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Water nanofluid
Case Studies in Thermal Engineering 14 (2019) 100416
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Energy and exergy analysis of a thermosiphon and forcedcirculation flat-plate solar collector using MWCNT/Water nanofluid
T
Mahmoud Eltaweela,∗, Ahmed A. Abdel-Rehima,b a b
The British University in Egypt, Cairo, Egypt Shoubra Faculty of Engineering, Benha University, Cairo, Egypt
A R T IC LE I N F O
ABS TRA CT
Keywords: Flat-plate solar collector Solar energy Thermosiphon Nanofluids Energy efficiency Exergy efficiency
The scientific community has focused on the use of nanofluids to enhance solar collectors' thermal efficiency. Solar energy is considered an extensively available energy resource with a weak effect on the environment. The flat-plate solar collector is considered to be the most common collector used, but the efficiency of the collector has limitations, such as the low effectiveness of the absorber in terms of energy absorption and the transmission of that energy to the working fluid. The flat-plate solar collector can be enhanced by replacing the working fluid with a nanofluid. Nanofluids are considered new media that exhibit thermophysical properties superior to those of ordinary working fluids. In this study, the effect of using multi-walled carbon nanotubes in a thermosiphon and forced-circulation flat-plate solar collector was investigated experimentally. A set of different concentrations was tested (0.01 wt%, 0.05 wt% and 0.1 wt%) with distilled water. The experimental results show an enhancement of the system's exergy efficiency and energy efficiency when a nanofluid was used compared with those attained using distilled water in both forced and thermosiphon systems.
1. Introduction Developing clean and efficient energy sources is a goal of many scientists and engineers. The solar energy that the earth receives within 1 h can cover the total energy needs of humans over an entire year [1]. That energy must be collected, and with the use of the appropriate technology, solar energy can be the most efficient energy resource worldwide. The average amount of solar radiation received per day in Egypt is between 19.9 and 23.1 MJ/m2, which is considered to be a relatively high value compared with that received in Europe [2]. Solar energy can be converted to thermal energy using a special type of heat exchanger as a thermal solar collector. Many different types of solar collectors have been developed; flat-plate solar collectors are the most common solar heaters. Such collectors have been developed since the 1980s without any significant change in their design. Flat-plate solar collectors are the cheapest method for solar applications, but the problem is that they have a low thermal efficiency and low outlet temperature. Over the past decade, most research has focused on using nanotechnology in various applications due to the favourable properties afforded. By adding a weighted fraction of nanoparticles to a soluble substance such as water, the heat transfer of a liquid can be significantly increased. Sunlight is absorbed by the flat plate, which is called the absorber, and heats the fluid in the tubes. Flat-plate solar
∗
Corresponding author. Department of mechanical engineering, The British University in Egypt, Cairo, Egypt. E-mail address: [email protected] (M. Eltaweel).
https://doi.org/10.1016/j.csite.2019.100416 Received 14 January 2019; Received in revised form 13 February 2019; Accepted 16 February 2019 Available online 19 February 2019 2214-157X/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
collectors can increase the fluid temperature by 30–80 °C. Collectors of this type consist of an absorbing surface, a trap for radiation loss, a heat transfer medium and thermal insulation behind the absorber surface [3]. A nanofluid consists of a small fraction of nanoparticles mixed with a liquid. This liquid could be water, oil or ethylene glycol. That small fraction of particles can completely change the properties of the liquid with respect to heat transfer. Metals usually have a higher thermal conductivity than fluids do. Therefore, placing metallic particles in a fluid produces a good heat-conducting liquid. However, the number of nanoparticles used in a nanofluid has must low, and the particles should not be larger than 100 nm to avoid problems such as creating an unstable mixture. Another reason is that large particles can lead to a pressure drop because the particles might stick to the walls of the surrounding container. The use of nanofluids offers many advantages, such as increasing heat conductivity, which in turn enhances heat transfer performance. In addition, nanofluid particles are so small that they can easily move through solid blocks. One of the main characteristics of nanoparticles is their large surface area-to-volume ratio, which increases the heat transfer between the solid particles and the fluid; in addition, the particles’ high thermal capacity and small size lead to low energy loss, increasing system performance. Nanoparticles also enhance the turbulent motion of fluids [3]. 1.1. Thermal performance Natarajan and Kiatsiriroat [4]experimentally investigated the use of multi-walled carbon nanotubes (MWCNTs) as a component of a working fluid in a solar plate collector and observed that MWCNTs were better than most other nanoparticles, particularly at high inlet temperatures. Faizal et al. [5] reported that by using MWCNT/water as a working fluid, the collector size can be reduced by 37% of its original size while maintaining the same efficiency. Said et al. [6] reported that by using single-walled carbon nanotubes (SWCNTs) mixed with water, the solar collector efficiency could be increased by 95.12%, but due to the high cost of SWCNTs, it is better to use MWCNTs because of their relatively similar efficiency and lower cost. Otanicar and Golden [7] performed economic and environmental experiments using nanofluids in flat-plate solar collectors and found that the use of nanofluids can increase the payback period of collectors but will provide the same economic savings due to the low cost of nanofluids. Nanofluids have also been found to contribute 3% more pollution than other working fluids. Ebrahim and Bajestan [8] performed a numerical analysis of using carbon nanotubes in working fluids (water and EG) at concentration ratios ranging from 0 to 6%. The authors observed temperature differences and indicated the best Reynolds number, pressure drop and pump power. The results showed that increasing the concentration will increase the heat transfer coefficient; additionally, the heat transfer coefficient was observed to scale inversely with the nanoparticle diameter. Using carbon nanotubes mixed with water at a Reynolds number of 1460 and a diameter-to-length ratio greater than 100 at a concentration of 0.038%, the temperature difference was 0.9 °C, and a pressure drop of 9625 Pa was observed at a pump power of 0.16 W. Raj et al. [3] found that the highest efficiency was achieved when carbon nanotubes were used. Sharafeldin and Grof [9] found that the volume fraction and the mass flux rate are directly proportional to the collector efficiency; with a mass flux rate of 0.019 kg/s m2 and volume fraction of CeO2 equal to 0.066%, the efficiency can increase by 10.74% compared to that attained using water. 1.2. Nanofluid stability Preparing nanofluids is a technical challenge because aggregation can occur due to van der Waals interactions. Stability can be achieved by chemical or physical treatment. For example, a surfactant can be used to increase stability; however, surfactant use is restricted to low-temperature applications because at high temperature, bubbles that affect the heat transfer efficiency significantly can form. Stability can also be achieved by applying a force on clusters of suspended particles [10]. Eastman et al. [11] examined the stability of copper nanoparticles in ethylene glycol at a concentration of 0.3% and found that after 2 days of preparation, the fluid exhibited higher thermal conductivity than that observed after 2 months of storage, due to the decrease in dispersion stability over time. Madni et al. [12] investigated the stability of nanofluids, demonstrating that one suitable way to enhance stability is by adding surfactants through an approach called non-covalent functionalization. Kilic et al. [13] studied the effect of using a 2 wt% titanium dioxide/water nanofluid mixed with 0.2 wt% Triton X-100 as a surfactant to maintain stability and eliminate the problem of agglomeration over time. The instantaneous efficiency of the corresponding collector was determined to be 36.2% for pure water and 48.67% for the mixture used. Raj et al. [3] reported that long-term stability and suitable dispersion of nanoparticles is fundamental to achieving sufficient absorption of solar radiation and increasing solar collector efficiency. 1.3. Challenges of using nanofluids Li et al. [14] investigated the effect of nanoparticle concentration with respect to viscosity and found that variations in concentration are directly proportional to those in viscosity. In addition, pumping power and pressure drop could be affected by these parameters. Mahbubul et al. [15] studied the viscosity characteristics of nanofluids by comparing the effects of different parameters on viscosity, such as preparation methods, nanoparticle size, concentration effect and temperature. They discovered that viscosity mainly depends on concentration and temperature differences. Nanoparticles tend to bond with each other, thereby preventing a high surface-to-area ratio. To overcome this limitation, particle dispersion additives are added to fluids containing nanoparticles. As a result, the surface properties of nanoparticles are altered, and the corresponding nanofluid will contain a high level of impurities. Research has countered the fluctuation of properties by limiting sample nanofluid volumes to less than 100 ml [16]. One of the problems with using nanofluids is sedimentation, which might occur during operation. To avoid this problem, it is preferred to use nanomaterial concentrations of less than 0.5% [17]. Although most researchers have performed experiments in nearly the same 2
Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
manner, their results consistently vary [18]. Additionally, collector design affects device performance, from the collector size to the different materials used at the experiment location, all of which affect experiments and alter results [19]. Raj et al. [3] found that increases in volume fraction are directly proportional to increases in collector efficiency; however, beyond a certain volume fraction, efficiency decreases, considering the corresponding degradation of the heat transfer rate and increase in viscosity. 1.4. Exergy efficiency in solar collectors Hepbasli [20] defined exergy as the maximum output that a system can achieve relative to the surrounding temperature. Energy system efficiency, design and simulation can be evaluated via exergy analysis, and high exergy efficiency is the key to obtaining sustainable development. Additionally, exergy analysis can determine whether a system is capable of increasing energy efficiency by reducing inefficiencies in existing systems. Luminosu and Fara [21] numerically studied the optimal operation of the flat-plate solar collector using exergy analysis, considering the thermal loss coefficient, its overall heat removal factor and other heat transfer coefficients to be constants. Bejan et al. [22] reported that entropy generation or exergy during heat transfer can be maximized in efficient solar power systems for thermodynamic optimization and could also help increase the efficiency of existing systems. Saha and Mahanta [23] investigated the thermodynamic optimization of a FPSC using entropy minimization. Kar [24] studied the optimum operation and efficiency of FPSC in terms of exergy. Shojaezadeh et al. [25] studied the effects of changing certain parameters on the exergy efficiency of a FPSC with an aluminium oxide/water nanofluid; the parameters observed to have an effect were ambient temperature, solar radiation, inlet fluid temperature, nanoparticle volume concentration and mass flow rate. The results showed that by using an aluminium oxide/water nanofluid, the maximum collector exergy efficiency was only increased by one percent. Said et al. [26] performed an exergy analysis of flat-plate solar collectors using a TiO2 /water nanofluid with polyethylene glycol as a surfactant to increase stability. The results showed that the system's energy efficiency could be increased by 76.6% and the exergy efficiency increased by 16.9% at a concentration of 0.1 vol%. Another study performed by Said et al. [27] examined the use of SWCNTs at a concentration of 0.3 vol% in water in a FPSC and its effect on thermophysical properties. It was found that the energy and exergy efficiency increased, with the energy efficiency increasing by 95.12% and the maximum exergy efficiency reaching 26.25%. The present work investigates the first and second laws of thermodynamics on a flat-plate solar collector with thermosiphon and forced circulation. Three different concentrations of multi-walled carbon nanotubes with water were examined and compared. The use of MWCNT/water nanofluid in FPSCs is an attractive topic which was investigated by several researchers, however there is a very limited publications which investigated this topic on thermosiphon principle. Similarly, up to the authors knowledge, there is no publications investigated the energy and exergy of a thermosiphon FPSC with the use of MWCNT/water nanofluid. Also, in this paper the effect of using thermosiphon and forced circulation were investigated on the same collector to minimize the errors as possible and have a reliable comparison, relative to previous work where experiments were done on two similar collectors with two types of circulations. 2. Experimental devices and methodology The purpose of this work was to investigate the effect of using an MWCNT/water nanofluid as the working fluid in a thermosiphon FPSC and its effect on energy and exergy efficiency. The system used in the experiment is a commercial FPSC placed on the roof of The British University in Egypt, El Shorouk, Cairo, Egypt. First, solar irradiation at the location was measured from 9:00 a.m. to 4:00 p.m. on the 24th of May 2018. Fig. 1 shows that solar irradiation on the 24th of May 2018 ranged from 520 W/m2 to 914 W/m2 . The FPSC used and a schematic diagram of the system is shown in Fig. 2. The specifications of the collector are presented in Table 1. The system tank, which has a capacity of 200 L, functions as a heat exchanger that absorbs heat from the collector. The tank has a heat exchanger that is connected to the collector to create closed circulation for the working fluid. This arrangement allows for the possibility of heat exchange between the collector fluid and the tank fluid, as shown in Fig. 2. The system circulation uses natural convection from the temperature difference between the inlet and outlet of the collector; thus, there was no need for a pump in the system when the thermosiphon was tested. A pump was used in the system when forced circulation was tested. Sensors were installed to measure the required parameters for evaluating the system's performance. First, 2 PTN thermocouples sensors were installed at the inlet and outlet of the collector, and a flow meter (YF-S201) was installed before the inlet to measure the flow rate of the fluid. For weather data, OMEGA type FMA 1000 series sensors were used to measure ambient temperature and wind speed and to measure the intensity of solar radiation. A PYR 1307 solar irradiation sensor was used with the same slope and position as the collector. The sensors were read every 5 min from 9:00 a.m. to 4:00 p.m. over several days. The nanofluid preparation method used was a two-step technique, to increase system stability and durability [28]. Distilled water was used in this study for both the distilled water experiment and the nanofluid experiment as the base fluid. Three concentrations were used in this study: 0.01%, 0.05% and 0.1% by weight fraction of MWCNTs. An ultrasonic homogenizer, also called a sonicator, type (Hielscher UP400s) was used to prepare the nanofluid samples for 2 h, 4 h and 8 h for 0.01%, 0.05% and 0.1% concentrations respectively, after a 1 h stirring in a magnetic stir for each concentration, the sonicator used ultrasonic waves with pulses of 0.5 s on and 0.5 s off at an amplitude of 60% and frequency of 24 kHz until each sample was completely dispersed. The specifications of the MWCNTs are shown in Table 2. 3
Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
Fig. 1. Radiation measurement for the selected location in May 2018.
Fig. 2. The Flat-plate Solar Collector (a): Photograph of the solar collector used. (b): Schematic illustration of the system used. (c): The tank used. Table 1 Specifications of the flat-plate solar collector used. Specifications
Dimensions
Absorption area
2 m2 3L 7 80% 4 mm Copper 12% 96%
Working fluid capacity Number of riser tubes Glass transmissivity Glass thickness Absorber plate material Absorber emissivity Absorber absorptivity
4
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Table 2 MWCNT specifications. MWCNT properties Specifications
Value
Purity Diameter Length Density
95 wt% 10–40 nm 20 μm
Specific surface area
460 m2/ gm 6150 W/m.K
1.3–2.4 gm/cm3
Thermal conductivity
3. Efficiency calculations 3.1. Energy efficiency The collector's useful energy can be calculated by either of the following two equations:
˙ P (Tout − Tin ) Qu = mC
(1)
Qu = AC FR [IT (τα ) − UL (Tin − Tamb)]
(2)
where m˙ is the mass flow rate, CP is the specific heat capacity of the fluid, Tout is the working fluid outlet temperature, Tin is the working fluid inlet temperature, AC is the collector area, IT is the total radiation, τ is the transmissivity of the glass, α is the absorptivity of the collector and Tamb is the ambient temperature. The heat removal factor (FR) can be calculated using the following equation:
˙ P (Tout − Tin ) mC AC [IT (τα ) − UL (Tin − Tamb)]
FR =
(3)
The total irradiation the collector receives can be calculated using the following equation: (4)
QT = Ac IT The specific heat of the nanofluid can be calculated using the following equation:
CPNF = CPNP (φ) + CPW (1 − φ)
(5)
where CPNF is the nanofluid specific heat, CPNP is the nanoparticle specific heat, CPW is the distilled water specific heat and φ is the concentration weight fraction. The weight fraction (φ ) can be calculated using the following equation:
φ=
Vnp Vnp + Vw
(6)
where Vnp is the nanoparticle volume and Vw is the water volume. The volume of the nanoparticles (Vnp ) can be calculated using the following equation:
Vnp =
Mnp ρnp
(7)
where Mnp is the nanoparticle mass and ρnp is the nanoparticle density. The collector instantaneous efficiency can be calculated as follows:
η=
Quseful QT
=
˙ P (Tout − Tin ) mC (T − Tamb) ⎤ = FR ⎡ (τα ) − UL in ⎥ ⎢ Ac IT IT ⎦ ⎣
(8)
3.2. Exergy efficiency In studying the exergy efficiency of a system, certain assumptions must be made, such as the system must operate in a steady state and under steady flow, the entrance effect is the only loss coefficient considered, and heat transfer to the system and work transfer from the system are positive. Onsager and Kreuzer [28,29]stated that because of viscous friction and heat transfer as a function of the design variables selected, it is extremely important to measure the entropy generated or the exergy destroyed. S. Farahat [30] reported that typical exergy stabilities can be expressed in rate form, as indicated in the equation below, where the effects of kinetic and potential energy deviations are ignored. 5
Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
˙ in + Ex ˙ out + Ex ˙ st + Ex ˙ leak + Ex ˙ dest = 0 Ex
(9)
˙ in is the inlet exergy rate to the system, Ex ˙ out is the output exergy rate from the system, Ex ˙ st is the stored exergy rate for the where Ex ˙ leak is the leakage exergy rate for the system, and Ex ˙ dest is the exergy destruction rate. system, Ex ˙ dest ) can be defined as follows: The exergy destruction rate (Ex
˙ dest = Ta S˙ gen Ex
(10)
˙ in ) is composed of a fluid flow component (Ex ˙ in, f ) and an absorbed solar energy component (Ex ˙ in, s ) The inlet exergy rate (Ex [29,30]: m˙ ΔPin ˙ in, f = mC ˙ p, NF ⎛Ti − Ta − Ta ln Ti Ta ⎞ + Ex ρ ⎝ ⎠
(11)
˙ in, s = IT AC ⎛1 − Ta ⎞ Ex Ts ⎠ ⎝
(12)
( )
⎜
⎟
where Ts is the surface temperature of the sun, which is equal to 5770 K, and Pin is the fluid pressure at the inlet. ˙ out ) features a fluid flow component and is computed as the inlet fluid exergy rate [29,30]: The outlet exergy rate (Ex
m˙ ΔPout ˙ out , f = mC ˙ p, NF ⎛To − Ta − Ta ln To Ta ⎞ − Ex ρ ⎝ ⎠
( )
(13)
where Pout is the fluid pressure at the outlet. A non-isothermal solar flat-plate collector's overall rate of entropy generation, reported by Bejan [31], is defined as follows:
q q ˙ p, NF ⎛ln To Ti ⎞ − abs + loss S˙ gen = mC Ts Ta ⎝ ⎠
( )
(14)
where qabs is the energy absorbed by the collector absorber surface, which can be calculated by the equation qabs = ατ (IT Ac ) , and qloss is the total heat lost to the environment, which can be calculated by the following equation:
˙ p, NF (To − Ti ) qloss = qabs − mC
(15)
The exergy efficiency can be calculated as follows [30,32]:
ηex =
˙ out , f − Ex ˙ in, f Ex ˙ in, s Ex
(16)
Using equations (9) and (10), and (11), the equation can be rewritten as follows:
ηex =
(
( )) − )
m˙ ⎡Cp, NF To − Ti − Ta ln To Ti ⎣ qabs 1 − Ta Ts
(
ΔP
ρ⎤
⎦ (17)
Using equations (12)–(14) the exergy efficiency can also be defined as follows:
ηex = 1 −
Ta S˙ gen ⎡1 − ⎣
( ) ⎤⎦ q Ta Ts
abs
(18)
4. Results and discussion 4.1. Thermosiphon circulation 4.1.1. Energy analysis The fluid used in the experiment was distilled water with a pH value of 7, and the concentrations of the MWCNTs used in the experiment were 0.01 wt%, 0.05 wt% and 0.1 wt%. First, the system efficiency was measured using distilled water. The reduced temperature parameter Ti − Ta IT was plotted on the X axis and the efficiency (η) on the Y axis. Fig. 3 presents a comparison of the efficiency of the thermosiphon FPSC for distilled water and three different concentrations of MWCNT/water nanofluid: 0.01 wt%, 0.05 wt% and 0.1 wt%. The average efficiencies of the collector were 36.54%, 51.29%, 56.41% and 70.67% for distilled water and the 0.01 wt%, 0.05 wt% and 0.1 wt% MWCNT/water nanofluids, respectively. As shown in Fig. 3, the efficiency of the collector increased due to the use of MWCNT/water nanofluid as the working fluid. The nanotubes increased the heat transfer between the collector and the working fluid and enhanced the thermal conductivity and absorptivity of the working fluid. The MWCNTs showed a significant increase in system efficiency with a small concentration fraction of 0.1 wt%, which could reduce the size of the collector by 34%, the collector size reduction can be calculated as [33]. The use of a thermosiphon FPSC affects the mass flow rate of the working fluid due to various factors. The higher the fluid concentration is, the higher the viscosity becomes; in turn, the mass flow rate decreases. In the thermosiphon collector, the mass flow
(
)
6
Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
Fig. 3. Variation of thermosiphon FPSC efficiency for distilled water and nanofluids of various concentrations.
rate is a parameter related to the temperature difference between the inlet and outlet of the collector and solar irradiation. The mass flow rate of the collector with distilled water was at its minimum; however, when MWCNTs were added, the mass flow rate increased, and as the concentration ratio increased, the mass flow rate decreased because of the increase in the fluid viscosity. The mass flow rate for each working fluid were measured, the average value of the mass flow rate throughout the day is shown in Table 3. The heat removal factor of the collector FR was calculated for each weight fraction of MWCNT nanofluid, and the factor was found to have a noticeable effect on the FPSC efficiency, as shown in Table 4. The heat removal factor of the nanofluid is higher than that of distilled water and increasing the weight fraction of MWCNTs increases the heat removal factor. Additionally, the absorbed energy parameter increases. The performance of the collector is enhanced by increasing these factors when using an MWCNT nanofluid. The addition of MWCNTs increased the absorbed energy parameter (FR(τα)) of distilled water by 31.2%, 51.1% and 65.1% at concentrations of 0.01 wt%, 0.05 wt% and 0.1 wt%, respectively. The value of FRUL for distilled water increased by 43.7%, 134.6% and 139.9%, respectively, at MWCNT concentrations of 0.01 wt%, 0.05 wt% and 0.1 wt%.
4.1.2. Exergy analysis The exergy efficiency of the thermosiphon FPSC was investigated. Fig. 4 shows that the exergy efficiency increased with the concentration of the MWCNT/water nanofluid. For MWCNT weight fractions of 0.01%, 0.05% and 0.1%, the exergy increased relative to that of distilled water by 2.18%, 4.98% and 9.039%, respectively. The highest exergy observed, 23.35%, was achieved by the 0.1 wt% MWCNT nanofluid. Fig. 5 shows that the increase in the MWCNT concentration in distilled water increased the exergy efficiency (ηex) of the system, from 14.55% for distilled water to 15.69%, 21.42% and 23.35% for the 0.01 wt%, 0.05 wt%, and 0.1 wt% MWCNTs, respectively, which could be obtained at the lowest (Ta/IT) values. The figure also shows that the increase in the (Ta/IT) value caused the optimum exergy efficiency to decrease exponentially, and the optimum exergy efficiency could be obtained at the lowest (Ta/IT) value.
Table 3 Mass flow rate of the four different working fluids. Nanoparticles Concentration
Mass Flow Rate (L/Min)
0 0.01 wt% 0.05 wt% 0.1 wt%
0.375529 0.5042 0.46835 0.41823
7
Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
Table 4 Values of FRUL and FR(τα) for MWCNT/water nanofluids and water-based working fluid. Weight fraction %
F R UL
FR(τα)
Distilled water 0.01 0.05 0.1
5.216 7.5 12.240 12.515
0.477 0.626 0.7209 0.78765
Fig. 4. A comparison of the change in exergy efficiency over time of the thermosiphon flat-plate solar collector for distilled water and different nanofluid concentrations.
Fig. 5. A comparison of the exergy efficiency and the ratio of ambient temperature to radiation intensity of the thermosiphon FPSC for distilled water and the different nanofluid concentrations.
4.2. Forced circulation A pump was used in the tested collector to study the difference between using forced and natural circulation for the same working fluid. The highest concentration of MWCNTs was used in the experiment for a comparison between distilled water under four different flow rate conditions: 0.5 L/min, 1 L/min and 1.5 L/min and thermosiphon circulation. The efficiency of the collector 8
Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
Fig. 6. Variation of FPSC efficiency for 0.1 wt% MWCNT/water nanofluid at three different flow rates and thermosiphon circulation.
increased by 34.13% using the 0.1 wt% MWCNT/water nanofluid, with an average efficiency of 70.67% for the nanofluid and 36.54% for distilled water with thermosiphon circulation. The average efficiencies of the nanofluid and distilled water were 76.92% and 49.3%, respectively, at a flow rate of 1.5 L/min with an increase in efficiency equal to 27.62%. The system efficiency for a flow rate of 1.0 L/min; the average efficiencies of the nanofluid and distilled water were 63.97% and 37.5%, respectively, with an increase in efficiency equal to 26.47%. The average system efficiencies for distilled water and the nanofluid were 35.4% and 58.17%, respectively, at a flow rate of 0.5 L/min, with an increase in efficiency equal to 22.77%. The highest increase in efficiency for the 0.1 wt % MWCNT/water nanofluid relative to the efficiency observed for distilled water was achieved with the forced circulation system at a flow rate of 1.5 L/min, followed by the thermosiphon circulation system. However, using nanofluid in a thermosiphon FPSC has a greater impact on the efficiency of the FPSC than using distilled water in forced circulation. Fig. 6 shows that the energy efficiency will increase by 35.2%,34.1% and 21.3% when compared with 0.1 wt % MWCNT/water nanofluid in thermosiphon FPSC with forced circulation flow rates 0.5 L/min, 1.0 L/min and 1.5 L/min in the same FPSC, respectively. The present work was compared with previous work done by other researchers for FPSC and MWCNT/water nanofluid (Table 5) and the results show the energy efficiency of the system increase is variable between the particle size of MWCNT and the concentration, however, karami et al. [33] used a very low concentration and the enhancement is close to the present work. Verna et al. [34] also used a similar particle size and a higher concentration but the enhancement is actually lower. MWCNT/water nanofluid with 0.1 wt% concentration achieved a higher efficiency in thermosiphon circulation than in 0.5 and 1 L/min because the temperature difference between the inlet and the outlet is higher in thermosiphon circulation, although the mass flow rate value was between 0.55 and 0.62 from 10:30 a.m. to 2:00 p.m. while the rest of the day the values are low, which gave a low average value of mass flow rate throughout the day. While in forced circulation the temperature difference is lower than thermosiphon circulation throughout the day. The result is a lower efficiency when the flow rate is 0.5 and 1 L/min, while when the flow rate is 1.5 L/min the efficiency increased in the times with low solar radiation on the contrary of thermosiphon. The increase in the reduced temperature parameter Ti − Ta IT reduce the increase in the collector efficiency between the thermosiphon and 1.5 L/min circulation because thermosiphon circulation has a high efficiency when the solar intensity is high.
(
)
5. Conclusions The efficiency of a flat-plate solar collector was investigated experimentally using multi-walled carbon nanotubes with a diameter of 10–40 nm and a length of 29 μm at concentrations of 0.01 wt%, 0.05 wt% and 0.1 wt%. The experiment, performed in Cairo, Egypt, shows that the use of 0.01 wt% MWCNTs increased the efficiency by 16% compared with that of distilled water; for 0.05 wt% MWCNTs, the efficiency increased by 21%, and for 0.1 wt% MWCNTs, the efficiency increased by 34.13% compared with the efficiency attained using distilled water. The experimental results indicate that using MWCNTs in water as a nanofluid will enhance the Table 5 Comparison between the present work and different papers using MWCNT/water nanofluid in FPSC. Researcher
Concentration
Particle size
Energy efficiency enhancement
Present work Verma et al. [34] Hawng et al. [35]
0.1 wt% 0.2 vol% 1 vol%
35.2% 23.47% 7%
Natrajan et al. [4]
1 vol%
Karami et al. [33]
0.015 vol%
10–40 nm diameter, 20 μm length 7 nm diameter, 20 μm length 10–30 nm diameter 10–50 μm length 10 nm diameter 270 nm length 10 nm diameter 5–10 μm length
9
41% 32%
Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
efficiency of the solar collector, and for a concentration of 0.1 wt% MWCNTs, the size of the solar collector can be reduced by 34%. Furthermore, using forced circulation increased the efficiency of the flat-plate solar collector by 6.21% at a flow rate of 1.5 L/min relative to that attained using a thermosiphon. Also, it is concluded that using a nanofluid in a thermosiphon FPSC will increase the efficiency more than using distilled water in a forced circulation FPSC. The experimental results show that the maximum exergy efficiency for the flat-plate solar collector is approximately 23.35% compared to what is achieved with distilled water, i.e., 14.55%. The results also show that the optimum exergy efficiency decreases exponentially with increasing Ta/IT values. The relatively low exergy efficiency shows that the flat-plate collector system still needs improvement. The experimental results suggest that using an MWCNT/water nanofluid in a thermosiphon flat-plate solar collector is a viable solution for enhancing the energy and exergy efficiency. Acknowledgements The authors would like to thank the renewable energy centre in The British University in Egypt for providing the testing facilities. Nomenclature Ac Surface area of the collector CP Heat capacity of the fluid Cp,bf Heat Capacity of the base fluid Cp,np Heat Capacity of the nanoparticles Cp,nf Heat Capacity of the nanofluid ˙ in Ex Inlet exergy rate to the system ˙ out Ex Output exergy rate from the system ˙ st Stored exergy rate for the system Ex ˙ leak Ex Leakage exergy rate for the system ˙ dest Ex Exergy destruction rate FR Heat Removal Factor IT Total Radiation ˙ m Mass flow rate Mass of the nanoparticles Mnp Density of the nanoparticles ρnp Fluid pressure at the inlet Pin Fluid pressure at the outlet Pout QT Total Irradiation received by the collector Quseful Useful Energy Gain Tamb Ambient Temperature Tin Inlet Fluid Temperature of the collector Tout Outlet Fluid Temperature of the collector Sun temperature Ts UL Overall heat loss coefficient of the solar collector Vnp Volume of nanoparticles Vw Volume of water Greek Symbols τ α ε η β φ θ Subscripts
Transmissivity Absorptivity Emissivity Efficiency of the Flat-Plate Solar Collector Slope of the collector (Inclination Angle) weight Fraction Latitude
Bf FPSC MWCNT Nf Np
Base-fluid Flat-plate solar collector Multi-walled carbon nanotubes Nanofluid Nanoparticle
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Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
References [1] Frank Kreith, Kreider, Principles of Solar Engineering, Hemisphere Pub. Corp., Washington, 2014 Jan F. [2] M.J. Muhammad, I.A. Muhammad, N.A.C. Sidik, M.N.A.W.M. Yazid, R. Mamat, G. Najafi, The use of nanofluids for enhancing the thermal performance of stationary solar collectors, Renew. Sustain. Energy Rev. 63 (2016) 226–236. [3] P. Raj, S. subudhi, A review of studies using nanofluids in Flat-Plate And direct absorption solar collectors. Renew. Sustain. Energy Rev. 84 (2018) 54–74. [4] E. N., R. S., Role of nanofluids in solar water heater, Int. J. Adv. Manuf. Technol. (2009) 1–5, https://doi.org/10.1007/s00170-008-1876-8. [5] M. F., R. S., S. Mekhilef, Potential of size reduction of flat-plate solar collectors when applying MWCNT nanofluid, Proceedings of the 4th International Conference on Energy and Environment (ICEE 2013), 2013, pp. 1–4. [6] Z. S., R. S., M. S., N. R., M. A., Thermophysical properties of single wall carbon nanotubes and its effect on exergy efficiency of a flat plate solar collector, Sol. Energy 115 (2015) 757–769. [7] T. Otanicar, J. Golden, Comparative environmental and economic analysis of conventional and nanofluid solar hot water technologies, Environ. Sci. Technol. 43 (September 2009) 6082–6087. [8] E. Ebrahimnia-Bajestan, H. Niazmand, W. Duangthongsuk, S. Wongwises, Numerical investigation of effective parameters in convective heat transfer of nanofluids flowing under a laminar flow regime, Int. J. Heat Mass Trans. 54 (2011) 4376–4388. [9] M. Sharafeldin, G. Grof, Experimental investigation of flat plate solar collector using CeO2-water nanofluid, Energy Convers. Manag. 155 (2018) 32–41. [10] Y. Hwang, H. Park, J. Lee, W. Jung, Thermal conductivity and lubrication char- acteristics of nanofluids, Curr. Appl. Phys. 6 (2006) 67–71. [11] J. Eastman, S. Choi, S. Li, W. Yu, L. Thompson, Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nano- particles, Appl. Phys. Lett. 78 (6) (2001) 718–720. [12] I. Madni, C.-Y. Hwang, S.-D. Park, Y.-H. Choa, H.-T. Kim, Mixed surfactant system for stable suspension of multiwalled carbon nanotubes, Colloids Surf., A A 358 (1–3) (2010) 101–107 https://doi.org/10.1016/j.colsurfa.2010.01.030. [13] F. Kiliç, T. Menlik, A. Sözen, Effect of titanium dioxide/water nanofluid use on thermal performance of the flat plate solar collector, Sol. Energy 164 (2018) 101–108. [14] J. Li, Z. Li, B. Wang, Experimental viscosity measurements for copper oxide nanoparticle suspensions, Tsinghua Sci. Technol. 7 (2002) 198–201. [15] I. Mahbubul, R. Saidur, M. Amalina, Latest developments on the viscosity of nanofluid, Int. J. Heat Mass Transf. 55 (2012) 874–885. [16] R. Saidur, K. Leong, H. Mohammad, A review on applications and challenges of nanofluids, Renew. Sust. Energ. Rev. 15 (3) (2011) 1646–1668. [17] G. P., M. C., I. M., P.K. D., Techniques for measuring the thermal conductivity of nanofluids:a review, Renew. Sustain. Energy Rev. 14 (2010) 1913–1924. [18] T. Y., F. V., E. S., S. Z., An experimental investigation on the effect of MWCNT-H2O nanofluid on the efficiency of flat-plate solar collectors, Exp. Therm. Fluid Sci. 39 (2012) 207–212. [19] G. C., E. F., R.A. d., D. L., A new solution for reduced sedimentation flat panel solar thermal collector using nanofluids, Appl. Energy 111 (2013) 80–93. [20] A. Hepbasli, A key review on exergetic analysis and assessment of renewable energy resources for a sustainable future, Renew. Sustain. Energy Rev. 12 (3) (2008) 593–661. [21] I. Fara, L. Luminosu, Determination of the optimal operation mode of a flat solar collector by exergetic analysis and numerical simulation, Energy 30 (2005) 731–747. [22] A. Bejan, G. Tsatsaronis, M. Moran, Thermal Design and Optimisation, John Wiley and Sons, New York, 1996. [23] S. Mahata, D. Saha, Thermodynamic optimisation of solar flat plate collector, Renew. Energy 23 (2001) 181–193. [24] K. Kar, Exergy efficiency and optimum operation of solar collectors, Appl. Energy 21 (1985) 301–314. [25] E. Shojaeizadeh, F. Veysi, A. Kamandi, Exergy efficiency investigation and optimization of an Al2O3-Water nanofluid based flat-plate solar collector, Energy Build. 101 (2015) 12–23. [26] Z.S.M. Said, R. Saidur, A. Hepbasli, N. Rahim, S. Mekhilef, T. Ward, Performance enhancement of a flat plate solar collector using titanium dioxide nanofluid and polyethylene glycol dispersant, J. Clean. Prod. 92 (1 April 2015) 343-3. [27] Z. S., R. S., M. S., N. R., M. A., Thermophysical properties of single wall carbon nanotubes and its effect on exergy efficiency of a flat plate solar collector, Sol. Energy 115 (2015) 757–769. [28] L. Onsager, ""Reciprocal relations in irreversible processes. I.,"," Phys. Rev., , vol. 37, pp. 405-426. [29] H. Kreuzer, Nonequilibrium Thermodynamics and its Statistical Foundations, 1 ed., Clarendon Press, Oxford and New York, 1981, p. 455. [30] S. Farahat, F. Sarhaddi, H. Ajam, Exergetic optimization of flat plate solar collectors, Renew. Energy 34 (2009) 1169–1174. [31] A. Bejan, Entropy Generation Minimization: the Method of Thermodynamic Optimization of Finite-Size Systems and Finite-Time Processes, CRC Pressi llc, Florida, 1996. [32] S.A. Kalogirou, S. Karellas, K. Braimakis, C. Stanciu, Exergy analysis of solar thermal collectors and processes, Progress in Energy and Combustion Science, vol. 56, 2016, pp. 106–137. [33] M. K., M.A. A. B., S. D., A. G., A new application of carbon nanotubes nanofluid as working fluid of low-temperature direct absorption solar collector, Sol. Energy Mater. Sol. Cells 121 (2014) 114–118. [34] S.K. Verma, A.K. Tiwari, D.S. Chauhan, Experimental evaluation of flat plate solar collector using nanofluids, Energy Convers. Manag. 134 (2017) 103–115. [35] Y. H., H.S. P., J.K. L., W.H. J., Thermal conductivity and lubrication characteristics of nanofluids, Curr. Appl. Phys. 6S (2006) 67–71.
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Case Studies in Thermal Engineering journal homepage: www.elsevier.com/locate/csite
Energy and exergy analysis of a thermosiphon and forcedcirculation flat-plate solar collector using MWCNT/Water nanofluid
T
Mahmoud Eltaweela,∗, Ahmed A. Abdel-Rehima,b a b
The British University in Egypt, Cairo, Egypt Shoubra Faculty of Engineering, Benha University, Cairo, Egypt
A R T IC LE I N F O
ABS TRA CT
Keywords: Flat-plate solar collector Solar energy Thermosiphon Nanofluids Energy efficiency Exergy efficiency
The scientific community has focused on the use of nanofluids to enhance solar collectors' thermal efficiency. Solar energy is considered an extensively available energy resource with a weak effect on the environment. The flat-plate solar collector is considered to be the most common collector used, but the efficiency of the collector has limitations, such as the low effectiveness of the absorber in terms of energy absorption and the transmission of that energy to the working fluid. The flat-plate solar collector can be enhanced by replacing the working fluid with a nanofluid. Nanofluids are considered new media that exhibit thermophysical properties superior to those of ordinary working fluids. In this study, the effect of using multi-walled carbon nanotubes in a thermosiphon and forced-circulation flat-plate solar collector was investigated experimentally. A set of different concentrations was tested (0.01 wt%, 0.05 wt% and 0.1 wt%) with distilled water. The experimental results show an enhancement of the system's exergy efficiency and energy efficiency when a nanofluid was used compared with those attained using distilled water in both forced and thermosiphon systems.
1. Introduction Developing clean and efficient energy sources is a goal of many scientists and engineers. The solar energy that the earth receives within 1 h can cover the total energy needs of humans over an entire year [1]. That energy must be collected, and with the use of the appropriate technology, solar energy can be the most efficient energy resource worldwide. The average amount of solar radiation received per day in Egypt is between 19.9 and 23.1 MJ/m2, which is considered to be a relatively high value compared with that received in Europe [2]. Solar energy can be converted to thermal energy using a special type of heat exchanger as a thermal solar collector. Many different types of solar collectors have been developed; flat-plate solar collectors are the most common solar heaters. Such collectors have been developed since the 1980s without any significant change in their design. Flat-plate solar collectors are the cheapest method for solar applications, but the problem is that they have a low thermal efficiency and low outlet temperature. Over the past decade, most research has focused on using nanotechnology in various applications due to the favourable properties afforded. By adding a weighted fraction of nanoparticles to a soluble substance such as water, the heat transfer of a liquid can be significantly increased. Sunlight is absorbed by the flat plate, which is called the absorber, and heats the fluid in the tubes. Flat-plate solar
∗
Corresponding author. Department of mechanical engineering, The British University in Egypt, Cairo, Egypt. E-mail address: [email protected] (M. Eltaweel).
https://doi.org/10.1016/j.csite.2019.100416 Received 14 January 2019; Received in revised form 13 February 2019; Accepted 16 February 2019 Available online 19 February 2019 2214-157X/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
collectors can increase the fluid temperature by 30–80 °C. Collectors of this type consist of an absorbing surface, a trap for radiation loss, a heat transfer medium and thermal insulation behind the absorber surface [3]. A nanofluid consists of a small fraction of nanoparticles mixed with a liquid. This liquid could be water, oil or ethylene glycol. That small fraction of particles can completely change the properties of the liquid with respect to heat transfer. Metals usually have a higher thermal conductivity than fluids do. Therefore, placing metallic particles in a fluid produces a good heat-conducting liquid. However, the number of nanoparticles used in a nanofluid has must low, and the particles should not be larger than 100 nm to avoid problems such as creating an unstable mixture. Another reason is that large particles can lead to a pressure drop because the particles might stick to the walls of the surrounding container. The use of nanofluids offers many advantages, such as increasing heat conductivity, which in turn enhances heat transfer performance. In addition, nanofluid particles are so small that they can easily move through solid blocks. One of the main characteristics of nanoparticles is their large surface area-to-volume ratio, which increases the heat transfer between the solid particles and the fluid; in addition, the particles’ high thermal capacity and small size lead to low energy loss, increasing system performance. Nanoparticles also enhance the turbulent motion of fluids [3]. 1.1. Thermal performance Natarajan and Kiatsiriroat [4]experimentally investigated the use of multi-walled carbon nanotubes (MWCNTs) as a component of a working fluid in a solar plate collector and observed that MWCNTs were better than most other nanoparticles, particularly at high inlet temperatures. Faizal et al. [5] reported that by using MWCNT/water as a working fluid, the collector size can be reduced by 37% of its original size while maintaining the same efficiency. Said et al. [6] reported that by using single-walled carbon nanotubes (SWCNTs) mixed with water, the solar collector efficiency could be increased by 95.12%, but due to the high cost of SWCNTs, it is better to use MWCNTs because of their relatively similar efficiency and lower cost. Otanicar and Golden [7] performed economic and environmental experiments using nanofluids in flat-plate solar collectors and found that the use of nanofluids can increase the payback period of collectors but will provide the same economic savings due to the low cost of nanofluids. Nanofluids have also been found to contribute 3% more pollution than other working fluids. Ebrahim and Bajestan [8] performed a numerical analysis of using carbon nanotubes in working fluids (water and EG) at concentration ratios ranging from 0 to 6%. The authors observed temperature differences and indicated the best Reynolds number, pressure drop and pump power. The results showed that increasing the concentration will increase the heat transfer coefficient; additionally, the heat transfer coefficient was observed to scale inversely with the nanoparticle diameter. Using carbon nanotubes mixed with water at a Reynolds number of 1460 and a diameter-to-length ratio greater than 100 at a concentration of 0.038%, the temperature difference was 0.9 °C, and a pressure drop of 9625 Pa was observed at a pump power of 0.16 W. Raj et al. [3] found that the highest efficiency was achieved when carbon nanotubes were used. Sharafeldin and Grof [9] found that the volume fraction and the mass flux rate are directly proportional to the collector efficiency; with a mass flux rate of 0.019 kg/s m2 and volume fraction of CeO2 equal to 0.066%, the efficiency can increase by 10.74% compared to that attained using water. 1.2. Nanofluid stability Preparing nanofluids is a technical challenge because aggregation can occur due to van der Waals interactions. Stability can be achieved by chemical or physical treatment. For example, a surfactant can be used to increase stability; however, surfactant use is restricted to low-temperature applications because at high temperature, bubbles that affect the heat transfer efficiency significantly can form. Stability can also be achieved by applying a force on clusters of suspended particles [10]. Eastman et al. [11] examined the stability of copper nanoparticles in ethylene glycol at a concentration of 0.3% and found that after 2 days of preparation, the fluid exhibited higher thermal conductivity than that observed after 2 months of storage, due to the decrease in dispersion stability over time. Madni et al. [12] investigated the stability of nanofluids, demonstrating that one suitable way to enhance stability is by adding surfactants through an approach called non-covalent functionalization. Kilic et al. [13] studied the effect of using a 2 wt% titanium dioxide/water nanofluid mixed with 0.2 wt% Triton X-100 as a surfactant to maintain stability and eliminate the problem of agglomeration over time. The instantaneous efficiency of the corresponding collector was determined to be 36.2% for pure water and 48.67% for the mixture used. Raj et al. [3] reported that long-term stability and suitable dispersion of nanoparticles is fundamental to achieving sufficient absorption of solar radiation and increasing solar collector efficiency. 1.3. Challenges of using nanofluids Li et al. [14] investigated the effect of nanoparticle concentration with respect to viscosity and found that variations in concentration are directly proportional to those in viscosity. In addition, pumping power and pressure drop could be affected by these parameters. Mahbubul et al. [15] studied the viscosity characteristics of nanofluids by comparing the effects of different parameters on viscosity, such as preparation methods, nanoparticle size, concentration effect and temperature. They discovered that viscosity mainly depends on concentration and temperature differences. Nanoparticles tend to bond with each other, thereby preventing a high surface-to-area ratio. To overcome this limitation, particle dispersion additives are added to fluids containing nanoparticles. As a result, the surface properties of nanoparticles are altered, and the corresponding nanofluid will contain a high level of impurities. Research has countered the fluctuation of properties by limiting sample nanofluid volumes to less than 100 ml [16]. One of the problems with using nanofluids is sedimentation, which might occur during operation. To avoid this problem, it is preferred to use nanomaterial concentrations of less than 0.5% [17]. Although most researchers have performed experiments in nearly the same 2
Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
manner, their results consistently vary [18]. Additionally, collector design affects device performance, from the collector size to the different materials used at the experiment location, all of which affect experiments and alter results [19]. Raj et al. [3] found that increases in volume fraction are directly proportional to increases in collector efficiency; however, beyond a certain volume fraction, efficiency decreases, considering the corresponding degradation of the heat transfer rate and increase in viscosity. 1.4. Exergy efficiency in solar collectors Hepbasli [20] defined exergy as the maximum output that a system can achieve relative to the surrounding temperature. Energy system efficiency, design and simulation can be evaluated via exergy analysis, and high exergy efficiency is the key to obtaining sustainable development. Additionally, exergy analysis can determine whether a system is capable of increasing energy efficiency by reducing inefficiencies in existing systems. Luminosu and Fara [21] numerically studied the optimal operation of the flat-plate solar collector using exergy analysis, considering the thermal loss coefficient, its overall heat removal factor and other heat transfer coefficients to be constants. Bejan et al. [22] reported that entropy generation or exergy during heat transfer can be maximized in efficient solar power systems for thermodynamic optimization and could also help increase the efficiency of existing systems. Saha and Mahanta [23] investigated the thermodynamic optimization of a FPSC using entropy minimization. Kar [24] studied the optimum operation and efficiency of FPSC in terms of exergy. Shojaezadeh et al. [25] studied the effects of changing certain parameters on the exergy efficiency of a FPSC with an aluminium oxide/water nanofluid; the parameters observed to have an effect were ambient temperature, solar radiation, inlet fluid temperature, nanoparticle volume concentration and mass flow rate. The results showed that by using an aluminium oxide/water nanofluid, the maximum collector exergy efficiency was only increased by one percent. Said et al. [26] performed an exergy analysis of flat-plate solar collectors using a TiO2 /water nanofluid with polyethylene glycol as a surfactant to increase stability. The results showed that the system's energy efficiency could be increased by 76.6% and the exergy efficiency increased by 16.9% at a concentration of 0.1 vol%. Another study performed by Said et al. [27] examined the use of SWCNTs at a concentration of 0.3 vol% in water in a FPSC and its effect on thermophysical properties. It was found that the energy and exergy efficiency increased, with the energy efficiency increasing by 95.12% and the maximum exergy efficiency reaching 26.25%. The present work investigates the first and second laws of thermodynamics on a flat-plate solar collector with thermosiphon and forced circulation. Three different concentrations of multi-walled carbon nanotubes with water were examined and compared. The use of MWCNT/water nanofluid in FPSCs is an attractive topic which was investigated by several researchers, however there is a very limited publications which investigated this topic on thermosiphon principle. Similarly, up to the authors knowledge, there is no publications investigated the energy and exergy of a thermosiphon FPSC with the use of MWCNT/water nanofluid. Also, in this paper the effect of using thermosiphon and forced circulation were investigated on the same collector to minimize the errors as possible and have a reliable comparison, relative to previous work where experiments were done on two similar collectors with two types of circulations. 2. Experimental devices and methodology The purpose of this work was to investigate the effect of using an MWCNT/water nanofluid as the working fluid in a thermosiphon FPSC and its effect on energy and exergy efficiency. The system used in the experiment is a commercial FPSC placed on the roof of The British University in Egypt, El Shorouk, Cairo, Egypt. First, solar irradiation at the location was measured from 9:00 a.m. to 4:00 p.m. on the 24th of May 2018. Fig. 1 shows that solar irradiation on the 24th of May 2018 ranged from 520 W/m2 to 914 W/m2 . The FPSC used and a schematic diagram of the system is shown in Fig. 2. The specifications of the collector are presented in Table 1. The system tank, which has a capacity of 200 L, functions as a heat exchanger that absorbs heat from the collector. The tank has a heat exchanger that is connected to the collector to create closed circulation for the working fluid. This arrangement allows for the possibility of heat exchange between the collector fluid and the tank fluid, as shown in Fig. 2. The system circulation uses natural convection from the temperature difference between the inlet and outlet of the collector; thus, there was no need for a pump in the system when the thermosiphon was tested. A pump was used in the system when forced circulation was tested. Sensors were installed to measure the required parameters for evaluating the system's performance. First, 2 PTN thermocouples sensors were installed at the inlet and outlet of the collector, and a flow meter (YF-S201) was installed before the inlet to measure the flow rate of the fluid. For weather data, OMEGA type FMA 1000 series sensors were used to measure ambient temperature and wind speed and to measure the intensity of solar radiation. A PYR 1307 solar irradiation sensor was used with the same slope and position as the collector. The sensors were read every 5 min from 9:00 a.m. to 4:00 p.m. over several days. The nanofluid preparation method used was a two-step technique, to increase system stability and durability [28]. Distilled water was used in this study for both the distilled water experiment and the nanofluid experiment as the base fluid. Three concentrations were used in this study: 0.01%, 0.05% and 0.1% by weight fraction of MWCNTs. An ultrasonic homogenizer, also called a sonicator, type (Hielscher UP400s) was used to prepare the nanofluid samples for 2 h, 4 h and 8 h for 0.01%, 0.05% and 0.1% concentrations respectively, after a 1 h stirring in a magnetic stir for each concentration, the sonicator used ultrasonic waves with pulses of 0.5 s on and 0.5 s off at an amplitude of 60% and frequency of 24 kHz until each sample was completely dispersed. The specifications of the MWCNTs are shown in Table 2. 3
Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
Fig. 1. Radiation measurement for the selected location in May 2018.
Fig. 2. The Flat-plate Solar Collector (a): Photograph of the solar collector used. (b): Schematic illustration of the system used. (c): The tank used. Table 1 Specifications of the flat-plate solar collector used. Specifications
Dimensions
Absorption area
2 m2 3L 7 80% 4 mm Copper 12% 96%
Working fluid capacity Number of riser tubes Glass transmissivity Glass thickness Absorber plate material Absorber emissivity Absorber absorptivity
4
Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
Table 2 MWCNT specifications. MWCNT properties Specifications
Value
Purity Diameter Length Density
95 wt% 10–40 nm 20 μm
Specific surface area
460 m2/ gm 6150 W/m.K
1.3–2.4 gm/cm3
Thermal conductivity
3. Efficiency calculations 3.1. Energy efficiency The collector's useful energy can be calculated by either of the following two equations:
˙ P (Tout − Tin ) Qu = mC
(1)
Qu = AC FR [IT (τα ) − UL (Tin − Tamb)]
(2)
where m˙ is the mass flow rate, CP is the specific heat capacity of the fluid, Tout is the working fluid outlet temperature, Tin is the working fluid inlet temperature, AC is the collector area, IT is the total radiation, τ is the transmissivity of the glass, α is the absorptivity of the collector and Tamb is the ambient temperature. The heat removal factor (FR) can be calculated using the following equation:
˙ P (Tout − Tin ) mC AC [IT (τα ) − UL (Tin − Tamb)]
FR =
(3)
The total irradiation the collector receives can be calculated using the following equation: (4)
QT = Ac IT The specific heat of the nanofluid can be calculated using the following equation:
CPNF = CPNP (φ) + CPW (1 − φ)
(5)
where CPNF is the nanofluid specific heat, CPNP is the nanoparticle specific heat, CPW is the distilled water specific heat and φ is the concentration weight fraction. The weight fraction (φ ) can be calculated using the following equation:
φ=
Vnp Vnp + Vw
(6)
where Vnp is the nanoparticle volume and Vw is the water volume. The volume of the nanoparticles (Vnp ) can be calculated using the following equation:
Vnp =
Mnp ρnp
(7)
where Mnp is the nanoparticle mass and ρnp is the nanoparticle density. The collector instantaneous efficiency can be calculated as follows:
η=
Quseful QT
=
˙ P (Tout − Tin ) mC (T − Tamb) ⎤ = FR ⎡ (τα ) − UL in ⎥ ⎢ Ac IT IT ⎦ ⎣
(8)
3.2. Exergy efficiency In studying the exergy efficiency of a system, certain assumptions must be made, such as the system must operate in a steady state and under steady flow, the entrance effect is the only loss coefficient considered, and heat transfer to the system and work transfer from the system are positive. Onsager and Kreuzer [28,29]stated that because of viscous friction and heat transfer as a function of the design variables selected, it is extremely important to measure the entropy generated or the exergy destroyed. S. Farahat [30] reported that typical exergy stabilities can be expressed in rate form, as indicated in the equation below, where the effects of kinetic and potential energy deviations are ignored. 5
Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
˙ in + Ex ˙ out + Ex ˙ st + Ex ˙ leak + Ex ˙ dest = 0 Ex
(9)
˙ in is the inlet exergy rate to the system, Ex ˙ out is the output exergy rate from the system, Ex ˙ st is the stored exergy rate for the where Ex ˙ leak is the leakage exergy rate for the system, and Ex ˙ dest is the exergy destruction rate. system, Ex ˙ dest ) can be defined as follows: The exergy destruction rate (Ex
˙ dest = Ta S˙ gen Ex
(10)
˙ in ) is composed of a fluid flow component (Ex ˙ in, f ) and an absorbed solar energy component (Ex ˙ in, s ) The inlet exergy rate (Ex [29,30]: m˙ ΔPin ˙ in, f = mC ˙ p, NF ⎛Ti − Ta − Ta ln Ti Ta ⎞ + Ex ρ ⎝ ⎠
(11)
˙ in, s = IT AC ⎛1 − Ta ⎞ Ex Ts ⎠ ⎝
(12)
( )
⎜
⎟
where Ts is the surface temperature of the sun, which is equal to 5770 K, and Pin is the fluid pressure at the inlet. ˙ out ) features a fluid flow component and is computed as the inlet fluid exergy rate [29,30]: The outlet exergy rate (Ex
m˙ ΔPout ˙ out , f = mC ˙ p, NF ⎛To − Ta − Ta ln To Ta ⎞ − Ex ρ ⎝ ⎠
( )
(13)
where Pout is the fluid pressure at the outlet. A non-isothermal solar flat-plate collector's overall rate of entropy generation, reported by Bejan [31], is defined as follows:
q q ˙ p, NF ⎛ln To Ti ⎞ − abs + loss S˙ gen = mC Ts Ta ⎝ ⎠
( )
(14)
where qabs is the energy absorbed by the collector absorber surface, which can be calculated by the equation qabs = ατ (IT Ac ) , and qloss is the total heat lost to the environment, which can be calculated by the following equation:
˙ p, NF (To − Ti ) qloss = qabs − mC
(15)
The exergy efficiency can be calculated as follows [30,32]:
ηex =
˙ out , f − Ex ˙ in, f Ex ˙ in, s Ex
(16)
Using equations (9) and (10), and (11), the equation can be rewritten as follows:
ηex =
(
( )) − )
m˙ ⎡Cp, NF To − Ti − Ta ln To Ti ⎣ qabs 1 − Ta Ts
(
ΔP
ρ⎤
⎦ (17)
Using equations (12)–(14) the exergy efficiency can also be defined as follows:
ηex = 1 −
Ta S˙ gen ⎡1 − ⎣
( ) ⎤⎦ q Ta Ts
abs
(18)
4. Results and discussion 4.1. Thermosiphon circulation 4.1.1. Energy analysis The fluid used in the experiment was distilled water with a pH value of 7, and the concentrations of the MWCNTs used in the experiment were 0.01 wt%, 0.05 wt% and 0.1 wt%. First, the system efficiency was measured using distilled water. The reduced temperature parameter Ti − Ta IT was plotted on the X axis and the efficiency (η) on the Y axis. Fig. 3 presents a comparison of the efficiency of the thermosiphon FPSC for distilled water and three different concentrations of MWCNT/water nanofluid: 0.01 wt%, 0.05 wt% and 0.1 wt%. The average efficiencies of the collector were 36.54%, 51.29%, 56.41% and 70.67% for distilled water and the 0.01 wt%, 0.05 wt% and 0.1 wt% MWCNT/water nanofluids, respectively. As shown in Fig. 3, the efficiency of the collector increased due to the use of MWCNT/water nanofluid as the working fluid. The nanotubes increased the heat transfer between the collector and the working fluid and enhanced the thermal conductivity and absorptivity of the working fluid. The MWCNTs showed a significant increase in system efficiency with a small concentration fraction of 0.1 wt%, which could reduce the size of the collector by 34%, the collector size reduction can be calculated as [33]. The use of a thermosiphon FPSC affects the mass flow rate of the working fluid due to various factors. The higher the fluid concentration is, the higher the viscosity becomes; in turn, the mass flow rate decreases. In the thermosiphon collector, the mass flow
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Case Studies in Thermal Engineering 14 (2019) 100416
M. Eltaweel and A.A. Abdel-Rehim
Fig. 3. Variation of thermosiphon FPSC efficiency for distilled water and nanofluids of various concentrations.
rate is a parameter related to the temperature difference between the inlet and outlet of the collector and solar irradiation. The mass flow rate of the collector with distilled water was at its minimum; however, when MWCNTs were added, the mass flow rate increased, and as the concentration ratio increased, the mass flow rate decreased because of the increase in the fluid viscosity. The mass flow rate for each working fluid were measured, the average value of the mass flow rate throughout the day is shown in Table 3. The heat removal factor of the collector FR was calculated for each weight fraction of MWCNT nanofluid, and the factor was found to have a noticeable effect on the FPSC efficiency, as shown in Table 4. The heat removal factor of the nanofluid is higher than that of distilled water and increasing the weight fraction of MWCNTs increases the heat removal factor. Additionally, the absorbed energy parameter increases. The performance of the collector is enhanced by increasing these factors when using an MWCNT nanofluid. The addition of MWCNTs increased the absorbed energy parameter (FR(τα)) of distilled water by 31.2%, 51.1% and 65.1% at concentrations of 0.01 wt%, 0.05 wt% and 0.1 wt%, respectively. The value of FRUL for distilled water increased by 43.7%, 134.6% and 139.9%, respectively, at MWCNT concentrations of 0.01 wt%, 0.05 wt% and 0.1 wt%.
4.1.2. Exergy analysis The exergy efficiency of the thermosiphon FPSC was investigated. Fig. 4 shows that the exergy efficiency increased with the concentration of the MWCNT/water nanofluid. For MWCNT weight fractions of 0.01%, 0.05% and 0.1%, the exergy increased relative to that of distilled water by 2.18%, 4.98% and 9.039%, respectively. The highest exergy observed, 23.35%, was achieved by the 0.1 wt% MWCNT nanofluid. Fig. 5 shows that the increase in the MWCNT concentration in distilled water increased the exergy efficiency (ηex) of the system, from 14.55% for distilled water to 15.69%, 21.42% and 23.35% for the 0.01 wt%, 0.05 wt%, and 0.1 wt% MWCNTs, respectively, which could be obtained at the lowest (Ta/IT) values. The figure also shows that the increase in the (Ta/IT) value caused the optimum exergy efficiency to decrease exponentially, and the optimum exergy efficiency could be obtained at the lowest (Ta/IT) value.
Table 3 Mass flow rate of the four different working fluids. Nanoparticles Concentration
Mass Flow Rate (L/Min)
0 0.01 wt% 0.05 wt% 0.1 wt%
0.375529 0.5042 0.46835 0.41823
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Table 4 Values of FRUL and FR(τα) for MWCNT/water nanofluids and water-based working fluid. Weight fraction %
F R UL
FR(τα)
Distilled water 0.01 0.05 0.1
5.216 7.5 12.240 12.515
0.477 0.626 0.7209 0.78765
Fig. 4. A comparison of the change in exergy efficiency over time of the thermosiphon flat-plate solar collector for distilled water and different nanofluid concentrations.
Fig. 5. A comparison of the exergy efficiency and the ratio of ambient temperature to radiation intensity of the thermosiphon FPSC for distilled water and the different nanofluid concentrations.
4.2. Forced circulation A pump was used in the tested collector to study the difference between using forced and natural circulation for the same working fluid. The highest concentration of MWCNTs was used in the experiment for a comparison between distilled water under four different flow rate conditions: 0.5 L/min, 1 L/min and 1.5 L/min and thermosiphon circulation. The efficiency of the collector 8
Case Studies in Thermal Engineering 14 (2019) 100416
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Fig. 6. Variation of FPSC efficiency for 0.1 wt% MWCNT/water nanofluid at three different flow rates and thermosiphon circulation.
increased by 34.13% using the 0.1 wt% MWCNT/water nanofluid, with an average efficiency of 70.67% for the nanofluid and 36.54% for distilled water with thermosiphon circulation. The average efficiencies of the nanofluid and distilled water were 76.92% and 49.3%, respectively, at a flow rate of 1.5 L/min with an increase in efficiency equal to 27.62%. The system efficiency for a flow rate of 1.0 L/min; the average efficiencies of the nanofluid and distilled water were 63.97% and 37.5%, respectively, with an increase in efficiency equal to 26.47%. The average system efficiencies for distilled water and the nanofluid were 35.4% and 58.17%, respectively, at a flow rate of 0.5 L/min, with an increase in efficiency equal to 22.77%. The highest increase in efficiency for the 0.1 wt % MWCNT/water nanofluid relative to the efficiency observed for distilled water was achieved with the forced circulation system at a flow rate of 1.5 L/min, followed by the thermosiphon circulation system. However, using nanofluid in a thermosiphon FPSC has a greater impact on the efficiency of the FPSC than using distilled water in forced circulation. Fig. 6 shows that the energy efficiency will increase by 35.2%,34.1% and 21.3% when compared with 0.1 wt % MWCNT/water nanofluid in thermosiphon FPSC with forced circulation flow rates 0.5 L/min, 1.0 L/min and 1.5 L/min in the same FPSC, respectively. The present work was compared with previous work done by other researchers for FPSC and MWCNT/water nanofluid (Table 5) and the results show the energy efficiency of the system increase is variable between the particle size of MWCNT and the concentration, however, karami et al. [33] used a very low concentration and the enhancement is close to the present work. Verna et al. [34] also used a similar particle size and a higher concentration but the enhancement is actually lower. MWCNT/water nanofluid with 0.1 wt% concentration achieved a higher efficiency in thermosiphon circulation than in 0.5 and 1 L/min because the temperature difference between the inlet and the outlet is higher in thermosiphon circulation, although the mass flow rate value was between 0.55 and 0.62 from 10:30 a.m. to 2:00 p.m. while the rest of the day the values are low, which gave a low average value of mass flow rate throughout the day. While in forced circulation the temperature difference is lower than thermosiphon circulation throughout the day. The result is a lower efficiency when the flow rate is 0.5 and 1 L/min, while when the flow rate is 1.5 L/min the efficiency increased in the times with low solar radiation on the contrary of thermosiphon. The increase in the reduced temperature parameter Ti − Ta IT reduce the increase in the collector efficiency between the thermosiphon and 1.5 L/min circulation because thermosiphon circulation has a high efficiency when the solar intensity is high.
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5. Conclusions The efficiency of a flat-plate solar collector was investigated experimentally using multi-walled carbon nanotubes with a diameter of 10–40 nm and a length of 29 μm at concentrations of 0.01 wt%, 0.05 wt% and 0.1 wt%. The experiment, performed in Cairo, Egypt, shows that the use of 0.01 wt% MWCNTs increased the efficiency by 16% compared with that of distilled water; for 0.05 wt% MWCNTs, the efficiency increased by 21%, and for 0.1 wt% MWCNTs, the efficiency increased by 34.13% compared with the efficiency attained using distilled water. The experimental results indicate that using MWCNTs in water as a nanofluid will enhance the Table 5 Comparison between the present work and different papers using MWCNT/water nanofluid in FPSC. Researcher
Concentration
Particle size
Energy efficiency enhancement
Present work Verma et al. [34] Hawng et al. [35]
0.1 wt% 0.2 vol% 1 vol%
35.2% 23.47% 7%
Natrajan et al. [4]
1 vol%
Karami et al. [33]
0.015 vol%
10–40 nm diameter, 20 μm length 7 nm diameter, 20 μm length 10–30 nm diameter 10–50 μm length 10 nm diameter 270 nm length 10 nm diameter 5–10 μm length
9
41% 32%
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efficiency of the solar collector, and for a concentration of 0.1 wt% MWCNTs, the size of the solar collector can be reduced by 34%. Furthermore, using forced circulation increased the efficiency of the flat-plate solar collector by 6.21% at a flow rate of 1.5 L/min relative to that attained using a thermosiphon. Also, it is concluded that using a nanofluid in a thermosiphon FPSC will increase the efficiency more than using distilled water in a forced circulation FPSC. The experimental results show that the maximum exergy efficiency for the flat-plate solar collector is approximately 23.35% compared to what is achieved with distilled water, i.e., 14.55%. The results also show that the optimum exergy efficiency decreases exponentially with increasing Ta/IT values. The relatively low exergy efficiency shows that the flat-plate collector system still needs improvement. The experimental results suggest that using an MWCNT/water nanofluid in a thermosiphon flat-plate solar collector is a viable solution for enhancing the energy and exergy efficiency. Acknowledgements The authors would like to thank the renewable energy centre in The British University in Egypt for providing the testing facilities. Nomenclature Ac Surface area of the collector CP Heat capacity of the fluid Cp,bf Heat Capacity of the base fluid Cp,np Heat Capacity of the nanoparticles Cp,nf Heat Capacity of the nanofluid ˙ in Ex Inlet exergy rate to the system ˙ out Ex Output exergy rate from the system ˙ st Stored exergy rate for the system Ex ˙ leak Ex Leakage exergy rate for the system ˙ dest Ex Exergy destruction rate FR Heat Removal Factor IT Total Radiation ˙ m Mass flow rate Mass of the nanoparticles Mnp Density of the nanoparticles ρnp Fluid pressure at the inlet Pin Fluid pressure at the outlet Pout QT Total Irradiation received by the collector Quseful Useful Energy Gain Tamb Ambient Temperature Tin Inlet Fluid Temperature of the collector Tout Outlet Fluid Temperature of the collector Sun temperature Ts UL Overall heat loss coefficient of the solar collector Vnp Volume of nanoparticles Vw Volume of water Greek Symbols τ α ε η β φ θ Subscripts
Transmissivity Absorptivity Emissivity Efficiency of the Flat-Plate Solar Collector Slope of the collector (Inclination Angle) weight Fraction Latitude
Bf FPSC MWCNT Nf Np
Base-fluid Flat-plate solar collector Multi-walled carbon nanotubes Nanofluid Nanoparticle
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