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Renewable Energy 29 (2004) 1489–1501

www.elsevier.com/locate/renene

Efficiency and exergy analysis of a new solarair heater

İrfan Kurtbas , Aydın Durmus̨Firat University, Technical Education Faculty, Mechanical Education Department, 23119 Elazığ, Turkey

Received 4 June 2003; accepted 18 January 2004

Abstract

It would be misleading to consider only the cost aspect of the design of a solar collector.High service costs increase total costs during the service life of solar collector. The mosteffective way to save energy is by increasing the efficiency in a solar collector by the heattransfer coefficient.

In our study, five solar collectors with dimensions of 0:9 0:4 m were used and the flow line

increased where it had narrowed and expanded geometrically in shape. These collectors wereset to four different cases with dimensions of 1 2 m. Therefore, heating fluids exit the solarcollector after at least 4.5 m displacement. According to the collector geometry, turbulenceoccurs in fluid flow and in this way heat transfer is increased. The results of the experimentswere evaluated on the days with the same radiation. The efficiencies of these four collectorswere compared to conventional flat-plate collectors. It was seen that heat transfer and pressureloss increased depending on shape and numbers of the absorbers.# 2004 Elsevier Ltd. All rights reserved.

Keywords: Air collector; Collector efficiency; Exergy loss

1. Introduction

The effects of material and construction of the absorber on the efficiency of thecollectors have been widely reported in the literature, but the influences of flow lineof the fluid on the efficiency of the collectors have not been studied in detail.

Flat-plate collectors have an important place among applications of solar energysystem. The main part of flat-plate collectors is black absorber surface. Because of

Corresponding author. Tel.: +90-424-2370000; fax: +90-424-2367064.

E-mail address: [email protected] (_II. Kurtbas).

0960-1481/$ - see front matter # 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.renene.2004.01.006

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this, several investigations were made on this subject in order to increase efficiencyof the collector and outlet temperature of fluid. The aim of these investigations is

Nomenclature

A collector surface area (m2)Ah channel cross-section area (m

2)C p specific heat (J/kg.K)DH hydraulic diameter (m)E exergy (W)E D dimensionless exergy loss () f friction coefficient ()h enthalpy (J/kg)I total solar radiation incident upon plate of the collector (W/m2)k adiabatic constant of the air (ffi1,4) ()

Nu Nusselt number ()P pressure (N/m2)Q useful heat gain (W)Pr Prandtl number ()R universal gas constant (J/kg. K)Re Reynolds number ()S entropy (J/kg.K)T temperature (K)T as surface temperature of the absorber (K)U channel perimeter exposed to air (m)

V average velocity of air (m/s)W work (J)a heat convection coefficient (W/m2.K)k heat conduction coefficient (W/m.K)g efficiency of air collector ()ṁ mass flow rate of air (kg/s)hT log logarithmic main temperature difference (K)q density of air (kg/m3)l dynamic viscosity of air (Pas. s)

Subscriptse environmenti inleto outletmax maximummin minimumR radiation

_I I . Kurtbas, A. Durmus̨ / Renewable Energy 29 (2004) 1489–15011490

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to develop a more efficient absorber, to increase the amount of energy obtained, todecrease the cost of energy provided from sun, to store the energy and to use itcontinuously.

Flat-plate collectors are classified into two groups according to fluid used. Wateris usually used in liquid collectors and air, in gas collectors. Since the air has worsethermodynamic properties in terms of heat transfer compared to liquid, theefficiency of air collectors is naturally of low value. Because of this, several types of solar air heaters have been proposed over the recent years in order to improve theirperformance. They are generally used for heating in conditioning and drying of agriculture situations.

A modified solar air heater, which incorporated aluminum wool on a perforatedplate placed diagonally on the passageway of the air to serve as a front absorbingmedium above the absorber plate was designed, conducted and tested [1]. Theefficiency of the air solar collector increased up to five-fold compared to the flat-surface collectors by using materials to increase the absorption surface area.Rectangular staggered fins are soldered on the collectors’ back [2]. The intersticesare inserted between two consecutive fins located in the same row. A turbulentfluid flow is developed which permits the improvement of the thermal heat transferof these collectors in comparison to the flat-plate. For the same fin configurations,the thermal heat transfer coefficient was evaluated with a selective or non-selectiveabsorber-plate. It was seen that the nature of the absorber plate (selective or non-selective) had no significant effect on the heat transfer and Nusselt number in

finned system collectors. In addition, there were no differences in friction factors. Itis only necessary to reduce the spacing between consecutive fin rows in order toincrease the heat transfer. A collector was designed in order to overcome the physi-cal problems of conventional flat-plate air collectors as well as the particular tech-nical problems of matrix air collectors [3]. The absorber of the collector consist of two parallel sheets of black oxidized or black galvanized industrial woven,fine-meshed wire screens which are made of copper. In this study, the followingresults are obtained; the thermal performance of the collector improved withincreasing mass flow rates due to an enhanced heat transfer to the air stream.There was little effect on its overall thermal efficiency at low mass flow rates

(10 g/s). The novel matrix air collector yielded an improved thermal performancewith higher heat transfer rates to the airflow and smaller friction losses comparedto flat-plate air collectors of conventional design. The surface of air collectors hav-ing V-corrugation surface, fin and flat-plate were designed. This surfaces were cov-ered with material of black copper-oxide having 0.15 emit coefficient and 0.9absorber coefficient [4]. In this study, the efficiency of the collectors was investi-gated by performing the experiments with different mass flow rate. It was seen thatparticular V-corrugation collector had both high thermal efficiency of collector andhigh loss of pressure. The efficiency of the collector was investigated by placing

parallel obstructions to the flow area in the flat-plate air collector [5]. The efficiencyof the collector increased with increasing numbers of fin. The experimental resultswere compared with the theoretical results. The optimization was also conducted

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for different intervals. It was seen that the optimum location of fins was in the mid-dle of the collector.

In this study, an absorber having five slices in a collector case was designed to

investigate the effect of the flow line of the fluid on the performance of solar collec-tors. This absorber slices have (0:9 0:4 m) dimensions, four different surfacegeometries, single passage, and narrowed-extended shape. In this way, the heattransfer was increased by being extended along the flow line of fluid (air) and chan-ging velocity and pressure in narrowed-extended area in which swirl and secondaryflows form. As known, swirl and secondary flows cause the convection coefficientof the heat transfer to increase.

2. Experimental set up

The experimental set up of the solar air collector is schematized as shown inFig. 1. Although, the collectors designed are composed of basically the sameelements present in the conventional flat-plate solar air collectors, it has specialconstructions due to the front absorption surface.

The absorbers were formed by a black-painted galvanized sheet with 0.8 mmthick. Type IV of the absorber is flat-plate with 25 mm gap between parallel plates.The air flow is provided as seen Fig. 1a–d. Type III is the onduline profile plate. Inthis type, the gap between plates is kept as 25 mm along the plates. The bottom

surface of type II is flat profile and the upper surface is onduline profile. In thetype I, the air to be heated leaves the absorber by passing from narrowed-extendedgap. The narrowest gap is 25 mm and the widest gap is 180 mm of the absorber.

Fig. 1. Experimental set-up.


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The sides of the air duct in the absorber were welded by soldering after thegalvanized sheet had been covered. Besides, the area welded was covered by jointseal in order to prevent heat leaking. Five slices were placed in the collector cases

with 1 2 m dimensions. The collector material of the cases was chosen from agalvanized sheet 0.4 mm thick. A single glazing was chosen in order to maximizethe radiation impact on the absorber surface and to reduce costs. To minimize theheat losses from the sides and from the bottom of the collector were insulated byglass wool, which has low heat conductive coefficient (k ¼ 0:038 W=m:K).

The air was provided by a radial fan with a maximum 0.31 m3/s mass flow rates.The radial fan placed at the outlet of the collectors sucked in the air. If the radialfan was placed at the inlet of collectors, the turbulence could have occurredbecause of blowing. However, sucking of the air prevented this condition. Thepressure loss was measured by means of a water U-manometer placed between

entrance and the exit and the velocity of the air was measured at the inlet of thecollector.

3. Analysis of exergy

Exergy is the amount of maximum work obtained theoretically at the end of a reversible process in which equilibrium with environment should be obtained.According to this definition, in order to calculate exergy, the environment

conditions should be known [6].Exergy balance in a steady state open system can be written as followsX

E i X

E o þX

E product ¼ 0 ð1Þ

The lost work as being described between differences of maximum work with realwork

W lost ¼ W max W real ¼ E ð2Þ

This expression is equal to exergy loss. Therefore, exergy loss in the open systems;

E ¼X

m:

i hi T eS eð Þ X

m:

o ho T eS oð Þ þX

Q 1 T e

T s

W ð3Þ

Eq. (3) gives the balance of exergy in the collector. If it is assumed that the collec-tor has a single entrance and exit and the air is ideal fluid and also the conditionsare at steady state [7], for Eq. (3)

E ¼ m:

ei eoð Þ þ E R ð4Þ

can be written. Here,

ei ¼ hi T eS i ð Þ he T eS eð Þ ð5Þ

eo ¼ ho T eS oð Þ he T eS eð Þ ð6Þ


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and inserting these into Eq. (4)

E ¼ m:

hi hoð Þ T e S o S i ð Þð Þ þ I :A: 1 T e=T sð Þ ð7Þ

for changing of enthalpy and entropy

Dh ¼ C pDT ð8Þ

DS ¼ C p:ln T o=T i ð Þ R:ln P o=P i ð Þ ð9Þ

If Eqs. (8) and (9) are inserted into Eq. (7)

E ¼ m::C p:DT m

:

:C p:T e:ln T o=T i ð Þ m::R:T e:ln P o=P i ð Þ þ I :A: 1 T e=T sð Þ ð10Þ

is obtained.

E D ¼E

Q¼

T e

DT :ln

T o=T i ð Þ

P o=P i ð Þk 1

k

!þ

1

g 1

T e

T s

1 ð11Þ

the equation of dimensionless exergy is obtained.The efficiency of solar heating systems extensively depends on the efficiency of

the collectors. Test methods based on incident measures are applied to the wholecollector throughout both liquid and gas flows. In this method, mass flow rate of the fluid, the temperature of the collector inlet and outlet and the radiation inten-sity are measured simultaneously [7].

Thermal collector efficiency is defined as the ratio of useful energy and theincident solar radiation.

g ¼ Q

I :A ð12Þ

The useful energy Q used in the calculation of collector efficiency can be estimatedby using following equation

Q ¼ m::C p: T o T i ð Þ ð13Þ

Air collectors (flat-plate solar air heaters) are adiabatic radiative heat exchangers,

transferring solar radiant energy into heat, which is transferred by convection fromthe absorber to the working fluid (air) [1]. According to this definition, heat trans-fer obtained can be given in terms of Nusselt number.

Nu ¼ a:DH

k ð14Þ

where DH is the hydraulic diameter and evaluated as

DH ¼ 4:Ah

U ð15Þ

Ah is the channel cross-section area, U is the channel perimeter exposed to air, aand k are the coefficients of convective heat transfer and of conductive heat trans-fer of air, respectively.


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For the coefficient of convective heat transfer

Q ¼ a:A:DT log ð16Þ

where D

Tlog is the logarithmic main temperature difference between temperature of absorber surface and air temperature. If Eq. (16) is equalized to Eq. (13), the coef-ficient of convective heat transfer can be calculated.

Then the Reynolds number, which depends strongly on the velocity of air, hasbeen written as

Re ¼ q:V :DH

l ð17Þ

The velocity (V ) of the air was measured at the collector entrance; the continuityequation permits us to obtain the velocity in any frontal section of collector duct.

m: ¼ q:Ah:V ð18Þ

Dynamics viscosity, density of air and specific heat of air are determined accordingto average air temperature between entrance and exist of the collector.

4. Methods and measurements

The experiments were conducted on the days of June, July and August in Elazığin Turkey. The collectors were located with 37

v

angles towards the south.

The experiments were carried out at the same time periods between 9.00 and17.00 of the days for a variety of mass flow rates. The air flow through the collec-tor was supplied by a radial fan and adjusted via a sliding valve located at the airinlet. The flow rate was kept constant and same in both the collector designed andconventional flat-plate collector.

The experiments were carried out using five different mass flow rates and thesliding valve at the radial fan changed these rates. The velocity of the air was mea-sured by wind rose. The collectors were tested according to the ASHARE 93-97standard [8].

The incident solar radiation was measured with a Kipp and Zonen piranometer.Copper-Constantan thermocouples were placed at the four points in the collector,as well as at the inlet and outlet ports of the air to measure by a multi-channeldigital micro voltmeter for 60-min periods. The information about the relativehumidity of the air and wind speed during the experiments were kindly supplied bymeteorology department in Elazığ.

In this study, errors came from sensitiveness of equipment and measurements.First; errors due to measurement of temperature; are sensitiveness of voltmeter isabout 0.1%

v

C, measurement error is 0.2% and sensitiveness of the thermo-couple is 0.1%

v

C. The sensitiveness was obtained from a catalog of the instru-

ments. The second came from the measurement of flow rate. The sensitiveness of the flow meter is about 0.1% and error due to measurement is about 0.1%. Intotal, errors for measurement of flow rate are about 0.2%. The empirical relations


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which are shown in Table 1 are constructed by the least square method. Themaximum errors caused by the assumptions and sensitivity in measurement werefound to 8%, 10% and 7% for the Nusselt number, friction coefficient ( f ) anddimensionless exergy loss, respectively.

The empirical formulas given above are valid for Reynolds number in the rangeof 2600 and 6500.

5. Results and discussion

In this study, the aim was to increase collector efficiency using passive method inair collectors. When a comparison was made between collectors the days having

approximately the same radiation were used. The results obtained from the collec-tors designed are depicted in Fig. 2. Moreover, the efficiency in each collector isalso given in the same figures in terms of mass flow rates. Increasing the mass flowrates resulted in 1.5- to 3.5-fold increase in each collector efficiency. However, theoutlet temperature of air significantly changes with the geometry of the absorber.As known, the incident solar radiation is one of the most important parameters inthe collector efficiency. The temperature of absorber surfaces increased up to 86

v

Cdepending on the incident solar radiation. In addition, the outlet temperature of airincreased 78.5

v

C in the lowest mass flow rate (0.012 kg/s), and 67 v

C in the high-

est mass flow rate (0.028 kg/s). This behavior may be explained by longer constanttimes of air with the hot surfaces inside the collector. As seen from the results, thecollector efficiency increased with increasing mass flow rate of fluid. When theradiation is maximum, collector efficiency is also maximum. The radiation valueschange in the range of 880 W/m2 and 480 W/m2 and it reaches the maximum inthe midday.

According to Fig. 2, maximum efficiency in type 1 is 29.2%, 44.3% in type 2,60.4% in type 3, 67% in type 4 and 16% in the conventional flat-plate collector. Itwas revealed from Fig. 1, that the effect of absorber construction on the collectorefficiency is fairly important.

The efficiency for mass flow rate 0.028 kg/s is given in Fig. 3 according to daytimes. The efficiency of flat-plate collector changed between 9% and 15%. In type 1,the efficiency of collector increased up to 29% at midday by extending the flow line

Table 1Empirical correlations obtained from results of experiment

Nusselt number (Nu) Friction coefficient ( f ) Dimensionless exergy loss (E D)

Type 1 11.353 Re0.168 0.122 Re0.612 933.29 Re0.634

Type 2 28.889 Re0.199 0.154 Re0.719 285.63 Re0.541

Type 3 37.244 Re0.243 0.188 Re0.774 178.34 Re0.516

Type 4 43.901 Re0.228 0.221 Re0.724 163.59 Re0.516

Flat-plate 8.917 Re0.168 0.075 R0.636 1364.1 Re0.624

Theoretical 0.0158 Re0.8 (0.79 ln Re 1.64)2 –


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without changing surface geometry. Extending the flow line two-fold apparently

increased the collector efficiency almost twice as much. In type 3, the extending of

flow line of the air as well as staggering of the flow line because of the ondulineprofile, the collector efficiency increased approximately three times compared to the

flat-plate collector at a level of 44%. In types 3 and 4, the surfaces geometry

increased the collector efficiency by 4.5-fold as shown in Fig. 3. By changing the

flow area at both upper and lower surfaces, the efficiency increased 12% compared

to changing the upper surface.The effect of extending the flow line and the surface geometry on the heat trans-

fer are clearly depicted in Fig. 4. In this figure, the changing of Nusselt number

with Reynolds number is given. The heat gained is proportional to collector

efficiency as given in Eq. (12). As is known, the same parameters such as ambientair temperature, collector overall heat loss coefficient and collector efficiency factor

are critical parameters for collector efficiency. Therefore, the comparison of the

Fig. 2. (a) For type 1, the collector efficiency as a function of day times for five mass flow rates. (b) Fortype 2, the collector efficiency as a function of day times for five mass flow rates. (c) For type 3, thecollector efficiency as a function of day times for five mass flow rates. (d) For type 4, the collectorefficiency as a function of day times for five mass flow rates.


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heat transfer between both collectors and correlations would be more practical.

For full developed turbulent flow of air between two plates with one side heated

and the other side insulated, the correlation was given by Kays and Crawford [9].

Nu ¼ 0:0158 Re0:8 ð19Þ

Fig. 3. Change of collector efficiency with day times for each absorber in ṁ =0.028 kg/s.

Fig. 4. Change of Nusselt number with Reynolds number for each absorber in ṁ =0.028 kg/s.


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According to this equation obtained for turbulent flow, Nusselt number changed at

between 8.6 to 17.7 level for 2600 < Re < 6500. The values of Nusselt number inflat-plate and type 1 absorber were found to be less than the theoretical values. The

lower useful heat gain (Q) and the higher logarithmic main temperature differencemay be the reason for decreasing the convective heat coefficient. In type 1,although the efficiency increased by two-fold by extending flow line compared toflat-plate collector, the magnitude of heat transfer was less than the theoretical

value. In types 2, 3 and 4, the heat transfer significantly increased. The heat trans-fer in type 2 increased 20–25%, 60–70% in type 3 and 90–95% in type 4 comparedto the theoretical value. The reason for that was most probably, the extending of the flow line and the forming of swirl and secondary flows by staggering the flow

line with surface geometry. Hence, the convective heat transfer coefficient increasedby introducing turbulence effect to the fluid and this also increased Nusseltnumber.

The changing of the pressure loss and friction coefficient in the each collectorwith Reynolds number are given in Fig. 5 for the maximum mass flow rates. In thecollector designed, the pressure loss increased approximately 1.5 to 4 N/m2 com-pared to the flat-plate collector. Petukhov developed the friction factor for smooth

tubes [10] as follows:

F ¼ ð0:79 ln Re 1:64Þ2 ð20Þ

According to this theoretical correlation, the friction coefficient in flat-plate

collector increased 2.9-fold, 4.8-fold in type 1, seven-fold in type 2, 8.6-fold in type3 and 9.7-fold in type 4. The increase in friction coefficient resulted in an increase

Fig. 5. Change of pressure loss and friction caefficient with Reynolds number for each absorber inṁ =0.028 kg/s.


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in pressure loss. As known, the pressure loss is an important parameter in terms of overall cost.

The total exergy loss is shown in Fig. 6. The dimensionless exergy loss obtained

from Eq. (11). increased significantly according to the result calculated for eachcollector. Since the exergy loss changes with ambient conditions, a theoretical cor-relation does not exist in the literature. However, in our study, same approxi-mation can be applied for minimum exergy loss. If the solar collectors areconsidered as a heat exchanger, the maximum heat transfer occurs in case of dis-charging the collector at the surface temperature of the air inlet. Therefore, a mini-mum heat loss occurs. According to this statement, for the maximum heat transferthe following equation can be used.

Qmax ¼ m::C p: T as T i ð Þ ð21Þ

Likewise, the minimum pressure loss occurred in collector (P o=P i ¼ 1) can bedefined as the minimum exergy loss. As seen in Fig. 6, the lowest exergy lossoccurred in type 4 as given in Eq. (11), there is a reverse relationship betweendimensionless exergy loss and collector efficiency, as well as temperature difference(hT ). It is clear that when the efficiency is maximum, the exergy loss is minimum.The minimum exergy loss is also given in Fig. 6 for type 4. The exergy loss in type4 is higher at 65% level compared to the minimum exergy loss. The experimentalresults revealed that the pressure loss significantly affected the exergy loss. Theeffect of pressure loss on the exergy loss is in the range of ca. 12–15%. Approxi-mately the similar results were also obtained for other collectors. The exergy lossfor type 1 increased 1.6-fold, 2.3-fold for type 2, 3.2-fold for type 3 and 3.5-fold fortype 4 compared to the flat-plate collector. The results obtained for exergy loss

Fig. 6. Change of dimensionless exergy loss with Reynolds number for each absorber inṁ =0.028 kg/s.


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gave us same information about collector overall heat loss coefficient and collectorefficiency.

6. Conclusion

The conclusions can be drawn from the experimental study of the new collectorsdesigned, and show the efficiency of the collector improves with increasing massflow rates due to an enhanced heat transfer to the air flow. The efficiency of aircollectors increases depending on the surface geometry of the collector andextension of the air flow line. When the surface roughness is increased, the heattransfer and pressure loss increases. The optimum slice number of the absorber canbe determined for heat transfer and pressure loss changes the number of absorber

slices in the collector. The exergy loss of the system decreases depending on theincrease of the collector efficiency. There is a reverse relationship between dimen-sionless exergy loss and heat transfer, as well as pressure loss. The more importantparameters in order to decrease the exergy loss are the collector efficiency, tempera-ture difference (T oT i ) of the air and pressure loss.

References

[1] Yildiz C, Togrul IC, Sarsilmaz C, Pehlivan D. Thermal efficiency of an air solar collector withextended absorption surface and increased convection. Int Comm Heat Mass Transf 2002;29:831–40.

[2] Hachemi A. Experimental study of heat transfer and fluid flow friction in solar heater with andwithout selective absorber. Renew Energy 1999;17:155–68.

[3] Kolb A, Winter ERF, Viskanta R. Experimental studies on a solar air collector with metal matrixabsorber. Solar Energy 1999;65:91–8.

[4] Close DJ. Solar air heaters. Solar Energy 1963;7(3):117–29.

[5] Yeh T, Lin T. Efficiency improvement of flat-plate solar air heaters. Energy 1995;21:435–43.[6] Durmus A. Heat transfer end exergy loss in a concentric heat exchanger with snail entrance. Int.

Comm. Heat Mass Transfer 2002;29:303–12.[7] Yorgancioglu H. Second low optimization of air-cooled flat-plate solar collectors, MS thesis, Mech-

anical Engineering Department, METU, 1996.[8] ASHARE (Methods of testing to determine the thermal performance of solar collectors), 1977.[9] Kays WM, Crawford ME. Convective heat and mass transfer. 3rd ed. New York: McGraw-Hill;

1993.[10] Petukhov BS. Heat transfer and friction in turbulent pipe flow with variable physical properties.

New York: Academic Press; 1970.


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