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distilled water nanofluid on the energy efficiency of evacuated tube solar collector
International Journal of Heat and Mass Transfer 108 (2017) 972–987
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
An experimental investigation on the effect of Al2O3/distilled water nanofluid on the energy efficiency of evacuated tube solar collector Javad Ghaderian, Nor Azwadi Che Sidik ⇑ Department of Thermofluid, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, UTM Skudai, Johor 81310, Malaysia
a r t i c l e
i n f o
Article history: Received 27 August 2016 Received in revised form 21 December 2016 Accepted 27 December 2016
Keywords: Solar energy Evacuated Tube Solar Collector (ETSC) All-glass passive circulation Thermosyphon Al2O3 nanofluid Thermal efficiency
a b s t r a c t The present study was designed to experimentally investigate the effect of using Al2O3/distilled water nanofluid as the working fluid, on the thermal efficiency of all-glass passive circulation evacuated tube solar collector with internal spherical coil inside the horizontal tank. The volume fraction of nanoparticles was 0.03% and 0.06% with the nanoparticles dimension 40 nm. Triton X-100 as surfactant was used in this experimental study. The performance of the ETSC using Al2O3 nanofluid and water was compared with the flow rate inside the coil was varied from 20 to 60 l/h. The present study revealed that the maximum efficiency was found to be 57.63% for 0.06 vol% of nanofluids and mass flow rate of 60 l/h. The collector efficiency shows greater enhancement with the increasing volume fractions of Al2O3 nanoparticles and flow rate. In conclusions, results suggest that Al2O3 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 2016 Elsevier Ltd. All rights reserved.
1. Introduction Solar energy is renewable and has a nonpolluting nature; hence it is commonly used in applications such as energy heating systems, chemical substance processing and electrical power generation. Evacuated Tube Solar Collectors (ETSC) are able to capture the available Solar Energy and then transfer it as thermal Energy to the water heating system [1–3]. The ETSCs are made up of multiple parallel or rows of glass tubes. Each tube incorporates a twin glass tube that is coated, which is able to absorb solar energy. Air from the space between the twin glasses is removed and forms vacuum. So, there is no air to cause convective losses or to conduct heat, and besides, convective and conductive heat losses are removed. There are two main categories of vacuum solar collector can be seen in the literature; all glass direct flow and heat pipe ETC [4–6]. Heat transfer improvement in solar collectors is one of the critical issues in energy saving and compact designs. One of the innovative techniques to increase heat transfer efficiency involves the employment of additives in base fluids [7–12]. Recently, the suspension of nanosized particles, which is an impressive substance, has been employed in conventional fluids [13–15]. Numerous scientists have currently made use of various kinds of nanofluids to examine the efficiency of different type of ETSC. Sabiha et al. [16] investigated the thermal performance enhancement of a heat pipe ⇑ Corresponding author. E-mail address: [email protected] (N.A.C. Sidik). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.12.101 0017-9310/Ó 2016 Elsevier Ltd. All rights reserved.
evacuated tube solar collector with using the SWCNT nanofluid as the working fluid. In this research, three different mass flow rate were carried out in the experimental study with having volume concentration of 0.05, 0.1 and 0.2 vol%. The results of their study indicated that the maximum efficiency obtained 93.43% for 0.2 volume fraction of SWCNTs nanofluids at a mass flow rate of 0.025 kg/ s. Other than that, Mahendran et al. [17] conducted an experiment to determine the efficiency of an ETSC using water-based Titanium Oxide. 99.5% purified titanium oxide nanoparticle of 40 wt% was used with an average particle size of 30–50 nm. For the preparation method, a two-step method was adopted by dispersing the TiO2 nanopowder into distilled water. In this study, the effect of clear sky and cloudy day against the time and the solar insolation was analysed extensively. Nevertheless, the temperature enhancement of nanofluid was about 19% higher than that of water. The maximum efficiency of the collector system using TiO2 was 0.73, as compared to water, which was (0.53). In other innovative research, Gao et al. [18] analysed a U-pipe ETSC in conjunction to thermal performance. In comparison to FPSC with U-pipe Evacuated solar collector, which is referred as U-pipe, the ETSC generally works better in cold climate and also associated with low heat loss advantage. Furthermore, the authors showed that the thermal collector efficiency did not increase certainly with the increase in length. Karami et al. [19] investigated using carbon nanotubes nanofluid (CNT) as the working fluid in direct absorption solar collector as a result of its high thermal conductivity as well as dispersion stability. Six different volume concentrations alkaline
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Nomenclature Atank Cp C p;bf C p;np C p;nf FR _ m S t1 ; t2 T T1; T2 Ti; To T in
surface area of the storage tank specific heat at constant pressure heat capacity of base fluid (water) heat capacity of nanoparticles heat capacity of nanofluid collector heat removal factor mass flow rate (natural circulation flow rate through the evacuated tube solar radiation on absorber initial and final time of measurement temperature initial and final average temperature of the system respectively during the period of measurement, averaged from nine thermocouple readings in the tank in flow and out flow temperature through the opening of the tube coil inlet temperature
functionalized carbon nanotubes (f-CNT) were considered in this study. The study has shown that improvements the thermal conductivity up to 32.1% obtained by using 150 ppm f-CNT to water. This study also found that generally effect of temperature is more than volume concentration for DASC. Park and Kim [20] conducted experiment on a solar collector with using oxidized MWCNT as the working fluid for improving the heat transfer efficiency of a heat pipe in a solar collector. This kind of nanofluids increases the operating temperature range and total heat. The thermal conductivity of the nanofluid increase 12.6% with using 0.1 vol% of oxidized MWCNT. Therefore, they recommended that the oxidized MWCNT nanofluid will certainly demonstrate to outstanding results as working fluid of a heat pipe of solar collector (see Table 1). A recent research study relating to the efficiency of an ETSC by utilising nanofluids has been carried out by Hussain et al. [21] Two different kinds of nanoparticles, silver (Ag) and also zirconium oxide (ZrO2) with 30 nm and 50 nm respectively, were spread in pure water at 1, 3, and also 5 vol%. In order to prepare the nanofluids was used two-step technique. Accordingly, the present study
T out T Ta
a b
c d
g u / bf nf np
coil outlet temperature average temperature of the tank during measurement period average ambient temperature during measurement period thermal diffusivity coefficient of volumetric expansion inclination angle delta efficiency volume fraction of nanoparticles (%) weight fraction of nanoparticles in nanofluid base-fluid nanofluids nanoparticle
attempts to examine the efficiency of an ETSC with verity of mass flow rate (30 and 90 l/h m2) as well as different volume concentration of Ag and ZrO2 nanoparticles. The finding of the present study is that, as a result of the greater thermal conductivity of Ag nanoparticles compared to ZrO2 nanoparticles therefore the efficiency of the ETSC was higher for 5 vol% Ag nanofluids compared with ZrO2 nanofluids. In the current experimental study, for the first time we proposed a system of passive circulation evacuated tube solar collector with internal spherical coil inside the horizontal tank Al2O3/ distilled water nanofluid as the working fluid. As shown in Fig. 1, the system consists of 18 evacuated tubes manufactured from Borumsilicate glass that were connected directly to the horizontal storage tanks, with 100 l as a nominal capacity, with the spiral coil inside the tank. The inner diameter tube and outer diameter tube of the evacuated glass tubes were 45 mm and 47 mm long, respectively, made of stainless steel by a polyurethane isolation layer. The evacuated tubes were connected with the storage tank by rubber sealing. The tube length was 1800 mm; the length of the storage tank was nearly 1700 mm, the thickness of the glass tubes was
Table 1 Summaries of previous studies on the performance of evacuated tube collectors based on working fluids. Author(s)
Year
Collector type
Methodology & type of study
NFs type and size of NP
Research finding
Mahendran et al. [17] Mahendran et al. [22] Lu et al. [23]
2013
HP-ETSC
Experimental
TiO2/water (20–30 nm)
The effect of volume concentration was studied using three different volumes 1–3%. Efficiency increased by 42% using 2% concentration of nanofluid
2012
HP-ETSC
Experimental
TiO2/water (30–50 nm)
2011
Experimental
Copper oxide 50 nm
Liu et al. [24]
2013
Evacuated tubular solar collectors Concentric Tube
The efficiency of collector estimated using TiO2 nanofluid of 0.3% concentration is about 73% compare to water 58% Efficiency enhanced by 16.7%. Fixed flow rate at 2.7 l per minute for both liquids The heat transfer coefficient has increased by 30% using water based CuO as substitute of water as working fluid
Experimental
Copper oxide 50 nm
Shahi et al. [25]
2010
ETC
Numerical
Akbari et al. [26]
2008
Horizontal tube
Numerical
Copper oxide 100 nm Al2O3
Open thermosyphon working fluid with using nanofluid has the higher collector efficiency than that using water. Two kinds of The solar collector integrated with thermosyphon are also compared with that integrated with common concentric tube They investigated the effect of nanofluid on natural convection in ETC. It was found that 40% increase in mass flow rate is obtained when a approaches 15–35° while enhancement is about 5% when a changes from 55° to 75° Heat transfer coefficient increased by 15% at 4 vol% Al2O3 and maximum heat transfer coefficient is achieved at angle of 45°. The effect of tube inclinations and effect of nanoparticles concentration on the thermal parameters and hydrodynamics are presented, But nanoparticles do not have effect on the hydrodynamics parameter
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41 41
1. Tube 1 2. Tube 3 3. Tube 6 4. Tube 9 5. Tube 12 6. Tube 15 7. Tube 18 8. The Bottom Rod 9. The Top Rod 10.The Middle Rod
11.Tank 12.Valve 13.Valve 14.Auxiliary Source 15.Valve 16.Coil 17.Flow meter 18.Rubber Seal 19.Inlet coil valve 20.Outlet coil valve
21.Thermocouple 22.Thermocouple 23.Thermocouple 24.Thermocouple 25.Thermocouple 26.Thermocouple 27.Thermocouple 28.Thermocouple 29.Thermocouple 30.Thermocouple
31.Thermocouple 32.Thermocouple 33.Location of temperature measurments 34.Location of temperature measurments 35.Location of temperature measurments 36.Location of temperature measurments 37.Location of temperature measurments 38.Location of temperature measurments 39.Exhaust pipe 40.Frame 41.Reflector Plate
Fig. 1. Schematic view of ETSC with experimental instrument.
1.6 mm and its internal diameter was 450 mm. The tilts of the actual tubes were 45° and 1.57 m2 was the surface area of the solar collector (Ac). The global absorptivity ðaÞ and global emissivity ðeÞ were at 0.96 and 0.06 respectively, with the selective coating for inner tube as according to the product brochure, refer to Table 2.
Table 2 Specification of evacuated tube solar collector. Specifications
Unit
Dimension
Gross area Aperture area Absorber area Length Width/width incl. Connection. Connection Max. operating pressure Temperature working Collector angle Absorber Absorption (a)/emission (e) Collector housing Collector glazing
m2 m2 m2 in mm
2.57 2.22 2.36 70.800 1560/1612
bar k o – – – –
Number of tubes Outer glass tube diameter Inner glass tube diameter Sealing material Frame materia
– in in – –
10 283–372 45 Aluminium 0.96/0.06 Aluminium Evacuated tubes (borosilicate glass) 18 evacuated tubes 2.2800 1.8500 Silicone Stainless steel
2. Material and preparation method Aluminium Oxide (Al2O3) nanoparticles were purchased from Sky Spring Nanomaterials, Inc. Houston, USA. An enhancement on ETSC was performed by implementing nanofluids (Al2O3/DW) instead of ordinary fluid water with different volume concentrations of nanoparticles aluminium oxide powders (alumina) 99% (Gamma), (0, 0.03 and 0.06) with APS = 40 nm morphology, near spherical shape. For dispersion of Al2O3, Triton X-100 was used as a natural surfactant and also distilled water was utilised in this study as a base fluid. Two- step methods were carried out to enable the reduction of Al2O3 aggregation and increase the scattering behaviour. Firstly, by applying Triton X-100 as surfactant and secondly, by using the ultrasonic vibration (Q500 ultrasonic system with standard probe model).
3. Experimental setup The experimental set up of evacuated tubes solar collector that was used in this research is displayed in Fig. 2. The dimensions and proportions of the ETSC that were used in this experiment are shown in Table 2. Physical properties of the Al2O3 nanoparticles and water used in the calculations are presented in Table 3. According to this design of the system and evacuated tube, this ETSC was working by thermosyphon and as mentioned, this
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Fig. 2. Front and Back view of ETSC under experimental work with part of the setup.
Table 3 Physical properties of nanomaterial and water. S. no
Nanoparticle & base fluid
Density [Kg m3]
Thermal conductivity [Wm1 K1]
Specific heat [J kg1 K1]
Fluid thermal diffusivity a 107 [m2 s1]
1. 3.
Al2O3 Water
3970 997.5
36 0.605
773 4178
131.7 1.47
system does not need the electrical pump, as the circulation inside the collector is a natural convection. The measurements were undertaken during sunny days between 10 am and 5 pm while the heat flux and the global solar radiation were not constant. The tank temperature was monitored in the period of the experimental work by using fifteen T-type thermocouples, positioned at different levels and multiple locations inside the tank, and the internal energy increase of the system could be calculated. Thermocouple location is shown in Fig. 3. For the measuring of the flow temperature, measurement at the inlet and outlet of the natural circulation inside the tubes was carried out for seven chosen vacuum tubes (tubes number 1, 3, 6, 9, 12, 15 and 18) which are shown in Fig. 3. In order to measure the temperature profile across the vertical plane at the opening of each tube, three T-type thermocouples were used; one in the bottom half at the centre of the tubes and two others uniformly distributed across the top half of the opening of the tubes. The outlet flow temperature from each tube was the average of the measurements from the two thermocouples at the top, and the inlet flow temperature was represented by the bottom thermocouple. In order to compute the total of useful energy per tube, this was achieved by the division of the total of useful energy from the collector in the number of tubes (see Table 4). The experimental part was conducted under Malaysian climate; latitude 3.13°N and longitude 101.68°E. The title angle of this solar collector (b = 45°) [1,25] is from horizontal, aspect ratio of 40, capacity of tank 100 l. A spiral coil was used inside the tank that
was working as a heat exchanger that transmitted the heat load of the working fluid inside the solar collector to the consuming water inside the coil. The temperature range 20–80 °C was considered for calibration of thermocouples against a platinum resistance thermometer, based on the ASHRAE Standard in experimental work. After the thermocouples were fixed inside the ETSC, the fluid working inside the collector should have a uniformed temperature before starting the experimental work. For this purpose, the separate circulation pump was used for mixing the working fluid and also the area of tubes were shaded with a cover for the prevention of solar radiation to the area of the tubes. The results of all the thermocouples’ readings have been found to be within (±0.1 K). During the peak solar radiation period, the working fluid circulating flow experienced temperature increase through an evacuated tube with the order of (2–4) K. Therefore, on each thermocouple, an error of (0.1) K in up to 10% error would be led in the designation of the natural circulation flow rate in the middle of the day (around 1 pm), and higher error will happen in the morning and in the afternoon. Errors could also result from the averaging of the two thermocouples’ readings to get the outlet flow temperature. The thermocouple readings’ data by data logger were averaged and recorded at 60 s intervals in the experiment work. The computation of energy and flow rate is dependent on the accuracy of the temperature measurements. Hence, to reduce the error due to short term fluctuations in the temperature readings, each temperature used in the energy and flow rate computation was obtained
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Fig. 3. (a) Schematic view of one of the evacuated close tubes for heat transfer analysis of the storage tank and coil, (b) seven chosen tubes in the setup with locations of thermocouples for inlet flow and outlet flow temperature measurements across the opening of the tubes.
Table 4 Volume concentration of Al2O3 nanoparticles with corresponding weight. No.
Volume concentration,u (%)
Weight of nanoparticles ðwAl2 O3 Þ (gm)
1 2
0.03 0.06
0.613 1.267
from the linear regression of the 30 s readings over a half-hour period across solar noon [27]. The change in the temperature over half-hour intervals is almost linear for most of the day during the experimental period on clear days, except in the early mornings and evenings.
water) nanofluids. In order to weigh the nanoparticles very accurately, a sensitive balance with 0.1 mg resolution is employed (see Figs. 6 and 7). 6. Weight of nanoparticles for different volume concentration The weight of the nanoparticles required for the preparation of 500 ml of Al2O3 nanofluids of a particular volume concentration, using distilled water as base fluid is calculated by using the standard expression:
/¼
V np V nf
ð6:1Þ
4. XRD and FESEM of nanoparticles
V nf ¼ V np þ V bf In this study, the methodology of performing structural characterisation of nanoparticles is able to be performed by using techniques such as BET, XRD, FESEM, SEM, TEM, and HRTEM. As this research work is more concerned about the application of nanofluids, XRD, FESEM analyses were carried out for the authentication of nanoparticles [16,24,28,29]. Samples of nanoparticles (Al2O3) were used for the experimental part as the working fluid in ETSC and were introduced into the XRD machine for the analysing of these samples. For qualification of mineral phases, 1 g of these samples was analysed using an X-ray Diffraction (XRD) at a scan speed of 1°/min from 5° to 70° under 40 kV/40 mA. This figure confirms that the material used in this study was aluminium oxide and copper oxide (see Figs. 4 and 5). 5. Preparation of Al2O3 nanofluids According to the literature review, a single step method is applicable for the preparation of metal nanofluid, while it is better to use a two-step method for the preparation of nanofluids including oxide nanoparticles [30–35]. The major concerns in the two-step methods are nanofluid stabilisation and low agglomeration [36– 40]. Several methods and techniques are applied to prepare an even stable suspension, such as using ultrasonic equipment, addition of stabilisers or pH control. In this research, the two-step method will be utilised for the preparation of (Al2O3/distilled
V bf ¼
V np ¼
W bf
qbf W np
qnp
ð6:2Þ ð6:3Þ
ð6:4Þ
Quantity of base fluid (Water), V f ¼ 500 ml Density of Al2O3 particles, qf ¼ 3:970 gm/cm3 Density of water, qf ¼ 997:5 kg/m3. 7. Collector efficiency The operation of evacuated tube solar collector was influenced by the following factors [41,42]. Collector construction. Solar factors: radiation intensity; diffuse fraction; incidence angle between the sun and the collector. Ambient conditions: ambient temperature and wind speed; sky temperature. Operating conditions: fluid inlet temperature; fluid flow rate and thermal properties; collector slope and orientation.
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600 Al2 O 3 sample
Intensity counts
500 400 300 200 100 0 5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
2θ(degree) Fig. 4. XRD analyses results from the nanoparticles for Al2O3 nanofluid.
Fig. 5. FESEM image of Al2O3 nanoparticle.
(Al 2 O 3 NF)Without surfactant and sonication
(Al 2 O 3 NF) With suitable sonication and Triton X-100 surfactant
Fig. 6. Stability of Al2O3 nanofluids after 72 h with and without Triton X-100 surfactant.
Evacuated tube solar collector efficiency ðgÞ was investigated, the reading taken during mid-day (noon). As the sun was perpendicular and heat flux at maximum, the efficiency was
extremely fluctuated due to the dependency on temperature difference and precision. It can be evaluated from the following equations.
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After Affteer 1 day A daay After Affte A terr 3 days daay d ays ys After7 Affte t r7 r7 days day ays ays
Fig. 7. Stability of the Al2O3 nanofluids preparation over a period of one week.
that in water, which is similar to the results published in literature by Putra et al. [43].
The thermal efficiency of an ETSC can be measured by both Eqs. (6.5) and (6.6);
g¼
Qu Ac G
ð6:5Þ
g¼
_ p ðT out T in Þ mC Ac G
ð6:6Þ
9. Thermal storage tank temperature The temperature of the tank had been one of the important aspects in the evacuated tubes solar collector system as it was found to be correlated in terms of circulation flow rate and thermosyphon flow within the collector. In this research, in order to determine the enhancement of internal energy of the system, the temperature of the tank had been monitored by using nine (9) Ttype thermocouples placed at 3 different levels (top, middle, and bottom) inside the tank. Despite the fact that working fluid in the tubes was made up by around 15% of the system thermal mass, the temperature of the tank represented the overall system heat range because throughout the day, almost all tanks were usually completely mixed and the increment of temperature throughout the tubes was only 2–3 K for water as the working fluid, whereas approximately 5 K and 8 K for working fluids that consisted of Al2O3 nanofluids, in which, the average temperature of the tube was near similar to that of tank temperature. Besides, the attainment of useful energy was determined from the raise of the tank temperature over a day time, as well as the mass of working fluid inside the tank and the tubes. It absolutely demonstrated that the temperature of the tank had an impact on
8. Results and discussion
C/C0
The stability of Al2O3 nanofluids in pure water against deposition was determined through sedimentation experiments illustrated in Fig. 8. The stability of Al2O3, in fact, had been dependent on the amount of ionic strength because the increase in ionic strength decreased the stability of nanoparticles (NPs) considerably. Furthermore, as portrayed in Figs. 9 and 10, as the concentrations of the nanofluid were increased, the thermal capacity was reduced; thus providing a certain amount of heat to the nanofluid and water, whereby nanofluid provided more output temperature. Other than that, the use of Al2O3 and CuO nanofluids caused the heat transfer coefficient to increase with the increasing concentration. Besides, Fig. 8, illustrates that the thermal conductivity enhancement in Al2O3 nanofluids had been more significant than
1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40
Al 2 O 3 in DW 0
20
40
60
80
100
120
140
160
Time (min) Fig. 8. Sedimentation test from Al2O3 nanofluids in pure water via UV–visible spectrophotometer.
180
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4300 4200
Cp [J/kg.k]
4100
Al 2O3NP
4000 3900 3800 3700 3600 3500 3400 0.00
0.01
0.02
0.03
0.04
0.05
0.06
Nanoparticle concentration (%) Fig. 9. Sedimentation test from Al2O3 nanofluids via UV–visible spectrophotometer.
1.35
Thermal conductivity (W/m.k)
1.32 1.29 1.26 1.23 1.20 1.17 1.14 1.11 Putra, 2002 (Al 2 O 3 /NFs) Effective Medium Theory (Al 2O 3 /NFs) 40 nmAl 2 O 3 /NFs after preparation
1.08 1.05 1.02 0.99 0.00
0.01
0.02
0.03
Nanoparticale concentration
0.04
0.05
0.06
%
Fig. 10. The experimental data of thermal conductivity for Al2O3 nanofluid at different volume fractions at 300 K with a thermal conductivity measurement device.
the rate of thermosyphon flow (natural circulation flow) throughout the tubes [44]. For this kind of ETSC, a greater tank temperature could lead to higher natural circulation flow rate with similar ray radiation input as a result of two effects. First of all, thermosyphon through the tubes was influenced by the change in density of the warmed working fluid, as well as the less warm fluid that replaced it from the tank. In greater temperatures, nevertheless, the actual density gradient of the working fluid (Al2O3 nanofluid and water) could be increased. Thus, with similar heating situation, the driving force for the natural circulation could increase with the operating temperature of the ETSC. Second, the viscosity of working fluid decreased as the temperature rose; therefore, frictional resistance was reduced at higher temperatures. The temperature of the tank had been monitored by using nine T-type thermocouples placed at 3 different levels (top, middle, and bottom) inside the tank. The working fluid inside the tank had different temperatures, therefore, these nine thermocouples were employed to monitor the temperature at different places of the tank at every hour. The average temperature reading at every hour had been the result from the sum of the thermocouples data recorded divided by the number of sensors. In fact, the data presented in Fig. 11, which showed that by using a lower mass flow, the average temperature of the tank was indeed higher. Moreover, as mentioned previously, the heat exchange time between the working fluid inside the collector and the fluid inside the coil
increased, and as a result of this, the heat exchange was performed better and the circulation flow rate was not only more rapid, but the number of circulation of thermosyphon was higher as well; consequently, the average temperature of the tank was increased. Meanwhile, Fig. 11, depict that when Al2O3 nanofluid (0.06 volume) had been used, the temperature of the tank was higher than that for other working fluids, but a remarkable discovery is that by increasing the volume of fraction by twice, the size of the maximum temperature displayed a difference of 8 K. The differences between using nanofluids and pure water as a working fluid are further highlighted from Fig. 11. From the figures, it was apparent that with the use of water, compared to Al2O3 nanofluid, the temperature in the tank was lower because in the circulation flow rate, the thermal conductivity had more effect in tank temperature. The tank temperature, in addition, depended on the effectiveness of the heat exchanger and the temperature difference between the outlet and the inlet fluid inside the coil. In contrast, the highest temperature was attained by using Al2O3 nanofluid (0.06 vol) as the heat transfer fluid was 345 K, which had been more than that of water. In this thermosyphon mode, the Al2O3/ DW (0.03 vol) nanofluids slowly transferred heat to the tank and increased the fluid temperature inside the coil at a slow rate. However, at a high flow rate (60 l/h), both working fluids (Al2O3/DW and water) displayed the lowest temperatures in the thermal storage tank due to the very little contact time with the coil (heat exchanger). The histogram in Fig. 11 indicates that Al2O3/DW
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Thermal storage tank avearge tempearture [k]
980
350 Water Al2 O3 NF (0.03 vol) Al2 O3 NF (0.06 vol)
340 330 320 310 300
Mass flow rate (20 l/h)
8
9
10
11
12
13
14
15
16
Thermal storage tank avearge tempearture [k]
Time (Hour) 340 Water Al 2O3 NF (0.03 vol) Al 2O3 NF (0.06 vol)
335 330 325 320 315 310
Mass flow rate (40 l/h)
305 8
9
10
11
12
13
14
15
16
Thermal storage tank avearge tempearture [k]
Time (Hour) 330 Water Al 2 O3 NF(0.03 vol) Al 2 O3 NF(0.06 vol)
325 320 315 310
Mass flow rate (60 l/h)
305 8
9
10
11
12
13
14
15
16
Time (Hour) Fig. 11. The average temperature of thermal storage tank for different working fluids inside the ETSC with a mass flow rate at 20 l/h, 40 l/h and 60 l/h inside the coil.
nanofluids with 0.03 volume fraction increased the useful temperature of the tank by 8.2% respectively when the mass flow rate was 40 l/h.
10. Coil outlet temperature with different working fluids inside the ETSC The coil outlet temperature depended of the effectiveness of the heat exchanger (coil), the difference of temperature between inflow and outflow temperatures of the tubes in the ETSC, as well as the temperature of the tank. Hence, this section presents the experimental results of the coil outlet temperature obtained for different working fluids. Fig. 12, obviously shows that as the rate of mass flow inside the coil declined, the temperature difference rose, and the reason might be that at lower mass flow rate, more heat was absorbed by water since the coil inside the tank functioned as heat exchanger. The working fluid inside the ETSC had an effect on the temperature of the fluid inside the coil. This model of collector is called thermosyphon phenomenon (natural circulation inside the ETSC). It is a physical effect and refers to a method of passive heat
exchange based on natural convection, which circulates the fluid without the necessity of a mechanical pump and thus, the difference of temperature for water inside the coil is indeed important. Fig. 12 shows that as the rate of mass flow increased at the same volumetric concentration, the temperature difference decreased. The maximum temperature difference was found in 20 l/h volume flow rate. Nonetheless, by using 0.03 Al2O3 nanofluid, the difference in temperature did not differ much compared to a higher volume of fraction, which had been due to lower thermal conductivity. Also, these figures illustrate that the difference in temperature increased with low mass flow rates before 12 pm. However, after 1 pm, the difference in temperature was almost similar, which had been due to the higher mass flow rate that increased the heat gain in comparison to the heat gain at low mass flow rate. Fig. 13 shows that Al2O3 (0.06) nanofluid had higher temperature difference before 1 pm as compared to water, but after 1 pm, the value of water rose slightly and this might be due to the fall in temperature in the case of nanofluid, which was more rapid, as compared to water. Meanwhile, Fig. 13, present the changes that took place in volume flow rate inside the coil, as well as the temperature difference in ETSC, by using different volume fractions
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50
Temperature Difference ( oC)
50 Working fluid water
(20 l/h) (40 l/h) (60 l/h)
40
40
30
30
20
20
10
10
0 9 AM
10AM
11AM
12 PM
1PM
2PM
3PM
4PM
0 5PM
Temperature Difference ( oC)
Hour (Time) 50 40
50 (20 l/h) (40 l/h) (60 l/h)
40
30
30
20
20
10
10
0 9 AM
Working fluid (Al2O 3 /DW (0.03vol))
10AM
11AM
12 PM
1PM
2PM
3PM
4PM
0 5PM
Hour (Time) Temperature Difference ( oC)
50 40
50 (20 l/h) (40 l/h) (60 l/h)
40
30
30
20
20
10
10
0 9 AM
Working fluid (Al 2 O 3 /DW(0.06vol))
10AM
11AM
12 PM
1PM
2PM
3PM
4PM
0 5PM
Hour (Time) Fig. 12. Variation in temperature difference against time at different mass flow rates inside the coil with different working fluid inside the ETSC.
of nanofluid Al2O3. It was also observed that at different time intervals, the difference in temperature was varied and in most of the cases, the temperature difference for Al2O3 was higher than water. This was due to the higher heat absorption capacity of the nanofluids compared to water. The increase of volume flow rate inside the coil of the tank for 60 l/h where the working fluid inside the evacuated tube and the collector was pure water without nanoparticle in the first experimental test, as well as the use of nanofluid Al2O3 with u ¼ 0:03 and u ¼ 0:06 for another test, is shown in Fig. 13. This figure shows that the difference in temperature for the case of low volumetric concentration (0.03 vol) was slightly higher than that of higher volumetric concentration (0.06 vol). This was because; at higher volumetric concentration, the problem of settling down of nanoparticles had been more serious, as compared to that with low volumetric concentration.
In addition, by observing the plots displayed from Fig. 13, it is clear that the difference in temperature for the nanofluids increased at lower volume. One possible reason for the lower performance at higher volume concentration was because the nanofluids were unstable at high volume concentration; which means, the particles at high volume fraction became agglomerated, heavy, and settled down, which all together, lowered the absorption capacity of the nanofluid. Meanwhile, the second reason could be that solar radiation was absorbed in the upper layers of nanofluids at higher volume fraction due to high concentration.
11. The measuring of flow rate in evacuated tubes The measured flow rates at four different evacuated tube positions (tubes 9, 12, 15, and 18) across the array are shown in Fig. 14.
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50
Temperature Difference (K)
50 40
Pure Water (Al2 O3 /DW (0.03%vol)) (Al2 O3 /DW (0.06%vol))
40
30
30
20
20
10
10
0 9 AM
Mass flow rate (20 l/h)
10AM
11AM
12 PM
1 PM
2 PM
3 PM
4 PM
0 5 PM
Hour (Time) 50
Temperature Difference (K)
50 40
Pure Water (Al 2 O 3 /DW(0.03% vol)) (Al 2 O 3 /DW(0.06% vol))
40
30
30
20
20
10
10 M ass flow rate (40l/h)
0 9 AM
10AM
11AM
12 PM
1PM
2PM
3PM
4PM
0 5PM
Temperature Difference (K)
Hour (Time) 50
50 40
Pure Water (Al 2 O 3 /DW(0.03% vol)) (Al 2 O 3 /DW(0.03% vol))
40
30
30
20
20 10
10 M ass flow rate 60 l/h
0 9 AM
10AM
11AM
12 PM
1PM
2PM
3PM
4PM
0 5PM
Hour (Time) Fig. 13. Variation in temperature difference against time for different working fluids inside the ETSC and various concentrations of nanofluids with the volume of flow rate at 20 l/h, 40 l/h, and 60 l/h inside the coil.
The measurements revealed that the flow rate of the circulation was relatively uniform across the tube array, with the exception of a tube near the edge (tube 18). Based on the flow rate calculations in Eq. (12.1), the presumed uniform heat input to all evacuated tubes had been valid for the tubes located in the centre, but perhaps not for tubes located near the edge of the array. During the middle of the day when these measures were made, the tubes located near the edges of the collector received less solar input compared to the other tubes because the outer tubes did not receive as significantly reflected radiation from the backing reflector; consequently, the temperature that increased the circulation of water through these tubes had been found lower. As a result of the idea of uniform heat output from each tube, the lower temperature that increased in the outer tubes might incorrectly imply a higher flow rate in these tubes. Thus, in order to quantify the actual distribution of flow rate over the array, a more detailed experimental investigation had been carried out.
In fact, the difference in temperature between the hot and the cold fluids could be the reason for the natural circulation to occur through the collector via thermosyphon. Besides, the gradient of water density increased with higher temperature, and at higher temperature operation, the viscosity of water experienced a reduction. The measured amount of natural circulation flow rates (thermosyphon) through four evacuated tubes at the peak of solar noon for different tank temperatures are shown in Fig. 14. The temperature of the tank in these figures had been considered as the average temperature over the half an hour test period, as recorded by the various thermocouples. 12. Performance evaluation tests The natural circulation of the flow rate, through each evacuated tube, can be specified with the useful energy that is gathered by
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1e-2
Circulation flowrate (kg/s )
9e-3 8e-3 7e-3 6e-3 5e-3 4e-3 3e-3 2e-3 Tube18
1e-3 Tube15
0 330
328
Tube12
326
324
Average Tank Te
322
320
mperatu
re (K )
po s be Tu
Tube9
ni it io
TS nE
C
1e-2
Circulation flowrate (kg/s )
9e-3 8e-3 7e-3 6e-3 5e-3 4e-3 3e-3 2e-3 1e-3 0 340
Tube18 338
336
334
Averag
332
e Tank
Tube12 330
328
326
Tempe
324
rature
322
Tube9
Tu
pos be
Tube15 C TS nE i n i ti o
Fig. 14. Variation of natural circulation flow rates across the collector tube array determined from the measured tank energy gain and the rise in tube temperature for three average tank temperatures with, Al2O3 (u ¼ 0:03, 0.06), as the working fluid.
each tube, and the increase of temperature is specified with the working fluid during the circulation inlet as well as outlet of the tube. These relations can be described in the following equations. _ through Natural circulation flow rate by thermosyphon ðmÞ each of the closed end of the evacuated tubes with different working fluids; (water, Al2O3/distilled water) inside the collector (tank and glass evacuated tube of solar collector) can be specified from the useful energy collected by the tank and the working fluid’s temperature increase inlet and outlet of the tube Eq. (12.1). Useful energy collected by the 18 tubes is: the amount of the total energy increase of working fluid in the tank, the evacuated tubes as well as another parameter that is the heat loss from the tank over the measurement period Eq. (12.2). Average flow rate through every tube was provided [44,45].
_ ¼ m
Qu NC f ðT o T i Þ
; N ¼ 18
Q u ¼ DEðsystemÞ þ Q ðTank
Z
t2
U ðTank
t1
U ðTankÞ ¼
ð12:2Þ
lossÞ
Q u ¼ ms C f ðT 2 T 1 Þ þ
U ðTubeÞ ¼
ð12:1Þ
T a Þdt
lossÞ AðTankÞ ðT
mf C f ðT i T f Þ Aa DtðT m T atm Þav
qf V T C f ðT i T f Þ Dt
T i T atm ln T f T atm
ð12:3Þ
ð12:4Þ ! ð12:5Þ
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the peak solar radiation, the circulation flow rate through each tube was between (4.72e3) and (6.94e3) kg/s for different tank temperatures. However, while using the Al2O3 nanofluid, the circulation flow rate through each tube calculated was between (6.11e3) and (9.72e3) kg/s.
13. Circulation flow rate in ETSC The natural circulation flow rates through single-ended evacuated tubes with an aspect ratio of 40 mounted at 45° inclination over a diffuse reflector for a range of heat inputs and operating temperatures are shown from Fig. 15. In fact, this figure shows the experimental and the computed data for using Al2O3 nanofluids with different volume fractions as the working fluids inside the ETSC. In this Figure, the data points linked to solid lines displayed the numerical values for several levels of heat input: 25, 50, 75, and 100 W per tube. The experimental data obtained from a number of measurements described in this chapter are demonstrated in the form of specific points from Fig. 15, in which the coloured points that are empty refers to the five evacuated tubes located in the array, while the filled points represent the tubes located at the edge. By using Eq. (12.2), the useful energy was computed from the collector. The thermosyphon circulation flow rate throughout the tubes was calculated by using Eq. (12.1) [46]; assuming uniform heat input across the collector into the 18 evacuated tubes. These kinds of experimental data points were extracted from measurements during the 30-minute period of time around solar noon, as well as the heat input that ranged between 55 and 75 W per tube. The heat input quantity to the vacuum tube and the circulation flow implicated a clear relationship; greater heat input contributed to higher flow rate. This was noticed from both experimental measurements and CFD simulation results, in which the circulation flow rate through the tubes was affected by the tank temperature. For this type of ETSC, a higher natural circulation flow rate could generate greater tank temperature results for the same radiation input due to two reasons. First, natural circulation is driven by the difference in density of the heated fluid and the colder fluid that replaces it from the tank. At higher temperatures, the density gradient with temperature increases for the same heating condition, therefore, the driving force for the natural circulation increases with the operating temperature. Second, water viscosity decreases as the temperature increases; hence, there is less frictional resistance at higher temperature. For the range of temperature investigated in this study, the decrease in water viscosity pointed out a third factor. During
14. Efficiency of ETSC with the presence of mass flow rate inside the coil A significant factor that had an impact on the performance of the evacuated tube solar water heater had been the performance of the solar collector, which possessed the ability to transform solar radiation directly into valuable energy. The optical efficiency refers to the performance of a collector when exposed to solar irradiation normal to the collector area operated at ambient temperature. The optical efficiency of ETSC depended on the absorption and the emittance of selective surface area used for absorber, the distance of tubes, and the configuration of reflector. Whenever an ETSC functioned at a temperature higher than ambient, the actual efficiency reduced due to heat loss from the absorber tubes to ambient. Fig. 16 present the efficiency of this ETSC, in which the efficiency points were obtained from midday measurements in the experimental part of the study. The actual straight line charts depicted from (Fig. 16) were attained by plotting the efficiency points obtained from midday measurements against reduced temperature parameters, ðT T a Þ=GT , the thermal efficiency of the ETSC. The efficiency of nanofluid with low volume concentration, in comparison to higher concentration nanofluid for both nanomaterials, which had been tested in this experimental work also, is also portrayed in these figures. Besides, Table 5 shows that the greatest enhancement in performance was actually discovered for Al2O3 (0.06 vol) nanofluid operation with 57.6% rather than water. Meanwhile, the efficiency of collector with water as the working fluid inside the collector was around 32.2%, which implied that by using (Al2O3/DW) nanofluids, the efficiency increased by 25.6%. This clarifies that the improved performance of nanofluids, compared to pure water, could be due to the higher thermal conductivity of nanoparticles, which causes Brownian motion and helps to absorb more solar energy.
370
Tank temperature (K)
340 330 320
100W(1PM)
75W(12PM)
Al 2 O 3 /DW(0.06)
50W(11AM)
350
Aluminium oxide nanofluid as a working fluid inside the ETSC Computed data (Heat input per tube) 25 W(10AM)
360
[25w] [50w] [75w] [100w] Tube 1 Tube 3 Tube 6 Tube 9 Tube 12 Tube 15 Tube 18 Tube 1 Tube 3 Tube 6 Tube 9 Tube 12 Tube 15 Tube 18
Al 2 O3 /DW(0.03)
Experimental data(Al2 O 3 /DW)0.06 Q:55-75W/tube Tube at collector edge(1,18) 5 tubes in the array(3,6,9,12,15)
310 Experimental data(Al 2 O 3 /DW)0.03 Q:55-75W/tube Tube at collector edge(1,18) 5 tubes in the array(3,6,9,12,15)
300 290 2.0e-3
3.0e-3
4.0e-3
5.0e-3
6.0e-3
7.0e-3
8.0e-3
9.0e-3
1.0e-2
1.1e-2
1.2e-2
Circulation flow rate(kg/s) Fig. 15. The comparison of experimental and computed results for natural circulation flow rate through ETSC with Al2O3 nanofluids as the working fluid.
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0.7 0.6
η= -2.3937x+0.588 R?= 0.9761
Water Al2 O3 (0.03% vol)/DW Al2O3 (0.06% vol)/DW
Efficiency %
0.5 0.4
η = -1.7979x+0.4514 R?= 0.978
0.3
η = -1.444x+0.3229, R?= 0.9596
0.2 0.1 0.0 0.00
Flow rate 60 l/h
0.01
0.02
0.03
0.04
0.05
0.06
(⎯T-Ta /G ) 0.7 η = -2.0186x+0.56977 R?= 0.964
Efficiency %
0.6
Water Al 2 O3 (0.03% vol)/DW Al 2 O3 (0.06% vol)/DW
0.5 0.4 η = -1.718x+0.4477 R?= 0.9648
0.3 0.2
η = -1.3648x+0.2412, R?= 0.9275
0.1 Flow rate 40 l/h
0.0 0.00
0.01
0.02
0.03
0.04
0.05
0.06
(⎯T-Ta /G ) 0.7 Water Al 2 O 3 (0.03% vol) /DW Al 2 O 3 (0.06% vol) /DW
Efficiency %
0.6 0.5
η= -0.7404x+0.4451 R?= 0.927
0.4 η = -0.7351x+0.3652 R?= 0.9496
0.3 0.2
η = -0.7344x+0.1958 R?= 0.9515
0.1 Flow rate 20 l/h
0.0 0.00
0.01
0.02
0.03
0.04
0.05
0.06
(⎯T-Ta /G ) Fig. 16. Extrapolation of ETSC efficiency test points to obtain the optical efficiency for different flow rate inside the coil with different working fluids inside the collector.
Table 5 The linear regression of efficiency data from the experimental results. No
Working fluid
_ m
Np
Vol %
Model
R2
1. 2. 3. 4. 5. 6. 7. 8. 9.
Water Water Water (Al2O3/DW) (Al2O3/DW) (Al2O3/DW) (Al2O3/DW) (Al2O3/DW) (Al2O3/DW)
20 40 60 20 40 60 20 40 60
– – – Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3
0.00 0.00 0.00 0.03 0.03 0.03 0.06 0.06 0.06
g ¼ 0:7344x þ 0:1958 g ¼ 1:3648x þ 0:2412 g ¼ 1:4744x þ 0:3234 g ¼ 0:7351x þ 0:3652 g ¼ 1:7180x þ 0:4477 g ¼ 1:8417x þ 0:4782 g ¼ 0:7404x þ 0:4450 g ¼ 2:0186x þ 0:5697 g ¼ 2:3734x þ 0:5891
0.9515 0.9275 0.9551 0.9496 0.9648 0.9605 0.9270 0.9640 0.9793
Furthermore, Fig. 16 shows the effects of Al2O3 with distilled water nanofluids on the efficiency of ETSC experimentally while the mass flow rate inside the coil was 40 l/h. The results showed that the use of Al2O3 with distilled nanofluids, in comparison to
lower volume fraction of nanoparticle as working fluid, increased the efficiency up to 12.7% and 8.55% respectively. In addition, compared to water as the working fluid, the efficiency increased by 31.81% and 23.24% respectively. Nonetheless, the least increment
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for efficiency was noted when the mass flow rate inside the coil was 20 l/h because the effect of mass flow rate on the efficiency was higher than the effect of temperature difference. Moreover, all the above figures display the performance curves of the solar collectors under the ASHRAE Standard with Al2O3 nanofluids at concentrations (0.03 and 0.06 vol) and mass flow rates (20, 40 and 60 l/h). It was found that the collector efficiency of the nanofluids (Al2O3/DW) at 0.06 vol concentration was higher than water due to high thermal conductivity. With Al2O3 nanofluid at the concentration of 0.06 vol and the mass flow rates at (20 and 60 l/h), the thermal solar characteristics values of F R ðsaÞ; F R U L were 0.4450, 0.7404 W/m2 k, 0.588, and 2.3734 W/m2 k. 15. Conclusions In this experimental research, the effective heat transfer enhancement and the variation of mass flow rate with the effect of different working fluids in evacuated tube solar collector (ETSC) had been studied. The experimental setup contains 18 evacuated tubes connected to the horizontal tank directly and the copper coil situated inside the tank that can be used as a heat exchanger. This kind of evacuated tube solar collector operates as the passive system and the circulation fluid inside the collector acts as a thermosyphon phenomenon. Al2O3/distilled water nanofluids with different volume concentrations were employed in the collector throughout the study. The obtained results demonstrated that the addition of 0.06 vol%. Al2O3 nanoparticles produced a reasonable heat transfer enhancement and increased the thermal conductivity in comparison to water. Heat transfer enhancement increased with the increase in volume concentration of Al2O3 nanoparticles. The total average energy efficiencies of the ETSC for water without nanofluid are 13.95%, 17.51%, and 22.85%; for 0.03 vol% Al2O3 nanofluids are 24.64%, 32.72%, and 39.52%; for 0.06 vol% Al2O3 nanofluids are 30.07%, 45.13%, and 58.65% with the mass flow rates of 20, 40, and 60 l/h respectively. The efficiency of the collector is higher for Al2O3 nanofluid compared to water due to the improved thermal properties of Al2O3 nanofluids. The collector efficiency surges up to 58.65% for 0.06 vol% Al2O3 nanofluid which is higher compared to water at a flow rate of 60 l/h. The maximum outlet temperature and temperature difference of water achieved at low mass flow rate of 20 l/h for 0.06 vol% Al2O3 nanofluids. References [1] G.L. Morrison, I. Budihardjo, M. Behnia, Water-in-glass evacuated tube solar water heaters, Sol. Energy 76 (2004) 135–140. [2] S.A. Kalogirou, Solar thermal collectors and applications, Prog. Energy Combust. Sci. 30 (2004) 231–295. [3] R. Tang, Y. Yang, W. Gao, Comparative studies on thermal performance of water-in-glass evacuated tube solar water heaters with different collector tiltangles, Sol. Energy 85 (2011) 1381–1389. [4] L. Ayompe, A. Duffy, M. Mc Keever, M. Conlon, S. McCormack, Comparative field performance study of flat plate and heat pipe evacuated tube collectors (ETCs) for domestic water heating systems in a temperate climate, Energy 36 (2011) 3370–3378. [5] D. Milani, A. Abbas, Multiscale modeling and performance analysis of evacuated tube collectors for solar water heaters using diffuse flat reflector, Renewable Energy 86 (2016) 360–374. [6] E. Zambolin, D. Del Col, Experimental analysis of thermal performance of flat plate and evacuated tube solar collectors in stationary standard and daily conditions, Sol Energy 84 (2010) 1382–1396. [7] M. Khattak, A. Mukhtar, S.K. Afaq, Application of nano-fluids as coolant in heat exchangers: a review, J. Adv. Rev. Sci. Res. 22 (2016) 1–11. [8] C.S. Nor Azwadi, I. Adamu, M. Jamil, Preparation methods and thermal performance of hybrid nanofluids, J. Adv. Rev. Sci. Res. 24 (2016) 13–23. [9] C.S. Nor Azwadi, O.A. Alawi, Computational investigations on heat transfer enhancement using nanorefrigerants, J. Adv. Res. Des. 1 (2014) 35–41.
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Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
An experimental investigation on the effect of Al2O3/distilled water nanofluid on the energy efficiency of evacuated tube solar collector Javad Ghaderian, Nor Azwadi Che Sidik ⇑ Department of Thermofluid, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, UTM Skudai, Johor 81310, Malaysia
a r t i c l e
i n f o
Article history: Received 27 August 2016 Received in revised form 21 December 2016 Accepted 27 December 2016
Keywords: Solar energy Evacuated Tube Solar Collector (ETSC) All-glass passive circulation Thermosyphon Al2O3 nanofluid Thermal efficiency
a b s t r a c t The present study was designed to experimentally investigate the effect of using Al2O3/distilled water nanofluid as the working fluid, on the thermal efficiency of all-glass passive circulation evacuated tube solar collector with internal spherical coil inside the horizontal tank. The volume fraction of nanoparticles was 0.03% and 0.06% with the nanoparticles dimension 40 nm. Triton X-100 as surfactant was used in this experimental study. The performance of the ETSC using Al2O3 nanofluid and water was compared with the flow rate inside the coil was varied from 20 to 60 l/h. The present study revealed that the maximum efficiency was found to be 57.63% for 0.06 vol% of nanofluids and mass flow rate of 60 l/h. The collector efficiency shows greater enhancement with the increasing volume fractions of Al2O3 nanoparticles and flow rate. In conclusions, results suggest that Al2O3 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 2016 Elsevier Ltd. All rights reserved.
1. Introduction Solar energy is renewable and has a nonpolluting nature; hence it is commonly used in applications such as energy heating systems, chemical substance processing and electrical power generation. Evacuated Tube Solar Collectors (ETSC) are able to capture the available Solar Energy and then transfer it as thermal Energy to the water heating system [1–3]. The ETSCs are made up of multiple parallel or rows of glass tubes. Each tube incorporates a twin glass tube that is coated, which is able to absorb solar energy. Air from the space between the twin glasses is removed and forms vacuum. So, there is no air to cause convective losses or to conduct heat, and besides, convective and conductive heat losses are removed. There are two main categories of vacuum solar collector can be seen in the literature; all glass direct flow and heat pipe ETC [4–6]. Heat transfer improvement in solar collectors is one of the critical issues in energy saving and compact designs. One of the innovative techniques to increase heat transfer efficiency involves the employment of additives in base fluids [7–12]. Recently, the suspension of nanosized particles, which is an impressive substance, has been employed in conventional fluids [13–15]. Numerous scientists have currently made use of various kinds of nanofluids to examine the efficiency of different type of ETSC. Sabiha et al. [16] investigated the thermal performance enhancement of a heat pipe ⇑ Corresponding author. E-mail address: [email protected] (N.A.C. Sidik). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.12.101 0017-9310/Ó 2016 Elsevier Ltd. All rights reserved.
evacuated tube solar collector with using the SWCNT nanofluid as the working fluid. In this research, three different mass flow rate were carried out in the experimental study with having volume concentration of 0.05, 0.1 and 0.2 vol%. The results of their study indicated that the maximum efficiency obtained 93.43% for 0.2 volume fraction of SWCNTs nanofluids at a mass flow rate of 0.025 kg/ s. Other than that, Mahendran et al. [17] conducted an experiment to determine the efficiency of an ETSC using water-based Titanium Oxide. 99.5% purified titanium oxide nanoparticle of 40 wt% was used with an average particle size of 30–50 nm. For the preparation method, a two-step method was adopted by dispersing the TiO2 nanopowder into distilled water. In this study, the effect of clear sky and cloudy day against the time and the solar insolation was analysed extensively. Nevertheless, the temperature enhancement of nanofluid was about 19% higher than that of water. The maximum efficiency of the collector system using TiO2 was 0.73, as compared to water, which was (0.53). In other innovative research, Gao et al. [18] analysed a U-pipe ETSC in conjunction to thermal performance. In comparison to FPSC with U-pipe Evacuated solar collector, which is referred as U-pipe, the ETSC generally works better in cold climate and also associated with low heat loss advantage. Furthermore, the authors showed that the thermal collector efficiency did not increase certainly with the increase in length. Karami et al. [19] investigated using carbon nanotubes nanofluid (CNT) as the working fluid in direct absorption solar collector as a result of its high thermal conductivity as well as dispersion stability. Six different volume concentrations alkaline
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Nomenclature Atank Cp C p;bf C p;np C p;nf FR _ m S t1 ; t2 T T1; T2 Ti; To T in
surface area of the storage tank specific heat at constant pressure heat capacity of base fluid (water) heat capacity of nanoparticles heat capacity of nanofluid collector heat removal factor mass flow rate (natural circulation flow rate through the evacuated tube solar radiation on absorber initial and final time of measurement temperature initial and final average temperature of the system respectively during the period of measurement, averaged from nine thermocouple readings in the tank in flow and out flow temperature through the opening of the tube coil inlet temperature
functionalized carbon nanotubes (f-CNT) were considered in this study. The study has shown that improvements the thermal conductivity up to 32.1% obtained by using 150 ppm f-CNT to water. This study also found that generally effect of temperature is more than volume concentration for DASC. Park and Kim [20] conducted experiment on a solar collector with using oxidized MWCNT as the working fluid for improving the heat transfer efficiency of a heat pipe in a solar collector. This kind of nanofluids increases the operating temperature range and total heat. The thermal conductivity of the nanofluid increase 12.6% with using 0.1 vol% of oxidized MWCNT. Therefore, they recommended that the oxidized MWCNT nanofluid will certainly demonstrate to outstanding results as working fluid of a heat pipe of solar collector (see Table 1). A recent research study relating to the efficiency of an ETSC by utilising nanofluids has been carried out by Hussain et al. [21] Two different kinds of nanoparticles, silver (Ag) and also zirconium oxide (ZrO2) with 30 nm and 50 nm respectively, were spread in pure water at 1, 3, and also 5 vol%. In order to prepare the nanofluids was used two-step technique. Accordingly, the present study
T out T Ta
a b
c d
g u / bf nf np
coil outlet temperature average temperature of the tank during measurement period average ambient temperature during measurement period thermal diffusivity coefficient of volumetric expansion inclination angle delta efficiency volume fraction of nanoparticles (%) weight fraction of nanoparticles in nanofluid base-fluid nanofluids nanoparticle
attempts to examine the efficiency of an ETSC with verity of mass flow rate (30 and 90 l/h m2) as well as different volume concentration of Ag and ZrO2 nanoparticles. The finding of the present study is that, as a result of the greater thermal conductivity of Ag nanoparticles compared to ZrO2 nanoparticles therefore the efficiency of the ETSC was higher for 5 vol% Ag nanofluids compared with ZrO2 nanofluids. In the current experimental study, for the first time we proposed a system of passive circulation evacuated tube solar collector with internal spherical coil inside the horizontal tank Al2O3/ distilled water nanofluid as the working fluid. As shown in Fig. 1, the system consists of 18 evacuated tubes manufactured from Borumsilicate glass that were connected directly to the horizontal storage tanks, with 100 l as a nominal capacity, with the spiral coil inside the tank. The inner diameter tube and outer diameter tube of the evacuated glass tubes were 45 mm and 47 mm long, respectively, made of stainless steel by a polyurethane isolation layer. The evacuated tubes were connected with the storage tank by rubber sealing. The tube length was 1800 mm; the length of the storage tank was nearly 1700 mm, the thickness of the glass tubes was
Table 1 Summaries of previous studies on the performance of evacuated tube collectors based on working fluids. Author(s)
Year
Collector type
Methodology & type of study
NFs type and size of NP
Research finding
Mahendran et al. [17] Mahendran et al. [22] Lu et al. [23]
2013
HP-ETSC
Experimental
TiO2/water (20–30 nm)
The effect of volume concentration was studied using three different volumes 1–3%. Efficiency increased by 42% using 2% concentration of nanofluid
2012
HP-ETSC
Experimental
TiO2/water (30–50 nm)
2011
Experimental
Copper oxide 50 nm
Liu et al. [24]
2013
Evacuated tubular solar collectors Concentric Tube
The efficiency of collector estimated using TiO2 nanofluid of 0.3% concentration is about 73% compare to water 58% Efficiency enhanced by 16.7%. Fixed flow rate at 2.7 l per minute for both liquids The heat transfer coefficient has increased by 30% using water based CuO as substitute of water as working fluid
Experimental
Copper oxide 50 nm
Shahi et al. [25]
2010
ETC
Numerical
Akbari et al. [26]
2008
Horizontal tube
Numerical
Copper oxide 100 nm Al2O3
Open thermosyphon working fluid with using nanofluid has the higher collector efficiency than that using water. Two kinds of The solar collector integrated with thermosyphon are also compared with that integrated with common concentric tube They investigated the effect of nanofluid on natural convection in ETC. It was found that 40% increase in mass flow rate is obtained when a approaches 15–35° while enhancement is about 5% when a changes from 55° to 75° Heat transfer coefficient increased by 15% at 4 vol% Al2O3 and maximum heat transfer coefficient is achieved at angle of 45°. The effect of tube inclinations and effect of nanoparticles concentration on the thermal parameters and hydrodynamics are presented, But nanoparticles do not have effect on the hydrodynamics parameter
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41 41
1. Tube 1 2. Tube 3 3. Tube 6 4. Tube 9 5. Tube 12 6. Tube 15 7. Tube 18 8. The Bottom Rod 9. The Top Rod 10.The Middle Rod
11.Tank 12.Valve 13.Valve 14.Auxiliary Source 15.Valve 16.Coil 17.Flow meter 18.Rubber Seal 19.Inlet coil valve 20.Outlet coil valve
21.Thermocouple 22.Thermocouple 23.Thermocouple 24.Thermocouple 25.Thermocouple 26.Thermocouple 27.Thermocouple 28.Thermocouple 29.Thermocouple 30.Thermocouple
31.Thermocouple 32.Thermocouple 33.Location of temperature measurments 34.Location of temperature measurments 35.Location of temperature measurments 36.Location of temperature measurments 37.Location of temperature measurments 38.Location of temperature measurments 39.Exhaust pipe 40.Frame 41.Reflector Plate
Fig. 1. Schematic view of ETSC with experimental instrument.
1.6 mm and its internal diameter was 450 mm. The tilts of the actual tubes were 45° and 1.57 m2 was the surface area of the solar collector (Ac). The global absorptivity ðaÞ and global emissivity ðeÞ were at 0.96 and 0.06 respectively, with the selective coating for inner tube as according to the product brochure, refer to Table 2.
Table 2 Specification of evacuated tube solar collector. Specifications
Unit
Dimension
Gross area Aperture area Absorber area Length Width/width incl. Connection. Connection Max. operating pressure Temperature working Collector angle Absorber Absorption (a)/emission (e) Collector housing Collector glazing
m2 m2 m2 in mm
2.57 2.22 2.36 70.800 1560/1612
bar k o – – – –
Number of tubes Outer glass tube diameter Inner glass tube diameter Sealing material Frame materia
– in in – –
10 283–372 45 Aluminium 0.96/0.06 Aluminium Evacuated tubes (borosilicate glass) 18 evacuated tubes 2.2800 1.8500 Silicone Stainless steel
2. Material and preparation method Aluminium Oxide (Al2O3) nanoparticles were purchased from Sky Spring Nanomaterials, Inc. Houston, USA. An enhancement on ETSC was performed by implementing nanofluids (Al2O3/DW) instead of ordinary fluid water with different volume concentrations of nanoparticles aluminium oxide powders (alumina) 99% (Gamma), (0, 0.03 and 0.06) with APS = 40 nm morphology, near spherical shape. For dispersion of Al2O3, Triton X-100 was used as a natural surfactant and also distilled water was utilised in this study as a base fluid. Two- step methods were carried out to enable the reduction of Al2O3 aggregation and increase the scattering behaviour. Firstly, by applying Triton X-100 as surfactant and secondly, by using the ultrasonic vibration (Q500 ultrasonic system with standard probe model).
3. Experimental setup The experimental set up of evacuated tubes solar collector that was used in this research is displayed in Fig. 2. The dimensions and proportions of the ETSC that were used in this experiment are shown in Table 2. Physical properties of the Al2O3 nanoparticles and water used in the calculations are presented in Table 3. According to this design of the system and evacuated tube, this ETSC was working by thermosyphon and as mentioned, this
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Fig. 2. Front and Back view of ETSC under experimental work with part of the setup.
Table 3 Physical properties of nanomaterial and water. S. no
Nanoparticle & base fluid
Density [Kg m3]
Thermal conductivity [Wm1 K1]
Specific heat [J kg1 K1]
Fluid thermal diffusivity a 107 [m2 s1]
1. 3.
Al2O3 Water
3970 997.5
36 0.605
773 4178
131.7 1.47
system does not need the electrical pump, as the circulation inside the collector is a natural convection. The measurements were undertaken during sunny days between 10 am and 5 pm while the heat flux and the global solar radiation were not constant. The tank temperature was monitored in the period of the experimental work by using fifteen T-type thermocouples, positioned at different levels and multiple locations inside the tank, and the internal energy increase of the system could be calculated. Thermocouple location is shown in Fig. 3. For the measuring of the flow temperature, measurement at the inlet and outlet of the natural circulation inside the tubes was carried out for seven chosen vacuum tubes (tubes number 1, 3, 6, 9, 12, 15 and 18) which are shown in Fig. 3. In order to measure the temperature profile across the vertical plane at the opening of each tube, three T-type thermocouples were used; one in the bottom half at the centre of the tubes and two others uniformly distributed across the top half of the opening of the tubes. The outlet flow temperature from each tube was the average of the measurements from the two thermocouples at the top, and the inlet flow temperature was represented by the bottom thermocouple. In order to compute the total of useful energy per tube, this was achieved by the division of the total of useful energy from the collector in the number of tubes (see Table 4). The experimental part was conducted under Malaysian climate; latitude 3.13°N and longitude 101.68°E. The title angle of this solar collector (b = 45°) [1,25] is from horizontal, aspect ratio of 40, capacity of tank 100 l. A spiral coil was used inside the tank that
was working as a heat exchanger that transmitted the heat load of the working fluid inside the solar collector to the consuming water inside the coil. The temperature range 20–80 °C was considered for calibration of thermocouples against a platinum resistance thermometer, based on the ASHRAE Standard in experimental work. After the thermocouples were fixed inside the ETSC, the fluid working inside the collector should have a uniformed temperature before starting the experimental work. For this purpose, the separate circulation pump was used for mixing the working fluid and also the area of tubes were shaded with a cover for the prevention of solar radiation to the area of the tubes. The results of all the thermocouples’ readings have been found to be within (±0.1 K). During the peak solar radiation period, the working fluid circulating flow experienced temperature increase through an evacuated tube with the order of (2–4) K. Therefore, on each thermocouple, an error of (0.1) K in up to 10% error would be led in the designation of the natural circulation flow rate in the middle of the day (around 1 pm), and higher error will happen in the morning and in the afternoon. Errors could also result from the averaging of the two thermocouples’ readings to get the outlet flow temperature. The thermocouple readings’ data by data logger were averaged and recorded at 60 s intervals in the experiment work. The computation of energy and flow rate is dependent on the accuracy of the temperature measurements. Hence, to reduce the error due to short term fluctuations in the temperature readings, each temperature used in the energy and flow rate computation was obtained
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Fig. 3. (a) Schematic view of one of the evacuated close tubes for heat transfer analysis of the storage tank and coil, (b) seven chosen tubes in the setup with locations of thermocouples for inlet flow and outlet flow temperature measurements across the opening of the tubes.
Table 4 Volume concentration of Al2O3 nanoparticles with corresponding weight. No.
Volume concentration,u (%)
Weight of nanoparticles ðwAl2 O3 Þ (gm)
1 2
0.03 0.06
0.613 1.267
from the linear regression of the 30 s readings over a half-hour period across solar noon [27]. The change in the temperature over half-hour intervals is almost linear for most of the day during the experimental period on clear days, except in the early mornings and evenings.
water) nanofluids. In order to weigh the nanoparticles very accurately, a sensitive balance with 0.1 mg resolution is employed (see Figs. 6 and 7). 6. Weight of nanoparticles for different volume concentration The weight of the nanoparticles required for the preparation of 500 ml of Al2O3 nanofluids of a particular volume concentration, using distilled water as base fluid is calculated by using the standard expression:
/¼
V np V nf
ð6:1Þ
4. XRD and FESEM of nanoparticles
V nf ¼ V np þ V bf In this study, the methodology of performing structural characterisation of nanoparticles is able to be performed by using techniques such as BET, XRD, FESEM, SEM, TEM, and HRTEM. As this research work is more concerned about the application of nanofluids, XRD, FESEM analyses were carried out for the authentication of nanoparticles [16,24,28,29]. Samples of nanoparticles (Al2O3) were used for the experimental part as the working fluid in ETSC and were introduced into the XRD machine for the analysing of these samples. For qualification of mineral phases, 1 g of these samples was analysed using an X-ray Diffraction (XRD) at a scan speed of 1°/min from 5° to 70° under 40 kV/40 mA. This figure confirms that the material used in this study was aluminium oxide and copper oxide (see Figs. 4 and 5). 5. Preparation of Al2O3 nanofluids According to the literature review, a single step method is applicable for the preparation of metal nanofluid, while it is better to use a two-step method for the preparation of nanofluids including oxide nanoparticles [30–35]. The major concerns in the two-step methods are nanofluid stabilisation and low agglomeration [36– 40]. Several methods and techniques are applied to prepare an even stable suspension, such as using ultrasonic equipment, addition of stabilisers or pH control. In this research, the two-step method will be utilised for the preparation of (Al2O3/distilled
V bf ¼
V np ¼
W bf
qbf W np
qnp
ð6:2Þ ð6:3Þ
ð6:4Þ
Quantity of base fluid (Water), V f ¼ 500 ml Density of Al2O3 particles, qf ¼ 3:970 gm/cm3 Density of water, qf ¼ 997:5 kg/m3. 7. Collector efficiency The operation of evacuated tube solar collector was influenced by the following factors [41,42]. Collector construction. Solar factors: radiation intensity; diffuse fraction; incidence angle between the sun and the collector. Ambient conditions: ambient temperature and wind speed; sky temperature. Operating conditions: fluid inlet temperature; fluid flow rate and thermal properties; collector slope and orientation.
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600 Al2 O 3 sample
Intensity counts
500 400 300 200 100 0 5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
2θ(degree) Fig. 4. XRD analyses results from the nanoparticles for Al2O3 nanofluid.
Fig. 5. FESEM image of Al2O3 nanoparticle.
(Al 2 O 3 NF)Without surfactant and sonication
(Al 2 O 3 NF) With suitable sonication and Triton X-100 surfactant
Fig. 6. Stability of Al2O3 nanofluids after 72 h with and without Triton X-100 surfactant.
Evacuated tube solar collector efficiency ðgÞ was investigated, the reading taken during mid-day (noon). As the sun was perpendicular and heat flux at maximum, the efficiency was
extremely fluctuated due to the dependency on temperature difference and precision. It can be evaluated from the following equations.
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After Affteer 1 day A daay After Affte A terr 3 days daay d ays ys After7 Affte t r7 r7 days day ays ays
Fig. 7. Stability of the Al2O3 nanofluids preparation over a period of one week.
that in water, which is similar to the results published in literature by Putra et al. [43].
The thermal efficiency of an ETSC can be measured by both Eqs. (6.5) and (6.6);
g¼
Qu Ac G
ð6:5Þ
g¼
_ p ðT out T in Þ mC Ac G
ð6:6Þ
9. Thermal storage tank temperature The temperature of the tank had been one of the important aspects in the evacuated tubes solar collector system as it was found to be correlated in terms of circulation flow rate and thermosyphon flow within the collector. In this research, in order to determine the enhancement of internal energy of the system, the temperature of the tank had been monitored by using nine (9) Ttype thermocouples placed at 3 different levels (top, middle, and bottom) inside the tank. Despite the fact that working fluid in the tubes was made up by around 15% of the system thermal mass, the temperature of the tank represented the overall system heat range because throughout the day, almost all tanks were usually completely mixed and the increment of temperature throughout the tubes was only 2–3 K for water as the working fluid, whereas approximately 5 K and 8 K for working fluids that consisted of Al2O3 nanofluids, in which, the average temperature of the tube was near similar to that of tank temperature. Besides, the attainment of useful energy was determined from the raise of the tank temperature over a day time, as well as the mass of working fluid inside the tank and the tubes. It absolutely demonstrated that the temperature of the tank had an impact on
8. Results and discussion
C/C0
The stability of Al2O3 nanofluids in pure water against deposition was determined through sedimentation experiments illustrated in Fig. 8. The stability of Al2O3, in fact, had been dependent on the amount of ionic strength because the increase in ionic strength decreased the stability of nanoparticles (NPs) considerably. Furthermore, as portrayed in Figs. 9 and 10, as the concentrations of the nanofluid were increased, the thermal capacity was reduced; thus providing a certain amount of heat to the nanofluid and water, whereby nanofluid provided more output temperature. Other than that, the use of Al2O3 and CuO nanofluids caused the heat transfer coefficient to increase with the increasing concentration. Besides, Fig. 8, illustrates that the thermal conductivity enhancement in Al2O3 nanofluids had been more significant than
1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40
Al 2 O 3 in DW 0
20
40
60
80
100
120
140
160
Time (min) Fig. 8. Sedimentation test from Al2O3 nanofluids in pure water via UV–visible spectrophotometer.
180
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4300 4200
Cp [J/kg.k]
4100
Al 2O3NP
4000 3900 3800 3700 3600 3500 3400 0.00
0.01
0.02
0.03
0.04
0.05
0.06
Nanoparticle concentration (%) Fig. 9. Sedimentation test from Al2O3 nanofluids via UV–visible spectrophotometer.
1.35
Thermal conductivity (W/m.k)
1.32 1.29 1.26 1.23 1.20 1.17 1.14 1.11 Putra, 2002 (Al 2 O 3 /NFs) Effective Medium Theory (Al 2O 3 /NFs) 40 nmAl 2 O 3 /NFs after preparation
1.08 1.05 1.02 0.99 0.00
0.01
0.02
0.03
Nanoparticale concentration
0.04
0.05
0.06
%
Fig. 10. The experimental data of thermal conductivity for Al2O3 nanofluid at different volume fractions at 300 K with a thermal conductivity measurement device.
the rate of thermosyphon flow (natural circulation flow) throughout the tubes [44]. For this kind of ETSC, a greater tank temperature could lead to higher natural circulation flow rate with similar ray radiation input as a result of two effects. First of all, thermosyphon through the tubes was influenced by the change in density of the warmed working fluid, as well as the less warm fluid that replaced it from the tank. In greater temperatures, nevertheless, the actual density gradient of the working fluid (Al2O3 nanofluid and water) could be increased. Thus, with similar heating situation, the driving force for the natural circulation could increase with the operating temperature of the ETSC. Second, the viscosity of working fluid decreased as the temperature rose; therefore, frictional resistance was reduced at higher temperatures. The temperature of the tank had been monitored by using nine T-type thermocouples placed at 3 different levels (top, middle, and bottom) inside the tank. The working fluid inside the tank had different temperatures, therefore, these nine thermocouples were employed to monitor the temperature at different places of the tank at every hour. The average temperature reading at every hour had been the result from the sum of the thermocouples data recorded divided by the number of sensors. In fact, the data presented in Fig. 11, which showed that by using a lower mass flow, the average temperature of the tank was indeed higher. Moreover, as mentioned previously, the heat exchange time between the working fluid inside the collector and the fluid inside the coil
increased, and as a result of this, the heat exchange was performed better and the circulation flow rate was not only more rapid, but the number of circulation of thermosyphon was higher as well; consequently, the average temperature of the tank was increased. Meanwhile, Fig. 11, depict that when Al2O3 nanofluid (0.06 volume) had been used, the temperature of the tank was higher than that for other working fluids, but a remarkable discovery is that by increasing the volume of fraction by twice, the size of the maximum temperature displayed a difference of 8 K. The differences between using nanofluids and pure water as a working fluid are further highlighted from Fig. 11. From the figures, it was apparent that with the use of water, compared to Al2O3 nanofluid, the temperature in the tank was lower because in the circulation flow rate, the thermal conductivity had more effect in tank temperature. The tank temperature, in addition, depended on the effectiveness of the heat exchanger and the temperature difference between the outlet and the inlet fluid inside the coil. In contrast, the highest temperature was attained by using Al2O3 nanofluid (0.06 vol) as the heat transfer fluid was 345 K, which had been more than that of water. In this thermosyphon mode, the Al2O3/ DW (0.03 vol) nanofluids slowly transferred heat to the tank and increased the fluid temperature inside the coil at a slow rate. However, at a high flow rate (60 l/h), both working fluids (Al2O3/DW and water) displayed the lowest temperatures in the thermal storage tank due to the very little contact time with the coil (heat exchanger). The histogram in Fig. 11 indicates that Al2O3/DW
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Thermal storage tank avearge tempearture [k]
980
350 Water Al2 O3 NF (0.03 vol) Al2 O3 NF (0.06 vol)
340 330 320 310 300
Mass flow rate (20 l/h)
8
9
10
11
12
13
14
15
16
Thermal storage tank avearge tempearture [k]
Time (Hour) 340 Water Al 2O3 NF (0.03 vol) Al 2O3 NF (0.06 vol)
335 330 325 320 315 310
Mass flow rate (40 l/h)
305 8
9
10
11
12
13
14
15
16
Thermal storage tank avearge tempearture [k]
Time (Hour) 330 Water Al 2 O3 NF(0.03 vol) Al 2 O3 NF(0.06 vol)
325 320 315 310
Mass flow rate (60 l/h)
305 8
9
10
11
12
13
14
15
16
Time (Hour) Fig. 11. The average temperature of thermal storage tank for different working fluids inside the ETSC with a mass flow rate at 20 l/h, 40 l/h and 60 l/h inside the coil.
nanofluids with 0.03 volume fraction increased the useful temperature of the tank by 8.2% respectively when the mass flow rate was 40 l/h.
10. Coil outlet temperature with different working fluids inside the ETSC The coil outlet temperature depended of the effectiveness of the heat exchanger (coil), the difference of temperature between inflow and outflow temperatures of the tubes in the ETSC, as well as the temperature of the tank. Hence, this section presents the experimental results of the coil outlet temperature obtained for different working fluids. Fig. 12, obviously shows that as the rate of mass flow inside the coil declined, the temperature difference rose, and the reason might be that at lower mass flow rate, more heat was absorbed by water since the coil inside the tank functioned as heat exchanger. The working fluid inside the ETSC had an effect on the temperature of the fluid inside the coil. This model of collector is called thermosyphon phenomenon (natural circulation inside the ETSC). It is a physical effect and refers to a method of passive heat
exchange based on natural convection, which circulates the fluid without the necessity of a mechanical pump and thus, the difference of temperature for water inside the coil is indeed important. Fig. 12 shows that as the rate of mass flow increased at the same volumetric concentration, the temperature difference decreased. The maximum temperature difference was found in 20 l/h volume flow rate. Nonetheless, by using 0.03 Al2O3 nanofluid, the difference in temperature did not differ much compared to a higher volume of fraction, which had been due to lower thermal conductivity. Also, these figures illustrate that the difference in temperature increased with low mass flow rates before 12 pm. However, after 1 pm, the difference in temperature was almost similar, which had been due to the higher mass flow rate that increased the heat gain in comparison to the heat gain at low mass flow rate. Fig. 13 shows that Al2O3 (0.06) nanofluid had higher temperature difference before 1 pm as compared to water, but after 1 pm, the value of water rose slightly and this might be due to the fall in temperature in the case of nanofluid, which was more rapid, as compared to water. Meanwhile, Fig. 13, present the changes that took place in volume flow rate inside the coil, as well as the temperature difference in ETSC, by using different volume fractions
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50
Temperature Difference ( oC)
50 Working fluid water
(20 l/h) (40 l/h) (60 l/h)
40
40
30
30
20
20
10
10
0 9 AM
10AM
11AM
12 PM
1PM
2PM
3PM
4PM
0 5PM
Temperature Difference ( oC)
Hour (Time) 50 40
50 (20 l/h) (40 l/h) (60 l/h)
40
30
30
20
20
10
10
0 9 AM
Working fluid (Al2O 3 /DW (0.03vol))
10AM
11AM
12 PM
1PM
2PM
3PM
4PM
0 5PM
Hour (Time) Temperature Difference ( oC)
50 40
50 (20 l/h) (40 l/h) (60 l/h)
40
30
30
20
20
10
10
0 9 AM
Working fluid (Al 2 O 3 /DW(0.06vol))
10AM
11AM
12 PM
1PM
2PM
3PM
4PM
0 5PM
Hour (Time) Fig. 12. Variation in temperature difference against time at different mass flow rates inside the coil with different working fluid inside the ETSC.
of nanofluid Al2O3. It was also observed that at different time intervals, the difference in temperature was varied and in most of the cases, the temperature difference for Al2O3 was higher than water. This was due to the higher heat absorption capacity of the nanofluids compared to water. The increase of volume flow rate inside the coil of the tank for 60 l/h where the working fluid inside the evacuated tube and the collector was pure water without nanoparticle in the first experimental test, as well as the use of nanofluid Al2O3 with u ¼ 0:03 and u ¼ 0:06 for another test, is shown in Fig. 13. This figure shows that the difference in temperature for the case of low volumetric concentration (0.03 vol) was slightly higher than that of higher volumetric concentration (0.06 vol). This was because; at higher volumetric concentration, the problem of settling down of nanoparticles had been more serious, as compared to that with low volumetric concentration.
In addition, by observing the plots displayed from Fig. 13, it is clear that the difference in temperature for the nanofluids increased at lower volume. One possible reason for the lower performance at higher volume concentration was because the nanofluids were unstable at high volume concentration; which means, the particles at high volume fraction became agglomerated, heavy, and settled down, which all together, lowered the absorption capacity of the nanofluid. Meanwhile, the second reason could be that solar radiation was absorbed in the upper layers of nanofluids at higher volume fraction due to high concentration.
11. The measuring of flow rate in evacuated tubes The measured flow rates at four different evacuated tube positions (tubes 9, 12, 15, and 18) across the array are shown in Fig. 14.
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50
Temperature Difference (K)
50 40
Pure Water (Al2 O3 /DW (0.03%vol)) (Al2 O3 /DW (0.06%vol))
40
30
30
20
20
10
10
0 9 AM
Mass flow rate (20 l/h)
10AM
11AM
12 PM
1 PM
2 PM
3 PM
4 PM
0 5 PM
Hour (Time) 50
Temperature Difference (K)
50 40
Pure Water (Al 2 O 3 /DW(0.03% vol)) (Al 2 O 3 /DW(0.06% vol))
40
30
30
20
20
10
10 M ass flow rate (40l/h)
0 9 AM
10AM
11AM
12 PM
1PM
2PM
3PM
4PM
0 5PM
Temperature Difference (K)
Hour (Time) 50
50 40
Pure Water (Al 2 O 3 /DW(0.03% vol)) (Al 2 O 3 /DW(0.03% vol))
40
30
30
20
20 10
10 M ass flow rate 60 l/h
0 9 AM
10AM
11AM
12 PM
1PM
2PM
3PM
4PM
0 5PM
Hour (Time) Fig. 13. Variation in temperature difference against time for different working fluids inside the ETSC and various concentrations of nanofluids with the volume of flow rate at 20 l/h, 40 l/h, and 60 l/h inside the coil.
The measurements revealed that the flow rate of the circulation was relatively uniform across the tube array, with the exception of a tube near the edge (tube 18). Based on the flow rate calculations in Eq. (12.1), the presumed uniform heat input to all evacuated tubes had been valid for the tubes located in the centre, but perhaps not for tubes located near the edge of the array. During the middle of the day when these measures were made, the tubes located near the edges of the collector received less solar input compared to the other tubes because the outer tubes did not receive as significantly reflected radiation from the backing reflector; consequently, the temperature that increased the circulation of water through these tubes had been found lower. As a result of the idea of uniform heat output from each tube, the lower temperature that increased in the outer tubes might incorrectly imply a higher flow rate in these tubes. Thus, in order to quantify the actual distribution of flow rate over the array, a more detailed experimental investigation had been carried out.
In fact, the difference in temperature between the hot and the cold fluids could be the reason for the natural circulation to occur through the collector via thermosyphon. Besides, the gradient of water density increased with higher temperature, and at higher temperature operation, the viscosity of water experienced a reduction. The measured amount of natural circulation flow rates (thermosyphon) through four evacuated tubes at the peak of solar noon for different tank temperatures are shown in Fig. 14. The temperature of the tank in these figures had been considered as the average temperature over the half an hour test period, as recorded by the various thermocouples. 12. Performance evaluation tests The natural circulation of the flow rate, through each evacuated tube, can be specified with the useful energy that is gathered by
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1e-2
Circulation flowrate (kg/s )
9e-3 8e-3 7e-3 6e-3 5e-3 4e-3 3e-3 2e-3 Tube18
1e-3 Tube15
0 330
328
Tube12
326
324
Average Tank Te
322
320
mperatu
re (K )
po s be Tu
Tube9
ni it io
TS nE
C
1e-2
Circulation flowrate (kg/s )
9e-3 8e-3 7e-3 6e-3 5e-3 4e-3 3e-3 2e-3 1e-3 0 340
Tube18 338
336
334
Averag
332
e Tank
Tube12 330
328
326
Tempe
324
rature
322
Tube9
Tu
pos be
Tube15 C TS nE i n i ti o
Fig. 14. Variation of natural circulation flow rates across the collector tube array determined from the measured tank energy gain and the rise in tube temperature for three average tank temperatures with, Al2O3 (u ¼ 0:03, 0.06), as the working fluid.
each tube, and the increase of temperature is specified with the working fluid during the circulation inlet as well as outlet of the tube. These relations can be described in the following equations. _ through Natural circulation flow rate by thermosyphon ðmÞ each of the closed end of the evacuated tubes with different working fluids; (water, Al2O3/distilled water) inside the collector (tank and glass evacuated tube of solar collector) can be specified from the useful energy collected by the tank and the working fluid’s temperature increase inlet and outlet of the tube Eq. (12.1). Useful energy collected by the 18 tubes is: the amount of the total energy increase of working fluid in the tank, the evacuated tubes as well as another parameter that is the heat loss from the tank over the measurement period Eq. (12.2). Average flow rate through every tube was provided [44,45].
_ ¼ m
Qu NC f ðT o T i Þ
; N ¼ 18
Q u ¼ DEðsystemÞ þ Q ðTank
Z
t2
U ðTank
t1
U ðTankÞ ¼
ð12:2Þ
lossÞ
Q u ¼ ms C f ðT 2 T 1 Þ þ
U ðTubeÞ ¼
ð12:1Þ
T a Þdt
lossÞ AðTankÞ ðT
mf C f ðT i T f Þ Aa DtðT m T atm Þav
qf V T C f ðT i T f Þ Dt
T i T atm ln T f T atm
ð12:3Þ
ð12:4Þ ! ð12:5Þ
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the peak solar radiation, the circulation flow rate through each tube was between (4.72e3) and (6.94e3) kg/s for different tank temperatures. However, while using the Al2O3 nanofluid, the circulation flow rate through each tube calculated was between (6.11e3) and (9.72e3) kg/s.
13. Circulation flow rate in ETSC The natural circulation flow rates through single-ended evacuated tubes with an aspect ratio of 40 mounted at 45° inclination over a diffuse reflector for a range of heat inputs and operating temperatures are shown from Fig. 15. In fact, this figure shows the experimental and the computed data for using Al2O3 nanofluids with different volume fractions as the working fluids inside the ETSC. In this Figure, the data points linked to solid lines displayed the numerical values for several levels of heat input: 25, 50, 75, and 100 W per tube. The experimental data obtained from a number of measurements described in this chapter are demonstrated in the form of specific points from Fig. 15, in which the coloured points that are empty refers to the five evacuated tubes located in the array, while the filled points represent the tubes located at the edge. By using Eq. (12.2), the useful energy was computed from the collector. The thermosyphon circulation flow rate throughout the tubes was calculated by using Eq. (12.1) [46]; assuming uniform heat input across the collector into the 18 evacuated tubes. These kinds of experimental data points were extracted from measurements during the 30-minute period of time around solar noon, as well as the heat input that ranged between 55 and 75 W per tube. The heat input quantity to the vacuum tube and the circulation flow implicated a clear relationship; greater heat input contributed to higher flow rate. This was noticed from both experimental measurements and CFD simulation results, in which the circulation flow rate through the tubes was affected by the tank temperature. For this type of ETSC, a higher natural circulation flow rate could generate greater tank temperature results for the same radiation input due to two reasons. First, natural circulation is driven by the difference in density of the heated fluid and the colder fluid that replaces it from the tank. At higher temperatures, the density gradient with temperature increases for the same heating condition, therefore, the driving force for the natural circulation increases with the operating temperature. Second, water viscosity decreases as the temperature increases; hence, there is less frictional resistance at higher temperature. For the range of temperature investigated in this study, the decrease in water viscosity pointed out a third factor. During
14. Efficiency of ETSC with the presence of mass flow rate inside the coil A significant factor that had an impact on the performance of the evacuated tube solar water heater had been the performance of the solar collector, which possessed the ability to transform solar radiation directly into valuable energy. The optical efficiency refers to the performance of a collector when exposed to solar irradiation normal to the collector area operated at ambient temperature. The optical efficiency of ETSC depended on the absorption and the emittance of selective surface area used for absorber, the distance of tubes, and the configuration of reflector. Whenever an ETSC functioned at a temperature higher than ambient, the actual efficiency reduced due to heat loss from the absorber tubes to ambient. Fig. 16 present the efficiency of this ETSC, in which the efficiency points were obtained from midday measurements in the experimental part of the study. The actual straight line charts depicted from (Fig. 16) were attained by plotting the efficiency points obtained from midday measurements against reduced temperature parameters, ðT T a Þ=GT , the thermal efficiency of the ETSC. The efficiency of nanofluid with low volume concentration, in comparison to higher concentration nanofluid for both nanomaterials, which had been tested in this experimental work also, is also portrayed in these figures. Besides, Table 5 shows that the greatest enhancement in performance was actually discovered for Al2O3 (0.06 vol) nanofluid operation with 57.6% rather than water. Meanwhile, the efficiency of collector with water as the working fluid inside the collector was around 32.2%, which implied that by using (Al2O3/DW) nanofluids, the efficiency increased by 25.6%. This clarifies that the improved performance of nanofluids, compared to pure water, could be due to the higher thermal conductivity of nanoparticles, which causes Brownian motion and helps to absorb more solar energy.
370
Tank temperature (K)
340 330 320
100W(1PM)
75W(12PM)
Al 2 O 3 /DW(0.06)
50W(11AM)
350
Aluminium oxide nanofluid as a working fluid inside the ETSC Computed data (Heat input per tube) 25 W(10AM)
360
[25w] [50w] [75w] [100w] Tube 1 Tube 3 Tube 6 Tube 9 Tube 12 Tube 15 Tube 18 Tube 1 Tube 3 Tube 6 Tube 9 Tube 12 Tube 15 Tube 18
Al 2 O3 /DW(0.03)
Experimental data(Al2 O 3 /DW)0.06 Q:55-75W/tube Tube at collector edge(1,18) 5 tubes in the array(3,6,9,12,15)
310 Experimental data(Al 2 O 3 /DW)0.03 Q:55-75W/tube Tube at collector edge(1,18) 5 tubes in the array(3,6,9,12,15)
300 290 2.0e-3
3.0e-3
4.0e-3
5.0e-3
6.0e-3
7.0e-3
8.0e-3
9.0e-3
1.0e-2
1.1e-2
1.2e-2
Circulation flow rate(kg/s) Fig. 15. The comparison of experimental and computed results for natural circulation flow rate through ETSC with Al2O3 nanofluids as the working fluid.
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0.7 0.6
η= -2.3937x+0.588 R?= 0.9761
Water Al2 O3 (0.03% vol)/DW Al2O3 (0.06% vol)/DW
Efficiency %
0.5 0.4
η = -1.7979x+0.4514 R?= 0.978
0.3
η = -1.444x+0.3229, R?= 0.9596
0.2 0.1 0.0 0.00
Flow rate 60 l/h
0.01
0.02
0.03
0.04
0.05
0.06
(⎯T-Ta /G ) 0.7 η = -2.0186x+0.56977 R?= 0.964
Efficiency %
0.6
Water Al 2 O3 (0.03% vol)/DW Al 2 O3 (0.06% vol)/DW
0.5 0.4 η = -1.718x+0.4477 R?= 0.9648
0.3 0.2
η = -1.3648x+0.2412, R?= 0.9275
0.1 Flow rate 40 l/h
0.0 0.00
0.01
0.02
0.03
0.04
0.05
0.06
(⎯T-Ta /G ) 0.7 Water Al 2 O 3 (0.03% vol) /DW Al 2 O 3 (0.06% vol) /DW
Efficiency %
0.6 0.5
η= -0.7404x+0.4451 R?= 0.927
0.4 η = -0.7351x+0.3652 R?= 0.9496
0.3 0.2
η = -0.7344x+0.1958 R?= 0.9515
0.1 Flow rate 20 l/h
0.0 0.00
0.01
0.02
0.03
0.04
0.05
0.06
(⎯T-Ta /G ) Fig. 16. Extrapolation of ETSC efficiency test points to obtain the optical efficiency for different flow rate inside the coil with different working fluids inside the collector.
Table 5 The linear regression of efficiency data from the experimental results. No
Working fluid
_ m
Np
Vol %
Model
R2
1. 2. 3. 4. 5. 6. 7. 8. 9.
Water Water Water (Al2O3/DW) (Al2O3/DW) (Al2O3/DW) (Al2O3/DW) (Al2O3/DW) (Al2O3/DW)
20 40 60 20 40 60 20 40 60
– – – Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3
0.00 0.00 0.00 0.03 0.03 0.03 0.06 0.06 0.06
g ¼ 0:7344x þ 0:1958 g ¼ 1:3648x þ 0:2412 g ¼ 1:4744x þ 0:3234 g ¼ 0:7351x þ 0:3652 g ¼ 1:7180x þ 0:4477 g ¼ 1:8417x þ 0:4782 g ¼ 0:7404x þ 0:4450 g ¼ 2:0186x þ 0:5697 g ¼ 2:3734x þ 0:5891
0.9515 0.9275 0.9551 0.9496 0.9648 0.9605 0.9270 0.9640 0.9793
Furthermore, Fig. 16 shows the effects of Al2O3 with distilled water nanofluids on the efficiency of ETSC experimentally while the mass flow rate inside the coil was 40 l/h. The results showed that the use of Al2O3 with distilled nanofluids, in comparison to
lower volume fraction of nanoparticle as working fluid, increased the efficiency up to 12.7% and 8.55% respectively. In addition, compared to water as the working fluid, the efficiency increased by 31.81% and 23.24% respectively. Nonetheless, the least increment
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for efficiency was noted when the mass flow rate inside the coil was 20 l/h because the effect of mass flow rate on the efficiency was higher than the effect of temperature difference. Moreover, all the above figures display the performance curves of the solar collectors under the ASHRAE Standard with Al2O3 nanofluids at concentrations (0.03 and 0.06 vol) and mass flow rates (20, 40 and 60 l/h). It was found that the collector efficiency of the nanofluids (Al2O3/DW) at 0.06 vol concentration was higher than water due to high thermal conductivity. With Al2O3 nanofluid at the concentration of 0.06 vol and the mass flow rates at (20 and 60 l/h), the thermal solar characteristics values of F R ðsaÞ; F R U L were 0.4450, 0.7404 W/m2 k, 0.588, and 2.3734 W/m2 k. 15. Conclusions In this experimental research, the effective heat transfer enhancement and the variation of mass flow rate with the effect of different working fluids in evacuated tube solar collector (ETSC) had been studied. The experimental setup contains 18 evacuated tubes connected to the horizontal tank directly and the copper coil situated inside the tank that can be used as a heat exchanger. This kind of evacuated tube solar collector operates as the passive system and the circulation fluid inside the collector acts as a thermosyphon phenomenon. Al2O3/distilled water nanofluids with different volume concentrations were employed in the collector throughout the study. The obtained results demonstrated that the addition of 0.06 vol%. Al2O3 nanoparticles produced a reasonable heat transfer enhancement and increased the thermal conductivity in comparison to water. Heat transfer enhancement increased with the increase in volume concentration of Al2O3 nanoparticles. The total average energy efficiencies of the ETSC for water without nanofluid are 13.95%, 17.51%, and 22.85%; for 0.03 vol% Al2O3 nanofluids are 24.64%, 32.72%, and 39.52%; for 0.06 vol% Al2O3 nanofluids are 30.07%, 45.13%, and 58.65% with the mass flow rates of 20, 40, and 60 l/h respectively. The efficiency of the collector is higher for Al2O3 nanofluid compared to water due to the improved thermal properties of Al2O3 nanofluids. The collector efficiency surges up to 58.65% for 0.06 vol% Al2O3 nanofluid which is higher compared to water at a flow rate of 60 l/h. The maximum outlet temperature and temperature difference of water achieved at low mass flow rate of 20 l/h for 0.06 vol% Al2O3 nanofluids. References [1] G.L. Morrison, I. Budihardjo, M. Behnia, Water-in-glass evacuated tube solar water heaters, Sol. Energy 76 (2004) 135–140. [2] S.A. Kalogirou, Solar thermal collectors and applications, Prog. Energy Combust. Sci. 30 (2004) 231–295. [3] R. Tang, Y. Yang, W. Gao, Comparative studies on thermal performance of water-in-glass evacuated tube solar water heaters with different collector tiltangles, Sol. Energy 85 (2011) 1381–1389. [4] L. Ayompe, A. Duffy, M. Mc Keever, M. Conlon, S. McCormack, Comparative field performance study of flat plate and heat pipe evacuated tube collectors (ETCs) for domestic water heating systems in a temperate climate, Energy 36 (2011) 3370–3378. [5] D. Milani, A. Abbas, Multiscale modeling and performance analysis of evacuated tube collectors for solar water heaters using diffuse flat reflector, Renewable Energy 86 (2016) 360–374. [6] E. Zambolin, D. Del Col, Experimental analysis of thermal performance of flat plate and evacuated tube solar collectors in stationary standard and daily conditions, Sol Energy 84 (2010) 1382–1396. [7] M. Khattak, A. Mukhtar, S.K. Afaq, Application of nano-fluids as coolant in heat exchangers: a review, J. Adv. Rev. Sci. Res. 22 (2016) 1–11. [8] C.S. Nor Azwadi, I. Adamu, M. Jamil, Preparation methods and thermal performance of hybrid nanofluids, J. Adv. Rev. Sci. Res. 24 (2016) 13–23. [9] C.S. Nor Azwadi, O.A. Alawi, Computational investigations on heat transfer enhancement using nanorefrigerants, J. Adv. Res. Des. 1 (2014) 35–41.
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