APPLICATIONS OF NANOFLUIDS IN SOLAR THERMAL SYSTEMS

PREPARED BY:SRIKANTH REDDY(M-TECH.)RENEWABLE ENERGY-NIT JAIPURContact:srikanthkonda.eee@gmail.com

Nanofluids: Introduction

A Nanofluid is a fluid containing nanometer-sized particles, called nanoparticles.

These fluids are engineered colloidal suspensions of nanoparticles in a base fluid.

The nanoparticles used in nanofluids are typically made of metals, oxides, carbides, or carbon nanotubes. Common base fluids include water, ethylene glycol and oil.

Nanofluid examples:

S.No Suspended particles Base fluid

1 Aluminum water

2 copper water

3 Al2O3 water

4 Graphite Therminol

5 Multi wall nano carbon tubes water

6 Graphite Propylene glycol

Solar thermal Applications of Nanofluids:

Solar 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 aspects

1) Efficiency view point

2) Economic and Environmental view point

Efficiency of nanofluid-based solar collectors

A schematic of the direct absorption collector is shown in the following figure.

The upper side of this collector is covered by a glass.

This allows the high frequency radiation entering the absorber and acts as trap for the low frequency radiation going out.

while the lower side is well insulated, so it is adiabatic.

This insulation prevents the heat loss from observer rear surface.

The working fluid is a mixture of water and aluminum nanoparticles.

The efficiency of the collector is obtained by the following equation:

Where is the mass flow rate kg/s, Cp is the specific heat co-efficient

Tout and Tin are temperatures at outlet and inlet of solar collector,

A is the Area of collector(m2) and Gt is the available solar energy(W/m2)

Effect of number/concentration of particles:

As 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 by Tyagi et al.[1] 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 beneficial.

Effect of size of particles:

The size of particles has less effect compared to that of increase in volume

of particles tyagi et.al [1] conducted experiments and the plot for

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.

Effect of type of nano particle used:

In other study[2] 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(CNT), 4) water with graphite nano-particles 5)water with silver as nano-particles

Highest 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 fraction.

Effect of % weight ratio of nanoparticles in base fluid:

Some of the works[3] focused on the effect of weight ratio of nanoparticles in base fluid in flat plate solar collectors with two weight fractions 0.2% and 0.4% and nanoparticle diameter of 15nm.

Key findings:

The efficiency of the solar collector with 0.2% weight fraction (wt.) nanofluid is greater than that with water by 28.3%.

2. For a wide range of the reduced temperature parameter (Ti -Ta)/GT, the efficiency of collector with 0.2% wt.% nanofluid is higher compared to 0.4 wt.% (see Fig. 12).

3. Using surfactant leads to a 15.63% enhancement of the efficiency.

Effect of % weight ratio of nanoparticles in base fluid:

When the same experiment of different weight ratios is conducted with carbon nanotubes as suspended particles the results[4] observed were as follows:

1.The efficiency of the collector by using of MWCNT–water nanofluid without surfactant is remarkably increased for 0.4 wt.% nanofluid, whereas with 0.2 wt.% the efficiency reduces compared to water as the working fluid.

2. For 0.2 wt.% nanofluid, using surfactant increases the

efficiency of the collector compared to water.

Nanofluids application in concentrated solar collectors for power production: Concentrated 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 oil.

In an theoretical study and experimental validation provided by Khullar et al [5] Aluminum nanoparticles with 0.05 vol.% were suspended in Therminol VP-1 as the base fluid for the analysis.

thermal efficiency of NCPSC compared to a conventional parabolic solar collector is about 5–10% higher under the same weather conditions.

Economic considerations:

An accepted way to determine the economic and environmental impacts of a product is life cycle assessment [6].

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 collector.

NOTE: useful life is 15 years is considered here.

Embodied energy considerations:

Embodied 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.

Emission/Environmental considerations

Environmental 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 collector.

Summery of Research

Application of nanofluids in Solar ponds

Salinity gradient solar ponds are great bodies of water between 2–5 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

Application 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.

Application of nanofluids in solar stills:

Solar 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 capacity.

Application of nanofluids in solar stills (cntd.)

Recently, Gnanadason et al. [7] reported that using nanofluids in a solar still can increase its productivity.

The schematic of their experimental set-up is shown below.

They investigated the effects of adding carbon nanotubes (CNTs) to the water inside a single basin solar still.

Their results revealed that adding nanofluids increases the efficiency by 50%. Nevertheless, they have not mentioned the amount of nanofluid added to the water inside the solar still.

Regarding the addition of nanofluids to the solar still,

the economic viability should be considered.

In literature, some works reported that adding dyes to

solar stills could improve the efficiency.

For instance, in [7] it’s concluded that adding violet dye

to the water inside the solar still increases the efficiency

by 29%, which is considerable.

On the other hand, it is evident that nanofluids

(especially CNTs) compared to dyes are more expensive

Future work and other possibilities:

Both technical and economical advancement in nanofluids is still going on and this trend has strong potential in facilitating greater deployment of nanofluids. Areas of improvement/application are listed as follows

Parabolic trough systems: As mentioned, only a theoretical work has been done on parabolic trough collectors, therefore some experimental studies can be performed on the effects of nanofluids on the efficiency of parabolic trough systems.

Photovoltaic/thermal systems: A photovoltaic/thermal (PV/T) system is a hybrid structure that converts part of the sun’s radiation to electricity and part to thermal energy. In this setup temperature that builds up on/under solar panel is removed using a liquid which can be used in process heating applications etc. Usage of nanofluids for heat removal from panels is under research[8].

Other possibilities: Besides the above ideas, nanofluids also can be used in:

1). Solar cooling systems[9]

2). Solar absorption refrigeration systems[10]

3). A combination of different solar devices[11].

Challenges:

Even though use of nanofluids in solar thermal system is propelling efficiency increment it suffers from some barriers which hampers its deployment in real world applications rather than laboratory environment.

Some of the challenges are discussed below:

High cost:

The first possible challenge in the use of nanofluids in solar thermal devices is the high cost of nanofluids because of difficulties in production.

The high cost of nanofluids to use in thermal engineering systems such as heat exchangers is emphasized as a disadvantage in some works [12, 13].

Instability and agglomerating:

Instability and agglomerating of the nanoparticles is another problem.

Therefore, using nanofluids in solar systems with natural circulation (such as thermosiphons) where there is no pump to circulate the fluid, is not reasonable.

It should be also noted that for high temperature gradients the agglomeration of nanoparticles

seems to be more serious [14].

Challenges (cntd.):

Pumping power and pressure drop:

Using a nanofluid with higher viscosity compared to the base fluid leads to the increase of pressure drop and consequently the increases in the required power for pumping.

For example, Duangthongsuk and Wongwises [15] found during their experiments that the pressure drop under a turbulent regime increases with an increase in volume fraction of TiO2/water nanofluid

In another experimental research, Razi et al. [16] also concluded that using CuO/oil nanofluid increases the pressure drop under a laminar regime.

Erosion and corrosion of components:

Existance of nanoparticles in nanofluid may lead to corrosion and erosion of thermal devices in a long time.

Celata et al. [17] recently investigated the effects of nanofluid flow effects using TiO2, Al2O3, SiC, ZrO2 nanoparticles with water as the base fluid flowing in different materials, i.e., aluminum, copper and stainless.

Results concluded that no erosion in steel pipe and highest erosion in aluminum. ZrO2 and TiO2 caused highest and SiC caused least erosion

References:

[1]. H. Tyagi, P. Phelan, R. Prasher, Predicted efficiency of a low-temperature nanofluid – based direct absorption solar collector, J. Solar Energy Eng. 131 (2009) 041004.

[2] T.P. Otanicar, P.E. Phelan, R.S. Prasher, G. Rosengarten, R.A. Taylor, Nanofluidbased direct absorption solar collector, J. Renew. Sustain. Energy 2 (2010) 033102.

[3] T. Yousefi, F. Veysi, E. Shojaeizadeh, S. Zinadini, An experimental investigation on the effect of Al2O3–H2O nanofluid on the efficiency of flat-plate solar collectors, Renew. Energy 39 (2012) 293–298.

[4] T. Yousefi, F. Veysi, E. Shojaeizadeh, S. Zinadini, An experimental investigation on the effect of MWCNT–H2O nanofluid on the efficiency of flat-plate solar collector, Exp. Therm. Fluid Sci. 39 (2012) 207–212.

[5] V. Khullar, H. Tyagi, P.E. Phelan, T.P. Otanicar, H. Singh, R.A. Taylor, Solar energy harvesting using nanofluids-based concentrating solar collector, in: Proceedings of MNHMT2012 3rd Micro/Nanoscale Heat & Mass Transfer International Conference on March 3–6, Atlanta, Georgia, USA, 2012

[6]. T.P. Otanicar, J. Golden, Comparative environmental and economic analysis of conventional and nanofluid solar hot water technologies, Environ. Sci. Technol. 43 (2009) 6082–6087.

[7]. M.K. Gnanadason, P.S. Kumar, S. Rajakumar, M.H.S. Yousuf, Effect of nanofluids in a vacuum single basin solar still, I.J.AERS 1 (2011) 171–177.

References: [8]. T. Hung, W. Yan, Enhancement of thermal performance in double-layered microchannel heat sink with

nanofluids, Int. J. Heat Mass Transfer 55 (2012) 3225–3238.

[9]. T.S. Ge, Y.J. Dai, Y. Li, R.Z. Wang, Simulation investigation on solar powered desiccant coated heat exchanger cooling system, Appl. Energy 93 (2012) 532– 540.

[10]. M. Ozgoren, M. Bilgili, O. Babayigit, Hourly performance prediction of ammonia water solar absorption refrigeration, Appl. Therm. Eng. 40 (2012) 80–90.

[11]. Z.S. Lu, R.Z. Wang, Z.Z. Xia, X.R. Lu, C.B. Yang, Y.C. Ma, G.B. Ma, Study of a novel solar adsorption cooling system and a solar absorption cooling system with new CPC collectors, Renew. Energy 50 (2013) 299–306.

[12]. M.N. Pantzali, A.A. Mouza, S.V. Paras, Investigating the efficacy of nanofluids as coolants in plate heat exchangers (PHE), Chem. Eng. Sci. 64 (2009) 3290–3300.

[13] J. Lee, I. Mudawar, Assessment of the effectiveness of nanofluids for singlephase and two-phase heat transfer in micro-channels, Int. J. Heat Mass Transfer 50 (2007) 452–463.

[14]. R.A. Taylor, P.E. Phelan, R.J. Adrian, A. Gunawan, T.P. Otanicar, Characterization of light-induced, volumetric steam generation in nanofluids, Int. J. Therm. Sci. 56 (2012) 1–11.

References:

[15] P. Razi, M.A. Akhavan-Behabadi, M. Saeedinia, Pressure drop and thermal characteristics of CuO-base oil nanofluid laminar flow in flattened tubes under constant heat flux, Int. Commun. Heat Mass Transfer 38 (2011) 964–971.

[16]. W. Duangthongsuk, S. Wongwises, An experimental study on the heat transfer performance and pressure drop of TiO2–water nanofluids flowing under a turbulent flow regime, Int. J. Heat Mass Transfer 53 (2010) 334–344.

[17] G.P. Celata, F.D. Annibale, A. Mariani, Nanofluid Flow Effects on metal surfaces, Energia Ambiente e Innovazione 4–5 (2011) 94–98.