Effects of CuOwater Nanofluid on the Efficiency of a Flat-plate Solar

8/12/2019 Effects of CuOwater Nanofluid on the Efficiency of a Flat-plate Solar

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Effects of CuO/water nanofluid on the efficiency of a flat-plate solar

collector

Ali Jabari Moghadam a,, Mahmood Farzane-Gord a, Mahmood Sajadi a, Monireh Hoseyn-Zadeh b

a Department of Mechanical Engineering, Shahrood University of Technology, Shahrood, Iranb Faculty of Science, Ferdowsi University, Mashhad, Iran

a r t i c l e i n f o

Article history:

Received 7 March 2014

Received in revised form 4 June 2014

Accepted 13 June 2014

Available online 24 June 2014

Keywords:

Flat plate solar collector

Nanofluid

Experimental study

Efficiency

a b s t r a c t

Solar water heating is an effective method for heat demands in domestic applications. Solar collector is a

main component of any solar water heating system. In this work, the effect of CuOwater nanofluid, as

the working fluid, on the performance and the efficiency of a flat-plate solar collector is investigated

experimentally. The volume fraction of nanoparticles is set to 0.4% and the mean particle dimension is

kept constant at 40 nm. The working fluid mass flow rate is varied from 1 to 3 kg/min. The experiments

are conducted in Mashhad, Iran with the latitude of 36.19. The experimental results reveal that utilizing

the nanofluid increases the collector efficiency in comparison to water as an absorbing medium. The

nanofluid with mass flow rate of 1 kg/min increases the collector efficiency about 21.8%. For any partic-

ular working fluid, there is an optimum mass flow rate which maximizes the collector efficiency. Adding

nanoparticles to a base fluid produces a nanofluid which has enhanced thermal characteristics compared

with its base fluid.

2014 Elsevier Inc. All rights reserved.

1. Introduction

Solar heat is being widely used for providing heat for many

houses. Solar Water Heating is an effective method of utilizing

solar heat to perform many useful tasks. The energy from the

sun can provide hot water for many applications, displacing the

need to burn fossil fuels. A solar collector is the main component

for absorbing heat from solar beam and utilizing it for heating

purposes. One way to absorb more heat from the solar beam is

to modify heat characteristics of the working fluid.

Nanofluids are suspensions of metallic or nonmetallic nanopar-

ticles in a base fluid; this term was introduced by Choi [1]. A sub-

stantial increase in liquid thermal conductivity, liquid viscosity,

and heat transfer coefficient are the unique characteristics ofnanofluids. It is well known that metals in solid phase have higher

thermal conductivities than those of fluids [2]. For example, the

thermal conductivity of copper at room temperature is about 700

times greater than that of water and about 3000 times greater than

that of engine oil. The thermal conductivity of metallic liquids is

much greater than that of nonmetallic liquids. Thus, fluids contain-

ing suspended metal particles are expected apparent enhanced

thermal conductivities rather than pure fluids [3]. Masuda et al.

[4] dispersed oxide nanoparticles (Al2O3 and TiO2 with 4.3 wt%)

in liquid and showed that the thermal conductivity is increased

by 32% and 11%, respectively. Grimm [5] dispersed aluminum

particles (180 nm) in a fluid and claimed a 100% increase in the

thermal conductivity of fluid for 0.510 wt%.

Using the nanofluids in solar collectors has been subjected to a

few recent studies. Yosefi et al. [6] investigated the effect of

MWCNT as an absorbing medium on the efficiency of a flat-plate

solar collector experimentally and reported 35% enhancement in

the collector efficiency for 0.4 wt%. Also the same researchers[7]

repeated the experiments with Al2O3Water nanofluid and

reported 28.3% enhancement in the collector efficiency for

0.2 wt%. Chaji et al. [8]used TiO2Water nanofluid as a working

fluid at a small flat plate solar collector and observed 15.7%enhancement in the collector efficiency (compared with pure

water). Polvongsri and Kiatsiriroat [9] investigated the thermal

enhancement of a flat plate solar collector with silver nanofluid.

They concluded that using this nanofluid can improve thermal

performance of flat plate collector compared with water especially

at high inlet temperature. He et al.[10]investigated the light-heat

conversion characteristics of two nanofluids, waterTiO2 and

watercarbon nanotube (CNT), in a vacuum tube solar collector

under sunny and cloudy weather conditions. The experimental

results show very good light heat conversion characteristics of

the CNTH2O nanofluid with the weight concentration of 0.5%.

http://dx.doi.org/10.1016/j.expthermflusci.2014.06.014

0894-1777/ 2014 Elsevier Inc. All rights reserved.

Corresponding author. Address: Mechanical Engineering, Shahrood University

of Technology, P.O. Box 316, Shahrood, Iran. Tel./fax: +98 273 3300258.

E-mail address:[email protected](A.J. Moghadam).

Experimental Thermal and Fluid Science 58 (2014) 914

Contents lists available at ScienceDirect

Experimental Thermal and Fluid Science

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Because of the better light-heat conversion characteristics of the

CNTH2O nanofluid compared to the TiO2H2O nanofluid, the

temperature of the CNTH2O nanofluid is higher than that of the

TiO2H2O one. Lue et al. [11] examined thermal performance of

an open thermo-siphon which uses CuoWater nanofluid for

high-temperature evacuated tubular solar collectors. They showed

that with optimal filling ratio 60% and optimal mass concentration

1.2%, evaporating heat transfer coefficients may increase by about

30% compared with those of pure water. Keshavarz and Razvarz

[12] experimentally studied the effect of Al2O3/Water nanofluid

on the efficiency enhancement of a heat pipe at different operating

conditions. They concluded that the thermal efficiency of a heat

pipe charged with nanofluids is higher than that of pure water as

working fluid. Saidur et al.[13]also theoretically investigated the

effect of using Al2O3/Water nanofluid on the performance of direct

solar collector. They showed that using nanofluids within 1.0% vol-

ume fraction gave a promising improvement on the direct solarcollector performance. Sani et al.[14]introduced a new nanofluid,

made from dispersing carbon nanohorn in ethylene glycol, for solar

energy applications. Their results show that this nanofluid is useful

for increasing the efficiency of solar thermal devices and costs

reduction (in comparison with carbon-black nanofluid). Natarjan

[15]investigated the thermal conductivity enhancement of a base

fluid using carbon nanotube (CNT). According to their results, if

these fluids are used as heat transport media, the efficiency of

the conventional solar water heater will be increased. Tyagi et al.

[16] studied the capability of using a non-concentrating direct

absorption solar collector (DAC) theoretically and compared its

performance with a conventional flat-plate collector. In their

research, a nanofluid composed of water and aluminum nanoparti-

cles, was used as the absorbing medium. According to their results,the efficiency of a DAC with nanofluid is up to 10% higher than that

of a flat-plate collector. Otanicar [17]studied environmental and

economical effects of using nanofluids to enhance the solar collec-

tor efficiency compared with conventional solar collectors. Otani-

car et al. [18] studied experimentally the effect of different

nanofluids on the efficiency of the micro-thermal-collector. He

reported an efficiency improvement up to 5% by utilizing the nano-

fluids as the absorption medium. Mahian et al. [19]examined the

nanofluids applications in solar thermal engineering systems; in

this review, the effects of nanofluids on the performance of solar

collectors and solar water heaters were investigated from the effi-

ciency, economic and environmental considerations viewpoints.

The aim of the current experimental work is to investigate the

effect of using particular nanofluid, CuOH2O, as an absorbingmedium (the working fluid) on the efficiency of a flat-plate solar

collector. A review of the literature shows that there is no work

on the flat-plate solar collector performance using CuO/water as

the working fluid. For this purpose, a commercial flat plate collec-

tor is selected to carry out the experiments in NorthEast of Iran

during summer 2012. The effect of the absorbing medium mass

flow rate on the collector efficiency is investigated. The efficiency

values of nanofluid and water (as two working fluids) are

compared.

2. Experimental device and method

2.1. Experimental procedure

A schematic diagram of the experimental setup and the picture

are shown inFigs. 1 and 2, respectively. The solar collector perfor-

mance has been experimentally investigated in Mashhad, Iran (lati-

tude N and longitude 59.37E). The collector specifications are given

inTable 1. The working fluid is circulated through the collector by

using an electrical pump. The solar system tank serves as a heat

exchanger for absorbing the heat loaded from the collector and then

delivering it to the cooling water. The tank capacity is nearly 20 l. A

heat exchanger has been placed inside the tank to transfer heat load

from the solar collector to the cooling water. A flow meter was

installed on the pipe after the electric pump. A simple valve was also

installed after the electric pump to control the working fluid mass

flow rate. Two temperature sensors were used to measure the fluid

temperature at the inlet and outlet of the solar collector. The ambi-

ent temperature was measured by a thermometer. The total solar

radiation was measured by a TES 1333 R solar meter. Also the wind

Nomenclature

AC surface area of solar collector (m2)

Cp heat capacity (J/kg K)Cp,bf heat capacity of base fluid (J/kg K)Cp,np heat capacity of nanoparticles (J/kg K)FR heat removal factor

GT global radiation (W/m2)_m mass flow rate (kg/min)

n number of day in yearPt error of parametersPY overall errorQu rate of useful energy gained (W)t time (s)Ta ambient temperature (K)Ti inlet fluid temperature of collector (K)To outlet fluid temperature of collector (K)

To,i collector outlet initial fluid temperature (K)To,s collector outlet fluid temperature after times (K)UL overall loss coefficient of solar collector (w/m2k)Xt measured parameters in error analysisY the calculated quantity from the measured results

Greek Symbolssa absorptancetransmittance products time constant of solar collector (s)gi efficiency of flat-plate solar collectorb slope of collectord declinationu volume fraction; latitude

Fig. 1. Schematic diagram of the experimental setup (1. Collector 2. Pump 3. Heat

exchanger4. Tank 5. Thermometer 6. Solar meter 7. Controlvalve 8. Automate valve9. Rotameter).

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speed was measured by a PROVA (AV M-0736.19) anemometer. The

instruments were calibrated for each individual experiment.

In this study, deionized water and water-based CuO nanofluidswere used as the working fluid. Copper oxide spherical nanoparti-

cles with purity 99.9% and mean diameter of 40 nm are purchased

from NANOSANY CORPORATION. The black CuO nanoparticles

which are insoluble in water have a bulk density and true density

of 0.79 g/cm3 and 6.4 g/m3, respectively. The nanofluids have been

prepared by dispersing CuO nanoparticles into the base fluid

directly; after which they were oscillated continuously for about

2 h in an ultrasonic homogenizer (XL-2020). The volume fraction

of nanoparticles in the fluid was 0.4%. The prepared nanofluid

solution is illustrated inFig. 3. The nanoparticle diameter and the

nanofluid volume fraction were selected based on the previous

studies [618]. Otanicar et al. [16] have investigated the effects

of variety of nanoparticles: carbon nano-tubes, graphite, and silver

on performance of a flat plate collector and reported a rapid effi-

ciency increases up to volume fraction of 0.4 followed by a leveling

off from 0.5. Here 0.4 is selected as volume fraction for higher

collector performance and also to avoid particle precipitation and

instability in the base fluid.

2.2. Testing method

The ASHRAE Standard 86-93 [20] for testing thermal perfor-

mance of a solar collector is certainly one of the most important

standards used to evaluate the performance of flat-plate and

concentrating solar collectors. The standard calculates the perfor-

mance by obtaining the values of instantaneous efficiency for var-

ious combinations of incident radiation, ambient temperature, and

inlet fluid temperature. This requires experimental measurement

of the incidence solar radiation rate as well as the energy rate

absorbed by the working fluid. It should be pointed out that the

experiments have to be performed under a steady state or quasi-

steady-state condition. In addition, some experiments should be

performed to examine the collector transient thermal response

characteristics.

The optimum collector slope is calculated as follows[21]:

b j; dj 1

where ; is the latitude and d is given by:

d 23=45sin 360n 284

365

2

in which, n is the number of day in a year. In this study, the opti-

mum slope of the collector is obtained 17for a two-month period.

The collector heat capacity can be defined in terms of a time con-

stant. It is also necessary to determine the time response of the

solar collector in order to evaluate the transient behavior of the col-lector and to select the correct time intervals for steady-state effi-

ciency tests. The time constant of a collector is the time required

for the fluid leaving the collector to arrive 63% of its final steady

state value after a step change in incident radiation:

To;sTo;iTo;iTi

0:368 3

where,To,s is the collector outlet fluid temperature after time t,To,iis the collector outlet initial fluid temperature, and Tiis the collector

inlet fluid temperature.

The mass flow rate accuracy must be held within 1%, irradia-

tion must be steady within 50 W/m2, the outdoor ambient

temperature must not vary more than 1.5 K, and the inlet temper-

ature must be within 0.1 K for the entire test period. The pre-dataperiod is defined to maintain steady-state conditions during a

specified time interval prior to the data period. A test period con-

tains both the pre-data and the data periods. For outdoor tests with

a fixed test setup, the pre-data period is 15 min and the data period

is set to a 5 min interval or an interval equal to the collector time

constant.

2.3. Efficiency calculation

The ASHRAE Standard suggests performing the experiments in

various inlet temperatures. An acceptable distribution of inlet tem-

peratures is obtained by setting Tito ambient air temperature, 30%,

60%, 90% the manufacturers recommended maximum operating

temperature for the collector. After steady state conditions aremet, the data of each test period are averaged and used in the

Fig. 2. A picture of the experimental setup.

Table 1

Specifications of the flat-plate solar collector.

Specification Dimension Unit

Occupied area 200 94 9.5 cmAbsorption area 1.51 m2

Weight 38.5 kg

Frame (Al6063 extruded)

Glass (float) t= 4 mm

Header pipe (cu) U = 22, t= 0.9 mm

Connector riser pipe to absorber sheet (cu) U = 10, t= 0.9 mm

Absorption sheet:

Thermal emission: 7%

Solar absorption: 96.2%

Coating method: vacuum

Magnetron sputtering

Fig. 3. Production of nanofluid.

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decreasing the mass flow rate leads to increasing the collector

efficiency; since the low mass flow rate and hence the small fluid

velocity causes absorbing more solar energy. In this case, the

highest efficiency is achieved for 1 kg/min. Therefore, it can be

concluded that the optimum mass flow rate depends upon the

working fluid thermal characteristics. The Reynolds numbers for

mass flow rates of 3, 2, and 1 kg/min are 1857, 1104 and 977,

respectively [24]. Increasing mass flow rate increases Reynolds

and Nusselt numbers, but decreases the temperature difference

between the collector inlet and outlet. Thus, the heat transfer rate

is influenced by these two parameters. Table 3 show that the FR(sa)

value of the collector is highest for 1 kg/min, and theFRULvalue in

this mass flow rate is lowest. Therefore, based on Eq.(8), the high-

est efficiency of the solar collector is attained for this mass flow

rate. According toFig. 5, the solar collector efficiency is optimum

for the 1 kg/min mass flow rate.

Figs. 6 and 7show the effect of working fluids on the collector

efficiency for each mass flow rate. As shown in Fig. 6, the efficiency

of the flat-plate solar collector with CuO nanofluid at 3 kg/min is

almost the same as water at 2 kg/min (Fig. 7) and 1 kg/min

(Fig. 8). Generally, the solar collector efficiency with nanofluid is

higher than that of water. This can be deduced by comparing the

value ofFR(sa) (the absorbed energy parameter) and UL(heat loses)for CuO nanofluid and water inTables 2 and 3. The tables values

show that when the absorbed energy parameter is high and the

collector heat loss is low, the efficiency is increased. Averagely,

the efficiency values for 2 and 1 kg/min mass flow rates in the case

CuO nanofluid are higher than that of water by 4.74% and 21.8%,respectively.

Fig. 9shows that the efficiency of CuO nanofluid (1 kg/min) is

higher than that of water (2 kg/min) by 16.7%; the errors were

calculated for each data set. In general, there were 96 data for effi-

ciency calculations. Error values for each data set were calculated

by using Eq. (9). Sampling error calculation for one data set is given

inTable 4. The maximum and minimum values of error are found

to be 8.4% and 1.7%, respectively. When a nanofluid is selected as

the working fluid, its nanoparticles serve as traveling media, and

their mass migration phenomenon helps the heat transfer

enhancement. Adding CuO nanoparticles to water produces a

nanofluid that has some advantages (compared with pure water)

to enhance heat transfer. The darkened working fluid with better

absorptivity than water (and also enhanced thermal conductivity),

the Brownian motion of the particles as well as enlarged heat

Table 3

Thermal characteristics of the collector with nanofluid as working fluid in various

mass flow rate.

Mass flow rate (kg/min) s (sec) FR (sa) UL

1 87 0.7574 4.38

2 98 0.7081 11.17

3 110 0.5231 8.76

Fig. 6. The efficiencyof the flat-platesolarcollector forwater andnanofluid in 3 kg/min mass flow rate.

Fig. 7. The efficiencyof theflat-plate solar collector for water andnanofluid in 2 kg/

min mass flow rate.

Fig. 8. The efficiencyof theflat-plate solar collector for water andnanofluid in 1 kg/

min mass flow rate.

Fig. 9. The flat-plate solar collector efficiency for water and nanofluid at their

optimum mass flow rates.

Table 4

Sampling error calculation for one data set.

Inlet

temperature

Outlet

temperature

Total

radiation

Mass flow

rate

Data (Xt) 58.4 68.2 1255 0.033

Error (Pt) 0.1 0.1 1 0.0017@Y@Xt

Pt 0.0055 0.005498 0.00043 0.027766

Efficiency (Y) 0.544

Error of efficiency (PY) 0.028838

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transfer surface (between the nanoparticles and the base fluid), all

together improve the heat transfer process; and the experiment

results are well justified.

4. Conclusions

The effects of using CuO nanofluid as the absorbing medium

on the flat-plate solar collector efficiency have been studied

experimentally. The experiments are carried out in Mashhad, Iran

(latitude 36.19N and longitude 59.37E). The influence of the mass

flow rate on the solar collector efficiency has also been investi-

gated. The working fluid mass flow rate has been selected in the

range of 13 kg/min. The volume fraction of nanoparticles is set

to 0.4%; the particle dimension is 40 nm. The results demonstrate

that using CuOH2O nanofluid increases the solar collector effi-

ciency in comparison with that of water by 16.7% (especially in

the optimum mass flow rate). The experimental results also prove

that the highest heat absorption by the collector occurs at different

mass flow rates for water and nanofluid. The optimum mass flow

rate depends on the working fluid thermal characteristics.

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Effects of CuOwater Nanofluid on the Efficiency of a Flat-plate Solar

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