FIELD OF THE INVENTION
The present invention generally relates to production of water based nanofluids. More particularly, the invention relates to a method of producing ceramic nanoparticles-based nanofluids. The invention further relates a testing method of ceramic nanoparticles-based nanofluids for solar heating applications.
BACKGROUND OF THE INVENTION
Conventional fluids, such as water, engine oil and ethylene glycol are normally used as heat transfer fluids. Although various techniques are applied to enhance the heat transfer performance, the low heat transfer performance of these conventional fluids, negates the efforts of performance enhancement and reduction in the size of the heat exchangers. The known technique for heat transfer enhancement, is the use of solid particles as an additive suspended into

the base fluid. Improving the thermal conductivity is the key idea to improve the heat transfer characteristics of conventional fluids. Since a solid metal has a larger thermal conductivity than a base fluid, suspending the metallic solid fine particles into the base fluid is expected to improve the thermal conductivity of that fluid. The enhancement of thermal conductivity of conventional fluids by the suspension of solid particles, such as millimeter- or micrometer-sized particles, is known for many years. However, such knowledge has not been found suitable for practical applications due to problems such as sedimentation leading to increased pressure drop in the flow channel. The recent advancement in materials technology research work has however, made it possible to produce heat transfer fluids by suspending nanometer-sizes particles in base fluids which can change the transport and thermal properties of the base fluid.
Nanqfluids are basically solid-liquid composite materials consisting of solid nanoparticles or nanofibers with sizes typically of 1-100 nm suspended in liquid. The nanofluid is not a simple liquid-solid mixture; the most important criterion of nanofluid is the capability of free stable suspension in agglomerate-free condition

for long duration without causing any chemical changes in the base fluid. This can be achieved by minimizing the density between solids and liquids or by increasing the viscosity of the liquid; by using nanometer size particles and by preventing particles from agglomeration, the settling down of particles can be avoided. Nanofluids have attracted great interest recently because of its enhanced thermal properties, which lead to many effective applications in heat transfer, automotives, electronics, biomedical, and nuclear fields.
Nanofluids based on water as the base solvent using various types of nanomaterials have been studied extensively worldwide in order to understand the effect on thermal conductivity as an important parameter. The extensive research has been carried out in Alumina-water and CuO-water systems besides few reports in Cu-water, and carbon nanotubes (MWCNT) based systems. The nanoparticles used in three main systems, such as Al2O3-based, CuO based and Cu-based nanofluids vary in the range of 13-300, 23-29 and 50-500 nm respectively. The improvement in the thermal conductivity in such systems varies

in the range of 1.10-1.29, 1.07-1.54 and 1.002-1.24 respectively. Limited reports are available in metal oxide based nanofluids in oil and ethylene glycol. The enhancement of thermal conductivity of the base fluid is indeed the most important requirement to improve the thermal efficiency of different systems.
Since the discovery of heat transfer enhancement properties of nanofluids, a number of patents related to nanofluids heat transfer have been filed. One of the first related patents was that by Choi and Eastman (US 6221275) described a method which involves dispersing nanocrystalline particles such as copper, copper oxide and aluminium oxide in the liquids. Davidson and Bradshaw (US 0151114) disclosed methods for using the nanopowders to transfer heat in a transformer or basically between a heat source and a heat sink. Lockwood et al. (US 0242566) described nanofluids for anti wear and corrosion inhibitor applications, by using high thermal conductivity graphite nanoparticles and carbon nanotubes. Hajikata et al. (US 0054217) disclosed a heat transport nanofluid in which the phase of the nanofluid changes corresponding to the temperature, and interalia achieves high heat transfer coefficients. Hong and

Marquis (US 0158610) outlines a method for suspending carbon nanoparticles in a hydrophilic heat transfer fluid to enhance the heat transfer characteristics such as thermal conductivity and freezing point. Hong et al. (US 0158609) describes a process of producing nanofluids in which carbon nanoparticles are directly dispersed into the fluid, wherein other additives are added in the presence of surfactants with occasional ultrasonication. Farmer (US 0291429) describes a method of utilization of nanofluid for energy conversion and storage device. Washan et al. (US 0234263) discloses a nanofluid based composition which can be used for cleaning soiled surfaces.
Baney et al. (US 0290577) discloses a nuclear reactor having efficient and highly stable thermal transfer nanofluids. McCants and Hayes (US 0288472) describes the application of nanofluids for thermal management in computers. Thermal management systems are enabled to cool a computer having integrated circuits that generate heat during use. The thermal management system includes a zinc-oxide nanofluid circulated through a series of tubes via a pump such that the heat produced by the electronic components of the computer can be captured by

the circulating nanofluid and then removed from the nanofluid by a radiator. Lee et al. (US 0049415) discloses a method of producing ZnO based nanofluids without the use of any surface active agents. The associated low-temperature, normal-pressure process produces few harmful materials and may be easily employed for production of ZnO nanoparticles. The above examples are only indicative and not exhaustive. More and more applications are being opened up by using the nanofluids.
World demand for energy is projected to be more than the double by 2050, and more than triple by the end of the century. Incremental improvements in existing energy networks are not considered to be adequate to meet this demand in a sustainable way. More energy from sunlight strikes the earth in one hour (4,3 x 1020 J) than all the energy consumed on the planet in a year (4.1 x 1020 J). The huge gap between the present use of solar energy ( <1 %) and its enormous untapped potential offers a great challenge in energy research. Sunlight is a compelling solution to the global need for clean, abundant sources of energy in the future. It is readily available, secured from geopolitical tension, and poses no threat to the environment through pollution through greenhouse gases.

Nanoparticles provide the following possible advantages in solar power plants: (1) the extremely small size of the articles ideally allows them to pass through pumps and plumbing without adverse effects, (2) nanofluids can absorb energy directly- skipping intermediate heat transfer steps, (3) the nanofluids can be optically selective -(i.e. high absorption in the solar range and low emittance in the infrared) (4) a more uniform receiver temperature can be achieved inside the collector (5) enhanced heat transfer via greater convection and thermal conductivity may improve receiver performance, (6) absorption efficiency may be enhanced by tuning the nanoparticle size and shape to the application.
The application of nanofluids for solar power heating is a new concept. Prior art is silent on adaptation of nanofluids for solar heating application, though some indirect method of using nanofluid for solar heating has been reported recently. Further, the applicability of nanofluids in high flux solar collectors is being attempted as evidenced in a recent papers Taylor et al (J. Ren. Sust. Energy, 3, 023104,2011). In a recent article, Han et al. (Nanoscale Res. Lett., 6,457,2011) have studied the thermal properties of carbon black aqueous nanofluids for solar

absorption. By performing an indirect experiment in laboratory using quartz tubes, the authors concluded that carbon black containing water has higher thermal absorption characteristics compared to that of pure water. Therefore, no report is available for water based nanofluids containing different ceramic nanoparticle for direct solar testing applications. Further, a large-scale ceramic nanoparticle based water nanofluid in actual solar heating system, poses further problems for example, producing a large quantity of nanofluid has several limitations by virtue of scale, and selecting an appropriate amount of nanoparticles for developing nanofluids for solar applications in an industrial set-up is time consuming and seasonal. It must be kept in mind that the fraction of nanoparticles in the base fluid water must be optimally selected in particular for solar heating application. If the nanoparticle concentration is too high, all the sunlight will be superficially absorbed on the surface and if the concentration is too low, a significant portion of the sunlight may not be effectively absorbed by the fluid. Further, the least amount of particles needed for effective solar absorption is cost-effective. Finally, low particles concentration in the fluid can be made stable for long time. Therefore, it is ideal to have a volumetric heating of the fluid by adjusting the optimum amount of nanoparticles dispersed in the fluid to form a stable suspension.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process to produce an improved water based nanofluid by dispersing metal oxide nanoparticles with surface active agents.
Another object of the invention is to propose a process to produce an improved water based nanofluid by metal oxide nanoparticles with surface active agents, in which the metal oxide nanoparticles are one of multiwalled carbon nanotubes (MWCNT), copper oxide (CuO), cerium oxide (CeO2), aluminum oxide (AI2O3) and Zirconium oxide (ZrO2).
A still another object of the present invention is to propose a method to produce a stable nanofluid consisting of metal oxide nanoparticles and surface active agents in a range of 100-150 L capacity.
A further object of the present invention is to propose a solar water heating system for testing of nanofluids and comparing the heating capability with that of water.

The final object of the invention is to demonstrate such large quantity fluids for solar heating capability.
SUMMARY OF THE INVENTION
The invention proposes a stable dispersion of nanopowders in water by ultrasonic disruption and using surface active agents for different nanomaterials. The water based nanofluids produced in this invention can be made stable at the room temperature with enhanced thermal conductivity. The range of production has been scaled up to industrial level so as to produce 100-150 Litres of such water based nanofluids. According to the invention, a high speed mixer instead of ultrasonication is used. A system has been further provided for testing nanofluids for determining solar heating capability by redesigning a domestic solar water heating device. Provision has been made to run two such devices simultaneously so as to compare the solar heating capability of water vis-a-vis nanofluid by maintaining a closed circuit configuration. The results of such experiments provide direct evidence for the use of nanofluids for similar closed loop applications especially in heat transfer and in electronics.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 - depicts a process flow sheet for producing of water based nanofluids including characterization according to the invention.
Figure 2 - shows a graphical comparison on the thermal conductivity of different water based nanofluids produced according to the invention as a function of volume fractions of respective ceramic nanoparticles.
Figure 3 is the schematic process flow for preparing a large scale water based nanofluids.
Figures 4(a) and (b) are the schematic of solar water heating system used for testing of thermal conductivity of nanofluids and water respectively.
Figures 5(a) and (b) are the graphical representation on solar heating capability of nanofluids of CNT based and CuO based respectively including a comparison with that of water.

Table 1 summarizes the characteristics of the ceramic nanoparticles used in the invention.
DETAIL DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described in detail to such an extent that those skilled in the art can easily implement the present invention with reference to the attached drawings.
The present invention deals with development of stable nanoparticles dispersed water based nanofluid, thermal characterization, large scale production of such fluids and demonstration on the solar heating capability.
As shown in Fig. 1, the nanofluid is prepared by ultrasonication process for dispersion of nanoparticles with surface active agents. The effect of both the process and the agents are complimentary and cannot be used independently to

produce stable nanofluids. The additives like MWCNT, CuO CeO2, ZrO2 and AI2O3 were used in this invention, and the detailed characteristics of such materials are given in table 1. A 250 watt probe type ultrasonic disrupter was used to homogenize the normal tap water and particle mixture and the time required for such operation varies for different materials and for different volume fractions of such materials. The ultrasonication was carried out at a stretch for 60 min. followed by a gap of 10 min. before resuming the next sonication step. In the case of MWCNT based fluids, extra care was taken in order to avoid breaking of CNT particles due to constant exposure to ultrasonication. A known hand held thermal conductivity meter was used for thermal conductivity measurement of the base fluid and the nanofluids followed transient heat source method of measurement. Fig. 2 shows that the thermal conductivity of nanofluids are higher compared to that of base fluids.
In the second part, and as shown in Fig. 3, a bucket was used for the preparation of at least 10 Liters of nanofluid with nanoparticle concentration

ranging from 0.05-0.1 vol.% for 150 Liters of final suspension, after initial ultrasonication of the concentrated suspension of about 1 L volume. The mixing was carried out using a high speed mixer capable to generate 100- 5000 rpm in the suspension. An amount of surface active agent corresponding to 150 L suspension was added to 1L water and ultrasonicated for about 15 min. The surface active agents are selected from polyacrylate based CR-3000, oleic acid, sodium dodecyl sulphate (SDS), TX-100 etc. In some experiments, no surfactants was used and in some cases, two surfactants were used in combination. This was followed by addition of nanoparticles and ultrasonication was continued for l-2h resulting in a highly concentrated 1 L suspension having nanoparticles in an amount suitable for 150 L final suspension of known concentration. The suspension thus produced was transferred to a larger bucket and diluted to about 10 L by mixing with water. The mixing was carried out by stirring with the high speed mixer at a speed of 1000-2000 rpm depending on the additives used for 6-8h. The suspension was used for dilution experiments to produce 150 L suspension by using a manual stirrer for 15 min.

In the third part of the present invention, a system was configured by modifying the 100 liters per day standard domestic solar water heating system. The modification was necessary in order to maintain the close circuit requirement of the testing (Figure 4a). In the system, an inlet water provision (1) to a fluid tank (2) is provided with a valve. The tank (2) has an opening on the top (3) for pouring nanofluids. After the concentrated nanofluid is poured from the top (3), it is mixed with water to make it diluted to 150 liters with a final solid concentration in the range of 0.05-0.1 vol. %. The diluted nanofluid is allowed to flow via a valve (4) to an insulated tank (5) followed by flow through an inlet pipe (6) to a solar collector (7). The sunlight gets absorbed by the circulated nanofluid and siphoned back through an outlet pipe (8) to the insulated storage tank (5) for storing the hot fluid. An outlet valve (9) is also provided in the collector (7) for draining the fluid out from the collector (7) and for cleaning. The temperature in the outlet (9) was measured at regular intervals using a k-type probe thermocouple (10) coupled with a digital temperature indicator (11) in every 30 min. intervals. Provision has been made for releasing the hot fluid from

the tank (5) after the experiment through a release valve (12). Two 100 LPD systems (Fig. 4a and b) were simultaneously run on the rooftop in order to compare the solar heating capability of water with that of nanofluid of particular composition in the similar sunlight condition. Each experiment was repeated next day in order to obtain the consistency of the data. The temperature readings were used to obtain the difference in temperature from the start of the experiment at different intervals and the solar heating capability was estimated based on such readings. It can be seen from figure 5 that a very small concentration of nanoparticles can enhance the solar heating capability of water in the range of 10-30 %.
The present invention can be better explained with suitable examples.
Example 1: MWCNT based Nanofluid
1.9-2.2 g of an anionic surface active agent was added in 1L water and ultrasonicated for 15 Min. This was followed by addition of 30-40 ml of another non-ionic surface active agent and continued ultrasonication for another 15 min.

180-220 g of MWCNT was added to the surface treated water and ultrasonicated for 1hr resulting in a highly concentrated suspension. This suspension was transferred to a 20 L bucket and diluted to 10 L mark using water. The 10 L suspension was stirred by a high speed mixer for 6-8 h at a varying rpm range of 500-2000. The thermal conductivity of such fluid showed an enhancement of 5-6 % over its base fluid. The prepared fluid was diluted to 150 Liter suspension by adding water in the solar heating tank and mixed uniformly using a manual stirrer. This fluid was used for passing through the solar collector for measurement of solar heating capability. It was noted that, the MWCNT based nanofluid has the potential to absorb heat by more than 4-6 deg C compared to that of conventional water in similar condition.
Example 2: CuO based Nanofluid
30-40 ml of an anionic surface active agent was added in 2L water and ultrasonicated for 30 min. .450-500 g of CuO was added to the surface treated water and ultrasonicated for 1h resulting in a highly concentrated suspension.

This suspension was transferred to a 20 L bucket and diluted to 10 L mark using water. The 10 L suspension was stirred by a high speed mixer for 6-8 h at a varying rpm range of 500-2000. The thermal conductivity of such fluid showed an enhancement of 2-3 % over its base fluid. The prepared fluid was diluted to 150 Liter suspension by adding water in the solar heating tank and mixed uniformly using a manual stirrer. This fluid was used for passing through the solar collector for measurement of solar heating capability. It was noted that, the CuO based nanofluid has the potential to absorb heat by more by 1-1.5 deg C compared to that of conventional water in similar condition.
Example 3 : CeO2 based Nanofluid
550-600 g of CeO2 was added to 2L water and ultrasonicated for 1h resulting in a highly concentrated suspension. This suspension was transferred to a 20 L bucket and diluted to 10 L mark using water. The 10 L suspension was stirred by a high speed mixer for 6-8 h at a varying rpm range of 500-2000.

The thermal conductivity of such fluid showed an enhancement of 8-10 % over its base fluid. The prepared fluid was diluted to 150 Liter suspension by adding water in the solar heating tank and mixed uniformly using a manual stirrer. This fluid was used for passing through the solar collector for measurement of solar heating capability. It was noted that, the CeO2 based nanofluid has the potential to absorb heat by more by 3 - 4 deg C compared to that of conventional water in similar condition.
Example 4 : ZrO2 based Nanofluid
30-40 ml of an anionic surface active agent was added in 2 L water and ultrasonicated for 30 min. 450-470 g of ZrO2 was added to the surface treated water and ultrasonicated for 1h resulting in a highly concentrated suspension. The suspension was transferred to a 20 L bucket and diluted to 10 L mark using water. The 10 L suspension was stirred by a high speed mixer for 6-8 h at a varying rpm range of 500-2000.

The thermal conductivity of such fluid showed an enhancement of 8-9 % over its base fluid. The prepared fluid was diluted to 150 Liter suspension by adding water in the solar heating tank and mixed uniformly using a manual stirrer. This fluid was used for passing through the solar collector for measurement of solar heating capability. It was noted that, the ZrO2 based nanofluid has the potential to adsorb heat by more by 2-3 deg C compared to that of conventional water in similar condition.
The above examples are not limited but are exemplary in nature. Many experiments have been carried out for each material by varying the volume fractions and the solar heating capability measured. The above examples along with, other experiments have established that the invention is made on the successful demonstration of large volume water based nanofluids with various types of nanomaterials resulting in enhancement of heat absorption capability of nanofluids.

While preferred embodiments have been shown and described, it should be understood that changes and modifications can be made therein without departing from the invention in its broader aspects. Various features of the invention are defined in the following claims:

We claim:
1. A method of suspending nanoparticles in water comprising the steps of:
producing surface treated water with known surfactants in the
range of 0.05-2 vol. %;
adding nanoparticles of 0.05-2 vol. % such as multiwalled carbon
nanotubes, copper oxide, cerium oxide, zirconium oxide and
aluminum oxide, into the fluid; and
producing nanofluid by ultrasonic disrupter exhibiting enhancement
of thermal conductivity in the range of 2-20 % over the base fluid.
2. A method of producing large volume nanofluid of selected composition,
comprising the steps of:
preparing concentrated slurry of 1-2 L batch having solid content of
0.05 and 0.1 vol. %, and producing 150 Liter suspension using
ultrasonication;
diluting the slurry to 10 Liter batch using a high speed mixing in the
range of 500-2000 rpm; and
diluting the 10 liter slurry to form a stable suspension of 150 liters
using a manual stirrer.

3. The method as claimed in claim 2, wherein the mixing time is varied from 6-8h in final stage for 10 L batch and 30-120 min. in the initial stage of ultrasonication.
4. A method of measuring the heat transfer capability of nanofluids by solar heating, comprising the steps of:
providing at least two modified 100 LPD domestic solar water
heating system by incorporating in each closed circuitry and
temperature measurement means;
passing the nanofluids through a solar collector in a first of the two
systems, and tap water in a second of the two systems;
measuring the temperature in the outlet in every 30 min. interval
for both the systems;
repeating the steps on the subsequent days in order to compare
the data.

5. The method as claimed in claim 4, wherein, the fluid requirement in each batch is about 150 liter exhibiting applicability of the process with large amount of nanofluid.
6. The method as claimed in claim 4, wherein, the testing steps are carried out for 4-6h during the peak sunlight period and repeated on the subsequent day to validate the result.
7. The method as claimed in claim 4, wherein, the temperature is recorded at regular time intervals both for water and for respective nanofluids.
8. The method as claimed in claim 4, wherein the nanofluids exhibits a
higher solar heating capability in the range of 10-30 % over its base fluid without
the nanoparticles.