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Energy performance of an evacuated tube solar collector using single walled carbon nanotubes nanofluids M.A. Sabiha a , R. Saidur b , S. Hassani a , Z. Said c , Saad Mekhilef d,a Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia b Centre of Research Excellence in Renewable Energy (CoRE-RE), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia c Department of Engineering Systems and Management, Masdar Institute of Science and Technology, PO Box 54224, Abu Dhabi, United Arab Emirates d Power Electronics and Renewable Energy Research Laboratory (PEARL), Department of Electrical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia article info Article history: Received 10 June 2015 Accepted 4 September 2015 Available online 19 September 2015 Keywords: Solar energy ETSC Heat pipe SWCNTs nanofluid Thermal efficiency abstract An experimental study was performed to determine the thermal efficiency of an Evacuated Tube Solar Collector (ETSC) using water based Single Walled Carbon Nanotubes (SWCNTs) nanofluids. Experiments were carried out using SWCNTs nanofluids having volume concentrations of 0.05, 0.1, and 0.2 vol.%. The performance of the collector was compared with SWCNTs nanofluid and water using the flow rates of 0.008, 0.017, and 0.025 kg/s. The experiments were undertaken according to ASHRAE standard 93-2003. The results show that, the collector efficiency improved with SWCNTs nanofluids com- pared to water as a working fluid. The maximum efficiency found to be 93.43% for 0.2 vol.% SWCNTs nanofluids at a mass flow rate of 0.025 kg/s. The collector efficiency shows greater enhancement with the increasing volume fractions of SWCNT nanoparticles and flow rate. In conclusions, results suggest that SWCNTs nanofluids can be used as the working fluids in an ETSC to absorb heat from solar radiation and to convert solar energy into thermal energy efficiently. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Renewable energy can be considered as an alternative energy source to meet the growing energy demand due to the scarcity and continuous depletion of conventional fuels. The most available source of renewable energy on earth is solar energy as the earth receives abundance of energy coming from the sun. Solar thermal collectors capture solar radiation which is then turned to thermal energy and transferred to a working fluid subsequently. Compared to other stationary collector such as flat plate solar collector (FPC), ETSCs have outstanding thermal performance due to lower heat loss, easy transportability, and quick installation. In addition, ETSCs are suitable for unfavorable climates [1–4]. The vacuum between the glass tubes in an ETSC reduces conduction and convection losses and allow the collector to operate at high temperatures [5–7]. The conventional fluids which are used as the heat transfer medium in solar collectors suffer from poor thermal and heat absorption properties. It has been found that these conventional fluids have a limited capacity to carry heat up, which in turn limits the collector performance. From literature, it has been observed that nanoparticles dispersed in conventional fluids (Nanofluids), have improved thermal properties [8,9]. Thus, nanofluids can be a good substitute of the conventional fluids in solar collectors [10–12]. Several researchers have already used different types of nanofluids to investigate the performance of an ETSC. A recent study regarding the performance of an ETSC by using nanofluids have been conducted by Hussain et al. [13]. Silver (Ag – 30 nm) and zirconium oxide (ZrO 2 – 50 nm) nanoparticles were dispersed in distilled water at 0, 1, 3, and 5 vol.% and two step method was applied to prepare the nanofluids. The efficiency of an ETSC was investigated as functions of mass flow rate (30 and 90 l/h m 2 ) of Ag and ZrO 2 nanofluids as well as the volume concen- trations of nanoparticles. The efficiency of the ETSC was higher for 5 vol.% Ag nanofluids compared to ZrO 2 nanofluids due to the higher thermal conductivity of Ag nanoparticles. The thermal conductivity of Ag and ZrO 2 nanoparticles are 429 W/m K and 22.7 W/m K respectively. In comparison with distilled water, higher thermal performances have been observed for both Ag and ZrO 2 nanofluids. Al-Mashat and Hasan [14] investigated the efficiency of a well instrumented ETSC consists of 16 evacuated tubes using Al 2 O 3 /water nanofluid. The performance of an ETSC is found to be proportional to volume concentration. The efficiency enhanced 28.4% with 1 vol.% of Al 2 O 3 and 6.8% with 0.6 vol.% of Al 2 O 3 . In addition, the efficiency increased by 7.08% using flat plate reflector, and 16.9% using curved plate reflector. http://dx.doi.org/10.1016/j.enconman.2015.09.009 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved. Corresponding author. Tel.: +603 7967 6851; fax: +603 7967 5316. E-mail addresses: [email protected] (M.A. Sabiha), [email protected] (S. Mekhilef). Energy Conversion and Management 105 (2015) 1377–1388 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Energy performance of an evacuated tube solar collector using single walled carbon nanotube nanofluids

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Page 1: Energy performance of an evacuated tube solar collector using single walled carbon nanotube nanofluids

Energy Conversion and Management 105 (2015) 1377–1388

Contents lists available at ScienceDirect

Energy Conversion and Management

journal homepage: www.elsevier .com/ locate /enconman

Energy performance of an evacuated tube solar collector using singlewalled carbon nanotubes nanofluids

http://dx.doi.org/10.1016/j.enconman.2015.09.0090196-8904/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +603 7967 6851; fax: +603 7967 5316.E-mail addresses: [email protected] (M.A. Sabiha), [email protected] (S.

Mekhilef).

M.A. Sabiha a, R. Saidur b, S. Hassani a, Z. Said c, Saad Mekhilef d,⇑aDepartment of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, MalaysiabCentre of Research Excellence in Renewable Energy (CoRE-RE), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi ArabiacDepartment of Engineering Systems and Management, Masdar Institute of Science and Technology, PO Box 54224, Abu Dhabi, United Arab Emiratesd Power Electronics and Renewable Energy Research Laboratory (PEARL), Department of Electrical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 June 2015Accepted 4 September 2015Available online 19 September 2015

Keywords:Solar energyETSCHeat pipeSWCNTs nanofluidThermal efficiency

An experimental study was performed to determine the thermal efficiency of an Evacuated Tube SolarCollector (ETSC) using water based Single Walled Carbon Nanotubes (SWCNTs) nanofluids.Experiments were carried out using SWCNTs nanofluids having volume concentrations of 0.05, 0.1, and0.2 vol.%. The performance of the collector was compared with SWCNTs nanofluid and water using theflow rates of 0.008, 0.017, and 0.025 kg/s. The experiments were undertaken according to ASHRAEstandard 93-2003. The results show that, the collector efficiency improved with SWCNTs nanofluids com-pared to water as a working fluid. The maximum efficiency found to be 93.43% for 0.2 vol.% SWCNTsnanofluids at a mass flow rate of 0.025 kg/s. The collector efficiency shows greater enhancement withthe increasing volume fractions of SWCNT nanoparticles and flow rate. In conclusions, results suggestthat SWCNTs nanofluids can be used as the working fluids in an ETSC to absorb heat from solar radiationand to convert solar energy into thermal energy efficiently.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction have improved thermal properties [8,9]. Thus, nanofluids can be

Renewable energy can be considered as an alternative energysource to meet the growing energy demand due to the scarcityand continuous depletion of conventional fuels. The most availablesource of renewable energy on earth is solar energy as the earthreceives abundance of energy coming from the sun. Solar thermalcollectors capture solar radiation which is then turned to thermalenergy and transferred to a working fluid subsequently. Comparedto other stationary collector such as flat plate solar collector (FPC),ETSCs have outstanding thermal performance due to lower heatloss, easy transportability, and quick installation. In addition, ETSCsare suitable for unfavorable climates [1–4]. The vacuum betweenthe glass tubes in an ETSC reduces conduction and convectionlosses and allow the collector to operate at high temperatures[5–7]. The conventional fluids which are used as the heat transfermedium in solar collectors suffer from poor thermal and heatabsorption properties. It has been found that these conventionalfluids have a limited capacity to carry heat up, which in turn limitsthe collector performance. From literature, it has been observedthat nanoparticles dispersed in conventional fluids (Nanofluids),

a good substitute of the conventional fluids in solar collectors[10–12]. Several researchers have already used different types ofnanofluids to investigate the performance of an ETSC.

A recent study regarding the performance of an ETSC by usingnanofluids have been conducted by Hussain et al. [13]. Silver (Ag –30 nm) and zirconium oxide (ZrO2 – 50 nm) nanoparticles weredispersed in distilled water at 0, 1, 3, and 5 vol.% and two stepmethod was applied to prepare the nanofluids. The efficiency of anETSC was investigated as functions of mass flow rate (30 and90 l/h m2) of Ag and ZrO2 nanofluids as well as the volume concen-trations of nanoparticles. The efficiency of the ETSC was higher for5 vol.% Ag nanofluids compared to ZrO2 nanofluids due to the higherthermal conductivity of Ag nanoparticles. The thermal conductivityof Ag and ZrO2 nanoparticles are 429W/m K and 22.7 W/m Krespectively. In comparison with distilled water, higher thermalperformances have been observed for both Ag and ZrO2 nanofluids.

Al-Mashat and Hasan [14] investigated the efficiency of a wellinstrumented ETSC consists of 16 evacuated tubes usingAl2O3/water nanofluid. The performance of an ETSC is found tobe proportional to volume concentration. The efficiency enhanced28.4% with 1 vol.% of Al2O3 and 6.8% with 0.6 vol.% of Al2O3. Inaddition, the efficiency increased by 7.08% using flat plate reflector,and 16.9% using curved plate reflector.

Page 2: Energy performance of an evacuated tube solar collector using single walled carbon nanotube nanofluids

Nomenclature

ETSC Evacuated Tube Solar CollectorSWCNT Single Walled Carbon NanotubeQu useful energy, WAc collector absorbance areaCp specific heat, J/kg �Cs transmittancea absorptanceG solar irradiance, W/m2

U uncertaintya sensitivityN number of measurementsS standard deviationg efficiency of the collector

Ti inlet temperature of the working fluid, �CTo outlet temperature of the working fluid, �CTa ambient temperature, �Cu nanoparticles volume fraction, %_m mass flow rate of working fluid, kg/skn thermal conductivity of nanofluidkbf thermal conductivity of base fluid

Subscript1 at the flow rate of 0.008 kg/s2 at the flow rate of 0.017 kg/s3 at the flow rate of 0.025 kg/s

1378 M.A. Sabiha et al. / Energy Conversion and Management 105 (2015) 1377–1388

Water based CuO nanofluid were used by Liu et al. [15] to inves-tigate the efficiency of special open thermosyphon and evacuatedtubular solar air collector combined with compound parabolic con-centrator (CPC). Using nanofluid, the maximum value of collectingefficiency of open thermosyphon has an increment of 6.6% and themean value of collecting efficiency has an increment of 12.4%.

Gao et al. [16] compared the thermal efficiency between waterin glass (WGETSC) & U pipe ETSC (UPETSC) using antifreeze fluid(40% glycol by volume) as a working fluid. From the experimentalstudy, the average thermal efficiency of WGETSC is found less thanUPETSC.

Mahendran and Sharma [17] conducted experiment on an ETSCusing TiO2 nanofluids and reported that 2.0 vol.% TiO2 nanofluidsincreases the efficiency by 42.5% compared to water. The efficiencyof collector shows greater enhancement at low volume flow rate ofnanofluids compared to water as a base fluid.

Deionized water and water based CuO nanofluids were used byLu et al. [18] on an evacuated tube solar air collector. The CuOnanoparticles have the potential to increase evaporation heattransfer coefficient by about 30%. The wall temperature of the openthermosyphon decreases due to the use of the CuO nanofluid.However, in this paper SWCNTs nanofluid is considered as theworking fluid which has been used by some researchers for othertypes of solar collector.

Karami et al. [19] introduced CNT nanofluid as an excellentworking fluid for direct absorption solar collector (DASC) due toits high thermal conductivity, good optical properties, and disper-sion stability. Functionalized CNT (f-CNT) nanoparticles were dis-persed in water and the thermal conductivity, optical propertyand stability were observed for six different volume concentrations(5, 10, 25, 50, 100, 150 ppm) of f-CNT particle. The extinction coef-ficient of nanofluid having 150 ppm CNT increased by 4.1 cm�1 andthermal conductivity increased by 32.2% compared to water. Theyalso found that the thermal conductivity is mainly dependent ontemperature than the volume concentration for solar collectorapplications. Consequently, they reported CNT nanofluid as a verysuitable working fluid for increasing overall efficiency of DASC.

To enhance the heat transfer efficiency of a heat pipe in a solarcollector Park and Kim [20] proposed a new method where thehydroxyl radicals were combined with oxidized MWCNTs to beused as the working fluid. The oxidized MWCNT nanofluids per-formed better than MWCNT nanofluids in increasing the operatingtemperature range and the total heat. From the experiment, at90 �C, the thermal conductivity of the nanofluid of 0.1 vol.% is12.6% higher than that of the base fluid (distilled water). Hence,they proposed that the oxidized MWCNT nanofluid will show out-standing effects as working fluid of a heat pipe of solar collector.

Quarter circular solar collector is a novel model for solar ther-mal system which was proposed by Rahman et al. [21]. Theenhanced performance was achieved using CNT/water nanofluidby compromising between two parameters which were the volumefraction of nanoparticles and tilt angle of the collector.

Chougule et al. [22] conducted a study on an FPC consists ofheat pipe and compared the performance using water and CNTnanofluid. The performance of collector using nanofluid is better.The average collector efficiencies at tilt angle 31.5� are 25% and45%, at 50� are 36% and 61% for water and nanofluid respectively.The maximum instantaneous efficiency obtained by using nano-fluid is 69% for 50� tilt angle. From their experimental results, itwas furthermore reported that the FPC consists of heat pipe (over-all efficiency 25–69%) gives better performance over conventionalFPC (overall efficiency 12–20%).

According to literature study summarized in Table 1, it can beobserved that no experimental study has been conducted usingSWCNT nanofluid as the working fluid in a heat pipe ETSC. Theobjective of this study is to investigate the thermal performanceenhancement of heat pipe ETSC using SWCNTs nanofluids as thenanofluids are capable to absorb solar thermal energy at all avail-able solar radiations. Due to the hydrophobic nature of CNTnanoparticles, Sodium Dodecyl Sulfates (SDS) have been used toprepare the nanofluid with the base fluid (distilled water).

2. Heat pipe solar collector

A heat pipe ETSC consists of a heat pipe inside the vacuumsealed tube, containing a temperature sensitive medium such asmethanol. There are condensation and evaporation sections inthe heat pipe. Solar radiation heats up and vaporizes the heat pipefluid in the evaporation section, and the vapor then rises to thecondenser where the vapor emits the heat and condensed back.The working fluid flowing through a manifold absorbs the emittedheat. The condensed fluid flows back to the bottom of the heat pipewhere the solar radiation begins heating it up again. To work prop-erly, the pipes must have a minimum tilt angle in order for vapor torise and the fluid to flow back.

This collector uses the copper heat pipe and the structure isillustrated in Fig. 1. The external diameter of the evaporator sectionfor this heat pipe is 8 mm and the length is 1630 mm. The externaldiameter of the condensation section for this heat pipe is 14 mmand the length is 70 mm. An aluminum fin covers the outside ofthe heat pipe evaporation section. There is Cu-SS-N/AL absorbentcoating on the fins; the absorptivity is more than 0.93. The heatpipe is inside a vacuum sealed glass tube made of a borosilicate

Page 3: Energy performance of an evacuated tube solar collector using single walled carbon nanotube nanofluids

Table 1Summaries of previous studies on the performance of evacuated tube collectors based on working fluids.

Author Type ofinvestigation

Working fluid Types of ETSC Performance

Tong et al. [23] Theoretical &Experimental

MWCNT Enclosed ETSC – Heat transfer coefficient increased by 8% using 0.24 vol.% MWCNT/waternanofluid compared to water– Collector efficiency increased 4% using nanofluid than water

Hussain et al.[13] Experimental Water based Ag andZrO2 nanofluids

ETSC consists of 20evacuated tubes

– The evacuated collector performed better using both Ag and ZrO2 nanofluidswith higher nanoparticle concentration (5 vol.%)– The performance of the collector was same as water for nanofluids lowerconcentration of nanoparticles (1 vol.%)

Al-Mashat andHasan [14]

Experimental& Theoretical

Nanofluid(Water–Al2O3)

Well instrumented collectorconsists of 16 evacuatedtubes

– The efficiency increased 7.08% with using flat plate reflector, and 16.9% withusing curved plate reflector– The volume concentration of Al2O3 was proportional to ETC performance,efficiency enhanced 28.4% with 1% of Al2O3and 6.8% with 0.6% of Al2O3, for 0.3%of Al2O3 did not make sensible enhancement

Liu et al. [15] Experimental Water based CuOnanofluid

Special open thermosyphonand evacuated tubular solarair collector combined withCPC

– Using nanofluid, the maximum value of collecting efficiency of openthermosyphon had an increment of 6.6%– The mean value of collecting efficiency of open thermosyphon had anincrement of 12.4%

Mahendran andSharma [17]

Experimental TiO2 nanofluid ETSC – Compared to water, 2.0% TiO2 nanofluids increased the efficiency of ETC by42.5%– The efficiency of collector showed greater enhancement at low volume flowrate and concentration of nanofluids compared to its base fluid which waswater

Chougule et al.[22]

Experimental Carbon nanotubenanofluid

Evacuated heat pipe andFPC

– The performance of collector using nanofluid is better– The average collector efficiencies at tilt angle 31.5� were 25% and 45% and attilt angle 50� were 36% and 61% for water and nanofluid respectively– The maximum instantaneous efficiency obtained by using nanofluid was 69%at 50� tilt angle– Solar heat pipe collector (overall efficiency 25–69%) gave better performanceover conventional FPC (overall efficiency 12–20%)

Lu et al. [18] Experimental Deionized water andwater-based CuOnanofluids

Evacuated tubular solar aircollector

– The CuO nanoparticles had the potential to increase evaporation heat transfercoefficient by about 30%– The wall temperature of the open thermosyphon decreased due to the use ofthe CuO nanofluid

Zhang andYamaguchi [24]

Experimental CO2 ETSC – Compared to water as working fluid, the supercritical CO2 had much morehigher efficiency– The collector efficiency was above 60% using supercritical CO2 as workingfluid

M.A. Sabiha et al. / Energy Conversion and Management 105 (2015) 1377–1388 1379

glass. The length of the vacuum glass tube is 1800 mm, outer tubediameter is 58 mm, and the inner tube diameter is 47 mm. Thethickness of the tube wall is 1.6 mm. The space between the glasstube and the metal pipe remains as vacuum and the vacuumdegree is 5 � 10�3 Pa.

The solar collector consists of 12 tubes and a manifold. Themanifold contains a main copper tube where the collectors’ con-densation sections are inserted. There are inlet and outlet watertubes at the two edges of the main copper tube, absorbs the heatfrom the heat pipe condensation section, experiences a tempera-ture rise and then flows out. The total collector area is 1.92 m2

while the absorbance area is 1.14 m2. The daily solar absorbancecapability of the collector is 7.9 MJ/m2 and the maximum workingpressure is 0.7 MPa.

3. Experimental conditions, setup and techniques

The climatic conditions, experimental setup and techniques(SEM and XRD, SWCNT nanofluid preparation); the testing and effi-ciency calculation methods used in this experiment are discussedin several subsections.

3.1. Climate condition in Malaysia

Malaysia is situated in central South-East Asia, borderingThailand in the north, with Singapore to the south and Indonesiato the south and west. It is composed of Peninsular Malaysia andthe states of Sabah and Sarawak on the north coast of the island

of Borneo, 650–950 km across the South China Sea. Malaysia’sstrategic geographical location offers it an advantage because theaverage solar energy received is between 1400 and 1900 kW h/m2 annually with the highest solar radiation estimated at6.8 kW h/m2 in August and November [25]. The weather of thiscountry benefits from a tropical climate with high temperaturesand high humidity throughout the year. Daytime temperatures riseabove 30 �C year-round and night-time temperatures rarely dropbelow 20 �C. Therefore, it is told that the weather condition inMalaysia is very feasible for solar energy system as the countryreceive abundant of sunshine throughout the year with an averageof 10 h of sunshine daily. Kuala Lumpur, the capital city of Malaysiais at 3�70N, 101�320E, 22 m. More than 7 h of sunshine each day isavailable in Kuala Lumpur except in November (5 h).

Malaysia is in the equatorial zone with an average daily solarradiation of more than 900W/m2 and can reach a maximum of1200W/m2 for most of the year. Fig. 2 shows that the solar radia-tion in the clear sky is more than 600W/m2 during the time periodof 11:30 until 16:00 h. The radiation at solar noon on most of thedays is above 1000 W/m2. The ambient temperature was measured28–36 �C and 24–33 �C on clear and cloudy days respectively inMalaysia. It is a good indication of the solar potential energy in thistest region.

3.2. Experimental setup

The ETSC was experimentally investigated at University ofMalaya using the setup shown in Fig. 3 and in the schematic

Page 4: Energy performance of an evacuated tube solar collector using single walled carbon nanotube nanofluids

Heat pipe Heat pipe fluid

Condensation sectionEvaporation section Absorption finGlass tube with high vacuum

Fig. 1. Heat pipe.

Fig. 2. Solar radiation vs time (on a clear day and a cloudy day).

1380 M.A. Sabiha et al. / Energy Conversion and Management 105 (2015) 1377–1388

diagram of the setup shown in Fig. 4. The specifications of the ETSCthat are used for this study is presented in Table 2. The experimentswere carried out both on clear and cloudy days during the period ofFebruary to October. According to literature, the heat pipe ETSCmust bemountedwith aminimum tilt angle of 25� in order to allowthe internal fluid of the heat pipe to return to the hot absorber [26].Therefore, the tilt angle of the collector was set at 33�. An electricpump is used in this solar collector system for the force convectionheat transfer. The solar system has a tank (Fig. 4), with capacity of100 l for absorbing the heat load from the collector cycle. A heatexchanger is used outside the tank which transfers the heat loadof the solar cycle to thewater. A flowmeter is connected to thewaterpipe before the electric pipe (Fig. 4) to measure the flow rate of theworkingfluids. A simple valve is used to control themass flowrate ofthe working fluid in the solar system.

Mass flow rate is one of the most important influencing param-eters for the thermal performance of an ETSC. According to litera-ture, the temperatures difference between top and bottom in thestorage tank decreases if the mass flow rate increases due to stron-ger convection. However, higher thermal stratification in the stor-age tank can be achieved by using a smaller flow rate but it isthermally less efficient [27,28]. Basically, flow rates that are toolow will not remove the heat efficiently from the collector, andthe efficiency of the system will be low. Flow rates that are toohigh will require larger pumps and plumbing that increase both

initial and operating costs. Though there is no standard for the flowrates but the manufacturers can suggest the range of flow ratesaccording to the area of the collector and the density of the work-ing fluids. Five K types thermocouples were used to measure theinlet, outlet fluid temperatures, ambient temperature and the tem-perature of water inside the tank. These sensors were connected toa 10 channels data logger for data storage and analysis. The globalradiation was recorded by a TES 133R solar meter. The pressuredifference between the entry and exit point, was measured usinga pressure sensor. A PROVA (AV M-07) Anemometer was used tomeasure the wind speed.

All the data were then transferred from data logger into thecomputer by an interface cable. Calibration of the entire systemwas carried out several times to obtain accurate data. All theinstruments used for this experiment were calibrated accordingto the provided standards.

3.3. SEM and XRD

The particle shape is an important characterization to analyzethe fundamental properties of nanoparticles. A Scanning ElectronMicroscope (SEM) provides topographical, morphological andcompositional information by tracing a sample of nanoparticleswith an electron beam. In addition, an SEM can detect and analyzesurface fractures, provide information in microstructures, examine

Page 5: Energy performance of an evacuated tube solar collector using single walled carbon nanotube nanofluids

Fig. 3. Evacuated tube solar collector at University of Malaya.

Nanofluid Tank

Pump

Pump

Cooling Water tank

Evacuated Tube Solar Collector

Insulation valve

Insulation valve

External water in valve

External water out valve

Flow Meter

Fig. 4. The schematic of the experimental setup of the evacuated tube solar collector.

M.A. Sabiha et al. / Energy Conversion and Management 105 (2015) 1377–1388 1381

surface contaminations, reveal spatial variations in chemical com-positions, provide qualitative chemical analyses and identify crys-talline structures. An SEM is the powerful tool to study the shapeand suspension uniformity. Therefore, a clear perception of pur-chased nanoparticles has been discovered by the SEM image anal-ysis. Fig. 5 exhibits SEM micrograph of SWCNT nanoparticles. TheSWCNT nanoparticles are primarily cylindrical. However, due to

strong Van der Wall’s attractive force, the nanoparticles are inthe form of dried agglomerates with larger dimensions than theprimary particles. In order to break down the large agglomerates,an ultrasonication was applied.

The XRD analysis of SWCNT nanoparticles is shown Fig. 6. Thepeaks of this pattern are fitted precisely with the available refer-ence patterns. The figure indicates that, all the peaks in this XRD

Page 6: Energy performance of an evacuated tube solar collector using single walled carbon nanotube nanofluids

Table 2Specifications of the ETSC.

Specification Dimension

Length 1800 mm (1.8 m)Outer tube diameter 58 mm (0.058 m)Inner tube diameter 47 mm (0.047 m)Material Borosilicate glass 3.3Absorbance 0.93Transmittance 0.89Collector area 1.92 m2

Absorbance area 1.14 m2

Distance between 2 tubes 7.5 cmThickness of the glass 1.6 mmHeat transfer coefficient 2.360 W/(m2 K)

0

10000

20000

30000

40000

50000

60000

70000

0 20 40 60 80 100

Cou

nts

2-Theta

(002)

(011)

(004)(112)

Fig. 6. XRD analysis of SWCNTs nanoparticles.

Table 3Properties of SWCNT nanoparticles.

Parameter SWCNT

Average particle diameter (nm) 1–2Purity (%) 90% CNTs, 60% SWCNTsDensity (kg/m3) 2100 [29]Thermal conductivity (W/m K) 6000 [30,31]Specific surface area (m2/g) 360–400Specific heat (J/kg K) 841 at 300 K [29,32,33]

1382 M.A. Sabiha et al. / Energy Conversion and Management 105 (2015) 1377–1388

pattern are matched distinctly with the hexagonal crystal systemof carbon nanotube. The reference pattern file code no is 98-061-7290. No peaks with other impurities were detected in the crys-talline phases. Thus, the XRD pattern is confirmed that thenanoparticles are carbon nanotube.

3.4. Preparation of SWCNTs nanofluids

Two step method was selected in order to prepare nanofluidsbased on the facility available in the University of Malaya. Thismethod can be used for large scale production of nanofluids. ShortSWCNT (90% CNTs, 60% SWCNTs) nanoparticles were purchasedfrom Nanostructured & Amorphous Materials, Inc., USA. Propertiesof SWCNT nanoparticles are listed in Table 3.

Distilled water used as the base fluid to prepare SWCNTs nano-fluid. Sodium Dodecyl Sulfate (SDS) surfactants were used to pre-vent nanoparticles agglomerates and to make the fluid stable. Eq.(3.1) was used to convert volume percentage to weight percentagefor weighing nanoparticles.

Concentration ðvol:%Þ ¼mp

qp

� �mp

qp

� �þ mbf

qbf

� � ð3:1Þ

Fig. 5. SEM image of SWCNT nanoparticles.

The suspension of SWCNTs, SDS and distilled water was thensonicated by a high pressure ultrasonic homogenizer (capacity upto 2000 bar) in order to overcome the strong cohesion forcesbetween tubes of SWCNT nanoparticles (see Table 4).

After preparation, to investigate the heat conductance capacityof SWCNTs nanofluid, the thermal conductivity of nanofluids wasmeasured using a KD2 Pro thermal property analyzer (DecagonDevice, USA) and a differential scanning calorimeter (model: DSC4000, Perkin Elmer, USA) was used to measure the specific heatof SWCNTs nanofluid.

3.5. Testing method

The ASHRAE 93-2003 standard describes test methods forsteady state or quasi-steady-state thermal performance, timeresponse and angular response tests. The purpose of performingthe tests under steady state conditions is to avoid transient influ-ences during the test that could inflate the ‘‘measured” perfor-mance of a collector. The standard rigorously defines theconditions for steady state conditions, as listed in Table 5. An aver-age daily measurement has been used in this experiment. Theexperiments are done for 9 h of a day from 9:30 am to 6:30 pmas the sunlight is available for these hours in Malaysia.

3.6. Efficiency calculations

The collector performance test is performed under steady stateconditions. The inlet and outlet temperatures were measured atdifferent mass flow rates (0.008, 0.017, and 0.025 kg/s). In such acase, the useful energy absorbed by the absorber can be calculatedusing Eq. (3.2).

Table 4Experimental conditions.

Nanoparticles SWCNTBase fluid WaterSurfactant SDSVolume concentration (vol.%) 0.05, 0.1, 0.2Mass flow rate (kg/s) 0.008, 0.017, 0.025

Page 7: Energy performance of an evacuated tube solar collector using single walled carbon nanotube nanofluids

Table 5ASHRAE standard.

Variable Absolutelimits

Units

Required environmental conditionsTotal solar irradiance (normal to sun) 790 (average) W/m2

Diffuse fraction 20(maximum)

%

Wind speed, u 2.2 < u < 4.5 m/sIncidence angle modifier 98% < normal incidence value < 102%

Maximum variation of key variableTotal solar irradiance (normal to

surface)±32 W/m2

Ambient temperature ±1.5 KVolume flow rate ±0.005 Gallon per minute

(Gpm)Inlet temperature 1 KIncident angle ±2.5 Degree (�)

M.A. Sabiha et al. / Energy Conversion and Management 105 (2015) 1377–1388 1383

Qu ¼ _mCpðTo � TiÞ ð3:2Þ

Qu is the rate of useful energy gained, _m is the mass flow rate ofworking fluid, Cp is the heat capacity of water or SWCNT nanofluid,Ti and To are the inlet, outlet temperature of the solar collectorrespectively. The input energy can be calculated using Eq. (3.3):

Qin ¼ AcG ð3:3Þ

where Ac is the absorbance area of the collector and G is the globalsolar radiation. The thermal efficiency can be obtained by dividinguseful energy with the energy input as in Eq. (3.4);

g ¼ Qu

Qinð3:4Þ

Thus; g ¼ _mCpðTo � TiÞAcG

ð3:5Þ

The thermal efficiency was calculated using experimental dataacquired according to Eq. (3.5).

4. Uncertainty analysis

Errors can arise from various factors for instance calibrationerrors, individual instrument uncertainty, data acquisition errors,and data reduction errors. Thus, to assign credible limits to theaccuracy of the presented results, an uncertainty analysis was per-formed using Eq. (4.1) [34]. The uncertainties of the measurementdevices used in this experiment are presented in Table 6.

U ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXR

i¼1a2i � S2i

rð4:1Þ

where a ¼ 1ffiffiffiN

p .

Here, U is the overall uncertainty, a is for sensitivity, N is thenumber of measurement to find the individual instrument uncer-tainty, and S is standard deviation. The maximum uncertaintyobtained to determine the collector efficiency is approximately±3.76% in the present study.

Table 6Accuracy of measurement devices.

Measurement devices Accuracy

K type thermocouples ±2.2 �CPyranometer ±5%PROVA (AV M-07) Anemometer ±3%,Flow meter <±2%Pressure transducer ±0.3% (at ±25 �C)

5. Results and discussion

Measured thermal conductivity and specific heat of SWCNTnanofluids and the thermal performance of the ETSC using SWCNTnanofluids and water both on sunny and cloudy days are presentedin several sections as follows.

5.1. Thermal conductivity and specific heat of SWCNT nanofluids

The thermal conductivity of the SWCNTs nanofluid were mea-sured for three volume concentrations which are 0.05, 0.1, and0.2 vol.% within range of 20–70 �C, illustrated in Fig. 7. FromFig. 7(a), it is observed that the thermal conductivity of SWCNTsnanofluid is much higher than water and the conductivityincreases with the increase in volume fraction, and temperature.The increase in thermal conductivity with temperature is advanta-geous for applications in collectors, as the solar radiation variesthroughout the day, with a minimum in the morning, reaching amaximum at about 14 h and reducing thereafter. Therefore,SWCNTs nanofluid is capable of absorbing more heat emitted bythe heat pipe while running through the manifold of the collectorcompared to water.

Fig. 7(b) presents the specific heat of 0.05, 0.1, and 0.2 vol.% ofSWCNTs nanofluids which was measured for every increase of5 �C within the temperature range 20–70 �C. The performance ofan ETSC was investigated using water and SWCNTs nanofluids.Before using the nanofluids as the working fluid, it was confirmedthat the suspension of SWCNTs and SDS is stable for more than1 month.

5.2. Thermal performance of the ETSC

Each test was performed in several days and the similar exper-imental data of solar radiation and ambient temperature have beenchosen in order to calculate and compare the effects of variousmass flow rates. The ETSC was tested at mass flow rates of 0.008,0.017, and 0.025 kg/s to investigate the efficiency of the collector.The results presented in Fig. 8(a)–(d), showed that the mass flowrate has an impact on the efficiency of the collector. The relation-ship between useful energy and the mass flow rate is shown inEq. (3.2) which shows that the useful energy is directly propor-tional to the mass flow rate. For every mass flow rates of waterand SWCNTs nanofluids, the efficiencies of the collector were plot-ted where g1, g2, and g3 represented the efficiency at mass flowrate of 0.008, 0.017, and 0.025 kg/s respectively. Besides mass flowrate and volume concentration of the SWCNTs nanofluid, the effi-ciency of the solar collector depended significantly on ambienttemperature and solar intensity. In this experiment the averageand maximum solar radiation was measured approximately693.53 W/m2 and 1075.575W/m2 respectively.

The heat pipe ETSC was exposed to solar radiation for half anhour before the readings were taken. The efficiency of the ETSCincreases with the increased temperature difference between theinlet and outlet temperature of the working fluids. According toFig. 8(a), the highest efficiency of ETSC for each mass flow rate ofwater was observed at noon when the temperature differencewas higher due to high solar radiation as well as ambient temper-ature. The efficiencies of the ETSC using water as working fluid are30.88%, 48.21%, and 54.37% at 0.008, 0.017, and 0.025 kg/s,respectively.

For solar radiation above 1050W/m2, the maximum tempera-ture difference between outlet and inlet fluids obtained by0.05 vol.% SWCNTs nanofluid are 16.28 �C, 12.79 �C, and 10.89 �C;0.1 vol.% SWCNTs nanofluid are 24.83 �C, 16.54 �C, and 11.77 �C;0.2 vol.% SWCNTs nanofluid are 27.38 �C, 20.35 �C, and 13.93 �C

Page 8: Energy performance of an evacuated tube solar collector using single walled carbon nanotube nanofluids

Fig. 7. (a) Thermal conductivity and (b) specific heat of SWCNT nanofluid and water.

1384 M.A. Sabiha et al. / Energy Conversion and Management 105 (2015) 1377–1388

for mass flow rates of 0.008, 0.017, and 0.025 kg/s, respectively. Itcan be observed the temperature difference between the outletand inlet fluid decreased with increasing mass flow rate. For thesame mass flow rates, the highest efficiency of the heat pipe ETSCusing 0.05 vol.% SWCNT nanofluid found to be 48.57%, 70.71%, and84.88% respectively, presented in Fig 8(b). Fig. 8(c) showed thehighest efficiency of the collector using 0.1 vol.% SWCNTs nanoflu-ids are 66.96%, 86.80%, and 90.98% whereas the maximum efficien-cies of the system using 0.2 vol.% SWCNTs nanofluid are 70.62%,91.62%, and 93.43%, illustrated at Fig. 8(d) for mass flow rates of0.008, 0.017, and 0.025 kg/s, respectively.

The highest efficiency of the collector illustrated at Fig. 9 using0.2 vol.% SWCNTs nanofluids at cloudy days was achieved as56.81% at 0.025 kg/s which was 4.5% higher than water with sameflow rate at clear days. Therefore, evacuated collector usingSWCNTs nanofluids is also suitable at cloudy days.

It is observed from Table 7 that the collector efficiency with thethree concentrations of SWCNTs nanofluids is higher than water asworking fluid. The highest efficiency of the collector is found to be93.43% using 0.2 vol.% with the mass flow rate of 0.025 kg/s. Eventhe collector efficiency at cloudy days using SWCNTs nanofluidworking fluid is higher than the efficiency of the collector usingwater at clear days. Besides better heat conductivity, nanofluidshave greater convective heat transfer capability because ofnanoconvection around nanotubes. Nanoconvection is one of themechanisms of energy transport in nanofluid due to the convectionof the nanoparticles caused by Brownian motion. The effect ofBrownian motion in evacuated collector system is impressive asnanofluid flows in tube at certain flow rates rather than being instatic situation. The efficiency improvement of the collector canalso be explained by cluster formation where clusters of nanotubesact as bridges to transfer more energy. The larger surface area ofnanoparticles increases the heat transfer rate of nanofluids as well.

Thus, SWCNTs nanofluid with higher volume concentrations pro-vides better efficiency.

This is the first experimental study to investigate the thermalperformance of an ETSC using SWCNT nanofluid. By comparingthe performance of ETSC using SWCNT nanofluids and other work-ing fluids presented in Table 1 and the obtained results in Table 7,it can be concluded that the SWCNT is the best choice for higherthermal performance.

Subsequently, this system can be used for domestic applicationssuch as water heating, laundry (Fig. 10) as well as for industrieslike textile, cement, clay brick production, wood and timber, wastewater treatment and dairy. in order to decrease fossil fuel con-sumption and to become more economical and environmentalfriendly.

5.3. Correlation development for thermal efficiency as a function ofthermal conductivity

From the experimental results, it can be observed that the effi-ciency of the ETSC system is highly dependent on the volume con-centration of the working fluids. The increasing volumeconcentration results in higher thermal conductivity as shown inFig. 7(a). Due to higher thermal conductivity, the SWCNTs nano-fluid is capable of absorbing better heat emitted by the heat pipewhich causes outlet temperature to be high. Thus, besides the massflow rates, temperature difference of the outlet and inlet fluid, areaof the collector and solar radiation, the thermal performance of thesolar system also depends on the thermal conductivity of theworking fluids.

In order to explore the positive impact of the thermal conduc-tivity enhancement on the efficiency of ETSC, an empirical correla-tion to predict the thermal efficiency of ETSC as function ofdifferent influencing physical parameters has been developed.

Page 9: Energy performance of an evacuated tube solar collector using single walled carbon nanotube nanofluids

Fig. 8. Efficiency of ETSC for (a) water, (b) 0.05 vol.% SWCNTs, (c) 0.1 vol.% SWCNTs and (d) 0.2 vol.% SWCNTs nanofluids as working fluids at three mass flow rates.

M.A. Sabiha et al. / Energy Conversion and Management 105 (2015) 1377–1388 1385

Page 10: Energy performance of an evacuated tube solar collector using single walled carbon nanotube nanofluids

0

10

20

30

40

50

60

0

100

200

300

400

500

600

700

Tem

pera

ture

(°C

) & E

ffic

ienc

y (%

)

Sola

r rad

iatio

n (W

/m2 )

Time (hr)

Radiation Ta Tin Tout η(%)

100

Fig. 9. Experimental data for 0.2 vol.% SWCNTs nanofluid 0.025 kg/s on a cloudy day.

Table 7Efficiencies of ETSC using SWCNTs nanofluids.

Mass flowrate (kg/s)

Efficiency on clear days (%) Cloudy day

Water 0.05 vol.%SWCNTnanofluid

0.1 vol.%SWCNTnanofluid

0.2 vol.%SWCNTnanofluid

0.2 vol.%SWCNTnanofluid

0.008 30.88 48.57 66.96 70.62 –0.017 48.21 70.71 86.80 91.62 –0.025 54.37 84.88 90.98 93.43 56.81

1386 M.A. Sabiha et al. / Energy Conversion and Management 105 (2015) 1377–1388

The present correlation has been elaborated usingVaschy–Buckingham theorem (Buckingham p theorem) to gener-ate two pi groups which included the different physical variablesinfluencing the thermal efficiency of the ETSC as shown in the fol-lowing equation:

g ¼ f ð _m; cp;DT;A;G; kn; kbf Þ ð5:1ÞThe pi groups are generated from several physical variables as

follows:

Fig. 10. Application of ETSC f

p1 ¼ _mcpDTAcG

ð5:2Þ

p2 ¼ knkbf

ð5:3Þ

By combining these two pi groups, the correlation is thenderived as:

g ¼ ap1pb2 ð5:4Þ

where a and b are empirical coefficients and determined by apply-ing non-linear regression to the experimental data. The final form ofthe obtained correlation is presented in Eq. (5.5)

g ¼ 0:91p1ffiffiffiffiffiffip2

p ð5:5ÞThe correlation (5.5) showed an excellent agreement with the

experimental data since the standard and mean deviation was±2.12% and ±1.75% respectively and the R2 value is 0.9946. Thecomparison between the experimental data and the predicted datausing the correlation (5.5) is presented in Fig. 11.

Using the correlation (5.1), to further explain the effect of ther-mal conductivity of the SWCNT nanofluids on the efficiency of the

or domestic applications.

Page 11: Energy performance of an evacuated tube solar collector using single walled carbon nanotube nanofluids

0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.9534

35

36

37

38

39

40

41

42

43

Thermal conductivity enhancement (W/m.K)

Ther

mal

eff

icie

ncy

(%)

Fig. 12. Thermal efficiency with the enhancement of thermal conductivity.

M.A. Sabiha et al. / Energy Conversion and Management 105 (2015) 1377–1388 1387

ETSC, an assumption is made as DT = 10 �C; G = 1000 W/m2;kbf = 0.6 W/m K (distilled water) and if the mass flow rate is fixedat 0.01 kg/s, the result showed that the thermal efficiencyincreased with the enhancement of thermal conductivity as pre-sented in Fig. 12.

It can be clearly observed from Fig. 12 that, at 0.6 W/m K whichrepresents the value of thermal conductivity of water, the thermalefficiency of the ETSC is lower comparing to the case of thermalconductivity of nanofluid.

For the same assumptions if the flow rate is not fixed, to achievean efficiency of 70%, the mass flow rate of the working fluidsdecreased with the enhancement of thermal conductivity whichis illustrated in Fig. 13. Therefore, it can be concluded that a work-ing fluid with an enhanced thermal conductivity requires a lowpumping power which ultimately lower the initial cost andincrease the exergetic efficiency of the solar system.

The present correlation is valid for ETSC consisting 12 evacu-ated tubes, SWCNT nanoparticles suspended in water with volumeconcentration of 0.05–0.2 vol.% and within inlet fluid temperaturerange of 25–46 �C.

0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.950.0165

0.017

0.0175

0.018

0.0185

0.019

0.0195

0.02

0.0205

Thermal conductivity enhancment (W/m.K)

Mas

s flo

w ra

te (k

g/s)

Fig. 13. Mass flow rate with thermal conductivity enhancement.

6. Challenges and future recommendations

From literature, an ETSC can be the best choice due to theirexcellent heat transfer capability and least environmental effectduring their manufacture but the initial cost of installing an ETSCis higher compared to an FPC system [35]. However in China, thecost of installing ETSC system is much lower compared to FPC[36,37]. An economic analysis of SWH using ETSC is carried outin China by Saxena and Srivastava [38]. The initial cost of installingthe SWH, having a 100 l storage tank reported is $377 and the pay-back period calculated is 4.41 years. Whereas, a payback period of17 years is calculated by Allen et al. [39] for SWH, consists of 110 lstorage tank using FPC which had the initial cost of $4823–5627.The simple payback period without considering the discount rateis calculated 0.7–3.0 years. Though the production cost of an ETSCis getting lower, using nanofluids as working fluid is not yet costeffective due to the requirement of complicated and advancedequipment needed to prepare stable nanofluids as well as the highcost of nanoparticles. Besides, the carbon nanotube nanofluidshave higher viscosity compared to conventional fluids which willincrease the pressure drop and pumping power of the system[40]. Moreover, stability of nanofluids is a major concern for thereal life applications of nanofluids. Therefore, ETSC system withSWCNT nanofluids is still a challenge from the economic and appli-cation point of views.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Experimental data

Cor

rela

ted

data

-3%

+3%

Fig. 11. Comparison of experimental data with predicted data using the correlation(5.1).

For further development in harvesting solar energy, solartracker can be installed to track the maximum sunlight throughoutthe day. Though the cylindrical shape of the ETC helps to track thesun passively throughout the whole day but it is not able to absorbthe maximum sunlight as the solar panel is positioned with a fixedangle. Solar tracker is able to orient itself along the direction of thesunlight and ensures the absorption of maximum sunlightthroughout the day by adjusting its orientation according to thesun. Another way to improve the overall efficiency of the systemis to use concentrating evacuated collector. To achieve high tem-perature, concentrating collectors use mirrors and lenses by con-centrating sunlight of a large area onto a small area.

To justify the aptness and to assess the environmental impact ofan ETSC using SWCNTs nanofluid, life cycle assessment (LCA) studycan be conducted. Life cycle assessment (LCA) study will give acomplete environmental view of a process, assisting in decisionmaking processes in conflicts between economic, social and envi-ronmental variables [41].

7. Conclusions

The effect of mass flow rates and volume concentrations ofSWCNTs nanofluids, as the working fluid, on the efficiency of anETSC was investigated experimentally. To study the energy effi-ciency of the collector, the mass flow rates of the working fluidsused were 0.008, 0.017, and 0.025 kg/s. The following conclusionshave been made from the experiments conducted.

Page 12: Energy performance of an evacuated tube solar collector using single walled carbon nanotube nanofluids

1388 M.A. Sabiha et al. / Energy Conversion and Management 105 (2015) 1377–1388

(1) The energy efficiencies of the ETSC for 0.05 vol.% SWCNTnanofluids are 48.57%, 70.71%, and 84.88%; for 0.1 vol.%SWCNT nanofluids are 66.96%, 86.80%, and 90.98%; for0.2 vol.% SWCNT nanofluids are 70.62%, 91.62%, and 93.43%with the mass flow rates of 0.008, 0.017, and 0.025 kg/srespectively.

(2) The efficiency of the collector is higher for SWCNTs nano-fluid compared to water due to the improved thermal prop-erties of SWCNTs nanofluids. The collector efficiency surgesup to 93.43% for 0.2 vol.% SWCNTs nanofluid which is 71.84%higher compared to water at a flow rate of 0.025 kg/s.

(3) The maximum outlet temperature and temperature differ-ence of water achieved at low mass flow rate of 0.008 kg/sfor 0.2 vol.% SWCNT nanofluids.

(4) The efficiency of the collector using 0.2 vol.% SWCNTnanofluids on cloudy days is better compared to water onsunny days for the same mass flow rate.

(5) An empirical correlation is developed in terms of importantparameters which have significant influence on the effi-ciency of the ETSC and the correlation shows an excellentagreement with the experimental data having a standardand mean deviation of ±2.12% and ±1.75% respectively.

Acknowledgements

The authors would like to acknowledge the Ministry of HigherEducation Malaysia (MOHE) for financial support. This work wassupported by UM-MOHE High Impact Research GrantScheme (HIRG) (Project no: UM.C/HIR/MOHE/ENG/40).

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