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water nanofluid in a flat plate solar water heater under natural and forced circulations
Energy Conversion and Management 95 (2015) 160–169
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Performance of copper oxide/water nanofluid in a flat plate solar water heater under natural and forced circulations Jee Joe Michael ⇑, S. Iniyan Institute for Energy Studies, Department of Mechanical Engineering, Anna University, Chennai 600 025, India
a r t i c l e
i n f o
Article history: Received 26 August 2014 Accepted 6 February 2015
Keywords: Flat plate Solar water heater Thermosyphon Nanofluid CuO
a b s t r a c t Flat plate solar water heater is widely used for heating of water in low-temperature residential applications. In this paper, Copper Oxide/water (CuO/H2O) nanofluid is prepared from Copper Acetate and its thermal performance was investigated experimentally on a 100 Liters per Day (LPD) thermosyphon based indirect-type flat plate solar water heater. The volumetric fraction of CuO/water nanofluid chosen was 0.05%. Significant improvement in performance was observed in thermosyphon circulation compared to forced circulation, for the low volumetric fraction considered. The CuO/water nanofluid was prepared with the inclusion of surfactant Sodium Dodecyl Benzene Sulfonate (SDBS), as it provided the best CuO nanoparticle dispersion stability compared to pure water suspension and Triton X-100 surfactant suspensions. Also, the thermophysical properties of the synthesized nanoparticle and prepared nanofluid were compared theoretically and experimentally. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Solar water heater is being used worldwide for low temperature applications mostly in the domestic sector for washing clothes and bathing purposes. Thermosyphon flat plate solar water heater is a solar passive system, which can produce hot water in the temperature range of 60–90 °C. Closed loop or heat-exchanger type or Indirect type solar water heater are used in which a primary fluid namely pure water or glycol–water mixture is added, to prevent the formation of scaling on the inner surface of the copper tubes due to passage of high saline water and to prevent damage to tubes due to water freezing in cold climates. Due to low thermal conductivity of water and the heat exchanger effectiveness, the temperature attained by the secondary fluid is reduced. In order to increase the outlet useful temperature and thermal efficiency, nanoparticles having high thermal properties are mixed with the primary fluid to form nanofluids, thereby increasing the effective thermal conductivity of the primary solution. The effect of nanofluids in several industrial and residential applications was experimentally and theoretically analyzed by several researchers all over the world. The thermal performance using nanofluids depends on several thermophysical properties of nanoparticle such as particle diameter, shape and the pH, viscosity, thermal conductivity, volume fraction, specific heat of nanofluid. ⇑ Corresponding author. Tel.: +91 9444338954. E-mail address: [email protected] (J.J. Michael). http://dx.doi.org/10.1016/j.enconman.2015.02.017 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
Different theories were established to understand the behavior of nanoparticle under temperature, pH, sonication etc. Theoretical analyses on the heat transfer coefficient based on the effect of particle size, extinction coefficient [1], Brownian motion and Thermophoresis were developed [2]. Consequently, a method to measure the thermal diffusivity and thermal conductivity of nanofluids based on the temperature oscillation technique was also developed [3]. The heat transfer coefficient, outlet temperature and entropy generation among four different nanofluids such as Cu/water, Al2O3/water, TiO2/water and SiO2/water provided interesting results about the effect of Nusselt number, Bejan number, density, heat capacity and efficiency on the performance of a solar collector [4]. Similarly, a theoretical study on the performance of Al2O3/water nanofluid in a flat plate solar water heater was analyzed for different particle sizes and volume fractions [5]; also the effect of Prandtl number on flow and heat transfer was analyzed using FEM method [6]. Experimentally, 15 nm particle size, 0.2% weight fraction Al2O3/water nanofluid with Triton X100 surfactant produced a 28.3% [7] and 30% [8] enhancement in thermal efficiency in a flat plate solar water heater. Different thermophysical properties of Al2O3 [9] and TiO2 nanofluid were measured for various temperatures and volume fractions and concluded that low volume fraction nanofluid performed better [10]. The thermal performance of a natural circulation system using Al2O3/water nanofluid was tested for 0.5% and 3% volume fractions [11], and a similar enhanced thermal performance up to 22% and 62% were obtained in the evaporator and condenser sections
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respectively [12]. The photoconductive and photothermal CuO nanoparticle [13] of 4% concentration, when mixed with water has impoved the thermal conductivity by 20% [6]. Similarly, a 40 nm particle size, 0.4% volume fraction CuO/water nanofluid found a 21.8% collector efficiency improvement between 1 and 3 kg/min flow rate [14]. Also, a 25.6% improvement was observed for 0.0083 kg/s mass flow rate and 0.1% weight fraction [15]. A compound parabolic evacuated tube air collector using CuO/water nanofluid in thermosyphon mode provided outlet air temperatures above 170 °C [16]. The effect of Cu/glycol nanofluid was tested for various volume fractions and Reynolds number experimentally [17] and analyzed theoretically using FEM analysis [18]. Also, a 23.83% thermal enhancement was observed using Cu/water nanofluid at 0.1% weight fraction and 140 L/h flow rate [19]. Enhanced thermal conductivity up to 32% was obtained in a direct absorber solar collector by adding 150 ppm carbon nanotubes in water [20]. SWCNT/water nanofluid produced a 15.33% increase in heat transfer coefficient with a 4.34% reduction in entropy generation [21]. MWCNT nanofluid of 0.2% weight fraction produced improved performance using Triton X-100 as the surfactant [22]. Another surfactant namely Sodium Dodecyl Sulfate (SDS) also performed as a suitable surfactant for CNT based nanofluids [23]. It was observed that decreasing the pH with respect to isoelectric point helped in producing higher efficiency [24]. An optimum volume fraction and tilt angle for enhanced heat transfer rate using CNT/water nanofluid was analyzed for a solar collector using FEM analysis [25]. Improved electrical and thermal performance of a solar photovoltaic thermal (PV/T) system was observed by the use of silica/water nanofluid [26]. TiO2/water nanofluid with twisted tape inserts in a solar collector enhanced the thermal performance up to 1.59 times at 0.21% volume fraction [27]. The efficiency, size, cost, savings, payback period and environmental impact of a solar collector were estimated for different nanofluids [28], also it was calculated that nanofluid reduces the embodied energy and carbon emission by approximately 9% and 3% respectively [29]. A review on the performance of solar collector using nanofluids [30], nanofluid applications in different types of solar collectors, photovoltaic systems, solar thermoelectrics, thermal energy storage systems [31], solar water heaters, solar stills, solar PV/T systems, solar ponds [32], industrial and transport applications [33] were presented. As per the literature survey, a fascinating amount of research papers are available regarding solar water heaters. Most of the research papers are devoted to thermosyphon or forced circulations. However, as per the authors knowledge, no journal paper was available focusing on the performance comparison of thermosyphon and forced circulation modes in a single system. In this paper, a flat-plate solar water heating system was tested in natural and forced circulations. However, the quantity of research involved in the application of nanofluids in solar water heaters is still in the nascent stage. Many researchers have experimented solar water heater using nanofluids based on Al2O3 and CNT nanoparticles for their high thermal conductivity and low density. In this paper, the performance of solar water heater using the common heat transfer fluid, water was compared to a 0.05% volume fraction CuO/water nanofluid. The CuO/water nanofluid was prepared inhouse from commercially available raw materials and their nanocharacterization tests were also presented. 2. Methodology The experiment was conducted by synthesizing the CuO nanoparticle, preparing the CuO/water nanofluid and comparing the performance of the prepared CuO/water nanofluid with water in a commercially available 100 LPD thermosyphon indirect type flat plate solar water heater.
161
2.1. Synthesis of CuO nanoparticles The volume of the primary working fluid in the 100 LPD indirect type flat plate solar water heater was measured as 8 L. The quantity of nanoparticles required to prepare a 0.05% volume concentration of CuO–water nanofluid was calculated to be 25.3 g. Due to the high cost of commercially available nanoparticles, it was synthesised locally from Copper Acetate by a simple method called Aqueous Precipitation method. Analytical grade Copper Acetate Monohydrate (Cu(CH3COO)2H2O) as the precursor and glacial Acetic acid (CH3COOH) to prevent hydrolysis, are taken in a flatbottomed glass beaker, and heated with constant stirring in a magnetic stirrer. Sodium Hydroxide (NaOH) pellets as stabilizing agents were added slowly till the pH of the solution reaches between 6 and 7, and the color of the solution slowly turns from blue to black as shown in Fig. 1. The solution is removed from the magnetic stirrer and cooled to room temperature. The black precipitate formed is centrifuged, and then washed with distilled water three times to remove the impure ions. It is then covered with aluminum foil and placed in a hot air oven to completely dry the contents. The dried precipitate is finely powdered, using Agate mortar to produce CuO solid particles in nano dimensions. 2.2. Preparing the nanofluid The high density (6310 kg/m3) of the CuO nanoparticles compared to the basefluid, water (1000 kg/m3), causes the immediate settlement of the CuO nanoparticles at the bottom of the beaker. Hence, a compatible amphiphilic surfactant, namely, Sodium Dodecyl Benzene Sulfonate (SDBS) of 10% weight of the amount of nanoparticles to be added, was first dispersed completely in double distilled water using a magnetic stirrer, and then CuO nanoparticles of the required quantity are slowly added with constant stirring for 30 min. The solution is further sonicated using an ultrasonicator for 60 min to break the agglomerated particles and facilitate a homogeneous mixture of the CuO nanoparticles and water, called CuO/water nanofluid. The CuO nanoparticles’ stability was checked with SDBS surfactant, with Triton X-100 surfactant and without any surfactant. In Fig. 2, the dispersibility of nanoparticles with the SDBS surfactant in water, compared to without-surfactant water after 24 h, clearly shows the stability of the nanoparticles using the surfactant. Similarly, the CuO/water nanofluid showed better stability with the SDBS surfactant, compared to the Triton X-100 surfactant after 3 days. Therefore, it was concluded that the SDBS surfactant performed better compared to the Triton X-100 surfactant for CuO/ water nanofluid. 2.3. Description of the system A 100 LPD thermosyphon based ladder type heat exchanger type solar water heater was used in the study. The detailed specification of the solar water heater is listed in Table 1. The flat plate collector was tilted at an angle of 13° corresponding to the latitude of the location and facing south, since Chennai is located in the Northern hemisphere. The temperature of the different locations was measured, using PT-100 RTD sensors. The solar radiation was measured on the plane of the solar collector using a pyranometer (Hukseflux LP02); the ambient temperature and the wind speed were measured using a weather station (WatchDog 2000). All the sensors were connected to a data acquisition system (Agilent 34970A), and recorded every 5 min continuously and plotted in graphs. A booster pump and a rotameter were used during forced circulation testing. The complete experimental setup and the schematic diagram of the solar water heating system are shown in Fig. 3.
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Fig. 1. (a) The Copper Acetate powder, (b) the dried black precipitate of CuO, (c) the black color solution after adding NaOH pellets, and (d) the pH strip showing pH between 6 and 7.
Fig. 2. Stability of nanofluid (a) after 24 h and (b) after 3 days.
J.J. Michael, S. Iniyan / Energy Conversion and Management 95 (2015) 160–169 Table 1 Technical specifications of 100 LPD solar water heater. S. No.
Description
Value
Flat plate collector 1 Collector size 2 Thermal absorber copper fin 3 Copper header tube 4 Copper riser tube 5 Riser to fin joint 6 Glazing 7 8
2080 mm 1050 mm 100 mm Quantity: 9, with selective black-chrome coating 25 mm diameter 12.5 mm diameter Ultrasonic metal welding Quantity: 1, with 4 mm thick toughened solar glass 50 mm thick glass wool
Bottom thermal insulation Side thermal insulation 25 mm thick glass wool
Hot water thermal storage tank 9 Tank material and shape 10 Size 11 Outer thermal insulation 12 Connecting tube
304 stainless steel horizontal cylinder 950 mm length and 360 mm diameter 50 mm thick Poly-Urethane Foam (PUF) Ethylene Propylene Diene Monomer (EPDM) rubber
The experiment was conducted in thermosyphon and two forced circulations using water and 0.05% volume concentration CuO–water nanofluid. 2.4. Uncertainty analysis The efficiency of the solar water heater is directly proportional to the mass flow rate & specific heat of the heat transfer fluid, temperature difference between the outlet and inlet fluid to the solar collector and inversely proportional to the area of the solar collector and incident solar radiation falling on the solar collector. Assuming negligible uncertainty in the specific heat and area of the solar collector, the overall uncertainty equation for the efficiency of the solar water heater can be calculated from the following function.
163
_ G; DTÞ gi ¼ f ðm; Therefore the overall uncertainty equation can be expressed as:
2 U gi
gi
¼
2 2 2 U m_ UG U DT þ þ _ m G DT
_ G and DT where U denotes uncertainty and the variables such as m, are the mass flow rate of water (kg/s), global solar radiation incident on the solar collector plane (W/m2) and the temperature difference between the outlet and inlet fluid of the solar collector (°C) respectively. The accuracy of the rotameter, RTD PT-100 temperature sensors and the pyranometer were ±5%, ±0.1 °C and ±5% respectively. The maximum uncertainty in the mass flow rate, solar radiation and temperature difference were calculated as 0.22%, 0.18% and 3.97% respectively. Therefore, the maximum uncertainty, thus calculated was 3.97%. 3. Results and discussion The properties of the synthesized CuO nanoparticle and the prepared CuO/water nanofluid were tested by different nano-characterization tests and its performance in the solar water heating system was compared to water in thermosyphon and two forced circulation conditions. 3.1. Nano-characterization tests A large number of techniques are available to measure the properties of nanoparticles, but different techniques measure different properties of the nanoparticle, and hence, the results are not identical. Since, the nanoparticles used in this study were synthesized; a few characterization tests were conducted to understand the properties of the prepared nanoparticle and nanofluid. 3.1.1. X-ray Diffractometer (XRD) method The XRD method was used to determine the crystal structure, size and the purity of the CuO nanoparticle. A beam of X-rays is passed through the nanoparticle sample, and the constructive
Fig. 3. Complete experimental setup and schematic diagram showing the positions of temperature sensors.
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Fig. 4. XRD image of the CuO nanoparticle.
interference of the mono-chromatic X-ray as it gets scattered by the different atoms, is used to estimate the crystalline size, using Bragg’s law. The average crystalline size of the CuO nanoparticle was calculated to be 0.3 nm and 0.21 nm for the peaks of 2h = 35.6648 and 2h = 38.8354 respectively, using the Debye– Scherrer equation. The intensities and position of peaks in XRD image as shown in Fig. 4, prove, that the values are in good agreement with the reported values of the ICDD file No. 96-101-1195. Also, it was observed that the sample was a single phase CuO with a monoclinic structure, and lattice parameters of a = 4.67 Å, b = 3.43 Å, c = 5.12 Å.
Fig. 5. SEM image of the CuO nanoparticle.
3.1.2. Scanning Electron Microscopy (SEM) method The morphology of the CuO nanoparticle was studied by passing a fine beam of high energy electrons on the surface of the sample. The image formed, as shown in Fig. 5, due to the scattered electron beam and sample interaction, shows that the synthesized nanoparticles are spherical in shape.
Fig. 6. The EDX graph of the sample.
Fig. 7. TEM image of the CuO nanoparticle.
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atmosphere. The CuO/water nanofluid and the basefluid, water samples of 13 mg weight each were heated at the rate of 10 °C/ min in aluminum crucibles placed in nitrogen gas atmosphere inside the DSC instrument (Netzsch DSC 204). From the Fig. 9, it is observed that the reaction is completely endothermic, with the first peak formed is called the Heat of Fusion, and the second peak formed is called the Heat of Vapourisation. It is observed that more heat energy was absorbed and the reactions happened at slightly lower temperatures due to the better thermal properties of CuO/ water nanofluid compared to water.
Fig. 8. Particle size analyzer image of CuO particles dispersed in water without surfactant.
3.1.3. Energy Dispersive X-ray Spectroscopy (EDX) method The EDX method is used to characterize the elemental composition of the sample, by analyzing the emitted X-rays from the sample after being bombarded by the SEM electron beam. From Fig. 6, it was observed that the peaks obtained correspond to the copper and oxygen elements. No other peaks were formed suggesting the absence of any impurity. 3.1.4. Transmission Electron Microscopy (TEM) method The actual size of the nanoparticles can be measured by passing a stream of very high energy electrons through a thin film of the sample. The transmitted electrons through the sample are focused on a screen to depict the image, as shown in Fig. 7. The TEM image shows that the sample is composed of spherical nanoparticles. The average size of the nanoparticle observed from the TEM image is 75 nm. 3.1.5. Photon correlation spectroscopy (PCS) method This method is used to determine the average size of the nanoparticle dispersed in very low concentration in water. Based on Dynamic Light Scattering (DLS), the temporal variation of the light intensity scattered by the Brownian diffusion of nanoparticles can be related to the particle diameter by Stokes Einstein relation. As shown in Fig. 8, the mean size of the nanoparticle obtained by this method was 111.3 nm, which is larger than the size obtained through other methods, because the van der Waals attractive force between the dry nanoparticles is large, and hence, agglomerates even after sonication. The agglomeration can be reduced by adding a surfactant, thereby helping in the effective dispersion stability of the nanoparticles. 3.1.6. Differential Scanning Calorimetry (DSC) method The DSC method is used to observe the temperature and heat flow changes due to physical and chemical transitions in the sample, when subjected to temperature and time in an inert gas 25
3.1.8. Transient hot-wire method The thermal conductivity, which is defined as the property of the material to conduct heat was measured by the transient hotwire method for the nanofluid, using a thermal properties analyzer (KD2 Pro, Decagon Devices). A value of 0.722 W/mK was observed as shown in Fig. 11, for a temperature of 34.25 °C. 3.1.9. Rheological method The viscosity, which is defined as the fluid’s resistance to flow, was measured using a parallel plate Rheometer (Malvern Bohlin Gemini Rotonetic 2 Drive) for the CuO/water nanofluid. The change in the viscosities of water and CuO/water nanofluid due to change in shear stress at 30 °C is shown in Fig. 12. The measured experimental values of the different thermo-physical properties of CuO/water nanofluid for a volume concentration of 0.05% were compared with the numerical values obtained from theoretical models, and shown in Table 2. 3.2. Experimental analysis The performance of the solar water heater was tested using 0.05% volume concentration CuO/water nanofluid in thermosyphon
CuO Water
20
DSC (mW/mg)
3.1.7. Modulated Differential Scanning Calorimetry (MDSC) method The MDSC method (TA Instruments Q200 DSC) was used to measure the thermal capacity, which is defined as the amount of heat required to increase the temperature of the sample by 1 °C. The value was measured by imposing a sinusoidal temperature modulation to the constant heating rate of 10 °C/min and separating the total heat flow into two components, namely, sensible heat which is reversible and temperature dependent, and latent heat which is non-reversible and time dependent. The average value of the obtained result is mathematically computed using Fourier analysis, and a curve as shown in Fig. 10, is obtained. It was observed that the specific heat of the prepared nanofluid was 3.965 kJ/kg K.
15 10 5 0 -10
10
30
50
70
90
110
130
150
Temperature (˚C) Fig. 9. The DSC graph of CuO/water nanofluid and water showing the heating rate at 10 °C/min (color picture available online).
Fig. 10. Specific heat graph of the CuO/water nanofluid.
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temperature difference between the outlet and inlet fluid. The CuO/water nanofluid in thermosyphon circulation produced the highest storage tank temperature of 91.4 °C, while the CuO/water nanofluid in 0.1 kg/s produced the lowest temperature of 66.5 °C as shown in Fig. 14. In contrast, the highest temperature reached using water as the heat transfer fluid is 83.9 °C, which is less than the CuO/water nanofluid. In thermosyphon mode, the CuO/water nanofluid slowly transfers the heat to the thermal storage tank and rises its water temperature. But, at a high flow rate of 0.1 kg/ s, both fluids produce the lowest temperatures in the thermal storage tank, because of the very small contact time with the heat exchanger. CuO/water nanofluid with 0.05% volume fraction has increased the thermal storage tank useful temperature by 8.9% in thermosyphon mode.
Fig. 11. Thermal conductivity meter with CuO/water nanofluid sample (color picture available online).
(Ra = 105, natural flow), 0.01 kg/s (Re = 1276, laminar flow) and 0.1 kg/s (Re = 12764, turbulent flow) mass flow rates and compared with water. 3.2.1. Thermal absorber fin temperature The temperature of the copper fins is the lowest at the cold inlet side of the collector, because of the higher heat transfer rate in the initial section of the riser tube, which slowly reduces causing a reduced temperature difference between the fluid and the fin. The temperature of the fin increases, as the fluid passes through the riser tubes due to the buoyancy effect inside the collector gaining heat. CuO/water nanofluid, due to its higher thermal conductivity and heat transfer rate compared to water, attains higher temperature compared to water. As shown in Fig. 13, the highest temperature of 94.5 °C was obtained for the CuO/water nanofluid, in the thermosyphon mode and the lowest temperature of 73.8 °C, was also found for the CuO/water nanofluid, but in the 0.1 kg/s flow rate. The higher temperature CuO/water nanofluid combined with the heat exchanger effectiveness causes the large temperature difference between the CuO/water nanofluid and water in thermosyphon circulation.
Viscosity (Pas)
3.2.2. Thermal storage tank temperature The temperature of the water in the thermal storage tank depends on the effectiveness of the heat exchanger and the
0.00098
CuO
0.00096
Water
0.00094 0.00092 0.0009 0.00088 0.00086 0.00084 0.00082
0
2
4
6
Shear Stress (Pa) Fig. 12. Viscosity graph.
8
10
12
3.2.3. Outlet temperature The temperature of the outlet fluid from the solar collector depends on the mass flow rate, solar insolation and ambient temperature. As shown in Fig. 15, the highest outlet temperature of 96.3 °C was found for the CuO/water nanofluid in thermosyphon and the lowest temperature of 69.8 °C was found for the CuO/water nanofluid in 0.1 kg/s flow rate. In thermosyphon mode of circulation, the CuO/water nanofluid absorbs more heat, due to its higher heat transfer coefficient and produces higher outlet fluid temperature, but in forced circulation, due to the very small contact time of the CuO/water nanofluid inside the riser tubes, it produces the lowest temperatures. 3.2.4. Thermal efficiency The thermal efficiency of the solar collector was calculated _ C p ðT o T i Þ=½Ac IðtÞ ¼ using the following equation, gi ¼ ½m _ Cp IðtÞðsaÞ U t ðT p T a Þ, where the useful heat, Q u ¼ m ðT o T i Þ, Ac = area of the collector (m2), I(t) = global solar radiation incident on the solar collector (W/m2), sa = transmittanceabsorptance product (dimensionless), Ut = top heat loss coefficient (W/m2 K), Ti = temperature of inlet fluid (°C), Ta = ambient temperature (°C). The thermophysical properties of the CuO/water nanofluid and water given in Table 2 were used for the theoretical calculations. As shown in Fig. 16, the highest efficiency of 57.98% was obtained for the CuO/water nanofluid in 0.1 kg/s flow rate. The lowest efficiency of 52.33% was found for the water in thermosyphon circulation. The increase in thermal efficiency was 6.3%, 2.7% and 0.4% in thermosyphon, 0.01 kg/s and 0.1 kg/s flow rates respectively. It was observed that the enhanced performance using CuO/water nanofluid was significant in thermosyphon flow compared to forced flows. As the flow rate increases, the improvement in efficiency compared to water at the same flow conditions were reducing. However, higher thermal efficiency was obtained at higher flow rates, primarily due to the higher Reynolds number. Even though the efficiency of the solar collector was high during turbulent flow due to increased heat transfer coefficient, the contribution of the nanofluid for the increase in performance was less in turbulent flow compared to laminar and thermosyphon flows. After two days in thermosyphon mode, the CuO/water nanofluid showed reduced performance, similar to water due to reduced dispersion stability of nanoparticles in stagnant condition, particularly during night time. Hence dispersion stability of nanoparticles in the basefluid is of prime importance for thermosyphon operation. The straight line graph in Fig. 17 obtained by plotting the (Ti Ta)/GT against the instantaneous efficiency shows the thermal efficiency of the solar collector. The slope of the straight line gives the value of removed energy parameter, FRUL. The intersection of the straight line with the Y-axis gives the absorbed energy parameter, FR(sa).
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Water (0.01 kg/s) Water (0.1 kg/s) Water (Thermosyphon)
100
90 85 80 75 70 65 15:30
15:15
14:45
15:00
14:30
14:15
14:00
13:45
13:15
13:30
13:00
12:45
12:15
12:30
11:45
12:00
11:30
11:15
10:45
11:00
10:30
10:15
10:00
60
Time (hours) Fig. 13. Thermal absorber fin temperature of the flat plate solar collector.
Water (0.1 kg/s) Water (0.01 kg/s) Water (Thermosyphon)
95 90
CuO (0.01 kg/s) CuO (0.1 kg/s) CuO (Thermosyphon)
Temperature (˚C)
85 80 75 70 65 60 55 15:30 15:30
15:00
15:15 15:15
14:45
14:30
14:15
14:00
13:45
13:30
13:15
13:00
12:45
12:30
12:15
12:00
11:45
11:30
11:15
11:00
10:45
10:30
10:15
10:00
50
Time (hours) Fig. 14. Thermal storage tank temperature.
Water (0.1 kg/s) Water (0.01 kg/s) Water (Thermosyphon)
100 95
CuO (0.01 kg/s) CuO (0.1 kg/s) CuO (Thermosyphon)
90 85 80 75 70 65 60 55
Time (hours) Fig. 15. Outlet temperature of the flat plate solar collector.
15:00
14:45
14:30
14:15
14:00
13:45
13:30
13:15
13:00
12:30
12:45
12:15
12:00
11:45
11:30
11:00
11:15
10:45
10:30
10:15
50 10:00
Temperature (˚C)
Temperature (˚C)
95
CuO (0.01 kg/s) CuO (0.1 kg/s) CuO (Thermosyphon)
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60
Water (Thermosyphon) Water (0.1 kg/s) Water (0.01 kg/s)
58 56
CuO (Thermosyphon) CuO (0.1 kg/s) CuO (0.01 kg/s)
Efficiency (%)
54 52 50 48 46 44 42 15:15
15:30
15:00
14:45
14:30
14:15
14:00
13:45
13:30
13:15
12:45
13:00
12:30
12:15
12:00
11:45
11:30
11:15
10:45
11:00
10:30
10:15
10:00
40
Time (hours) Fig. 16. Flat plate solar collector efficiency.
1 Water (thermosyphon) Water (0.1 kg/s) Water (0.01 kg/s)
0.9 0.8
CuO (thermosyphon) CuO (0.1 kg/s) CuO (0.01 kg/s)
Efficiency (%)
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
0
0.05
0.1
0.15
0.2
0.25
(Ti - Ta) / Gt Fig. 17. Straight line graph between thermal efficiency and reduced temperature parameter.
Table 3 Values of FRUL, FR(sa) and R2 for the CuO/water nanofluid and water.
CuO (thermosyphon) Water (thermosyphon) CuO (0.01 kg/s) Water (0.01 kg/s) CuO (0.1 kg/s) Water (0.1 kg/s)
4. Conclusion
FR(sa)
UL
R2
0.925 0.860 0.859 0.774 0.896 0.809
11.519 8.120 5.557 3.831 5.676 3.970
0.991 0.968 0.985 0.977 0.994 0.997
The performance of nanofluid with low volume concentration is preferred compared to higher concentration nanofluid which suffers from stability issues. From Fig. 17 and Table 3, it is seen that the addition of nanoparticles in the basefluid, water improves the absorbed energy parameter, FR(sa), but also has increased losses. This is due to the higher operating temperature during the use of nanofluid as heat transfer fluid. The operating temperature can be reduced by increasing the effectiveness of the heat exchanger. It is also seen that the largest improvement in performance is observed for thermosyphon operation rather than forced circulation. This explains that the enhanced performance of nanofluid is due to the higher thermal conductivity of the nanoparticles, which causes Brownian motion and helps to absorb more solar energy. With increase in flow rate, the higher heat transfer coefficient due to increase in Reynolds number overshadows the performance improvement due to nanofluids.
Many researchers, over the decades have increased the efficiency of solar water heater to the present level. Yet, the current efficiency of the solar water heater is less compared to other conventional technologies. Addition of nanoparticles in the heat transfer fluid contributes to a significant improvement in the thermal efficiency. Here in this paper, CuO nanoparticles are synthesized, CuO/water nanofluid prepared for a low volume concentration of 0.05% improved the thermal performance of the solar water heater by 6.3%. Also, while comparing the mass flow rate, the highest improvement in efficiency was observed in thermosyphon circulation compared to forced circulations. However, the highest efficiency of the solar water heater was obtained at the high flow rate of 0.1 kg/s. Further increase in efficiency is possible, if the effectiveness of the heat exchanger inside the thermal storage tank is designed for nanofluid operation. Few nano-characterization tests of the CuO nanoparticle and CuO/water nanofluid conducted were also presented. Acknowledgements The authors wish to express their sincere thanks to Anna University, for providing the necessary funds through the ‘‘Anna Centenary Research Fellowship’’. The author gives special thanks to Anna University, Indian Institute of Technology Madras and Sathyabama University for providing the required facilities to conduct the nano-characterization tests.
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Performance of copper oxide/water nanofluid in a flat plate solar water heater under natural and forced circulations Jee Joe Michael ⇑, S. Iniyan Institute for Energy Studies, Department of Mechanical Engineering, Anna University, Chennai 600 025, India
a r t i c l e
i n f o
Article history: Received 26 August 2014 Accepted 6 February 2015
Keywords: Flat plate Solar water heater Thermosyphon Nanofluid CuO
a b s t r a c t Flat plate solar water heater is widely used for heating of water in low-temperature residential applications. In this paper, Copper Oxide/water (CuO/H2O) nanofluid is prepared from Copper Acetate and its thermal performance was investigated experimentally on a 100 Liters per Day (LPD) thermosyphon based indirect-type flat plate solar water heater. The volumetric fraction of CuO/water nanofluid chosen was 0.05%. Significant improvement in performance was observed in thermosyphon circulation compared to forced circulation, for the low volumetric fraction considered. The CuO/water nanofluid was prepared with the inclusion of surfactant Sodium Dodecyl Benzene Sulfonate (SDBS), as it provided the best CuO nanoparticle dispersion stability compared to pure water suspension and Triton X-100 surfactant suspensions. Also, the thermophysical properties of the synthesized nanoparticle and prepared nanofluid were compared theoretically and experimentally. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Solar water heater is being used worldwide for low temperature applications mostly in the domestic sector for washing clothes and bathing purposes. Thermosyphon flat plate solar water heater is a solar passive system, which can produce hot water in the temperature range of 60–90 °C. Closed loop or heat-exchanger type or Indirect type solar water heater are used in which a primary fluid namely pure water or glycol–water mixture is added, to prevent the formation of scaling on the inner surface of the copper tubes due to passage of high saline water and to prevent damage to tubes due to water freezing in cold climates. Due to low thermal conductivity of water and the heat exchanger effectiveness, the temperature attained by the secondary fluid is reduced. In order to increase the outlet useful temperature and thermal efficiency, nanoparticles having high thermal properties are mixed with the primary fluid to form nanofluids, thereby increasing the effective thermal conductivity of the primary solution. The effect of nanofluids in several industrial and residential applications was experimentally and theoretically analyzed by several researchers all over the world. The thermal performance using nanofluids depends on several thermophysical properties of nanoparticle such as particle diameter, shape and the pH, viscosity, thermal conductivity, volume fraction, specific heat of nanofluid. ⇑ Corresponding author. Tel.: +91 9444338954. E-mail address: [email protected] (J.J. Michael). http://dx.doi.org/10.1016/j.enconman.2015.02.017 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
Different theories were established to understand the behavior of nanoparticle under temperature, pH, sonication etc. Theoretical analyses on the heat transfer coefficient based on the effect of particle size, extinction coefficient [1], Brownian motion and Thermophoresis were developed [2]. Consequently, a method to measure the thermal diffusivity and thermal conductivity of nanofluids based on the temperature oscillation technique was also developed [3]. The heat transfer coefficient, outlet temperature and entropy generation among four different nanofluids such as Cu/water, Al2O3/water, TiO2/water and SiO2/water provided interesting results about the effect of Nusselt number, Bejan number, density, heat capacity and efficiency on the performance of a solar collector [4]. Similarly, a theoretical study on the performance of Al2O3/water nanofluid in a flat plate solar water heater was analyzed for different particle sizes and volume fractions [5]; also the effect of Prandtl number on flow and heat transfer was analyzed using FEM method [6]. Experimentally, 15 nm particle size, 0.2% weight fraction Al2O3/water nanofluid with Triton X100 surfactant produced a 28.3% [7] and 30% [8] enhancement in thermal efficiency in a flat plate solar water heater. Different thermophysical properties of Al2O3 [9] and TiO2 nanofluid were measured for various temperatures and volume fractions and concluded that low volume fraction nanofluid performed better [10]. The thermal performance of a natural circulation system using Al2O3/water nanofluid was tested for 0.5% and 3% volume fractions [11], and a similar enhanced thermal performance up to 22% and 62% were obtained in the evaporator and condenser sections
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respectively [12]. The photoconductive and photothermal CuO nanoparticle [13] of 4% concentration, when mixed with water has impoved the thermal conductivity by 20% [6]. Similarly, a 40 nm particle size, 0.4% volume fraction CuO/water nanofluid found a 21.8% collector efficiency improvement between 1 and 3 kg/min flow rate [14]. Also, a 25.6% improvement was observed for 0.0083 kg/s mass flow rate and 0.1% weight fraction [15]. A compound parabolic evacuated tube air collector using CuO/water nanofluid in thermosyphon mode provided outlet air temperatures above 170 °C [16]. The effect of Cu/glycol nanofluid was tested for various volume fractions and Reynolds number experimentally [17] and analyzed theoretically using FEM analysis [18]. Also, a 23.83% thermal enhancement was observed using Cu/water nanofluid at 0.1% weight fraction and 140 L/h flow rate [19]. Enhanced thermal conductivity up to 32% was obtained in a direct absorber solar collector by adding 150 ppm carbon nanotubes in water [20]. SWCNT/water nanofluid produced a 15.33% increase in heat transfer coefficient with a 4.34% reduction in entropy generation [21]. MWCNT nanofluid of 0.2% weight fraction produced improved performance using Triton X-100 as the surfactant [22]. Another surfactant namely Sodium Dodecyl Sulfate (SDS) also performed as a suitable surfactant for CNT based nanofluids [23]. It was observed that decreasing the pH with respect to isoelectric point helped in producing higher efficiency [24]. An optimum volume fraction and tilt angle for enhanced heat transfer rate using CNT/water nanofluid was analyzed for a solar collector using FEM analysis [25]. Improved electrical and thermal performance of a solar photovoltaic thermal (PV/T) system was observed by the use of silica/water nanofluid [26]. TiO2/water nanofluid with twisted tape inserts in a solar collector enhanced the thermal performance up to 1.59 times at 0.21% volume fraction [27]. The efficiency, size, cost, savings, payback period and environmental impact of a solar collector were estimated for different nanofluids [28], also it was calculated that nanofluid reduces the embodied energy and carbon emission by approximately 9% and 3% respectively [29]. A review on the performance of solar collector using nanofluids [30], nanofluid applications in different types of solar collectors, photovoltaic systems, solar thermoelectrics, thermal energy storage systems [31], solar water heaters, solar stills, solar PV/T systems, solar ponds [32], industrial and transport applications [33] were presented. As per the literature survey, a fascinating amount of research papers are available regarding solar water heaters. Most of the research papers are devoted to thermosyphon or forced circulations. However, as per the authors knowledge, no journal paper was available focusing on the performance comparison of thermosyphon and forced circulation modes in a single system. In this paper, a flat-plate solar water heating system was tested in natural and forced circulations. However, the quantity of research involved in the application of nanofluids in solar water heaters is still in the nascent stage. Many researchers have experimented solar water heater using nanofluids based on Al2O3 and CNT nanoparticles for their high thermal conductivity and low density. In this paper, the performance of solar water heater using the common heat transfer fluid, water was compared to a 0.05% volume fraction CuO/water nanofluid. The CuO/water nanofluid was prepared inhouse from commercially available raw materials and their nanocharacterization tests were also presented. 2. Methodology The experiment was conducted by synthesizing the CuO nanoparticle, preparing the CuO/water nanofluid and comparing the performance of the prepared CuO/water nanofluid with water in a commercially available 100 LPD thermosyphon indirect type flat plate solar water heater.
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2.1. Synthesis of CuO nanoparticles The volume of the primary working fluid in the 100 LPD indirect type flat plate solar water heater was measured as 8 L. The quantity of nanoparticles required to prepare a 0.05% volume concentration of CuO–water nanofluid was calculated to be 25.3 g. Due to the high cost of commercially available nanoparticles, it was synthesised locally from Copper Acetate by a simple method called Aqueous Precipitation method. Analytical grade Copper Acetate Monohydrate (Cu(CH3COO)2H2O) as the precursor and glacial Acetic acid (CH3COOH) to prevent hydrolysis, are taken in a flatbottomed glass beaker, and heated with constant stirring in a magnetic stirrer. Sodium Hydroxide (NaOH) pellets as stabilizing agents were added slowly till the pH of the solution reaches between 6 and 7, and the color of the solution slowly turns from blue to black as shown in Fig. 1. The solution is removed from the magnetic stirrer and cooled to room temperature. The black precipitate formed is centrifuged, and then washed with distilled water three times to remove the impure ions. It is then covered with aluminum foil and placed in a hot air oven to completely dry the contents. The dried precipitate is finely powdered, using Agate mortar to produce CuO solid particles in nano dimensions. 2.2. Preparing the nanofluid The high density (6310 kg/m3) of the CuO nanoparticles compared to the basefluid, water (1000 kg/m3), causes the immediate settlement of the CuO nanoparticles at the bottom of the beaker. Hence, a compatible amphiphilic surfactant, namely, Sodium Dodecyl Benzene Sulfonate (SDBS) of 10% weight of the amount of nanoparticles to be added, was first dispersed completely in double distilled water using a magnetic stirrer, and then CuO nanoparticles of the required quantity are slowly added with constant stirring for 30 min. The solution is further sonicated using an ultrasonicator for 60 min to break the agglomerated particles and facilitate a homogeneous mixture of the CuO nanoparticles and water, called CuO/water nanofluid. The CuO nanoparticles’ stability was checked with SDBS surfactant, with Triton X-100 surfactant and without any surfactant. In Fig. 2, the dispersibility of nanoparticles with the SDBS surfactant in water, compared to without-surfactant water after 24 h, clearly shows the stability of the nanoparticles using the surfactant. Similarly, the CuO/water nanofluid showed better stability with the SDBS surfactant, compared to the Triton X-100 surfactant after 3 days. Therefore, it was concluded that the SDBS surfactant performed better compared to the Triton X-100 surfactant for CuO/ water nanofluid. 2.3. Description of the system A 100 LPD thermosyphon based ladder type heat exchanger type solar water heater was used in the study. The detailed specification of the solar water heater is listed in Table 1. The flat plate collector was tilted at an angle of 13° corresponding to the latitude of the location and facing south, since Chennai is located in the Northern hemisphere. The temperature of the different locations was measured, using PT-100 RTD sensors. The solar radiation was measured on the plane of the solar collector using a pyranometer (Hukseflux LP02); the ambient temperature and the wind speed were measured using a weather station (WatchDog 2000). All the sensors were connected to a data acquisition system (Agilent 34970A), and recorded every 5 min continuously and plotted in graphs. A booster pump and a rotameter were used during forced circulation testing. The complete experimental setup and the schematic diagram of the solar water heating system are shown in Fig. 3.
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Fig. 1. (a) The Copper Acetate powder, (b) the dried black precipitate of CuO, (c) the black color solution after adding NaOH pellets, and (d) the pH strip showing pH between 6 and 7.
Fig. 2. Stability of nanofluid (a) after 24 h and (b) after 3 days.
J.J. Michael, S. Iniyan / Energy Conversion and Management 95 (2015) 160–169 Table 1 Technical specifications of 100 LPD solar water heater. S. No.
Description
Value
Flat plate collector 1 Collector size 2 Thermal absorber copper fin 3 Copper header tube 4 Copper riser tube 5 Riser to fin joint 6 Glazing 7 8
2080 mm 1050 mm 100 mm Quantity: 9, with selective black-chrome coating 25 mm diameter 12.5 mm diameter Ultrasonic metal welding Quantity: 1, with 4 mm thick toughened solar glass 50 mm thick glass wool
Bottom thermal insulation Side thermal insulation 25 mm thick glass wool
Hot water thermal storage tank 9 Tank material and shape 10 Size 11 Outer thermal insulation 12 Connecting tube
304 stainless steel horizontal cylinder 950 mm length and 360 mm diameter 50 mm thick Poly-Urethane Foam (PUF) Ethylene Propylene Diene Monomer (EPDM) rubber
The experiment was conducted in thermosyphon and two forced circulations using water and 0.05% volume concentration CuO–water nanofluid. 2.4. Uncertainty analysis The efficiency of the solar water heater is directly proportional to the mass flow rate & specific heat of the heat transfer fluid, temperature difference between the outlet and inlet fluid to the solar collector and inversely proportional to the area of the solar collector and incident solar radiation falling on the solar collector. Assuming negligible uncertainty in the specific heat and area of the solar collector, the overall uncertainty equation for the efficiency of the solar water heater can be calculated from the following function.
163
_ G; DTÞ gi ¼ f ðm; Therefore the overall uncertainty equation can be expressed as:
2 U gi
gi
¼
2 2 2 U m_ UG U DT þ þ _ m G DT
_ G and DT where U denotes uncertainty and the variables such as m, are the mass flow rate of water (kg/s), global solar radiation incident on the solar collector plane (W/m2) and the temperature difference between the outlet and inlet fluid of the solar collector (°C) respectively. The accuracy of the rotameter, RTD PT-100 temperature sensors and the pyranometer were ±5%, ±0.1 °C and ±5% respectively. The maximum uncertainty in the mass flow rate, solar radiation and temperature difference were calculated as 0.22%, 0.18% and 3.97% respectively. Therefore, the maximum uncertainty, thus calculated was 3.97%. 3. Results and discussion The properties of the synthesized CuO nanoparticle and the prepared CuO/water nanofluid were tested by different nano-characterization tests and its performance in the solar water heating system was compared to water in thermosyphon and two forced circulation conditions. 3.1. Nano-characterization tests A large number of techniques are available to measure the properties of nanoparticles, but different techniques measure different properties of the nanoparticle, and hence, the results are not identical. Since, the nanoparticles used in this study were synthesized; a few characterization tests were conducted to understand the properties of the prepared nanoparticle and nanofluid. 3.1.1. X-ray Diffractometer (XRD) method The XRD method was used to determine the crystal structure, size and the purity of the CuO nanoparticle. A beam of X-rays is passed through the nanoparticle sample, and the constructive
Fig. 3. Complete experimental setup and schematic diagram showing the positions of temperature sensors.
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Fig. 4. XRD image of the CuO nanoparticle.
interference of the mono-chromatic X-ray as it gets scattered by the different atoms, is used to estimate the crystalline size, using Bragg’s law. The average crystalline size of the CuO nanoparticle was calculated to be 0.3 nm and 0.21 nm for the peaks of 2h = 35.6648 and 2h = 38.8354 respectively, using the Debye– Scherrer equation. The intensities and position of peaks in XRD image as shown in Fig. 4, prove, that the values are in good agreement with the reported values of the ICDD file No. 96-101-1195. Also, it was observed that the sample was a single phase CuO with a monoclinic structure, and lattice parameters of a = 4.67 Å, b = 3.43 Å, c = 5.12 Å.
Fig. 5. SEM image of the CuO nanoparticle.
3.1.2. Scanning Electron Microscopy (SEM) method The morphology of the CuO nanoparticle was studied by passing a fine beam of high energy electrons on the surface of the sample. The image formed, as shown in Fig. 5, due to the scattered electron beam and sample interaction, shows that the synthesized nanoparticles are spherical in shape.
Fig. 6. The EDX graph of the sample.
Fig. 7. TEM image of the CuO nanoparticle.
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atmosphere. The CuO/water nanofluid and the basefluid, water samples of 13 mg weight each were heated at the rate of 10 °C/ min in aluminum crucibles placed in nitrogen gas atmosphere inside the DSC instrument (Netzsch DSC 204). From the Fig. 9, it is observed that the reaction is completely endothermic, with the first peak formed is called the Heat of Fusion, and the second peak formed is called the Heat of Vapourisation. It is observed that more heat energy was absorbed and the reactions happened at slightly lower temperatures due to the better thermal properties of CuO/ water nanofluid compared to water.
Fig. 8. Particle size analyzer image of CuO particles dispersed in water without surfactant.
3.1.3. Energy Dispersive X-ray Spectroscopy (EDX) method The EDX method is used to characterize the elemental composition of the sample, by analyzing the emitted X-rays from the sample after being bombarded by the SEM electron beam. From Fig. 6, it was observed that the peaks obtained correspond to the copper and oxygen elements. No other peaks were formed suggesting the absence of any impurity. 3.1.4. Transmission Electron Microscopy (TEM) method The actual size of the nanoparticles can be measured by passing a stream of very high energy electrons through a thin film of the sample. The transmitted electrons through the sample are focused on a screen to depict the image, as shown in Fig. 7. The TEM image shows that the sample is composed of spherical nanoparticles. The average size of the nanoparticle observed from the TEM image is 75 nm. 3.1.5. Photon correlation spectroscopy (PCS) method This method is used to determine the average size of the nanoparticle dispersed in very low concentration in water. Based on Dynamic Light Scattering (DLS), the temporal variation of the light intensity scattered by the Brownian diffusion of nanoparticles can be related to the particle diameter by Stokes Einstein relation. As shown in Fig. 8, the mean size of the nanoparticle obtained by this method was 111.3 nm, which is larger than the size obtained through other methods, because the van der Waals attractive force between the dry nanoparticles is large, and hence, agglomerates even after sonication. The agglomeration can be reduced by adding a surfactant, thereby helping in the effective dispersion stability of the nanoparticles. 3.1.6. Differential Scanning Calorimetry (DSC) method The DSC method is used to observe the temperature and heat flow changes due to physical and chemical transitions in the sample, when subjected to temperature and time in an inert gas 25
3.1.8. Transient hot-wire method The thermal conductivity, which is defined as the property of the material to conduct heat was measured by the transient hotwire method for the nanofluid, using a thermal properties analyzer (KD2 Pro, Decagon Devices). A value of 0.722 W/mK was observed as shown in Fig. 11, for a temperature of 34.25 °C. 3.1.9. Rheological method The viscosity, which is defined as the fluid’s resistance to flow, was measured using a parallel plate Rheometer (Malvern Bohlin Gemini Rotonetic 2 Drive) for the CuO/water nanofluid. The change in the viscosities of water and CuO/water nanofluid due to change in shear stress at 30 °C is shown in Fig. 12. The measured experimental values of the different thermo-physical properties of CuO/water nanofluid for a volume concentration of 0.05% were compared with the numerical values obtained from theoretical models, and shown in Table 2. 3.2. Experimental analysis The performance of the solar water heater was tested using 0.05% volume concentration CuO/water nanofluid in thermosyphon
CuO Water
20
DSC (mW/mg)
3.1.7. Modulated Differential Scanning Calorimetry (MDSC) method The MDSC method (TA Instruments Q200 DSC) was used to measure the thermal capacity, which is defined as the amount of heat required to increase the temperature of the sample by 1 °C. The value was measured by imposing a sinusoidal temperature modulation to the constant heating rate of 10 °C/min and separating the total heat flow into two components, namely, sensible heat which is reversible and temperature dependent, and latent heat which is non-reversible and time dependent. The average value of the obtained result is mathematically computed using Fourier analysis, and a curve as shown in Fig. 10, is obtained. It was observed that the specific heat of the prepared nanofluid was 3.965 kJ/kg K.
15 10 5 0 -10
10
30
50
70
90
110
130
150
Temperature (˚C) Fig. 9. The DSC graph of CuO/water nanofluid and water showing the heating rate at 10 °C/min (color picture available online).
Fig. 10. Specific heat graph of the CuO/water nanofluid.
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temperature difference between the outlet and inlet fluid. The CuO/water nanofluid in thermosyphon circulation produced the highest storage tank temperature of 91.4 °C, while the CuO/water nanofluid in 0.1 kg/s produced the lowest temperature of 66.5 °C as shown in Fig. 14. In contrast, the highest temperature reached using water as the heat transfer fluid is 83.9 °C, which is less than the CuO/water nanofluid. In thermosyphon mode, the CuO/water nanofluid slowly transfers the heat to the thermal storage tank and rises its water temperature. But, at a high flow rate of 0.1 kg/ s, both fluids produce the lowest temperatures in the thermal storage tank, because of the very small contact time with the heat exchanger. CuO/water nanofluid with 0.05% volume fraction has increased the thermal storage tank useful temperature by 8.9% in thermosyphon mode.
Fig. 11. Thermal conductivity meter with CuO/water nanofluid sample (color picture available online).
(Ra = 105, natural flow), 0.01 kg/s (Re = 1276, laminar flow) and 0.1 kg/s (Re = 12764, turbulent flow) mass flow rates and compared with water. 3.2.1. Thermal absorber fin temperature The temperature of the copper fins is the lowest at the cold inlet side of the collector, because of the higher heat transfer rate in the initial section of the riser tube, which slowly reduces causing a reduced temperature difference between the fluid and the fin. The temperature of the fin increases, as the fluid passes through the riser tubes due to the buoyancy effect inside the collector gaining heat. CuO/water nanofluid, due to its higher thermal conductivity and heat transfer rate compared to water, attains higher temperature compared to water. As shown in Fig. 13, the highest temperature of 94.5 °C was obtained for the CuO/water nanofluid, in the thermosyphon mode and the lowest temperature of 73.8 °C, was also found for the CuO/water nanofluid, but in the 0.1 kg/s flow rate. The higher temperature CuO/water nanofluid combined with the heat exchanger effectiveness causes the large temperature difference between the CuO/water nanofluid and water in thermosyphon circulation.
Viscosity (Pas)
3.2.2. Thermal storage tank temperature The temperature of the water in the thermal storage tank depends on the effectiveness of the heat exchanger and the
0.00098
CuO
0.00096
Water
0.00094 0.00092 0.0009 0.00088 0.00086 0.00084 0.00082
0
2
4
6
Shear Stress (Pa) Fig. 12. Viscosity graph.
8
10
12
3.2.3. Outlet temperature The temperature of the outlet fluid from the solar collector depends on the mass flow rate, solar insolation and ambient temperature. As shown in Fig. 15, the highest outlet temperature of 96.3 °C was found for the CuO/water nanofluid in thermosyphon and the lowest temperature of 69.8 °C was found for the CuO/water nanofluid in 0.1 kg/s flow rate. In thermosyphon mode of circulation, the CuO/water nanofluid absorbs more heat, due to its higher heat transfer coefficient and produces higher outlet fluid temperature, but in forced circulation, due to the very small contact time of the CuO/water nanofluid inside the riser tubes, it produces the lowest temperatures. 3.2.4. Thermal efficiency The thermal efficiency of the solar collector was calculated _ C p ðT o T i Þ=½Ac IðtÞ ¼ using the following equation, gi ¼ ½m _ Cp IðtÞðsaÞ U t ðT p T a Þ, where the useful heat, Q u ¼ m ðT o T i Þ, Ac = area of the collector (m2), I(t) = global solar radiation incident on the solar collector (W/m2), sa = transmittanceabsorptance product (dimensionless), Ut = top heat loss coefficient (W/m2 K), Ti = temperature of inlet fluid (°C), Ta = ambient temperature (°C). The thermophysical properties of the CuO/water nanofluid and water given in Table 2 were used for the theoretical calculations. As shown in Fig. 16, the highest efficiency of 57.98% was obtained for the CuO/water nanofluid in 0.1 kg/s flow rate. The lowest efficiency of 52.33% was found for the water in thermosyphon circulation. The increase in thermal efficiency was 6.3%, 2.7% and 0.4% in thermosyphon, 0.01 kg/s and 0.1 kg/s flow rates respectively. It was observed that the enhanced performance using CuO/water nanofluid was significant in thermosyphon flow compared to forced flows. As the flow rate increases, the improvement in efficiency compared to water at the same flow conditions were reducing. However, higher thermal efficiency was obtained at higher flow rates, primarily due to the higher Reynolds number. Even though the efficiency of the solar collector was high during turbulent flow due to increased heat transfer coefficient, the contribution of the nanofluid for the increase in performance was less in turbulent flow compared to laminar and thermosyphon flows. After two days in thermosyphon mode, the CuO/water nanofluid showed reduced performance, similar to water due to reduced dispersion stability of nanoparticles in stagnant condition, particularly during night time. Hence dispersion stability of nanoparticles in the basefluid is of prime importance for thermosyphon operation. The straight line graph in Fig. 17 obtained by plotting the (Ti Ta)/GT against the instantaneous efficiency shows the thermal efficiency of the solar collector. The slope of the straight line gives the value of removed energy parameter, FRUL. The intersection of the straight line with the Y-axis gives the absorbed energy parameter, FR(sa).
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Water (0.01 kg/s) Water (0.1 kg/s) Water (Thermosyphon)
100
90 85 80 75 70 65 15:30
15:15
14:45
15:00
14:30
14:15
14:00
13:45
13:15
13:30
13:00
12:45
12:15
12:30
11:45
12:00
11:30
11:15
10:45
11:00
10:30
10:15
10:00
60
Time (hours) Fig. 13. Thermal absorber fin temperature of the flat plate solar collector.
Water (0.1 kg/s) Water (0.01 kg/s) Water (Thermosyphon)
95 90
CuO (0.01 kg/s) CuO (0.1 kg/s) CuO (Thermosyphon)
Temperature (˚C)
85 80 75 70 65 60 55 15:30 15:30
15:00
15:15 15:15
14:45
14:30
14:15
14:00
13:45
13:30
13:15
13:00
12:45
12:30
12:15
12:00
11:45
11:30
11:15
11:00
10:45
10:30
10:15
10:00
50
Time (hours) Fig. 14. Thermal storage tank temperature.
Water (0.1 kg/s) Water (0.01 kg/s) Water (Thermosyphon)
100 95
CuO (0.01 kg/s) CuO (0.1 kg/s) CuO (Thermosyphon)
90 85 80 75 70 65 60 55
Time (hours) Fig. 15. Outlet temperature of the flat plate solar collector.
15:00
14:45
14:30
14:15
14:00
13:45
13:30
13:15
13:00
12:30
12:45
12:15
12:00
11:45
11:30
11:00
11:15
10:45
10:30
10:15
50 10:00
Temperature (˚C)
Temperature (˚C)
95
CuO (0.01 kg/s) CuO (0.1 kg/s) CuO (Thermosyphon)
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J.J. Michael, S. Iniyan / Energy Conversion and Management 95 (2015) 160–169
60
Water (Thermosyphon) Water (0.1 kg/s) Water (0.01 kg/s)
58 56
CuO (Thermosyphon) CuO (0.1 kg/s) CuO (0.01 kg/s)
Efficiency (%)
54 52 50 48 46 44 42 15:15
15:30
15:00
14:45
14:30
14:15
14:00
13:45
13:30
13:15
12:45
13:00
12:30
12:15
12:00
11:45
11:30
11:15
10:45
11:00
10:30
10:15
10:00
40
Time (hours) Fig. 16. Flat plate solar collector efficiency.
1 Water (thermosyphon) Water (0.1 kg/s) Water (0.01 kg/s)
0.9 0.8
CuO (thermosyphon) CuO (0.1 kg/s) CuO (0.01 kg/s)
Efficiency (%)
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
0
0.05
0.1
0.15
0.2
0.25
(Ti - Ta) / Gt Fig. 17. Straight line graph between thermal efficiency and reduced temperature parameter.
Table 3 Values of FRUL, FR(sa) and R2 for the CuO/water nanofluid and water.
CuO (thermosyphon) Water (thermosyphon) CuO (0.01 kg/s) Water (0.01 kg/s) CuO (0.1 kg/s) Water (0.1 kg/s)
4. Conclusion
FR(sa)
UL
R2
0.925 0.860 0.859 0.774 0.896 0.809
11.519 8.120 5.557 3.831 5.676 3.970
0.991 0.968 0.985 0.977 0.994 0.997
The performance of nanofluid with low volume concentration is preferred compared to higher concentration nanofluid which suffers from stability issues. From Fig. 17 and Table 3, it is seen that the addition of nanoparticles in the basefluid, water improves the absorbed energy parameter, FR(sa), but also has increased losses. This is due to the higher operating temperature during the use of nanofluid as heat transfer fluid. The operating temperature can be reduced by increasing the effectiveness of the heat exchanger. It is also seen that the largest improvement in performance is observed for thermosyphon operation rather than forced circulation. This explains that the enhanced performance of nanofluid is due to the higher thermal conductivity of the nanoparticles, which causes Brownian motion and helps to absorb more solar energy. With increase in flow rate, the higher heat transfer coefficient due to increase in Reynolds number overshadows the performance improvement due to nanofluids.
Many researchers, over the decades have increased the efficiency of solar water heater to the present level. Yet, the current efficiency of the solar water heater is less compared to other conventional technologies. Addition of nanoparticles in the heat transfer fluid contributes to a significant improvement in the thermal efficiency. Here in this paper, CuO nanoparticles are synthesized, CuO/water nanofluid prepared for a low volume concentration of 0.05% improved the thermal performance of the solar water heater by 6.3%. Also, while comparing the mass flow rate, the highest improvement in efficiency was observed in thermosyphon circulation compared to forced circulations. However, the highest efficiency of the solar water heater was obtained at the high flow rate of 0.1 kg/s. Further increase in efficiency is possible, if the effectiveness of the heat exchanger inside the thermal storage tank is designed for nanofluid operation. Few nano-characterization tests of the CuO nanoparticle and CuO/water nanofluid conducted were also presented. Acknowledgements The authors wish to express their sincere thanks to Anna University, for providing the necessary funds through the ‘‘Anna Centenary Research Fellowship’’. The author gives special thanks to Anna University, Indian Institute of Technology Madras and Sathyabama University for providing the required facilities to conduct the nano-characterization tests.
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