T) solar collector: Experimental validation of a 2-D theoretical model

Renewable Energy 85 (2016) 1052e1067

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Bi-fluid photovoltaic/thermal (PV/T) solar collector: Experimental validation of a 2-D theoretical model Hasila Jarimi a, c, *, Mohd Nazari Abu Bakar a, Mahmod Othman b, Mahadzir Hj Din a a

Faculty of Applied Sciences, Universiti Teknologi MARA, Perlis, 02600 Arau, Perlis, Malaysia Faculty of Computer Sciences and Mathematics, Universiti Teknologi MARA, Perlis, 02600 Arau, Perlis, Malaysia c Faculty of Applied Sciences, Universiti Teknologi MARA Shah Alam, 40450 Shah Alam, Selangor, Malaysia b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 December 2014 Received in revised form 28 May 2015 Accepted 6 July 2015 Available online xxx

This paper discusses theoretical and indoor experimental studies of a bi-fluid type photovoltaic/thermal PV/T solar collector. 2D steady-state analysis was developed and computer simulation was performed using MATLAB. Experiments were conducted for steady-state analysis under the solar simulator at Solar Energy Research Lab UiTM Perlis, Malaysia. The test includes all three modes of fluid operation under the same PV/T system, namely: the air mode, the water mode, and the simultaneous mode of water and air. The simulation results were validated against the experimental results using three methods of error analysis: root mean square percentage deviation (RMSPD), mean absolute percentage error (MAPE), and coefficient of determination (R2). On average, the MAPE, RMSPD, and R2 computed for the fluid output temperature are approximately 0.92%, 1.19%, and 0.98, respectively. Thus, we concluded that the simulation and experimental results are in good agreement. The PV/T collector designed in this study has a variety of applications as it can be operated in three different modes of fluid operation, and the theoretical model is useful in modelling all three modes without further modification. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Photovoltaic/thermal Solar collector Bi-fluid (PV/T) 2-D steady-state Error analysis

1. Introduction Theoretical and experimental studies of PV/T solar collectors were carried out and documented as early as in the mid-1970s. In 1976, Wolf [1] investigated the performance of a combined solar heating and photovoltaic electric power generation system for a single family residence. The study was carried out in Boston, USA. In 1979, Florschuetz [2] came up with the famous extension of the well-known HotteleWhilliereBliss model for thermal analysis of flat-plate collectors to the analysis of combined photovoltaic/thermal collectors. More studies were carried out by S. D. Hendrie [3] to prove the technical feasibility of the collectors. Since then, studies on collector designs have continued and performance analysis, including development of thermal and electrical modelling together with the influence of various parameters on the overall performance of the collectors. When both fluids are utilized as the working fluids, the collector

* Corresponding author. Faculty of Applied Sciences, Universiti Teknologi MARA, Perlis, 02600 Arau, Perlis, Malaysia. E-mail address: [email protected] (H. Jarimi). http://dx.doi.org/10.1016/j.renene.2015.07.014 0960-1481/© 2015 Elsevier Ltd. All rights reserved.

is known as a bi-fluid PV/T solar collector and this was first introduced by Tripanagnostopoulos [4]. However, his research focused mainly on the experimental studies of the collector during the independent mode of fluid operation. Another study has been conducted by Assoa et al. [5] such that a bi-fluid PV/T solar collector was developed and analysed experimentally and theoretically using 2-D steady-state analysis. However, as far as the present authors are concerned the design of the collector is such that the position of air and water alternates. Hence, the 2-D analysis for the water and air components were carried out separately for each of the working fluid. A novel design of a hybrid solar collector which integrates the use of both water and air as the working fluids known as a ‘bi-fluid type photovoltaic/thermal (PV/T) solar collector’ has been proposed by our research group. The design concept has been discussed in detail in Abu Bakar et al. [6]. The performance of the collector is then further improved by the researchers by introducing the use of low-cost heat transfer enhancement technique via the use of set of fins parallel to the direction to the air flow [7] as shown in Fig. 1, while the inputs and outputs of the working fluids are shown in Fig. 2. When both fluids are to be operated simultaneously, the results found seem more appealing.

H. Jarimi et al. / Renewable Energy 85 (2016) 1052e1067

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Fig. 1. (a) Side-view cross-section, and (b) front and top-view cross-section of the designed bi-fluid PV/T solar collector.

In this study, following the developed 2-D mathematical model as discussed in Abu Bakar et al. [6], a slight modification to the model that is suitable for the finned air channel configuration is discussed. Additionally, different from the previous published research by the current authors, the mathematical model was validated against the indoor experimental results which were performed at Solar Energy Research Lab, UiTM Perlis, Malaysia. 2. Theoretical analysis In terms of practical heat conduction problems which involve complicated geometries with complex boundary conditions or variable properties, the thermal balance equations associated with the heat transfer cannot be solved analytically [8]. In this study, when both fluids are to be operated simultaneously, the system is considered as complex and complicated [7,9]. In general, the system is analysed such that both fluids operated simultaneously and flow transversely to one another. Since the heat transfer takes

place in all three dimensions, the system is deemed complicated to be analysed analytically. Therefore, by referring to Craft [10], the system is analysed numerically in which 2D steady-state analysis with finite difference method is employed using the following procedure as discussed in Abu Bakar et al. [6]; i) the construction of the temperature nodes; ii) the construction of thermal nodal networks for each sub-segment, m; ii) the development of the energy balance equations for the PV laminate, the back surface of Tedlar, back panel, air and water nodes; iii) the calculation of the heat transfer coefficients using the guessed temperatures for the average and each temperature node; vi) solving the thermal nodal network model using matrix solution procedure. In order to construct the nodes, the serpentine-shaped copper pipe is assumed as a long straight tube as in Ref. [6] and the solar collector has been segmented into segment M along the y-direction. The system is then further divided into sub-segments m. The size of the segments in the y-direction are assumed small enough so that the temperature gradient in the y-direction along the layer of PV cells, the rear surface of Tedlar, and the back plate

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Fig. 2. Schematic of the test-rig facilities: 1: Water outlet, 2: Water inlet, 3: Air ducting, 4: Water flow rate measurement, 5: Water input storage and heating tank (Tank no.1), 6: Flow rate calibration facility, 7: Water output storage tank, A: Solar simulator, B: Wind Simulator, and C: Air heating segment.

are assumed to vary almost linearly. The temperature nodes are then constructed for the collector's layers in the x-z plane. Meanwhile, the position of the nodes in the x-y plane is considered at the average point. Additionally, the heat conduction term in the y eaxis is considered as negligible for all the sub-segments m. Due to the different in the area of PV cells and surface area involved in the heat transfer process, area correction factors gs,m, for the corresponding surface area of As,m at sub-segment m, are introduced. For example; gbsbp,m, which is the area correction factor from the back surface of Tedlar to the back plate; gbsf1,m, which is the area correction factor from the back surface of Tedlar to the air

Later in the forthcoming Section 4, the flexibility of the modelling which is also usable to model the system during independent mode of fluid operation will be elaborated. Nevertheless, interested readers can refer to [6,7] for detailed discussions on the modelling. 2.1. Energy balance equation Following the work by Refs. [6,11] the energy balance equations for each temperature nodes can be written and explained as follow: For the nodes of the solar cells of the PV module, the energy balance equation is written as:

    kpv dpv   kpv dpv   tg ac ðPFÞG  tg ac ðPFÞðhele ÞG ¼ Ut;m Tpv;m  Ta þ hpvbs;m Tpv;m  Tbs;m  Tpv;mþ1 eTpv;m þ Tpv ;m  Tpv;m1 2 2 ðDxÞ ðDxÞ |fflfflfflfflfflfflffl{zfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl ffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} 1

2

3

4

flow; gbpf1,m, which is the area correction factor from the surface of the back plate to the air flow; gbsf2,m, which is the area correction factor from the back surface of Tedlar to the water flow; and gbp,m,

(1)

5

The heat transfer terms are defined as follow: 1: The rate of the solar energy received by solar cells of the PV

        hpvbs;m Tpv;m  Tbs;m þtg aT ð1  PFÞG ¼ hcvbsf 1;m gbsf 1;m Tbs;m  Tf 1;m þ hrbsbp;m gbsbp;m Tbs;m  Tbp;m þhcvbsf 2;m gbsf 2;m Tbs;m  Tf 2;m |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} 4

6

7

8

9

(2)

which is the area correction factor from the surface of the back plate to the ambient. In this paper, the energy balance equations for the collector designed during the simultaneous mode are concisely discussed.

module after transmission per unit area. 2: The rate of electrical energy available per unit area. 3: The rate of the top heat lost to the ambient per unit area. 4: The rate of heat transferred to the back surface of Tedlar per unit area.5: The rate of heat conducted along

H. Jarimi et al. / Renewable Energy 85 (2016) 1052e1067

  m_ f 1 Cpf 1 Tf 1;m  Tf 1;m1

the x-direction aside the tube per unit area. For the back surface of Tedlar nodes, for sub-segments with copper tube, an additional term to account for the heat transferred from the back surface of Tedlar to the water flow is introduced and represented as term no 9:

ðDxDyPV Þ

Nfin

dTfin kfin Ac;fin dz





¼ hcvfinf 1;m Nfin Ac;fin Tfin;m  Tf 1;m



In general, the energy balance equations for the water flow can be written as:

  m_ f 2 Cpf 2 Tf 2o;m M  Tf 2i;m M ðDxDyPV Þ



However, (8) can also be written in terms of the average fluid temperature Tf 2;m as follow:

(3)



m_ f 2 Cpf 2



¼ Ubp;m gbp;m Tbp;m  Ta |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} 11

(4) The heat transfer terms are defined as follows: 10: The rate of heat transfer from the back plate with fins to the air flow per unit area. 11: The rate of heat lost to the ambient through the back plate per unit area. With hp ¼ 1gfinf1,m(1hf) is the overall surface efficiency and hf ¼ tanh(mfinhfin)/mfinhfin is the fin efficiency. Now, for the fluid flow in this system, the energy source is determined by the heat convection between the fluid and the wall surface per unit area of the PV sub-segment m. Following the discussion above for the sub-segment m, the energy balance equations for the air temperature nodes for each sub-segment m, can be written as:





The computer programme using MATLAB is used to perform the computer simulation. The program starts with all the heat transfer coefficients and thermophysical properties of the fluids calculated using the set up parameters and guessed temperature nodes. Similar to the method carried out by [Zondag et al. [13], Ong [14], Dehra [15]] the actual temperature of each node was calculated using matrix-solving procedure. The matrices [h] for the coefficient matrix, [Tm,actual] for the new temperature nodes and [q] for the heat flux sources were then set up: [h]484  484[Tm,actual] 484  1 ¼ [q]484  1. Using the matrix inverse function in MATLAB, the inverse matrix ½h1 484484 is obtained and the values of the actual temperature nodes were estimated and compared with the initially guessed temperatures. If the difference calculated is less than 0.01  C, the iteration process stops and the values of the old temperatures were replaced with the new computed ones. The average temperatures for each solar collector layer were then calculated by integrating the corresponding temperature nodes on the respective surface. 2.3. Energy analysis The electrical efficiency of the collector is modelled as a function of temperature based on Florschuetz [2] equation as follow:



m_ f 1 Cpf 1 Tf 1;mþ1=2  Tf 1;m1=2 ¼ hcvbpf 1;m gbpf 1;m hp Tbp;m   Tf 1;m ðDxDyPV Þ  þ hcvbsf 1;m gbsf 1;m Tbs;m   Tf 1;m ðDxDyPV Þ

M

   2Tf 2;m M 2Tf 2i;m M  ¼ hcvbsf 2;m gbsf 2;m Tbs;m Tf 2;m M ðDxDyPV Þ

2.2. Numerical solution



10





(9)

hrbsbp;m gbsbp;m Tbs;m  Tbp;m  hcvbpf 1;m gbpf 1;m hp Tbp;m  Tf 1;m |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} 8

   ¼ hcvbsf 2;m gbsf 2;m Tbs;m  Tf 2;m M (8)

In addition, at steady-state, the temperature of the fins is also considered as equal to the temperature nodes of the base of the back plate; hence, the heat conduction term can also be expressed in terms of the back plate temperature nodes. However, in reality the temperature of the fin drops along the fin. In order to take into account this effect, overall surface efficiency must be included in the heat transfer equation. The heat transfer equation for the temperature nodes for the surface of the back plate with fins is then written as follows:



  ¼ hcvbpf 1;m gbpf 1;m hp Tbp;m  Tf 1;m   þ hcvbsf 1;m gbsf 1;m Tbs;m  Tf 1;m (7)

The heat transfer terms are defined as follow: 6: The rate of the solar energy absorbed by the back surface of Tedlar of the PV module per unit area. 7: The rate of heat transfer to the air flow per unit area. 8: The rate of energy radiated to the surface of the back plate and fins per unit area. At steady-state, the heat convection rate from the fins surface to the working fluid is equal to the heat conducted into the fin elements. The heat conduction equation can then be expressed as:



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   hele ¼ href 1  bref Tpv  Tref

(5)

Now, from upwind differencing scheme as discussed in Apsley [12], the left hand side of (5) can be written as:

    m_ f 1 Cpf 1 Tf 1;mþ1=2  Tf 1;m1=2 ¼ m_ f 1 Cpf 1 Tf 1;m  Tf 1;m1

href, and bref are the electrical efficiency and temperature coefficient at the reference mean PV cells temperature Tref respectively. The overall instantaneous useful thermal energy produced Qth,inst by each of the working fluid used in the solar collector is computed using the following equation as defined in many heat transfer text books and literature [8,16]:   Qth;inst ¼ m_ f Cpf Tfo  Tfi

(6) The following energy balance equation for the air nodes is then obtained:

(10)

(11)

Therefore, the total instantaneous thermal energy produced by the collector is given by the summation of the thermal energy produced by both air and water as follow [6]:

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X

H. Jarimi et al. / Renewable Energy 85 (2016) 1052e1067

Qth;inst

    ¼ m_ f 1 Cpf 1 Tf 1o  Tf 1i þ m_ f 2 Cpf 2 Tf 2o  Tf 2i

(12)

The total instantaneous thermal efficiency of the solar collector can then be computed as:

X

hth;inst ¼

    m_ f 1 Cpf 1 Tf 1o  Tf 1i þ m_ f 2 Cpf 2 Tf 2o  Tf 2i GAc

(13)

However, as cited by Abu Bakar and Hj Othman [17], as recommended by Hill et al. [18], the steady-state thermal performance of the collector is normally evaluated by averaging the performance over certain period of time, depending on the time constant of the collector. Therefore, by modifying the equations as recommended by Refs. [19e22], the total thermal efficiency can be computed as:

Z  X

hth ¼

    m_ f 1 Cpf 1 Tf 1o  Tf 1i þ m_ f 2 Cpf 2 Tf 2o  Tf 2i dt Z Ac  Gdt (14)

In order to evaluate the total yields of a collector system, PVT Roadmap [23] suggested four different types of analysis: 1. Calculate the total energy by simple addition 2. Calculate the primary energy (Fossil fuel energy required to produce the amount of useful thermal and electrical energy. This differs from the above due to plant and heater efficiencies) 3. Calculate the saved cost from the tariffs for heating energy and electrical energy 4. Calculate the exergy. The energetic performance or the overall energy gain is the common performance evaluating method. It is worth emphasising here that even though each energy is evaluated using the law of thermodynamic (the first law) with the same standard unit, electric energy and thermal energy are different in nature. Therefore, the ‘combined’ or ‘overall’ performance of the collector should be performed by carefully taking into consideration the different nature of the energies [19,24e26]. Additionally, in PVT Roadmap [23], the primary energy saving efficiency or also known as ‘equivalent thermal efficiency’ which has been originally introduced and discussed in detail in Huang et al. [27] is chosen to compute the overall performance of PV/T. Therefore, the same type of analysis is used in this study. Researches; [28e30], also used the same form of evaluation. Following the work by Huang et al. [27], the instantaneous primary energy saving is written as:

X

QPVT;inst ¼

X

Qth;inst þ

Pele Cf

QPVT ¼

Fig. 4. Indoor experimental setup.

Therefore, similar to (14) the total yield of the PV/T system or the P primary energy saving efficiency hPVT can be written as:

Z 

(15) X

Equation (15) can also be written as:

X

Fig. 3. DAQ modules, solar simulator, and collector under testing.

Z      m_ f 1 Cpf 1 Tf 1o  Tf 1i þ m_ f 2 Cpf 2 Tf 2o  Tf 2i dt Z Pele dt þ Cf (16)

The equation is defined as follow: The overall equivalent thermal output (primary energy saving) from a PV/T system ¼ Overall thermal energy collected by the collector þ Overall electrical power output/Conversion power factor(Cf).

hPVT

    m_ f 1 Cpf 1 Tf 1o  Tf 1i þ m_ f 2 Cpf 2 Tf 2o  Tf 2i dt Z ¼ Ac  Gdt 1 0 Z P dt ele C 1 B C Z þB A C @ f Ac  Gdt (17)

Cf is known as the power plant conversion factor. Its value can range from 0.2 to 0.4 depending on the quality of the coal used [27]. In this study, the value of 0.34 is used. It is the average thermal efficiency of a conventional coal power plant based on the 2008e2012 data given by Suruhanjaya Tenaga (Energy Commissions) [31].

H. Jarimi et al. / Renewable Energy 85 (2016) 1052e1067

3. Experimental study In order to evaluate the performance of the bi-fluid type hybrid solar collector, a prototype, and indoor test-rig facilities have been set-up and fabricated at Solar Energy Research Lab, UiTM Perlis, Malaysia. The prototype is non-optimized and fabricated to validate the computer simulation results from the developed 2-D mathematical model for the collector under study. The indoor test-rig facilities are shown in Fig. 3 and Fig. 4. The facilities comprise of three important parts namely; solar simulator, air and water heat exchange system, and the power-supply and control system. In addition, many low-cost solar simulators which are suitable for solar collector testing have been fabricated worldwide starting as early as in 1974 [17,32]. For the collector indoor testing, the ideal solar simulator would consist of lamps in which the spectral distribution resembles the spectral distribution of the sunlight. However, due to economical and physical constraint, this is impossible to achieve [33]. In the research published by several researchers [17,32e35] the minimum characteristics or standards as listed in ASHRAE were followed in their research, tungsten halogen lamps were employed for the solar simulator and concluded as sufficient and reliable for the indoor collector testing. While, a variety of types of lamps have been suggested, in this study, by referring to the work of [Othman et al. [21], Garg, et al. [32], Hussain, et al. [33], Agrawal, et al. [34]], a low-cost simulator which consists of a 3-phase array of 30 quartz-halogen tungsten lamps of Philips Plusline Small Double-Ended Linear Lamps of 500 W each installed on a lamp casing with a reflector arranged on a steel support structure with (1.4 m  1.4 m) in size was fabricated for the collector indoor testing. The simulator is capable to produce solar intensities of up to 800 W/m2. The intensity of these lamps were made to vary using the United Automation CSR2-E series Power Regulator installed at the power-supply control box for the solar simulator. A calibrated Eppley Black & White 8e48 pyranometer was used to measure the incidental solar radiation. It is important to note that when the collector was tested under the solar simulator, the electrical characteristics measured experimentally are somewhat different from the values given by the manufacturer. A similar condition is obtained in the study performed by Abu Bakar and Hj Othman [17]. This may be attributed to the fact that the quartz tungsten halogen lamps used as the indoor solar simulator in this study have a colour temperature of less than 3400 K. Therefore, in terms of the spectral characteristics, they radiate weaker in the shorter wavelengths (blue and UV portion) but stronger in the infrared portion [36]. This leads to lower electrical performance, but slightly higher thermal performance of the PV/T solar collector indoors in comparison to the performance under real sky conditions or outdoors. This discrepancy will be further discussed in the future studies which involve the outdoor experimental validation of the theoretical model. Despite the aforementioned issue, the indoor testing facility is still considered reliable since the primary focus of the indoor collector testing is to validate the developed mathematical model and to investigate the ‘behaviour’ of the thermal and electrical performance of the collector when operated with different sets of fluids' mass flow rate and imposed with different set of solar radiation. Following this, similar to the study conducted by Ref. [5] for the mathematical validation purposes, for indoor collector testing, the local walls and the sky temperature are assumed equal to the ambient temperature. Meanwhile the electrical performance was also predicted by taking into account its electrical characteristics when imposed by the radiation from the solar simulator. Additionally, the ageing factor of the cells could also lead to the lower

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performance of the PV cells. In order to use (10), for the mathematical validation purposes, href is obtained experimentally as 0.04 at Tref ¼ 40  C. Therefore, in this study the measured href and Tref were employed for the theoretical computation of the electrical performance. Water was pumped into the collector by an SP737 Tideway Electric Submersible Water Pump from the primary water tank. Its speed was controlled using a United Automation CSR2-E series Power Regulator. In order to fix the temperature of the water at constant temperature, a secondary water tank was used to discard the heated water from the panel. Prior to entering the collector, the water flow rate was first measured using a Blue-White F-440 inline flow meter. Meanwhile, air was pumped into the collector using an air blower which consists of a unit of fan driven by a Siemens 3-Phase induction AC motor connected to an AC driver to control the air flow rate into the collector. The air flow rate was measured using an SDL 350 hot wire thermo-anemometer of Extech instruments. The temperatures of the PV module both at the top and back surface, back plate, and fluids were measured using K-type thermocouples which were calibrated prior to collector testing. All the thermocouples, and the pyranometer were connected to the channels of Advantech DAQ modules (ADAM) for data logging via a PC. By following the guideline given by Ref. [37] in principle, the mean rear temperature of the PV module can be used to estimate the mean temperature of the PV cells TPV. Nevertheless, at a steady state using the developed mathematical model, the mean temperature of PV cells can be approximated by performing thermal resistance analysis between the tempered glass cover temperature and the cells using the following relation:

Tpn ¼

  Ut þ hpg Tg  Ut Ta hpg

(18)

The electrical characteristics of the PV module for indoor testing were measured using a VS-6810 IV Tracer. The thermal readings were recorded at the time interval of 1 s and being averaged over the period when the system was in the steady-state. The combination of both heat exchange system into the collector leads to a new test-rig design which must be tailored to the dual heat exchange demands. Therefore, the fabrication of the test rig facilities discussed in this study serves as a good starting platform to further develop experimental research on a bi-fluid type PV/T solar collector.

4. Results and discussion From the 2-D steady-state analysis discussed earlier, an algorithm to simulate the collector's performance under controlled environmental parameters has been developed using MATLAB. The experiments were performed under a solar simulator at Solar Energy Research Lab UiTM Perlis, Malaysia. The model is then validated using the experimental results from the collector indoor testing. Various thermophysical parameters used in the calculations are summarised in Table 1 [38e42]. Covering all the three modes of fluid operation, at average solar radiation of 700 W/m2 and an average wind speed of 1 ms1, the influence of the mass flow rate to the energy efficiency and fluid temperature rise of the collector are discussed. The average wind speed of 1 ms1 is an acceptable value since it is within the range of 0 ms1 to 2 ms1 as recommended by PV catapult [37] for the indoor collector testing of an unglazed PV/T solar collector.

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H. Jarimi et al. / Renewable Energy 85 (2016) 1052e1067

Table 1 Thermophysical properties and design parameters [38e42]. Parameter Width of collector Length of collector Bond width Outer diameter tube Inner diameter tube Height of air channel Width of air channel Length of tube segment Tube spacing

Wc L B Do Di H Wair Dytube W

1.8

Value

Parameter

0.53 m 1.183 m 0.01 m 0.01 m 0.008 m 0.05 m 0.53 m 0.42 m 0.09 m

EVA thermal conductivity Thickness of Tedlar Tedlar thermal conductivity Emissivity of PV Emissivity of the back surface Absorptivity of PV Absorptivity of Tedlar Thickness of tempered glass PV cells thermal conductivity Thickness of PV cells

Increase in solar radiation

1.6 1.4

dT kT εpv εbs

apv aT dg kpv

dpv

W 0:35 mK 0.000175 m W 0:2 mK 0.85 0.85 0.95 0.5 0.003 m W 148 mK 0.0003 m

700 W/m2 , Tpv = 44.70 °C 600 W/m2, Tpv = 43.6 °C

1.2 I (A)

Value kEVA

500 W/m2, Tpv = 40.9 °C

1.0 400 W/m2, Tpv =39.9 °C

0.8 0.6 0.4 0.2 0.0 4

6

8

10

12 14 16 18 Voltage (V) PV/T Bi-fluid 700 W/m² PV/T Bi-fluid 600 W/m² PV/T Bi-fluid 500 W/m²

20

PV/T Bi-fluid 400 W/m²

Fig. 5. Current against voltage or IeV curve for PV/T bi-fluid with mass flow rate of water at 0.0066 kg/s and air at 0.0262 kg/s.

Fig. 6. Current against voltage or IeV curve for PV/T air, water, and bi-fluid with mass flow rate of water and air at 0.0066 kg/s and 0.0262 kg/s respectively.

4.1. Electrical performance In order to simulate real system operation, the photovoltaic electrical output was connected to an electronic load of a VS 1680 IeV tracer. The tracer was connected to a computer using a USB 2.0 port of which an IeV curve with the value of the electrical

characteristics which are the short circuit current Isc, open circuit voltage Voc,, maximum current Imax, voltage Vmax,, and maximum power point Pmax, are displayed using a suitable software installed in the computer. As shown in Fig. 5, for the same mass flow rate of air and water of 0.0262 kg/s and 0.0066 kg/s during the simultaneous mode of fluid operation, when the solar radiation increased

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20

PV/T bi-fluid

18

16 PV/T air

Power (W)

14 12 10 PV/T water

8 6 4 2 0 0

2

4

6

8

PV/T Bi-fluid 700 W/m²

10 12 Voltage (V) PV/T air 700 W/m²

14

16

18

20

PV/T water 700 W/m²

Fig. 7. Power against voltage or PeV curve for PV/T air, water, and bi-fluid with mass flow rate of water and air at 0.0066 kg/s and 0.0262 kg/s respectively.

from 400 W/m2 to 700 W/m2 with the bin size of 100 W/m2, the average temperature of the PV cells increases from 39.9  C to 44.70  C. The increase in the solar radiation also has a significant effect to the electric current produced by the collector. The electrical current increases as the radiation increases. Likewise, it is also reflected by the increase in the average temperature of the cells. Whereas, as can be seen from Fig. 6, for the same amount of solar radiation G of 700 W/m2, the simultaneous use of both fluids led to the decrease in the temperature of the PV panel. With the air and water flow fixed at 0.0262 kg/s and 0.0066 kg/s respectively, the effect of using different modes of fluid operation to the production of electric current is not as significant as to the previous condition shown in Fig. 5. However, a marginal decrease in the value of Isc and also an increase in the Voc were observed when the fluids operated simultaneously. As shown in Fig. 6, the increase in Voc is more significant and the pattern of the curves reflect smaller parasitic resistance during the simultaneous mode (bi-fluid) operation. This may have caused by the temperature distribution across the PV

panel that is more uniformed in comparison to the independent mode of fluid operation. As illustrated in Fig. 7, for the same set of solar radiation, the increase in the peak power point and the open circuit voltage for the panel during simultaneous mode of fluid operation is higher than the independent mode. The increase in the open circuit voltage is observed owing to the low mean temperature of PV cells during the simultaneous mode of fluid operation. From the above mentioned graph, it can be concluded that using of both working fluids simultaneously as one of the effective electricity enhancement techniques for a PV module. As shown in Fig. 8, during simultaneous mode of fluid operation, the increase in the solar intensity led to an increase in the peak power point for the same fluids' mass flow rates. 4.2. Photovoltaic/thermal performance 4.2.1. Experimental validation In the theoretical study of a PV/T solar collector, a number of researchers have emphasised on the validation method of the

20 Increase in solar radiation

18 16 Power (W)

14 12 10 8 6 4 2 0 0

2

4

6

8

10 12 Voltage (V)

14

16

18

PV/T Bi-fluid 700 W/m²

PV/T Bi-Fluid 600 W/m²

PV/T Bi-fluid 500 W/m²

PV/T Bi-fluid 400 W/m²

20

Fig. 8. Power against voltage PeV curve at PV/T bi-fluid with mass flow rate of water at 0.0066 kg/s and air at 0.0262 kg/s with different solar radiation.

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Table 2 Summary of error analysis in the study of PV/T solar collector. Type of PV/T and testing

Type of modelling

Parameters being studied

Error analysis

Literature

Remarks

PV/T air type (flat plate and single pass), outdoor testing.

1-D analytical analysis. (Thermal)

Fluid output temperature, back surface of Tedlar temperature, and cell temperature.

RMSPD of 4.58%, 8.71% and 12.09% for fluid output, back surface and cell temperature respectively.

[11]

PV/T water type (flat plate), outdoor testing.

1-D analytical analysis. (Thermal)

Fluid output temperature and cell temperature.

RMSPD of 7.22% and 5.87% for the cell and the fluid output temperature respectively.

[44]

PV/T air type (flat plate), outdoor testing.

1-D analytical analysis. (Thermal and electrical)

Fluid output, cell and back surface temperatures, and overall efficiency ƞtot, thermal efficiency ƞth, and electrical efficiency ƞele.

RMSPD of 2.37%, 12.58% and 8.63% for the fluid output, cell and back surface temperature respectively, and RMSPD of 9.81% 28.56% and 11.02% for ƞtot, ƞth,and ƞele respectively. RMSPD of 1.85% and 2.32% for design A and B respectively

[41]

The authors have concluded a fair agreement between the experimental and theoretical studies. The higher RMSPD values for the back surface and cell temperatures are due to sudden change in wind speed. Despite of the higher predicted values predicted theoretically, they have concluded that the RMSPD values obtained reflect the validity of the mathematical model. The authors concluded that the simulation and experimental measurements are in good agreement even though the RMSPD for the computed efficiencies are high. The researchers concluded that the experimental and predicted results are concluded to be in good agreement.

.

PV/T water type collector with two different type of absorber; Type A; sheet and tube design, Type B: Parallel channels design, outdoor testing. PV/T bi-fluid type solar collector, indoor testing.

1-D analytical analysis. (Thermal)

Fluid output temperature

2-D analytical analysis. (Thermal)

All temperature nodes.

Relative difference for the main temperatures is approximately 2% except for the surface insulation which gives 17% of error.

[5]

PV/T air type (CPC and fins), indoor testing.

1-D analytical analysis (Thermal)

Thermal efficiency ƞth, and electrical efficiency ƞele.

Relative difference of 3.13% and 58.97% respectively.

[17]

PV/T air type, outdoor testing

1-D analytical analysis (Thermal)

Fluid output temperature, cell temperature, thermal efficiency ƞth, and electrical efficiency ƞele.

MAPE of 1.74%, 13.62%, 24.27%, and 4.87% respectively.

[26]

developed mathematical model with respect to their research. As discussed in Refs. [19,21,43], the validation method was performed by comparing the results obtained experimentally and theoretically based on the trends shown on the related graphs. The developed model is considered valid if the trends shown by both theoretical and experimental curves are in good agreement. Nevertheless, in alignment to report by previous researches, the validation of the developed model can be done using the following six different types of error analysis proposed. Some of the related error analysis which were performed by previous researchers are summarised in Table 2:

[45]

They have concluded that the experimental and simulation measurements are in good agreement The error is due to the ideal assumption that the radiation is uniformly distributed across the panel. The small relative difference obtained for the ƞth reflects the validity of the mathematical model. Whereas the high value of relative difference in ƞele is due to the different experimentally obtained electrical characteristics than those provided by the manufacturer. They have concluded that the experimental and simulation measurements are in good agreement. They have also included other types of error analysis in their study

i) Root mean square percentage deviation (RMSPD) as given in Refs. [11,41,44,45] xsim,i and xexp,i are the simulated and measured values;

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !2ffi u   n u1 X x  x  100 sim;i exp;i RMSPD ¼ t n i¼1 xsim;i

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ii) Root mean square error RMSE as given in Ref. [26];

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Table 4 The MAPE, R2 and RMSPD of the PV/T bi-fluid mode solar collector at G ¼ 700 W/m2.

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u1 X  2 RMSE ¼ t xsim;i  xexp;i n i¼1

iii) Mean absolute percentage error (MAPE) analysis as discussed in Ref. [26];

!   x n 1X sim;i  xexp;i  100 MAPE ¼ n i¼1 xexp;i

Type

PV/T Bi-Fluid at fixed air flow rate of 0.0262 kg/s

Parameter/Error analysis

MAPE

R2

RMSPD

MAPE

R2

RMSPD

Tg Tbs Tpv Tbp Tf1o Tf2o

6.57% 2.67% 6.69% 1.54% 0.93% 0.59% 8.40% 3.92% 5.32% 0.98% 4.91%

0.6633 0.7783 0.6527 0.6630 0.9602 0.9863 0.7620 0.9728 0.9170 0.2167 0.9216

7.50% 2.92% 7.66% 1.76% 1.11% 0.88% 9.82% 5.35% 5.76% 1.14% 5.44%

4.76% 4.26% 5.09% 2.61% 1.19% 0.27% 7.83% 4.21% 2.12% 3.42% 2.27%

0.9807 0.9701 0.9163 0.9497 0.9727 0.9926 0.9891 0.9796 0.9430 0.8410 0.9439

5.19% 4.71% 5.39% 2.76% 1.51% 0.34% 10.01% 4.29% 2.52% 3.71% 2.57%

hthf1 h

thf2 P hth

h

ele P hPVT

PV/T Bi-Fluid at fixed water flow rate of 0.0066 kg/s

iv) Mean bias error (MBE) as given in, [26,46]; Table 3 and Table 4. These tables include fluid output temperature, thermal, electrical, and total efficiency; and the mean temperatures of the collector's components.

n   1X MBE ¼  xexp;i x n i¼1 sim;i

v) Coefficient of determination, (R2) as given in Refs. [41,47];

  Pn  xsim;i  xexp;i i¼1 xsim;i  xexp;i #" # R ¼" 2 Pn  2 Pn   x x  x x sim;i exp;i sim;i exp;i i¼1 i¼1 2

vi) t-statistic method (t-stat) as given in Ref. [26];

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðn  1ÞMBE2 t  stat ¼ RMSE2  MBE2 In this study, in addition to the direct comparison between the simulation and theoretical curves, the validation of the mathematical model is further justified using three of the proposed error analysis which are the RMSPD, R2 and MAPE. The three analysis are chosen since these three are the most common types of error analysis being employed and discussed by researchers and hence deemed reliable. The results of the error analysis are summarised in Table 3 The MAPE, R2 and RMSPD of the PV/T air mode and PV/T water mode solar collector at G ¼ 700 W/m2. Mode

Parameter

MAPE

R2

RMSPD

PV/T air

Tg Tbs Tpv Tbp Tf1o (Tf1oTf1i)

3.39% 3.44% 5.15% 4.04% 1.34% 7.01% 6.47% 3.33% 5.10% 7.48% 11.73% 8.12% 3.43% 1.21% 5.81% 5.47% 1.10% 4.10%

0.9539 0.9407 0.9538 0.9535 0.9873 0.9841 0.9879 0.8396 0.9904 0.8259 0.9134 0.8235 0.8935 0.9918 0.9901 0.9478 0.5587 0.9485

3.75% 5.78% 4.71% 4.47% 1.63% 7.82% 7.14% 3.71% 5.62% 8.97% 13.99% 9.00% 3.57% 1.66% 6.68% 6.17% 1.30% 4.78%

hthf1 hele P

PV/T Water

hPVT Tg Tbs Tpv Tbp Tf2o (Tf2oTf2i)

hthf2 hele P

hPVT

4.2.2. PV/T air and PV/T water (independent mode) Fig. 9 and Fig. 10, show the theoretical and indoor experimental analysis of the PV/T collector during air mode and water mode operation respectively under the solar simulator of 700 W/m2 in radiation. Even though the mathematical model is developed for the system to allow both fluids to operate simultaneously, by setting one the fluids at zero flow rate, the model is usable and reliable in simulating the collector performance during the independent mode of fluid operation. The experimental and simulation measurements are concluded to be in good agreement such that the theoretical and experimental curves are found to be in similar trend. Both the curves reflect the same influence of mass flow rate to the performance of the collector in the sense that the air mass flow rate m_ f 1 , increased from 0.0074 kg/s to the maximum of _ f 2 , increased from 0.0900 kg/s, and water mass flow rate m 0.0017 kg/s to 0.0265 kg/s. Both the thermal and electrical efficiencies increase while the value of the change in the fluid temperature rise decreases. Despite that, at the very low flow rate of water ranging between 0.0017 kg/s to 0.0033 kg/s i.e. at laminar flow region of Reynolds number less than 1000, the predicted efficiency and temperature rise are higher than the experimental results. The entrance region effect which has been assumed negligible in the selection of Nusselt correlation seem to be the most possible explanation to this finding. For the air mode of operation, it is important to note that the increase in the thermal efficiency is significant in the laminar region which is from mass flow rate of 0.0074 kg/s to 0.0221 kg/s with the experimental thermal efficiency increases from 23.43% to 47.92%. Meanwhile, the theoretical thermal efficiency increases from 26.84% to 44.33%. However, as the mass flow rate continues to increase, the experimental thermal efficiency will reach its ‘optimum point’ and approaches plateau at the end-of-the laminar to transition and turbulent region of flow rate with the highest achievable efficiency of 59.85% at mass flow rate of 0.09 kg/s. The physical explanation to this condition is such that as the mass flow rate increases, at steady-state condition, larger amount of heat is extracted since the volume of fluid available to extract the heat per second is higher. It is also worth to note that for the water mode of operation, as the mass flow rate of water increased from 0.0017 kg/s to 0.0066 kg/s, the experimental and theoretical thermal efficiency of the collector increased from 29.76% to 50.53%, and 33.63% to 46.82% respectively. However, unlike in the case of air mode of fluid operation, due to the high heat capacity of water, the thermal

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Fig. 9. Comparison between the theoretical and experimental results on the influence of air mass flow rate to the: (a) air thermal efficiency (hthf1) and change in the air temperature P rise (Tf1oTf1i); and (b) electrical efficiency (hele) and primary energy saving efficiency ( hPVT ) during air mode (at stagnant water) [G ¼ 700 W/m2].

efficiency tends to plateau even in the laminar flow region. As reported by de Vries [48], a ‘slight bum’ in his simulation result is obtained due to the change in the laminar etransition flow region of the fluid used as the water mass flow rate increased from the laminar to turbulent flow region. In line with the abovementioned study, when analysed in detail, a small-almost negligible fluctuation in the thermal efficiency is observed during the transitional flow region i.e. between 0.0050 and 0.013 kg/s for both theoretically and experimentally. Looking at the total efficiency of the PV/T solar collector, as discussed in Section 2.3, for the energy analysis, primary energy saving as also used in Fudholi et al. [29], is employed. Therefore, using (17), the values for the electrical and primary energy saving efficiency were first computed. As shown in Fig. 9(b) and Fig. 10(b) a marginal increase in the efficiency with both air and water mass flow rate are obtained. Furthermore, the computed experimental primary energy saving efficiency of the PV/T collector increases from 34.87%to 72.59%; and 41.35% to 64.79% for air and water respectively. Whereas, theoretically, the values computed ranged from 37.93% to 70.59%; and 44.96% to 63.77%, respectively. It is important to state that the trend of the curve is similar to the curve of thermal efficiency as the electrical efficiency has only a slight

influence on the primary energy saving efficiency of the PV/T collector with the increase in fluid flow rate. In addition, as mentioned by de Vries [48] a PV/T collector could be operated at high fluid flow rate, resulting in high efficiency. However, it comes with a small increase in fluid temperature and high requirement of pumping power. This implies that the collector needs to be operated at its optimum flow rate to ensure optimum efficiencies at the optimum temperature rise. Hence, determining the optimum mass flow rate of the working fluid is important and necessary. In this study, the graphs of the influence of mass flow rate to the collector performance are also useful in order to determine the optimum flow rate for the fluids. As illustrated in Fig. 9, at air mode operation, the optimum mass flow rate is achieved at 0.0262 kg/s, with the experimental thermal efficiency obtained is 51.40% with 8.72  C in temperature rise and primary energy saving efficiency of 63.26%. Meanwhile, for the water mode, as illustrated in Fig. 10, the optimum mass flow rate is achieved at 0.0066 kg/s, and the thermal efficiency obtained is 50.53% with 8.00  C in temperature rise and primary energy saving efficiency of 62.31%. In order to further justify the validity of the mathematical model, error analysis for the air mode and water mode of fluid operation for solar radiation of 700 W/m2 were conducted and

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Fig. 10. Comparison between the theoretical and experimental results on the influence of water mass flow rate to the: (a) water thermal efficiency (hthf2) and change in the water P temperature rise (Tf2oTf2i); and (b) electrical efficiency (hele) and primary energy saving efficiency ( hPVT ) during water mode (at stagnant air) [G ¼ 700 W/m2].

summarised in Table 3. In most of the error analysis conducted by previous researchers, the fluid output temperature was amongst the main focus. In this study, for the air mode, the values of mean absolute percentage error (MAPE), coefficient of determination R2 and root mean square percentage deviation (RMSPD) for the fluid output temperature are 1.34%, 0.9873, and 1.63%, respectively. Meanwhile for water, the values are 1.21%, 0.9918, and 1.66%, respectively. On average, the computed errors on the PV module back surface temperature for water mode is the highest with MAPE and RMSPD in the range of 3%e15% since the temperatures predicted are higher than the measured ones. This may be attributed to the assumption that the thermal resistance between the PV module and the tube bonding is negligible. 4.2.3. PV/T Bi-fluid (simultaneous mode) The collector performance analysis during simultaneous mode of operation or known as a PV/T bi-fluid system is mainly segregated into two parts namely, PV/T bi-fluid system at fixed air flow rate and PV/T bi-fluid system at fixed water flow rate with each test is performed under the average solar radiation of 700 W/m2. In this study, the investigation on the thermal behaviour of each fluid to

the collector's performance when they are being operated simultaneously is deemed interesting since both of the heat absorbers are directly associated with one another. Even though the fluids are not physically intact, their thermal performance influences one another. As presented in Fig. 11(a), when the water flow rate is increased during the fixed air flow rate of 0.0262 kg/s, both the theoretical and experimental analysis show that the thermal efficiency of water also increases and tends to plateau at flow rate beyond 0.0066 kg/s. However, the increase in the water flow rate has resulted to a decrease in the air thermal efficiency. This also tends to plateau as the water flow rate increases. A similar condition is obtained when the water is fixed at 0.0066 kg/s as shown in Fig. 12(a). However with an increase in the air thermal efficiency and a decrease in water thermal efficiency, both tend to plateau as the air flow rate increases. This is expected since an increase in one of the fluids flow rate implies that more energy is removed by the fluid with an increase in flow rate. Hence, lower energy could be extracted by the other fluid component. Nevertheless, the optimum fluid flow rate is lower since the simultaneous use of both fluids will compensate one another in extracting the unwanted heat from the collector into useful energy. This leads to another advantage of

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Fig. 11. Comparison between the theoretical and experimental results on the influence of water mass flow rate to the: (a) fluids' thermal efficiency (hthf1 and hthf2), total thermal P P efficiencyð hth Þ , electrical efficiency (hele), and primary energy saving efficiency ( hPVT ); and (b) change in the fluids' temperature rise [(Tf1oTf1i) and (Tf2oTf2i) ]; during simultaneous mode at fixed air flow rate of 0.0262 kg/s [G ¼ 700 W/m2].

the bi-fluid system. Similar comment has been reported by Tripanagnostopoulos [4] in his study of a bi-fluid PV/T system. Nevertheless, at a very low flow rate of water, the increase in the water temperature is overestimated by the simulation result. Similar to the discussion in the 4.2.2, this is believed to have occurred due to the assumption that the fluid is in a steady-state with negligible effect of entrance region. The effect of secondary flow which may have existed in the pipe bends which are assumed negligible in the modelling. This might have contributed to the overestimated results. In order to perform the experiment, the optimum flow rate of air and water obtained during the independent mode of fluid operation are chosen as the fixed flow rate. It is important to note that these two flow rates are chosen for the experiment for the mathematical validation purposes and do not mark as the optimum flow rates during simultaneous mode of fluid operation. As seen in Figs. 11 and 12, the experimental and simulation results are concluded to be in good agreement since similar trends of graphs are shown both experimentally and theoretically. As presented in Fig. 11(a), when air is fixed at 0.0262 kg/s, the water flow rate is set to vary between 0.0017 kg/s to 0.0265 kg/s of which the total experimental thermal efficiency and primary energy saving efficiency increases from 51.88% to 65.70%; and 64.02% to 77.90%, respectively. In comparison to the water mode operation,

the primary energy saving efficiency is found higher by approximately 12%. As for the temperature rise, as shown in Fig. 11(b), at the lowest water mass flow rate (i.e. 0.0017 kg/s) an increase of 10.50  C and 5.90  C are achieved by both water and air respectively. With both fluids operating simultaneously, the measured mean PV cells temperature is as low as 49.22  C during the water lowest flow rate of 0.0017 kg/s, whereas temperature of 57.79  C was recorded for the same lowest flow rate during the water mode operation. This low temperature has led to an increase in the electrical efficiency of the collector. Its electrical efficiency and electrical equivalent thermal efficiency which are of values of 4.13% and 12.14% respectively have been measured. As a result, primary energy saving efficiency of 64.02% is computed even at 0.0017 kg/s; the lowest flow rate of water in the experiment. This is in fact one of the significant advantages of the bi-fluid system. The findings show that high efficiency and temperature rise are achievable even at a very low flow rate of fluid operation which requires low pumping power and an acceptable net power production even though the system uses two pumps mechanism. Meanwhile, as presented in Fig. 12(a), when water is fixed at 0.0066 kg/s, the air flow rate is set to vary between 0.0074 kg/s to 0.090 kg/s of which the total experimental thermal efficiency and primary energy saving efficiency increase from 51.87% to 66.12%; and 64.01% to 78.98%, respectively. As for the temperature rise, as shown in Fig. 12(b), at

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Fig. 12. Comparison between the theoretical and experimental results on the influence of air mass flow rate to the: (a) fluids' thermal efficiency (hthf1 and hthf2), total thermal P P efficiencyð hth Þ , electrical efficiency (hele), and primary energy saving efficiency ( hPVT ); and (b) change in the fluids' temperature rise [(Tf1oTf1i) and (Tf2oTf2i) ]; during 2 simultaneous mode at fixed water flow rate of 0.0066 kg/s [G ¼ 700 W/m ].

the lowest water mass flow rate, an increase of 7.04  C and 6.60  C are achieved by both water and air respectively. With both fluids operating simultaneously, the measured mean PV cells temperature is as low as 51.42  C during the air lowest flow rate of 0.0074 kg/s; whereas temperature of 62.77  C is recorded for the same lowest flow rate during the air mode operation. This low temperature has led to an increase in the electrical efficiency of the collector and also its electrical equivalent thermal efficiency which is of the values of 4.13% and 12.14% respectively. As a result, primary energy saving efficiency of 64.01% is computed even at 0.0074 kg/s; the lowest flow rate of air in this experiment. Error analysis for the bi-fluid system under the solar simulator of 700 W/m2 in radiation is summarised in Table 4. When air flow rate is fixed at 0.0262 kg/s, the values of MAPE, R2 and RMSPD for the air and water output temperature are; 0.93%, 0.9602 and 1.11%; and 0.59%,0.9863 and 0.88% respectively. Whereas, at fixed water flow rate of 0.0066 kg/s, the values computed are 1.19%, 0.9727 and 1.51%; and then 0.27%, 0.9926 and 0.34% respectively. On average, the values of the MAPE, R2 and RMSPD, are 3.87%, 0.7722, and 4.49%; and 3.46%, 0.9526, and 3.91% for the fixed air and water flow rate configuration respectively. Following this

and also by referring to Fig. 11 to Fig. 12, the simulation results for the bi-fluid system are concluded to be in good agreement with the experimental results and hence are useful in giving reliable prediction for the collector performance. Nevertheless, errors do exist due to the assumption that the panel is uniformly illuminated by the solar simulator. Error also occurs due to inconsistent wind speed which may have been caused by irregular power supply. This validation is crucial to mark the significant contribution of this study in the field of PV/T technology. The developed 2-D mathematical model is usable to simulate three different modes of PV/T solar collectors (3-in 1 modelling) namely air mode, water mode, and simultaneous mode of solar collector under the same design of PV/T bi-fluid solar collector without having to modify the mathematical model while simulating the collector performance. This has never been discussed in any published literature. 5. Conclusion The theoretical and experimental analysis of a PV/T solar

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collector which incorporate the use of dual heat exchanger systems using air and water were performed in this study. It is important to emphasise that a simple design concept is used for the collector. This involves a flat plate unglazed collector which was fabricated with low fabrication and operating cost. Despite the simple design, three different modes of PV/T solar collector operation namely; PV/ T air mode, PV/T water mode and PV/T bi-fluid (simultaneous mode) can be generated from the collector. This becomes the highlight in this study. Upon completing the experiment the following conclusions are made in this study;  The designed collector is practical in generating three different modes of fluid operation without further modification with the exception that the exit of the air channel may need to be closed during PV/T water mode for better performance.  The developed 2-D mathematical model is novel and flexible in the sense that three different modes of fluid operation can be simulated using the same mathematical model without any modification. Up to now, this type of modelling has never been discussed in any related literature.  The developed 2-D mathematical model has been validated against indoor experimental results for all three modes of operation, with the average MAPE, RMSPD, and R2 computed for the fluids' output temperature are approximately 0.92%, 1.19%, and 0.9818, respectively.  Considering the simple and economical collector design, the performance of the collector is comparable to or higher than the existing design of unglazed PV/T solar collector for the independent mode of fluid operation i.e. the first two modes; with the primary energy saving efficiency of the collector at the optimum flow rate are 58.10% and 62.31% for air and water respectively.  In conclusion, both the theoretical and experimental studies are believed to be able to contribute as the starting platform in the research of a bi-fluid type PV/T solar collector. Acknowledgement This work is funded by the Malaysian Fundamental Research Grant Scheme (FRGS) 600-RMI/ST/FRGS 5/3/Fst (160/2010) and Solar Energy Research Lab, Universiti Teknologi Mara (UiTM) Perlis, Malaysia. References [1] M. Wolf, Performance analyses of combined heating and photovoltaic power systems for residences, Energy Convers. 16 (1976) 79e90. [2] L.W. Florschuetz, Extension of the Hottel-Whillier model to the analysis of combined photovoltaic/thermal flat plate collectors, Sol. Energy 22 (1979) 361e366. [3] S.D. Hendrie, Photovoltaic/thermal Collector Development Program, Rapport Final, Massachusetts Institute of Technology, Cambridge, Mass, USA, 1982. [4] Y. Tripanagnostopoulos, Aspects and improvements of hybrid photovoltaic/ thermal solar energy systems, Sol. Energy 81 (2007) 1117e1131. [5] Y.B. Assoa, C. Menezo, G. Fraisse, R. Yezou, J. Brau, Study of a new concept of photovoltaicethermal hybrid collector, Sol. Energy 81 (2007) 1132e1143. [6] M.N. Abu Bakar, M. Othman, M. Hj Din, N.A. Manaf, H. Jarimi, Design concept and mathematical model of a bi-fluid photovoltaic/thermal (PV/T) solar collector, Renew. Energy 67 (2014) 153e164. [7] H. Jarimi, M.N. Abu Bakar, N.A. Manaf, M. Othman, M. Din, Mathematical modelling of a finned bi-fluid type photovoltaic/thermal (PV/T) solar collector, in: Clean Energy and Technology (CEAT), 2013 IEEE Conference on, 2013, pp. 163e168. lu, Heat and Mass Transfer: Fundamentals [8] Y.A. Çengel, A.J. Ghajar, M. Kanog and Applications, McGraw Hill Higher Education, 2011. [9] M.N. Abu Bakar, M. Othman, M. Hj Din, N.A. Manaf, H. Jarimi, Development of an improved photovoltaic/thermal (PV/T) solar collector with bi-fluid configuration, Int. J. Chem. Environ. Eng. 4 (2013). [10] T.J. Craft, Finite Difference Schemes [PDF Document], 2010 [Available: Lecture Notes Online Web site], http://cfd.mace.manchester.ac.uk/twiki/pub/Main/ TimCraftNotes_All_Access/cfd1-findiffs.pdfhttp://cfd.mace.manchester.ac.uk/

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gs,m: area correction factor

k: thermal conductivity of fluids

t: transmittance a: absorptance rd: reflectance (ta)eff: effective transmittance absorptance product h: efficiency hthf1: thermal Efficiency of air hthf2: thermal Efficiency of water P hth : total thermal efficiency

hele: electrical Efficiency heth: electrical efficiency converted to equivalent thermal efficiency P hPVT : primary energy saving or equivalent thermal efficiency

Nomenclature

Subscripts

Ac: solar collector surface area (m2) Cp: heat capacity of fluid (J/ (kg K)) Dh: hydraulic diameter (m) G: solar irradiance (W/m2) h: heat transfer coefficient (W/(m2 K)) k: thermal conductance (W/(m K)) M: the main segment of the collector m: nodes number with respect to x axis m_ fr : fluid mass flow rate (kg/s) Pele: instantaneous electrical power produced by the PV/T system. qu: rate of useful energy flux (W/m2) Re: Reynolds number S: the effective thermal irradiation (W/m2) T: temperature (K) U: overall heat loss coefficient (W/(m2 K)) v: speed (m/s) Dx: sub segment spacing in the x-direction. DyPV: the width of the sub segment for PV laminate nodes. Dybs: the width of the sub segment for back surface nodes. Dybp: the width of the sub segment for back panel nodes. Dytube: the tube length for a segment.

a: ambient bs: back surface of Tedlar bp: back plate/panel c: solar cell cvw,m: convection due to wind from the PV laminate node cvbsf1,m: convection from the back surface node to the air node cvbsf2,m: convection from the back surface node to the water node f1: fluid 1 (air) f2: fluid 2 (water) g: PV panel glass cover/tempered glass i: input inst: instantaneous o: output pv: solar cells of PV module pvbs,m: conduction from the PV laminate node to the back surface of Tedlar node pvg,m: conduction from the PV laminate node to the PV glass node rbsbp,m: radiation from the back surface of Tedlar node to the back panel node. rpvsky,m: radiation to the sky from the PV laminate node T: Tedlar t: top w: wind

Greek Letters

m,: dynamic viscosity of fluid d: thickness (m)

Abbreviations ave: average PF: packing factor