using only copper nanospheres (which show a plasmon absorbance peak in the region 570-590 nm), a mixture of two different particle shapes of the same material (copper) shows two distinct maxima in UV-VIS-NIR spectrum, to effectively increase the absorption of incident solar radiation, which results in heating of the nanofluid. So, a formulation consisting of cocktail of plasmonic particles shows multiple maxima in UV-VIS-NIR spectrum; thereby increases the attenuation of a wider range of wavelengths from the incident spectrum. The performance of nanofluids over base fluids is analyzed in terms of the Figure of Merit (FOM), which compares the performance of various nanofluids.
Accordingly,
wherein,
Since the Figure of Merit represented above is a dimensionless number, it has no unit.
Figure 17d shows a schematic representation of copper nanospheres of different sizes with loose aggregation having the particle characteristics are as under:
Particle Material Particle Shape Particle Size Range Absorbance Range
Copper (Cu) Nanospheres 10nm-80 nm 0 565-590 nm
Copper (Cu) Aggregates 100 nm-800 mm 0 565-590 nm
Particle Material I Particle Shape I Particle Size Range I Absorbance Range
Copper (Cu) Nanospheres 10 nm-80 nm Ø 565-590 nm
Copper (Cu) Aggregates 100 nm-800 mm Ø 565-590 nm

Figure 17e shows SEM images copper nanospheres with loose aggregation shown in Figure 17d
Figure 17f shows a schematic representation of copper nanospheres, copper nanocubes and copper nanorods of different sizes and having the particle characteristics are as under:
Particle Material Particle Shape Particle Size Range Absorbance
Range
Copper (Cu) Nanospheres 1Onm-8Onm0 565-590 nm
Copper (Cu) Nanocubes 40 nm - 200 nm (Edge length) 575-660 nm
Copper (Cu) Nanorods 25nm-1OOnm0; 500 nm - 10 micron (length) 580-760 nm
Particle Material Particle Shape Particle Size Range
Range
Copper (Cu) Nanospheres 10 nm - 80 nm Ø 565-590 nm
40 nm - 200 nm
Copper (Cu) Nanocubes 575-660 nm
(Edge length)
25 nm - 100nm Ø
Copper (Cu) Nanorods 580-760 nm
500 nm - 10 micron (length)
Figure 17g shows SEM images of copper nanospheres, copper nanocubes and copper nanorods shown in Figure 17f
Figure 17h shows a schematic representation of copper nanospheres, copper nanocubes and copper nanowires of different sizes and having the particle characteristics are as under
Particle Material Particle Shape —
Particle Size Range Absorbance Range
Copper (Cu) Nanospheres 10nm-80 nm 0 565-590 nm
Copper (Cu) Nanocubes 40 nm - 200 nm (Edge length) 575-680 nm
Copper (Cu) Nanowires 2Onm-5Onm0; 1 micron - 80 micron (length) 580-760 nm
I I Absorbance
Particle Material Particle Shape Particle Size Range
Range
Copper (Cu) Nanospheres 10 nm - 80 nm Ø 565-590 nm
40 nm - 200 nm
Copper (Cu) Nanocubes 575-680 nm
(Edge length)
20 nm - 50 nm Ø;
Copper (Cu) Nanowires . 580-760 nm
1 micron - 80 micron (length)
Figure 17i shows SEM images of copper nanospheres, copper nanocubes and copper nanowires shown in Figure 17h.
Figure 17j shows a SEM images of a nanofluid formulation optimized to match the incident solar spectral profile by using a mixture of different particle sizes and shapes discussed above. The absorption characteristics of the particles of the same material change with particle size and shape due to shift in the localized

surface plasmon resonance (LSPR) frequency. Using a mixture of particle shapes gives rise to a broad absorption profile spanning from ~400 nm to 1100 nm wavelengths, which corresponds to the maxima of solar spectrum. The broadband spectral profile in turn is a composite of partially overlapping LSPR absorption bands of individual particle shapes; covering the maximum of the solar spectral profile incident on earth's surface.
Figure 17k shows SEM images of another nanofluid formulation optimized to match the incident solar spectral profile, similar to the one shown in figure 17j, but with a different relative composition of multiple particle shapes of plasmonic copper nanoparticles.
Figure 18 shows FOM v/s. Concentration: for spherical copper nanoparticles at varying concentrations, i.e. for cocktail of spherical and cubic copper nanoparticles at varying concentrations. CuNP (spherical)+CuNP (spherical+cubic) nanoparticles were compared to see the effect of broadband radiation absorption achieved by this cocktail of particle shapes.
Figure 19 shows nanofluids FOM plotted at different times during heating cycle. Figure 20 shows FOM for different Insolation.
Figure 21 shows a schematic representation of thermal resistances in conventional solar heating systems and direct absorption system. This uses conventional methods, in which the incident sunlight falls on a metal (of high thermal conductivity) surface and subsequently, this heated metal surface transfers the absorbed heat to the working fluid. Thermal resistance network is the comparison between a conventional solar thermal plant and a nanofluid solar thermal plant. Here, Rabs, Rcd, Rcv. RH.EX, and Rabs' refer to the thermal resistance of solid surface absorption conduction, convection, fluid-to-fluid heat exchange, and volumetric solar absorption heat transfer steps, respectively. [Ref.: Taylor et al. J. Renewable Sustainable Energy, 3, 023104,2011]. So, the above schematic diagram shows the thermal resistances for a conventional solar thermal system and the volumetric absorber system.

Figure 22a shows a perspective view of the conventional parabolic trough solar concentrator collecting solar radiations. Here, cold water CW is supplied into the one end of the metallic receiver pipe RP optimally placed before the parabolic trough solar water concentrator reflector Rf and hot water HW is obtained from the other end of the receiver pipe RP. Fluids inherently have a poor thermal conductivity (e.g. Water: 0.607 W/mK, Ethylene glycol: 0.256 W/mK, Transformer oil: 0.15 W/mK as against solids e.g. Copper 401 W/mK). Therefore, there are thermal losses and lowering of efficiency at each of the interfaces.
Figure 22b shows side view of the conventional parabolic reflector PRf of Fig. 22a. Here, the concentrated rays of light SL fall on a metal pipe MP which gets heated UD and the fluid FL flowina through this heated metallic pipe MP gets heated up by conduction.
Figure 22c shows an enlarged side view of the conventional parabolic reflector PRf of Fig. 22b. In this system, the heating of the fluid FL is a boundary layer phenomena and since the fluid carrying metal pipe MP is illuminated on only about 25% of the radial area, there is inhomogeneous heating of the fluid.
Figure 23a shows a side view of the Direct absorption nanofluid-based solar thermal collector. The parallel rays of sunlight SL are incident on the parabolic reflector PRf and then reflected towards the solar heat collector, i.e. an absorber glass tube TAbs with a selective surface enclosed within a glass envelop GE, through which the concentrated sunrays transmit to heat the nanoparticles present in the fluid. These nanoparticles in turn heat up the fluid, which is heated internally and throughout the bulk of the fluid and not just at the boundary layer, as observed in the conventional parabolic trough solar collector of Figs.22a, 22b.
Figure 23b shows an enlarged side view of the Direct absorption nanofluid-based collector of Figure 23a. Here, the fluid FL mixed with nanofluid NF gets heated up inside the glass pipe GE, as opposed to the boundary layer heating of the conventional system of Figures 23b, 23c. So, the thermal and optical losses are eliminated by minimizing the boundary layer phenomena. Since the heating is radiative heating by the absorbing "nanoheaters", and not conduction heating

as in the conventional system of Figures 23b, 23c, the limitation of the low conductivity of liquids is excellently addressed. Therefore, this nanofluid-based system heats the fluid throughout the bulk of the fluid volume. Besides the reduced thermal losses due to less number of interfaces, this direct solar absorption system uses very little amount of material such as the copper. So, the copper requirement in the fluids is just in the range of 50 ppm, which translates into a cost of less than 500 INR (~ USD 8) per 1000 litre of the additive mixed in the base fluids, thus negligibly increasing the material cost.
Figure 24 shows a typical commercial application of the invention, which employs a plurality of parabolic mirrors PM placed in parallel and provided with optimally positioned glass pipes GP (or Glass envelops GE) facing these parabolic mirrors PMs. The system includes heat storage HS in the oil flowing through the glass pipes GP. A heat exchanger HE and a condenser CD are also connected to the steam turbine and generator T&G for generating electricity (distributed through transmission towers TT) from the solar heat harnessed by the Direct Absorption nanofluid-based solar thermal collector.
Such nanofluids-based solar heat collector can be used in many commercial applications as follows:
• Nanofluids as working fluids in the existing evacuated tube solar thermal systems for domestic and industrial uses.
• Direct volumetric absorbing solar thermal systems for solar heating.
• Nanofluids as working fluids for generating process heat for industrial purposes in industries such as textiles, petrochemicals, agriculture and food processing.
• Oil-based nanofluids for steam generation by using solar thermal energy with reflecting concentrators for power generation industry.
In accordance with the present invention, the formulation composition is finalized by the synthesis of nanofluids, with nanoparticles of different but defined particle
sizes and shapes, e.g. nanospheres, nanocubes, nanowires, nanoprisms and
loose agglomerates.

Further, a physico-chemical characterization of the fluids (e.g. UV-VIS spectroscopy, X-ray Diffractometry, Scanning Electron Microscopy, Transmission Electron Microscopy, Particle Size Distribution by dynamic light scattering and Zeta potential measurements by electro-kinetic effect) is thoroughly completed for fluid characterization and for verifying the stability and applicability of the fluids.
Stable formulations are obtained in water, water-ethylene glycol mixtures (30:70; 50:50; 70:30 by volume), Paraffin oil and transformer oil. Heating profiles under various conditions like particle loading, concentration, pH, surface potential, insulation, Solar radiation intensity (insolation) with a time step of 1 second over a period of at least 10000 seconds are also measured. Long term stability of fluids is also tested over a period of 270 days at constant room temperature and under repeated cycles of heating and cooling under solar radiation.
Optimization of the solar absorption profile is done in the broad absorption range in the VIS and NIR spectrum, i.e. from 450 nm to 1400 nm coinciding with the solar radiation spectral profile in order to cover the entire solar spectrum and thus high efficiency is achieved at extremely low particle loadings (less than 50 ppm) to reduce the cost of the additive to less than 100 INR (~ USD 2) per kilolitre.
High temperatures of up to 73°C are achieved with just 50 ppm of additive as against 38°C for pure base fluid (water/ethylene glycol, without any additive).
However, the upper temperature achieved by radiative heating is not limited by the fluid formulation, but by the atmospheric cooling which is directly proportional to the temperature difference between the fluid (system) temperature and the surrounding atmospheric (global) temperature. The heating rates could be further enhanced by optimizing the geometry of the system and optimizing the insulation. The heating fluid formulations is successfully completed.
Further, the tunability of the absorption profile (wavelength) and absorptivity is also established successfully by optimizing the particle shape, particle size and relative concentration of each particle shape.

POTENTIAL APPLICATIONS OF THE PRESENT INVENTION
The nanofluid formulation comprises nanoparticles additives obtained in accordance with the present invention can advantageously be used in direct absorption solar collectors requiring:
• An additive capable of solar radiation absorption in the range of 250 to 2000 nm, with a peak around 550 nm to match the peak of solar spectrum, typically having an absorption coefficient at least 1000 to 10000 times higher than of pure water.
• an additive to effectively convert the absorbed electromagnetic radiation into heat.
• an additive to effectively dissipate the heat to the surrounding water medium (matrix) without radiative losses, such as fluorescence.
• an additive stable as a stable dispersion in the nanofluid formulation, because it prevents the particles from separating/ settling down in the dispersion.
• an additive made of inexpensive materials with low cost of materials, energy and processing.
• An additive effective for all above stated functions at the lowest possible concentrations.
TECHNICAL ADVANTAGES AND ECONOMIC SIGNIFICANCE
The nanofluid-based direct absorption solar collector configured in accordance with the present invention has the following technical and economic advantages:
• Relies on optical absorption of solar radiation and radiative heating of the
fluid.
• Uses very little material, i.e. the quantity of metallic nanomaterials used is
just in the range of 10 to 100 ppm, thus only negligibly increasing the cost
of heat-exchange fluid required for the direct absorption solar collector.

• Nanofluids based direct absorption solar collector works like "microwave based heating" in which the heating occurs from within the medium, thereby it reduces the surface thermal losses.
• Provides the highest efficiency of the order to 90-95% with only 50-70 ppm of nanoparticles dispersed in the nanofluid formulation.
Throughout this specification, the word "comprise", or variations such as "comprises" or "comprising", shall be understood to implies including a described element, integer or method step, or group of elements, integers or method steps, however, does not imply excluding any other element, integer or step, or group of elements, integers or method steps.
The exemplary embodiments described in this specification are intended merely to provide an understanding of various manners in which this embodiment may be used and to further enable the skilled person in the relevant art to practice this invention. The description provided herein is purely by way of example and illustration.
Although the embodiments presented in this disclosure have been described in terms of its preferred embodiments, the skilled person in the art would readily recognize that these embodiments can be applied with modifications possible within the spirit and scope of the present invention as described in this specification by making innumerable changes, variations, modifications, alterations and/or integrations in terms of materials and method used to configure, manufacture and assemble various constituents, components, subassemblies and assemblies, in terms of their size, shapes, orientations and interrelationships without departing from the scope and spirit of the present invention.
While considerable emphasis has been placed on the specific features of the preferred embodiment described here, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiments without departing from the principles of the invention.

These and other changes in the preferred embodiment of the invention will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

WE CLAIM:
1. A nanofluid-based direct absorption solar collector comprising a parabolic reflector with a glass pipe running parallel to the reflector axis and disposed at the focal point of the reflector for carrying fluid formulation therethrough to facilitate in harnessing the broadband absorption of substantially the entire range of solar radiations by using the the absorption profile based on plasmonic excitation of the fluid formulation.
2. Direct absorption solar collector as claimed in claim 1, wherein the fluid formulation comprises a cocktail of different shaped metallic nanoparticle additives dispersed in fluids such as water or ethylene glycol or Glycol-water mixtures or high boiling fluids like oils and the plasmonic excitation peak thereof are adjusted by tuning the sizes and shapes of the nanoparticles.
3. Direct absorption solar collector as claimed in claim 2, wherein the cocktail of different shaped metallic nanoparticles in fluid comprises defined particle sizes and shapes, such as nanospheres, nanocubes, nanorods, nanowires, nanoprisms and nanoparticle clusters.
4. Direct absorption solar collector as claimed in claim 3, wherein the fluid formulation comprises a cocktail of metallic nanoparticles of coinage metals such as Gold, Silver and Copper, preferably copper nanoparticles.
5. Direct absorption solar collector as claimed in claim 4, wherein the concentration of said copper nanoparticles is of the order of of 5 ppm to 500 ppm; particularly 10 -10010-50 ppm of copper or 10-50 mg/litre of nanofluid.
6. Direct absorption solar collector as claimed in claim 4, wherein the absorption profile of said cocktail is tuned by optimizing the particle shape, size and relative concentration of each particle shape for maximizing the efficiency of solar absorption.

7. A method of harnessing solar thermal power by using a nanofluid formulation in a direct absorption solar collector as claimed claims 1 to 6, wherein nanoparticles additives of the nanofluid formulation are synthesized by a single-step method by a top down approach using wet chemistry techniques.
8. Method as claimed claim 7, wherein the nanofluid formulation comprises nanoparticles additives synthesized by a single-step method.
9. Method as claimed claim 8, wherein the nanofluid formulation comprises nanoparticles additives synthesized by a top down approach using wet chemistry techniques.
10. Method as claimed in claim 9, wherein said method of synthesizing the nanoparticles additives includes the method steps of:

• synthesizing copper nanoparticles by reducing a copper amine complex in an aqueous medium at room temperature during a chemical reduction by using a strong reducing agent in presence of a capping agent;
• selecting the capping agents from amongst, but not limited to octylamine, dodecylamine, teradecylamine, hexadecylamine, octadecylamine; ethylenediamine, cetyltrimethylammonium bromide, gelatin, chitosan and hydrolysed chitosan;
• selecting the reducing agents from amongst, but not limited to, hydrazine hydrate, sodium borohydride, Dextrose, citric acid and ascorbic acid;
• centrifuging and re-dispersing the synthesized nanoparticles in fresh deionized (Dl) water and repeating this process 2 to 4 times to eliminate any excess capping agent to obtain a nanoparticle dispersion;
• using a secondary dispersing agent from amongst but not limited to polyvinylpyrrolidone (PVP), Polyvinylalcohol (PVA), hydroxypropyl cellulose (HPC), polyamines, polyethylene glycols, gelatin, chitosan, gum arabic, xanthan gum and guar gum in the nanofluid formulation;

• diluting said copper nanoparticle solution with fresh distilled water for obtaining a final concentration of 1000 ppm of Cu in water;
• bubbling said copper nanoparticle solution with nitrogen to eliminate any dissolved oxygen for preventing the surface oxidation of copper;
• adding an anti-oxidant agent to the copper nanoparticle solution for obtaining enhanced surface oxidation resistance by scavenging oxygen;
• using said copper nanoparticle solution as the stock solution;
• centrifuging said stock solution part to obtain a solid part thereof; and
• drying said film at ambient temperature and conditions inside a desiccator.