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Performance analysis of photovoltaic thermal (PVT) water collectors

Ahmad Fudholi a,, Kamaruzzaman Sopian a, Mohammad H. Yazdi a, Mohd Hafidz Ruslan a,Adnan Ibrahim b, Hussein A. Kazem c

a Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysiab Universiti Kuala Lumpur Institute of Product Design and Manufacturing (UniKL IPROM), 56100 Cheras, Kuala Lumpur, Malaysiac Faculty of Engineering-Sohar University, PO Box 44, Sohar PCI 311, Oman

a r t i c l e i n f o

Article history:

Received 21 June 2013

Accepted 11 November 2013

Available online 18 December 2013

Keywords:

Electrical performance

Thermal performance

Photovoltaic thermal (PVT)

PVT performance

Primary-energy saving efficiency

a b s t r a c t

The electrical and thermal performances of photovoltaic thermal (PVT) water collectors were determined

under 500800 W/m2 solar radiation levels. At each solar radiation level, mass flow rates ranging from

0.011 kg/s to 0.041 kg/s were introduced. The PVT collectors were tested with respect to PV efficiency,

thermal efficiency, and a combination of both (PVT efficiency). The results show that the spiral flow

absorber exhibited the highest performance at a solar radiation level of 800 W/m2 and mass flow rate

of 0.041 kg/s. This absorber produced a PVT efficiency of 68.4%, a PV efficiency of 13.8%, and a thermal

efficiency of 54.6%. It also produceda primary-energy savingefficiency ranging from 79% to 91% at a mass

flow rate of 0.0110.041 kg/s.

2013 Elsevier Ltd. All rights reserved.

1. Introduction

The idea of combining photovoltaic (PV) and solar thermal col-

lector to provide electrical and heat energy is not new, yet it has

received limited attention. Growing concern about energy sources

and their usage has consequently increased interest in photovol-

taic thermal (PVT) solar collectors. PVT solar collectors, which basi-

cally combine the functions of a flat plate solar collector and a

photovoltaic panel, convert solar radiation directly into both elec-

trical and thermal energies. Research on PVT started during the

mid-1970s focused on PVT collectors, with the primary aim of

increasing PV efficiency. Domestic application was regarded as

the main market. Initially the focus was on air- and water-based

glazed collectors. Given these problems, the cost of a complete

PVT system is incredibly high and therefore unaffordable for

industrial and residential owners. One of the most attractive

applications of air- or water-based PVT collectors is the building-

integrated photovoltaic thermal (BIPVT) system, which has under-

gone rapid development in recent years. However air-based PVT

systems have undergone more developed. The PVT system has

potential in generating both type of energies because of its higher

reliability and lower environment impact. Generally, a water-

based PVT system consists of a PV module, an absorber collector

in the form of tubes, a transparent glass cover, and an insulated

container. Over the next few years, BIPVT publications are

expected to increase, and PVT products are expected to undergo ra-

pid growth[13].Several studies on PVT solar collectors have been conducted.

Fig. 1shows PVT water collector with glass cover. The purpose of

the transparent cover, firstly to reduce the conduction losses from

the absorber collector through the restraint of the stagnant air

layer between the absorber collector and the glass and secondly

to reduce the radiation losses from the collectors. As shown in

Fig. 3, produced a hybrid PVT systems consist of PV modules made

from polycrystalline and amorphous solar cells with heat extrac-

tion unit mounted together using the copper sheet and pipes con-

cept. The application aspects in the industry of PVT systems with

water heat extraction has been studied thoroughly and analyzed

with TRNSYS program. The study includes the industrial process

heat system that operated at two different (load supply) tempera-

tures. The result shows that the electrical production using poly-

crystalline solar cell is more than when using amorphous solar

cells but in term of solar thermal fraction gives slightly lower re-

sults[3].

Theoretically analyses were based on a modified HottelWhil-

lier model, and the results were validated using experimental data

from a prototype PVT collector[4]. The effects of design parame-

ters, such as fin efficiency, thermal conductivity between the PV

cells and their supporting structure, and lamination method, on

both the electrical and thermal efficiencies of the PVT were also

determined. Furthermore, PVT can be prepared using of lower cost

materials, such as precoated color steel, without significantly

decreasing the efficiency. Integration of PVT into rather than onto

0196-8904/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.enconman.2013.11.017

Corresponding author.

E-mail address: [email protected](A. Fudholi).

Energy Conversion and Management 78 (2014) 641651

Contents lists available at ScienceDirect

Energy Conversion and Management

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a building can also lower the system cost. In one study on a water-

based PVT system, a numerical model of wall-mounted PVT water

collector systems was developed by modifying the HottelWhillier

model, which was originally used for the thermal analysis of flat-

plate solar thermal collectors. Recently, performance analysis

was conducted to analyze the exergy of PVT. The performance

and life cycle cost of PVT systems with PV technology different

from that of a similar PVT systemwere evaluated. The results showthat the use of PVT systems is generally advantageous over that of

similar PVT systems both from the efficiency and economic point

of view. Mono-crystalline silicon PVT systems have higher energy

and exergy efficiencies and are suitable for applications that have

higher energy and exergy demands or have limited space for

mounting, such as in multistory buildings[5].

A computer simulation was performed to analyze the system

performance. The combined effects of solar cell packing factor

and water mass flow rate on the electrical and thermal efficiencies

were investigated. The simulation results showed that the increase

in working fluid mass flow rate is beneficial for PV cooling. How-

ever, the advantage brought by the increased flow rate diminishes

when the critical flow rate is exceeded, thereby decreasing thermal

efficiency. System operation at the optimum mass flow rate cannot only improve the thermal performance of the system but also

meet the PV cooling requirement to achieve higher electrical per-

formance [6]. A centralized PV and hot-water collector wall system

mounted at vertical facades was experimentally studied [7]. The

results showed that the thermal efficiency was 38.9% at reduced

(zero) temperature and electrical efficiency was 8.56% during late

summer. A dynamic simulation model of a PVT and water heating

system was developed. This modeling approach was validated by

comparison with experimental data[8]. The results showed that

the electrical performance is affected by on-site shading. Moreover,

the output from the model showed high agreement with the exper-imental observations.

A computer simulation of a water-based PVT solar collector

system using energy models was developed. Higher economical

advantages relative to that of a conventional PV system were ob-

tained. The annual average thermal and cell conversion efficiencies

of a specific PVT system, which was mounted on a vertical wall of a

fully air-conditioned building with collectors equipped with a flat-

box-type thermal absorber and polycrystalline silicon cell, were

37.5% and 9.39%, respectively, compared with the normal building

faade[9]. A computational fluid dynamic (CFD) model for a novel

PVT collector was developed and experimentally validated [10].

The results indicated that PV cell efficiency can be increased to

5.3% and the outlet water temperature of the collector is suitable

for domestic hot-water use. The effect of flow distribution on thePV performance of a PVT water collector was also investigated

[11]. The results showed that parameters such as the manifold-

to-riser pipe ratio, array geometry, manifold flow direction, and

mass flow rate affect the flow distribution, which, in turn, affects

PV conversion.

Innovative applications of PVT collector were performed re-

cently [1218]. PVT applications are cost-effective solar energy

applications. However, additional studies must still be conducted,

particularly on the design of new thermal absorber collectors.

Alternative designs of PVT solar collectors are presented in this pa-

per. A prototype of this new absorber was constructed. To date,

studies on water-based PVT collectors have been few. Therefore,

further experimental and analytical research should be performed

to improve the electrical and thermal performance of water-basedPVT solar collectors using new absorber collector designs.

Nomenclature

Ac frontal area solar collector (m2)

b collector width (m)Cb conductance of the bond between the fin and square

tubeCp specific heat of working fluid (J/kg C)

D diameter (m)Dh hydraulic diameter (m)F fin efficiency factorF0 collector efficiency factorFR heat removal efficiency factorGT solar radiation at NOCT (W/m

2)hfi heat transfer coefficient of fluid (W/m

2C)

k thermal conductivity (W/m C)L tube length (m)l thickness (m)_m mass flow rate (kg/s)

N number of glass covern number of tubep collector perimeter (m)Qu actual useful heat gain (W)

S solar radiation (W/m2)T temperature (C)UL overall heat transfer coefficient (W/m

2C)

Ut top loss coefficient (W/m2

C)

v wind velocity (m/s)W tube spacing (m)a absorptanceh collector tilte emittance

s transmittanceg efficiencyr Stefans Boltzmann constant (W/m2C4)

Subscriptsa ambientabs absorber thicknessc cellfi inlet fluidg glassi inleto outletp platepm mean platePV photovoltaicPVT photovoltaic thermalr referencet tubew wind

Fig. 1. PVT water collector with glass cover[3].

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2. Materials and methods

The fabricated PVT water collector was tested at the Solar

Energy Laboratory, Faculty Science and Technology, Universiti Ke-

bangsaan Malaysia. The solar simulator at this laboratory consists

of 23 units of halogen tungsten lamps (Brillanta), with an output

power of 500 W each. The lamps are arranged in nine columns

with each column.Indoor testing is important because in the laboratory, solar radi-

ation can be controlled, and accurate measurements can be ob-

tained. The control parameters for indoor tests include the input

and output temperatures, ambient temperature, wind velocity at

the collector surface, useful current and voltage, and water flow

rate inside the tube. These parameters were matched to the stan-

dards set for all PVT absorber collectors.

2.1. New design of absorbers

Fig. 2shows three PVT water collector designs. The parameters

of these fabricated design configurations are listed inTable 1. The

first collector (Fig. 2a) is a web flow absorber, the second collector

(Fig. 2b) is direct flow absorber, and the third collector (Fig. 2c) isspiral flow absorber.Fig. 3shows the example of a complete PVT

water collector design with spiral flow absorber. The absorber col-

lectors consisted of either rectangular or round hollow stainless-

steel tubes. The tubes were connected using a tungsten inert gas

welding method. Nipples were welded to the rear surface for man-

ifold attachment. Silicone was used to seal the ends and to glue the

top absorber sheet into place. The absorber collector, which con-

sists of a single unilateral channel for water to flow, was inserted

underneath the standard PV module 1 m high, 0.65 m long, and

0.3 m thick. The thermal insulator was packed beneath the absor-

ber collector to prevent heat from escaping further and to provide

more uniform temperatures throughout the system. A standard PV

module, represented as a flat-plate single glazing sheet of polycrys-

talline silicon, was laminated and bonded using a high-tempera-ture silicone adhesive and sealant. Once sealed and watertight,

the absorber collectors were attached to be the bottom side of

the PV module and then encapsulated in a polyvinyl resin to form

a complete PVT system 815 mm628 mm30 mm in size. The

absorber collectors were designed in the form of a continuous coil

or configured tube and consist of at least one inlet and outlet to al-

low the medium (water) to enter and exit a coil, respectively. The

inlet and outlet nipples were arranged to allow the medium

(water) to flow in and out and cover the entire PV module. The con-figuration is as follows: a low-temperature (cold) medium (water)

enters the coil, flows in and out, and leaves the absorber collector

as hot water, which can be consumed or stored for later use. In this

manner, solar energy can be fully utilized. Normally, in a PVT sys-

tem, the hot-water storage tank is located near the ground level,

whereas the solar module is mounted on the roof. In this PVT sys-

tem, the hot-water storage was located as near as possible to the

collector to maintain the water pressure from the pump to the col-

lector and vice versa. The hot-water storage was connected to the

flat-plate collector through sets of pipes. The water was circulated

by a pump. The pipes were assumed to be well-insulated to pre-

vent heat loss. Energy was absorbed only by the collector unit.

The energy absorbed by the collector was used in heating the

water. Only the water storage tank was assumed to lose energy.The inlet water temperature of the collector was assumed to be

the same as the mean water temperature in the storage tank.

Fig. 2. (a) Web flow absorber, (b) direct flow absorber and (c) spiral flow absorber.

Fig. 3. PVT water collector design with spiral flow absorber.

Table 1

The parameters configurations of PVT solar collectors.

Absorber type Unit

Web flow (Fig. 2a) Absorber material: round hollow tubes of stainless steel

Absorber collector module: 1 channel each of size 12.7 mm1 mm1000 mm (length) and 640 mm (width)

Method of joining: welding

Inlet/outlet no: 2

Direct flow (Fig. 2b) Absorber material: rectangular hollow tubes of stainless steel

Absorber collector module: 19 channel each of size 12.7 mm12.7 mm1 mm1000 mm (length) and 640 mm (width)


Inlet/outlet no: 4

Spiral flow (Fig. 2c) Absorber material: rectangular hollow tubes of stainless steel

Absorber collector module: 1 channel each of size 12.7 mm12.7 mm1 mm700 mm (length) and 640 mm (width)


Inlet/outlet no: 4

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The temperature of the water entering the tank was assumed to be

the same as the outlet temperature of the collector.

2.2. Method

The three PVT water collectors were tested in a laboratory

atmosphere at temperatures and solar radiation levels similar to

those used during the PV module testing. The solar collector wastested using a simulator based on the testing procedures of ISO

9806-1: 1994 and the ANSI/ASHRAE Standard 93: 2003. The testing

procedures include the following features:

(i) The testing procedure can be used to evaluate the perfor-

mance of PVT solar collectors.

(ii) The thermal performance of the collectors can be deter-

mined partly by obtaining the instantaneous efficiencies of

different combinations of incident solar radiation, ambient

temperature, and inlet fluid temperature.

(iii) The test measures the rate of the incident solar radiation

falling onto the solar collector as well as the rate of energy

addition to the transfer fluid as it passes through the

collector.(iv) The testing should be conducted under steady-state or

quasi-steady-state conditions.

The PVT collectors were exposed to solar radiation of 500, 600,

700 and 800 W/m2. When the load was applied to the collectors,

the changes in current and voltage were recorded for each solar

radiation level. During this testing, mass flow rate of 0.011,

0.024, 0.032 kg/s, and 0.041 kg/s were set. Any changes in the col-

lector due to these mass flow rates were recorded. Data were col-

lected and stored every minute using a data-acquisition system,

and these data were subsequently used to calculate the PV effi-

ciency and thermal efficiency of the collector. The water inlet

and outlet during testing were controlled and connected back to

the storage tank to form a close-loop system. The complete mea-

suring setup for the PVT collector is shown in Fig. 4.

3. Analysis of the PVT solar collectors

The performance of the PVT collectors can be expressed by a

combination of efficiency expressions [19] consisting of thermal

efficiency (gth) and electrical efficiency (gPV). These efficiencies

usually include the ratio of the useful thermal gain and electrical

gain of the system to the incident solar irradiation on the collector

gap within a specific time or period. The analytical parameters of

the PVT collector are presented inTable 2. The total efficiencies,

known as total efficiency or PVT efficiency (gPVT), are used to eval-uate the overall performance of the system [1921]:

gPVT gth gPV 1

Electrical energy (gf) is a high-grade form of energy gain, theprimary energy-saving efficiency is proposed as another perfor-

mance indicator of the energy-grade difference between electricity

and thermal energy and is given by[1921]:

gf

gPVgp gth 2

where gp is the electric-power generation efficiency of a conven-tional power plant; its value can be taken as 38%. The evaluation

indicator of the primary energy-saving efficiency also considers

the quality and quantity of the energy that the PVT system converts

into solar energy.

Fig. 4. Schematic diagram of IPVT water collector.

Table 2

PVT solar collector characteristics.

Description Symbol Value Unit

Ambient temperature Ta 20 C

Collector area Ac 0.65 m2

Number of glass cover N 1

Emittance of glass eg 0.88

Emittance of plate ep 0.95

Collector tilt h 14 Fluid thermal conductivity kf 0.613 W/m C

Specific heat of working fluid Cp 4180 J/kg C

Back insulation conductivity kb 0.045 W/m C

Back insulation thickness lb 0.05 m

Insulation conductivity ke 0.045 W/m C

Edge insulation thickness le 0.025 m

Absorber conductivity kabs 51 W/m C

Absorber thickness labs 0.002 m

Fin conductivity kf 84 W/m C

Fin thickness d 0.0005 m

Heat transfer coefficient from cell to absorber hca 45 W/m C

Heat transfer inside tube hfi 333 W/m C

Transmittance s 0.88 Absorptance a 0.95


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3.1. Analysis of flat plate PVT collector

The thermal performance of PVT is affected by a number of sys-

tem design parameters and operating conditions. In this study, the

system was analyzed using various configurations of solar radia-

tion, ambient temperature, and flow rate conditions. The collector

was assumed to be represented as a flat-plate collector with a sin-

gle glazing sheet. Based on this assumption, the thermal and PVperformance of the PVT unit was evaluated by the deriving the effi-

ciency parameters based on the HottelWhillier equations [22].

The thermal efficiency of a conventional flat-plate solar collector

is the ratio of the useful thermal energy (Qu) to the overall incident

solar radiation (S) and can be expressed as:

gthQuS

3

The useful collected heat absorbed by the flat-plate solar collec-

tor can be given as the combined results of the average mass flow

rate _m, heat capacity of flowing medium (Cp) and temperature

difference at the collector inlet (Ti) and outlet (To) and can be ex-

pressed as:

Qu

_mCp

ToT

i 4

The difference between the absorber solar radiation and ther-

mal heat losses is determined using the HottelWhillier equation

[23]:

Qu AcFRGTsaPVULTiTa 5

where Ac is the collector area, Ta is the ambient temperature, Ti is

the inlet temperature, UL is the overall collector heat loss, (sa)PVis the PV thermal efficiency, GTis the solar radiation at NOCT (radi-

ation level 800 W/m2, wind velocity 1 m/s, and ambient tempera-

ture 26 C), andFR is the heat removal efficiency factor introduced

[22,23]. This factor is expressed as follows:

FR _mCpAcUL

1 exp AcULF

0

_mCp 6whereF0 is the collector efficiency factor, which is calculated using

F0 1UL

ULDh W DhF

" #

1

Cb

1

2abhfi7

where a is the duct width, b is the duct height, Cb is the conductance

of the bond between the fin and square tube,hfiis the heat-transfer

coefficient of the fluid,Dhis the hydraulic diameter, andFis the fin

efficiency factor given by

Ftanh MWDh

2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiMWDh

2

q 8

where

Dh 2ab

ab 9

The coefficientMin Eq.(8)considers both the thermal conduc-

tivity of the absorber and the PV cell. Mis calculated using[11,24]

M

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiUL

kabslabskPVlPV

s 10

where kabsis the absorber thermal conductivity, labsis the absor-

ber thickness, kPVis the PVT conductivity, and lPVis the PV collector

thickness. The overall loss coefficient (UL) of the collector is the

sum of the edge (Ue) and top (Ut) loss coefficients and can be ex-

pressed as

UL UeUt 11

Ue kepl

LeAc12

Ut N

CTpm

TpmTaNf

h ie 1hw8>:

9>=

>;

1

rTpmTa T

2pmT

2a

ep 0:00591Nhw

1

2Nf10:133epeg

N13

where

C 5201 0:000051b2 14

f 1 0:089hw 0:1166hwep1 0:07866N 15

e 0:43 1 100

Tpm

16

Tpm TiQ=Ac

FRUL1 FR 17

wherep is the collector perimeter, Nis the number of glass covers,ris the StefanBoltzmann constant,epis the plate emittance,egis the

glass emittance,b is the collector tilt,Tpmis the mean plate temper-

ature, andhw is the wind heat-transfer coefficient.

The heat transfer coefficients such as the forced convection (hw)

can be calculated using Eq. (18) [25], whereas the natural heat

transfers (hnat) can be calculated using Eq.(19)[25], as follows:

hw 2:8 3:0v 18

hnat 1:78TpmTa 19

A combination of the natural and forced convection heat trans-

fer (Eqs. (18) and (19)) determines the overall convection heattransfer (hc) and possibly the overall top loss heat-transfer coeffi-

cient for the collector[26].

hc

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffih

3wh

3nat

q 20

The useful heat gain produced by the PVT collector can then be

calculated using Eqs. (320). By rearranging Eq. (3), the thermal

efficiency of the collector is expressed as [27]

gth FRsaPVFRULTiTaGT

21

3.2. Principle of the PV module

The electrical efficiency of the PV module (gPV), which is a func-tion of module temperature, is given by [3,28]

gPV gr1 cTcTr 22

where gris the reference efficiency of the PV module (gr= 0.12),cisa temperature coefficient (c= 0.0045 C),Tcis the cell temperature,andTris the reference temperature.

4. Results and observations

The performance and efficiency of the PVT collectors are deter-

mined by their electrical and thermal characteristics. The analyses

of the PVT collectors are segregated into three sections, namely PVefficiency, thermal efficiency, and combination of both.

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4.1. Effects of the PV module temperature on the PVT collector PV

efficiency

Figs. 58and simplified inTables 35show the analysis results

of the PVT collectors after exposure to 500800 W/m2 of solar radi-

ation at 0.0110.041 kg/s mass flow rates. The results show that

the PV efficiencies of the collectors significantly changed under

various PV temperature and mass flow rates. Among the basicPVT absorber collectors, the web system at 500 W/m2 exhibited a

decreased PV efficiency of 0.35%, from 11.42% at 47.88 C to

11.07% at 50.24 C. At 600 W/m2, the total PV efficiency increased

to 0.39% but showed an average decrease from 11.94% at

48.57 C to 11.55% at 51.65 C. At 700 W/m2, the total PV efficiency

increased by 0.42% from 12.13% at 49.65 C to 11.71% at 52.85 C.

When the solar radiation further increased to 800 W/m2, the total

PV efficiency increased by 0.46% from 12.37% at 50.03 C to 11.91%

at 53.54 C. Meanwhile, the change trend of direct flow is nearly

similar to that of web flow, with the total PV efficiency increasing

by 0.37% from 11.78% at 47.28 C to 11.41% at 50.15 C. When the

solar radiation was increased to 600 W/m2, the efficiency further

increased by 0.42, from 12.18% at 48.5 C to 11.76% at 51.75 C.

At 700 W/m2, the total efficiency increased by 0.45% from 12.38%

at 49.22 C to 11.93% at 52.95 C. Further increase in the solar radi-

ation to 800 W/m2 resulted in a 0.5% increase in the total PV effi-

ciency, from 12.69% at 49.89 C to 12.19% at 53.64 C. On the

other hand, the spiral flow exhibited a 0.53% increase in the total

PV efficiency, from 12.52% at 46.24 C to 11.99% at 50.86 C. When

the solar radiation was further increased to 600 W/m2, the total PV

efficiency increased by 0.58% from 13.07% at 46.84 C to 12.49% at

51.95 C. At 700 W/m2, the total PV efficiency increased by 0.63%

from 13.47% at 47.64 C to 12.84% at 52.83 C. Finally, at 800 W/

m2, the PV efficiency increased by 0.63%. This inclination is result-

ing from the average efficiency, which dropped due to the cooling

effect of the collector. The data collected shows when the temper-

ature increased, mass flow rate dropped, the efficiency dropped

simultaneously. As result, the collector efficiency dropped from

13.81% at 48.03 C to 13.01% at 53.35 C.

4.2. Effect of mass flow rate on the PVT collectors

The mass flow rate through the collectors and into the desig-

nated channels indirectly affects PV module cooling. The effects

of the mass flow rate on the absorber collectors are shown in

Figs. 911. The mass flow rates used in this analysis (0.011

0.041 kg/s) were later applied under various solar radiation levels.

The results show that increasing the mass flow rate simultaneously

decreased the PV temperature of the PVT collectors at all solar radi-

ation levels. At the same mass flow rate, the PV temperatures in-

creased in the solar radiation level. Figs. 59 show that from

0.011 kg/s to 0.041 kg/s mass flow rates and under 500 W/m2 solar

radiation, the temperature decreased from 50.20 C to 47.76 C and

PV efficiency simultaneously increased from 11.07% to 11.42%.

When solar radiation was increased to 800 W/m2, temperature

dropped from 53.5 C to 50 C, whereas PV efficiency increased

from 11.91% to 12.37%. For the direct flow absorber as (Fig. 10)

and referring to Figs. 58, at solar radiation of 500 W/m2 and when

mass flow rate increased from 0.011 kg/s to 0.041 kg/s, PV temper-

ature dropped from 50.11 C to 47.18 C, and PV efficiency in-

creased from 11.41% to 11.78%. The same result was obtained

when the solar radiation increased to 800 W/m2: temperature de-

creased from 53.6 C to 49.8 C, whereas PV efficiency increased

from 12.19% to 12.69%.

For the spiral flow absorber collector (Figs. 58 and 11), PVtem-

perature decreased from 50.76 C to 46.2 C, and PV efficiency in-creased from 11.99% to 12.52% at a solar radiation of 500 W/m2

Fig. 5. Changes in PV efficiency with the mean PV temperature of the PVT absorbercollectors under 500 W/m2 of solar radiation.

Fig. 6. Changes in PV efficiency with the mean PV temperature of the PVT absorber

collectors under 600 W/m2 of solar radiation.






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with the mass flow rate increased from 0.011 kg/s to 0.014 kg/s. At800 W/m2 solar radiation level, PV temperature was 53.3 C and

decreased to 48C, whereas PV efficiency increased from 13.01%

to 13.81%.

Table 3

Results of PV efficiency (gPV) and PV temperature (TPV) for web flow under various mass flow rates and solar radiations.

500 W/m2 600 W/m2 700 W/m2 800 W/m2

_m(kg/s) TPV(C) gPV(%) TPV(C) gPV(%) TPV(C) gPV(%) TPV(C) gPV(%)

0.011 50.24 11.07 51.65 11.55 52.85 11.71 53.54 11.91

0.013 49.93 11.12 51.04 11.62 52.33 11.78 53.03 12.00

0.016 49.65 11.18 50.63 11.69 51.95 11.82 52.65 12.07

0.020 49.26 11.22 50.26 11.75 51.37 11.93 52.06 12.15

0.024 48.83 11.27 49.76 11.80 50.89 11.98 51.69 12.20

0.027 48.58 11.30 49.59 11.84 50.69 12.01 51.17 12.22

0.029 48.46 11.32 49.27 11.87 50.45 12.02 51.05 12.26

0.032 48.27 11.35 49.09 11.90 50.22 12.04 50.84 12.28

0.035 48.09 11.38 48.87 11.92 50.01 12.07 50.55 12.32

0.038 47.90 11.40 48.76 11.93 49.82 12.10 50.14 12.35

0.041 47.88 11.42 48.57 11.94 49.65 12.13 50.03 12.37

Table 4

Results of PV efficiency (gPV) and PV temperature (TPV) for direct flow under various mass flow rates and solar radiations.

500 W/m2 600 W/m2 700 W/m2 800 W/m2


0.011 50.15 11.41 51.75 11.76 52.95 11.93 53.64 12.19

0.013 50.04 11.45 51.34 11.84 52.54 11.99 53.25 12.240.016 49.56 11.50 50.86 11.90 51.93 12.04 52.86 12.30

0.020 48.84 11.56 50.47 11.96 51.36 12.13 52.13 12.41

0.024 48.53 11.60 49.95 12.01 50.74 12.18 51.56 12.49

0.027 48.17 11.62 49.63 12.05 50.57 12.23 51.04 12.52

0.029 48.35 11.65 49.46 12.09 50.15 12.25 51.03 12.55

0.032 48.06 11.68 49.38 12.12 49.94 12.26 50.66 12.59

0.035 47.85 11.70 48.99 12.14 49.63 12.31 50.23 12.62

0.038 47.67 11.74 48.76 12.15 49.47 12.35 50.06 12.66

0.041 47.28 11.78 48.50 12.18 49.22 12.38 49.89 12.69

Table 5

Results of PV efficiency (gPV) and PV temperature (TPV) for spiral flow under various mass flow rates and solar radiations.

500 W/m2 600 W/m2 700 W/m2 800 W/m2


0.011 50.86 11.99 51.95 12.49 52.83 12.84 53.35 13.01

0.013 49.94 12.06 51.13 12.53 52.15 12.90 52.73 13.20

0.016 49.33 12.15 50.44 12.66 51.23 13.00 52.05 13.33

0.020 48.72 12.22 49.53 12.75 50.26 13.14 50.83 13.44

0.024 48.05 12.27 48.76 12.84 49.73 13.22 50.25 13.56

0.027 47.82 12.32 48.43 12.90 49.21 13.29 49.73 13.63

0.029 47.40 12.36 48.06 12.92 48.86 13.31 49.26 13.66

0.032 47.07 12.40 47.82 12.94 48.48 13.36 48.97 13.71

0.035 46.74 12.44 47.58 13.00 48.28 13.42 48.68 13.75

0.038 46.43 12.49 47.15 13.05 47.86 13.44 48.34 13.79

0.041 46.24 12.52 46.84 13.07 47.64 13.47 48.03 13.81

Fig. 9. Changes in PV temperature of web flow absorber with the mass flow rates

under different solar radiation levels. Fig. 10. Changes in PV temperature of the direct flow absorber with the mass flow

rate under different solar radiation levels.


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4.3. Thermal performance of PVT collectors

Fig. 12 and Table 6 show the increase in thermal efficiency with

the mass flow rates of the PVT collectors under 800 W/m2 of solarradiation. The web flow absorber exhibited an increase in the ther-

mal efficiency from 41.11% to 48.07% as a result of the increased

mass flow rate from 0.011 kg/s to 0.041 kg/s. The other absorbers

exhibited the same trend. The direct flow absorber exhibited an in-

creased thermal efficiency from 46.43% to 54.13% when the mass

flow rate increased from 0.011 kg/s to 0.041 kg/s. The highest ther-

mal efficiency was produced by the spiral flow absorber, which

exhibited an increase from 58.01% to 68.42% when the mass flow

rate increased from 0.011 kg/s to 0.041 kg/s. Fig. 13andTable 7

show that the outlet temperature of the collectors decreased with

increased mass flow rates under 800 W/m2 of solar radiation. The

outlet temperature of the web flow absorber dropped from

63.34 C to 49.54 C, whereas those for the direct flow and spiral

flow absorbers dropped from 62.57 C to 50.32 C and 65 C to

52.4 C, respectively.

4.4. PVT performance of PVT water collectors

The performance of the PVT collectors can be represented by a

combination of efficiency expressions, which consists of electrical

efficiency and thermal efficiency. The sum of both efficiencies,

known as the PVT efficiency, is used to evaluate the overall perfor-

mance of the system. Based on the testing performed on the collec-

tors, both efficiencies increased when the mass flow rate increased.

Therefore, the total efficiency (PVT efficiency) and primary energy-

saving efficiency increased concurrently when the mass flow rate

increased.

Figs. 1417 and Table 6 show PV, thermal, PVT, and primary en-

ergy-saving efficiencies, respectively, of the web flow, direct flow,

and spiral flows at 0.0110.041 kg/s mass flow rates and 800 W/

m2 solar radiation. The PVT efficiency of the web, direct, and spiral

flow absorbers further increased by 7.4%, 8.2%, and 10.4%, respec-

tively, and their primary energy-saving efficiency increased by

8.2%, 9%, and 11.7%, respectively. The web flow absorber exhibited

an increase PVT efficiency from 53.0% to 60.44% when the mass

flow rate increased from 0.011 kg/s to 0.041 kg/s. The other

absorbers exhibited the same trend. The direct flow absorber

showed an increased PVT efficiency from 58.62% to 66.82% when

the mass flow rate increased. The highest PVT efficiency obtained

was that of the spiral flow absorber, which exhibited an increase

from 58.01% to 68.42%.

The web flow absorber showed an increase in primary energy-

saving efficiency from 72.45% to 80.62%. Meanwhile, the direct

flow absorber exhibited an increased primary energy-saving

Fig. 11. Changes in PV temperature of the spiral flow absorber with the mass flow

rate under different solar radiation levels.

Fig. 12. Changes in thermal efficiency of 800 W/m2 solar radiation with the mass

flow rate for different flow absorbers.

Table 6

Results of efficiencies at different mass flow rates under 800 W/m 2 of solar radiation.

Web flow Direct flow Spiral flow_m(kg/s) Efficiencies (%) Efficiencies (%) Efficiencies (%)

gth gPV gPVT gf gth gPV gPVT gf gth gPV gPVT gf

0.011 41.11 11.91 53.02 72.45 46.43 12.19 58.62 78.51 45.00 13.01 58.01 79.24

0.013 42.66 12.00 54.66 74.24 48.53 12.24 60.77 80.74 48.49 13.20 61.69 83.23

0.016 43.98 12.07 56.05 75.74 50.01 12.30 62.31 82.38 49.64 13.33 62.97 84.72

0.020 46.66 12.15 58.81 78.63 50.98 12.41 63.39 83.64 51.05 13.44 64.49 86.42

0.024 47.03 12.20 59.23 79.14 52.47 12.49 64.96 85.34 51.73 13.56 65.29 87.41

0.027 47.25 12.22 59.47 79.41 52.67 12.52 65.19 85.62 51.95 13.63 65.58 87.82

0.029 47.37 12.26 59.63 79.63 52.84 12.55 65.39 85.87 52.07 13.66 65.73 88.02

0.032 47.52 12.28 59.80 79.84 52.97 12.59 65.56 86.10 52.25 13.71 65.96 88.33

0.035 47.66 12.32 59.98 80.08 53.08 12.62 65.70 86.29 52.38 13.75 66.13 88.56

0.038 47.79 12.35 60.14 80.29 53.19 12.66 65.85 86.51 52.51 13.79 66.30 88.80

0.041 48.07 12.37 60.44 80.62 54.13 12.69 66.82 87.52 54.61 13.81 68.42 90.95

Fig. 13. Changes in outlet temperature of 800 W/m2 solar radiation with the mass

flow rates for different flow absorbers.


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efficiency from 78.51% to 87.52%. The highest primary-energy sav-

ing efficiency was exhibited by the spiral flow absorber, which

showed an increase from 79.24% to 90.95%.

Figs. 1820 and Table 7 show inlet, outlet, and PV temperatures,

respectively, of the web flow, direct flow, and spiral flows at

0.0110.041 kg/s mass flow rates and 800 W/m2 solar radiation.

The results show that increasing the mass flow rate simultaneously

decreased the temperatures (inlet, outlet and PV) of the PVT

collectors.

4.5. Comparison with other absorber collector designs

A performance evaluation was conducted to investigate theintegration of PV and thermal system. The performance of a

conventional solar water heater, which is a PVT system known as

the integrated photovoltaic/thermal system was studied. The re-sults showed that the solar PVT collector made from a corrugated

polycarbonate module produced high thermal efficiency. Huang

et al. [29]suggested that further improvements can be achieved

by using proper insulation for the PVT design. They managed to

achieve a PV efficiency of 9%, a thermal efficiency of 38%, and a to-

tal efficiency (PVT efficiency) of 47%.

A PVT collector system for domestic application in China was

developed [30,31]. In this experiment, an aluminum-alloy flat

box with a square or rectangular channel was designed and

constructed. The test results showed the high efficiency of the

Table 7

Results of temperatures at different mass flow rates under 800 W/m2 of solar radiation.

Web flow Direct flow Spiral flow

_m(kg/s) Ti (C) To (C) ToTi (C) TPV(C) Ti (C) To (C) ToTi (C) TPV(C) Ti (C) To (C) ToTi (C) TPV(C)

0.011 51.00 63.34 12.34 53.54 50.90 62.57 11.67 53.64 54.00 65.00 11.00 53.35

0.013 48.32 60.39 12.07 53.03 47.22 58.67 11.45 53.25 47.32 61.00 13.68 52.73

0.016 46.32 58.25 11.93 52.65 45.00 56.36 11.36 52.86 45.78 59.04 13.26 52.05

0.020 41.04 52.90 11.86 52.06 43.78 55.08 11.30 52.13 43.63 56.97 13.34 50.83

0.024 40.93 52.13 11.20 51.69 41.22 52.50 11.28 51.56 42.78 54.99 12.21 50.25

0.027 40.85 51.36 10.51 51.17 41.12 52.00 10.88 51.04 42.64 54.84 12.20 49.73

0.029 40.81 50.59 9.78 51.05 40.96 51.50 10.54 51.03 42.58 54.72 12.14 49.26

0.032 40.79 50.12 9.33 50.84 40.91 50.93 10.02 50.66 42.42 54.54 12.12 48.97

0.035 40.72 49.85 9.13 50.55 40.88 50.63 9.75 50.23 42.34 53.78 11.44 48.68

0.038 40.64 49.70 9.06 50.14 40.82 50.48 9.66 50.06 42.25 53.18 10.93 48.34

0.041 40.20 49.54 9.34 50.03 39.00 50.32 11.32 49.89 42.09 52.40 10.31 48.03

Fig. 14. Changes in web flow absorber efficiency with mass flow rate under 800 W/

m2 of solar radiation.

Fig. 15. Changes in direct flow absorber efficiency with mass flow rate under

800 W/m2

of solar radiation.

Fig. 16. Changes in spiral flow absorber efficiency with mass flow rate under

800 W/m2 of solar radiation.

Fig. 17. Comparison of the PVT and primary energy-saving efficiencies of theabsorbers at different mass flow rates under 800 W/m2 of solar radiation.


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combined system, with a nearly 65% primary energy saving for dai-ly exposure at reduced (zero) temperature operation. Chow et al.

[30,31]managed to achieve a PV efficiency of 11%, a thermal effi-

ciency of 51%, and a total efficiency (PVT efficiency) of 62%. A sim-

ilar experiment was performed using an aluminum-alloy flat box

with a square or rectangular channel and polycrystalline silicone

cells, which utilized water as a [32]. The results showed that the

thermal efficiency reached 40% when the initial temperature in

the system matched the daily mean ambient temperature. He

et al.[32]managed to achieve a PV efficiency of 9.87%, a thermal

efficiency of 40%, and a total efficiency of 49.87%.

Another experiment on a natural-circulation hybrid PVT water-

heating system was conducted[33]. In this experiment, a sensitiv-

ity study of the system showed that by combining different

systems, the installation area produce more energy per unit surfacearea than a system consisting of one PV module and one hot-water

system. Ji et al. [33] managed to achieve a PV efficiency of 10.15%, a

thermal efficiency of 45%, and a total efficiency of 55.15%. Table 8

shows the summarized comparison results between the proposed

absorber configurations and other absorber collector designs

[20,2933]. According to the literature[20], the PVT water-based

collector can achieve a maximum electrical efficiency of around9.5% and a thermal efficiency of approximately 50%.

5. Conclusion

PVT water collector consisting of a combined PV module and an

absorber collector were investigated. The performances of three

PVT water collectors were determined. The results indicate that a

solar radiation level of 800 W/m2 and a mass flow rate of

0.041 kg/s, the spiral flow absorber produced a PVT efficiency of

approximately 65%, a PV efficiency of 13%, and a thermal efficiency

of 52%. It also exhibited a primary energy-saving efficiency of 79

91% at mass flow rates ranging of 0.0110.041 kg/s.

The results show that the efficiency of the PV module increase

when the temperature decrease. The decrease in temperature isnot linear with the mass flow rate increase. However, temperature

significantly decrease after the mass flow rate reaches 0.024 kg/s.

Overall, the efficiency of the PVT water collectors increases with

the mass flow rates under various solar radiation levels. This result

is due to the increase in the cooling factor of the PV module cells

when the mass flow rate increases. Therefore, mass flow rate

indirectly contributes to the increase in PVT water collector

temperature.

Acknowledgements

The authors would like to thank the Ministry of Science,

Technology, and Innovation Malaysia for funding this research

(Sciencefund 03-01-02-SF0039) and the Solar Energy Research

Institute (SERI), Universiti Kebangsaan Malaysia for providing the

laboratory facilities and technical support.

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