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Case Studies in Thermal Engineering

Case Studies in Thermal Engineering 5 (2015) 70–78

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Investigations for effect of Al2O3–H2O nanofluid flow rate onthe efficiency of direct absorption solar collector

Hemant Kumar Gupta n, Ghanshyam Das Agrawal, Jyotirmay MathurDepartment of Mechanical Engineering, Malaviya National Institute of Technology, Jaipur, India

a r t i c l e i n f o

Article history:Received 20 October 2014Received in revised form24 December 2014Accepted 16 January 2015Available online 17 January 2015

Keywords:Direct absorption solar collectorAl2O3–water nanofluidFlow rateCollector testingEfficiency enhancement

x.doi.org/10.1016/j.csite.2015.01.0027X/& 2015 Published by Elsevier Ltd. This isreativecommons.org/licenses/by-nc-nd/4.0/)

esponding author.ail address: hemant.rin2001@gmail.com (H.K

a b s t r a c t

The efficiency of conventional tube‐ in plate type solar collectors is limited due to higherheat losses for surface based solar energy absorption and indirect transfer of heat from hotabsorber surface to working fluid having poor heat transfer properties flowing throughtubes. In this paper, a prototype direct absorption solar collector having gross area 1.4 m2

working on volumetric absorption principle is developed to investigate the effect of usingAl2O3–H2O nanofluid as heat transfer fluid at different flow rates. Experimentation wascarried using distilled water and 0.005% volume fractions of 20 nm size Al2O3 nano-particles at three flow rates of 1.5, 2 and 2.5 lpm. ASHRAE standard 93-86 was followed forcalculation of instantaneous efficiency of solar collector. Use of nanofluid improves theoptical and thermo physical properties that result into an increase in the efficiency of thecollector in all cases of using nanofluids in place of water. Collector efficiency enhance-ment of 8.1% and 4.2% has been observed for 1.5 and 2 lpm flow rate of nanofluid re-spectively. Optimum flow rate of 2.5 and 2 lpm towards maximum collector efficiencyhave also been observed for water and nanofluid respectively.& 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND

license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Solar thermal energy is one of the most popular renewable sources of sustainable energy with least environmentalimpact, no requirement of transportation and free availability for every human being all over the world [1–3]. Solar thermalcollector is a widely used system for collection and conversion of solar energy into thermal energy. Among these differenttypes of solar collectors, the conventional ‘tube in plate’ type flat plate collector absorbs incident solar radiation through ablack solid surface, and transfers heat to working fluid flowing in tubes called risers, brazed onto the surface of the absorberplate. The efficiency of a solar thermal collector relies on the effectiveness of absorbing incident solar radiant energy andheat transfer from the absorber to the carrier, which is normally fluid. Due to surface heat absorption and indirect transfer ofheat to working fluid, the conversion of sunlight into thermal energy suffers from relatively low efficiencies [4].

In order to improve the efficiency of solar thermal collector, researchers proposed the concept of directly absorbing thesolar energy within the fluid volume in the 1970s called Direct Absorption Solar Collector (DASC) [5,6]. However, the ef-ficiency of direct absorption collector is limited by the absorption properties of the conventional working fluid, which is verypoor over the range of wavelength in solar spectrum [7]. In the beginning, black liquids containing millimeter to micrometersized particles were also used as working fluid in direct absorption solar collectors to enhance the absorption of solar

an open access article under the CC BY-NC-ND license.

. Gupta).

H.K. Gupta et al. / Case Studies in Thermal Engineering 5 (2015) 70–78 71

radiation that had showed efficiency improvement. The applications of micron-sized particles into the base fluid for DASCslead to pipe blockage, erosion, abrasion and poor stability. Particle sedimentation from the suspensions resulted in cloggedchannels [5].

Advance material synthesis technologies provide us an opportunity to produce the nano-size materials (nanoparticles),when suspended in conventional fluids considered as nanofluids [8]. The use of nanofluid has a dramatic improvement onthe liquid thermo physical properties such as thermal conductivity [9,10]. Studies suggested the thermal conductivity en-hancement due to dispersion of nanoparticles [11], intensification of turbulence [12], Brownian motion [13,14] and ther-mophoresis [15].

Masuda et al. [16] dispersed Al2O3 and TiO2 nanoparticles in water and found thermal conductivity improvement by 32%and 11%, respectively. Grimm [17] dispersed aluminum metal particles (1–80 nm) into water and claimed 100% increase inthermal conductivity of the nanofluid for 0.5–10 wt%. Natarajan and Sathish [18] investigated the thermal conductivityenhancement of base fluids using carbon nanotube (CNT) and suggested efficiency enhancement of the conventional solarwater heater by using CNT based nanofluids as a heat transport medium. Nanoparticles also offer the potential of improvingthe radiative properties of liquids, leading enhanced efficiency of direct absorption solar collectors [19].

Recently Sheikholeslami et al. [20–24] used nanofluid and simulated nanofluid flow and heat transfer by differentmethods for different kind of problems to enhance the heat transfer rate.

Yousefi et al. [25] reported the experimental results on a tube in plate type conventional solar collector (size 2 m2) usingAl2O3–H2O nanofluid of 0.2 wt% and 0.4 wt% concentrations for three different mass flow rates and found 28.3% im-provement in efficiency with 0.2 wt% of nanofluid in comparison to water. Yousefi et al. [26] also examined the effects ofmultiwall carbon nanotubes–water nanofluid and observed remarkable efficiency increase with 0.4 wt% nanofluid.

Tyagi et al. [27] numerically studied a direct absorption solar collector using aluminum nanoparticles in water forperformance evaluation and reported efficiency improvement up to 10% than that of a flat-plate collector. Otanicar et al. [28]experimentally studied the role of different nanofluids as the absorption medium on the efficiency of horizontal micro size(3 cm�5 cm) direct absorption collector in indoor environment and reported efficiency improvement up to 5%.

Very few studies on the thermal performance evaluation of flat plate solar collector with nanofluids are available. As suchno study on full size (1.4 m2) tilted DASC under actual outdoor condition is available. An attempt has been made in thepresent paper, to experimentally study the effect of Al2O3–H2O nanofluid flowing as thin film over the glass absorber plateas a direct absorbing medium on the efficiency of a tilted direct absorption solar collector under outdoor condition. Effect ofthree different nanofluid flow rate i.e. 1.5 lpm, 2 lpm and 2.5 lpm were considered on the DASC efficiency and the collectorperformance was also compared with base fluid distilled water.

2. DASC experimental setup

Schematic diagram explaining the working of direct absorption collector is shown in Fig. 1.An experimental setup of direct absorption solar collector of size 1.54 m�0.9 m (gross area of 1.4 m2) has been devel-

oped as shown in Fig. 2.

2.1. Experimental apparatus and procedure

For experimental study, a setup of DASC was developed and erected at the roof top of Mechanical Engineering De-partment, Malaviya National Institute of Technology, Jaipur (26.01° latitude and 75.52° longitudes). The collector was or-iented due south with a tilt angle of 26°. Photograph of experimental setup (Fig. 2) showing direct absorption collector, twotanks and instruments used along specification of the collector components used in Table 1. It mainly consists of a glass baseplate (1.5 m long, 0.9 mwide, 0.006 m thick), mounted on a wooden box with inner glass wall on all four sides and equipped

Fig. 1. The schematic of direct absorption solar collector (DASC).

Fig. 2. Experimental setup of direct absorption solar collector.

Table 1The specifications of DASC components.

S. no. Component Dimension Remark

1 Collector 1.54 m�0.9 m Gross area¼1.40 m2

2 Absorber 1.44 m�0.80 m Effective area¼1.15 m2

3 Transparent cover 6 mm Material toughened glass4 Base plate 6 mm thick Material toughened glass5 Collector box inner glass wall 6 mm thick Material -plain glass6 Film formation system ¾″ Header pipe with 1 mm dia holes-106 no, pitch 1 mm Aluminum pipe7 Bottom insulation 50 mm thick Glass wool8 Side insulation 25 mm thick Glass wool9 Frame 200 mm height Material-M.S.

H.K. Gupta et al. / Case Studies in Thermal Engineering 5 (2015) 70–7872

with a spray system for film formation over the glass base plate. In DASC no tubes are used for carrying fluid and nanofluidflows directly over the glass plate, which is used in place of black absorber plate. A perforated header pipe (106 holes of1 mm diameter with 1 mm pitch) is used to obtain a uniform nanofluid film on the glass plate.

Experimental test setup consists of a solar collector, working fluid loop and data acquisition system. The working fluidloop has two tanks called bottom storage reservoir and upper reservoir. A simple manual globe valve is used to control flowrate of working fluid and flow rate is measured with the help of electromagnetic digital flow meter (Make-Electronet, range0–5 lpm, accuracy 71%). A centrifugal pump circulates the collected fluid in the system.

Three J-type thermocouples were installed to measure collector inlet and outlet fluid temperatures and the ambienttemperature. These readings were collected and stored in a computer through a data logger (Make-Agilent, model-34970A,16 channels). Intensity of total solar radiation was recorded using digital solar meter (Range 1–1300 W/m2, accuracy 75% ofmeasurement). The experiments were performed at different inlet temperatures of working fluid according to ASHRAEStandard 93-86 [29].

Before every test run, the experimental test loop was cleaned using distilled water to remove oxides of nanoparticles andfouling residue that could affect the collector performance.

Table 2Physical properties of Alumina (Al2O3) nanoparticles.

Size of particles 20–30 nmShape of particles Spherical particlesDensity 3700 kg/m3

Surface area per unit weight 15–20 m2/gCrystal form GammaAl2O3 content 99.99%

H.K. Gupta et al. / Case Studies in Thermal Engineering 5 (2015) 70–78 73

2.2. Nanofluid preparation

Preparation of stable nanofluid with uniform dispersion is an important requirement for improving heat transfer per-formance of conventional fluids and nanofluid needs to be prepared in a systematic and careful manner. Three methodsavailable for preparation of stable nanofluids are [30].

i.

Surfactant addition to the base fluid. ii. Acid treatment of base fluid. iii. Ultrasonic mixing of nanopowder in base liquid.

Thermo physical properties of nanofluids are affected with the use of surfactants and acid treatment may cause materialdegradation after some days of continuous usage of nanofluids in practical applications. The sonication is an approvedtechnique for dispersing the aggregated nanoparticles [31,32]. In the present study ultrasonic vibration mixer is used forpreparation of nanofluid with minimum aggregation of nanoparticles and improved dispersion behavior. Dry Al2O3 nano-particles of 99.99% purity and average size of 20–30 nm (procured from Nanoshel-Intelligent Materials Private Ltd., USAbased company) are used with distilled water as base fluid in nanofluid preparation. Properties of the Al2O3 nanoparticlesused are tabulated in the Table 2.

The quantity of Al2O3 nanoparticles required for preparation of nanofluid of different volume concentrations is calculatedusing formula in Eq. (1). A sensitive balance (make-citizen, model-CTG 602 resolution-0.1 mg) is used to weigh the Al2O3

nanoparticles very accurately.

m V VF (1)np t np npρ=

where mnp is the mass of nanoparticles (kg), Vt is the total volume of nanofluid (m3), VFnp is the volume fraction of na-noparticles and ρnp is the density of nanoparticles (kg/m3).

Ultrasonication was applied for 6–7 h to mix calculated amount of Al2O3 nanoparticles in distilled water using ultrasonicvibration mixer (Make-Toshniwal, model-UP-600S, power-600 W, frequency-2773 kHz,) as shown in Fig. 3.

The Al2O3 nanofluid thus prepared was kept for observation and no particle settlement was observed at the bottom ofthe flask even after twenty four hours. During the experimentation, the time taken to complete the experiment is less thanthe time required for first sedimentation to take place and hence surfactants are not mixed in the Al2O3 nanofluids. Fourdifferent volume concentrations of 0.001, 0.005, 0.01, and 0.05% were used in the study.

Fig. 3. Ultrasonic cleaner apparatus for sonication process of Al2O3–water nanofluids.

H.K. Gupta et al. / Case Studies in Thermal Engineering 5 (2015) 70–7874

3. Testing method

Thermal performance of solar collectors is commonly evaluated using ASHRAE Standard 93-86. Collector thermal per-formance is calculated by determining collector instantaneous efficiency for different incident solar radiations, ambienttemperatures, and inlet fluid temperatures. Intensity of incident solar radiations as well as useful heat gain by the workingfluid is measured under steady state conditions.

3.1. Time attempt

As per ASHRAE Standard 93-86 steady-state conditions should be maintained during the data period and also during aspecified time interval prior to the data period, called the pre-data period. For attaining steady state conditions the massflow rate must be within 71%, irradiation must be within 750 W/m2, the outdoor ambient temperature must not varymore than 71.5 K, and the inlet temperature must be within 70.1 K for the entire test period.

3.2. Governing equations for efficiency calculation

The experiments were performed at different inlet temperatures of working fluid according to ASHRAE Standard. Themeasurements were taken for ambient, inlet & outlet temperature, global solar intensity and the mass flow rate of workingfluid. The useful heat gain by the fluid can be calculated using Eq. (2).

Q mC T T A F I U T T( ) [ ( ) ( )] (2)u p o i c R T L i aτα= − = − −

where Q u is the useful heat gain (W), m is the mass flow rate of fluid (kg/min), Cp is the heat capacity of water or nanofluid(J/kg K), To is the outlet fluid temperature of solar collector (K), Ac is the surface area of solar collector (m2), FR is the heatremoval factor, (τα) is absorptance–transmittance product, IT is the global solar radiation (W/m2), UL is the overall losscoefficient of solar collector, and Ta is the ambient temperature (K).

The heat capacity of nanofluid is calculated with the help of equation [33].

C C C( ) (1 ) (3)p nf p np p bf, , ,ϕ ϕ= + −

where ϕ indicates the volume fraction of nanoparticles, and Cp np, and Cp bf, are heat capacities of nanoparticles (773 J/kg K)and base fluid (4180 J/kg K) respectively. Instantaneous collector efficiency relates the useful heat gain to the incident solarenergy by Eqs. (4) and (5).

QA I

mC T T

I

( )

(4)iu

c T

p o i

Tη = =

−

F F UT T

I( )

( )(5)i R R L

i a

Tη τα= −

−

If the thermal efficiency test is performed at the normal incidence conditions then FR(τα), and F UR L is constant for thetemperature range of the collector. When the efficiency values obtained from averaged data is plotted against T T I( / )i a T− astraight line will result according to Eq. (5). Intersection of the line with the vertical efficiency axis equals to absorbedenergy parameter, FR(τα). At this point the temperature of the fluid entering the collector equals the ambient temperatureand collector efficiency is at its maximum. Slope of the line indicates energy loss from the collector that is nominated asenergy loss parameter F UR L. At the intersection of the line with the horizontal axis collector efficiency is zero and designatedas stagnation point, usually occurs when no fluid flows in the collector.

3.3. Experimental uncertainty analysis

As per ASME guidelines, absolute measurements do not exist and errors arise from many sources. Some of the commonsources of error are: Calibration errors, data acquisition errors and data reduction errors. The uncertainty of the experi-mental results in the present work was determined by following ASME guidelines on reporting uncertainties in experi-mental measurements based on the deviation in experimental parameters. The major components to uncertainty in

Table 3Results of uncertainty analysis.

S. no. Parameter Uncertainty (%)

1 Solar intensity 75.62 Volumetric flow 71.43 Temperature difference 71.7

H.K. Gupta et al. / Case Studies in Thermal Engineering 5 (2015) 70–78 75

collector efficiency are the inaccuracy in flow rate measurement, temperature measurement and solar radiation intensitymeasurement. The results of uncertainty analysis of the measurements including all the sources of errors are presented inTable 3.

General form of equation for uncertainty analysis is as given below.

U U(6)

yi

n

X2

1

2i∑=

=

where Uy is the total uncertainty of calculated parameter and Uxi is the root sum square of scatter and measuring uncertaintyof each measured parameter.

The combined uncertainty for evaluating collector efficiency, Uη, was obtained by the root sum square method (RSS),based on Eq. (4) by the following relation.

⎜ ⎟⎡

⎣⎢⎢

⎛⎝

⎞⎠

⎛⎝⎜

⎞⎠⎟

⎛⎝⎜

⎞⎠⎟

⎤

⎦⎥⎥( )U

mm

T TT T

II

( )

(7)

out in

out in

22 2 2

= Δ

+Δ −

−+ Δ

η

The maximum uncertainty obtained in the present study in determining the collector efficiency, at various tests wasaround 6%.

4. Results and discussions

4.1. Water as working fluid

Experimental tests were performed with distilled water on direct absorption solar collector from 10 AM to the time atwhich stagnation point is reached for three flow rates of 1.5, 2 and 2.5 lpm on several days in October 2013. The best datasatisfying conditions of ASHRAE standard have been taken. Fig. 4 shows the variation of collector efficiency versus thereduced temperature parameters, (mTi�Ta)/IT, for each flow rate. The experimental data are best fitted with linear equationsto provide the performance characteristic parameters of the collector for different flow rates. The efficiency parameters, FRUL

and FR(τα), at each flow rate are presented in the Table 4.It is observed from Fig. 4 that the collector maximum efficiency (Ti¼Ta) increases 7.2%, when the flow rate is increased

from 1.5 lpm to 2 lpm, while increasing the flow rate to 2.5 lpm causes the maximum efficiency to increase only 0.9%.Table 3 shows that the energy loss parameter FRUL is the minimum for 2.5 lpm flow rate. So, the efficiency of the collector isthe highest at optimum value of 2.5 lpm flow rate.

It can also be seen from Fig. 4 that the efficiency of solar collector increases with increasing the flow rate. These resultsshow that by increasing the Reynolds number the efficiency is increased.

4.2. Al2O3–water as working fluid-effect of flow rates

Al2O3 nanoparticles are mixed in base fluid distilled water to get nanofluid of 0.005 volume fraction concentration andinvestigations are performed to determine the effect of different flow rates at 1.5, 2 and 2.5 lpm. Nanofluid is collected in thebottom tank and then pumped to overhead tank. At each flow rate experiments with several test periods at different inletfluid temperature in quasi steady state conditions were conducted from 10.00 AM to time when stagnation temperature isachieved on a day. The experimental results are plotted as shown in Fig. 5. The efficiency parameters FR(τα) and FRUL ofcollector for three flow rate of Al2O3 nanofluid are presented in Table 5.

Fig. 5 shows that the collector efficiency lines for 2 and 2.5 lpm intersect each other at reduced temperature value of

Fig. 4. Efficiency versus (Ti�Ta)/IT curve at three flow rates for water.

Table 4Collector efficiency parameters at three flow rates for water as working fluid.

S. no. Flow rate (lpm) FRUL FR(τα) R2

1 1.5 11.197 0.4926 0.98172 2 11.56 0.528 0.9683 2.5 9.9727 0.5326 0.9557

Fig. 5. Efficiency versus (Ti�Ta)/IT curve at three flow rates for Al2O3–water nanofluid.

Table 5Collector efficiency parameters for Al2O3–water nanofluid.

S. no. Flow rate (lpm) FRUL FR(τα) R2

1 1.5 10.903 0.5455 0.93492 2 15.663 0.7372 0.9823 2.5 9.4418 0.5322 0.925

H.K. Gupta et al. / Case Studies in Thermal Engineering 5 (2015) 70–7876

0.035. For low temperature range, (Ti�Ta)/ITo0.035, the collector efficiency is greater at 2 lpm due to higher value ofabsorbed energy parameter. However for high temperature range, (Ti�Ta)/IT40.035, the collector efficiency is greater at2.5 lpm due to reduced heat losses. As collector is operated most of the time in the low temperature range hence, 2 lpm flowrate is observed as an optimum value of the flow rate for maximum collector efficiency.

At very low flow rates, the fluid residence time in the collector was high so greater absorption of solar energy allowingmore temperature rise. But emission of radiation from the fluid scales with the fourth power of temperature, the fluidsuffered higher emissive losses at lower velocities, which resulted in smaller collector efficiencies. Thus, the heat transferrate is influenced by these two parameters.

At higher velocities, though, the temperature rise in the fluid itself was small, but this resulted in a progressively weakereffect of emissive losses hence, collector efficiencies were seen to be independent of flow rates at higher values.

Figs. 6, 7 and 8 present the relative effect of water and nanofluid on the collector efficiency for 1.5, 2 and 2.5 lpm flow raterespectively. It is seen that for all the considered flow rates, the collector efficiency using nanofluid is higher than usingwater. Efficiency can be estimated by comparing the values of absorbed energy and heat loss parameters in Tables 4 and 5.Collector efficiency value for 1.5 and 2 lpm flow rates in the case of Al2O3 nanofluid are higher than that of water up to 8.1%and 4.2% respectively and for 2.5 lpm efficiency is almost same. As the flow rate increase from 1.5 lpm to 2.5 lpm, thecollector efficiency enhancement with nanofluid decreases due to the less residence time for fluid in the collector at higherflow rates resulting negligible effect of nanofluid at higher flow rate in direct absorption solar collector.

y = -11.197x + 0.4926R² = 0.9902

η

(Ti -Ta)/IT (m2K/W) →

→

Fig. 6. Efficiency versus (Ti�Ta)/IT curve at 1.5 lpm flow rate for water and nanofluid.

y = -11.561x + 0.528R² = 0.9796

y = -15.67x + 0.7374R² = 0.9888

η

(Ti -Ta)/IT (m2K/W) →

→

Fig. 7. Efficiency versus (Ti�Ta)/IT curve at 2 lpm flow rate for water and nanofluid.

y = -9.9734x + 0.5326R² = 0.9813

y = -9.4831x + 0.533R² = 0.9682

η

(Ti -Ta)/IT (m2K/W) →

→

Fig. 8. Efficiency versus (Ti�Ta)/IT curve at 2.5 lpm flow rate for water and nanofluid.

H.K. Gupta et al. / Case Studies in Thermal Engineering 5 (2015) 70–78 77

Fig. 9 shows the comparison of efficiency at optimum flow rate for water (2.5 lpm) and nanofluid (2 lpm). It is seen thatefficiency lines intersect each other. For low temperature range (operating range of collector), collector efficiency usingnanofluid is higher due to higher absorbed energy parameter.

Adding solid nanosize particles to base fluid at such small volume concentration has many advantages (compared withpure water) other than the improved thermo physical properties of fluid such as thermal conductivity and heat transfercoefficient, which is responsible for high efficiency. Nanoparticles mass migration phenomenon in the nanofluid workingmedia enhances the heat transfer enhancement. Mixing of a small amount of nanoparticles to the base fluid also makes theclear fluid water completely opaque to the naked eye, improving the absorptivity hence extinction coefficient of nanofluidthan pure water (optical properties) as well as enlarge heat transfer surface area and much larger absorption efficiency incomparison to the scattering efficiency due to very tiny size particles, all together increase the efficiency of direct-absorptionsolar collectors, and the indicated experimental results are well justified.

5. Conclusion

The effect of using Al2O3–water nanofluid on the direct absorption solar collector efficiency with three different flowrates 1.5, 2, 2.5 lpm have been studied experimentally. The volume fraction of nanoparticles has been selected as 0.005%. Thecollector efficiency increased with nanofluid than pure water for all flow rates. Collector efficiency enhancement of 8.1% and4.2% has been observed for 1.5 and 2 lpm flow rate of nanofluid respectively. The experimental results prove that the

y = -9.9734x + 0.5326R² = 0.9813 y = -15.67x + 0.7374

R² = 0.9888

η

(Ti -Ta)/IT (m2K/W) →

→

Fig. 9. Efficiency versus (Ti�Ta)/IT curve at optimum flow rate for water and nanofluid.

H.K. Gupta et al. / Case Studies in Thermal Engineering 5 (2015) 70–7878

optimum flow rate for maximum collector efficiency occurs at different flow rate for water and nanofluid i.e. 2.5 and 2 lpmrespectively in this study.

Significant enhancement in solar radiation absorption and collector efficiency makes nano-fluids as a suitable heattransfer fluid for solar thermal applications and can also be used in solar collectors for effectively capturing and transportingthermal energy.

Acknowledgment

The authors would like to thank the Department of Science & Technology, India for supporting fund (Project no.075370015) under the program of Solar Energy Research Initiatives (SERI) Project. The authors would like to thank theDepartment of Mechanical Engineering, Malaviya National Institute of Technology, for supporting testing facilities.

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