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Investigation of single and double pass solar air heater with transverse fins and a package wire mesh layer
Energy Conversion and Management 89 (2015) 599–607
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Investigation of single and double pass solar air heater with transverse fins and a package wire mesh layer A.J. Mahmood a,⇑, L.B.Y. Aldabbagh b, F. Egelioglu a a b
Mechanical Engineering Department, Eastern Mediterranean University, Famagusta, North Cyprus, via Mersin 10, Turkey Mechatronics Engineering Department, College of Engineering, Mosul University, Mosul, Iraq
a r t i c l e
i n f o
Article history: Received 6 June 2014 Accepted 10 October 2014
Keywords: Thermal efficiency Solar air heater (SAH) Double-pass Single-pass Fins Steel wire mesh
a b s t r a c t The purpose of this work is to construct and test single-pass and double-pass solar air heaters (SAHs) with four transverse fins. These fins were painted dark black and placed transversely to create four equal-spaced sections. Sixteen steel wire mesh layers were located between these fins as an alternative to an absorber plate; they had a cross-sectional area of 0.18 cm 0.18 cm and an internal diameter of 0.02-cm. In this project, the thermal efficiency and outlet temperature were studied in a geographic area located in the city of Famagusta, North Cyprus. The experimental results indicate that the thermal efficiency increases as the air flow rate increases for the range of (0.011-0.032) kg/s. The maximum efficiency obtained using the 7.5-cm high collector was 62.50% for the double-pass SAH and 55% for the single-pass SAH at an air flow rate of 0.032 kg/s. Moreover, the thermal efficiency further increases by decreasing the height of the lower air pass of the double-pass SAH. The difference between the inlet temperature and outlet temperature, DT, indicated an inverse relationship with air flow rate: DT increased as the air mass flow rate decreased. The maximum differences (DT) observed were 45.30 K for the double-passes SAH and 39.9 K for the single-pass SAH at 0.011 kg/s, which were recorded during the middle of the day with a maximum solar intensity. The results demonstrate a significant improvement in the thermal efficiency and outlet air temperature. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Heating air with solar energy is much cheaper and cleaner than heating air with fossil fuel, the delivered heat from an air solar device can be used by various industries for drying agricultural products and as auxiliary heaters in buildings to save energy during winter [1]. The classical design of a solar air heater consists of a duct made of wood or other material in which the top side of the duct is covered with either a glass or transparent plastic sheet. The other sides of the duct and the bottom are thermally insulated. The absorber plate is placed inside the hot air duct parallel to the transparent cover, in which air blowers are used if it is an active system. Different factors affect the air heater efficiency such as collector dimensions, type and shape of absorber plate, glass cover, inlet temperature, wind speed, humidity, air path, and height of the channel. Among all, the collector glass cover, the absorber plate shape factor, and air path are the most important parameters in the design of any type of air heater. Major heat losses from flat⇑ Corresponding author. E-mail address: [email protected] (A.J. Mahmood). http://dx.doi.org/10.1016/j.enconman.2014.10.028 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.
plate solar collectors are found to be through the top cover; heat losses from the bottom and the sides of the collector are low as they are adequately insulated. Minimising the heat loss from the cover definitely will lead to increase the thermal performance of SAH. For this reason a double glazing was performed [2–7] or by using the counter flow (double pass) [5,8–12]. In double pass SAH, air will pass between the first glass, cover, and the second glass and then changing its direction to pass between the second glass and the bottom of the channel. Several researchers have suggested inserting the absorber plate at mid channel to divide the channel into two equal parts [3,4]. The air in this case will pass above and under the absorber plate in the same direction. El-Sebaii and Shalaby [4] recorded that the maximum difference between inlet and outlet air temperatures was about 28 °C for upper channel at a 0.0223 kg/s flow rate and 580 W/m2 solar intensity. It has been also suggested to insert an absorbing plate into a panel to have a double pass channel where the air flows from above and then below the absorber plate [13–15] or vice versa [16–18]. In general, the principle of using doubles pass or passing the flow from above and then from below is to increase the airflow path length inside collector. This leads to increase the heat transfer coefficient between the flowing air, the glass cover and the absorber
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Nomenclature Ac Cp h I m p Q Tin Tout DP DT
area of the collector (m2) specific heat of the fluid (kJ/kg K) fluid deflection inside the incline manometer (m) solar radiation (W/m2) air mass flow rate (kg/s) pressure (Pa) volume flow rate (m3/s) inlet temperature (K) outlet temperature (K) pressure difference, DP = qgh sin15° (N/m2) temperature difference (Tout Tin) (K)
plate. The transversely fins along the bed used by [19] divided the channel to three; five and seven equally spaced sections. In this way, air flows in a snake path picking up heat as it goes along the passage channel. As the path length was increased by increasing the number of fins, the velocity of air was also increased for the same air mass flow rate as the cross-sectional area of the air passage channel decreased. The longitudinal fins used by Omojaro and Aldabbagh [10] divided the lower channel to five equal spaces. They set up a bed with seven wire mesh layers (1.5 m 1 m) and fins with a height of 7-cm. The maximum efficiency obtained from their work is 51% at a mass flow rate of 0.035 kg/s. As we mentioned before the absorber plate is the most important parameter that can play important roles in a solar air heater. Increasing the absorber area definitely will increase the thermal efficiency. Different modifications have been suggested and applied to increase the surface area of the absorber plate [20]. Used roughness obstacles and baffles to increase the surface area of the absorber plate and in same time to increase the turbulence inside the flowing channel. For the same purpose [3] used v-corrugated absorber plate. Many researchers have used transverse ribs [2,9,21] or longitudinal fins [10,22,23] as such obstacles and some used transversely fins [19]. A gravel-packed bed used by El-Sebaii [5] and the iron chips packed bed used by Sharma et al. [24] improved the thermal performances of SAH by increasing the outlet air temperature and the thermal efficiency of the system. Another modification to absorber plate of solar air heater is by [8]. He presented an analysis of a double pass (counter-flow) solar air heater with various porous media (different porosity) in the lower channel. The use of porous media tends to increase the surface per unit volume ratio substantially and was found to improve the thermal efficiency in the air solar heater [10]. [5,11,12,14,20,25,26] also use of porous media in constructed their SAH. While wire mesh screen was used as the porous material packed into the solar air collector to serve as the porous media by [8,10,19,22,27]. These modifications, using the porous matrix, enhance the thermal efficiency significantly but also increase the pressure drop, which becomes important at high volume flow rates of the air. For this reason [10,19,27,28] found some method in arranging the wire mesh inside the collector to reduce the pressure drop through the solar air heater. The first aim of this study in this article is to study the effect of increasing the path of the following air inside the channel of a single and double pass solar air heater with porous media in the lower channel without an absorber plate. For this purpose a transverse fin was used and fixed inside the duct to direct the air inside the channel to give a path shape like 8 letters (Fig. 1). Those fins will increase the area of the absorber bed besides they will divide the channel to four equal sections. In this case the air at the entrance will be divided into two equal parts, one part moved to the left and the other part to right, and then the two parts will enter to
DTbed DTg
q g xm xp xTair xI xg U
temperature difference of bed (Tbed Tin) (K) temperature difference of glass (Tg Tin) (K) density of air (kg/m3) efficiency of the solar collector () uncertainty for the mass flow rate () uncertainty for the pressure differences () uncertainty for the film air temperature () uncertainty for the solar radiation () uncertainty for the solar thermal efficiency () porosity ()
the second section of the channel through the two opening made at the two sides. The mixed two parts of the air flow at the mid section of the second part of the channel will pass to the third section through the opening made at the mid second fin, and in this way the path of the air will be repeated. The wire meshes used in this collector are similar to the ones which were used by [10,19,27,28] with a difference in the total number of layer, number of layer in each matrix and the distance between the matrices in order to reduce more the pressure drop. The second aim of this work is to investigate the effect of the second pass height on the thermal performance of the solar air heater. Tests were conducted under actual outdoor conditions.
2. Experimental set-up and equipment 2.1. Experimental set-up Flat-plate SAHs were constructed to perform thermal efficiency experiments in Famagusta, North Cyprus. This investigation used a collector made of wood that was 147 cm 100 cm and 7.5-cm high (Fig. 1), and a wooden frame that was 4-cm thick with an upper rectangular 36 cm 4 cm hole for the air flow inlet. The design and operating parameters are shown in Table 1. All the sides of the bed were painted dark black, and three sides were insulated with Styrofoam with the exception of the upper side of the channel. Normal window glass of 4-mm thicknesses was used as glazing. The distance between the first glass and the second glass, was 2.5-cm. The single pass air collector could be achieved by removing the first glass at the top of the collector. Sixteen wire mesh layers with a cross-sectional opening of 0.181 cm 0.181 cm were also used, which were constructed in three group’s located 0.5-cm apart from each other. The first and second sets each contained six wire mesh layers that were fixed to the bottom of the channel and parallel to the glass cover. The third group that consisted of four wire mesh layers was located 0.5-cm above the first two groups. All of the layers (U = 0.981) were painted black before being installed in the channel. Four aluminium fins were also painted black and positioned transversely along the channel to divide it into four equal parts. These fins were all 7.2-cm high and 0.3-cm thick. Two of them were 80-cm long, while the other two were 45-cm long. A black slot rubber band, 0.5-cm wide and 0.3-cm thick, was used to prevent the fins from touching the glass and to prevent air from passing between them. In this way, the air flows along the pattern, gaining heat as it passes through the channel (Fig. 1a and b). In the case of the double-pass SAH, the flow first enters from above the exit part of the lower channel and passes inside the upper channel. The flow then reverses direction in the lower channel prior to turning to flow from the top side to the bottom side through a 36 cm 4 cm opening in the middle of
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(a)
1- Converging section. 2- Converging duct. 3- Orifice meter. 4- Diverging duct. 5- Air blower. 6- Glass thermocouples.
7- Fins. 8- Bed thermocouples 9- Glass. 10- Speed controller. 11- Incline manometer. 12- Outlet air thermocouples.
(c) (b) Fig. 1. (a) Schematic assembly of the single-pass solar air heater system, (b) section A-A, side view of single-pass solar air heater, and (c) schematic assembly of the doublepass solar air heater system.
the second glass pane (Fig. 1c). The calibrated orifice meter used to measure the air flow rate was of Holman’s design [29]. Flow straighteners were placed before and after the orifice meter to cre-
ate uniform flow through it. These straighteners were plastic straw tubes of 0.46-cm in diameter and 2-cm long. A 0.62-kW blower was joined to the discharge side. The incline alcohol manometer
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Table 1 The basic design and operating parameters used for the simulation study.
xm
¼
_ m
Parameters
Value
Location of collector Collector till from horizontal Collector orientation Date Length of collector Width of collector Air channel depth Number of glazing Glass covers space Fan type Mesh layer
Famagusta, North Cyprus 37° Due to the geographical location of Cyprus 35.125 °N and 33.95 °E South August 2012 147 cm 100 cm 7.5 cm 1 or 2 2.5 cm 0.62 kW Absorptivity of 0.96 Emissivity of 0.87
tube, at a 15° angle and a density of 803 kg/m3, was used to measure the pressure difference across the orifice. Different mass flow rates were obtained by using a speed controller which is connected to the blower in order to control the speed of the fan. Two holes were made one at the inlet of the SAH and one at the outlet to measure the pressure drop across the SAH. The pressure drop across the collector is recorded by using the inclined alcohol manometer tube for the various flow rates.
" #1=2 2 1 xT air 1 xP 2 þ 4 T air 4 P
ð2Þ
The equation of the thermal efficiency, g, of the solar bed is:
g¼
m C p ðT out T in Þ I Ac
ð3Þ
Because Ac is constant and assuming Cp is constant for the range of working temperatures, the fractional uncertainty of the solar thermal efficiency of the bed is related to m, I, and DT. The fractional uncertainty [28,29] is given as:
xg xm 2 xDT 2 xI 2 1=2 ¼ þ þ _ g DT I m
ð4Þ
The air flow rates were determined and the average values of results were calculated each day to obtain the fractional uncertainty. For the single-pass SAH, the average of all the variables of DT, Tin, Tout, Tair, m, I and g, were calculated to be 21.1 K, 306.7 K, 327.8 K, 318.3 K, 0.027 kg/s, 727.65 W/m2 and 39.2%, respectively. For the double-pass SAH, DT, Tin, Tout, Tair, m, I and g were calculated to be 23.7 K, 307.3 K, 331 K, 319.1 K, 0.027 kg/s, 717.15 W/m2 and 45%, respectively. The fractional uncertainty of the air flow rate
(a) 1100 2.2. Experimental proceedings
1000
3. Uncertainty analysis The errors associated with the practical measurements were obtained prior to performing the experiment. The uncertainty analysis for the fluid flow and the thermal efficiency are presented in this section. The mass flow rate of air passing across the bed is defined as:
m ¼ q:Q
Solar intensity, I
900 800 700 600 500 Day 1, m=0.011 kg/s Day 2, m=0.016 kg/s Day 3, m=0.022 kg/s Day 4, m=0.028 kg/s Day 5, m=0.032 kg/s
400 300 200
8
10
12
14
16
Time of the day (hrs)
(b) 1100 1000 900
Solar intensity, I
The ambient temperature, Tin, was recorded using two mercury thermometers that hung underneath the solar panel. Nine T-type calibrated thermocouples were distributed in three groups of three thermocouples each. These groups were used to measure the temperatures at three locations. The first group was used to measure the outlet air temperatures Tout, which were fixed 5 cm in front of the orifice meter. The second group was used to measure the bed temperatures Tbed recorded inside the wire mesh. The last group was used to measure the glass temperatures Tg recorded on the lower side of the glass inside a channel. The position of the thermocouple in the last two groups was shown in Fig. 1. All the measured temperatures were obtained using digital thermometers (OMEGASAYS) with ±0.51 °C accuracy. The value of the global solar radiation was measured using an Eppley Radiometer Pyranometer (PSP) coupled to an instantaneous solar radiation meter model HHM1A digital, Omega 0.25% basic dc accuracy and a resolution of ±0.5% from 0 to 2800 W/m. The pyranometer was fixed beside the glass cover of the collector. In order to maximize the solar radiation incident on the glass covers, the SAH was oriented facing south and tilted with an angle of 37° with respect to the horizontal [30]. The air is circulated for 30 min prior to the period in which the data were taken. The measured variables Tin, Tout, Tbed, Tg, wind speed, relative humidity ratio, manometer reading and solar radiation, I, were recorded every hour from 8 am morning and ended at 5 pm afternoon.
800 700 600 500 Day 1, m=0.011 kg/s Day 2, m=0.016 kg/s Day 3, m=0.022 kg/s Day 4, m=0.028 kg/s Day 5, m=0.032 kg/s
400 300 200
8
10
12
14
16
Time of the day (hrs)
ð1Þ
The fractional uncertainty (xm/m) for the rate flow of air is [19,29]:
Fig. 2. (a) Solar intensity versus different standard local time of days for: singlepass SAH, and (b) double-pass SAH.
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(a)
(a) 310
40
308 30
ΔΤ (Κ)
Τin (Κ)
306
304 Day 1, m=0.011 kg/s Day 2, m=0.016 kg/s Day 3, m=0.022 kg/s Day 4, m=0.028 kg/s Day 5, m=0.032 kg/s
302
300
8
10
12
14
20
m=0.011 kg/s m=0.016 kg/s m=0.022 kg/s m=0.028 kg/s m=0.032 kg/s
10
0
16
8
10
Time of the day (hrs)
12
14
16
Time of the day (hrs)
(b) 310
(b) 40
308
30
ΔΤ (Κ)
Τin (Κ)
306
304 Day 1, m=0.011 kg/s Day 2, m=0.016 kg/s Day 3, m=0.022 kg/s Day 4, m=0.028 kg/s Day 5, m=0.032 kg/s
302
300
8
10
12
14
20
m=0.011 kg/s m=0.016 kg/s m=0.022 kg/s m=0.028 kg/s m=0.032 kg/s
10
16
Time of the day (hrs) Fig. 3. Ambient temperatures versus different standard local time of days for: (a) single-pass SAH, and (b) double-pass SAH.
and the efficiency for SAH device were determined to be 0.0016 and 0.0073, respectively.
4. Results and discussion This work has presented the experimental test results and a comparison between two solar air devices, single and double-pass SAHs with 4 transverse fins and a wire mesh package consisting of 16 layers, in prevalent weather conditions during August 2012 in the city of Famagusta in North Cyprus. Figs. 2 and 3 show the hourly variation of the measured solar intensity of the single and double-pass SAHs and the inlet temperature when the air flow rate was (0.011–0.032) kg/s. The peak values of solar intensity, I, at 12:00 pm and 1:00 pm, were 1081 W/m2 for the single-pass SAH and 1005 W/m2 for the double-pass SAH. However, the maximum value of the inlet temperature, Tin, between 12:00 and 1:00 pm was 310 K and 309.9 K for single and double-pass SAHs, respectively, which depend on the environmental state including wind speed and humidity value. The average values recorded were 306.7 K and 307.3 K. The temperature difference (DT = Tout Tin) versus time for different mass flow rates increased from morning as the solar intensity increased, reaching its maximum value between 12:00 and 1:00 pm, as shown in Fig. 4, and then decreased at the
0
8
10
12
14
16
Time of the day (hrs) Fig. 4. Temperature difference versus standard local time of the day at different mass flow rates for: (a) single-pass SAH, and (b) double-pass SAH.
end of the day. The maximum DT values for single and double-pass SAHs were determined to be 40 K and 45.3 K, respectively. It is also clear that the highest DT is obtained for the double-pass SAH at an air mass flow of 0.011 kg/s, where the outlet temperature of the passing air through the bed decreases with increasing air flow rate. A comparison between the new design results obtained and the previously published data, as mentioned earlier in the introduction, shows an enhancement in the heat transfer performance. Figs. 5 and 6 show the variation of the bed temperature difference, DTbed = Tbed Tin, and the glass temperature difference, DTg = Tg Tin, with the time of the day for single and double-pass SAHs. It was observed that a minimum air flow rate of 0.011 kg/s produced the highest temperature difference: the maximum values of DTbed were recorded to be 47.6 K for the single-pass SAH and 52.7 K for the double-pass SAH. The maximum DTg was 32.6 K for the single-pass SAH and 38.4 K for double-pass SAH. For the maximum air flow rate of 0.032 kg/s, the maximum DTbed value was 28.3 K and DTg was 19.7 K for the double-passes SAH. From these results, its evidence that the temperature of the bed and the second glass is higher in double passes than in single pass SAH. This is as a result of the air in counter flow will preheated in the first channel before it turns down to enter the lower channel through the 36 cm 4 cm opening in the second glass and flow in
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(a)
(a) 40 50
30
30
ΔΤ g (Κ)
ΔΤbed (Κ)
40
20 m=0.011 m=0.016 m=0.022 m=0.028 m=0.032
10
0
8
10
12
kg/s kg/s kg/s kg/s kg/s
14
20
m=0.011 m=0.016 m=0.022 m=0.028 m=0.032
10
16
0
Time of the day (hrs)
8
10
12
kg/s kg/s kg/s kg/s kg/s
14
16
Time of the day (hrs)
(b)
(b) 40
50
40
30
ΔΤ g (Κ)
ΔΤbed (Κ)
30
20
m=0.011 m=0.016 m=0.022 m=0.028 m=0.032
10
0
8
10
12
14
kg/s kg/s kg/s kg/s kg/s
20
m=0.011 m=0.016 m=0.022 m=0.028 m=0.032
10
16
Time of the day (hrs) Fig. 5. Bed temperature difference versus standard local time of the day at different mass flow rates for: (a) single-pass SAH, and (b) double-pass SAH.
0
8
10
12
14
kg/s kg/s kg/s kg/s kg/s 16
Time of the day (hrs) Fig. 6. Glass temperature difference versus standard local time of the day at different mass flow rates for: (a) single-pass SAH, and (b) double-pass SAH.
second channel in reverse direction [8]. We have to mention here that the temperature of the bed and the glass is the average temperature of the three thermocouple positioned at three different positions, as we mentioned in experimental set-up section, inside the solar air heater. The thermal efficiencies versus hours of the day at different air flow rates are presented in Fig. 7. These efficiencies increase consequently with an increase in the fluid flow rate. The maximum values obtained for single-pass and double-pass SAHs with a maximum rate of flowing air of 0.032 kg/s were 55% and 62.5%, respectively. Depending on the solar intensity and the outlet air temperatures of the bed, the efficiency results generally continue to increase from morning until evening. These observations are similar to those of El-khawajah et al. [19]. The increases in the efficiencies are the result of heat stored inside the porous (wire mesh) media and fins. After noon, the solar intensity starts to decrease while the air mass flow rate continues to carry the same amount of energy, which is the energy received from the sun plus the energy absorbed from the mesh layers. Another explanation of this observation is a continual addition of heat to the inlet temperature from morning until evening on a day with no wind (Fig. 3). At all cases studied in this work, the efficiency in 4-fins double-pass SAH is greater than that of a single-pass SAH. The average efficiency of double pass is found to be bigger than single pass for
the entire air mass flow rate used (Fig. 8). The comparison of the average efficiencies in the present work with the reported data for double-pass SAHs shows that there is an improvement in the proposed SAH (Fig. 9). Fig. 10 shows the efficiency of the single-pass SAH compared to double-pass SAH versus standard local time of the day when decreasing lower channel heights, 7.5-cm, 5-cm and 3-cm were applied. The thermal efficiency was found to increase by decreasing the space between the second cover and bottom bed for the double-pass SAH. The maximum efficiency of 65.8% for height 3cm at m = 0.032 kg/s was shown to be the best for double pass solar collector compared to 64% and 62.5% for 5-cm and 7.5-cm respectively. For minimum air flow rate of 0.011 kg/s, the result shows similar behaviour as for the maximum air flow rate of 0.032 kg/s. In general, increasing the gap between second cover and the bottom of the duct reduces the average air velocity and decreases the heat transfer coefficient. Hence, the space of the lower air flow channel has a considerable effect on the performance of the solar SAH. The influence of changes in lower bed height on the thermal efficiency with and without the glass wool as a porous medium was considered [26]. They recommended that the best thermal efficiency of the duct with of 2.4-m channel length and air flow depth of 3-cm. El-Sebaii et al. [5] suggests designing equivalent
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(a)
70 60
60
50
50
η av %
η%
40
30
20 m=0.011 m=0.016 m=0.022 m=0.028 m=0.032
10
0
8
10
12
30
kg/s kg/s kg/s kg/s kg/s
14
40
Ramadan et al. [9] Omajaro [20] Sopian et al. [7] Mousa et al. [22] Present data, double pass
20 16
10
Time of the day (hrs)
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
Mass flow rate (kg/s)
(b)
Fig. 9. Efficiency comparison between the double-pass SAH with some double-pass SAHs in literature.
60
50
70
60
30
50
20
m=0.011 m=0.016 m=0.022 m=0.028 m=0.032
10
0
40
kg/s kg/s kg/s kg/s kg/s
η%
η%
40
30 Single-pass, m=0.011 kg/s Double- pass, h=3, m=0.011 kg/s Double- pass, h=5, m=0.011 kg/s Double- pass, h=7.5, m=0.011 kg/s Single-pass, m=0.032 kg/s Double- pass, h=3, m=0.032 kg/s Double- pass, h=5, m=0.032 kg/s Double- pass, h=7.5, m=0.032 kg/s
20 8
10
12
14
16
Time of the day (hrs)
10
Fig. 7. Variation of collector efficiency at different mass flow rates for: (a) singlepass SAH, and (b) double-pass SAH.
0
8
10
12
14
16
Time of the day (hrs) Fig. 10. Effect of second pass height on the thermal efficiency of a solar air heater for air mass flow rate of 0.011 kg/s and 0.032 kg/s.
800 55
50
Pressure drop (pa)
600
η av %
45
40
35
400
Ramadan et al. [9] El-khawajah et al. [16] Bashria et al. [23] Present data, double pass
200
Single pass Double pass 30
25 0.01
0 0.01 0.015
0.02
0.025
0.03
0.02
0.03
0.04
0.05
0.06
0.07
Time of the day (hrs)
Mass flow rate (kg/s) Fig. 8. Average efficiency versus air mass flow rate for single and double pass SAH.
Fig. 11. Pressure drop comparison between the double-pass SAH with some double-pass solar air heaters in literature.
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across the double-pass SAH that was filled with mesh layers was greater than that of the single-pass SAH at an equal rate of flowing air, as shown in Fig. 12. For 0.011 kg/s, the pressure drop for the single-pass SAH was observed to be 12 Pa and 14.5 Pa for the double-pass SAH. The maximum pressure drop values were 64 Pa and 66 Pa at a maximum air flow rate of 0.032 kg/s for single and double-pass SAHs, respectively. The pressure drop increases as the efficiency is increased for the same air mass flow rate (Fig. 13).
Pressure drop (pa)
60
40
5. Conclusions
45
The single and double pass SAH with wire mesh used as an absorber plate was constructed and tested for different air mass flow rate and different height of the lower channel in case of double pass. The obtained results of the proposed design show that the thermal efficiency and the average efficiency for the double-pass SAH is higher than that of the single-pass SAH for the same air mass flow rate, and will increase by increasing the air mass flow rate. The thermal efficiency also found to be reduced with increasing the height of the lower channel. Moreover, the temperature difference between the outlet and the inlet increases with decreasing the air mass flow rate. The pressure drop across the double-pass was greater than that of the single-pass SAH for the same air mass flow rate. The increase in the pressure drop within the bed with this proposed design is not too high when compared to the available published data. Finally, the comparison of the average efficiencies in the present work with the reported data for double-pass SAHs shows that there is an improvement in the proposed SAH.
40
References
20 Double pass Single pass
0 0.01
0.015
0.02
0.025
0.03
Mass flow rate (kg/s) Fig. 12. Pressure drop across the bed versus mass flow rates for single-pass and double-pass SAH.
55
η av %
50
35
30 Single pass Double pass 25
20 10
20
30
40
50
60
70
Pressure drop (pa) Fig. 13. Average efficiency versus pressure drop for the range of air mass flow rate between 0.011 and 0.032 kg/s.
height for both lower and upper air flow channel to present higher temperature output when porous media is used along with it. The disadvantage of using porous media instead of the absorber plate is the increases in the pressure drop through the channel. However, the increase in the pressure drop within the bed with this proposed design, as shown in Fig. 11, is not too high when compared to the available published data. Bashria et al. [12] determined that the use of porous media in the double flow V groove absorber increased the pressure drop by 3–25 Pa more than the pressure drop in the same double flow V groove absorber without the porous media. For 0.02 kg/s, the pressure drop was 7 Pa. The maximum pressure drop was 78 Pa with a maximum air flow rate of 0.08 kg/s. Romdhane et al. [21] indicated that the pressure drop for a double pass SAH with limestone was 100 Pa for an air flow rate of 0.01 kg/s and 800 Pa at an air flow rate of 0.05 kg/s. El-khawajah et al. [19] experimentally determined that the minimum pressure drop (Pd) was 14 Pa for a six-fin SAH with a minimum rate of flowing air of approximately 0.0121 kg/s. In addition, the greatest pressure drop (Pd) was 84.5 Pa for an air flow rate of 0.042 kg/s when mesh layers between transverse fins were used as an alternative to an absorber plate. The pressure drop (Pd)
[1] Akpinar EK, Sarsılmaz C, Yıldız C. Mathematical modeling of a thin layer drying of apricots in a solar energized rotary dryer. Int J Energy Res 2004;28:739–52. [2] Deniz A, Emin B, Ertekin C, Osman Y. Experimental investigation of three different solar air heaters: energy and exergy analyses. Appl Energy 2010;87:2953–73. [3] Bhargava AK, Garg HP, Sharma VK, Mahajan RB. Investigation on double-glazed solar air heater connected in series with rock bed solar collector-cum-storage system. Energy Convers Manage 1985;25:139–46. [4] El-Sebaii AA, Shalaby SM. Experimental investigation of an indirect-mode forced convection solar dryer for drying thymus and mint. Energy Convers Manage 2013;74:109–16. [5] El-Sebaii AA, Abou-Enein S, Ramadan MRI, El-Bialy E. Year round performance of double pass solar air heater with packed bed. Energy Convers Manage 2007;48:990–1003. [6] Yeh H, Ho C, Lin C. Effect of collector aspect ratio on the collector efficiency of upward type baffled solar air heaters. Energy Convers Manage 2000;41:971–81. [7] Gill RS, Singh S, Singh PP. Low cost solar air heater. Energy Convers Manage 2012;57:131–42. [8] Mohamad AA. High efficiency solar air heater. Sol Energy 1997;60:71–6. [9] El-Sebaii AA, Aboul-Enein S, Ramadan MR, Shalaby SM, Moharram BM. Thermal performance investigation of double pass-finned plate solar air heater. Appl Energy 2011;88:1727–39. [10] Omojaro AP, Aldabbagh LBY. Experimental performance of single and double pass solar air heater with fins and steel wire mesh as absorber. Appl Energy 2010;87:3759–65. [11] Musa EA, Sopian K, Shahrir A. Heat transfer analysis and pressure drop correlations for the double-pass solar collector with porous media. Energy Environ 2004;3:15–24. [12] Bashria A, Yousef A, Adam NM, Sopian K, Zaharim A, Alghoul M. Analysis of single and double passes V-grooves solar collector with and without porous media. Int J Energy Environ 2007;1(2). [13] Yang M, Yang X, Li X, Zhifeng WZ, Wang P. Design and optimization of a solar air heater with offset strip fin absorber plate. Appl Energy 2014;113:1349–62. [14] Sopian K, Supranto WR, Daud MY, Othman BY. Thermal performance of the double-pass solar collector with and without porous media. Renew Energy 1999;18:557–64. [15] Fudholi A, Sopian K, Othman MY, Ruslan MH, Bakhtyar B. Energy analysis and improvement potential of finned double-pass solar collector. Energy Convers Manage 2013;75:234–40. [16] Ho CD, Yeh HM, Wang RC. Heat-transfer enhancements in double-pass flat plate solar air heaters with recycle. Energy 2005;30:2796–817. [17] Lertsatitthanakorn C, Khasee N, Atthajariyakul S, Soponronnarit S, Therdyothin A, Suzuki RO. Performance analysis of a double-pass thermoelectric solar air collector. Solar Energy Mater Solar Cells 2008;92:1105–9.
A.J. Mahmood et al. / Energy Conversion and Management 89 (2015) 599–607 [18] Sevik Seyfi. Design, experimental investigation and analysis of a solar drying system. Energy Convers Manage 2013;68:227–34. [19] El-khawajah MF, Aldabbagh LBY, Egelioglu F. The effect of using transverse fins on a double pass flow solar air heater using wire mesh as an absorber. Sol Energy 2011;85:1479–87. [20] Mishra CB, Sharma SP. Performance study of air-heated packed bed solar energy collectors. Int J Energy 1981;6:153–7. [21] Romdhane, Ben Salma. The air solar collectors: Comparative study, introduction of baffles to favor the heat transfer. Solar Energy 2007;81:139–49. [22] Bharadwaj SS, Singh D, Bansal NK. Design and thermal performance of a matrix solar air heater. Energy Convers Manage 1981;21:253–6. [23] Fudholi A, Sopian K, Mohd HR, Mohd YO, Muhammad Y. Thermal efficiency of double pass solar collector with longitudinal fins absorbers. Am J Appl Sci 2011;8:254–60.
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[24] Sharma VK, Rizzi G, Garg HP. Design and development of a matrix type solar air heater. Energy Convers Manage 1991;31:379–88. [25] Choudhury C, Garg HP. Performance of air-heating collectors with packed airflow passage. Sol Energy 1993;30:205–21. [26] Yousef BAA, Adam NM. Performance analysis for flat plate collector with and without porous media. J Energy Southern Africa 2008;19:4. [27] Aldabbagh LBY, Egelioglu F, Ilkan M. Single and double pass solar air heaters with wire mesh as packing bed. Energy 2010;35:3783–7. [28] Nowzari R, Aldabbagh LBY, Egelioglu F. Single and double pass solar air heaters with partially perforated cover and packed mesh. Energy 2014;73:694–702. [29] Holman JP. Experimental methods for engineers. New York: McGraw-Hill; 1989. [30] Tiwari GN. Solar energy fundamentals, design, modeling and applications, vol. 142. Narosa Publishing House; 2002.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Investigation of single and double pass solar air heater with transverse fins and a package wire mesh layer A.J. Mahmood a,⇑, L.B.Y. Aldabbagh b, F. Egelioglu a a b
Mechanical Engineering Department, Eastern Mediterranean University, Famagusta, North Cyprus, via Mersin 10, Turkey Mechatronics Engineering Department, College of Engineering, Mosul University, Mosul, Iraq
a r t i c l e
i n f o
Article history: Received 6 June 2014 Accepted 10 October 2014
Keywords: Thermal efficiency Solar air heater (SAH) Double-pass Single-pass Fins Steel wire mesh
a b s t r a c t The purpose of this work is to construct and test single-pass and double-pass solar air heaters (SAHs) with four transverse fins. These fins were painted dark black and placed transversely to create four equal-spaced sections. Sixteen steel wire mesh layers were located between these fins as an alternative to an absorber plate; they had a cross-sectional area of 0.18 cm 0.18 cm and an internal diameter of 0.02-cm. In this project, the thermal efficiency and outlet temperature were studied in a geographic area located in the city of Famagusta, North Cyprus. The experimental results indicate that the thermal efficiency increases as the air flow rate increases for the range of (0.011-0.032) kg/s. The maximum efficiency obtained using the 7.5-cm high collector was 62.50% for the double-pass SAH and 55% for the single-pass SAH at an air flow rate of 0.032 kg/s. Moreover, the thermal efficiency further increases by decreasing the height of the lower air pass of the double-pass SAH. The difference between the inlet temperature and outlet temperature, DT, indicated an inverse relationship with air flow rate: DT increased as the air mass flow rate decreased. The maximum differences (DT) observed were 45.30 K for the double-passes SAH and 39.9 K for the single-pass SAH at 0.011 kg/s, which were recorded during the middle of the day with a maximum solar intensity. The results demonstrate a significant improvement in the thermal efficiency and outlet air temperature. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Heating air with solar energy is much cheaper and cleaner than heating air with fossil fuel, the delivered heat from an air solar device can be used by various industries for drying agricultural products and as auxiliary heaters in buildings to save energy during winter [1]. The classical design of a solar air heater consists of a duct made of wood or other material in which the top side of the duct is covered with either a glass or transparent plastic sheet. The other sides of the duct and the bottom are thermally insulated. The absorber plate is placed inside the hot air duct parallel to the transparent cover, in which air blowers are used if it is an active system. Different factors affect the air heater efficiency such as collector dimensions, type and shape of absorber plate, glass cover, inlet temperature, wind speed, humidity, air path, and height of the channel. Among all, the collector glass cover, the absorber plate shape factor, and air path are the most important parameters in the design of any type of air heater. Major heat losses from flat⇑ Corresponding author. E-mail address: [email protected] (A.J. Mahmood). http://dx.doi.org/10.1016/j.enconman.2014.10.028 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.
plate solar collectors are found to be through the top cover; heat losses from the bottom and the sides of the collector are low as they are adequately insulated. Minimising the heat loss from the cover definitely will lead to increase the thermal performance of SAH. For this reason a double glazing was performed [2–7] or by using the counter flow (double pass) [5,8–12]. In double pass SAH, air will pass between the first glass, cover, and the second glass and then changing its direction to pass between the second glass and the bottom of the channel. Several researchers have suggested inserting the absorber plate at mid channel to divide the channel into two equal parts [3,4]. The air in this case will pass above and under the absorber plate in the same direction. El-Sebaii and Shalaby [4] recorded that the maximum difference between inlet and outlet air temperatures was about 28 °C for upper channel at a 0.0223 kg/s flow rate and 580 W/m2 solar intensity. It has been also suggested to insert an absorbing plate into a panel to have a double pass channel where the air flows from above and then below the absorber plate [13–15] or vice versa [16–18]. In general, the principle of using doubles pass or passing the flow from above and then from below is to increase the airflow path length inside collector. This leads to increase the heat transfer coefficient between the flowing air, the glass cover and the absorber
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Nomenclature Ac Cp h I m p Q Tin Tout DP DT
area of the collector (m2) specific heat of the fluid (kJ/kg K) fluid deflection inside the incline manometer (m) solar radiation (W/m2) air mass flow rate (kg/s) pressure (Pa) volume flow rate (m3/s) inlet temperature (K) outlet temperature (K) pressure difference, DP = qgh sin15° (N/m2) temperature difference (Tout Tin) (K)
plate. The transversely fins along the bed used by [19] divided the channel to three; five and seven equally spaced sections. In this way, air flows in a snake path picking up heat as it goes along the passage channel. As the path length was increased by increasing the number of fins, the velocity of air was also increased for the same air mass flow rate as the cross-sectional area of the air passage channel decreased. The longitudinal fins used by Omojaro and Aldabbagh [10] divided the lower channel to five equal spaces. They set up a bed with seven wire mesh layers (1.5 m 1 m) and fins with a height of 7-cm. The maximum efficiency obtained from their work is 51% at a mass flow rate of 0.035 kg/s. As we mentioned before the absorber plate is the most important parameter that can play important roles in a solar air heater. Increasing the absorber area definitely will increase the thermal efficiency. Different modifications have been suggested and applied to increase the surface area of the absorber plate [20]. Used roughness obstacles and baffles to increase the surface area of the absorber plate and in same time to increase the turbulence inside the flowing channel. For the same purpose [3] used v-corrugated absorber plate. Many researchers have used transverse ribs [2,9,21] or longitudinal fins [10,22,23] as such obstacles and some used transversely fins [19]. A gravel-packed bed used by El-Sebaii [5] and the iron chips packed bed used by Sharma et al. [24] improved the thermal performances of SAH by increasing the outlet air temperature and the thermal efficiency of the system. Another modification to absorber plate of solar air heater is by [8]. He presented an analysis of a double pass (counter-flow) solar air heater with various porous media (different porosity) in the lower channel. The use of porous media tends to increase the surface per unit volume ratio substantially and was found to improve the thermal efficiency in the air solar heater [10]. [5,11,12,14,20,25,26] also use of porous media in constructed their SAH. While wire mesh screen was used as the porous material packed into the solar air collector to serve as the porous media by [8,10,19,22,27]. These modifications, using the porous matrix, enhance the thermal efficiency significantly but also increase the pressure drop, which becomes important at high volume flow rates of the air. For this reason [10,19,27,28] found some method in arranging the wire mesh inside the collector to reduce the pressure drop through the solar air heater. The first aim of this study in this article is to study the effect of increasing the path of the following air inside the channel of a single and double pass solar air heater with porous media in the lower channel without an absorber plate. For this purpose a transverse fin was used and fixed inside the duct to direct the air inside the channel to give a path shape like 8 letters (Fig. 1). Those fins will increase the area of the absorber bed besides they will divide the channel to four equal sections. In this case the air at the entrance will be divided into two equal parts, one part moved to the left and the other part to right, and then the two parts will enter to
DTbed DTg
q g xm xp xTair xI xg U
temperature difference of bed (Tbed Tin) (K) temperature difference of glass (Tg Tin) (K) density of air (kg/m3) efficiency of the solar collector () uncertainty for the mass flow rate () uncertainty for the pressure differences () uncertainty for the film air temperature () uncertainty for the solar radiation () uncertainty for the solar thermal efficiency () porosity ()
the second section of the channel through the two opening made at the two sides. The mixed two parts of the air flow at the mid section of the second part of the channel will pass to the third section through the opening made at the mid second fin, and in this way the path of the air will be repeated. The wire meshes used in this collector are similar to the ones which were used by [10,19,27,28] with a difference in the total number of layer, number of layer in each matrix and the distance between the matrices in order to reduce more the pressure drop. The second aim of this work is to investigate the effect of the second pass height on the thermal performance of the solar air heater. Tests were conducted under actual outdoor conditions.
2. Experimental set-up and equipment 2.1. Experimental set-up Flat-plate SAHs were constructed to perform thermal efficiency experiments in Famagusta, North Cyprus. This investigation used a collector made of wood that was 147 cm 100 cm and 7.5-cm high (Fig. 1), and a wooden frame that was 4-cm thick with an upper rectangular 36 cm 4 cm hole for the air flow inlet. The design and operating parameters are shown in Table 1. All the sides of the bed were painted dark black, and three sides were insulated with Styrofoam with the exception of the upper side of the channel. Normal window glass of 4-mm thicknesses was used as glazing. The distance between the first glass and the second glass, was 2.5-cm. The single pass air collector could be achieved by removing the first glass at the top of the collector. Sixteen wire mesh layers with a cross-sectional opening of 0.181 cm 0.181 cm were also used, which were constructed in three group’s located 0.5-cm apart from each other. The first and second sets each contained six wire mesh layers that were fixed to the bottom of the channel and parallel to the glass cover. The third group that consisted of four wire mesh layers was located 0.5-cm above the first two groups. All of the layers (U = 0.981) were painted black before being installed in the channel. Four aluminium fins were also painted black and positioned transversely along the channel to divide it into four equal parts. These fins were all 7.2-cm high and 0.3-cm thick. Two of them were 80-cm long, while the other two were 45-cm long. A black slot rubber band, 0.5-cm wide and 0.3-cm thick, was used to prevent the fins from touching the glass and to prevent air from passing between them. In this way, the air flows along the pattern, gaining heat as it passes through the channel (Fig. 1a and b). In the case of the double-pass SAH, the flow first enters from above the exit part of the lower channel and passes inside the upper channel. The flow then reverses direction in the lower channel prior to turning to flow from the top side to the bottom side through a 36 cm 4 cm opening in the middle of
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(a)
1- Converging section. 2- Converging duct. 3- Orifice meter. 4- Diverging duct. 5- Air blower. 6- Glass thermocouples.
7- Fins. 8- Bed thermocouples 9- Glass. 10- Speed controller. 11- Incline manometer. 12- Outlet air thermocouples.
(c) (b) Fig. 1. (a) Schematic assembly of the single-pass solar air heater system, (b) section A-A, side view of single-pass solar air heater, and (c) schematic assembly of the doublepass solar air heater system.
the second glass pane (Fig. 1c). The calibrated orifice meter used to measure the air flow rate was of Holman’s design [29]. Flow straighteners were placed before and after the orifice meter to cre-
ate uniform flow through it. These straighteners were plastic straw tubes of 0.46-cm in diameter and 2-cm long. A 0.62-kW blower was joined to the discharge side. The incline alcohol manometer
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Table 1 The basic design and operating parameters used for the simulation study.
xm
¼
_ m
Parameters
Value
Location of collector Collector till from horizontal Collector orientation Date Length of collector Width of collector Air channel depth Number of glazing Glass covers space Fan type Mesh layer
Famagusta, North Cyprus 37° Due to the geographical location of Cyprus 35.125 °N and 33.95 °E South August 2012 147 cm 100 cm 7.5 cm 1 or 2 2.5 cm 0.62 kW Absorptivity of 0.96 Emissivity of 0.87
tube, at a 15° angle and a density of 803 kg/m3, was used to measure the pressure difference across the orifice. Different mass flow rates were obtained by using a speed controller which is connected to the blower in order to control the speed of the fan. Two holes were made one at the inlet of the SAH and one at the outlet to measure the pressure drop across the SAH. The pressure drop across the collector is recorded by using the inclined alcohol manometer tube for the various flow rates.
" #1=2 2 1 xT air 1 xP 2 þ 4 T air 4 P
ð2Þ
The equation of the thermal efficiency, g, of the solar bed is:
g¼
m C p ðT out T in Þ I Ac
ð3Þ
Because Ac is constant and assuming Cp is constant for the range of working temperatures, the fractional uncertainty of the solar thermal efficiency of the bed is related to m, I, and DT. The fractional uncertainty [28,29] is given as:
xg xm 2 xDT 2 xI 2 1=2 ¼ þ þ _ g DT I m
ð4Þ
The air flow rates were determined and the average values of results were calculated each day to obtain the fractional uncertainty. For the single-pass SAH, the average of all the variables of DT, Tin, Tout, Tair, m, I and g, were calculated to be 21.1 K, 306.7 K, 327.8 K, 318.3 K, 0.027 kg/s, 727.65 W/m2 and 39.2%, respectively. For the double-pass SAH, DT, Tin, Tout, Tair, m, I and g were calculated to be 23.7 K, 307.3 K, 331 K, 319.1 K, 0.027 kg/s, 717.15 W/m2 and 45%, respectively. The fractional uncertainty of the air flow rate
(a) 1100 2.2. Experimental proceedings
1000
3. Uncertainty analysis The errors associated with the practical measurements were obtained prior to performing the experiment. The uncertainty analysis for the fluid flow and the thermal efficiency are presented in this section. The mass flow rate of air passing across the bed is defined as:
m ¼ q:Q
Solar intensity, I
900 800 700 600 500 Day 1, m=0.011 kg/s Day 2, m=0.016 kg/s Day 3, m=0.022 kg/s Day 4, m=0.028 kg/s Day 5, m=0.032 kg/s
400 300 200
8
10
12
14
16
Time of the day (hrs)
(b) 1100 1000 900
Solar intensity, I
The ambient temperature, Tin, was recorded using two mercury thermometers that hung underneath the solar panel. Nine T-type calibrated thermocouples were distributed in three groups of three thermocouples each. These groups were used to measure the temperatures at three locations. The first group was used to measure the outlet air temperatures Tout, which were fixed 5 cm in front of the orifice meter. The second group was used to measure the bed temperatures Tbed recorded inside the wire mesh. The last group was used to measure the glass temperatures Tg recorded on the lower side of the glass inside a channel. The position of the thermocouple in the last two groups was shown in Fig. 1. All the measured temperatures were obtained using digital thermometers (OMEGASAYS) with ±0.51 °C accuracy. The value of the global solar radiation was measured using an Eppley Radiometer Pyranometer (PSP) coupled to an instantaneous solar radiation meter model HHM1A digital, Omega 0.25% basic dc accuracy and a resolution of ±0.5% from 0 to 2800 W/m. The pyranometer was fixed beside the glass cover of the collector. In order to maximize the solar radiation incident on the glass covers, the SAH was oriented facing south and tilted with an angle of 37° with respect to the horizontal [30]. The air is circulated for 30 min prior to the period in which the data were taken. The measured variables Tin, Tout, Tbed, Tg, wind speed, relative humidity ratio, manometer reading and solar radiation, I, were recorded every hour from 8 am morning and ended at 5 pm afternoon.
800 700 600 500 Day 1, m=0.011 kg/s Day 2, m=0.016 kg/s Day 3, m=0.022 kg/s Day 4, m=0.028 kg/s Day 5, m=0.032 kg/s
400 300 200
8
10
12
14
16
Time of the day (hrs)
ð1Þ
The fractional uncertainty (xm/m) for the rate flow of air is [19,29]:
Fig. 2. (a) Solar intensity versus different standard local time of days for: singlepass SAH, and (b) double-pass SAH.
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(a)
(a) 310
40
308 30
ΔΤ (Κ)
Τin (Κ)
306
304 Day 1, m=0.011 kg/s Day 2, m=0.016 kg/s Day 3, m=0.022 kg/s Day 4, m=0.028 kg/s Day 5, m=0.032 kg/s
302
300
8
10
12
14
20
m=0.011 kg/s m=0.016 kg/s m=0.022 kg/s m=0.028 kg/s m=0.032 kg/s
10
0
16
8
10
Time of the day (hrs)
12
14
16
Time of the day (hrs)
(b) 310
(b) 40
308
30
ΔΤ (Κ)
Τin (Κ)
306
304 Day 1, m=0.011 kg/s Day 2, m=0.016 kg/s Day 3, m=0.022 kg/s Day 4, m=0.028 kg/s Day 5, m=0.032 kg/s
302
300
8
10
12
14
20
m=0.011 kg/s m=0.016 kg/s m=0.022 kg/s m=0.028 kg/s m=0.032 kg/s
10
16
Time of the day (hrs) Fig. 3. Ambient temperatures versus different standard local time of days for: (a) single-pass SAH, and (b) double-pass SAH.
and the efficiency for SAH device were determined to be 0.0016 and 0.0073, respectively.
4. Results and discussion This work has presented the experimental test results and a comparison between two solar air devices, single and double-pass SAHs with 4 transverse fins and a wire mesh package consisting of 16 layers, in prevalent weather conditions during August 2012 in the city of Famagusta in North Cyprus. Figs. 2 and 3 show the hourly variation of the measured solar intensity of the single and double-pass SAHs and the inlet temperature when the air flow rate was (0.011–0.032) kg/s. The peak values of solar intensity, I, at 12:00 pm and 1:00 pm, were 1081 W/m2 for the single-pass SAH and 1005 W/m2 for the double-pass SAH. However, the maximum value of the inlet temperature, Tin, between 12:00 and 1:00 pm was 310 K and 309.9 K for single and double-pass SAHs, respectively, which depend on the environmental state including wind speed and humidity value. The average values recorded were 306.7 K and 307.3 K. The temperature difference (DT = Tout Tin) versus time for different mass flow rates increased from morning as the solar intensity increased, reaching its maximum value between 12:00 and 1:00 pm, as shown in Fig. 4, and then decreased at the
0
8
10
12
14
16
Time of the day (hrs) Fig. 4. Temperature difference versus standard local time of the day at different mass flow rates for: (a) single-pass SAH, and (b) double-pass SAH.
end of the day. The maximum DT values for single and double-pass SAHs were determined to be 40 K and 45.3 K, respectively. It is also clear that the highest DT is obtained for the double-pass SAH at an air mass flow of 0.011 kg/s, where the outlet temperature of the passing air through the bed decreases with increasing air flow rate. A comparison between the new design results obtained and the previously published data, as mentioned earlier in the introduction, shows an enhancement in the heat transfer performance. Figs. 5 and 6 show the variation of the bed temperature difference, DTbed = Tbed Tin, and the glass temperature difference, DTg = Tg Tin, with the time of the day for single and double-pass SAHs. It was observed that a minimum air flow rate of 0.011 kg/s produced the highest temperature difference: the maximum values of DTbed were recorded to be 47.6 K for the single-pass SAH and 52.7 K for the double-pass SAH. The maximum DTg was 32.6 K for the single-pass SAH and 38.4 K for double-pass SAH. For the maximum air flow rate of 0.032 kg/s, the maximum DTbed value was 28.3 K and DTg was 19.7 K for the double-passes SAH. From these results, its evidence that the temperature of the bed and the second glass is higher in double passes than in single pass SAH. This is as a result of the air in counter flow will preheated in the first channel before it turns down to enter the lower channel through the 36 cm 4 cm opening in the second glass and flow in
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(a)
(a) 40 50
30
30
ΔΤ g (Κ)
ΔΤbed (Κ)
40
20 m=0.011 m=0.016 m=0.022 m=0.028 m=0.032
10
0
8
10
12
kg/s kg/s kg/s kg/s kg/s
14
20
m=0.011 m=0.016 m=0.022 m=0.028 m=0.032
10
16
0
Time of the day (hrs)
8
10
12
kg/s kg/s kg/s kg/s kg/s
14
16
Time of the day (hrs)
(b)
(b) 40
50
40
30
ΔΤ g (Κ)
ΔΤbed (Κ)
30
20
m=0.011 m=0.016 m=0.022 m=0.028 m=0.032
10
0
8
10
12
14
kg/s kg/s kg/s kg/s kg/s
20
m=0.011 m=0.016 m=0.022 m=0.028 m=0.032
10
16
Time of the day (hrs) Fig. 5. Bed temperature difference versus standard local time of the day at different mass flow rates for: (a) single-pass SAH, and (b) double-pass SAH.
0
8
10
12
14
kg/s kg/s kg/s kg/s kg/s 16
Time of the day (hrs) Fig. 6. Glass temperature difference versus standard local time of the day at different mass flow rates for: (a) single-pass SAH, and (b) double-pass SAH.
second channel in reverse direction [8]. We have to mention here that the temperature of the bed and the glass is the average temperature of the three thermocouple positioned at three different positions, as we mentioned in experimental set-up section, inside the solar air heater. The thermal efficiencies versus hours of the day at different air flow rates are presented in Fig. 7. These efficiencies increase consequently with an increase in the fluid flow rate. The maximum values obtained for single-pass and double-pass SAHs with a maximum rate of flowing air of 0.032 kg/s were 55% and 62.5%, respectively. Depending on the solar intensity and the outlet air temperatures of the bed, the efficiency results generally continue to increase from morning until evening. These observations are similar to those of El-khawajah et al. [19]. The increases in the efficiencies are the result of heat stored inside the porous (wire mesh) media and fins. After noon, the solar intensity starts to decrease while the air mass flow rate continues to carry the same amount of energy, which is the energy received from the sun plus the energy absorbed from the mesh layers. Another explanation of this observation is a continual addition of heat to the inlet temperature from morning until evening on a day with no wind (Fig. 3). At all cases studied in this work, the efficiency in 4-fins double-pass SAH is greater than that of a single-pass SAH. The average efficiency of double pass is found to be bigger than single pass for
the entire air mass flow rate used (Fig. 8). The comparison of the average efficiencies in the present work with the reported data for double-pass SAHs shows that there is an improvement in the proposed SAH (Fig. 9). Fig. 10 shows the efficiency of the single-pass SAH compared to double-pass SAH versus standard local time of the day when decreasing lower channel heights, 7.5-cm, 5-cm and 3-cm were applied. The thermal efficiency was found to increase by decreasing the space between the second cover and bottom bed for the double-pass SAH. The maximum efficiency of 65.8% for height 3cm at m = 0.032 kg/s was shown to be the best for double pass solar collector compared to 64% and 62.5% for 5-cm and 7.5-cm respectively. For minimum air flow rate of 0.011 kg/s, the result shows similar behaviour as for the maximum air flow rate of 0.032 kg/s. In general, increasing the gap between second cover and the bottom of the duct reduces the average air velocity and decreases the heat transfer coefficient. Hence, the space of the lower air flow channel has a considerable effect on the performance of the solar SAH. The influence of changes in lower bed height on the thermal efficiency with and without the glass wool as a porous medium was considered [26]. They recommended that the best thermal efficiency of the duct with of 2.4-m channel length and air flow depth of 3-cm. El-Sebaii et al. [5] suggests designing equivalent
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(a)
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Ramadan et al. [9] Omajaro [20] Sopian et al. [7] Mousa et al. [22] Present data, double pass
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Fig. 9. Efficiency comparison between the double-pass SAH with some double-pass SAHs in literature.
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30 Single-pass, m=0.011 kg/s Double- pass, h=3, m=0.011 kg/s Double- pass, h=5, m=0.011 kg/s Double- pass, h=7.5, m=0.011 kg/s Single-pass, m=0.032 kg/s Double- pass, h=3, m=0.032 kg/s Double- pass, h=5, m=0.032 kg/s Double- pass, h=7.5, m=0.032 kg/s
20 8
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Fig. 7. Variation of collector efficiency at different mass flow rates for: (a) singlepass SAH, and (b) double-pass SAH.
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Time of the day (hrs) Fig. 10. Effect of second pass height on the thermal efficiency of a solar air heater for air mass flow rate of 0.011 kg/s and 0.032 kg/s.
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Fig. 11. Pressure drop comparison between the double-pass SAH with some double-pass solar air heaters in literature.
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across the double-pass SAH that was filled with mesh layers was greater than that of the single-pass SAH at an equal rate of flowing air, as shown in Fig. 12. For 0.011 kg/s, the pressure drop for the single-pass SAH was observed to be 12 Pa and 14.5 Pa for the double-pass SAH. The maximum pressure drop values were 64 Pa and 66 Pa at a maximum air flow rate of 0.032 kg/s for single and double-pass SAHs, respectively. The pressure drop increases as the efficiency is increased for the same air mass flow rate (Fig. 13).
Pressure drop (pa)
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5. Conclusions
45
The single and double pass SAH with wire mesh used as an absorber plate was constructed and tested for different air mass flow rate and different height of the lower channel in case of double pass. The obtained results of the proposed design show that the thermal efficiency and the average efficiency for the double-pass SAH is higher than that of the single-pass SAH for the same air mass flow rate, and will increase by increasing the air mass flow rate. The thermal efficiency also found to be reduced with increasing the height of the lower channel. Moreover, the temperature difference between the outlet and the inlet increases with decreasing the air mass flow rate. The pressure drop across the double-pass was greater than that of the single-pass SAH for the same air mass flow rate. The increase in the pressure drop within the bed with this proposed design is not too high when compared to the available published data. Finally, the comparison of the average efficiencies in the present work with the reported data for double-pass SAHs shows that there is an improvement in the proposed SAH.
40
References
20 Double pass Single pass
0 0.01
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Mass flow rate (kg/s) Fig. 12. Pressure drop across the bed versus mass flow rates for single-pass and double-pass SAH.
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35
30 Single pass Double pass 25
20 10
20
30
40
50
60
70
Pressure drop (pa) Fig. 13. Average efficiency versus pressure drop for the range of air mass flow rate between 0.011 and 0.032 kg/s.
height for both lower and upper air flow channel to present higher temperature output when porous media is used along with it. The disadvantage of using porous media instead of the absorber plate is the increases in the pressure drop through the channel. However, the increase in the pressure drop within the bed with this proposed design, as shown in Fig. 11, is not too high when compared to the available published data. Bashria et al. [12] determined that the use of porous media in the double flow V groove absorber increased the pressure drop by 3–25 Pa more than the pressure drop in the same double flow V groove absorber without the porous media. For 0.02 kg/s, the pressure drop was 7 Pa. The maximum pressure drop was 78 Pa with a maximum air flow rate of 0.08 kg/s. Romdhane et al. [21] indicated that the pressure drop for a double pass SAH with limestone was 100 Pa for an air flow rate of 0.01 kg/s and 800 Pa at an air flow rate of 0.05 kg/s. El-khawajah et al. [19] experimentally determined that the minimum pressure drop (Pd) was 14 Pa for a six-fin SAH with a minimum rate of flowing air of approximately 0.0121 kg/s. In addition, the greatest pressure drop (Pd) was 84.5 Pa for an air flow rate of 0.042 kg/s when mesh layers between transverse fins were used as an alternative to an absorber plate. The pressure drop (Pd)
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