Study of the performance of double pass solar air heater of a new designed absorber: An experimental work

Solar Energy 198 (2020) 479–489

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Study of the performance of double pass solar air heater of a new designed absorber: An experimental work

T

⁎

Saleh Abo-Elfadla, Hamdy Hassana,b, , M.F. El-Dosokya,c a

Mechanical Engineering Department, Faculty of Engineering, Assiut University, Assiut, Egypt Energy Resources Eng. Department, Egypt-Japan University of Science and Technology (E-JUST), Alexandria, Egypt c College of Engineering, Alasala University, P.O.B. 12666, Dammam 31483, Saudi Arabia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Tubular solar air heater Double pass Energy Performance Efficiency Temperature

An experimental study on the performance of double pass Solar Air Heater (SAH) having a new designed absorber is performed. The new SAH absorber is constructed from conductive aluminum tubes adjacent to each other and installed in the same direction of the air flowing inside the SAH. The new SAH performance (called Tubular Solar Air Heater (TSAH)) is studied at various mass flow rates of air inside the SAH. Moreover, it is compared with the performance of Flat plate Solar Air Heater (FSAH) having the same dimensions and materials of the TSAH except the absorber design. Findings indicate that the TSAH has higher outlet air temperature, efficiency and net energy gain and lower top heat loss compared to FSAH. TSAH achieves maximum air temperature rise more than 6 °C compared to FSAH at 0.025 kg/s. TSAH efficiency is greater than FSAH efficiency by about 19.4%, 21%, and 40.3%, at inlet air flow rate of 0.075 kg/s, 0.05 kg/s and 0.025 kg/s, respectively. The outlet air temperature and top thermal energy loss of the TSAH decrease with increasing the air mass flow rate, while its efficiency, energy output, and pressure drop increase. Moreover, TSAH efficiency in case of double air pass is greater than single air pass. Despite the TSAH pressure drop is greater than FSAH pressure drop, but its value is very small to impact the TSAH net thermal energy gain. The designed TSAH performance is found greater than the performance of FSAH and previous designs of published SAHs.

1. Introduction Energy and water are some of the vital needs which impact the civilized human life (Yousef et al. 2017; Hassan et al., 2019a,b). Recently, energy problems have forced nations to utilize resources of renewable energies like solar energy instead of fossil fuel which can contribute to alleviate the environmental pollution and energy crisis (Gouda et al., 2019). In solar thermal applications, solar energy can be transformed into thermal energy by utilizing solar collectors. A flat plate solar collector is utilized for moderate and low temperature applications and can be classified as solar liquid heater and solar air heater (SAH) (Ravi and Saini, 2016). SAH absorbs the solar radiation which is transported to the SAHs air (Manikandan et al., 2019; Patel and Lanjewar, 2019). SAH is one of the simplest solar collectors that utilizes the energy from solar to heat the air which can be utilized in various heating applications. SAH is simple and has low construction costs. Furthermore, it works for a long time with the same efficiency and it is thermally stable and easy to be maintained (Chabane, 2014). Despite the merits of the SAHs, they have various limitations as low air thermal conductivity,

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low settling time of the air through the SAH, and low surface area of its absorber plate. These limitations reduce its efficiency and influence the heat transfer rate from the absorber plate to the air and the reflected and energy loss from its top plus its sides and back surfaces (Hassan and Abo-elfadl, 2018; Varun Siddhartha, 2010). A great number of researches have been performed experimentally or theoretically to overcome these limitations and to enhance SAH efficiency by developing new designs, replacing single air pass solar air heater (SPSAH) by double air pass solar air heater (DPSAH), diminishing SAH losses, utilizing heat transfer enhancers, etc. One of the SAHs enhancements that has been studied extensively is utilizing double pass airflow up and down its absorber plate to reduce top heat losses and hence raising its performance (Alam and Kim, 2017). Other researchers have tried to develop the SAH design to enhance its performance. Naphon (2005) investigated theoretically the double pass FSAH performance with and without porous media. He found that the FSAH with porous media had 25.9% higher efficiency than FSAH without porous media. The performance of DPSAH and SPSAH with fins and steel wire meshes as an absorber plate was

Corresponding author. E-mail addresses: [email protected], [email protected] (H. Hassan).

https://doi.org/10.1016/j.solener.2020.01.091 Received 1 October 2019; Received in revised form 29 January 2020; Accepted 31 January 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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cans. Three absorber plate configurations were studied: flat, flat with aligned distribution cans and flat with staggered cans distribution. They revealed that the efficiency obtained in case of staggered distribution cans is 68% at airflow rate 0.05 kg/s. The effect of DPSAH with various airflow ratios at various configurations of the absorber plate was investigated experimentally by Hassan and Abo-elfadl (2018). They investigated corrugated, flat, and pin finned absorber configurations. The results were that the efficiency of the DPSAH is greater than the efficiency of the SPSAH. Furthermore, the maximum SAH efficiency is about 70% for the flat plate absorber at 100% of the air flow passing up the SAH absorber plate and is around 79% for the pin finned absorber plate in case of 100% air flow down the absorber plate. Moreover, Singh and Singh (2018) investigated theoretically the curved SAH performance. It is found that a slight rise in the pressure drop of the curved SAH compared to the FSAH was obtained. Furthermore, an increase of the outlet air temperature of the SPSAH was found in case of curved V-corrugated plate compared to the smooth plate. The performance of helical channels flow paths SAH was presented theoretically and experimentally by Heydari and Mesgarpour (2018). It is noticed that the DPSAH with helical channels has a thermal efficiency of 14.7% greater than the SAH without helical channels and 8.6% greater than the finned SAH. The SAH efficiency with twisted rib over the SAH absorber plate was examined theoretically and experimentally by Kumar and Layek (2018) at various rib inclination angles, roughness parameters and twist ratios. Their outcomes revealed that, at Reynolds number of 21000, the optimal value of the thermo hydraulic performance (the performance of the equipment based on the balance between the heat transfer coefficient (or the heat transfer capacity) and the pressure drop) is 2.13 corresponding to optimal roughness parameter at Reynolds number of 21000. Recently, Singh et al. (2019) investigated experimentally the performance of DPSAH with finned wire mesh packed ped with two glass covers. They discovered that the SAH efficiency reaches maximumly 80% at air MFR 0.03 kg/s and solar radiation 823 W/m2. Saravanakumar et al. (2019) presented an analytical work on improving the working of SAH by integrating arc-shaped rib roughened barrier with fins and baffles at its absorber plate. The findings are that the proposed SAH modifications enhanced the energy efficiency and effectiveness by 28.3% and 27.1%, respectively with respect to arc shape rib roughened without fins and baffles. Komolafe et al. (2019) examined experimentally the SAH performance of rectangular rib roughness installed at the absorber plate. They concluded that the thermal efficiency ranges between 14.0% and 56.5% for the tested SAH with an average value of 33%. V ribs geometry in the back of the SAH absorber plate was studied by Patel and Lanjewar (2019) to enhance the heat transfer from the absorber plate to the air. Various V ribs parameters were investigated (rib height to hydraulic diameter, rib pitch to height, and rib size to rib height). The results showed that the increase in the heat transfer to the air is attained at rip pitch/height equal to 10 while the maximum friction factor occurs at rip pitch/height equal to 8. The thermal performance of trail SAH was presented by Mzad et al. (2019). They concluded that selecting the appropriate absorbers, glass covers, and best insulation materials improve the heat transfer within the air vein and decline the top and bottom SAH heat losses. Jin et al. (2019) investigated numerically multiple V-shaped ribs SAH. The results indicated that the optimal spanwise rib number decreases with reducing the relative rib pitch, attack angle, and channel height. Wang et al. (Wang et al., 2019) studied the heating performance of transpired SAH. Its cover has infiltration holes that pass the solar energy to the SAH absorber plate. The results demonstrated that rising the height of these holes from 0.3 to 0.9 augments the outlet temperature difference by about 6.8 °C and the efficiency of the heat collection by 25%. The working of spiral SAH compared to serpentine and conventional SAHs was investigated experimentally by Jia et al. (2019). They stated that the spiral SAH ultimate efficiency was enhanced with an increase of 3% with respect to the conventional one. Singh (2020) presented

investigated by Omojaro and Aldabbagh (2010). They found that SAH efficiency augmented with rising the air Mass Flow Rate (MFR). Moreover, the DPSAH efficiency was higher than the efficiency of the SPSAH by nearly 7 to 19.4% at the same air MFR. At MFR of air 0.038 kg/s, the obtained maximum efficiency of double and single pass was 63.74% and 59.62%, respectively. The performance of SPSAH and DPSAH with steel wire mesh at various MFRs of the air through the heater was experimentally investigated by Aldabbagh et al. (2010). It was stated that SAH efficiency increased with augmenting the air MFR in a range between 0.012 and 0.038 kg/s. DPSAH efficiency is bigger than single pass. Furthermore, the SAHs performance of double pass finned and v corrugated absorber was inspected theoretically by El-sebaii et al. (2011). They demonstrated that double pass v-corrugated SAH achieved an efficiency of 9.3–11.9% greater than double pass-finned SAH. Ho et al. (2012) evaluated theoretically the performance of DPSAH with baffles and fins at a recycling operation. It revealed that SAH efficiency reached about 60% at air MFR of 0.077 kg/s, recycling of 0.5 and solar radiation of 830 W/m2. Furthermore, the thermal behavior of solar air heaters of double-pass counter flow and double parallel flow was presented theoretically by Hernández and Quiñonez (2013). They stated that at air MFR of 0.04 kg/s, the achieved maximum temperature for the double pass is 46.4 °C while for double flow collector is 42.5 °C and for single pass is 40.7 °C. Yang et al. (2014) inspected theoretically SAH efficiency of an absorber plate of offset strip fin. They demonstrated that SAH thermal efficiency changes between 32% and 60%. The performance of DPSAH and SPSAH with pack mesh and perforated cover at air MFR varying from 0.11 kg/s to 0.037 kg/s was investigated experimentally by Nowzari et al. (2014). It is revealed that DPSAH efficiency is greater than single pass efficiency by around 5–22%. The efficiency of the double pass SAH is 53.67% at air MFR of 0.037 kg/s. Additionally, the SAH performance of SPSAH and DPSAH with four transverse fins and package wire mesh layers was investigated experimentally by Mahmood et al. (2015). Moreover, the maximum obtained efficiency at MFR of 0.032 kg/s by using 7.5 cm high collector is 62.50% for the double pass and 55% for the single pass. The best configuration of DPSAH and SPSAH was inspected experimentally by Nowzari et al. (2015). They replaced the normal absorber plate of the SAH by perforated cover with layers of wire mesh. The outcomes showed that the efficiency of DPSAH having quarter perforated cover and hole to hole spacing 3 cm at air MFR of 0.0032 kg/ s is greater than the other investigated configurations. Dissa et al. (2016) presented experimentally the performance of SAH of an absorber plate consisting of joining two absorbers. One absorber was a porous absorber from an aluminum mesh while the second was nonporous absorber from a corrugated iron sheet. It was stated that the modified SAH efficiency attained was 61% at solar noon. Zheng et al. (2017) analyzed theoretically the thermal performance of metal corrugated packing SAH. They concluded that its thermal efficiency varies between 47% and 66%, and the rise of the SAH output air temperature varies among 2.95 K and 49.87 K. Abdullah et al. (2017) performed an experimental work to investigate the performance of three configurations of the SAH. The first SAH had a circular absorber plate where half the circle is a glass cover and the second is an absorber. The second has a flat glass cover and the absorber plate is a semi-circle geometry. The third has an absorber of half circle including triangular cover plate. They demonstrated that the highest efficiency (64%) is obtained for the first configuration. Different SAH designs were presented theoretically and experimentally at the review paper of Kabeel et al. (2017). These configurations are such as fins with various configurations, artificially roughened absorbers, storage materials, utilizing backing beds and selective coating absorbers. Also, Abdullah et al. (2018) investigated experimentally the performance of DPSAH with and without turbulator at the bottom and top surfaces of the absorber plate by utilizing aluminum 480

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external surface of the tubes are painted with the same black painting material of the flat absorber plate to increase also their absorptivity to the solar radiation. The tubes are fixed together by using three sheets of aluminum joining all the absorber tubes. The distance between the upper surface of the tubes and the glass cover is adjusted to have the same value of the distance between the absorber flat plate and the glass cover of the FSAH. Two identical fans (8), one for each SAH are used to extract the air through the connecting insulated tube (9) where the air enters the SAH at inlet port (2) and exits from port (3) as shown in Fig. 1(b). Each SAH is assembled and ported in an inclined frame of an inclination angle 27° of Assiut city with the help of the metal stands (10). The SAHs are oriented to the south during all experiments to receive the maximum solar radiation. The design of each SAH can change the distance between the centerline of the absorber plate and glass cover. However, in this study, the distance from the SAH base to the glass cover is fixed at 7 cm. Moreover, the SAH design is flexible to have one or two glass covers but the results presented in this study are for only one glass cover.

experimentally and numerically an examination of DPSAH serpentine wavy wire-mesh packed bed. Results confirmed that the maximum instantaneous efficiency of the studied DPSAH is approximately 74% with a rise of 17% compared to SPSAH. From the aforementioned extensive literature survey and despite several works that have been presented, the efficiency of the SAH is still relatively low despite its various merits which requires further researches. Furthermore, from the indicated previous studies and to the authors' best survey, it is found that the efficiency of the DPSAH is greater than the SPSAH. So, in this work, an experimental investigation of new designed SAH absorber is presented in case of DPSAH which has not been presented before. The new SAH absorber consists of adjacent aluminum tubes placed longitudinally in the same direction of the air flow inside the SAH. The performance of the new SAH called tubular solar air heater (TSAH) is compared with the performance of flat plate solar air heater (FSAH) at various inlet air MFRs. The impact of the new design on the output air temperature, efficiency, top heat loss, pressure drop, energy gained, etc., is considered. Moreover, a comparison of the efficiency of this new design for double and single pass air flow is presented. Hence, this work presents a contribution to the design of the SAHs which results in an enhancement of SAHs performance to solve the problem of its low efficiency. The rest of this work includes the experimental work presented in Section 2 which covers the experimental setup, measured values and the measuring accuracy. Section 3 presents and discusses the results while Section 4 closes with the conclusions of the results and provides potential future research directions.

2.2. Measured values During the experimental work, various parameters are measured and registered from 8:00 AM to 16:00 PM. The measurements are carried out through three days (29/10/2018, 31/10/2018 and 2/11/ 2018). Different temperatures are measured during this experimental work at the same locations for the two SAHs and are registered at the same time. The SAHs temperatures are measured by using K type thermocouples (11) connected to a data logger (12) of type NI cDAQ9172 (Electronic Test Equipment company, USA). The temperatures are registered every two seconds on a computer connected to the data logger. The temperature of the air is measured at the inlet port (1) and exit port (2) from each SAH as stated in Fig. 3. Three temperatures (3, 4 and 5) are measured on each SAH absorber at a line passing at the center of the absorber through its length with a space between every two measuring points of 37.5 cm. Furthermore, two temperatures (6 and 7) are measured at the glass cover of each SAH at an equal spacing between every two measuring points of 50 cm through the cover length. Additionally, the temperature at the return air position (8) is measured. Besides, the ambient temperature is measured and registered. The solar radiation is measured using a pyranometer of type sp lite2 silicon (KIPP &ZONEN company, Netherlands) and the output of the pyrometer is also registered every two seconds on the previous computer. A fan meter of type MEITAV M4000MD (Meitav-tec company, Europe) is utilized to measure the air velocity each 15 min and the pressure drop through each SAH is measured by using an inclined U tube measuring device (14) each 15 min as stated in Fig. 1(b).

2. Experimental work An experimental setup is constructed to examine the impact of a new designed absorber on the SAH performance (outlet air temperature, energy gain, energy loss, pressure drop, etc.) and its performance is compared with the performance of conventional SAH of flat absorber plate. To perform this work, two similar SAHs (Tubular solar air heater (TSAH) with tubular absorber and flat solar air heater (FSAH) with flat plate absorber) are constructed with the same design and materials and having the same dimensions except the design of the absorber of each one. These two SAHs are constructed and installed on the roof of the power laboratory building, mechanical engineering department, faculty of engineering, Assiut University, Assiut, Egypt of latitude 27° N and longitude 18° E. 2.1. Experimental setup construction An image of the experimental setup taken from the front is shown in Fig. 1(a) and layout of the experimental setup of the TSAH is illustrated in Fig. 1(b). Each SAH consists of a rectangular frame constructed from thick wood to minimize the heat loss from the SAH. The frame has inside dimensions (length × width × height) of 150 × 75 × 8 cm and wall thickness of 10 cm. The rear wall of each SAH is fabricated from thick wood (1) because it can be easily fabricated and constructed. Moreover, it has a lower thermal conductivity which minimizes the heat loss from the SAH. The air enters at the top of the SAH from the tubular inlet port (2) of internal diameter 9.5 cm and exits from the exit port (3) of internal diameter 9.5 cm as illustrated in Fig. 1(b). Each SAH has another inlet port (4) at its lower section of inside diameter 9.5 cm to be used in case of single pass air flow. The inlet ports at the upper and lower have valves to control the mass flow rate inlet to each SAH. Each SAH has a glass cover (5) of dimensions 150 × 75 cm with thickness 3 cm. The conventional FSAH has an aluminum absorber flat plate (6) of dimensions 150 × 75 cm and thickness 1 mm. The absorber plate is painted in black paint of higher thermal conductivity to increase its absorptance to the solar energy. The TSAH absorber (7) consists of circular aluminum tubes as stated in Fig. 1(a). Each absorber tube has length 150 cm, diameter 2.5 cm and wall thickness 1 mm and the

2.3. Solar air heater efficiency The SAH efficiency η is governed by (Mahmood et al., 2015; Sahu and Prasad, 2016):

η=

Qout Qin

(1)

where Qout is the energy gain rate in W and is calculated by:

̇ p (T3 − T2) Qout = mc

(2)

where Cp is the air specific heat of (1.004 kJ/kg.K). The rate of the thermal energy gain from the top surface is calculated by:

̇ p (T8 − T1) Qout = mc

(3)

Qin is the input thermal energy rate in W and is calculated by

Qin = GA

(4)

where G is the incident solar radiation to the surface of the SAH in W/ m2, A is the surface area of the collector absorber in m2, ṁ is the total 481

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a

Fig. 1a. (a) Experimental setup image of the FSAH and TSAH (side view).

mass flow rate of air in kg/s, T2 is the outlet air temperature from the SAH (Fig. 2) and T1 is the inlet air temperature to the SAH in °C. The MFR of the air is determined by:

ṁ = ρAV

(5)

where ṁ is the MFR of the air at the lower inlet port in kg/s, and ρ is the air density in kg/m3, A is the cross-sectional area of the inlet port in m2 and V is the average velocity of the air at the inlet port in m/s. The density of the air is calculated from:

ρ=

101.3 0.287 × T1

Fig. 2. Positions of the temperature measuring points.

is the top convection heat transfer coefficient due to wind speed in W/ m2.K, hr,o is the radiation heat transfer coefficient on the top surface in W/m2.K. The convection heat transfer coefficient at the top surface due to wind speed (hw) is calculated from the wind velocity as follows (Ansari and Bazargan, 2018, Hassan et al., 2019a,b):

(6)

The top heat loss is calculated based on (John Duffie, 2013; Soteris Kalogirou, 2014):

Qt =

Tg − Ta 1 Ahw − Ahr , o

h w = 5.7 + 3.8Vw

(7)

(8)

where Vw is the wind velocity in m/s. The radiation convection heat transfer coefficient hr,o is calculated as following (John Duffie, 2013;

where Qt is the top heat loss in W, Tg and Ta are the average glass surface temperature and ambient air temperature, respectively in °C, hw

b

Fig. 1b. (b). Layout of the experimental setup of the TSAH Wooden frame (1), Lower inlet port (2), Exit port (3), Upper inlet port (4), Glass cover (5), Absorber (7), Fan (8), Insulated tube (9), Stand (10), Thermocouples (11) and data logger (12). 482

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Table 1 Uncertainty of measured devices. Instrument

Uncertainty

Pyranometer Digital temperature measuring device MEITAV measuring velocity Thermocouples

0.5% ± 0.5 °C ± 0.1 m/s ± 1. 4 °C

Soteris Kalogirou, 2014):

hr , o =

σ ∊g (Tg + Ts + 546)((Tg + 273)2 + (Ts + 273)2)(Tg − Ts ) Tg − Ts

(9)

where σ is the Stefan Boltzmann constant (5.667 × 10 W/m .K ), εg is the emissivity of the glass material whose value is taken as 0.88, and Ts is the sky temperature in °C. The sky temperature is calculated from (Soliman and Hassan, 2018; Karn et al., 2019; Nasef et al., 2019): -8

Ts = 0.0522(Ta + 273)1.5 − 273

2

4

(10)

2.4. Uncertainty and error analysis The experimental errors and uncertainty are considered in this study. Besides, a calibration is performed for the utilized sensors like thermocouples, MFR measuring instrument, pressure measuring technique, et., and the results of this work consider the calibration procedures. Uncertainty analysis is accomplished in agreement with the instructions provided by Taylor (1997). Table 1 demonstrates the uncertainties of the measured parameters from the utilized devices, instruments, etc. All required precautions are performed to minimize the measuring errors. The uncertainty (δl) of any calculated values (l) as SAH efficiency, heat loss, etc., from uncertainties (δ1 and δ2) of two measured values (x and y) (Taylor, 1997; Abd Elbar and Hassan, 2019) is computed based on the following formula:

δl =

Fig. 3. Variation of solar radiation and temperatures at 0.025 kg/s.

2

2

⎛ ∂l ⎞ δx2 + ⎛ ∂l ⎞ δy2 ⎝ ∂x ⎠ ⎝ ∂y ⎠ ⎜

the measuring days is imposed on these figures. The results show that the measured air temperatures rise with time from the morning until reaching maximum temperature before the midday. Then, they decline with time having the same trend of the solar energy. It is noted that the maximum solar radiation occurs at a time earlier than the maximum SAH temperatures rising. The time lag between the maximum temperature increase and maximum solar radiation may be due to the time taken to heat the SAHs and the air. The absorber plate temperature is greater than the glass cover temperature and the former is greater than the outlet air temperature because of the transferred heat from the absorber plate to the air. All these previous SAHs temperatures are greater than the ambient temperature as expected. By comparing the results of Figs. 3(a), 4(a), 5(a) with the corresponding Figs. 3(b), 4(b), 5(b) of the TSAH and FSAH at different inlet MFRs, it is found that the absorber plate temperature and glass temperatures in case of FSAH are greater than those temperatures in case of TSAH. Nevertheless, the outlet air temperature from the SAH in case of TSAH is greater than the case of FSAH because of the greater contact area with the air in case of TSAH absorber compared with FSAH absorber plate area. It is found that the TSAH absorber contact area is 3.14 times that of the FSAH because it represents the peripheral area of all absorber tubes. Likewise, the exposed surface to the solar radiation of the FSAH is smaller than that of the TSAH because TSAH exposed surface represents nearly half the total peripheral area of the absorber. In addition, the TSAH absorber design reduces the reflected solar radiation back to the surrounding due to the internal reflections between the surfaces of the absorber tubes which decreases the top heat loss as will be seen later. This indicates that the heat transfer to the air in case of TSAH is larger than the case of FSAH. For example, at an inlet air

⎟

(11)

Based on the earlier formula, the uncertainty of the SAH efficiency in Eq. (1) is 1.6%, and for the energy gained in Eq. (9) is 1.9%. and for the MFR is 3%. 3. Results and discussions The experiments are carried out from 30/10/2018 to 3/11/2018 at clear sky conditions. The results are presented for the TSAH and FSAH for three average air MFR to the SAHs port (0.025 kg/s, 0.05 kg/s and 0.075 kg/s). The temperature results represent the average temperature of the measuring values. For example, the absorber plate temperature represents the average temperature of thermocouples readings located at points 3, 4 and 5. To obtain precise results for the comparison of the TSAH and FSAH performances, the experiments and measurements are carried out at the same time under the same climate conditions. 3.1. Temperature The principal aim of the SAH is to heat the air to be used in industrial applications, housings heating, etc. The variation of the temperatures of the outlet air from the SAH (outlet air), return air (T return), inlet air to the SAH (Inlet air), absorber plate (T plate) and glass (T glass) with time is shown in Fig. 3 at air MFR of 0.025 kg/s. However, the temperature variations with time at air mass flow rate of 0.05 kg/s and 0.075 kg/s are illustrated in Figs. 4 and 5, respectively. The results of the TSAH temperatures are indicated in Figs. 3(a), 4(a) and 5(a) while Figs. 3(b), 4(b), and 5(b) demonstrate the temperatures of the FSAH. Moreover, the variation of the solar radiation with time at 483

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Fig. 5. Variation of solar radiation and temperatures with time at 0.075 kg/s. Fig. 4. Variation of solar radiation and temperatures at 0.05 kg/s with time.

absorber tubes is greater than the contact area in case of FSAH. This means that the positive impact of the double pass flow TSAH is smaller than that of FSAH. Likewise, the temperature difference between inlet and outlet air decreases in both SAHs with rising the air mass flow rate due to rising the convection heat transfer which decreases the absorber temperature. This will increase the SAH useful energy and efficiency as will be seen later. Increasing the MFR from 0.025 to 0.05 m/s (100%), decreases the outlet temperature by about 11.1% and 11.14% in case of FSAH and TSAH, respectively which means that the impact of the MFR on the outlet temperature is greater in case of TSAH than FSAH.

MFR of 0.025 kg/s, the maximum temperature of the absorber and glass cover are 81.72 °C and 53.1 °C, respectively in case of FSAH and 72.6 °C and 53.1 °C, respectively in case of TSAH. While the maximum outlet air temperature is 49.64 °C in case of FSAH and 55.93 °C in case of TSAH. This means that an increase of about 6.29 °C is obtained in the outlet air temperature because of using the new absorber at inlet air MFR 0.025 kg/s which represents an augmentation of nearby 12.6% in the air temperature. From the temperature results at 0.05 kg/s, the maximum average temperatures of the absorber, glass cover and outlet air are 67.64 °C, 47.74 °C, and 44.05 °C, respectively in case of FSAH and 58.34 °C, 49.74 °C, and 49.7 °C, respectively in case of TSAH. Nevertheless, these temperatures in case of 0.075 kg/s are 59.23 °C, 45.12 °C, and 40.13 °C, respectively for FSAH and 51.22 °C, and 43.33 °C and 42.06 °C, respectively for TSAH. The percentage rising of the outlet air temperature from the TSAH compared to FSAH is 12.91% and 4.8% at 0.05 kg/s and 0.075 kg/s, respectively. This indicates that the percentage increase in the outlet air temperature decreases with increasing the mass flow rate inside the SAH. In addition, the glass cover and absorber temperatures decrease with rising the air mass flow rate. Figs. 3–5 reveal that the maximum outlet air temperature at return point is 49.34 °C, 42.101 °C and 38.58 °C for FSAH and 43.94 °C, 38.9 °C and 36.7 °C for TSAH at air MFR 0.025 kg/s, 0.05 kg/s and 0.075 kg/s, respectively. This signifies that the heat gain from the upper surface of the absorber plate in the case of TSAH is lower than that of FSAH while the air heat gain flowing down the absorber plate for the FSAH is much lower than that of the air flowing inside the absorber tubes. The explanation of this is that the contact area between the air and the

3.2. Useful energy gain In this work, the hourly and daily accumulated energy gains are presented.

3.2.1. Hourly energy gain The transient variation of the hourly useful energy extracted by the flowing air calculated from Eq. (2) for TSAH and FSAH at the studied air MFRs is illustrated in Fig. 6. It shows that the hourly useful thermal energy has the same tendency of the SAHs temperatures and solar radiation stated formerly which rises with time from the morning until its maximum value is attained at approximately 12:30 PM, then it decreases with time. The cause is that the useful thermal energy depends mainly on the SAH outlet air temperature as stated in Eq. (2). The maximum increase of the useful energy for the TSAH compared to FSAH at 0.075 kg/s, 0.05 kg/s and 0.025 kg/s is about 31.97%, 21.17%, and 19.54%, respectively. 484

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Fig. 8. Variation of the hourly top thermal energy loss with time. Fig. 6. Variation of the hourly useful energy rate with time.

SAH is the heat loss from its top surface where its rising reduces its efficiency. 3.3.1. Hourly energy loss Fig. 8 illustrates the variation of the hourly energy losses rate from the top surfaces of the SAHs (top losses) with time at different inlet MFRs, while Fig. 9 reveals the variation of the accumulated hourly top losses with time. Fig. 8 also demonstrates the same trend as the hourly outlet air temperature and the energy gain because the top energy losses depend on the absorber and glass cover temperatures. Also, Fig. 9 reveals the same trend of the accumulated useful energy and outlet temperature form the SAHs stated previously. Figs. 8 and 9 reveal that the top losses in cases of FSAH are greater than the top losses of the TSAH and they reduce by increasing the inlet air MFRs because of decreasing the absorber temperature as mentioned before. This explains the reason for increasing the thermal energy gain with rising the inlet air MFR and the energy gain of TSAH is greater than the energy gain of FSAH as indicated early. Fig. 9 illustrates that the daily top losses of the TSAH decrease by about 25.4% and 40.4% because of rising the inlet air flow rate two and three times, respectively.

Fig. 7. Variation of the accumulated useful energy with time.

3.2.2. Accumulated energy gain Fig. 7 reveals that the accumulated useful energy rises with time from starting the measurements until the end of the readings. The outcomes of Fig. 7 reveal that the useful energy augments with rising the inlet air MFR due to the increase of the convective heat transfer coefficient at the absorber surface for the two SAHs. The increase of the daily accumulated useful energy rate due to using TSAH instead of FSAH is about 31.98%, 21% and 19.3% at 0.025 kg/s, 0.05 kg/s and 0.075 kg/s, respectively. This indicated that the impact of using TSAH absorber reduces by increasing the MFR until a certain value and after that, this impact will be fixed. This is so since the increasing of the air MFR rises the convection coefficient at the absorber surface which reduces the positive impact of increasing the absorber surface area. Fig. 7 indicates that the augmentation of the daily useful energy due to rising the air MFR two and three times is around 35.4% and 52.7%, respectively in case of TSAH and approximately 28%, and 68.9%, respectively for FSAH. This signifies that the impact of increasing the air MFR on the FSAH is greater than the TSAH. This is attributed to the fact that increasing the air MFR increases the convection heat transfer over the absorber surface which decreases the impact of increasing the absorber contact surface area. In general, it can be stated that for all cases, the extracted thermal energy from the absorber to the air for the TSAH is greater than FSAH.

3.3.2. Daily energy loss Fig. 10 illustrates the percentage of the daily top energy losses from the different SAHs to the daily useful energy of each SAH calculated previously at various inlet air mass flow rates. It is clear from the figure that the top losses represent a relatively small value of the energy gain for the TSAH and FSAH. It is also obvious that the percentage of the top losses to the gained energy rises with decreasing the inlet air MFR because of increasing the absorber temperature. The daily top energy loss of the TSAH represents 87.8%, 94% and 97.1% of those of the FSAH at

3.3. Top energy loss Fig. 9. Variation of the accumulated top thermal energy loss with time.

One of the main parameters that governs the performance of the 485

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Fig. 10. Total daily top thermal energy loss. Fig. 12. Variation the efficiency with time.

0.075 kg/s, 0.05 kg/s and 0.025 kg/s, respectively as stated in Fig. 10. This reflects that the top losses of the TSAH with respect to the FSAH reduce with increasing inlet air mass flow rate. The percentage of the daily energy gain by the air below the absorber for FSAH and inside tubes for TSAH (lower energy gain) to the total energy gain of the FSAH and TSAH is demonstrated in Fig. 11. It reveals that the lower heat gain for TSAH is greater than the case of FSAH. The cause of this is that the contact area between the air and the absorber tube of the lower flow in case of TSAH is greater than the contact area in case of flat absorber. This figure also indicates that the lower energy gain for the TSAH is almost the same as the upper air flow. However, most of the energy gain in case of FSAH is obtained for the upper air flow. It is detected that the lower energy gain percentage of the TSAH increases with augmenting the air MFR. It also demonstrates that the lower energy gain of the FSAH over TSAH represents nearly 43.3%, 36.4% and 10.4% for MFR of 0.075 kg/s, 0.05 kg/s and 0.025 kg/s, respectively. These outcomes state that double pass effect decreases greatly with reducing the MFR, while the double pass impact rises with increasing the air MFR for TSAH.

increases from the morning to the midday due to increasing the incident solar radiation. However, at afternoon, the efficiency increases despite the decrease of the incident solar radiation as stated before. This may be attributed to the fact that during the morning, some of the solar energy is stored in the SAH body which is recovered at afternoon owing to the reduction of the ambient temperature below the midday temperature. Nevertheless, at higher MFR (0.075 kg/s), it rises with time until about 1 PM, then it reduces with time because the SAH efficiency is proportional to the solar radiation (Hassan and Abo-elfadl, 2018). Moreover, the higher convection owing to higher flow rate decreases significantly the stored energy in the SAH body and hence the previous impact of restoring this stored energy decreases highly. Fig. 12 also indicates that the efficiency of the TSAH for all studied inlet air MFRs is greater than the corresponding cases of the FSAH as a result of the greater contact area of the absorber with the air and the lower top losses as illustrated previously. Results demonstrate that the SAHs efficiencies increase by increasing the inlet MFRs as a result of increasing the convective heat transfer coefficient which rises the gained energy and reduces the absorber temperature and hence reduces the top losses. The maximum SAH efficiency reaches about 90.1% in case of TSAH and 75.37% in case of FSAH at 0.075 kg/s with an increase of 32.5% and 55% for TSAH and FSAH, respectively corresponding to the values at 0.025 kg/ s.

3.4. Efficiency The SAH efficiency measures its actual performance and at the same time, it represents an indicator for future research to enhance this performance. 3.4.1. Hourly efficiency The evolution of the hourly efficiency of the TSAH and FSAH with time at various inlet air MFRs is stated in Fig. 12. This figure reveals that the SAH efficiency at lower flow rate increases slightly with time from the morning until the end of the reading time. The efficiency

3.4.2. Average efficiency The average TSAH and FSAH daily efficiency at the studied inlet MFRs is demonstrated in Fig. 13 for DPSAH. Moreover, measurements

Fig. 13. Average daily efficiency at different air flow conditions.

Fig. 11. Percentage of lower energy gain compared to the total heat gain. 486

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Table 2 Comparison of the TSAH efficiency with the previous studied designs. SAH design

Maximum solar radiation, W/m2

Air mass flow rate kg/ s

Maximum efficiency %

TSAH (present work), single TSAH (present work), double TSAH (present work), double TSAH (present work), double DPSAH with turbulator (Abdullah et al., 2018) (experimental) Single pass plastic SAH with circular cross section (Abdullah et al., 2017) experimental Double pass serpentine wavy wire-mesh packed bed SAH (Singh, 2020) experimental and theoretical DPSAH with Corrugated absorber plate (Hassan and Abo-elfadl, 2018) experimental DPSAH with baffles and fins (Ho et al., 2012) experimental and theoretical. DPSAH double pass (Hernández and Quiñonez, 2013)., theoretical SPSAH with finned plate (Kabeel et al., 2018), experimental DPSAH with perforated absorber plate (Nowzari et al., 2014) experimental Spiral solar air heater (Jia et al., 2019) experimental DPSAH with fins and steel wire meshes (Omojaro and Aldabbagh, 2010) experimental SPSAH with conical absorber plate (Abus, 2018) theoretical

750 750 750 750 1100 1000 900

0.075 0.075 0.05 0.025 0.05 0.1 0.05

83.6 86.03 79.5 61 68 47 74

1100 830 1050 1030 730 750 1050 900

0.09 0.077 0.044 0.04 0.032 0.025 0.038 0.04

82 60 62 57 54.76 56 63.74 62

were carried out on the single pass TSAH and FSAH at air MFR of 0.075 kg/s at the next day of the double pass measurements at 0.075 kg/s. Fig. 13 shows that the DPSAH efficiency is greater than the single pass for TSAH and FSAH as stated previously. Moreover, it reveals that the average daily DPSAH efficiency reaches about 86.03% and 72.11% in case of TSAH and FSAH, respectively at inlet air MFRs 0.075 kg/s, while this value is 83.65% and 58.29% for single pass TSAH and FSAH, respectively at the same conditions. The average efficiency of the TSAH is about 86.03%, 76.3% and 59.8% for inlet air MFRs of 0.075 kg/s, 0.05 kg/s and 0.025 kg/s, respectively with an increase of about 19.4%, 21%, 40.3%, respectively compared to FSAH. These results indicate that using the TSAH solves the problem of lower thermal efficiency of the FSAH used for supplying higher hot air at low air flow rate. Moreover, the TSAH is more efficient than the FSAH in case of targeting higher temperature from the SAH. This signifies that TSAH uses the majority incident solar radiation for heating the air at a higher mass flow rate inside the SAH.

Fig. 14. Pressure drop through the SAHs.

3.4.3. Comparison with previous work Table 2 compares the efficiency of the present new design (TSAH) with the efficiency of the published previous designs of the SAHs. Table 2 illustrates that the present design has higher efficiency compared to the published previous designs for the different air MFRs inlet to the SAH. 3.5. Pressure drop Another parameter that measures the SAH performance is the pressure drop due to the resistance to the air flow through the SAH. It is stated that as the mass flow rate of the air increases, the extracted heat gain from the SAH increases. However, at the same time, the pressure drop of flowing air through the SAH increases resulting in an increase of the fan power. Fig. 14 indicates the pressure drop through the SAHs at different inlet air MFRs. It shows that the pressure drop across the TSAH is greater than the FSAH and generally the pressure drop through the SAHs rises by increasing the inlet air MFR. Results state that the pressure drop through the SAH of the TSAH is greater by around 53.8%, 41.7%, and 42.8% compared to FSAH for inlet air MFR 0.075 kg/s, 0.05 kg/s and 0.025 kg/s, respectively. To show the impact of increasing the pressure drop through the different SAHs, it is assumed that the value of the fan power required for flowing the air inside SAH is equivalent to a part of the useful energy rate from the SAH. The variation of the net accumulated useful energy of the TSAH and FSAH with time considering the equivalent fan power that overcomes the pressure drop at various inlet air MFRs is stated in Fig. 15. If the results of Figs. 7 and 15 are compared, it is found that they have approximately the same

Fig. 15. Variation of the net accumulated useful energy with time.

values which means that the pressure drop doesn’t have a sensible impact on the output useful thermal energy because the pressure drop value is very low compared to the equivalent useful SAH output energy. 3.6. Effect of the air flow rate Table 3 summarizes the impact of the air mass flow rates on the net daily accumulative energy, efficiency, pressure drop and percentage of the top energy loss. Table 3 illustrates that rising the air mass flow rate increases the SAH accumulated energy, efficiency and pressure drop but it decreases the percent of the top energy loss from the SAH. It also shows that the efficiency, net accumulated energy of the TSAH and pressure drop is greater than the FSAH while the percent of top energy 487

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Table 3 Impact of air MFR on SAHs energy, efficiency and pressure drop. Air mass flow rate

0.075 kg/s 0.05 kg/s 0.025 kg/s

FSAH TSAH FSAH TSAH FSAH TSAH

Net accumulated energy, KJ

Efficiency, %

Pressure drop, Pa

Percent top energy loss, %

18073.8 21400.7 16064.13 19403.6 10953.66 14454.2

71.1 86.03 65.72 79.5 46.3 61.15

26 40 12 17 3.5 5

22.1 19.4 26.95 25.34 36.37 35.4

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loss in case of FSAH is greater than TSAH. The results indicate that increasing the air mass flow rate two or three times increases the net daily energy by 34.2% and 48.1%, respectively and average daily efficiency by 30% and 40.7%, respectively for TSAH. However, it increases the net energy by 46.7% and 65%, respectively and efficiency by 41.9% and 53.6%, respectively for FSAH. 4. Conclusion An experimental investigation is carried out on the performance of double pass SAH of new designed absorber plate (TSAH) at different inlet air MFRs through the SAH. Furthermore, TSAH performance is compared with the performance of double flat solar air heater (FSAH) and presented previous designed SAHs. The findings indicate that TSAH has greater output air temperature, greater output useful power, higher efficiency and lower heat losses compared to FSAH. Moreover, the efficiency of the studied TSAH is greater than that of the previous presented SAH designs. The achieved increase of the average daily efficiency of the double pass TSAH is 41% and double pass FSAH is 55.7% due to increase air MFR three times. The daily TSAH useful energy rises by about 35.4% and 52.7% due to increasing air MFR two and three times, respectively and approximately 28%, and 68.9% respectively for FSAH. The useful output energy of the SAH increases and the energy losses decrease with increasing the air mass flow rate passing inside the SAH. The top TSAH heat losses represent 87.8% and 97.1% of the top losses of FSAH for inlet air MFR 0.075 kg/s and 0.025 kg/s, respectively. The required fan power to overcome pressure drop is very small compared to the gained useful energy rate from the SAH. Further studies could be presented on the impact of the SAH enhancement modifications presented in the literature on the performance of the present TSAH. Also, a parametric study could be performed on selecting the best design conditions of the new absorber plate for an efficient SAH. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References Abd Elbar, A.R., Hassan, H., 2019. An experimental work on the performance of new integration of photovoltaic panel with solar still in semi-arid climate conditions. Renew. Energy 146, 1429–1443. Abdullah, A.S., Al-sood, M.M.A., Omara, Z.M., Bek, M.A., Kabeel, A.E., 2018. Performance evaluation of a new counter flow double pass solar air heater with turbulators. Sol. Energy 173, 398–406. https://doi.org/10.1016/j.solener.2018.07. 073. Abdullah, A.S., El-samadony, Y.A.F., Omara, Z.M., 2017. Performance evaluation of plastic solar air heater with different cross sectional configuration. Appl. Therm. Eng. 121, 218–223. https://doi.org/10.1016/j.applthermaleng.2017.04.067. Abus, M., 2018. Energy and exergy analysis of solar air heater having new design absorber plate with conical surface. Appl. Therm. Eng. 131, 115–124. https://doi.org/ 10.1016/j.applthermaleng.2017.11.129. Alam, T., Kim, M., 2017. Performance improvement of double-pass solar air heater – A state of art of review. Renew. Sustain. Energy Rev. 79, 779–793. https://doi.org/10. 1016/j.rser.2017.05.087.

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