Investigation of Solar Heat Pipe Collector Using Nanofluid and Surfactant

Available online at www.sciencedirect.com ScienceDirect

Energy Procedia 90 ( 2016 ) 481 – 491

5th International Conference on Advances in Energy Research, ICAER 2015, 15-17 December 2015, Mumbai, India Investigation of Solar Heat Pipe Collector Using Nanofluid and Surfactant.

Gargee.A. Pisea, Sanjay.S.Salveb, Ashok.T.Pisea,b,1*,Amey.A.Pisea,b,2 aPG Student,Pimpri Chinchwad College of Engineering University of Pune, Pune, India- b Faculty of Engineering, Pimpri Chinchwad College of Engineering,Univeristy of Pune, Pune, India- a,b,1Deputy Director, Directorate of Technical Education, Mumbai, India a,b,2Student,Mechanical Department, VIT,Pune, India

Abstract

The basic aim of conducted experiments was to investigate the thermal performance of serpentine shape thermosyphon heat pipe flat plate solar collector under real operating conditions. Distil water, (Al2O3-water) nanofluid with varying concentrations (0.05, 0.25 and 0.5 by wt ), (Al2O3-water) + surfactant , water + surfactant were used as working fluids. Effect of coolant rate, concentration of nanomaterial on performance of solar heat pipe collector was studied experimentally for different tilt angles. Collector wastested for tilt angles (18.53, 33.5, 40, 50 and 60˚). Nanofluids of Al2O3 nanoparticles by wt having average size <50 nm dispersed in water with different concentrations by two step process. Two step processes comprising of magnetic string and ultra-sonication is used.From results it is observed that as the efficiency of the collector increases as coolant flow rate increases upto certain limit, then decreases. Also similar trend is observed for tilt angle and it is found maximum at 50˚. Surfactant, Surfactant-Nanofluids, nanofluids as working fluid in the heat pipe collector gives better performance as compared pure water and performance enhances with increase in the concentration of the nanofluid. ©© 20162016 The The Authors. Authors. Published Published by Elsevierby Elsevier Ltd. ThisLtd. is an open access article under the CC BY-NC-ND license (Peer-reviewhttp://creativecommons.org/licenses/by-nc-nd/4.0/ under responsibility ofthe organizing). committee of ICAER 2015. Peer-review under responsibility of the organizing committee of ICAER 2015 Keywords:Heat Pipe; Nanofluid; Collector; Tilt angle ;Surfactant

1.Introduction

Solar energy is widely used for providing heat to many houses. Solar water heating is an effective method of using solar energy to perform many useful tasks. In a conventional solar collector, the absorber plate is often backed ------* Corresponding author. Tel.: +91-022-30233356; fax:+91-022-22692102. E-mail address:[email protected]

1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICAER 2015 doi: 10.1016/j.egypro.2016.11.215 482 Gargee A. Pise et al. / Energy Procedia 90 ( 2016 ) 481 – 491

up with a grid or tubes containing water as working fluid in it. The solar energy is absorbed by the working fluid and transferred to the storage tank either by natural or forced circulation. The drawbacks associated with conventional solar collectors include the pump and its power requirement, more space to obtain the natural circulation of working fluid, night cooling due to reverse flow of cooled water, pipe corrosion and limited heat carrying capacity of working fluid, high heat loss factor. Heat pipes are found to provide an alternative solution to the above problems. Heat pipes, contain small amount of vaporizable fluid, transfers heat by evaporating the working fluid in a heating zone and condensing vapor in the cooling zone. Subsequently, the return flow of condensate to the heating zone occurs through the capillary structure (wick) which lies in the inner wall of the heat pipe. However, in certain applications, the return flow of the condensate to the heating zone occurs due to the gravity force and termed as gravity assisted heat pipes, wickless heat pipes or thermosyphon. Because of its ability to transfer large quantity of heat energy, the heat pipes have been widely used in several industrial applications, namely boilers, air-conditioning system, heat exchangers and solar water heating system. The wickless heat pipes are usually integrated with flat absorber plate of the solar heating system, to enhance the thermal performance. Numerous studies have been carried out to understand the thermal performance of two phase thermosyphon used in solar water heaters.Karagholi et al.[1] obtained 38% average efficiency which is enhancement in collector efficiency with thermosyphon compared to conventional solar collector. Nada et al [2] reported the results obtained through experimental investigation of a TPCT solar collector. They investigated the performance for different coolant rate and found that collector efficiency increases with increasing flow rate from 0.0.125 kg/s to 0.0292 kg/s and further increase in coolant rate reduces the performance. The thermal performance of a two-phase thermosyphon solar collector with various refrigerants was reported by Esen and Esen [3] The thermal performance of the heat pipe with R410A as working fluid was found to be better compared to other refrigerants. The maximum collector efficiencies of solar collector throughout the day were found to be 50.84%, 49.43% and 48.72% for R410A, R407C and R-134a, respectively. The thermal performance of solar collector by using various refrigerants such as: R-134a, R12, and ethanol was reported by Enaburekhan and Yakasai [4]. The heat pipe that utilizes R-134a as working fluid exhibits maximum collection efficiency compared to other working fluids. Since thermosyphon utilizes phase change of the working ﬂuid to transfer heat, selection of a working ﬂuid is essential to achieve the maximum heat transfer capacity. Initially, Choi [5] had made an attempt to use nanofluid (nanofluid-suspension) in thermal engineering due to its anomalous heat transfer characteristics. The colloidal suspension of nanoparticles in the base fluid with uniform dispersion is termed as nanofluid. Because of its enhanced heat transfer characteristics, nanofluids have been used in thermosyphon to improve the thermal performance. Hung et al. [6] reported effect of concentration on heat pipe performance charge with water/Al2O3 nanofluid with three different concentrations (0.5, 1.0, and 3.0 wt %), the maximum heat flux apparently increase with the increase of the mass concentration when the mass concentration is less than 1.0 wt. %. Then, they begin to decrease slowly after the mass concentration is over 1.0 wt. %,they also studied the thermal performance of a heat pipe with charged volume ratio of the working fluid (20%, 40%, 60%, and 80%). Results show that 40% charge heat pipe shows better performance. The thermal performance of the closed two-phase thermosyphon by using various nanofluids (titanium-ethanol and titanium-water suspension) was reported by Naphon et al. [7]. The heat transfer enhancement of the thermosyphon with titanium-ethanol suspension was found to be 10.6% higher compared to the heat pipe with ethanol.Moravej et al. [8]investigated the effect of aluminum oxide nanofluid on the thermal efficiency enhancement of a heat pipe. It observed that Al203 nanofluid has remarkable potential as a working fluid for heat pipe and thermosyphon of higher thermal prefaces.Manimaran et al [9]has been conducted experiment on the heat pipe for various parameters such as angle of inclination and fill ratio. The thermal efficiency for DI water and nanofluid increases when the fill ratio increases and the maximum efficiency are obtained for 75% fill ratio and for an angle of 30o Chougule et al. [10] investigated the thermosyphon heat pipe for nanofluids , surfactant at different inclination angles and found that the performance of the collector increases from 20˚ to 50˚ and further increase in tilt angle reduces the performance.They compared the results of pure water with (2EH) surfactant as working fluid and found an average efficiency of 54%Jung et al. [11]evaluated the effect of 1-octanol, 1-heptanol, 3-octanol and 2-ethyl-1-hexanol (9000 ppm) on mass transfer in aqueous LiBr solution in a falling film absorber. A maximum of 20% enhancement was obtained with 2EH.Kyung and Herold [11]tested additives 2EH by varying its concentrations from 0-500 ppm. Above 80 ppm, there was no significant Gargee A. Pise et al. / Energy Procedia 90 ( 2016 ) 481 – 491 483 enhancement in heat transfer coefficient. The enhancement in heat flux observed was 1.67 times of that without surfactant. Most of the work studies show change in heat pipe solar collector performance also observes with different working fluid in heat pipe. Though the numerous studies are on enhancement of solar heat pipe collector still their results don't quantify properly with optimum quantity and less work is observed for the case of open loop thermosyphon heat pipe. So in view of this, proposed work is to find the optimized coolant rate , tilt angle , effect of the nanofluid concentration , surfactant compared to that of water. In this study, attempt has been made to evaluate the performance of two phase thermosyphon solar collector using water and nanofluid +surfactant , water +surfactant, three concentration of water- nanofluid. In view of this a solar collector with thermosyphon loop heat pipe have been fabricated for the investigation. Nomenclature 2 A c Collector effective area (m ) Subscripts

Cp Specific heat of coolant (J/kgK) i Inlet It Total solar radiation intensity (W/m2) o Outlet T Temperature (ºC) p plate Qs Heat supplied (W) a ambient Qg Useful heat gain (W) hp heat pipe τα Absorptance-transmittance product s supplied

FR Heat Removal factor e evaporator

UL Overall heat loss coefficient c condenser coll Collector g gain L Length w Width Greek symbol T Thickness ƞ Efficiency S Surfactant ( 2 Ethyl 1 hexanol) Ɵ Tilt angle C Nanomaterial concentration

2.Experimental Method

2.1 Experimental Set-Up

Figure 1 shows the schematic of the solar collector with open loop thermosyphon heat pipe. The collector consists of a absorber plate brazed to the loop heat pipe filled with working fluid. The design parameters of various

Fig. 1 Schematic of thermosyphon solar collector. 484 Gargee A. Pise et al. / Energy Procedia 90 ( 2016 ) 481 – 491

components heat pipe flat plate collectors are summarized in Table 1. Thermosyphon loop heat pipe made of copper tubes (OD x L; 12 x 675 mm) is fabricated. Absorber plate (l x w x t ; 600 x 540 x 1mm) made of copper were painted mat black to enhance the ability of absorbing the solar radiation. The heat pipe was brazed to the absorber plate at 120 mm pitch. The condenser exchanges heat energy from heat pipe to coolant, thick insulation glass wool behind the absorber plate was used in order to avoid any loss of energy to the surroundings. Transparent glass was used as the upper glazing for the collector.

2.2 Experimental Procedure

In the present study Cu-constantan thermocouples, pyranometer, and rotameter were used to measure (inlet, outlet, plate, air) temperature, solar irradiation and coolant flow rate, respectively. The mass flow rate of cooling water was measured by using a rotameter with an accuracy of ±0.1 l/min. and also measured by using a stop watch with ±0.01 s accuracy and by using a graduated glass bottle of tolerance ±1 cm3. Temperature indicator was used for measurement. The instantaneous value of global solar radiation intensity was measured by the pyranometer (Model-DWR 8101, Make-DynaLab) having accurancy ± 1 W/m2.

Table 1. Specifications of the Collector.

1. HEAT PIPE (single pipe) Details 3. Cover plate Details

LHP 675 L*w*t 660*540*3 Le 600 Material Glass Lc 75 5. Glass cover to absorber spacing 70mm

do 12 7. Insulation

di 10 Thickness 30mm Material Copper Density 48kg/m³ 2. Absorber Plate Details (mm) Material Glass wool L*w*t 600*480*1 8. Tube spacing 120mm

Material Copper

1. HEAT PIPE (single pipe) Details 3. Cover plate Details

LHP 675 L*w*t 660*540*3 Le 600 Material Glass

Lc 75 5. Glass cover to absorber spacing 70mm

do 12 7. Insulation di 10 Thickness 30mm Material Copper Density 48kg/m³ 2. Absorber Plate Details (mm) Material Glass wool

L*w*t 600*480*1 8. Tube spacing 120mm

The solar collector was installed at a tilt angle stand facing south and tested for outdoor conditions of Pune, India (latitude 18.53 N; longitude 73.82 E). Initially experiments were carried out as per the ASME standards from 10.00 am to 4.00 a.m for varying coolant rates then experiments were carried for tilt angles ranging from 18.53˚ to 60˚ with the horizontal. During each test run the solar radiation intensity (It), ambient air temperature (Ta), inlet cooling water temperature (Ti) , outlet cooling water temperature (To) and plate temperature (Tp) of collector were recorded at half hour interval. The experiments were carried into three groups: for varied range of coolant rates from 2-14 kg/hr at standard tilt angle of 33.53˚ ,varied tilt angles from 18.53˚ -60˚, varied working fluids (water , nanofluid with three concentration 0.05 wt% , 0.25 wt%,0.5 wt%, nanofluid + surfactant , water + surfactant and with no working fluid charged into it. Gargee A. Pise et al. / Energy Procedia 90 ( 2016 ) 481 – 491 485

3 Theoretical Analysis

Figure 2 shows solar flat plate heat pipe collector without TES unit, designed and fabricated for experimental study. The collector is theoretically modelled so as to validate with the experimental results.

Fig. 2 Sectional view of Solar Heat Pipe Collector.

3.1 Assumptions

The following assumptions have been made to facilitate the analysis. The assumptions limit the accuracy with which collector performance can be analytically predicted. • The system is considered in steady state conditions. • The temperature gradient in the longitudinal direction of the collector can be neglected. • Temperature gradient in absorber plate is zero.

3.2 Solar Collector Efficiency

The well-known basic formula to evaluate Performance of solar collector is used to find theoretical efficiency,

(1)

For a single glazed flat-plate collector the useful energy gained by the absorber, is the difference between solar energy absorbed and the heat loss to the ambient over the length of the absorber. Under steady-state conditions, the useful heat delivered by a solar collector is equal to the energy absorbed by the heat transfer fluid minus the direct or indirect heat losses from the surface to the surrounding. The transitivity , absorptivity and the overall losses are calculated considering all the four sides of the collector.[12]

Useful heat gain (Qu) = solar energy absorbed- heat loss to the ambient (2)

Fig. 3. Different Losses from Collector

Solar energy absorbed= (3) Heat loss to the ambient= (4)

So, Ul = Top loss + Side loss + Bottom loss From Eqn (2) - (4) and considering the heat removal factor 486 Gargee A. Pise et al. / Energy Procedia 90 ( 2016 ) 481 – 491

) ] ] (5)

Heat supplied is given by Eqn (2)

Putting value of Qu and Qs into Eqn (1),

ɳth (6)

Using Eqn 6 it is possible to find theoretical value of collector efficiency using instantaneous value of Tp, Ta and It.

3.3 Data reduction

The rate of thermal energy input (Qin), the rate of the thermal energy gain (Qg) and the instantaneous efficiency (η) of each collector were calculated as below [12],

Qin = It Acoll (7) Qu = m x Cpx ∆T (8) η = Qu/Qin (9) The useful energy gain (Qu) and the efficiency (η) of the collector were calculated by using various basic quantities such as: mass flow rate of coolant, temperature of inlet and outlet coolant and solar radiation intensity. Considering the errors of various basic quantities, the maximum uncertainty of the energy gain (Qu) and the efficiency (η) of the collector were found to be less than 3% and 6%, respectively.

4. Preparation of Nano-Fluids

Nanofluids were prepared by using two-step method to produce uniform and stable fluid. Nanoparticles of Al2O3 were purchased from Intelligent Materials Pvt Ltd. Haryana (India) with nominal average particle diameter of <50 nm. Thermo physical properties of these nanoparticles are shown in Table 2. Nanofluid preparation of Al2O3–water, is as follows: (1) weight the mass of Al2O3 nanoparticles by a digital electronic balance; (2) put Al2O3, nanoparticles into the weighed distilled water gradually and agitate the Al2O3–water mixtures separately; (3) sonicate the mixtures continuously for 2 h at 1500 RPM and 24 kHz to produce uniform dispersion of nanoparticles in distilled water. The nanofluids were made in different mass fractions (0.05%, 0.25%, and 0.5%) and no surfactant was used as they may have some influence on the thermo physical properties of nanofluids. Fig. 4 shows a sample image of magnetic stirrer of Al2O3 -water nanofluid.Fig. 5 shows a sample image of Al2O3 - water nanofluid kept for ultra sonification.

Fig 4 Magnetic Stirring of Al2O3 Fig 5 Ultrasonification of Al2O3

Gargee A. Pise et al. / Energy Procedia 90 ( 2016 ) 481 – 491 487

Table 2 Thermo physical properties of Al2O3 nanoparticles (Intelligent Materials Pvt Ltd)

C(J/kg) K) α ρ (kg/m3) k (W/m K) 765 1317 3970 40

5. Results and Discussion

Experiments were carried out to investigate the optimal thermal performance of heat pipe solar collector at different coolant rates , tilt angles , working fluids for heat pipes such as: pure water, water-nanofluid , nanofluid- surfactant , water- surfactant. The variation of various parameters , namely, the rate of solar energy input, ambient air temperature, inlet cooling water temperature, outlet water temperature and plate temperature for thermosyphon heat pipe flat plate solar collectors at 33.53˚ and 50˚ tilt angle, effect of working fluid, no fluid charged in the heat pipe and validation with the theoretical analysis are shown in Figs. 6-14.

5.1 Solar Intensity

Fig 6 shows the variation of solar intensity for different tilt angle with horizontal. It shows that solar intensity increases with time and reaches its maximum value and then falls down with time. It resembles nature of a parabola. The maximum value of solar intensity was observed at 18.53˚ at Pune location (latitude 18.53° N; longitude 73.8°E) and reduces as angle increases.

Fig 6 Variation of Solar Intensity with tilt angle

Fig 7 Variation of Instantaneous Efficiency with coolant rate.

488 Gargee A. Pise et al. / Energy Procedia 90 ( 2016 ) 481 – 491

5.2 Effect of coolant rate

Figure 7 shows the variation of instantaneous efficiency with different coolant rate at standard tilt angle of 33.53˚ for water as working fluid. From nature of graph it is observed that efficiency of the collector is minimum at coolant rate 2 kg/hr and increase in efficiency is observed with coolant rate up to 12kg/hr and it starts reducing with further increase in coolant rate. Further increase in the coolant rate results into subcooling of the condenser section which results in increase on evaporator section.

5.3 Effect of tilt angle

Tests were performed for several tilt angles in the solar heat pipe collector. Initially collector was charged with pure water and tests were taken for different tilt angles from 18.53˚-60˚.Fig 8 shows the average variation of efficiency for different tilt angles.It is observed that the efficiency increases upto an angle of 50˚ and then decreases.It may be because of increasing tilt angle increases, buoyancy force on up going vapour and gravity force on down coming liquid which gives rise to enhancement in performance with angle. But after 50° increase in angle (60°) reduces the performance because, the gravitational force accelerates hindering the heat transfer process at the condenser end. Fig 9 and fig 10 shows the variation of temperatures with time at 33.53˚ and 50˚ tilt angle.It is observed that the temperature difference between inlet and outlet coolant temperature is higher for 50˚ tilt angle than 33.53˚ tilt angle.

Fig 8 Average Variation of Efficiency with tilt angle.

Fig 9 Temperature variation with time at 33.53˚tilt angle. Fig 10 Temperature variation with time at 50˚tilt angle

5.4 Effect of working fluid

Fig 11 shows the average variation of efficiency with working fluid : water ,water-nanofluid (0.05% wt , 0.25% wt , 0.5%wt ), nanofluid +surfactant , water +surfactant. Water shows lower performance and 0.5 wt% nanofluid shows highest performance for both the tilt angles. This may be due to performance of thermosyphon heat pipe Gargee A. Pise et al. / Energy Procedia 90 ( 2016 ) 481 – 491 489 governed by the formation of vapor bubble at the liquid–solid interface with the nanofluids the thermal resistance reduces. In the case of water a larger bubble nucleation size creates a higher thermal resistance that prevents the transfer of heat from the solid surface to the liquid and retard the performance. Also the nanoparticles suspension in the fluid has significant effect on the enhancement of heat transfer due toits higher heat capacity and higher thermal conductivity of working fluid. Therefore, the heat pipe thermal efficiency heat pipe increases with nanofluids as compared to that of the base working fluids like water, ethylene glycol etc.The presence of the surfactant in the nanofluid enhances the performance by acting as a catalyst.The mixture of surfactants and nanofluids gave better results as compared to water+ nanofluid may be because the presence of surfactant helped to stabilize the nanofluid resulting into better performance.It is found that the water with surfactant shows best results because of the reduction in the surface tension reducing the boiling point ultimately resulting into the decrease of evaporator load.Fig 12 shows show the % enhancement observed because of different working fluids compared to that of pure water for collector at 33.53˚ and 50˚ tilt angles. The maximum enhancement is observed for water-surfactant working fluid. Presence of surfactant in water reduces the surface tension which ultimately results into reduction of boiling point, so the power input required by the evaporator reduces. Requirement of less power input increases the performance. Almost 45 % enhancement compared to pure water was observed for collector with water+ surfactant as working fluid.

Fig 11 Average variation of efficiency with different working fluids.

Fig. 12 Percentage enhancement variation for different working fluids compared to pure water at Ɵ=33.53˚ and Ɵ=50˚.

5.5 No Working Fluid

To validate the dependency of heat pipe working on the working fluid presence test was conducted without charging any working fluid inside the collector. There was negligible temperature difference obtained as there was 490 Gargee A. Pise et al. / Energy Procedia 90 ( 2016 ) 481 – 491

only conduction mode of medium left for the transfer of heat to the coolant which hardly had any effect on the working of the heat pipe .

Fig. 13 Variation of instantaneous efficiency for collector with no working fluid charged into it at Ɵ=33.53˚.

5.6 Validation

Validation of experimental results are done with theoretical result obtain from theoretical analysis. From Eqn 6 instantaneous theoretical efficiency of solar collector can be calculated by knowing instantaneous value of plate temperature and ambient temperature. Figure 14 shows that experimental and theoretical value of instantaneous efficiency for water as a working fluid and 33.53° inclination angle with 12 kg/hr coolant rate of collector. The theoretical value of efficiency is slightly higher than that of experimental value cause number of assumption made in during theoretical analysis does not practically exist, also value of glass transitivity and absorptivity of absorber plate may vary from standard one. Fig 14 shows that the theoretical efficiency is almost equal to the experimental efficiency which states that the experimental values are matching the theoretical the reason may be that most of the conditions which are considered in the theoretical analysis, factors like heat loss coefficient, collector efficiency factor, friction factor etc are considered dependent upon time.

Fig. 14 Time variation of instantaneous efficiency for theoretical and experimental analysis at Ɵ=33.53˚.

5.7 Effect of Heat Loss factor:

The magnitude of heat removal factor (FRUL) and heat absorption factor ( FR(τα)) for water, water-nanofluid , Water-Surfactant working fluids are obtained in the present investigation and summarized in Table 3.It is observed that collector which uses water-surfactant , water-nanofluid as the working fluid exibits highest FR (τα ) and lowest (FRUL). This indicates that the thermal performance of solar collector with water-nanofluid is higher than water as working fluid in heat pipe.

Table 3 FRτα and FRUL values of collector for different working fluids. Working fluid Pure water Al2O3/ Water nanofluid Water + Surfactant

FRUL 2.07 1.88 1.76

FR (τα) 0.31 0.40 0.49 Gargee A. Pise et al. / Energy Procedia 90 ( 2016 ) 481 – 491 491

Conclusions

From the experimental investigation of thermosyphon flat plate solar collector from the variation of coolant rate, tilt angles, different working fluids following conclusions are drawn. • Performance of heat pipe solar collector depends on coolant flow rate, tilt angle and working fluid. • Increasing the coolant rate increases the thermal performance of heat pipe collector upto certain limit after that, increase in coolant rate has no effect on performance, coolant rate increased upto 12 kg/hr further increase decreased the performance. • The heat transfer rate through heat pipe collector increases from 18.53˚ -50˚ further increase in tilt angle reduces the performance, maximum efficiency of 36.072% was found at 50˚ tilt angle.

• Water-Al2O3 showed better performance compared to pure water, Enhancement observed for collector compared to pure water at 0.05 wt% , 0.25 wt%,0.5 wt% was found to be 3.79% , 10.72%,15.24%. • Water- Al2O3 + surfactant showed better performance compared to pure water , Enhancement observed for collector compared to pure water and nanofluids, the enhancement compared to water was found to be 20%. • Water-surfactant showed better performance compared to rest all of the working fluids The enhancement compared to water was found to be 45.23%.

Acknowledgement

The authors are indebted to the financial support from Govt College Karad from TEQIP , All India Shri Shivaji Memorial Societies College of Engineering, Pune and Pimpri Chinchwad College of Engineering, Pune for providing the necessary resources to complete this study.

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