water nanofluid

Renewable Energy 83 (2015) 463e473

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Effects of thermal performance of enclosed-type evacuated U-tube solar collector with multi-walled carbon nanotube/water nanofluid Yijie Tong a, Jinhyun Kim b, Honghyun Cho c, * a

Department of Electromechanical Engineering, Hangzhou Vocational and Technical College, Hangzhou, China Graduate School of Chosun University, Chosun University, Gwangju, 501-759, Republic of Korea c Department of Mechanical Engineering, Chosun University, Gwangju 501-759, Republic of Korea b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 July 2014 Accepted 11 April 2015 Available online

An enclosed-type evacuated U-tube solar collector (EEUSC) with high efficiency and low cost was designed and constructed. A copper fin was employed in the U-tube to assume a constant heat flux. The thermal performance of the EEUSC was evaluated under a wide range of operating conditions. Moreover, to increase the heat transfer efficiency in the U-tube over the thermal resistance of the air gap, a novel method was developed, which entailed filling the gap with high-thermal-conductivity liquid. Multiwalled carbon nanotube (MWCNT) nanofluid was used as the working fluid. Evaluation results showed that the efficiency of the EEUSC is influenced primarily by the air gap and that it increases by 4% with the use of the MWCNT nanoliquid. Calculations based on this improvement revealed that the annual CO2 and SO2 emissions will reduce by 1600 kg and 5.3 kg, respectively, when 50 solar collectors are employed. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Apparent conductance Solar collector U-tube Heat transfer Nanofluid

1. Introduction A solar collector is one of the most important renewable energy devices for converting sunlight to useful heat energy. In the last two decades, many researchers have focused on methods for improving the efficiency of solar collection and conversion systems. In the future, the use of fossil fuels will be limited owing to a shortage of their resources as well as their adverse environmental impacts. This scenario has been motivating researchers to determine possible alternative sources of energy. One of the key factors that could help achieve energy savings and a compact design of solar thermal collectors is heat transfer enhancement. With this background, the technology for utilization of solar energy has developed rapidly in recent years. Moreover, solar energy can be comparable to other renewable energy sources. Nevertheless, the amount of usable solar energy is insufficient for meeting present-day energy usage demands. Several papers and books have been devoted to the study of solar insolation, and have contributed numerous precise equations for calculation of total amount of solar radiation. In this study, these previous theories are utilized for evaluation of the efficiency of an

* Corresponding author. Tel.: þ82 62 230 7050; fax: þ82 62 230 7055. E-mail address: [email protected] (H. Cho). http://dx.doi.org/10.1016/j.renene.2015.04.042 0960-1481/© 2015 Elsevier Ltd. All rights reserved.

enclosed-type evacuated U-tube solar collector (EEUSC), and enhanced performance is demonstrated through the study results. It has been reported that in both developed and developing countries, solar water heating technology can be an economical choice owing to the simplicity of solar radiation as a renewable energy source [1,2]. Presently, a glass evacuated tube-type solar collector is widely employed for solar thermal utilization. Because of its lower heat loss, this type of collector has proved highly useful in residential applications that require higher temperatures [3e5]. Therefore, evacuated solar collectors, especially glass evacuated Utube solar collectors, are widely employed for supplying domestic hot water or heating. Evacuated solar collectors are also used in various applications, e.g., for generating solar electricity and in air-conditioning systems, cookers, and water heaters [6]. The solar collector, as a solar energy recovery device, collects solar energy and converts it to heat. Solar collector devices include solar water heaters and solar air heaters, which provide hot water and air, respectively. It is known that solar collectors convert solar radiation to heat. Solar radiation, which includes a high amount of energy, can conduct the energy of the sun through photons to a fluid [7]. It has been shown that solar collectors play a significant role in reducing energy consumption. Moreover, the use of nanoparticles within the working fluid (i.e., nanofluid) enhances heat transfer [8]. A nanofluid (liquid

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Nomenclature

r T

area (m2) bond width (m) conductance (W/m$K) specific heat capacity (J/kg$K) diameter (m) electric quantity (kWh) electricity solar collector efficiency factor friction factor solar radiation (W/m2) heat transfer coefficient (W/m2$K) heat transfer coefficient between fluid and tube (W/ m2$K) hga convective heat transfer coefficient from outer glass tube to surroundings (W/m2$K) hpgd heat transfer coefficient through thermal conductivity between absorber tube and glass tube (W/m2$K) hpgc radiative heat transfer coefficient between absorber tube and glass tube (W/m2$K) I solar radiation absorbed by absorber coating (W/m2) MWCNT Mutilwalled carbon nanotube k thermal conductivity (W/m$K) L length (m) _ m mass flow rate (kg/s) Nu Nusselt number Pr Prandtl number Q heat (W) Re Reynolds number

d

A B C Cp d E ele F0 F G h hfu

nanocomposite) is a mixture of a liquid substance (basefluid) and nanometer-sized material (nanoparticles) [9,10]. Nanoscience has played a crucial role in improving the performance of HVAC technology and systems. Roberts et al. [11] investigated the use of a heat pump as an alternative supporting device for solar collectors for water heating in Brazil and found that the payback can be in 2.1e3.3 years when the lifespan of the system is 20 years. The solar collector used in their study consisted of eight concentric evacuated tube collectors, which could be reliably operated for generating heat for residential applications. Further, a multi-walled carbon nanotube (MWCNT) was selected as the working fluid because of its good thermal properties. Amrollahi et al. [12] estimated the convective heat transfer coefficient of a MWCNT/water nanofluid and found an improvement of 33%e40% for a 0.25 wt% concentration under laminar and turbulent flows. Further, Kumaresan et al. [13] found the value of the Prandtl number of nanofluids with 0.45 vol% MWCNT to increase by 115.8% at 0  C and by 180.2% at 400  C relative to the Prandtl number of the base fluid. Natarajan and Sathish [14] investigated the thermal conductivity improvement of base fluids employing CNTs and recommended that the use of these fluids as a heat transport medium could result in an increased efficiency of a conventional solar water heater. Shuichi [15] studied the convective heat transfer behavior of aqueous suspensions of nanoparticles flowing through a horizontal tube heated under a constant heat flux condition and found that (i) a significant enhancement of the heat transfer performance occurred owing to suspension of nanoparticles in the circular tube flow in comparison to that when pure water was used as the working fluid, (ii) the enhancement intensified with increasing Reynolds number and nanoparticle concentration, and (iii) the substantial enhancement

W U

radius (m) temperature (K) thickness (m) circumferential distance between U-tubes (m) overall heat transfer coefficient (W/m2$K)

Greek symbols h efficiency ε emissivity m viscosity 4 volume concentration of nanoparticles Subscripts a ambient, apparent bf base fluid bulk bulk material c absorber coating, copper fin e edge f fluid g glass tube i inlet, inside l loss nf nanofluid o outlet p pipe s solid particle t tube u useful x length of section of U-tube

of the heat transfer performance is not attributed only to the enhancement of thermal conductivity due to the suspension of nanoparticles. Given the remarkable advantages of MWCNT nanofluid reported in previous studies [12e14], in the present study, it was selected as the working fluid in the solar collector for enhancing the heat transfer efficiency. The thermal performance of the individual EEUSC was investigated based on the knowledge of energy balance for evacuated U-tube solar collectors [16]. The effects of the heat loss coefficient and heat efficiency factor on the heat transfer efficiency were also studied. Further, the influence of the thickness of the air gap between the copper fin and the absorber filled with the filling liquid on the heat transfer efficiency was also evaluated. Finally, the cost savings from conserved energy and reductions in CO2 and SO2 emissions in the case of using multiple solar collectors were analyzed. 2. Simulation of solar collector 2.1. Simulation method of EEUSC First, the thermal performance of an individual EEUSC was investigated numerically. To this end, a one-dimensional (1D) analytical investigation of a single unit of the EEUSC was performed. This solar collector consists of a two-layered glass evacuated tube and an absorber tube. Vacuum is formed between the two glass tubes. The length and the outer and inner diameters of the glass tube are set as 1200, 47, and 37 mm, respectively. U-tubes are fabricated inside a circular fin. Fig. 1 shows the all-glass evacuated tubes. Solar radiation (denoted by G) absorbed by the selective absorbing coating is the incident solar radiation. Owing to the

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465

Fig. 1. Schematic of enclosed-type evacuated U-tube.

reflectance and absorptance of the outer glass tube and the heat loss of the absorber tube, the useful heat gain, which refers to the solar energy transferred to the working fluid, is equal to the difference between G and the thermal loss through the glass tube due to radiation, conduction, and convection heat transfer. To simplify calculations without losing accuracy significantly, several assumptions are made: the overall heat transfer coefficient of the evacuated glass tube to the surroundings is fixed as constant, the overall heat transfer coefficient from the header tube is also fixed as constant, and the air convection in the evacuated tube is neglected. In addition, the heat transfer considered in this model is assumed to be steady [17]. According to the energy balance equation, the useful energy gained from the solar collector is equal to the solar radiation collected by the solar collector minus the energy loss to the environment, as shown in Fig. 2, and it is expressed as

Qu ¼ I  QL

(1)

where I is the amount of solar energy absorbed by the selective absorbing coating.

The total overall heat transfer coefficient of the solar collector is calculated as

UL ¼ Ut þ Ue

(2)

Here, Ue is the overall heat transfer coefficient of the header tube edge, which is strongly affected by the thermal conductivity and thickness of the insulation and the surface areas of the header tube. The overall heat transfer coefficient through the edge should be referenced to the outer surface area of the absorber tube, As [18], as follows:

Ue ¼

ðUAÞp

(3)

As

Moreover, the overall heat transfer coefficient from the absorber tube to the ambient (Ut) can be expressed as

Ut ¼

1 hga

1 þ h1

(4)

pg

Here, hga is the convective heat transfer coefficient from the outer glass tube to the surroundings and is referenced from Tian

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Fig. 2. Thermal network of enclosed-type evacuated U-tube solar collector.

[19], with a value of about 12.7 W/(m2$K). hpg is the sum of hpgd and hpgc, which denote the heat transfer coefficient through the thermal conductivity and the radiative heat transfer coefficient between the absorber tube and the glass tube, respectively.

hpg ¼ hpgd þ hpgc

(5)

According to a previous study [20] and the heat transfer coefficient of the outer surface area of the absorber tube of this study, hpgc is calculated to be 0.2796 W/(m2$K). hpgd changes with the absorber tube temperature Tt and the outer glass tube temperature Tg, and can be expressed as

hpgd ¼

sεp 1þ

εp d ε g dg

ð1  εp Þ

ðT2b þ T2b ÞðTb þ Tg Þ

(6)

where εp is the emissivity of the absorber coating; εg , the emissivity of the inner surface of the outer glass tube; dg, the outer glass tube diameter; and s, the StefaneBoltzmann constant, equal to 5.67  108. Generally, when εp is very small, as is the case in this study, it is expressed as

hpgd ¼




sεp T2b

þ

T2g

  Tb þ Tg

(7)

kd

 kd

dT j dx xþDx

 þ Qu Dx ¼ 0

(10)

Considering the air gap between the absorber tube and the fin, the above equation can be rewritten as

Qu ¼ d

Tt  T ab

kab

þ kdcom

¼

com

Tt  T Ca

(11)

Here, dab and dcom are the thicknesses of the absorber tube and the filled component, respectively, and kab and kcom are their corresponding thermal conductivities. Considering the effect of filling material, liquid is an ideal choice. Then, water, benzene, and NaeK alloy, with thermal conductivities of 0.62 W/(m$K), 0.16 W/(m$K), and 23 W/(m$K), respectively, are chosen as the filling materials instead of air. Ca is the apparent conductance of the filling material. The boundary condition is expressed as

 dT  ¼ 0; dx x¼0

Tjx¼Wd ¼ Tb

(12)

2

To simplify the calculation, let m be defined as

m2 ¼

From Fig. 1(b), the overall heat transfer coefficient of the evacuated tube can be expressed as



UL



(13)

L kd 1 þ U Ca

The temperature variation can be calculated as

    Ut ðTt  Ta Þ ¼ hpgd Tt  Tg þ hpgc Tt  Tg

(8) cosmx

Based on all the above equations, it can be summed up that D, Dg, εp, εg, s, hga, and hpgc are all known, and the unknown parameters are Tt, Ta, Ut, hpgc, and Tg. If the ambient temperature and absorber temperature are given, the other parameters can be determined using these equations. In fact, according to Tian [19], UL is not a constant. It is a function of Tt and Ta, and this relation can be written as

T¼

8   0:0025009Tp  Ta  þ 0:70032 > > < 0:0025418Tp  Ta  þ 0:74336 UL ¼ 0:0025764Tp  Ta  þ 0:78825 > > : 0:0024599 Tp  Ta þ 0:85418

q0u ¼

263K 273K 283K 293K



dT j  dx x

 Ta < 273K  Ta < 283K  Ta < 293K  Ta < 308K (9)

To simplify the analysis, some simple assumptions are made. The first assumption is that the absorber tube is parallel to the copper fin, and can thus be regarded as a flat plate. Because the absorber coating is very thin, the temperature gradient in the radial direction is negligible, as is the temperature gradient in the flow direction along the tube. Based on the assumptions above, the heat balance on the copper fin can be analyzed, as shown in Fig. 3. From the configuration of the copper fin, its length is (Wd)/2, with a width of Dx in an elemental region and a unit length in the flow direction; then, the heat balance equation can be expressed as

Þ cosð mðWdÞ 2

ðTb  Ta 

I I Þ þ Ta þ UL UL

(14)

The heat gain is equal to sum of energies collected from both sides of the U-tube and the heat gain in the tube. The heat gain is also equal to the energy transferred to the fluid. These relations can be written as

F¼

ððW  dÞF þ dÞðI  UL ðTb  Ta ÞÞ L 1þU Ca


tanh mðWdÞ 2

q0u ¼

mðW  dÞ=2 Tb  Tf 1 hfu pd

þ k1

(15)

(16)

(17)

c

where kc is the thermal conductivity of the copper fin on the Utube. Furthermore, the thermal resistance of the tube's thickness is neglected. Through combination of the equations in order to solve tb and their substitution into Eq. (15), the net heat gain can be expressed as

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467

Fig. 3. Energy analysis of copper fin in EEUSC.



 q0u ¼ WF 0 I  UL Tf  Ta

(18)

Further, the collector efficiency factor (F0 ) and the heat transfer coefficient between the fluid and the copper tube (hfu) can be calculated by the following equations:

F0 ¼

1 UL

0

1

(19)

U C B 1þ CaL 1 1C WB @UL ððWdÞFþdÞ þ hfu pd þ kc A

reported a good agreement on the density between their experimental values and the equational values which can be obtained by Xuan and Roetzel [22].

  ð1  4Þrbf Cpðbf Þ þ 4rs Cp s   Cp nf ¼ rnf

(26)

rnf ¼ ð1  4Þrbf þ 4rs

(27)

4¼W

1 hnf

!

(20)

þ kdt

t

The heat gained by the fluid in the pipe is measured and given by

_ p ðTo  Ti Þ Qu ¼ mC

(21)

The thermal efficiency of the solar collector is defined as

h¼

_ p ðTo  Ti Þ mC AG

m_ ¼ rnf V_

(22) (23)

where m_ and Cp are the mass flow rate in the manifold and the specific heat of the working fluid in the manifold, respectively. 2.2. Calculation of nanofluid properties

knf ¼ kbf ð1  4Þ þ bks 4 0:75ds =ls 0:75ds =ls þ 1

(24)

(25)

where, kbf and ks are the thermal conductivities of the base fluid and nanoparticles, respectively; kbulk, ds, and ls are the thermal conductivity of the bulk material, characteristic length of the nanoparticles, and the mean free path of heat carriers in the nanoparticles, respectively. Cp is the specific heat of the working fluid with nanoparticles; in this study, we used the equation of Cp proposed by Xuan and Roetzel [22], which assumes thermal equilibrium between the nanoscale solid particles and the liquid phase. Pak and Cho [23]

(28)

water þW r water

Here, rs is the nanoparticle density, whose value is 2100 kg/m3; 4 is the volume concentration; Wwater ¼ 100 g; Wnf is the mass of the nanocomposite; and rwater ¼ 998.5 kg/m3. Generally, considering the small size and low volume fraction of particles in most nanofluids, it might be reasonable to treat nanofluids as pure liquids. The tube is not completely under a heat flux, but for simplification, it is assumed to be completely under a constant heat flux. The predicted values for a single-phase fluid can be referenced from the existing equation of Shah [24]:

8 > > > > > <

1  d 3 1:953 RePr i x Nu ¼   > > di > > 4:364 þ 0:0722 RePr > : x

 : :

RePr

di x

  33:33

  d RePr i < 33:33 x (29)

hnf ¼

Let knf be the thermal conductivity of the nanofluid; it is a sum of the thermal conductivities of the base fluid and nanoparticles [21].

ks ¼ kbulk

nf

rnf

1

hfu ¼

Wnf rnf

Nuknf di

(30)

Kumaresan et al. [13] validated the correlation in Eq. (29) with experimental data by using an MWCNT/water fluid, and they reported that the modeling results agreed well (±5%e12.5%) with the experimental results; in particular, the deviation was very small when x/di was larger than 200, which is similar to the results in this study. The Reynolds number for flow in a circular tube is defined as in Eq. (31), and viscosity can be calculated by the equation of Brinkman [25].

Re ¼

mnf ¼

4m_ pdmnf mwater ð1  4Þ2:5

(31)

(32)

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2.3. Environmental and economic analyses The unit for the energetic evaluation of various energy carriers has been specified by the European Nuclear Society (ENS) [26] as follows: 1 kg of coal equivalent corresponds to an energy value of about 29.3 MJ, and 1 kg of coal can release 2.62 kg CO2 and 0.0085 kg SO2, equivalently. Here, the mass of coal equivalent saved using the solar collector can be calculated as follows:

mcoal ¼

Qu Q coal

(33)

where Qu is the useful heat gained by the solar collector and Qcoal is the heat gain released by 1 kg of coal equivalent. According to the statistical data of Lindsay [27] the electricity prices in China, USA, UK, and Germany are 8, 12, 20 35 US cent/ kWh, respectively [27]. Conversion of the heat gain into consumed electricity according to Eq. (34) can also provide the energy cost savings in different countries.

Еele ¼

Qu Q ele

(34)

3. Experimental setup and method 3.1. Preparation of MWCNT/water nanofluid Some previous studies reported the usefulness of various surfactants such as gum arabic as dispersing agents for the preparation of MWCNT/water nanofluids. Gum arabic is capable of withstanding higher temperatures and better maintaining the colloidal stability of nanofluids than other surfactants [28]. In several previous works [28,29], the concentration of 0.25 wt% of gum arabic has been used for keeping MWCNT/water nanofluids stable; therefore, this concentration of gum arabic was used in the present study as well. To conduct a performance test of MWCNT/water nanofluid, the required amount of base fluid (water) was taken, and then the pristine MWCNT was added to it after probe sonication for 5 h, because the maximum nanotube surface can be sure to exposure to gum arabic for encapsulation. After probe sonication, the requisite amount of gum arabic was added to the nanofluid, which was then stirred in a magnetic stirrer to enable encapsulation of the MWCNT. This method enabled the preparation of stable nanofluids with a maximum MWCNT concentration of 0.5 wt%. According to Eq. (28), the maximum concentration of MWCNTs is limited to 0.24 vol%. Thus, in this study, the concentration of MWCNT nanoparticles was set to 0.2 vol% for the performance test under stable conditions. In general, the absorbance of MWCNTs is 10%e20% higher than that of water. Moreover, the absorbance of MWCNTs increases with their increasing concentration. Therefore, the MWCNT/water nanofluid is selected as the working fluid to absorb solar radiation directly. However, in this study, in the evacuated double-tube with the U-tube solar collector, the nanofluid worked to transfer heat from the solar collector to the heat storage tank, which implies that the improving effect of absorbance was negligible. 3.2. Test facility and conditions The solar collector of this study is a concentric EEUSC, which can be operated reliably to generate heat for residential applications. MWCNT/water nanofluid was used as the working fluid to improve the heat transfer efficiency. In consideration of energy usage in winter, the solar collector was installed facing south with a tilt

angle of about 45 ; the installation location was Gwangju, Korea, at a latitude and longitude of 35 and 126 , respectively. The closed-loop configuration of the test setup for measuring efficiency is shown in Fig. 4. The setup consists of the evacuated tube collector with the U-tube, two constant-temperature baths, a pump, two heat exchangers, and measurement devices. The specifications of the solar collector are listed in Table 1. The volume/ surface area and volume/cross-sectional area of the solar collector are 11.75 and 36.9 m3/m2, respectively. Generally, those of commercial solar collectors are 11e14 m3/m2 and 35e45 m3/m2, respectively. With decreasing volume/surface area, the heat loss increases, leading to a deterioration of the thermal performance of the solar collector. Further, the amount of heat collected by the solar collector decreases with a decrease in its volume/crosssectional area. The employed measurement devices were a pyranometer, flowmeter, and thermocouples, whose measurement ranges and accuracies are listed in Table 2. The constant-temperature bath employed here is also used in the 10RT refrigeration system with a 1-ton capacity for the working fluid. The measured physical quantities were the working fluid temperatures at the inlet and outlet of the collector, the ambient temperature, the flow rate of the circulating water, and the incident solar irradiance. The solar radiation was measured with the pyranometer, which was installed on the solar collector. The aperture of the pyranometer was leveled with that of the collector without casting a shadow on the collector. The radiation was continuously recorded along with the remaining data streams. An ambient temperature sensor was installed behind the collector and away from direct irradiance. The temperatures were measured with T-thermocouples placed in the inlet and outlet of each filled component. The flowmeter was used to measure the flow rate of the circulating fluid, which was circulated by the pump. Water flowed through the circulating pump and to both the collectors, and then collected in the constant-temperature bath in order to exchange heat with the water inside the tank. After that it flows into the flowmeter and water heater. The mass flow rate of the working fluid was kept constant at 0.01 kg/s. 4. Results and discussion Fig. 5 shows the variation of the specific heat, density, and thermal conductivity of the nanofluid with the volume concentration of the nanoparticles. The density of the nanofluid was directly proportional to the volume concentration of the nanoparticles; this relation can be explained by Eq. (28). The density increased from 992.2 kg/m3 to 1003 kg/m3 when the volume concentration increased from 0 vol% to 1 vol%. Table 3 lists the properties of the MWCNTs and water. Similarly, the thermal conductivity of the nanofluid was also directly proportional to the volume concentration of the nanoparticles, and it increased by 15% at the volume concentration of 0.3 vol% and by about 50% at 1 vol%. In contrast, the specific heat of the nanofluid was inversely proportional to the volume concentration of the nanoparticles. Substitution of lower specific heat of nanoparticles from Table 3, According to Eq. (26), a decrease in the specific heat of the nanoparticles will cause a reduction in the overall specific heat of the nanofluid. Specific heat refers to the energy required for raising the temperature of a unit mass of a substance by 1  C. This implies that different amounts of heat energy are required for increasing the temperatures of similar masses of different substances by 1  C. A lower specific heat of a nanofluid will result in a smaller amount of energy being required to increase its temperature. Fig. 6 shows the heat transfer coefficient between the working fluid and the U-tube as a function of the Reynolds number for various MWCNT volume concentrations. It can be seen that the

Y. Tong et al. / Renewable Energy 83 (2015) 463e473

469

Fig. 4. Schematic diagram of test facility for estimating efficiency of solar collector.

Nusselt number increases with increasing Reynolds number at a given MWCNT volume concentration. The Nusselt number of a nanofluid with nanoparticles is significantly enhanced compared with that of water, and the Nusselt number is directly proportional to the Reynolds number. In addition, the heat transfer coefficient between the working fluid and the U-tube wall at the concentration of 0.24 vol% is higher than that at other concentrations in the steady state, and the average increment of the heat transfer coefficient of the nanofluid relative to that of water is 8%. Even though the 0.24 vol% concentration shows the highest heat transfer coefficient in the simulation, in the experiment, the concentration of the MWCNT nanofluid was set to 0.2 vol% to ensure stable properties of the nanofluid. specific heat of the working fluid corresponding to the 0.2 vol% concentration of the MWCNT nanofluid were applied in the experiment under varying operating conditions. To examine the reliability of the analytical method, the

Table 1 Specifications of solar collector. Parameter

Specification

Outer tube outer diameter (mm) Outer tube thickness (mm) Transmittance Inner tube outer diameter (mm) Inner tube thickness (mm) Absorptivity of absorber tube Solar collector length (mm) Emissivity of absorber tube Copper fin thickness (mm) Thermal conductivity of copper fin (W/m$K) Air gap (mm) Thermal conductivity of air gap (W/m$K) U-tube outer diameter (mm)

47 2 0.907 37 2 0.93 1200 0.06 0.6 307 1.5 0.025 8

simulated efficiency of the solar collector was compared with the experimental results, as shown in Fig. 7. The data in the figure show good agreement between the simulation and experimental results. Even though some nanoparticles would have deposited in some parts of the experimental equipment, the difference in the mean efficiency of the EEUSC between the experimental and simulation results was only about 2.7%, which indicates that the developed analysis method is reasonable and highly accurate in the analysis of the thermal performance of solar collectors, especially when the normalized temperature is low. However, the difference between the simulation and experimental results increased with an increase in the normalized temperature. The fitted equation for the experimental results can be written as Y ¼ 220.35Х þ 57.5, and the Rsquared value is 0.93. This difference between the simulation and experimental results is attributed to the thermal conductivity being regarded as a constant, when it will actually increase with an increase in temperature. Furthermore, some heat loss is neglected during the calculation, which will also contribute to the differences between the simulation and experimental results. As mentioned earlier, the data in Fig. 7 demonstrate the good agreement between the simulation and experimental results. Fig. 8

Table 2 Measurement range and accuracy of variables. Item Solar radiation Temperature Flow rate

Equipment Pyranometer T-type thermocouple Flowmeter

Range

Accuracy 2

0e2000 (W/m ) 200e200 ( C) 2e350 (LPM)

±0.15% ±0.75% ±0.15%

Fig. 5. Variation of nanofluid properties with volume concentration of nanoparticles.

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Table 3 Properties of nanomaterial and base fluid. Material

Specific heat (J/kg$K)

Thermal conductivity (W/m$K)

Density (kg/m3)

MWCNT Water

711 4182

3000 0.6178

2100 992.2

shows the 3D plot of the relationship between the collector efficiency and the operating conditions. Here, the mass flow rate of the working fluid and the inlet temperature of the solar collector were set to 0.01 kg/s and 303 K, respectively. The efficiency of the solar collector increases with the incident solar radiation, and a higher ambient temperature will result in a higher efficiency at a constant collector inlet temperature. Under the given ambient temperature condition, the efficiency of the collector increased sharply to almost 300 W/m2, and then, the rate of increase reduced gradually and finally became almost constant. The reason for this trend is that under low radiation conditions, the heat flux is very low, which results in the relatively lower heat transfer rate between the U-tube and the working fluid.

The variation in the heat transfer coefficient through the thermal conductivity between the absorber tube and the glass tube with the difference between the absorber tube temperature and the ambient temperature (Tp  Ta) is shown in Fig. 9. The radiative heat transfer coefficient increased with increasing Tp  Ta. When Tp  Ta increased from 0 to 70 K, hpgd increased by nearly 60%. The relation of hpgd with Tp  Ta was not linear. In addition, the temperature increment of the outer glass tube with increasing Tp  Ta was also not linear. Furthermore, given a constant difference Tp  Ta, hpgd increases when the ambient temperature Ta increases, which indicates that the thermal loss will increase with an increase in the absorber tube temperature. Owing to the larger radiative heat transfer coefficient from the absorber tube to the glass tube, when Tp  Ta increased from 0 to 70 K, the outer glass tube temperature increased while the difference of Tg between air temperature of 263 K and 283 K was about 6 K. Fig. 10 shows the variations of the solar collector efficiency and the absorber coating temperature with the apparent conductance Ca when the solar radiation was set as 1000 W/m2, the ambient temperature was 283 K, and the inlet temperature of the working fluid of the solar collector was 313 K. The influence of the apparent conductance was found to be significant and the efficiency increased from 45% to 53% when Ca changed from 5 to 30 W/(m$K). It was also found that when Ca was larger than 80 W/(m$K), the change in the efficiency and coating temperature could be neglected, and in this case, the change was within 0.1% per 10 W/ (m$K) increment. In other words, the thermal resistance of the air gap can be ignored. According to Eq. (11), when the thickness and thermal conductivity of the absorber tube are 1.2 mm and 1.2 W/ (m$K), respectively, and the thickness of the air gap is 1.5 mm, the critical point of the filled component's thermal conductivity is about 0.17 W/(m$K). Additionally, Ca also has a significant influence on the absorber coating temperature. Generally, an air gap exists between the U-tube and the copper fin owing to a limitation of the manufacturing process. Fig. 11 shows the variation of the solar collector efficiency with solar radiation for various filling materials. The efficiency increased with increasing solar radiation when the ambient temperature was fixed and gradually became constant. In this study, four different materials between the copper fin and the absorber tube were studied. The results indicate that a higher thermal conductivity will result in

Fig. 6. Heat transfer coefficient as a function of Reynolds number.

Fig. 7. Efficiency variation of enclosed-type evacuated U-tube solar collector.

Fig. 8. Analysis of efficiency of EEUSC as a function of Ta and G.

Y. Tong et al. / Renewable Energy 83 (2015) 463e473

Fig. 9. Variation of hpgd and Tg with changes in absorber coating temperature.

a higher efficiency, especially compared to the air one. The difference in efficiencies between the NaeK alloy and air was about 2.3% for 100 W/m2 solar radiation and 4% for 900 W/m2, and this difference increased with increasing solar radiation. However, when the apparent conductance of the material is larger than 0.17 W/ (m$K), the change in the efficiency can be neglected. The results also show that the efficiency difference between C6H6 and NaeK alloy was 0.3% for the 100 W/m2 solar radiation and increased to 0.6% for the 900 W/m2 solar radiation; in contrast, the corresponding efficiency differences between water and NaeK alloy were 0.1% and 0.2% for the 100 W/m2 and 900 W/m2 solar radiations, respectively. These results validate the conclusion made from Fig. 10 that the efficiency will increase with the increase in the conductance of the material between the copper fin and the absorber tube, but it will gradually become constant with a further increase in conductance. Because of the similar results for benzene, water, and NaeK alloy, some factors can be taken into consideration, such as avoiding a chemical reaction between them and the copper fin. Moreover, because NaeK is a hazardous substance, water is a good choice as a filling material owing to its relatively higher thermal conductivity; the critical conductance is 0.17 W/ (m$K), whereas the conductance of the air gap is just 0.025 W/

Fig. 10. Influence of apparent conductance of filling material on solar collector efficiency and absorber coating temperature.

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(m$K). In addition, use of water will not increase the system cost significantly, because it will not be needed in large quantities and it will not corrode the copper fin. Fig. 12 shows the variation of the solar collector efficiency and the absorber coating temperature with the mean temperature of the working fluid for different filling materials. Fig. 12 indicates that the efficiency of the solar collector decreases with increasing mean temperature of the working fluid in the U-tube. NaeK shows the best efficiency out of all the filling materials. The difference in mean efficiencies between NaeK and air is about 3.6%, but that between NaeK and water is just 0.1%. This also signifies that the absorber coating temperature is proportional to the mean temperature of the working fluid in the U-tube, and the higher the thermal conductivity of the filling material, the lower is the absorber coating temperature. The mean difference in the absorber coating temperatures between NaeK and air is nearly 42 K, whereas that between water and NaeK is 3 K, and as mentioned in the discussion of Fig. 9, a higher absorber tube temperature corresponds to a higher thermal loss and a lower efficiency of the solar collector. Fig. 13 shows the relationship between the solar collector efficiency and the difference between the collector inlet temperature Tin and the ambient temperature Ta (i.e., Tin  Ta) in the case of the MWCNT/water nanofluid. It is seen that Tin  Ta is an important factor affecting the thermal performance. When Tin  Ta is changed from 10 to 90 K, the changes in the collector efficiency for radiation ranging from 200 to 1000 W/m2 are 28.6%, 15%, 11%, 8.6%, and 7%, respectively. This result indicates that a smaller Tin  Ta corresponds to a higher efficiency. Fig. 14 shows the energy saving achieved when water with 0.24 vol% MWCNT nanofluid is filled in the air gap. The measured data reveal that October shows the highest solar radiation in a year whereas July shows the lowest. This is because in Gwangju, July falls in the monsoon season and there are many rainy days in this month even though the ambient temperature is the highest in this month. It can also be seen from the figure that the daily energy enhancement for one solar collector ranges from 785 kJ in July to 1200 kJ in October. Calculation for the monthly energy enhancement reveals that October shows the highest monthly energy enhancement, of 37.2 MJ per solar collector, whereas July shows the lowest monthly energy enhancement, of about 25 MJ per solar collector. Generally, numerous solar collectors are used in solar

Fig. 11. -Variation of solar collector efficiency with amount of solar radiation for different filling materials.

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Fig. 14. Energy enhancement for each month. Fig. 12. Solar collector efficiency and absorber coating temperature as functions of mean temperature of working fluid for different filling materials.

Fig. 13. Variation of solar collector efficiency with varying Tin  Ta.

collector arrays which indicates that the enhancement margin will widen with the passage of time. According to the results of this study, the use of 50 solar collectors would result in a yearly energy enhancement of about 18,008 MJ under the considered operating conditions. Generally, the coal equivalent of energy is 29,306 kJ, which means that the complete combustion of 1 kg of coal equivalent can release 29,306 kJ [26]. However, this will simultaneously release CO2 and SO2 to the atmosphere and may intensify the greenhouse effect. After calculation, the yearly energy enhancement for a solar collector is equal to 12.3 kg of coal equivalent, and the corresponding amounts of CO2 and SO2 emissions prevented by the use of each collector would be 32.3 kg and 0.105 kg, respectively. The variations of the mass of coal equivalent and amounts of pollutant emissions prevented according to the number of solar collectors are shown in Fig. 15(a). It is seen that when the array has more than 50 solar collectors, very large amounts of pollutant emissions are prevented, and these amounts would actually be similar to those present in a large area, such as a city or even a state. Specifically, when the array has 50 solar collectors, the release of 1611.3 kg of

Fig. 15. Analyses of environmental and economic aspects relating to use of solar collectors.

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CO2 and 5.228 kg of SO2 would be prevented in a year. Therefore, when numerous solar collectors are installed, the prevented amounts of pollutant emissions can be significant and very meaningful. Further, as shown in Fig. 15(b), from an economic viewpoint, conversion of energy saved in a year to electricity power reveals savings of 5002 kWh of electricity power for 50 solar collectors [27]. Further, the energy cost saving for 50 solar collectors is calculated to be more than USD 1750 for Germany, and about USD 1000, 600, and 400 for UK, USA, and China, respectively. 5. Conclusions This study theoretically investigated an enclosed-type evacuated U-tube solar collector with MWCNT/water nanofluid. Results of the study demonstrated that nanofluid with a 0.24 vol% concentration showed the highest heat transfer coefficient between the tube and the working fluid and the heat transfer coefficient was about 8% higher than that when only water was used. Further, an analytical method (simulation model) was developed for calculating the solar collector efficiency and the outlet temperatures. It was found that the collector efficiency increased rapidly with increasing radiation in low-radiation conditions but remained relatively constant in high-radiation conditions. The developed method was validated through comparisons with experimental results, and good agreement was observed between the simulation and experimental results of the solar collector efficiency. It was also found that when the conductance of the filling material was larger than 0.17 W/(m$K), the enhancement of thermal performance of the solar collector could be neglected. From viewpoints of safety and cost, water is an ideal choice for the filling material, with a conductance higher than 0.17 W/(m$K); it will neither corrode the copper tube nor increase the cost significantly. When water is substituted for air in the gap between the copper fin and the absorber tube, the solar collector efficiency increases by about 4%. Further, the use of 50 solar collectors can save about 615 kg of coal yearly, which would have released 1600 kg CO2 and 5.3 kg SO2. Evaluation from environmental and economic viewpoints reveals that the performance improvement of solar collectors reported in this study can contribute to mitigation of greenhouse effects and enable significant cost savings. References [1] Liu L, Wang Z, Zhang H, Xue Y. Solar energy development in China e a review. Renew Sust Energy Rev 2010;14:301e11. [2] Badran AA, Yousef IA, Joudeh NK, Hamad RA, Halawa H, Hassouneh HK. Portable solar cooker and water heater. Energy Convers Manag 2010:1605e9. [3] Frier D, Cable RG. An overview and operation optimization of the Kramer junction solar electric generating system. ISES World Congr 1999:241e6. [4] Georgiev A. Simulation and experimental results of a vacuum solar collector system with storage. Energy Convers Manag 2005;46:1423e42.

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