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Efficiency and exergy analysis of a new solar air heater
Renewable Energy 29 (2004) 1489–1501 www.elsevier.com/locate/renene
Efficiency and exergy analysis of a new solar air heater I˙rfan Kurtbas , Aydın Durmus˛ Firat University, Technical Education Faculty, Mechanical Education Department, 23119 Elazıg˘, 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 most effective way to save energy is by increasing the efficiency in a solar collector by the heat transfer 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 were set to four different cases with dimensions of 1 2 m. Therefore, heating fluids exit the solar collector after at least 4.5 m displacement. According to the collector geometry, turbulence occurs in fluid flow and in this way heat transfer is increased. The results of the experiments were evaluated on the days with the same radiation. The efficiencies of these four collectors were compared to conventional flat-plate collectors. It was seen that heat transfer and pressure loss 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 the collectors have been widely reported in the literature, but the influences of flow line of 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 energy system. 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: ikurtbas@firat.edu.tr (I_ . Kurtbas).
0960-1481/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2004.01.006
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Nomenclature A Ah Cp DH E ED f h I k Nu P Q Pr R Re S T Tas U V W a k g m ˙ hTlog q l
collector surface area (m2) channel cross-section area (m2) specific heat (J/kg.K) hydraulic diameter (m) exergy (W) dimensionless exergy loss () friction coefficient () enthalpy (J/kg) total solar radiation incident upon plate of the collector (W/m2) adiabatic constant of the air (ffi1,4) () Nusselt number () pressure (N/m2) useful heat gain (W) Prandtl number () universal gas constant (J/kg. K) Reynolds number () entropy (J/kg.K) temperature (K) surface temperature of the absorber (K) channel perimeter exposed to air (m) average velocity of air (m/s) work (J) heat convection coefficient (W/m2.K) heat conduction coefficient (W/m.K) efficiency of air collector () mass flow rate of air (kg/s) logarithmic main temperature difference (K) density of air (kg/m3) dynamic viscosity of air (Pas. s)
Subscripts e environment i inlet o outlet max maximum min minimum R radiation
this, several investigations were made on this subject in order to increase efficiency of the collector and outlet temperature of fluid. The aim of these investigations is
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to develop a more efficient absorber, to increase the amount of energy obtained, to decrease the cost of energy provided from sun, to store the energy and to use it continuously. Flat-plate collectors are classified into two groups according to fluid used. Water is usually used in liquid collectors and air, in gas collectors. Since the air has worse thermodynamic properties in terms of heat transfer compared to liquid, the efficiency 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 their performance. They are generally used for heating in conditioning and drying of agriculture situations. A modified solar air heater, which incorporated aluminum wool on a perforated plate placed diagonally on the passageway of the air to serve as a front absorbing medium above the absorber plate was designed, conducted and tested [1]. The efficiency of the air solar collector increased up to five-fold compared to the flatsurface collectors by using materials to increase the absorption surface area. Rectangular staggered fins are soldered on the collectors’ back [2]. The interstices are inserted between two consecutive fins located in the same row. A turbulent fluid flow is developed which permits the improvement of the thermal heat transfer of 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-selective absorber-plate. It was seen that the nature of the absorber plate (selective or nonselective) had no significant effect on the heat transfer and Nusselt number in finned system collectors. In addition, there were no differences in friction factors. It is only necessary to reduce the spacing between consecutive fin rows in order to increase the heat transfer. A collector was designed in order to overcome the physical problems of conventional flat-plate air collectors as well as the particular technical 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 following results are obtained; the thermal performance of the collector improved with increasing 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 performance with higher heat transfer rates to the airflow and smaller friction losses compared to flat-plate air collectors of conventional design. The surface of air collectors having V-corrugation surface, fin and flat-plate were designed. This surfaces were covered with material of black copper-oxide having 0.15 emit coefficient and 0.9 absorber coefficient [4]. In this study, the efficiency of the collectors was investigated by performing the experiments with different mass flow rate. It was seen that particular V-corrugation collector had both high thermal efficiency of collector and high 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 efficiency of the collector increased with increasing numbers of fin. The experimental results were 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 middle 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 collectors. This absorber slices have (0:9 0:4 m) dimensions, four different surface geometries, single passage, and narrowed-extended shape. In this way, the heat transfer was increased by being extended along the flow line of fluid (air) and changing velocity and pressure in narrowed-extended area in which swirl and secondary flows form. As known, swirl and secondary flows cause the convection coefficient of the heat transfer to increase.
2. Experimental set up The experimental set up of the solar air collector is schematized as shown in Fig. 1. Although, the collectors designed are composed of basically the same elements present in the conventional flat-plate solar air collectors, it has special constructions due to the front absorption surface. The absorbers were formed by a black-painted galvanized sheet with 0.8 mm thick. 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. In this 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 the type I, the air to be heated leaves the absorber by passing from narrowed-extended gap. 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 the galvanized sheet had been covered. Besides, the area welded was covered by joint seal 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 a galvanized sheet 0.4 mm thick. A single glazing was chosen in order to maximize the radiation impact on the absorber surface and to reduce costs. To minimize the heat losses from the sides and from the bottom of the collector were insulated by glass 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 radial fan was placed at the inlet of collectors, the turbulence could have occurred because of blowing. However, sucking of the air prevented this condition. The pressure 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 the collector.
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 follows X X X Ei Eo þ Eproduct ¼ 0 ð1Þ The lost work as being described between differences of maximum work with real work Wlost ¼ Wmax Wreal ¼ E
ð2Þ
This expression is equal to exergy loss. Therefore, exergy loss in the open systems; X : X : X Te E¼ mi ðhi Te Se Þ mo ðho Te So Þ þ Q 1 W ð3Þ Ts Eq. (3) gives the balance of exergy in the collector. If it is assumed that the collector has a single entrance and exit and the air is ideal fluid and also the conditions are at steady state [7], for Eq. (3) :
E ¼ mðei eo Þ þ ER
ð4Þ
can be written. Here, e i ¼ ð hi T e S i Þ ð he T e S e Þ
ð5Þ
eo ¼ ðho Te So Þ ðhe Te Se Þ
ð6Þ
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and inserting these into Eq. (4) :
E ¼ mððhi ho Þ Te ðSo Si ÞÞ þ I:A:ð1 Te =Ts Þ
ð7Þ
for changing of enthalpy and entropy Dh ¼ Cp DT
ð8Þ
DS ¼ Cp :lnðTo =Ti Þ R:lnðPo =Pi Þ
ð9Þ
If Eqs. (8) and (9) are inserted into Eq. (7) :
:
:
E ¼ m:Cp :DT m:Cp :Te :lnðTo =Ti Þ m:R:Te :lnðPo =Pi Þ þ I:A:ð1 Te =Ts Þ
ð10Þ
is obtained. E Te ðTo =Ti Þ :ln ED ¼ ¼ k1 Q DT ðPo =Pi Þ k
! þ
1 Te 1 1 g Ts
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 whole collector 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 intensity are measured simultaneously [7]. Thermal collector efficiency is defined as the ratio of useful energy and the incident solar radiation. g¼
Q I:A
ð12Þ
The useful energy Q used in the calculation of collector efficiency can be estimated by using following equation :
Q ¼ m:Cp :ðTo Ti 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 from the absorber to the working fluid (air) [1]. According to this definition, heat transfer 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, a and k are the coefficients of convective heat transfer and of conductive heat transfer of air, respectively.
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For the coefficient of convective heat transfer Q ¼ a:A:DTlog
ð16Þ
where DTlog is the logarithmic main temperature difference between temperature of absorber surface and air temperature. If Eq. (16) is equalized to Eq. (13), the coefficient of convective heat transfer can be calculated. Then the Reynolds number, which depends strongly on the velocity of air, has been written as Re ¼
q:V :DH l
ð17Þ
The velocity (V) of the air was measured at the collector entrance; the continuity equation 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 according to 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 Elazıg˘ v in Turkey. The collectors were located with 37 angles towards the south. The experiments were carried out at the same time periods between 9.00 and 17.00 of the days for a variety of mass flow rates. The air flow through the collector was supplied by a radial fan and adjusted via a sliding valve located at the air inlet. The flow rate was kept constant and same in both the collector designed and conventional flat-plate collector. The experiments were carried out using five different mass flow rates and the sliding valve at the radial fan changed these rates. The velocity of the air was measured by wind rose. The collectors were tested according to the ASHARE 93-97 standard [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-channel digital micro voltmeter for 60-min periods. The information about the relative humidity of the air and wind speed during the experiments were kindly supplied by meteorology department in Elazıg˘. In this study, errors came from sensitiveness of equipment and measurements. First; errors due to measurement of temperature; are sensitiveness of voltmeter is v about 0.1% C, measurement error is 0.2% and sensitiveness of the thermov couple is 0.1% C. The sensitiveness was obtained from a catalog of the instruments. 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%. In total, errors for measurement of flow rate are about 0.2%. The empirical relations
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Table 1 Empirical correlations obtained from results of experiment Nusselt number (Nu) Type 1 Type 2 Type 3 Type 4 Flat-plate Theoretical
0.168
11.353 Re 28.889 Re0.199 37.244 Re0.243 43.901 Re0.228 8.917 Re0.168 0.0158 Re0.8
Friction coefficient (f) 0.612
0.122 Re 0.154 Re0.719 0.188 Re0.774 0.221 Re0.724 0.075 R0.636 (0.79 ln Re 1.64)2
Dimensionless exergy loss (ED) 933.29 Re0.634 285.63 Re0.541 178.34 Re0.516 163.59 Re0.516 1364.1 Re0.624 –
which are shown in Table 1 are constructed by the least square method. The maximum errors caused by the assumptions and sensitivity in measurement were found to 8%, 10% and 7% for the Nusselt number, friction coefficient (f) and dimensionless exergy loss, respectively. The empirical formulas given above are valid for Reynolds number in the range of 2600 and 6500.
5. Results and discussion In this study, the aim was to increase collector efficiency using passive method in air collectors. When a comparison was made between collectors the days having approximately the same radiation were used. The results obtained from the collectors designed are depicted in Fig. 2. Moreover, the efficiency in each collector is also given in the same figures in terms of mass flow rates. Increasing the mass flow rates resulted in 1.5- to 3.5-fold increase in each collector efficiency. However, the outlet temperature of air significantly changes with the geometry of the absorber. As known, the incident solar radiation is one of the most important parameters in v the collector efficiency. The temperature of absorber surfaces increased up to 86 C depending on the incident solar radiation. In addition, the outlet temperature of air v v increased 78.5 C in the lowest mass flow rate (0.012 kg/s), and 67 C in the highest mass flow rate (0.028 kg/s). This behavior may be explained by longer constant times of air with the hot surfaces inside the collector. As seen from the results, the collector efficiency increased with increasing mass flow rate of fluid. When the radiation is maximum, collector efficiency is also maximum. The radiation values change in the range of 880 W/m2 and 480 W/m2 and it reaches the maximum in the 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. It was revealed from Fig. 1, that the effect of absorber construction on the collector efficiency is fairly important. The efficiency for mass flow rate 0.028 kg/s is given in Fig. 3 according to day times. 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
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Fig. 2. (a) For type 1, the collector efficiency as a function of day times for five mass flow rates. (b) For type 2, the collector efficiency as a function of day times for five mass flow rates. (c) For type 3, the collector efficiency as a function of day times for five mass flow rates. (d) For type 4, the collector efficiency as a function of day times for five mass flow rates.
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 onduline profile, 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 transfer 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 ambient air temperature, collector overall heat loss coefficient and collector efficiency factor are critical parameters for collector efficiency. Therefore, the comparison of the
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Fig. 3. Change of collector efficiency with day times for each absorber in m ˙ =0.028 kg/s.
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
Fig. 4. Change of Nusselt number with Reynolds number for each absorber in m ˙ =0.028 kg/s.
ð19Þ
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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 in flat-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 difference may be the reason for decreasing the convective heat coefficient. In type 1, although the efficiency increased by two-fold by extending flow line compared to flat-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 transfer in type 2 increased 20–25%, 60–70% in type 3 and 90–95% in type 4 compared to 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 increased by introducing turbulence effect to the fluid and this also increased Nusselt number. The changing of the pressure loss and friction coefficient in the each collector with Reynolds number are given in Fig. 5 for the maximum mass flow rates. In the collector designed, the pressure loss increased approximately 1.5 to 4 N/m2 compared 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 type 3 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 in m ˙ =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 each collector. Since the exergy loss changes with ambient conditions, a theoretical correlation does not exist in the literature. However, in our study, same approximation can be applied for minimum exergy loss. If the solar collectors are considered as a heat exchanger, the maximum heat transfer occurs in case of discharging the collector at the surface temperature of the air inlet. Therefore, a minimum heat loss occurs. According to this statement, for the maximum heat transfer the following equation can be used. :
Qmax ¼ m:Cp :ðTas Ti Þ
ð21Þ
Likewise, the minimum pressure loss occurred in collector (Po =Pi ¼ 1) can be defined as the minimum exergy loss. As seen in Fig. 6, the lowest exergy loss occurred in type 4 as given in Eq. (11), there is a reverse relationship between dimensionless 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 type 4 is higher at 65% level compared to the minimum exergy loss. The experimental results revealed that the pressure loss significantly affected the exergy loss. The effect of pressure loss on the exergy loss is in the range of ca. 12–15%. Approximately the similar results were also obtained for other collectors. The exergy loss for type 1 increased 1.6-fold, 2.3-fold for type 2, 3.2-fold for type 3 and 3.5-fold for type 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 in m ˙ =0.028 kg/s.
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gave us same information about collector overall heat loss coefficient and collector efficiency. 6. Conclusion The conclusions can be drawn from the experimental study of the new collectors designed, and show the efficiency of the collector improves with increasing mass flow rates due to an enhanced heat transfer to the air flow. The efficiency of air collectors increases depending on the surface geometry of the collector and extension of the air flow line. When the surface roughness is increased, the heat transfer and pressure loss increases. The optimum slice number of the absorber can be 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 the increase of the collector efficiency. There is a reverse relationship between dimensionless exergy loss and heat transfer, as well as pressure loss. The more important parameters in order to decrease the exergy loss are the collector efficiency, temperature difference (ToTi) of the air and pressure loss. References [1] Yildiz C, Togrul IC, Sarsilmaz C, Pehlivan D. Thermal efficiency of an air solar collector with extended 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 and without selective absorber. Renew Energy 1999;17:155–68. [3] Kolb A, Winter ERF, Viskanta R. Experimental studies on a solar air collector with metal matrix absorber. 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, Mechanical 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.
Efficiency and exergy analysis of a new solar air heater I˙rfan Kurtbas , Aydın Durmus˛ Firat University, Technical Education Faculty, Mechanical Education Department, 23119 Elazıg˘, 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 most effective way to save energy is by increasing the efficiency in a solar collector by the heat transfer 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 were set to four different cases with dimensions of 1 2 m. Therefore, heating fluids exit the solar collector after at least 4.5 m displacement. According to the collector geometry, turbulence occurs in fluid flow and in this way heat transfer is increased. The results of the experiments were evaluated on the days with the same radiation. The efficiencies of these four collectors were compared to conventional flat-plate collectors. It was seen that heat transfer and pressure loss 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 the collectors have been widely reported in the literature, but the influences of flow line of 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 energy system. 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: ikurtbas@firat.edu.tr (I_ . Kurtbas).
0960-1481/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2004.01.006
1490
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Nomenclature A Ah Cp DH E ED f h I k Nu P Q Pr R Re S T Tas U V W a k g m ˙ hTlog q l
collector surface area (m2) channel cross-section area (m2) specific heat (J/kg.K) hydraulic diameter (m) exergy (W) dimensionless exergy loss () friction coefficient () enthalpy (J/kg) total solar radiation incident upon plate of the collector (W/m2) adiabatic constant of the air (ffi1,4) () Nusselt number () pressure (N/m2) useful heat gain (W) Prandtl number () universal gas constant (J/kg. K) Reynolds number () entropy (J/kg.K) temperature (K) surface temperature of the absorber (K) channel perimeter exposed to air (m) average velocity of air (m/s) work (J) heat convection coefficient (W/m2.K) heat conduction coefficient (W/m.K) efficiency of air collector () mass flow rate of air (kg/s) logarithmic main temperature difference (K) density of air (kg/m3) dynamic viscosity of air (Pas. s)
Subscripts e environment i inlet o outlet max maximum min minimum R radiation
this, several investigations were made on this subject in order to increase efficiency of the collector and outlet temperature of fluid. The aim of these investigations is
I_. Kurtbas, A. Durmus˛ / Renewable Energy 29 (2004) 1489–1501
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to develop a more efficient absorber, to increase the amount of energy obtained, to decrease the cost of energy provided from sun, to store the energy and to use it continuously. Flat-plate collectors are classified into two groups according to fluid used. Water is usually used in liquid collectors and air, in gas collectors. Since the air has worse thermodynamic properties in terms of heat transfer compared to liquid, the efficiency 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 their performance. They are generally used for heating in conditioning and drying of agriculture situations. A modified solar air heater, which incorporated aluminum wool on a perforated plate placed diagonally on the passageway of the air to serve as a front absorbing medium above the absorber plate was designed, conducted and tested [1]. The efficiency of the air solar collector increased up to five-fold compared to the flatsurface collectors by using materials to increase the absorption surface area. Rectangular staggered fins are soldered on the collectors’ back [2]. The interstices are inserted between two consecutive fins located in the same row. A turbulent fluid flow is developed which permits the improvement of the thermal heat transfer of 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-selective absorber-plate. It was seen that the nature of the absorber plate (selective or nonselective) had no significant effect on the heat transfer and Nusselt number in finned system collectors. In addition, there were no differences in friction factors. It is only necessary to reduce the spacing between consecutive fin rows in order to increase the heat transfer. A collector was designed in order to overcome the physical problems of conventional flat-plate air collectors as well as the particular technical 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 following results are obtained; the thermal performance of the collector improved with increasing 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 performance with higher heat transfer rates to the airflow and smaller friction losses compared to flat-plate air collectors of conventional design. The surface of air collectors having V-corrugation surface, fin and flat-plate were designed. This surfaces were covered with material of black copper-oxide having 0.15 emit coefficient and 0.9 absorber coefficient [4]. In this study, the efficiency of the collectors was investigated by performing the experiments with different mass flow rate. It was seen that particular V-corrugation collector had both high thermal efficiency of collector and high 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 efficiency of the collector increased with increasing numbers of fin. The experimental results were 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 middle 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 collectors. This absorber slices have (0:9 0:4 m) dimensions, four different surface geometries, single passage, and narrowed-extended shape. In this way, the heat transfer was increased by being extended along the flow line of fluid (air) and changing velocity and pressure in narrowed-extended area in which swirl and secondary flows form. As known, swirl and secondary flows cause the convection coefficient of the heat transfer to increase.
2. Experimental set up The experimental set up of the solar air collector is schematized as shown in Fig. 1. Although, the collectors designed are composed of basically the same elements present in the conventional flat-plate solar air collectors, it has special constructions due to the front absorption surface. The absorbers were formed by a black-painted galvanized sheet with 0.8 mm thick. 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. In this 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 the type I, the air to be heated leaves the absorber by passing from narrowed-extended gap. 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 the galvanized sheet had been covered. Besides, the area welded was covered by joint seal 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 a galvanized sheet 0.4 mm thick. A single glazing was chosen in order to maximize the radiation impact on the absorber surface and to reduce costs. To minimize the heat losses from the sides and from the bottom of the collector were insulated by glass 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 radial fan was placed at the inlet of collectors, the turbulence could have occurred because of blowing. However, sucking of the air prevented this condition. The pressure 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 the collector.
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 follows X X X Ei Eo þ Eproduct ¼ 0 ð1Þ The lost work as being described between differences of maximum work with real work Wlost ¼ Wmax Wreal ¼ E
ð2Þ
This expression is equal to exergy loss. Therefore, exergy loss in the open systems; X : X : X Te E¼ mi ðhi Te Se Þ mo ðho Te So Þ þ Q 1 W ð3Þ Ts Eq. (3) gives the balance of exergy in the collector. If it is assumed that the collector has a single entrance and exit and the air is ideal fluid and also the conditions are at steady state [7], for Eq. (3) :
E ¼ mðei eo Þ þ ER
ð4Þ
can be written. Here, e i ¼ ð hi T e S i Þ ð he T e S e Þ
ð5Þ
eo ¼ ðho Te So Þ ðhe Te Se Þ
ð6Þ
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and inserting these into Eq. (4) :
E ¼ mððhi ho Þ Te ðSo Si ÞÞ þ I:A:ð1 Te =Ts Þ
ð7Þ
for changing of enthalpy and entropy Dh ¼ Cp DT
ð8Þ
DS ¼ Cp :lnðTo =Ti Þ R:lnðPo =Pi Þ
ð9Þ
If Eqs. (8) and (9) are inserted into Eq. (7) :
:
:
E ¼ m:Cp :DT m:Cp :Te :lnðTo =Ti Þ m:R:Te :lnðPo =Pi Þ þ I:A:ð1 Te =Ts Þ
ð10Þ
is obtained. E Te ðTo =Ti Þ :ln ED ¼ ¼ k1 Q DT ðPo =Pi Þ k
! þ
1 Te 1 1 g Ts
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 whole collector 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 intensity are measured simultaneously [7]. Thermal collector efficiency is defined as the ratio of useful energy and the incident solar radiation. g¼
Q I:A
ð12Þ
The useful energy Q used in the calculation of collector efficiency can be estimated by using following equation :
Q ¼ m:Cp :ðTo Ti 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 from the absorber to the working fluid (air) [1]. According to this definition, heat transfer 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, a and k are the coefficients of convective heat transfer and of conductive heat transfer of air, respectively.
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For the coefficient of convective heat transfer Q ¼ a:A:DTlog
ð16Þ
where DTlog is the logarithmic main temperature difference between temperature of absorber surface and air temperature. If Eq. (16) is equalized to Eq. (13), the coefficient of convective heat transfer can be calculated. Then the Reynolds number, which depends strongly on the velocity of air, has been written as Re ¼
q:V :DH l
ð17Þ
The velocity (V) of the air was measured at the collector entrance; the continuity equation 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 according to 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 Elazıg˘ v in Turkey. The collectors were located with 37 angles towards the south. The experiments were carried out at the same time periods between 9.00 and 17.00 of the days for a variety of mass flow rates. The air flow through the collector was supplied by a radial fan and adjusted via a sliding valve located at the air inlet. The flow rate was kept constant and same in both the collector designed and conventional flat-plate collector. The experiments were carried out using five different mass flow rates and the sliding valve at the radial fan changed these rates. The velocity of the air was measured by wind rose. The collectors were tested according to the ASHARE 93-97 standard [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-channel digital micro voltmeter for 60-min periods. The information about the relative humidity of the air and wind speed during the experiments were kindly supplied by meteorology department in Elazıg˘. In this study, errors came from sensitiveness of equipment and measurements. First; errors due to measurement of temperature; are sensitiveness of voltmeter is v about 0.1% C, measurement error is 0.2% and sensitiveness of the thermov couple is 0.1% C. The sensitiveness was obtained from a catalog of the instruments. 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%. In total, errors for measurement of flow rate are about 0.2%. The empirical relations
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Table 1 Empirical correlations obtained from results of experiment Nusselt number (Nu) Type 1 Type 2 Type 3 Type 4 Flat-plate Theoretical
0.168
11.353 Re 28.889 Re0.199 37.244 Re0.243 43.901 Re0.228 8.917 Re0.168 0.0158 Re0.8
Friction coefficient (f) 0.612
0.122 Re 0.154 Re0.719 0.188 Re0.774 0.221 Re0.724 0.075 R0.636 (0.79 ln Re 1.64)2
Dimensionless exergy loss (ED) 933.29 Re0.634 285.63 Re0.541 178.34 Re0.516 163.59 Re0.516 1364.1 Re0.624 –
which are shown in Table 1 are constructed by the least square method. The maximum errors caused by the assumptions and sensitivity in measurement were found to 8%, 10% and 7% for the Nusselt number, friction coefficient (f) and dimensionless exergy loss, respectively. The empirical formulas given above are valid for Reynolds number in the range of 2600 and 6500.
5. Results and discussion In this study, the aim was to increase collector efficiency using passive method in air collectors. When a comparison was made between collectors the days having approximately the same radiation were used. The results obtained from the collectors designed are depicted in Fig. 2. Moreover, the efficiency in each collector is also given in the same figures in terms of mass flow rates. Increasing the mass flow rates resulted in 1.5- to 3.5-fold increase in each collector efficiency. However, the outlet temperature of air significantly changes with the geometry of the absorber. As known, the incident solar radiation is one of the most important parameters in v the collector efficiency. The temperature of absorber surfaces increased up to 86 C depending on the incident solar radiation. In addition, the outlet temperature of air v v increased 78.5 C in the lowest mass flow rate (0.012 kg/s), and 67 C in the highest mass flow rate (0.028 kg/s). This behavior may be explained by longer constant times of air with the hot surfaces inside the collector. As seen from the results, the collector efficiency increased with increasing mass flow rate of fluid. When the radiation is maximum, collector efficiency is also maximum. The radiation values change in the range of 880 W/m2 and 480 W/m2 and it reaches the maximum in the 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. It was revealed from Fig. 1, that the effect of absorber construction on the collector efficiency is fairly important. The efficiency for mass flow rate 0.028 kg/s is given in Fig. 3 according to day times. 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
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Fig. 2. (a) For type 1, the collector efficiency as a function of day times for five mass flow rates. (b) For type 2, the collector efficiency as a function of day times for five mass flow rates. (c) For type 3, the collector efficiency as a function of day times for five mass flow rates. (d) For type 4, the collector efficiency as a function of day times for five mass flow rates.
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 onduline profile, 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 transfer 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 ambient air temperature, collector overall heat loss coefficient and collector efficiency factor are critical parameters for collector efficiency. Therefore, the comparison of the
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Fig. 3. Change of collector efficiency with day times for each absorber in m ˙ =0.028 kg/s.
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
Fig. 4. Change of Nusselt number with Reynolds number for each absorber in m ˙ =0.028 kg/s.
ð19Þ
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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 in flat-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 difference may be the reason for decreasing the convective heat coefficient. In type 1, although the efficiency increased by two-fold by extending flow line compared to flat-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 transfer in type 2 increased 20–25%, 60–70% in type 3 and 90–95% in type 4 compared to 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 increased by introducing turbulence effect to the fluid and this also increased Nusselt number. The changing of the pressure loss and friction coefficient in the each collector with Reynolds number are given in Fig. 5 for the maximum mass flow rates. In the collector designed, the pressure loss increased approximately 1.5 to 4 N/m2 compared 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 type 3 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 in m ˙ =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 each collector. Since the exergy loss changes with ambient conditions, a theoretical correlation does not exist in the literature. However, in our study, same approximation can be applied for minimum exergy loss. If the solar collectors are considered as a heat exchanger, the maximum heat transfer occurs in case of discharging the collector at the surface temperature of the air inlet. Therefore, a minimum heat loss occurs. According to this statement, for the maximum heat transfer the following equation can be used. :
Qmax ¼ m:Cp :ðTas Ti Þ
ð21Þ
Likewise, the minimum pressure loss occurred in collector (Po =Pi ¼ 1) can be defined as the minimum exergy loss. As seen in Fig. 6, the lowest exergy loss occurred in type 4 as given in Eq. (11), there is a reverse relationship between dimensionless 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 type 4 is higher at 65% level compared to the minimum exergy loss. The experimental results revealed that the pressure loss significantly affected the exergy loss. The effect of pressure loss on the exergy loss is in the range of ca. 12–15%. Approximately the similar results were also obtained for other collectors. The exergy loss for type 1 increased 1.6-fold, 2.3-fold for type 2, 3.2-fold for type 3 and 3.5-fold for type 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 in m ˙ =0.028 kg/s.
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gave us same information about collector overall heat loss coefficient and collector efficiency. 6. Conclusion The conclusions can be drawn from the experimental study of the new collectors designed, and show the efficiency of the collector improves with increasing mass flow rates due to an enhanced heat transfer to the air flow. The efficiency of air collectors increases depending on the surface geometry of the collector and extension of the air flow line. When the surface roughness is increased, the heat transfer and pressure loss increases. The optimum slice number of the absorber can be 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 the increase of the collector efficiency. There is a reverse relationship between dimensionless exergy loss and heat transfer, as well as pressure loss. The more important parameters in order to decrease the exergy loss are the collector efficiency, temperature difference (ToTi) of the air and pressure loss. References [1] Yildiz C, Togrul IC, Sarsilmaz C, Pehlivan D. Thermal efficiency of an air solar collector with extended 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 and without selective absorber. Renew Energy 1999;17:155–68. [3] Kolb A, Winter ERF, Viskanta R. Experimental studies on a solar air collector with metal matrix absorber. 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, Mechanical 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.