Experimental Study of New Solar Air Heater Design

1 INTRODUCTION

One of the practical applications of solar energy is to produce hot air for industrial purposes using solar air heater (SAH). Hot air is used to remove moisture from timber, bagasse, vegetables, aqua products, chemicals, and so forth and to provide comfort air-conditioning in cold climatic conditions. However, lower heat transfer rates with air as working fluid, increased heat losses to surroundings at higher working temperatures and non-uniform availability of solar radiation are main limitations of SAH. Therefore, increasing the thermal performance of SAH has been taken up as an urgent challenge. Use of double passages, packed beds, corrugated absorber plates, encapsulated phase change material (PCM), air-blowers, and thermal storage units either directly or indirectly integrated with SAH are some of the techniques to improve thermal efficiency of SAH. Complete review on various design configurations of SAHs and methods to improve their performance is presented in Kabeel et al. ¹ Review on SAHs with thermal storage units is presented by Jegadheeswaran and Pohekar, ² Tyagi et al., ³ Saxena and Goel, ⁴ and Castro et al. ⁵ Performance of SAHs attached with latent heat thermal storage systems can be improved with the use of extended surfaces, multiple PCM's, microencapsulated PCM. ^4-10 Review on recent developments in new type of PCMs and their applications in SAHs is available in Farid et al., ⁶ Sharma and Sagara, ⁷ Lalit et al., ⁸ and Belen Zalba et al. ⁹ It was found that paraffin waxes and hydrated salts are popular PCMs used for solar energy storage and conversion into heat in SAHs. Paraffin waxes have lower thermal conductivity compared to hydrated salts and hence are better suited for moderate thermal storage capacities in SAHs. ^1-3 Use of ribs/obstacles on SAH absorber plate is one of the passive techniques to improve heat transfer rates in SAHs. Ribs on absorber plate creates more turbulence near the plate and hence local heat transfer coefficients increases with simultaneous increase in efficiency of SAH. Gupta and Garg, ¹⁰ Esen, ¹¹ El-Sebaii et al., ^{12, 13} and Nandkishor and Mohan ¹⁴ verify this fact experimentally.

Effective operation of SAH can be extended to off-sunshine hours by separately attaching a thermal storage unit packed with either sensible or latent heat storage material. Heat transferred from hot air to storage material in the thermal storage unit during daytime is reutilized in the night-time. Findings on SAH with additional storage system are subsequently presented here in this paragraph. It was experimentally found by Farid et al. ¹⁵ That performance of SAH improved with the use of cylindrical capsules filled with different PCMs, with a narrow melting range. Fath ¹⁶ proposed to integrate PCM filled tubes directly with the absorbed plate of SAH. Tubes were placed perpendicular to the airflow direction either in straight or corrugated position. It was found from his analytical solutions that SAH efficiency increased to 63.65% with the use of corrugated tubes filled with paraffin wax on its absorber plate. It was also observed that heat availability in SAH extended from 16 to 21 hours with decrease in mass flow rate of air from 0.02 to 0.01 kg/s. Krishnananth and Murugavel ¹⁷ conducted experimental analysis on an SAH with PCM storage tubes on its absorber plate. Tubes filled with paraffin wax are placed along the length of SAH (ie, parallel to the direction of airflow). It found that efficiency of SAH increased with the use of PCM tubes above the absorber plate. Reddy et al. ¹⁸ conducted experiments to study a thermal energy storage system with sodium thiosulfate pentahydrate (Na₂S₂O₃.5H₂O) as PCM filled in capsules of stainless steel. It is observed from the results that the effect of mass flow rate and inlet temperature of air is more on charging time of PCM. Hence, higher flow rates and higher inlet temperatures of air are recommended for better performance of SAH. Shalaby and Bek ¹⁹ conducted experiments on an indirect SAH with PCM with and without PCM and observed that the temperature of air at exit of SAH is 7.5°C higher than the ambient temperature with the use of PCM in storage chamber. Purnanand et al. ²⁰ conducted experiments on a concentric solar collector integrated with a thermal storage unit. It is found from their results that the charging and overall efficiency of thermal storage system are 12% and 5%, respectively, higher with the use of PCM. Average temperature of the working fluid reached 10°C higher with the use of PCM in the thermal storage unit. Bhardwaj et al. ²¹ conducted experiments on an indirect SAH using paraffin RT42 to remove moisture from Valeriana Jatamansi (high value medicinal plant which is found in many countries of Asia). It is observed from their results that the outlet temperature of air is 8°C to 10°C higher than the ambient temperature up to duration of 7 hours even after the sunset. The moisture content in Valeriana Jatamansi reduced up to 9% with the use of the solar dryer. Rabha and Muthukumar ²² conducted experiments on a solar dryer integrated with PCM thermal storage. The dryer consists of two double pass SAH's in series, a shell and tube heat exchanger with paraffin wax as PCM and a drying chamber to remove moisture from red chilli. It was found that PCM storage produced uniform air temperatures and even reduced the drying time of red chilli. Aymen et al. ²³ experimentally tested an indirect type of forced convection solar dryer using PCM. The results show that drying chamber temperature were 4°C to 16°C higher than the ambient temperature after sunset up to 10 to 12 hours. It was found experimentally by Nidal and Khaled ²⁴ that, acetamide is a good choice as PCM in solar stove application. Performance of solar stove improved to 43% with the use of PCM based thermal storage to supply hot air to solar stove. Mesut et al. ²⁵ experimentally found that use of honeycomb structural fins in a PCM storage panel increases heat transfer rates in SAHs. Efficiency of SAH and exit temperature of hot air increased considerably with the use honeycomb structural fins. Chaitanya and Mohan ²⁶ conducted experiments to know the thermal performance of an SAH integrated with a PCM-based storage unit to supply hot air during night period. They found that the temperature drop across SAH during charging of PCM was 7°C to 8°C and temperature rise during discharging of PCM was 4°C to 5°C. Kumar and Mohan ²⁷ experimentally found the thermal performance of double pass SAH with and without packed bed. Iron chips are used as packed bed material and mass flow rate of air changed between 0.036 and 0.625 kg/s. It is found that efficiency of SAH increased by 20.4% with the use of packed bed in the lower channel of SAH.

The other way of improving the thermal performance of SAH is by directly placing storage material on its absorber plate. Alkilani et al. ²⁸ placed cylindrical capsules filled with PCM on SAH absorber plate and conducted experiments to study its thermal performance. It is found that paraffin wax (PCM) thermal conductivity increased with the use of aluminum powder as ingredient. It is observed that the charging time reduced to 70% when paraffin wax-aluminum composite is used as storage material. Thermal storage efficiency is increased to 78% at an optimum mass flow rate of air equal to 0.7 kg/s. Sami et al. ²⁹ experimentally proved that SAH with packed bed of spherical capsules as latent heat storage improves the indoor temperature of greenhouse during night by up to 8°C higher than the ambient temperature. Dilip and Pratibha ³⁰ conducted experiments on a solar crop dryer attached with a packed bed filled with paraffin wax as PCM. They observed that the temperature in drying chamber increased by 6°C above ambient and the thermal efficiency of solar crop dryer increased to 28.2%. Akram ³¹ conducted experiments to enhance the thermal storage efficiency and thermal behavior for SAH integrated with cylindrical capsules. When paraffin wax and sand were used as thermal storage materials it was observed from the results that the outlet temperature of air is increased by 9.2% with the use of sand and PCM (20%) combination.

Kabeel et al. ³² experimentally found that PCM placed in bottom channel of the SAH fixed with v-corrugations on its absorber plate improves daily efficiency of SAH by 12% compared to that without the use of PCM. Efficiency of SAH with same configuration is 15% and 21.3% higher compared to SAH with plane absorber plate, with and without the use of PCM, respectively. Kabeel et al. ³³ also found that daily efficiency of SAH fixed with finned absorber plate and PCM (placed in its bottom channel) improved by 12.2% when compared to without the use of PCM. Moradi et al. ³⁴ conducted both experimental and numerical study on an SAH integrated with PCM. The PCM packed into thin plastic packs and two sheets of aluminum placed inside each package placed underneath the absorber plate. The average temperature difference across the SAH and efficiency of SAH are obtained as 4.5°C and 35% respectively. Amol Wadhawan et al. ³⁵ conducted experimental and numerical analysis for internal flow through a rectangular duct consisting of lauric acid as PCM placed in series of cylindrical tubes across the flow of air. It is observed that average air temperature at the duct exit is increased by 86.47% with a simultaneous increase in friction factor by 36.47% with the use of PCM based thermal energy storage device along with the duct.

Attaching capsules filled with PCM on absorber plate of SAH is a promising passive technique to improve its performance. Even the heat transfer rates are found to increase with the use of encapsules, it also increased pressure drop across SAH. Hence, alignment and shape of encapsules play an important role in the optimum design of encapsules on absorber plate of SAH. In literature, experimental analysis on SAH with cylindrical and spherical encapsules with inline arrangement were presented by many of the researchers. However, studies with staggered arrangement are not available. Encapsules in different shapes including square in cross-section and in different alignments including staggered, circular and so forth, may further improve heat transfer rates and performance of SAH. In order to fill the gap in literature in this aspect, experiments were conducted on SAH with two different types of encapsules: circular, square in cross section and with two alignments: inline (square pitch), staggered (triangular pitch). In this article, results in terms of heat transfer rates, efficiency, and temperature rise of air across SAH are presented from the experimental analysis.

2 EXPERIMENTAL SETUP AND PROCEDURE

Schematic diagram and physical model of the experimental setup are shown in Figure 1A,B, respectively. Experimental setup consists of a double pass SAH, an air blower, U-tube differential manometer connected across an orificemeter, thermocouples, and temperature indicators. Solar air heater is prepared with 1.8 cm thick plywood sheet in the form of a rectangular box. It is divided into two channels: upper and lower channel separated by an encapsulated absorber plate. The size of each channel is 120 cm × 80 cm × 10 cm and each has an aspect ratio equal to 8 (ASHRAE standards 93-97 ³⁶ ). Upper channel is covered with double-glazing (to reduce convection heat loss) with surface area of the single glass equal to 9600 cm². A normal window glass having a thickness of 0.5 cm is used for glazing and the distance between the two glasses is 2 cm. Packed bed with mild steel scrap of 4 kg was prepared and placed in the lower channel of the SAH. The absorber plate is made of G.I sheet of thickness of 0.4 cm and coated with black board paint to absorb maximum solar radiation. Absorber plate was prepared 10 cm shorter than the SAH length so that when it was fixed between lower and upper channels of the SAH it creates a space for air to move from upper to lower channels of the SAH. Absorber plate is fixed with 90 aluminum capsules (coated externally with black board paint) in inline or staggered arrangement as in Figures 2-5. Capsules in two different cross sections: square (4 cm side and 5 cm height) and circular (4 cm diameter and 4 cm height) are used on absorber plate. As indicated in Figures 2-5, cylindrical capsules in staggered arrangement are name as "type 1," square capsules in staggered arrangement as "type 2," cylindrical capsules in inline arrangement as "type 3," and square capsules in inline arrangement as "type 4." Each capsule is filled with molten paraffin wax up to 70% of its volume. Top side of each capsule is closed with rubber cocks to completely seal the paraffin wax. Black board paint is applied on the absorber plate including outer surface of all capsules to improve their absorptivity for solar radiation.

Experimental setup. A, Schematic diagram. B, Physical model

Absorber plate with cylindrical capsules in staggered arrangement: type 1

Absorber plate with square capsules in staggered arrangement: type 2

Absorber plate with cylindrical capsules in inline arrangement: type 3

Absorber plate with square capsules in inline arrangement: type 4

An air blower (1 HP and rated speed 2800 rpm) is used to supply air at different mass flow rates (0.00879-0.01484 kg/s) to the SAH. Flow rate of air is changed by manually adjusting a cap attached to the suction side of the air blower. Air is initially allowed to pass through the lower channel formed between absorber plate and packed bed. It takes a U-turn at the end of the channel and is then allowed to flow through the upper channel formed in between bottom glass and absorber plate. SAH is insulated with 2 cm thick polystyrene sheet as insulating material on its bottom and sides. The total experimental set up is placed on a metal frame which is oriented toward South and tilted at an angle of 15°. The total solar radiation was recorded by placing the solar power meter (WACO TM-206 model) having maximum range of 2000 W/m². Temperatures at different locations of the experimental setup are recorded manually using DIGIQUAL 12-P temperature indicator by connecting calibrated J (Iron-Constantan) type thermocouples with one end connected to the point of measurement and the other to the indicator. Twelve thermocouples are fixed inside encapsules to measure PCM temperature. Two thermocouples each are fixed to the absorber plate, lower glass, upper glass, and packed bed to measure their respective temperatures. Two more thermocouples are used to measure the air temperature at inlet and exit section of SAH. All thermocouples are connected to digital temperature indicator as shown in Figure 2, to manually record the temperature. Dimensions of SAH and the equipment used in the experiment are shown in Tables 1 and 2, respectively. Properties of paraffin wax used in the present analysis are shown in Table 3.

TABLE 1. Dimensions of solar air heater and materials of different parts

Part	Specification
Upper channel	120 cm × 80 cm × 10 cm
Aspect ratio	8 as per ASHRAE standards 93-97 36
Glass	Thickness 0.5 cm
Packed bed	4 kg of mild steel chips
Absorber plate	Aluminum sheet of 0.4 cm thickness
Insulation	2 cm thick polystyrene
Absorber coating	Black board paint

TABLE 2. Specifications of equipment used

Equipment	Specification
Air blower	0.5 hp variable speed motor
Temperature indicator	Digiqual 12, 18, 18 channels
Thermocouples	J-type (22 in number)
Solar power meter	WACO TM-206 model

TABLE 3. Thermophysical properties of paraffin wax ^{6, 9}

Parameter	Value
Melting temperature (°C)	64
Thermal conductivity (W/m.K)	0.167 (solid state) 0.346 (liquid state)
Density (kg/m3)	916 (solid state) 790 (liquid state)
Latent heat of fusion (kJ/kg)	173.6
Specific heat (kJ/kg.K)	2.0 (solid state) 2.15 (liquid state)

Exit side of the SAH is attached with an orificemeter and pressure tapings across the orificemeter are connected to two limbs of a U-tube differential manometer to measure flow rate of air. Water is used as a manometric fluid in the manometer. Deflection of water in U-tube differential manometer is measured and pressure head difference across orifice meter is measured to know the volume flow rate of air in exit section of the SAH. This is used to compute the velocity of air at the inlet to SAH by dividing with flow cross section at inlet to SAH. Velocity of air at inlet to SAH is presented in Table 4.

TABLE 4. Velocity of air at the inlet to SAH used for the present analysis

S. No.	Deflection in manometer (cm)	Volume flow rate of air (m3/s)	Mass flow rate of air (kg/s)	Velocity of air at inlet to SAH (m/s)
1	10	0.01309	0.01484	0.2332
2	8	0.01167	0.01328	0.2085
3	7	0.01095	0.01242	0.1955
4	6	0.01012	0.01145	0.1811
5	5	0.00934	0.01049	0.1644
6	3.5	0.00796	0.00879	0.1376

• Abbreviation: SAH, solar air heater.

J-type thermocouples (iron-constantan) with a digital temperature indicator are used to measure temperature at various locations in the experimental setup. The operating range of J-type thermocouples (class 1 tolerance) is −10°C to 375°C. In order to avoid measurement error all thermocouples were calibrated using temperature measurement trainer before using them in the experiment.

2.1 Thermal performance parameters of SAH

Heat gained by air across SAH is calculated using Equation (1),

(1)

Heat incident on SAH is calculated using Equation (2),

(2)

(3)

In Equation (3), A _p is calculated from

(4)

In Equation (3), A _C for cylindrical and square encapsules is calculated from,

(5)

In Equation (3), A _SC for cylindrical and square encapsules is calculated from,

(6)

Efficiency of SAH is calculated from Equation (7),

(7)

Thermal performance factor (thermal performance factor (TPF); adapted from Maradiya ³⁷ ) is calculated from Equation (8),

(8)

In Equation (8), and f are, respectively, Nusselt number and friction factor for flow through SAH with absorber plate attached with encapsulated PCM, and f _o are the Nusselt number and friction factor for flow through SAH with plane absorber plate (all the expressions are adapted from Incropera and Dewitt ³⁸ ).

(9)

(10)

In Equation (9), C ₁, C, and n values are dependent on transverse and longitudinal pitch of encapsules on the absorber plate and are taken from Incropera and Dewitt. ³⁸

(11)

(12)

2.2 Experimental procedure

Experiments were conducted on the prepared setup of double pass SAH with four types of absorber plates. The absorber plate with cylindrical capsules in staggered arrangement (type 1) was attached to SAH on the first day of the experiment and the blower was operated to supply air at a mass flow rate of 0.01484 kg/s. The direct solar radiation was recorded by placing solar power meter (WACO TM-206) perpendicular to the upper glass of the experimental setup. Thermocouples placed at different locations of SAH were connected to a multi-point temperature indicator to manually record temperatures at respective locations. The blower was allowed to run continuously for 30 minutes for each reading and observations were recorded at every 5 minutes within these 30 minutes of time interval. PCM inside the capsules are charged during daytime from 9:45am to 2:15pm. The temperature of the air at SAH inlet and exit, temperatures of packed bed, absorber plate, lower glass, upper glass, and PCM in capsules were manually recorded. In between 2:20pm and 4:20pm, blower was switched off and the experimental setup was completely placed under insulation covers to minimize the heat loss to the surroundings. After 4:20pm insulation covers were removed and blower was switched on to supply the air. This time PCM discharges stored heat to the air flowing over the capsules. The experiment for discharging process was conducted between 4:20pm to 5:40pm. The temperature of the air at SAH inlet and exit, temperatures of packed bed, absorber plate, lower glass, upper glass, and PCM in capsules were once again manually recorded. In subsequent 5 days, experiments were repeated with type 1 absorber plate with five different mass flow rates of air. Similar experiments are conducted on SAH with the remaining type of absorber plates. Details of mass flow rate of air, day of experiment and category of absorber plate used in SAH is presented in Table 5.

TABLE 5. Details of experiments conducted on different day intervals

Mass flow rate of air (m), kg/s	Day of experiment	Category of absorber plate used in SAH
0.01484	1/7/13/19	Type 1/2/3/4
0.01328	2/8/14/20
0.01242	3/9/15/21
0.01145	4/10/16/22
0.01049	5/11/17/23
0.00879	6/12/18/24

• Abbreviation: SAH, solar air heater.

During each day of experimentation, temperature at different locations of SAH and intensity of solar radiation measured at different time intervals during charging and discharging of PCM. PCM temperatures in capsules, air temperature at inlet and exit sections of SAH also measured at same time intervals. Average values of temperature of packed bed, absorber plate and glazing were calculated from the observations.

2.3 Uncertainty analysis

Accuracy of experimental analysis depends on errors due to instrumentation in the experiment. Uncertainty analysis investigates the uncertainty of variables that are used in the experiment. In the present investigation, uncertainty analysis was carried out to predict error in measurement of SAH efficiency and TPF. Procedure adapted from Schultz and Cole ³⁹ and used to determine percentage of uncertainty in the measurement of SAH efficiency and TPF was given below. Measurement of uncertainty in measurements of temperature of air, deflection in manometric liquid, mass flow rate of air, efficiency of SAH, TPF of SAH, and so forth, are summarized in Table 6.

TABLE 6. The uncertainties in measurements and calculations

S. No.	Parameter	Measurement uncertainty
1	Coefficient of discharge of orificemeter	±0.1
2	Temperature of air (°C)	±0.2128
3	Deflection in manometer (cm)	±0.01414
4	Velocity of air at inlet section of SAH inlet (m/s)	±0.0196
5	Incident solar radiation (W/m2)	±0.3272
6	Mass flow rate of air (kg/s)	±0.0071
7	Heat gained by air (W)	±0.5953
8	Incident energy on solar air heater (W)	±0.0724
9	Efficiency of SAH	±0.1448
10	Maximum velocity of air across the capsules (m/s)	±0.0539
11	Reynolds number	±0.0789
12	Nusselt number	±0.0587
13	Heat transfer coefficient (W/m2.K)	±0.0837
14	Friction factor	±0.0167
15	Nusselt number of plane SAH	±0.0631
16	Friction factor of plane SAH	±0.0197
17	Pressure drop across SAH (Pa)	±0.1245
18	TPF	±0.1582

• Abbreviations: SAH, solar air heater, TPF, thermal performance factor.

3 RESULTS AND DISCUSSION

Variation of temperature with time for SAH of type 1 is shown in Figure 6A during charging of PCM and in Figure 6B during discharging of PCM. Similarly, variation of temperature with time for SAH of type 2 is shown in Figure 7A during charging of PCM and in Figure 7B during discharging of PCM. Temperature with time for SAH of type 3 is shown in Figure 8A during charging of PCM and in Figure 8B during discharging of PCM. Temperature with time for SAH of type 4 is shown in Figure 9A during charging of PCM and in Figure 9B during discharging of PCM. Mass flow rate of air = 0.01484 kg/s is considered for the above cases.

Variation of average temperatures with time for solar air heater (SAH) of type 1

Variation of average temperatures with time for solar air heater (SAH) of type 2

Variation of average temperatures with time for solar air heater (SAH) of type 3

Variation of average temperatures with time for solar air heater (SAH) of type 4

It can be observed from Figures 6A to 9A that, at any instant of time T _ab  >T _PCM  >T _o  >T _G  >T _PB  >T _i . It can be observed from Figure 5A that, at 11:30am. T _PCM crosses T _ab and remains at higher value comparing with temperature at the end of the experiment. Due to phase change of PCM more amount of heat is transferred to PCM compared to absorber plate from 11:30am onward and hence T _PCM >T _ab. This trend was observed after the intensity of solar radiation reached maximum. It was observed from Figure 8A that, from 11:00am. T _PCM was crossing T _ab and remains at higher value comparing with temperature at any location. The temperature difference between T _PCM and T _ab increased from 11:00am and reached maximum at end of the experiment. Due to phase change of PCM more amount of heat is transferred to PCM compared to absorber plate from 11:00am onward and hence T _PCM >T _ab.

This trend was observed after the intensity of solar radiation reached maximum. It can be observed from Figure 9A that, T _PCM  >T _ab from 10:30am itself. The temperature difference between T _PCM and T _ab remains more or less constant after 11:00am until the end of the experiment. PCM changed its phase much earlier due to higher values of solar intensity during morning of the experimentation. It can be observed from Figures 6B to 9B that, at any instant of time T _PCM >T _o >T _ab  >T _G >T _PB >T _i. Glazing temperature starts decreasing rapidly after 5:00pm due to increase in velocity of atmosphere air at its surface with simultaneous decrease in atmosphere temperature.

Variation of intensity of solar radiation with time for SAH of type 1 (on first day of experimentation), type 2 (on seventh day of experimentation), type 3 (on 13^th day of experimentation), and type 4 (on 19^th day of experimentation) is shown in Figure 10. It can be observed that the intensity of radiation increased from 10:00am to 12:00pm and then decreased up to 2:00pm on all days of experimentation. It reached a maximum value of 1027.2 W/m² (day 1), 1027.5 W/m² (day 7), 1065 W/m² (day 13), and 1041.4 W/m² (day 19) at 12:00pm. It was observed that, intensity of solar radiation was almost zero during discharge of PCM from 4:30pm to 5:30pm for all days of experimentation and hence they are not presented.

Variation of solar radiation intensity with time

It was found from the experimental observations that T _PCM is a strong function of solar radiation intensity. At any instant of time, it was found that, T _PCM or T _ab is higher compared to temperature at any location in the experimental setup. The temperature difference between air temperature at exit and inlet sections of SAH is steadily increasing with increase in time during charging of PCM.

The average value of air temperature difference between exit and inlet sections of the SAH is given in Table. 7. It can be seen from Table 7 that staggered grid arrangement of encapsules gives higher temperature differences compared to inline grid arrangement. At lower mass flow rate of air, cylindrical capsules in staggered grid arrangement produces higher temperature differences compared to square capsules in staggered grid arrangement and vice a versa.

TABLE 7. Average air temperature difference between exit and inlet sections of the SAH during charging of PCM

S. No	Category of SAH	Mass flow rate of air (kg/s)	Temperature at SAH inlet T in (°C)	Temperature at SAH exit T out (°C)	Temperature difference (°C)
1	SAH: type 1	0.00879	30.80	44.28	13.48
		0.01049	30.82	44.65	13.82
		0.01145	33.62	45.91	12.28
		0.01242	32.88	46.37	13.48
		0.01328	32.65	45.42	12.77
		0.01484	32.54	44.77	12.25
2	SAH: type 2	0.00879	30.28	43.02	12.74
		0.01049	30.57	43.75	13.18
		0.01145	33.62	46.20	12.57
		0.01242	30.94	44.20	13.25
		0.01328	30.08	43.40	13.31
		0.01484	30.37	46.92	16.55
3	SAH: type 3	0.00879	31.31	43.37	12.05
		0.01049	32.57	44.08	11.51
		0.01145	32.48	44.54	12.05
		0.01242	32.37	43.97	11.60
		0.01328	33.11	45.22	12.11
		0.01484	31.88	43.91	12.02
4	SAH: type 4	0.00879	31.31	43.37	12.05
		0.01049	31.54	44.17	12.62
		0.01145	32.68	45.22	12.54
		0.01242	32.25	43.91	11.65
		0.01328	31.65	44.37	12.71
		0.01484	31.31	43.60	12.28

• Abbreviations: PCM, phase change material; SAH, solar air heater.

Heat gained by air, heat incident on SAH and efficiency of SAH with different absorber plates at different mass flow rates of air was calculated by using Equations (1) to (7-1) to (7). Variation of heat gained by air vs mass flow rate of air is shown in Figure 11. It was can be observed that as mass flow rate of air increased from 0.00879 to 0.01484 kg/s heat gained by air increases with all types of encapsules. Heat gained by air in SAH of type 1 (cylindrical encapsules in staggered arrangement) increased from 0.1053 to 0.1793 kW, that is, around 70% to its initial value. Heat gained by air in SAH of type 2 (square encapsules in staggered arrangement) increased from 0.1095 to 0.3533 kW, that is, more than two times of its initial value. Heat gained by air in SAH of type 3 (cylindrical encapsules in inline arrangement) increased from 0.1064 to 0.1597 kW, that is, around 51% to its initial value. Heat gained by air in SAH of type 4 (square encapsules in inline arrangement) increased from 0.1063 to 0.2264 kW, that is, more than double its initial value. Heat gained by air in SAH of type 2, up to a mass flow rate of air equal to 0.01328 kg/s is at an average of 7% higher than the corresponding heat gained by air in other types of SAH. Whereas, for mass flow rate of 0.01484 kg/s, it was at average of 47% higher than heat gained in other types of SAH. In staggered grid arrangement, from second row onward encapsules in each row were placed in wake region of their upward encapsules. As a result, more turbulence was created which enhanced heat transfer rates and hence increased temperature of air at exit section of SAH. In addition encapsules in square cross section are blunt in nature when compared to cylindrical encapsules and hence more turbulence was created and hence heat transfer rates are increased.

Variation of heat gained by air with mass flow rate of air

Variation of heat incident on SAH vs mass flow rate of air (m _a ) is shown in Figure 12. Heat incident on SAH (Q _i ) is a strong function of solar intensity of radiation (I) falling on SAH. It was observed from Figure 12 that, for SAH of type 1, Q _i changed between 1.2135 and 1.1558 kW as m _a changed between 0.00879 and 0.01484 kg/s. Q _i changed between 1.0278 and 1.0434 kW for SAH of type 2, 1.2136 to 1.2325 kW for SAH of type 3 and 1.0758 to 1.0629 kW for SAH of type 4 as m _a changed between 0.00879 and 0.01484 kg/s. Overall average value of Q _i was around 1.1254 kW. Average value of Q _i was 4% and 8% higher than overall average of Q _i for SAH of types 1 and 3, respectively. Average value of Q _i was 5% and 7% lower than overall average of Q _i for SAH of types 2 and 4, respectively.

Variation of heat incident on solar air heater with mass flow rate of air

Variation of efficiency with mass flow rate of air (m _a ) is shown in Figure 13. It can be observed that efficiency of SAH fitted with absorber plate having any type of encapsulated PCM is increasing with increase in mass flow rate of air (m _a ). Values of percentage increase in SAH efficiency at different m _a compared to initial m _a  = 0.0879 kg/s is given in Table 8. At any constant m _a , efficiency of SAH fitted with square encapsules in staggered arrangement is higher than the remaining encapsules arrangement. Increase in SAH efficiency with type 2 encapsules arrangement compared to other encapsules arrangement is given in Table 9. Average values of percentage increase in SAH efficiency are represented in Tables 8 and 9 with bold numbers. Average efficiency of SAH at different m _a under consideration with type 2 arrangement of encapsules is found to be higher and equal to 15.8% when comparing with other arrangement of encapsules. It is due to increase in turbulence of air over the absorber plate with simultaneous increase in heat transfer coefficients in the case of type 2 arrangement of encapsules on absorber plate of SAH. Hence, it was proved that SAH fitted with absorber plate having square encapsules in staggered arrangement gives higher efficiencies compared to the other arrangements.

Variation of efficiency of solar air heater (SAH) with mass flow rate of air

TABLE 8. Efficiency of SAH and % increase in efficiency with initial m _a  = 0.00879 kg/s

	Efficiency of SAH
m a	Cylindrical staggered	Square staggered	Cylindrical inline	Square inline	% Increase in efficiency of SAH compared with m a  = 0.00879 kg/s
kg/s	Type 1	Type 2	Type 3	Type 4	Type 1	Type 2	Type 3	Type 4
0.00879	8.104	10.625	8.706	9.806	—	—	—	—
0.01049	12.132	13.82	9.91	12.296	49.71	30.08	13.83	25.4
0.01145	12.848	14.139	11.595	13.14	58.54	33.08	33.19	34
0.01242	14.092	15.342	11.991	14.598	73.89	44.4	37.74	48.87
0.01328	14.082	17.012	13.241	15.818	73.77	60.12	52.1	61.31
0.01484	15.398	19.614	14.392	16.604	90.01	84.61	65.32	69.33
0.01188	12.776	15.092	11.6392	13.7104	57.66	42.05	33.7	39.82

• Note: Bold numbers indicates average values.
• Abbreviation: SAH, solar air heater.

TABLE 9. Efficiency of SAH and % increase in efficiency of type 2 SAH when compared to types 1, 3, and 4 SAH

	Efficiency of SAH
m a	Cylindrical staggered	Square staggered	Cylindrical inline	Square inline	% Increase in efficiency of type 2 SAH compared to
kg/s	Type 1	Type 2	Type 3	Type 4	Type 1	Type 3	Type 4	Avg.
0.00879	8.104	10.625	8.706	9.806	23.73	18.07	7.71	16.51
0.01049	12.132	13.82	9.91	12.296	12.22	28.3	11.03	17.19
0.01145	12.848	14.139	11.595	13.14	9.14	18	7.07	11.41
0.01242	14.092	15.342	11.991	14.598	8.15	21.85	4.85	11.62
0.01328	14.082	17.012	13.241	15.818	17.23	22.17	7.02	15.48
0.01484	15.398	19.614	14.392	16.604	21.5	26.63	15.35	21.16
0.01188	12.776	15.092	11.6392	13.7104	15.35	22.88	9.16	15.8

• Note: Bold numbers indicates average values.
• Abbreviation: SAH, solar air heater.

Average temperature difference of air between exit and inlet sections of the SAH during discharging of PCM is given in Table 10. Difference between these two temperatures (∆T) was found to be increasing with increase in mass flow rate of air for all types of absorber plates used in the present analysis. As the mass flow rate of air increased from 0.00879 to 0.01484 kg/s, ∆T for SAH of type 1 (cylindrical encapsules in staggered arrangement) increased from 3.6°C to 6.6°C. ∆T for SAH of type 2 (square encapsules in staggered arrangement) increased from 3.9°C to 8°C. ∆T for SAH of type 3 (cylindrical encapsules in inline arrangement) increased from 3.1°C to 6.2°C. ∆T for SAH of type 4 (square encapsules in inline arrangement) increased from 3.9°C to 7.2°C. ∆T for SAH of type 2 for a mass flow rate of air equal to 0.00879 kg/s is at an average of 9.41% higher than the corresponding efficiency of other types of SAH. It was at average of 14.82%, 22.41%, 0.62%, 11.29%, and 16.67% higher than efficiency for other types of SAH at m _a equal to 0.0105, 0.01146, 0.01239, 0.01326, and 0.01484 kg/s, respectively. Hence, it proves that SAH fitted with absorber plate having square encapsules in staggered arrangement produced hot air at higher temperatures compared to SAH with other absorber plates in the present analysis.

TABLE 10. Average air temperature difference between exit and inlet sections of the SAH during discharging of PCM

S. No.	Category of SAH	Mass flow rate of air (kg/s)	Temperature at SAH inlet T in (°C)	Temperature at SAH exit T out (°C)	Temperature difference (°C)
1	SAH: type 1	0.00879	26.8	30.4	3.6
		0.01049	27.1	31	3.9
		0.01145	27.2	31.9	4.7
		0.01242	27.2	33	5.8
		0.01328	28.1	34.2	6.1
		0.01484	27.5	34.2	6.7
2	SAH: type 2	0.00879	27.8	31.7	3.9
		0.01049	27.5	32	4.5
		0.01145	26.6	32.7	6.1
		0.01242	27.6	33	5.4
		0.01328	26.5	33	6.5
		0.01484	27	35	8
3	SAH: type 3	0.00879	27.1	30.3	3.2
		0.01049	26.8	30.6	3.8
		0.01145	27.2	31.3	4.1
		0.01242	27	31.7	4.7
		0.01328	27.2	32.5	5.3
		0.01484	27.2	33.4	6.2
4	SAH: type 4	0.00879	27.2	31.1	3.9
		0.01049	27.6	31.6	4
		0.01145	27.8	33.2	5.4
		0.01242	27.8	33.4	5.6
		0.01328	26	32	6
		0.01484	27.2	34.4	7.2

• Abbreviations: PCM, phase change material; SAH, solar air heater.

Thermal performance factor (TPF) was calculated using Equation (8) for SAH with absorber plate fixed with capsules in inline and staggered grid arrangements. Values of TPF for different mass flow rates of air (m _a ) are given Table 11. The table also presents values of friction factor (f), average Nusselt number (), and pressure drop across the SAH (∆P). It was found that f slightly decreased as m _a increased from 0.00879 to 0.01484 kg/s, whereas ∆P significantly improved for the same range of increase in m _a . It was found that TPF was decreasing with increasing m _a for both the arrangements of capsules. At any m _a , it was found that TPF for staggered grid arrangement was higher than that of inline grid arrangement of capsules. Average value of TPF for staggered arrangement was 50.88% higher than TPF value for inline grid arrangement. Therefore, encapsules in staggered grid arrangement were found to be better than encapsules in inline grid arrangement.

TABLE 11. Variation of , ∆P, and TPF with mass flow rate of air

• Abbreviation: TPF, thermal performance factor.

4 CONCLUSIONS

Experiments were conducted on a double pass packed bed SAH with encapsulated PCM on its absorber plate. Encapsules in two different geometries, square and circular in cross sections were used. Four absorber plates with encapsules in inline and staggered grid arrangements were attached one by one to SAH and experimental analysis was carried out with mass flow rate of air changed in between 0.00879 and 0.01484 kg/s. It was found that intensity of solar radiation steadily increases before noon, reaching to its maximum at noon and decreases after the noon. Accordingly absorber plate temperature and PCM temperature in encapsules were recorded as higher compared to other temperatures in the experimental setup during charging period, that is, from 10:00am to 2:00pm.

• Efficiency of SAH was found to be increasing with increase in mass flow rate of air for all the arrangements.
• Efficiency of SAH with square encapsules on its absorber plate is more efficient than SAH with cylindrical encapsules on its absorber plate.
• Square encapsules in staggered arrangement increases efficiency of SAH by 9.16% compared to square encapsules in inline arrangement. Similarly it increases efficiency of SAH by 15.35% and 22.88%, respectively, when compared to cylindrical encapsules in staggered and inline grid arrangements.
• In discharging period, that is, from 4.30pm to 5.30pm, air temperature at the SAH exit was at an average of 5.17°C greater than the ambient temperature for all arrangements.
• Average TPF of SAH with absorber plate fixed with encapsules in staggered grid arrangement was 50.88% higher than that of SAH with absorber plate fixed with encapsules in inline grid arrangement.

Hence, it is concluded that use of encapsulated PCM on absorber plate of an SAH improves SAH performance to transfer heat to air.

Nomenclature

Greek symbols

η

efficiency of solar air heater given by Equation (7) (—)

∆P

pressure difference across solar air heater (Pa)

μ

dynamic viscosity of air at an average temperature of air at SAH inlet and exit (Ns/m²)

μ _w

dynamic viscosity of air at capsule wall temperature (Ns/m²)

μ _b

dynamic viscosity of air at bulk mean temperature of air (Ns/m²)

ρ

density of air (kg/m³)

Subscripts

i

at inlet section of solar air heater

G

glass

o

at exit section of solar air heater

ab

absorber plate

PB

packed bed

PCM

phase change material in capsule

Open Research

DATA AVAILABILITY STATEMENT

Datasets related to this article can be found at https://doi.org/10.17632/t4rt98ykj5.3, an open-source online data repository hosted at Mendeley Data. ⁴⁰

REFERENCES

• 1 Kabeel AE, Mofreh Hamed H, Omara ZM, Kandeal AW. Solar air heaters: design configurations, improvement methods and applications—a detailed review. Renew Sustain Energy Rev. 2017; 70: 1189- 1206. https://doi.org/10.1016/j.rser.2016.12.021.
• 2 Jegadheeswaran S, Pohekar SD. Performance enhancement in latent heat thermal storage system: a review. Renew Sustain Energy Rev. 2009; 13: 2225- 2244. https://doi.org/10.1016/j.rser.2009.06.024.
• 3 Tyagi VV, Panwar NL, Rahim NA, Kothari R. Review on solar air heating system with and without thermal energy storage system. Renew Sustain Energy Rev. 2012; 16: 2289- 2303. https://doi.org/10.1016/j.rser.2011.12.005.
• 4 Saxena A, Goel V. Solar air heaters with thermal heat storages. Chin J Eng. 2013; 2013: 1- 11. https://doi.org/10.1155/2013/190279.
• 5 Pradeep Castro P, Karthick Selvam P, Suthan C. Review on the design of PCM based thermal energy storage systems. Imp J Interdiscip Res. 2016; 2(2): 203- 215.
• 6 Farid MM, Khudhair AM, Razack SAK, Al-Hallaj S. A review on phase change energy storage: materials and applications. Energ Conver Manage. 2004; 45: 1597- 1615. https://doi.org/10.1016/j.enconman.2003.09.015.
• 7 Sharma SD, Sagara K. Latent heat storage materials and systems: a review. Int. J Green Energy. 2005; 2: 1- 56. https://doi.org/10.1081/GE-200051299.
• 8 Bal LM, Satya S, Naik SN. Solar dryer with thermal energy storage systems for drying agricultural food products: a review. Renew Sustain Energy Rev. 2010; 14: 2298- 2314. https://doi.org/10.1016/j.rser.2010.04.014.
• 9 Zalba B, Marín JM, Cabeza LF, Mehling H. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Appl Therm Eng. 2003; 23(3): 251- 283. https://doi.org/10.1016/S1359-4311(02)00192-8.
• 10 Gupta CL, Garg HP. Performance studies on solar air heaters. Solar Energy. 1967; 11(1): 25- 31.
• 11 Esen H. Experimental energy and exergy analysis of a double-flow solar air heater having different obstacles on absorber plates. Build Environ. 2008; 43: 1046- 1054. https://doi.org/10.1016/j.buildenv.2007.02.016.
• 12 El-Sebaii AA, Aboul-Enein S, Ramadan MRI, Shalaby SM, Moharramb BM. Investigation of thermal performance of-double pass-flat and v-corrugated plate solar air heaters. Energy. 2011; 36: 1076- 1086. https://doi.org/10.1016/j.energy.2010.11.042.
• 13 El-Sebaii AA, Aboul-Enein S, Ramadan MRI, Shalaby SM, Moharramb BM. Thermal performance investigation of double pass-finned plate solar air heater. Appl Energy. 2011; 88: 1727- 1739. https://doi.org/10.1016/j.apenergy.2010.11.017.
• 14 Sah N, Mohan JKM. Thermal performance of a double-pass solar air heater (SAH) with ribbed absorber surface—an experimental study. World J Eng. 2020; 17(3): 373- 380. https://doi.org/10.1108/WJE-08-2019-0217.
• 15 Mohammed Farid M, Kim Y, Kansawa A. Thermal performance of a heat storage module using pcm's with different melting temperature: experimental. J Sol Energy Eng. 1990; 112: 125- 131.
• 16 Fath HES. Thermal performance of a simple design solar air heater with built-in thermal energy storage system. Energ Conver Manage. 1995; 36(10): 989- 997.
• 17 Krishnananth SS, Murugavel KK. Experimental study on double pass solar air heater with thermal energy storage. J King Saud Univ Eng Sci. 2013; 25: 135- 140. https://doi.org/10.1016/j.jksues.2012.05.004.
• 18 Reddy KD, Venkataramaiah P, Lokesh TR. Parametric study on phase change material based thermal energy storage system. Energy Power Eng. 2014; 6: 537- 549. https://doi.org/10.4236/epe.2014.614047.
• 19 Shalaby SM, Bek MA. Experimental investigation of a novel indirect solar dryer implementing PCM as energy storage medium. Energ Conver Manage. 2014; 83: 1- 8. https://doi.org/10.1016/j.enconman.2014.03.043.
• 20 Purnanand Bhale V, Rathod KM, Sahoo L. Thermal analysis of a solar concentrating system integrated with sensible and latent heat storage. Energy Procedia. 2015; 75: 2157- 2162. https://doi.org/10.1016/j.egypro.2015.07.357.
• 21 Bhardwaj AK, Chauhan R, Kumar R, Sethi M, Rana A. Experimental investigation of an indirect solar dryer integrated with phase change material for drying Valeriana Jatamansi (medicinal herb). Case Stud Ther Eng. 2017; 10: 302- 314. https://doi.org/10.1016/j.csite.2017.07.009.
• 22 Rabha DK, Muthukumar P. Performance studies on a forced convection solar dryer integrated with a paraffin wax–based latent heat storage system. Solar Energy. 2017; 149: 214- 226. https://doi.org/10.1016/j.solener.2017.04.012.
• 23 el Khadraoui A, Bouadila S, Kooli S, Farhat A, Guizani A. Thermal behavior of indirect solar dryer: nocturnal usage of solar air collector with PCM. J Clean Prod. 2017; 148: 37- 48. https://doi.org/10.1016/j.jclepro.2017.01.149.
• 24 Nidal Abu-Hamdeh H, Khaled Alnefaie A. Assessment of thermal performance of PCM in latent heat storage system for different applications. Solar Energy. 2019; 177: 117- 123. https://doi.org/10.1016/j.solener.2018.11.035.
• 25 Abuşka M, Şevik S, Kayapunar A. A comparative investigation of the effect of honeycomb core on the latent heat storage with PCM in solar air heater. Appl Therm Eng. 2019; 148: 684- 693. https://doi.org/10.1016/j.applthermaleng.2018.11.056.
• 26 Dosapati C, Mandapati MJK. Thermal performance of a packed bed double pass solar air heater with a latent heat storage system: an experimental investigation. World J Eng. 2019; 17(2): 203- 213. https://doi.org/10.1108/WJE-08-2019-0221.
• 27 Kumar IV, Kumar MMJ. Design, fabrication and testing of a double pass solar air heater. Advances in Fluid Dynamics, Lect Notes Mech Eng. 2020; 529- 536. https://doi.org/10.1007/978-981-15-4308-1_41.
• 28 Mahmud Alkilani M, Sopian K, Mat S. Fabrication and experimental investigation of pcm capsules integrated in solar air heater. Am J Environ Sci. 2011; 7: 542- 546.
• 29 Kooli S, Bouadila S, Lazaar M, Farhat A. The effect of nocturnal shutter on insulated greenhouse using a solar air heater with latent storage energy. Solar Energy. 2015; 115: 217- 228. https://doi.org/10.1016/j.solener.2015.02.041.
• 30 Jain D, Tewari P. Performance of indirect through pass natural convective solar crop dryer with phase change thermal energy storage. Renew Energy. 2015; 80: 244- 250. https://doi.org/10.1016/j.renene.2015.02.012.
• 31 Abed AH. Thermal storage efficiency enhancement for solar air heater using a combined SHSm and PCM cylindrical capsules system: experimental investigation. J Eng Technol. 2016; 34(5): 999- 1011.
• 32 Kabeel AE, Khalil A, Shalaby SM, Zayed ME. Experimental investigation of thermal performance of flat and v-corrugated plate solar air heaters with and without PCM as thermal energy storage. Energ Conver Manage. 2016; 113: 264- 272. https://doi.org/10.1016/j.enconman.2016.01.068.
• 33 Kabeel AE, Khalil A, Shalaby SM, Zayed ME. Improvement of thermal performance of the finned plate solar air heater by using latent heat thermal storage. Appl Therm Eng. 2017; 123: 546- 553. https://doi.org/10.1016/j.applthermaleng.2017.05.126.
• 34 Moradi R, Kianifar A, Wongwises S. Optimization of a solar air heater with phase change materials: experimental and numerical study. Exp Therm Fluid Sci. 2017; 17: 1- 27. https://doi.org/10.1016/j.expthermflusci.2017.07.011.
• 35 Wadhawan A, Dhoble AS, Gawande VB. Analysis of the effects of use of thermal energy storage device (TESD) in solar air heater. Alex Eng J. 2018; 57: 1173- 1183. https://doi.org/10.1016/j.aej.2017.03.016.
• 36 ASHRAE 93-97. Methods of Testing to Determine Thermal Performance of Solar Collectors. New York, NY: ASHRAE; 1977: 345.
• 37 Maradiya C, Vadher J, Agarwal R. The heat transfer enhancement techniques and their thermal performance factor. Beni-Suef Univ J Basic Appl Sci. 2018; 7: 1- 21. https://doi.org/10.1016/j.bjbas.2017.10.001.
• 38 Frank PI, David PD, Theodore LB, Adrienne SL. Incropera's Principles of Heat and Mass Transfer, Wiley India Edition. India: Wiley India; 2018.
• 39 Schultz RR, Cole R. Uncertainty analysis in boiling nucleation. Proc AICHE Symp Ser. 1979; 75(189): 32- 38.
• 40 Mandapati K. Observations made during experimentation on SAH with encapsulated PCM on its absorber plate. Mendeley Data. 2020; V3. https://doi.org/10.17632/t4rt98ykj5.3.

Experimental Study of New Solar Air Heater Design

Source: https://onlinelibrary.wiley.com/doi/full/10.1002/est2.256

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