An experimental investigation on performance analysis of air type photovoltaic thermal collector system integrated with cooling fins design

Accepted Manuscript Title: An experimental investigation on performance analysis of air type photovoltaic thermal collector system integrated with cooling fins design Author: Juwel Chandra Mojumder Wen Tong Chong Hwai Chyuan Ong K.Y. Leong Abdullah-Al-Mamoon PII: DOI: Reference:

S0378-7788(16)30736-8 http://dx.doi.org/doi:10.1016/j.enbuild.2016.08.040 ENB 6939

To appear in:

ENB

Received date: Revised date: Accepted date:

24-5-2016 1-8-2016 12-8-2016

Please cite this article as: Juwel Chandra Mojumder, Wen Tong Chong, Hwai Chyuan Ong, K.Y.Leong, Abdullah-Al-Mamoon, An experimental investigation on performance analysis of air type photovoltaic thermal collector system integrated with cooling fins design, Energy and Buildings http://dx.doi.org/10.1016/j.enbuild.2016.08.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

An experimental investigation on performance analysis of air type photovoltaic thermal collector system integrated with cooling fins design Juwel Chandra Mojumder a, Wen Tong Chong a, Hwai Chyuan Ong a *, K.Y. Leong b, AbdullahAl-Mamoon a

a

Department of Mechanical Engineering, Faculty of Engineering, University of Malaya,

50603 Kuala Lumpur, Malaysia b

Department of Mechanical Engineering, Universiti Pertahanan Nasional Malaysia,

Kem Sungai Besi, 57000 Kuala Lumpur, Malaysia

Corresponding author: Tel: +60 16 5903110; fax: +60 3 7967 5317, E-mail address: [email protected], [email protected] (H.C. Ong)

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Highlights:    

Designed and fabricated a prototype PV/T collector with cooling fins. Induced air as a working fluid in a range between 0.02 kg/s - 0.14 kg/s. Fins increased the thermal efficiency about 28.1% - 56.19%. Experimental value was validated with theoretical and statistical analysis.

Abstract: Photovoltaic thermal (PV/T) system was introduced to meet the thermal and electrical energy. The heat removal by air or water prevents the deterioration of the PV cell efficiency due to the overheating of cells. In this study, an air type single pass PV/T collector system was proposed where a number of thin rectangular fins were introduced for heat dissipation. The collector’s performance was analyzed with a fin system that was integrated by a thin flat metallic sheet (TFMS). Then, the temperature parameters were measured and compared to several operating conditions and configurations. Analytical expression was derived from the energy balance equations for each component of the design. Average temperatures from the top/rear PV surfaces, the collector back wall surface and the collector inlet/outlet temperatures were experimentally recorded under different fin numbers (0-4), mass flow rates (0.02 kg/s- 0.14 kg/s) and solar radiations (200 W/m2-700 W/m2). These readings were used in calculating the thermal and electrical efficiency of the proposed PV/T system. The maximum thermal efficiency and PV efficiency were obtained about 56.19% and 13.75% respectively for four fins at 0.14 kg/s of mass flow rate and 700 W/m2 of solar radiation. Besides, the root mean square percentages of deviation (e) and coefficient of correlation (r) were used to validate the result while also discussing the uncertainty values. This research will be helpful to design thermal collectors and provide valuable information regarding performance improvement methods in PV/T systems.

Keywords: Photovoltaic thermal (PV/T); Solar air collector; Thermal efficiency; Heat gain; Green energy; cooling fin. Nomenclature SAC TFMS PV PV/T A C dx e

solar air collector thin flat metallic sheet photovoltaic photovoltaic thermal area (m2) specific heat (Jkg-1 K-1) elemental length (m) root mean square of percentage

v W (ατ)eff Xex Xth Greek symbols 2

air flow velocity (ms-1) width (m) the product of effective absorptivity and transmittivity experimental data predicted or theoretical data

FR hp1 hp2 hc,fin hbs→f H I(t) k kc→bs k' L

 m N Nu P Pr Qu Re r T Ub

UL

UOHTC(c→a) Ubs

deviation (%) heat removal factor first Penalty factor second Penalty factor convective heat transfer coefficient for fin (Wm-2 K-1) heat transfer coefficient from solar cell to flowing air through PV back sheet (Wm-2 K-1) height (m) irradiation (Wm-2) thermal conductivity (Wm-1 K-1) conductive heat transfer coefficient from solar cell to PV back (Wm-2 K-1) kinematic viscosity (Wm-1 K-1) length (m) mass flow rate (kg/m3) numerical number for fin Nusselt number electrical power (W) Prandl number heat transfer rate (W) Reynolds number coefficient of correlation temperature (⁰C) overall back loss coefficient from flowing air to ambient through the insulator (Wm-2 K1 ) overall heat transfer coefficient from solar cell to ambient through top and back surface of insulation (Wm-2 K-1) overall heat transfer coefficient from solar cell to ambient (Wm-2 K-1) overall heat transfer coefficient from PV back sheet through the solar cell (Wm-2 K-1)

α β τ δ η

absorptivity temperature coefficient transmittivity thickness (m) efficiency (%)

ρ µ Subsacripts a bs c ca ch cs cf eff f g G in lam n o op out

density (kg m-3) dynamic viscosity (kg m-1s-1)

ref th tur w

reference thermal turbulence collector back wall surface

ambient back surface of PV module solar cell characteristic channel cross section of channel cross section of fin effective fluid glass gross inlet laminar number of observations overall operating outlet

1. Introduction The demand for energy is increasing in every energy sector with industrial development day by day. Since the last few decades, renewable energy resources have gained extensive popularity as an alternative source of energy. Photovoltaic (PV) technology places at the top of this list as it is well-known to generate electrical energy from the sun as an energy source that has a lot of potentials. Hereby, the total solar energy system can be classified into three major sources

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depending on the application [1] such as (a) solar thermal energy (b) solar PV power (c) combined solar PV thermal (PV/T) energy. Out of proposed different types of solar air collectors (SAC), the flat rectangular type has been the most widely investigated [2, 3]. On the other hand, PV cell can convert the solar radiation into electricity with peak efficiency in the range of 5-20% and the rest is converted to heat, reported by Charalambous et al. [4]. The PV cell’s electrical efficiency can decrease by ≈0.5% with the increment of the temperature of 1˚C, for a typical silicon (Si) based PV panel [1]. To overcome such deteriorations, studies were undertaken to improve the PV module’s electrical efficiency. Different studies tested the PV module’s performance using different working fluids, such as water, air, hybrid (water and air) [5, 6] and nano-fluid [7]. In further studies, the PV system was added to the SAC to maximize the total energy output by producing electrical power and heat simultaneously, which is known as PV thermal (PV/T) co-generation system. There are two types of PV/T collectors in terms of fluid flow path such as (1) single pass (2) double pass. Moreover, the cooling effect can be produced by either natural or force ventilation process. Tonui et al. [8] reported that force ventilation makes a higher heat transfer by convection rather than the natural flow circulation because, in the case of force ventilation, the stack force of the fluid is generated by using pumps (for water) or fans (for air). In brief, it has the advantage of generating low grade of thermal and high grade of electrical energy from the same unit. In the application the collected heat can be utilized widely in domestic and other many purposes such as pre-water heating, air heating, low-temperature heating etc. [4, 9]. In cold regions, PV/T systems can be utilized to combat freezing conditions, which was analyzed for three typical climate areas in China [10]. The objectives of PV/T design are attaining the maximum heat gain and spontaneous reduction of PV cell surface temperature by a controlled fluid flow. The importance of liquid based thermal collector was reviewed by Daghigh et al. [11], and reported that water has the highest heat gain, compared to air as an effective cooling fluid although the air type SAC design offers more advantages in control and design simplicity. Many studies have been carried out in different investigations on performance improvement in the PV/T technology. Such as, a tri-functional PV/T solar collector was introduced to generate electricity, reported by Ji et al. [12]. Thereafter, a high concentration PV–thermal (HCPVT) design was critically analyzed by Zimmermann et al. [13], to ensure the optimum thermal and electrical outputs. Researchers have also carried out investigations on different flow path directions for coolant fluid in different configurations [14], as well as the glazing effect in PV/T systems [15]. Also, the effect of channel dimensions on the performance of PV/T was extensively investigated. It was observed that smaller channel depth and high flow rates increased heat extraction [16]. Different collector channel depth-to-length ratios (Hch/Lch ) to optimize solar air heaters were also investigated [17]. Then, based on the results found, 2.5 ×10-3 was suggested as an optimal ratio in channel geometry for four collector models under variable mass flow rates, described by Hegazy [18]. Besides, the annual energy and exergy produced in PV/T systems under different weather conditions were also investigated [19].

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In some rigorous reviews, recent development and applications of PV/T were discussed and analyzed on different configurations with several improved methods for flat plate PV/T type collector models [1, 5, 20-22]. Several reviews offer the advancements of collector design in recent years and the future modifications required. To get better performances, insert devices such as twisted tapes, wire coils, and conical ridges [23], corrugated polycarbonate material [16] were effectively integrated into the channel. Also, graphite was introduced into the PV/T system in a recent study reported by Liang et al. [9] and hybrid micro-channels were used reported by Rahimi et al. [24]. Galvanized steel was used as a heat absorber material in numerical modeling in the thermal collector [25]. The further addition of commercial phase change material (PCM) to the cooling system, improved the indoor thermal comfort condition for winter and summer seasons [26]. The increase of heat transfer coefficient by convection is alternatively considered to indicate the increase of the collector efficiency, pointed by Sandhu et al. [27], where active and passive type heat removal methods were discussed too. In a different study, the importance of using absorber plates in the PV/T system was discussed by Charalambous et al. [4]. Afterward, different absorber materials have been used in thermal collectors as an important component in the collector to make a better thermal effect [28]. More recently, thermoelectric (TE) and heat sink modules were integrated with PV module to form hybrid PV/TE system which improved the PV efficiency [24]. But, most of the methods consist of several mechanical and thermodynamic systems such as air flowing through the rear surface of PV panel through collector channel [19, 29]. Different heat augmented materials have been used in the collector such as metallic sheet and fin to create a higher heat transfer area that improved the PV/T’s overall performance [8]. Othman et al. [30] investigated a double pass air type PV/T system integrated with fins. Subsequently, the similar arrangement of fins in the collector was followed by Elsafi et al. [31], where, finned double pass flat (FDPF) and compound parabolic concentrated (CPC) type PV/T collectors were discussed widely. Commonly used fin materials chosen were aluminum (Al), brass, nickel, and copper (Cu), based on the thermal conductivity performance in the PV/T system, discussed by Elsafi et al. [31]. They also studied the effect of fin shapes such as triangular, rectangular and parabolic, as a proper selection criterion. Although the number of fins was roughly analyzed during an investigation on the influence of the external recycling effect in the double pass solar air heaters by Ho et al. [7], a better model development regarding the fin arrangements and numbers under the extensive operating conditions and configurations need to be established. Hereby, PV and SAC have introduced various advanced technologies for building integration. Therefore, PV/T is gaining popularity in building technology due to its simultaneous electrical and thermal output from the PV system. It could be possible to reduce the energy consumption in buildings if PV/T can be adjoined. This conjugate system leads to rising attention for the ventilation of PV facades as well. A review disclosed the resourceful application of solar thermal collector in buildings technologies and presented various existing buildings integrated PV/T designs [32]. 5

To the best of our knowledge, there have not been any evidently reported results of the changes in thermal and electrical performance of air type PV/T systems due to the changes of fin numbers, mass flow rates, and solar irradiance in the collector. It is imperative to develop a numerical model with further thermal analysis and validation with experimental work for a wide range of mass flow rates and solar radiations. In this study, as different from the literature on PV/T structural study and investigation, a new and more efficient unglazed air type PV/T system has been designed and experimentally analyzed.

2. Methodology Several geometrical design concepts were found from previous literature studies to improve the thermal and PV performances of the PV/T system. During this study, rectangular fins were used in the collector. The fins were positioned longitudinally and integrated with a thin flat metallic sheet (TFMS) as illustrated in Fig. 1. Those are links between PV rear and back wall surfaces, so that heat can be conducted through the fin body from the PV rear surface and released into the air effectively. Fin efficiency can be enhanced by the proper selection of materials based on higher thermal conductivity. Aluminum (Al) is commonly being used for its good thermal conductivity, lower cost and lighter weight. Here, the impact of fin system and TFMS on PV/T overall system was studied. A prototype was built using the fin and TFMS, which was then studied under different parameters such as varying fin numbers, mass flow rates (0.02 kg/s-0.14 kg/s) and solar radiations (200-700 W/m2). Firstly, collector performance was investigated without fin (N=0) system in design, and then the results were compared with the performance calculated for increasing the number of fins to two (N=2) and four (N=4). The complete schematic of PV/T collector design is illustrated in Fig. 1. Fig. 1: Schematic of the modified PV/T collector design The width-to-length (Wch/Lch) and depth-to-length (Hch/Lch) ratios were 1.23 and 4.33 respectively as optimal ratios in channel geometry, which are close to the geometry discussed by Farshchimonfared et al. [33]. Hereby, the other basic collector parameters are shown in Table 1. A range of mass flow rates (0.02 kg/s – 0.14 kg/s) and solar radiations (200 W/m2 – 700 W/m2) were investigated. However, the mass flows are turbulent and thus, forced convection was dominated. In the investigation, each parameter was varied independently with respect to the other. Such as irradiation level is kept at 700 W/m2, while air flow rates were varied as 0.02 kg/s, 0.04 kg/s, 0.06 kg/s, 0.08 kg/s, 0.1 kg/s, 0.12 kg/s and 0.14 kg/s. The fins act as a secondary heat exchanger placed parallel and at equal distances with each other and from two parallel boundary walls. Besides, a thin, flat aluminum metal sheet thickness of 7×10-4 m was used which has the thermal conductivity of 205 Wm-1K-1 [34]. In addition, insulation material of 3 mm thickness assists in getting higher heat gain in the collector. The collector boundary walls were sealed with insulator materials. The air duct is properly sealed with silica and adhesive tape to minimize heat loss to the ambiance.

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Table 1 Physical characteristics of prototype PV/T model. 2.1. Experimental work The complete PV-thermal system consists of solar PV panel, storage batteries with solar controller device, mass flow rate unit and electrical connections, illustrated in Fig. 2(a). Polycrystalline Silicon (poly-Si) cell type PV module with rated output power of 40W was chosen for the experiment. The module consists of 36 poly-Si solar cells in series. When cells are in series at a given power output, it decreases the current value, but increases the output voltage and thus reduces the ohmic losses. Table 2 Nominal electrical data of the PV modules used in PV/T (at standard test conditions). The nominal electrical data listed in Table 2 is based on the standard test condition (STC: the cell temperature at 25⁰C, irradiation of 1000 Wm-2). Furthermore, the useful measuring instruments are listed in Table 3 with individual specifications, numbers, types and functional descriptions. Two lead (Pb) acid batteries were used as a primary electrical energy storage device for the PV module to meet the electrical energy requirement for fans (electric load) as much as needed (Fig. 2). A solar controller device (CMP24, 12V-20A) controls the power storage and distribution by preventing batteries from over-discharging. Alternative current (AC) section with AC appliances is not connected to the present study as the thermal performance was analyzed based on solar irradiance and mass flow rate. Fig. 2: (a) Complete PV/T connection (b) sensor position on PV surface (c) Position of anemometer to the outlet for measuring air velocity Table 3 The list of devices was used in the experimental work. i.

Measurement of mass flow rates

Electrical DC (direct current) fan provides the required mass flow to the channel in terms of force circulation process. A variable wire-wound resistor (Table 3) controls the flow rate into the channel to acquire the expected mass flow rate. A converging section was mounted in the outlet section of the collector aperture area and next to that, a cylindrical tube (straighter) is continued to make a steady air flow to the outlet. The fan is run by the DC power output from the PV system, illustrated in Fig. 2(a). A conventional instrument known as metal vane thermo-Anemometer (working range: 0.2-50 m/s) measures the average collector outlet/inlet air velocity from four different symmetric points on the outlet area, according to the dimension described in Fig. 2(c).

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ii.

Measurement of temperatures

Numbers of K-type thermocouple sensors with ±0.2 0C calibrated accuracy were used to measure the temperatures of PV cell surface, PV back surface, collector inlet/outlet and collector back wall surface. Thermocouple sensor positions were set on the PV cell surface area, according to the points (S1-S5) demonstrated in Fig. 2(b). All the temperatures were read out and saved in 4-channel temperature meter data logger. The temperature sensor is preferred to position at near to 200 mm from the collector inlet/outlet, and insulation can be placed around the pipe work both upstream and downstream, recommended by Buker et al. [35]. iii.

Measurement of solar radiations

Halogen bulbs were employed to imitate the solar irradiation during the testing period, measured by pyranometer, specification details shown in Table 3. The halogen lamps embedded in a rack are arranged in a matrix for uniform distribution of irradiance incident on the PV cell surface, connected in series operated by 240 V AC (alternating current) supply. Intensity can be varied by changing the perpendicular distance between PV surface and halogen bulbs and also by changing the number of operating bulbs. The incident solar irradiance was measured from the nine positions on the PV surface to ensure the full area coverage (Fig. 2(b)), while tests were conducted following the ISO standard [36]. Fig. 3: Schematic of the setup designed for measuring solar radiations Such as three units 500 W and two units 1000 W of halogen bulbs were able to produce average 700 ± 50 W/m2 of equivalent solar irradiance on the PV panel surface when the height was 1550 mm between the staking point of the solar simulator and ground level (Fig. 3). The percentage of a difference of experimentally found module average solar irradiance to the target (200-700 W/m2) was varied between 1.43% - 6.19%, which is highly reliable. As the irradiance at a particular point shall not differ from the mean or module total average irradiance on the aperture area by more than ±15% [37].

2.2. Thermal mathematical model In the energy balance of PV/T collector design, the following assumptions were considered during making a series of analytical solutions. a. A portion of the radiation energy incident on the panel produces electrical energy and the rest is absorbed by the PV cell. b. Discrete surface temperature assumed as the mean temperature value on the entire area of PV panel. c. Back wall surface and fin surface temperatures were taken as a uniform in the entire working area. d. Heat capacities were ignored for the PV back sheet, solar cell material and insulation materials. 8

e. One-dimensional heat conduction and quasi-steady state were considered. Fig. 4: (a) Thermal distribution model (b) Flow pattern for elemental length (dx) (c) Schematic diagram of fin dimensions and (d) Longitudinal fin placement in the collector In this study, energy balance equations for each component of the PV/T systems were developed based on the flow model and thermal distribution, illustrated in Fig. 4(a)-(b) [29, 38, 39]. i.

The heat energy balance for PV section for the present design can be written as

I t c c  bs 1  c  g dA  UOHTC c a  Tc  Ta   hbs f (Tbs  T f )dA  I t c c g dA

(1)

Here, the collector elemental area, dA  Wch dx As the rate of heat transfer from PV back surface to flowing air is equal to the rate of heat transfer from solar cell to the PV back surface [29], the thermal balance relation can be written as

hbs f (Tbs  T f )  kcbs Tc  Tbs 

(2)

In Eq. (1), PV panel area (APV) can be determined directly from panel configuration, UOHTC(c→a), hbs→f and other variable parameters αc , βc , τg are available in Table 4. Temperatures parameters (Tc , Ta , Tbs , Tf) and irradiance, I(t) can be determined experimentally. ii.

For the fluid, heat gained is distributed to the flowing air from the PV back surface by convection represented by hbs→f and thermal losses to the bottom plate by overall heat transfer coefficient represented by Ub, depending on the air flow rates (ṁf) in the channel (Fig. 4(b)). Resulting the following heat energy balance can be established, followed by Joshi et al. [29].

hbs f (Tbs  T f )  m f C f

d T f  dx  U b (T f  Ta ) dx

(3)

Air mass flow rate, m  f   f Acs vin iii.

Alternatively, the rate of heat carried out by the flowing air from the collector can be written as follows [29]:

m f C f iv.

d T f  dx

dx  m f C f Tout  Tin 

(4)

Heat balance relation for fin

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The heat carried out by fins releases to the moving air by convection is equal to the heat comes through the conduction from PV rear. The thermal balance relation for number of fins (N) used in the collector can be written as follows, by Tonui et al. [40]:

N × (heat conduction by fins) = N × (heat convection from fins both sides) dT fin  N   k fin Acf dy 

  N  y 0

y  H fin

 2h

c , fin

L fin T fin  T f dy

(5)

y 0

Where, Tfin indicates the fin surface temperature at any point along to the y-axis, illustrated in Fig 4(c)-(d). For the both condition of without and with fin uses in the PV/T collector, solving the Eq. (1), we get the PV cell temperature (Tc), Tc 

 eff I t   U OHTC ca Ta  hbs f Tbs U OHTC ca   hbs f

By substituting Tc into Eq. (2), back surface temperature, Tbs of the PV module, we get, Tbs 

h p1  eff I t   U bsTa  hbs f T f U bs  hbs f

Where,

 eff

   1     g c  c  c   bs 1      c    Solving Eq. (3) using Tc and Tbs, then we can conclude by the following relation.

 f Cf m

d T f dx

 dx  W





U L T f  Ta   Wch h p1h p 2  eff I t 

ch

(6)

The relations are shown in Eq. (6), first penalty factor, hp1 were demonstrated due to the presence of the solar cell, PV back surface and second penalty factor, hp2 for the interface between the back surface to flowing fluid, indicated in Appendix), UL comprises all losses (conduction, convection and radiation) from the PV/T collector.

Table 4 The values of operating parameters and parametric studies of PV/T air collector. Here, τg , αc , αbs , βc values are obtained from the similar design parameters calculated for thermal balance equation, discussed by Joshi et al. [29]. 10

To get the outlet air temperature, Tout and mean air temperature Tf, boundary conditions will be expressed, referred by Solanki et al. [41]. This boundary value defines the air temperature along to the flow distance from 0 to Lch for the current condition, in Fig. 4(b). Then, integrating the Eq. (3), under the initial boundary condition at x=0; Tf= Tin and the final boundary condition at x=Lch; Tf=Tout, we get,  h h  eff Tout   p1 p 2 UL 

Tf 

Here,

1 Lch

x  Lch

T

f

x 0

WchU L x m f C f

     W U x   W U x   ch L   ch L    m  m I t     C   C   Ta   1  e  f f    Tin e  f f      

 h h  eff dx   p1 p 2 UL 

         WchU L x    WchU L x            m   m    fCf    fCf  I t    1 e  1  e   Ta   1   Tin             WchU L x    WchU L x            m    m     C C   f f f f       

is a unit less factor, the value of which is decreased with the increase of mass flow

rate. 2.3. Thermal performance The thermal efficiency (ηth) relies on the usefully collected heat and the leaving air temperature under particular mass flow rate in the collector channel. This is known as the ratio of heat power (Qu) to the product of corresponding radiation, I(t) and the solar aperture area, AG which has been discussed extensively in several articles [1, 2, 8, 16, 30], expressed as follows:

th 

m C T  T  Qu  f f out in AG I (t ) AG I (t )

(7)

For the proposed PV/T design, the useful heat gain can be calculated by the following relation, which is identical to the relation described for the rate of useful energy for a flat plate collector [39, 42] and widely known as a “Hottel-Whillier-Bliss” equation.





Qu  FR AG I (t ) eff  U L Tin  Ta 

(8)

This characteristics equation evolved for conventional flat SAC makes a difference with current study is the presence of hp1 and hp2, which reduces the overall efficiency of a PV/T system. It also can be used to get an expression for the instantaneous thermal efficiency (ηth) of the PV/T system [41, 43], is then given by (T  T ) th  FR  eff  FRU L in a I (t ) (9)

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Which indicates that the thermal efficiency plotted against (Tin –Ta)/I(t), then FRUL represents the slope and FR(τα)eff is equal to y-axis intercept of the linear line.

2.4. Overall thermal efficiency The overall thermal efficiency (ηo) can be expressed by a combination of efficiency expressions consisting of PV efficiency (ηPV) and thermal efficiency (ηth). Power generated from PV relies on its cell efficiency, which is affected by the cell temperature. PV efficiency (ηPV) decreases with the operating cell temperature increasing. This is applicable for all types of polycrystalline silicon solar cell. The linear correlation between PV cell operating temperature and PV efficiency is described by empirical relation [2, 8]:  PV   c,ref 1   ref TPV  Tref (10)







Where, at lower solar radiation, Eq. (10) gives the equal PV efficiency with at reference conditions (ηPV ≈ ηc,ref =0.15). βref is the temperature coefficient, which is equivalent to 0.0045 ºC−1, depends on the panel quality and solar irradiation. PV efficiency, ηPV has been converted to equivalent thermal energy (ηth,PV) using electric power generation factor (ηe,power) due to the different form of energy and not the correct way to compare directly the kWh of electricity with a kWh of heat from a thermodynamic point of view [9].    o   PV   th e, power 



 th. pv  th

(11)

The value for electric power generation factor (ηe,power) suggested as 0.38 [2, 9, 15]. In the present study, fan power (Pfan) was ignored due to consuming a small amount of electricity from the storage battery itself and other electrical losses to gross benefits in real time application. However, the net power (Pnet) can be calculated by following: Pnet= power generated by PV module (s) – power consumed by fan (s) Pnet  PV  Ac  N c  I t   Pfan

(12)

2.5. Statistical analysis Root mean square of percentage deviation (e) and the coefficient of correlation (r) are analyzed statistically. These values have been evaluated to compare the results of the calculations as theoretical (Xth) with the experimental (Xex) results.

 e 

2

e

i

(13)

n

 X th  X ex    100  X th 

Where, ei  

12

And r

n X th  X ex    X ex    X th 

n

 X    X  2 ex

2

ex

 n

 X    X  2 th

(14)

2

th

3. Result and discussion Firstly, the effect of PV cell temperature on PV efficiency for various configurations was described. Secondly, the temperature parameters of proposed design were performed to establish the numerical model validation with individual statistical analysis. Thirdly, the effect of changing mass flow rates and solar radiations on the performance of PV/T system was investigated. 3.1. Impact of temperature on PV performance In this section, the experimental result of PV temperatures (TPV) and the change of PV efficiency (ηPV) for fins are presented in Fig. 5(a)-(c) at 700 W/m2 at constant solar radiation. Here, it was considered as TPV ≈ Tcell, for the identical properties of the PV. Fig. 5: Effect of PV surface temperature on PV efficiency (a) no fins, (b) two fins and (c) four fins PV efficiency, ηPV noticeably drops with the rise of cell temperature, as expected under the active mass flow rates ṁ1 (0.02 kg/s)-ṁ4 (0.14 kg/s) in the channel, which also satisfy the Eq. (10). In Fig. 5(a), the initial values of ηPV were calculated as 14.80% when the TPV was recorded 27.9 °C under 0.02 kg/s of controlled mass flow rate and no fin. Based on the results of this study, PV efficiency (ηPV) has decreased by 0.063% for every 1°C temperature increase of TPV. Therefore, the result shows the good agreement with previous results, as examples, ηPV was decreased by 0.05% and 0.03% outdoor experiment carried out by Chandrasekar et al. [22] and Bahaidarah et al. [44], respectively. But, more close to the result as 0.06%, found by Rahman et al.[45] in indoor test conditions. This deviation occurred due to different operational conditions. The halogen lamp has less spectral irradiance than sunlight but generates higher heat than sunlight [46]. The flowing air in the channel mitigates the TPV and it was found 49.3 °C at a maximum mass flow rate (0.14 kg/s), in Fig. 5(a). When the mass flow rate is increased to 0.14 kg/s, the final mitigated TPV, ηPV becomes 49.3 °C and 13.35% respectively, which is 0.43% of the ηPV increment than the efficiency calculated for 0.02 kg/s, in Fig. 5(a). When the number of fins is increased to two and four, consequently, it contributes to enhancing the ηPV as well. The final TPV were recorded 46.52°C for two fins, shown in Fig. 5(b) and 43.75 °C for four fins, shown in Fig. 5(c) under the 0.14 kg/s of mass flow rate. Finally, the ηPV improved by 0.77% under 0.14 kg/s of mass flow rate compared to 0.02 kg/s (Fig. 5(c)) in PV/T design for four fins. Meanwhile, final TPV were recorded as 55.25 °C, 43.75 °C under the mass flow rate of 0.02 kg/s and 0.14 kg/s respectively. But ηPV improved little higher by 0.81% when compared without the fins and at 0.02 kg/s of mass flow

13

rate (Fig. 5(a)). Thus, it significantly improved the electrical efficiencies by the addition of fins in the collector, and the higher the mass flow rate, the higher ηPV will be and vice versa. 3.2.Temperature effect in PV/T components The effect of mass flow rates on PV rear (Tbs) and collector back wall (Tw) surface temperatures are shown in Fig. 6 as the rest components of PV/T. The temperature variation of Tbs, Tw under the adjusting air flow rates of 0.02 kg/s, 0.14 kg/s and 700 W/m2 of constant solar radiation was observed. Fig. 6: PV rear and back wall surface temperature (°C) under (a) no fins, (b) two fins and (c) four fins For each condition employed in PV/T model, the performance was determined under seven controlled mass flow rates, spanning the range of ṁ=0.02-0.14 kg/s. Also clearly shown in Fig. 6(a)-(c) is that Tbs, Tw from the design with fins is generally more than that of design without the fin. Air temperature rises for both Tbs, Tw is seen to drop with increasing mass flow rates in the range of ṁ ≥ 0.02 kg/s, as expected. In Fig. 6(a), maximum Tbs was recorded as 55.4 ºC when the mass flow rate in the collector was 0.02 kg/s and the incident solar radiation is maximum (700 W/m2) too. Because the higher the air flow in the channel, the faster the PV back surface temperature mitigates. It is noticeable that integrating fins make an effective result in performance enhancement. More importantly, Tw was decreased to 30.15 ºC (Fig. 6(c)) from 32.45 ºC (Fig. 6(a)) at 0.14 kg/s due to higher heat transfer rate by fins to the flowing air although, the Tw at minimum air flow rate (0.02 kg/s) in the channel increased slightly. Therefore, all the surface temperatures rapidly increased for the first fifteen minutes because the PV cell and collector partly absorbed the heat coming from incident solar radiation. After that, it reaches to the near of saturation level; as a result, the temperature does not increase as rapidly during rest of the operating time. 3.3.Validation of working fluid (air) temperatures A comparison between theoretical and experimental result for collector air outlet temperature (Tout) and mean air temperature (Tf) was established, shown in Fig. 7 and Fig. 8 respectively. After that, the root mean square of percentage deviation (e) and the coefficient of correlation (r) was acquired using Eq. (13) - (14) as a validation for the values of Tout and Tf. Fig. 7: Comparison between the theoretical and experimental value of outlet temperature (°C) under (a) no fins, (b) two fins and (c) four fins By substituting the specified values of ambient temperature (Ta), collector inlet temperature (Tin) and other parameters available in Table 4 into the appropriate equations developed for Tout and Tf, the theoretical predictions were obtained, represented by Fig. 7-8 respectively. In addition, the physical properties of air are given in Table A in the appendix.

14

In Fig. 7(a)-(c), it shows the fair agreement in Tout between theoretical and experimental with the root mean square percentages of deviation (e) and coefficient of correlation (r) value. It demonstrates that analytical expression developed from the improved design is comparable to the experimental values. It was increased the Tout by 6.75 ºC after thirty minutes at 0.02 kg/s, shown in Fig. 7(a). As a result, due to integrating the fins in the collector, Tout was increased by about 8.25 ºC and 9.9 ºC comparing with the initial condition for two and four fin respectively under the same air flow rate condition, shown in Fig. 7(b)-(c). The flowing air takes the higher meeting time with rear and back wall surface in the channel at the lowest mass flow rate. Meanwhile, TFMS and additional fin surfaces increase the heat transfer rate in the channel during flowing. Fig. 8: Comparison between theoretical and experimental value of mean air temperature (°C) (a) no fins, (b) two fins and (c) four fins The various fluid mean temperatures (Tf) are shown in Fig. 8 as a function of mass flow rate. A very close agreement with ‘e’ and ‘r’ value with the range between 0.72-1.71% and 0.977-0.986 respectively for all cases are illustrated in Fig. 8(a)-(c) was established. However, enhancement in the value of (Tf) is remarkably lower as compared to the increase in the number of fins. Here, Tf is always less than Tout due to the high influence of solar irradiance and mass flow rate ṁ. Referring to the Fig. 4(b), it is clearly has shown that temperature rises into the channel along with the elemental length dx. Ambient air enters to the inlet at x=0 and travels until at x=Lch to the outlet, while solar radiation incident on the PV surface. So, Tfin is mainly influenced by the incident solar irradiance and the contact surface between fins and PV rear surface. As a result, the convection effect occurs to increase the temperature of working fluid and makes higher at an outlet. 3.4. Effect of irradiations on PV/T performance The result of solar irradiances and fin numbers to air temperature rising at constant air mass flow rate of 0.02 kg/s is shown in Fig. 9, where the air temperature rises exponentially with the solar irradiance increases. Fig. 9: Variation in the collector temperature difference (°C) with solar irradiance (W/m2) Fig. 10: Plot of instantaneous thermal efficiency of the PV/T collector A significant convection effect was developed in the channel due to an increase of the fin surface area and the results show the higher collector outlet temperature as expected. ηth|N=0= 0.357- 14.12 ηth|N=2= 0.420– 18.77 ηth|N=4= 0.473- 24.10

𝑇𝑖𝑛 −𝑇𝑎

(19)

𝐼(𝑡) 𝑇𝑖𝑛 −𝑇𝑎

(20)

𝐼(𝑡) 𝑇𝑖𝑛 −𝑇𝑎

(21)

𝐼(𝑡)

15

Linear curves, shown in Fig. 10, represent the instantaneous thermal efficiency (ηth) under different ṁ, Tin and N that comprises its overall performance. It shows that ηth decreases with increasing of (Tin-Ta)/I(t). From the above linear relations, 0.357, 0.420, 0.473 values represent the FR(τα)eff part of the Eq. (9) that intercept of the efficiency line with the y-axis and the values with 14.12, 18.77, 24.10 represents the slope (FRUL) of the efficiency line (Fig. 10). These three lines are expected to intersect at three different points of x-axis where heat gain values are considered as zero. These points are known as stagnation temperatures due to carry maximum temperature in the collector, by Bejan et al. [47]. It is clearly expressed that ηth increased with the increase in the number of fins (Fig. 10). The fan power requirement increases too, however, with increasing solar radiation rate, so that the net power (Pnet) continuously increases, following the Eq. (12). Moreover, it was found that, ηth slope developed for N=0, is very close to the slope found in research carried out by J. Ji et al. [12]. Thus, higher ṁ led to higher heat transfer coefficient (hbs→f), lower heat removal factor (FR) and lower TPV. 3.5. Effect of mass flow rates on PV/T performance The result of mass flow rates (kg/s) and fin numbers to air temperature rising at constant 700 W/m2 of irradiation level is shown in Fig. 11, where the air temperature decreased rapidly with the mass flow rates, ṁ increases. Fig. 11: Variation of temperature changes (°C) with mass flow rates (kg/s) Table 5 Temperatures, power output, efficiencies of PV/T system for four fins and 700 W/m2 of irradiation level. It shows that the design with four fins has a slightly better efficiency than those of other designs. The data also reveals that the design of fins is more efficient than without fins by converting solar irradiance to electrical and thermal energy; therefore, makes better overall performance (ηo), has shown for four fins at 700 W/m2 of irradiation (Table 5). Also, the useful heat gain, Qu is significantly increased with mass flow rate, ṁ. Heat is transferred to flowing air higher since fins provide the higher heat transfer area and TFMS increases the better convection effect. The overall thermal efficiency, ηo is calculated using Eq. (11), that increased by 7.9% for the mass flow rate changed 0.02 kg/s to 0.14 kg/s, while thermal efficiency, ηth and equivalent thermal efficiency, ηth,PV (%) also increased (Table 5) as well. Fig. 12: Variation of thermal efficiency with mass flow rate (kg/s) It has been observed that when the flow rate is increased, the collector temperature difference (∆T) decreased rapidly as expected outcome from Eq. (7). It shows that an almost relatively constant level was noticed for a value higher than 0.1 kg/s, while collector fluid mean temperature reaches a saturation level with respect to the incoming irradiance (Fig. 12). As a result, thermal efficiency increased with mass flow rate as described in Eq. (7). Thermal efficiency was calculated 16

as ≈56.19% for the system operating at higher air flow rate (0.14 kg/s) and the number of fins (N=4) in the collector (Fig. 12). It was more significant that the thermal efficiency was 6.63% higher than efficiency found at 0.02 kg/s. Similarly, the maximum thermal efficiencies were found 49.70% and 54.03% under no fins and two fins respectively at 0.14 kg/s. Uncertainty analysis is a very powerful tool to measure the error between measured and experimental value, according to the method suggested by Moffat [48]. The uncertainties in this study were determined by the root-sum-square method. The uncertainties for the calculated variables used in this study are shown in Table 6. Table 6 Average uncertainty values of the variables (0.02 kg/s - 0.14 kg/s). Hereby, authors recommend using more low-power halogen lamps to ensure the uniform irradiance and temperature distribution on the module surface for more accuracy of the testing procedure to the existing system. A smart controller can be used for measuring of air velocity with higher accuracy to find optimized mass flow rate, solar radiation and fin numbers in the future work. As it is important for the extensive analysis of thermal and electrical efficiency calculation in PV/T system, which can be continued for future BIPV application. Authors recommend to analysis the performance for the building integrated photovoltaic thermal (BIPV/T) application and observation the output for numbers of PV/T connected in series/parallel comparing for a range of mass flow rates and solar radiations. Authors are suggesting to use advance simulating software (e.g. COMSOL multi-physics, ANSYS-Fluent/CFD module etc.) in energy consumption analysis from PV/T individuals. Again, heat transfer fluid has a major contribution in the performance analysis of PV/T system and transforms solar energy into internal energy of medium fluid e.g air, water, ethylene and propylene glycol (antifreeze), hydrocarbon oils, refrigerants/phase change fluids etc. A comparative study of using such medium fluid is suggested to exercise.

4. Conclusion It is a big challenge in PV/T technology in terms of energy savings to overcome heat losses and running the system effectively. It can be integrated with heating, ventilating, and air conditioning (HVAC) system by ensuring adequate hot air supply. The design of BIPV/T can be modified using different thermal augmented materials to ensure the optimal thermal and electrical efficiency. In this paper, the thermal and electrical performance of the PV/T system was demonstrated for the employed designs in various conditions, through an analytical model development and experimental values. Some important findings from the proposed design are expressed as follows: 17

a. PV/T design integrated with TFMS and fins has better electrical and thermal efficiency compared to the design without fins. In addition, PV top, rear surface, collector back wall and outlet air temperatures were significantly affected by fin numbers and mass flow rates. b. The analytical and experimental values for the collector outlet (Tout) and mean air temperatures (Tf) was validated by fair agreement by the root mean square percentages of deviation (e) and coefficient of correlation (r) values. c. The air mass flow rates and numbers of fins strongly affect the PV performance. PV efficiency was increased by ≈0.81% for four fins at 0.14 kg/s of mass flow rate, compared to no fins at 0.02 kg/s. d. At the highest air velocity in channel makes a higher heat convection effect, which offers the maximum heat gain. Alternatively, heat gain is relatively higher at higher solar radiation value. The thermal efficiency was improved by 37.82% (no fins) to 56.19% (four fins) at 700 W/m2, when the mass flow rate was increased from 0.02 kg/s to 0.14 kg/s. Finally, it is concluded that such a structural improvement, will improve the overall PV/T performance. Therefore, operating cost becomes lower because the aluminium fins and TFMS are employed are usually thin, as compared with the width-to-length (Wch/Lch) and depth-to-length (Hch/Lch) ratios of the PV/T collector. Such a unit can be effectively fitted with ventilated PV system in buildings and can make economically viable and environmentally friendly.

Acknowledgements: The authors would like to acknowledge the Ministry of Higher Education of Malaysia and The University of Malaya, Kuala Lumpur, Malaysia for the financial support under UM.C/625/1/HIR/MoHE/ENG/15 (D000015-16001), SATU: RU021B-2015and PPP:PG 2392014B.

APPENDIX: Overall heat transfer coefficient (UL): U L  U b  U f ,a And U OHTC ca hbs f Ub  hbs f  U OHTC ca  Penalty factors ( h p1 , h p 2 ):

18

h p1 

hbs f hbs f  U OHTC ca 

And h p 2 

hbs f hbs f  U bs

Convective heat transfer for fin ( hc , fin ) was calculated by follows: The function for Nusselt number in force convection can be expressed as

Nu  f Re, Pr 

In this study, calculating an average Nusselt number ( Nua ) for force convection process was within a certain flow velocity span (0.13×10-3 to 1.33×102), referred by Krauter et al. [49]:

hc, fin 

k  Num ; Where, Num  Lca

Nu

2

lam

 Nu 2 tur



(A.1)

Nulam = 0.664Re1 / 2 Pr 1 / 3 Nutur 

(A.2)

0.8

0.037 Pr Re 1  2.433 Re  0.1 Pr 2 / 3  1





(A.3)

With, Re=

 a vin Lca C f and Pr =  k

Here, characteristic length (Lca) was considered equal to the fin length (Lfin), which was used to calculate Nusselt Number and Reynold Number. The flow velocity with higher than 1 m/s, heat transfer by force convection could be more than natural convection. As air properties are mainly the function of temperature, including other relative properties of air such as dynamic viscosity (μ), kinematic viscosity (k′), density (ρa), inlet velocity (vin). These values can be found from Table A under different operating temperature. Table A Properties of air at different temperatures (data collected from J.P.Holman [50]).

19

References: 1.

2. 3. 4. 5. 6.

7.

8. 9. 10.

11. 12. 13.

14.

15.

16. 17. 18. 19.

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39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50.

Solanki, S., S. Dubey, and A. Tiwari, Indoor simulation and testing of photovoltaic thermal (PV/T) air collectors. Applied energy, 2009. 86(11): p. 2421-2428. Tonui, J.K. and Y. Tripanagnostopoulos, Air-cooled PV/T solar collectors with low cost performance improvements. Solar Energy, 2007. 81(4): p. 498-511. Solanki, S.C., S. Dubey, and A. Tiwari, Indoor simulation and testing of photovoltaic thermal (PV/T) air collectors. Applied Energy, 2009. 86(11): p. 2421-2428. Duffie, J.A. and W.A. Beckman, Solar engineering of thermal processes. Vol. 3. 1980: Wiley New York etc. Shatat, M., M. Worall, and S. Riffat, Opportunities for solar water desalination worldwide: review. Sustainable cities and society, 2013. 9: p. 67-80. H. Bahaidarah, A.S., P. Gandhidasan, S. Rehman, Performance evaluation of a PV (photovoltaic) module by back surface water cooling for hot climatic conditions. Energy, 2013. 59: p. 445-453. Rahman, M., M. Hasanuzzaman, and N. Rahim, Effects of various parameters on PV-module power and efficiency. Energy Conversion and Management, 2015. 103: p. 348-358. education.org p, spectral-irradiance; 2015. . Bejan, A., D.W. Kearney, and F. Kreith, Second Law Analysis and Synthesis of Solar Collector Systems. Journal of Solar Energy Engineering, 1981. 103(1): p. 23-28. Moffat, R.J., Describing the uncertainties in experimental results. Experimental thermal and fluid science, 1988. 1(1): p. 3-17. Krauter, S., et al., Combined photovoltaic and solar thermal systems for facade integration and building insulation. Solar Energy, 1999. 67(4–6): p. 239-248. J.P.Holman, Heat Transfer. 1997. 8th edition, McGraw-Hill: p. 646.

22

(VΙΙ)

(VΙΙΙ)

(Ι) (ΙΙ) (ΙΙΙ) (ΙV)

(Χ)

Ι- PV module ΙΙ- converging section ΙΙΙ- back heat absorber ΙV-wooden wall V-TFMS VΙ- insulation VΙΙ-Exhaust fan VΙΙΙ- outlet ΙΧ- longitudinal fin Χ-inlet

(V) (VΙ)

(ΙΧ)

Fig. 1: Schematic of the modified PV/T collector design

23

Solar simulator

Air outlet Variable wirewound resistor

PV system

DC Fan (24 V)

F U

TFMS

Solar charge controller

S

DC output

Air inlet Battery1

Battery2

F- Fuse S-Switch

Inverter

AC appliances

(a) 23 mm

35 mm

90 mm

S

45 mm - Anemometer position S - Fan enclosure area

(b)

(c)

Fig. 2: (a) Complete PV/T connection (b) sensor position on PV surface (c) Position of anemometer to the outlet for measuring air velocity 24

Stacking point Solar simulator

15º

1110 mm

PV module PV center line along to mid-point 15º 440 mm 90º

Fig. 3: Schematic of the setup designed for measuring solar radiations

25

(a)

(b)

Thermal paste Fin array

(c)

(d)

Fig. 4: (a) Thermal distribution model (b) Flow pattern for elemental length (dx) (c) Schematic diagram fin dimensions and (d) Longitudinal fin placement

26

T_pv (0.14 kg/s) Ƞpv (0.14 kg/s)

60

15

50

14.5

40

14

30 13.5

20 10

13

700 W/m2

0

Electrical efficiency (%)

PV temperature (°C)

T_pv (0.02 kg/s) Ƞpv (0.02 kg/s)

12.5 0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 Time (minutes)

(a)

T_pv (0.14 kg/s) Ƞe (0.14 kg/s)

60

15

50

14.5

40

14

30 13.5

20 10

13

700 W/m2

0

12.5 0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 Time (minutes)

(b)

27

Electrical efficiency (%)

PV temperature (°C)

T_pv (0.02 kg/s) Ƞe (0.02 kg/s)

T_pv (0.14 kg/s) Ƞe (0.14 kg/s)

60

15

50

14.5

40

14

30 13.5

20 10

13

700 W/m2

0

12.5 0

2

4

6

Electrical efficiency (%)

PV temperature (°C)

T_pv (0.02 kg/s) Ƞe (0.02 kg/s)

8 10 12 14 16 18 20 22 24 26 28 30 32 Time (minutes)

(c) Fig. 5: Effect of PV surface temperature on PV efficiency (a) no fins, (b) two fins and (c) four fins

Tbs (0.02 kg/s) Tbs (0.14 kg/s)

Tw (0.02 kg/s) Tw (0.14 kg/s)

60

Temperature (°C)

55

700 W/m2

50 45 40 35 30 25 20 0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 Time (minutes)

(a)

28

Temperature (°C)

Tbs (0.02 kg/s) Tbs (0.14 kg/s)

60 55 50 45 40 35 30 25 20

Tw (0.02 kg/s) Tw (0.14 kg/s)

700 W/m2

0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 Time (minutes)

Temperature (°C)

(b) Tbs (0.02 kg/s) Tbs (0.14 kg/s)

60 55 50 45 40 35 30 25 20

Tw (0.02 kg/s) Tw (0.14 kg/s)

700 W/m2

0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 Time (minutes)

(c)

Fig. 6: PV rear and back wall surface temperature(°C) under (a) no fins, (b) two fins and (c) four fins

29

Tout_ex (0.02 kg/s) Tout_ex (0.14 kg/s)

Outlet temperature (°C)

40

Tout_th (0.02 kg/s) Tout_th (0.14 kg/s)

700 W/m2

35 30 25

e0.02kg/s=1.70,r=0.956 e0.14kg/s=1.01,r=0.986

20 15 0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 Time (minutes)

(a)

Outlet temperature (°C)

Tout_ex (0.02 kg/s) Tout_ex (0.14 kg/s)

Tout_th (0.02 kg/s) Tout_th (0.14 kg/s)

700 W/m2

35 30 25

e0.02kg/s=2.44,r=0.960 e0.14kg/s=1.37,r=0.956

20 15 0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 Time (minutes)

(b)

30

Tout_ex (0.02 kg/s) Tout_ex (0.14 kg/s)

Outlet temperature (°C)

40

Tout_th (0.02 kg/s) Tout_th (0.14 kg/s)

700 W/m2 35 30 25 e0.02kg/s=3.10, r=0.968 e0.14kg/s=2.05, r=0.959

20 15 0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 Time (minutes)

(c)

Collector mean air temperature (°C)

Fig. 7: Comparison between the theoretical and experimental value of outlet temperature (°C) under (a) no fins, (b) two fins and (c) four fins

35 33 31 29 27 25 23 21 19 17 15

Tf_exp (0.02 kg/s) Tf_exp (0.14 kg/s)

Tf_th (0.02 kg/s) Tf_th (0.14 kg/s)

700 W/m2

e0.02kg/s=0.84, r=0.982 e0.14kg/s=0.55, r=0.986 0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 Time (minutes)

(a)

31

Collector mean air temperature (°C)

Tf_exp (0.02 kg/s) Tf_exp (0.14 kg/s)

35 33 31 29 27 25 23 21 19 17 15

Tf_th (0.02 kg/s) Tf_th (0.14 kg/s)

700 W/m2

e0.02kg/s=1.32, r=0.982 e0.14kg/s=0.72, r=0.983 0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 Time (minutes)

(b) Tf_exp (0.02 kg/s) Tf_exp (0.14 kg/s)

Collector mean air temperature (°C)

35 33 31 29 27 25 23 21 19 17 15

Tf_th (0.02 kg/s) Tf_th (0.14 kg/s)

700 W/m2

e0.02kg/s=1.71, r=0.984 e0.14kg/s=1.24, r=0.977 0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 Time (minutes)

(c) Fig. 8: Comparison between theoretical and experimental value of mean air temperature (°C) under (a) no fins, (b) two fins and (c) four fins

32

Temperature difference (⁰C)

ΔT (N=0)

6

ΔT (N=2)

ΔT (N=4)

5 4 3 2 1 0 100

200

300 400 500 600 2 Solar irradiance (W/m )

700

800

Fig. 9: Variation in the collector temperature difference (°C) with solar irradiance (W/m2)

Thermal efficiency

0.55

η_th (N=0) η_th (N=4) Linear (η_th (N=2))

η_th (N=2) Linear (η_th (N=0)) Linear (η_th (N=4))

0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.0005

0.0015

0.0025

0.0035

0.0045

0.0055

0.0065

(Ti-Ta)/I(t) (°C m² / W)

Fig. 10: Plot of instantaneous thermal efficiency of the PV/T collector

33

ΔT (N=0)

Temperature difference (°C)

6.0

ΔT (N=2)

ΔT (N=4)

5.0 4.0 3.0 2.0 1.0 0.0 0

0.02

0.04

0.06 0.08 0.1 Mass flow rate (kg/s)

0.12

0.14

0.16

Fig. 11: Variation of temperature changes (°C) with mass flow rate (kg/s)

34

η_th (N=0)

Thermal efficiency

0.7

η_th (N=2)

η_th (N=4)

0.6 0.5 0.4 0.3 0.2 0

0.02

0.04

0.06 0.08 0.1 Mass flow rate (kg/s)

0.12

0.14

0.16

Fig. 12: Variation of thermal efficiency with mass flow rate (kg/s)

35

Table 1 Physical characteristics of prototype PV/T model. (a) Collector parameters Length, Lch Width, Wch Height, Hch Cross sectional area, Acs (b) Fin geometry parameters Length, Lfin Height, Hfin Thickness, δfin Thermal conductivity, kfin Fin inter spacing (for N=2) Fin inter spacing(for N=4)

Values 0.630 m 0.510 m 0.130 m 0.065 m2 0.300 m 0.160 m 2 ×10-3 m 205 Wm-1K-1 0.168 m 0.100 m

Table 2 Nominal electrical data of the PV modules used in PV/T (at standard test conditions). Parameter PV module area, APV Solar cell area, Ac Maximum voltage, Vmax Maximum current, Imax Fill factor (FF) Packing factor (PF) Open circuit voltage, Voc Short circuit current, Isc Operating temperature, Top

Value 0.31 m2 0.0069 m2 17.40 V 2.30 A 0.73 81% 21.50 V 2.53 A -40 ⁰C to 85 ⁰C

36

Table 3 The list of devices was used in the experimental work. Apparatus (numbers) Solar regulator (1) Storage battery (2) Pyranometer (1) Anemometer (1)

Electric fan (1) Temperature sensor (12) Voltage sensor (1) Current sensor (1) Data logger (3) Variable wirewound resistor (1)

Type

Function

Pb acid battery Data logging solar power meter Metal vane thermoAnemometer DC K-type thermocouple sensors 4-channel temperature meter Slide-type wave

Over discharging controller To storage electrical energy To measure the incoming solar radiation To measure the inlet/outlet air velocity

Specification (Accuracy) CMP24,12V-20A 12V,24Ah TES,1333R (±10 W/m2) Extech (± 0.3 m/s)

To assist the mass flow rate To measure the temperatures

24V,0.12 A (±0.2 ⁰C)

To measure DC voltage of PV To measure DC current of PV To record measured data Every 2-min intervals record To change the air flow rate 450 Ω, max 1A, Resistance tolerance (±10%)

Table 4 The values of operating parameters and parametric studies of PV/T air collector.

Parameter ηc,ref τg βc αbs αc ρa hp1 hp2

UOHTC(c→a) hbs→f

Value 0.15 0.95 0.83 0.50 0.90 1.23 0.69 0.44 2.80 6.50

Parameter Ubs Cf Ub (ατ)eff Uf,a UL Re Num hc,fin

Value 8.11 1005 2.96 0.69 0.62 2.58 9972.97 78.85 6.82

37

Table 5 Temperatures, power output, efficiencies of PV/T system for four fins and 700 W/m2 of irradiation level.

Mass flow rate, (kg/s)

Collector temperature difference, ΔT (°C)

0.02 0.04 0.06 0.08 0.10 0.12 0.14

5.35 2.88 1.97 1.46 1.20 1.00 0.87

Thermal efficiency, ηth (%)

Heat gain, Qu (W)

PV surface temperature, TPV (°C)

PV efficiency, ηPV (%)

Equivalent thermal efficiency, ηth,PV (%)

Overall thermal efficiency, ηo (%)

49.56 53.41 54.65 54.34 55.58 55.58 56.19

107.54 115.91 118.59 117.92 120.60 120.60 121.94

46.48 44.40 43.30 41.80 40.40 39.80 39.40

13.55 13.69 13.76 13.87 13.96 14.00 14.03

35.66 36.03 36.22 36.49 36.74 36.84 36.92

85.21 89.44 90.87 90.83 92.31 92.42 93.11

Table 6 Average uncertainty values of the variables (0.02 kg/s - 0.14 kg/s). Symbol PV cell temperature, TPV PV rear surface temperature, Tbs Collector back wall temperature, Tw Inlet temperature, Tin Outlet temperature, Tout Mass flow rate, m Solar radiation, I(t) Collector gross area, AG Fin area, Afin Fin inter spacing

Uncertainty range (%)

Average Uncertainty (%)

1.186-1.847 0.478-1.282 0.882-1.534 2.797-2.824 0.739-1.458 2.86-20 2.19-10 -

1.516 0.880 1.20 2.810 1.099 11.43 6.10 3.55 2.75 1.90

Table A Properties of air at different temperatures.



T (K)

(kg/m )

250 275 300 325

1.412 1.284 1.177 1.086

3

Cf  105 -1 -1 (kJ kg K ) (kg m-1s-1) 1.0053 1.599 1.0055 1.725 1.0057 1.846 1.0063 1.962

38

 105 (m2 s-1) 1.131 1.343 1.569 1.807

k  103 -1

Pr -1

(Wm K ) 0.022 0.024 0.026 0.028

0.722 0.713 0.708 0.701