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Nano Fluids-An IntroductionFluids containing Nano sized particles,called Nano particlesengineeredcolloidal suspensionsof nanoparticles in a basefluid. nanoparticles used in nano fluids are typically made of metals, oxides, carbides, orcarbon nanotubes. Common base fluids include water,ethylene glycoland oil.Nano fluid examples: Aluminum-Water Copper-Water etc

ENERGY AND ECONOMIC ADVANTAGES OF NANO FLUID BASED DIRECT ABSORPTION IN SOLAR THERMAL SYSTEMS1Solar thermal Applications of Nano fluidsSolar collectors are of particular kind of heat exchangers which absorbs the incoming solar radiation and convert it into thermal energy.The collected solar thermal energy is carried through a working fluid (liquid/gas) which could be used for hot water or space conditioning or may be for thermal storage to use during cloudy days/night times.In the conventional solar thermal collectors water, glycol and organic/synthetic oils were used as working fluid.However, these fluids have relatively low thermal conductivity and thus cannot reach high heat exchange rates in thermal engineering devices.A way to overcome this barrier is using ultra fine solid particles suspended in common fluids to improve their thermal conductivity.The use of nanofluids in solar water heaters is discussed with respect to two aspects1) Efficiency view point 2) Economic and Environmental view point

ENERGY AND ECONOMIC ADVANTAGES OF NANO FLUID BASED DIRECT ABSORPTION IN SOLAR THERMAL SYSTEMS2Efficiency of nano fluid-based solar collectorsENERGY AND ECONOMIC ADVANTAGES OF NANO FLUID BASED DIRECT ABSORPTION IN SOLAR THERMAL SYSTEMS3Effect of number/concentration of particlesAs the number of nano particles increases or the volume fraction of particels incrases as a percentage of total working fluid volume conductive nature and there by heat transfer capacity should increase there by leading to better efficiency.A study done resulted in the following observations.In this % volume of nano particles is increased from 0.1% to 5% and efficiency got asymptotic gradually.They attributed the increase of collector efficiency to the increase in attenuation of sunlight passing through the collector due to the nanoparticles addition that leads to the increase of collector efficiency.However, for a volume fraction higher than 2%, the efficiency remains nearly constant, so adding more nanoparticles is not beneficialENERGY AND ECONOMIC ADVANTAGES OF NANO FLUID BASED DIRECT ABSORPTION IN SOLAR THERMAL SYSTEMS4Effect of size of particlesThe size of particles has less effect compared to that of increase in volume efficiency variation with particle size is given as follows:The variation is less due to the fact that for a given volume even though size is varied conductivity of fluid remains the same.ENERGY AND ECONOMIC ADVANTAGES OF NANO FLUID BASED DIRECT ABSORPTION IN SOLAR THERMAL SYSTEMS5Effect of type of nano particle usedIn other study variation of efficiency using different types of nano-particles is estimated.Working fluids considered in this study are 1) water with reflective coating on back side of collector, 2) water with black painted back side of collector 3) water with carbon nanotubes, 4) water with graphite nano-particles 5)water with silver as nano-particlesHighest efficiency is reported in silver nanoparticle which is due to high conductivity of silver compared to other nano-particles.In CNT and graphite nanofluids efficiency increased when particle volume fraction is increased up to 0.5% and then reduced which is attributed to high absorption of fluid at larger particle volume fractionENERGY AND ECONOMIC ADVANTAGES OF NANO FLUID BASED DIRECT ABSORPTION IN SOLAR THERMAL SYSTEMS

6Nano fluids application in concentrated solar collectors for power productionConcentrated solar collectors produce more thermal energy compared to flat plate collectors. So efficiency is superior than that of flat plate collectors.This efficiency can be further improved if nanofluids are used in place of normally used fluids like therminol oilthermal efficiency of NCPSC compared to a conventional parabolic solar collector is about 510% higher under the same weather conditions.

ENERGY AND ECONOMIC ADVANTAGES OF NANO FLUID BASED DIRECT ABSORPTION IN SOLAR THERMAL SYSTEMS

7Economic considerationsAn accepted way to determine the economic and environmental impacts of a product is life cycle assessment.The table given below shows the economic parameters of flat plate collectors with water and Al2O3 as working fluids.The economic analysis shows that the capital cost and maintenance costs of the nanofluid-based solar collector compared to the conventional one are $120 and $20 higher, respectively.However, because of the higher efficiency and annual solar fraction of the nanofluid-based solar collector, the fuel cost savings per year, for both electricity and natural gas, is greater than that of the conventional solar collectorENERGY AND ECONOMIC ADVANTAGES OF NANO FLUID BASED DIRECT ABSORPTION IN SOLAR THERMAL SYSTEMS

8Embodied energy considerationsEmbodied energy: Energy used in life of a product from raw material to finished product stage.Embodied energy for both solar collectors, including conventional and nanofluid-based collectors are compared[6] as follows:The embodied energy for the nanofluid-based collector is about 9% lower which makes it energy conservative.

ENERGY AND ECONOMIC ADVANTAGES OF NANO FLUID BASED DIRECT ABSORPTION IN SOLAR THERMAL SYSTEMS

9Emission/Environmental considerationsEnvironmental emission reduction is one of the driving forces that is leading renewable energy sources use.The environmental effects of both conventional and nanofluid based solar flat plate collectors are given by,

The manufacturing of the nanofluid-based solar collector leads to 34 kg fewer CO2 emissions while during its operation it saves 50 kg year when compared to the conventional solar collector.The magnitudes of other emissions including SOx and NOx are very small, so the differences are not considerable. Over the 15-year expected lifetime of the solar collectors, the nanofluid- based solar collector would offset more than 740 kg of CO2 in comparison to a conventional collectorENERGY AND ECONOMIC ADVANTAGES OF NANO FLUID BASED DIRECT ABSORPTION IN SOLAR THERMAL SYSTEMS

10Application of nanofluids in Solar pondsSalinity gradient solar ponds are great bodies of water between 25 m deep, which could collect solar radiation and store it in the form of heat. Structure of conventional solar pond is shown below.If concentration gradient is not there the continuous water circulation of water between will cause heat loss. So concentration of lower section is increased by adding salt (NaCl) there by making it dense, thus reducing circulation and heat loss.Heat energy at temperatures 70-80 C can be extracted from the bottom and can be used for applications like dairy heat requirements, space heating, moisture removal from live stock etc.Normally fresh water is used for heat extraction, but at low temperatures 70-80 C heat exchange rates are very less and may cause delay there by heat losses. This can Be improved by using nanofluids with superior heat exchange rates

ENERGY AND ECONOMIC ADVANTAGES OF NANO FLUID BASED DIRECT ABSORPTION IN SOLAR THERMAL SYSTEMS

11Application of nanofluids in Solar ponds (cntd.)As seen, a nanofluid flows through a heat exchanger mounted at the bottom of the solar pond to absorb the heat. It expects that nanofluids could enhance the rate of heat removal from the bottom of the solar pond.An experimental setup is proposed in literature to improve solar pond efficiency by employing nanofluid.In this a closed loop consisting of nanofluid transfers heat energy from solar pond to system in which heat energy is required. Nonofluid acts as moderator here circulating between heat exchanger and nanofluid tank.

ENERGY AND ECONOMIC ADVANTAGES OF NANO FLUID BASED DIRECT ABSORPTION IN SOLAR THERMAL SYSTEMS

12Application of nanofluids in solar stillsSolar still:Solar stills are very useful in desalination and water purification.This system consists of a container in which sea water/ waste water is collected. Top of the container is covered with glass cover which acts as heat trap.As the solar energy gets trapped inside water temperature rises and starts evaporating. The evaporated water vapour gets to top and cools down to condense into water which can be collected along the edges of glass cover.This produces around 25 liters of fresh water per day but very slow at starting due to very low temperatures and low absorption rates.By adding synthetic dyes made of carbon nanotubes(CNT) heat absorption capacity can be increased thus increasing production capacityENERGY AND ECONOMIC ADVANTAGES OF NANO FLUID BASED DIRECT ABSORPTION IN SOLAR THERMAL SYSTEMS

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