concentratin collector

Electronic Packaging

Xinqiang Xu EP-13-1103 1

Performance Analysis of a Combination System of Concentrating PV/T Collector

and TEGs Xinqiang Xu Department of Mechanical Engineering Binghamton University - SUNY Binghamton, NY, 13902 [email protected] Siyi Zhou Department of Mechanical Engineering Binghamton University - SUNY Binghamton, NY, 13902 [email protected] Mark M Meyers Applied Optics Lab GE Global Research Niskayuna, NY, 12309 [email protected] Bahgat G Sammakia Department of Mechanical Engineering Binghamton University - SUNY Binghamton, NY, 13902 [email protected] Bruce T Murray Department of Mechanical Engineering Binghamton University - SUNY Binghamton, NY, 13902 [email protected]

Journal of Electronic Packaging. Received September 11, 2013; Accepted manuscript posted July 23, 2014. doi:10.1115/1.4028060 Copyright (c) 2014 by ASME

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Downloaded From: http://electronicpackaging.asmedigitalcollection.asme.org/ on 09/08/2014 Terms of Use: http://asme.org/terms



ABSTRACT

Thermoelectric modules utilize available temperature differences to generate electricity by the Seebeck effect. The current study investigates the merits of employing thermoelectrics to harvest additional electric energy instead of just cooling concentrating photovoltaic (CPV) modules by heat sinks (heat extractors). One of the attractive options to convert solar energy into electricity efficiently is to laminate TE modules between CPV modules and heat extractors to form a CPV-TE/thermal hybrid system. In order to perform an accurate estimation of the additional electrical energy harvested, a coupled field model is developed to calculate the electrical performance of TE devices, which incorporates a rigorous interfacial energy balance including the Seebeck effect, the Peltier effect, and Joule heating, and results in better predictions of the conversion capability. Moreover, a 3D multiphysics computational model for the hybrid concentrating PV-TE/thermal (CPV-TE/T) water collector system consisting of a solar concentrator, 10 serially-connected GaAs/Ge PV cells, 300 couples of bismuth telluride TE modules, and a cooling channel with heat-recovery capability, is implemented by using the commercial FE–tool COMSOLTM. A conjugate heat transfer model is used, assuming laminar flow through the cooling channel. The performance and efficiencies of the hybrid system are analyzed. As compared with the traditional PV/T system, a comparable thermal efficiency and a higher 8% increase of the electrical efficiency can be observed through the PV-TE hybrid system. Additionally, with the identical convective surface area and cooling flow rate in both configurations, the PV-TE/T hybrid system yields higher PV cell temperatures but more uniform temperature distributions across the cell array, which thus eliminates the current matching problem; however, the higher cell temperatures lower the PV module’s fatigue life, which has become one of the biggest challenges in the PV-TE hybrid system.

INTRODUCTION

Currently, renewable sources of energy are being widely advocated as a

substitute for traditional fossil fuels, which are the main sources of greenhouse gas

emissions into the atmosphere. Photovoltaic (PV) systems represent one of the most

promising options, since solar energy is pollution-free and inexhaustible. Concentrating

sunlight onto PV cells, and the replacement of expensive photovoltaic area with less

expensive concentrating optics, such as mirrors or lenses, is a novel solution to reduce

the cost of solar electricity. Light concentration leads to a significant semiconductor

material saving by a much higher power density at the cell surface. However, a common

PV system converts only 10%-25% of the incoming solar radiation into electricity, which

means that much of the incident solar energy simply heats the PV cells. The high cell


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temperatures have two undesirable consequences: 1) a sharp drop in cell electrical

efficiency (generally 0.5% per degree C rise for Si cells [1]); 2) permanent structural

damage and shorter fatigue life of the modules [2]. Therefore, in order to achieve higher

electrical performance and longer lifetime, a concentrating photovoltaic (CPV) module

must be forcedly cooled, and simultaneously, the available thermal energy captured and

stored by coolant can be used for other useful applications. It is generally accepted that

hybrid concentrating photovoltaic/thermal (HCPV/T) systems [3-8] have higher and

more stable performance when compared to individual solar devices.

Furthermore, there is another technology for converting thermal energies into

electricity, namely: thermoelectric (TE) technology, which can operate from a low grade

heat source such as waste heat energy and has drawn increasing interest.

Thermoelectric conversion is based on the Seebeck effect, where electromotive force is

generated due to the temperature difference between the two ends of thermoelectric

couples, consisting of n-type and p-type thermoelectric elements. Enormous

simulations, as well as experimental studies have been reported on solar-driven TE

generators. Chen [9] developed a thermodynamic model to analyze the performance of

a solar-driven TE power generator. The model based on a well-insulated flat plate

collection, in practice, might be difficult to achieve. Gunter et al. [10] constructed a

prototype of a solar thermoelectric generator. The hot side of the TE module was

heated by solar hot water, and the heat was released at the cold side by a heat sink.

Test results showed that the electrical efficiency reached a maximum value of 1.1% of

the incoming solar radiation. Omer et al. [11] derived a design procedure and performed


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a thermal performance analysis of a solar combined heat and thermoelectric power

cogeneration system based on a two-stage solar energy concentrator. Maneewan et al.

[12] conducted a numerical and laboratory-scale investigation on attic heat gain

reduction by means of a thermoelectric roof solar collector (TE-RSC). The electrical

conversion efficiency of the proposed TE-RSC system was 1~4%. Lertsatitthanakorn et al.

[13] developed and tested a double-pass thermoelectric solar air collector to study the

performance under the tropical climate of Mahasarakham, Thailand. Recently, Peng et

al. [14] addressed a detailed experimental and theoretical analysis of a solar

concentration system using a TE generator, in which the necessary concentration

degree and the different materials for the TE generator in a wide temperature range (up

to 800K) were considered.

The electrical energy generation in solar energy systems is considered to be the

most important. Combining HCPV/T systems with TE modules and making the heat flux

originating between PV cells and a heat extractor through TE modules is another

possibility to increase the electricity production in solar energy systems. In this system,

temperature differences across the TE modules generate additional power driven by the

Seebeck effect. With greater electrical and overall efficiencies, a so-called hybrid

concentrating photovoltaic-thermoelectric/thermal system (HCPV-TE/T) can be

achieved.

In this paper, a multiphysics simulation of an innovative hybrid solar collector

system, which contains a solar concentrator, a string of series-connected GaAs/Ge PV

cells, commercial TE modules applying bismuth telluride as a basic semiconductor


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material, and a water-fed cooling system, is modeled computationally using the

commercial FE-tool COMSOL [15]. The enhanced performance of a PV-TE hybrid system

is demonstrated under concentrated solar radiation and at relatively low temperatures

(<2000C). Also, the effects of the TE generator's geometric parameter (thickness) and

the physical property (figure of merit) on the hybrid system efficiency are thoroughly

investigated.

COMPUTATIONAL MODELS

Fig. 1(a) sketches the proposed HCPV-TE/T system. A Fresnel lens concentrates

the incidence radiation by a factor of 20 over the active solar array area. The PV cell

array panel composed of a single string of 10 serially-connected Gallium

arsenide/Germanium (GaAs/Ge) solar cells [16] is attached to the TE modules, which is

cooled by a heat sink containing water channels. Good contact between the bottom of

the PV cells and the top of the TE layer is assured by utilizing a thin-film thermal

cladding [17]. The schematic of the TE generator panel is depicted in Fig. 1(b). The TE

couples are arranged into 6 rows containing 50 cells each as shown in Fig. 2. The

dimensions and physical parameters of the HCPV-TE/T water collector are tabulated in

Table 1.

The thermoelectric material is a fundamental component of a TEG.

Semiconductor material Bi2Te3-based compounds are used in the current study. The

thermoelectric leg in the simulations shown in Fig. 2 is 1mm by 1mm by 1.2mm, capped

by thin copper electrodes with the height of 0.3mm. The material properties are


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specified in Table 2. Usually these are temperature-dependent and may be anisotropic,

but in this study we take them to be isotropic and constant.

METHODOLOGY

The three-dimensional Navier-Stokes and energy equations combined with the

continuity equation are solved numerically using the finite-element method to

determine the temperature and velocity fields. The flow is assumed to be steady,

incompressible and laminar (inlet Re number <350). The fluid properties are assumed to

be constant. The governing equations are expressed as follows:

Continuity equation:

0)( u (1)

Momentum Conservation equation:

Fuuu 2 P (2)

Energy equation:

PP C

QT

CT

2

u (3)

ρ is the fluid density, u is the flow velocity, F represents body forces acting on

the fluid, μ is dynamic viscosity , P is the pressure, T is the temperature, κ is thermal

conductivity, Cp is heat capacity, and Q is the rate of internal heat generation (e.g.,

chemical, electrical and nuclear energy) within the solid domain.


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The energy equation is solved in the fluid and solid domains where the heat

transfer is strictly dominated by convection and conduction, respectively. For the fluid

region, the conductive term and Q are zero; for the solid region, the convective term is

zero. Therefore, Eqn. (3) can be expressed in Cartesian tensor forms.

Fluid Domain:

2

2

2

2

2

2

x

T

y

T

x

T

C

k

z

Tu

y

Tu

x

Tu

P

zyx

(4)

Solid Domain:

PP C

Q

x

T

y

T

x

T

C

k

2

2

2

2

2

2

0 (5)

The no-slip and no flow-through boundary conditions are specified at all solid

surfaces. The identical inlet velocity boundary is given and the water temperature is

taken to be 20oC. A pressure outlet boundary condition is used at the exit of the

channel. Both convection and radiation are applied on the top surface of the

computational domain. The average convection heat transfer coefficient given by

McAdams et al. is used on the outside surface of the glass cover. In the absence of

forced convection, a heat transfer coefficient of 5 W/(m2·K) is designated [19]. The glass

cover of the system is assumed to have an emissivity of 0.88 [20]. In practice, a layer of

thermal insulation is added below the collector. If the heat loss by radiation at the back

of the collector is negligible, then

)/(31.15

1

036.0

04.01 2 WmKhk

Rin

in

insulation (6)


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Where, Rinsulation is the thermal resistance of the insulation layer, δin is the thickness of

the insulation layer and ink is the thermal conductivity of the insulation layer.

PV Model

The GaAs/Ge PV cell efficiency is a function of irradiance and cell temperature.

For an identical irradiance, the electrical efficiency is taken to be a linear function of the

cell temperature. The linear relationship for 20 times the concentrated irradiance is

given by Xu et al. [21] as,

])][301(0016.01[172.0 KTce (7)

Where, 0.172 and -0.0016 represent the nominal electrical efficiency and the

temperature coefficient of the solar cell, respectively.

The electrical energy ceE generated by the PV cell is computed as follows:

GpE gcece (8)

Where, p is the module packing factor, and p=1 [22]; G is the incident solar radiation.

The PV module electrical efficiency ePV , is equal to the sum of each cell's

power over the total incident solar energy,

GA

AE cce

ePV

10

1, (9)


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TE Model

The conversion efficiency of a TE module based on Carnot cycle can be roughly

estimated by the thermal-based model as [23],

]

1

11][[max

h

ch

ch

T

TTZ

TZ

T

TT

(10)

Where, Th is the temperature at the hot junction, Tc is the temperature at the surface

being cooled, and Z is the figure of merit for the TE module.

However, the diversity and complexity of thermoelectric applications

necessitates a fully coupled-field model, which, in addition to Joule heating, accounts for

Seebeck, Peltier, and Thomson effects as coupling mechanisms between thermal and

electric fields. In this model, the equations of heat flow Eqn. (11) and of continuity of

electric charge [24] Eqn. (12) are coupled by the set of thermoelectric constitutive

equations, Eqns. (13) and (14), involving Seebeck, Peltier and Thomson effects and the

constitutive equation for a dielectric medium Eqn. (15).

q

t

TCq P (11)

0)(

t

DJ (12)

T ][][ Jq (13)

)][(][ T EJ (14)

ED ][ (15)


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where, q is the heat flux vector, J is the electric current density vector, D is the electric

flux density vector, E is the electric field intensity, [κ] is the thermal conductivity matrix,

[σ] is the electrical conductivity matrix, [δ] is the Seebeck coefficient matrix, [Π]=T[ δ] is

the Peltier coefficient matrix, and [ε] is the dielectric permittivity matrix. In the absence

of time-varying magnetic fields, E is irrotational, and can be derived from an electric

scalar potential φ.

E (16)

The flow field needs to be incorporated into the thermoelectric schemes, which

requires the addition of Navier-Stokes model.

For the electrical part, the boundary conditions of TEGs’ outer surfaces are set as

electrical insulation. This means the current must be parallel to the TEG surface. The

voltage at the end of the circuit is set to zero to close the electrical circuit as shown in

Figure 3.

Therefore, the electric power 'W generated by TE modules can be calculated as

)(2'

2

loadTE

OC

RR

VW

(17)

Where, Voc is the open circuit voltage, RTE is the internal thermoelectric resistance, and

Rload is the electric resistance of the external load. For the maximum output electrical

power, Rload=RTE.

Thermal Efficiency of Hybrid System

The thermal efficiency t of the hybrid system is defined by


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GA

TTCm infoutfp

t

)( ,,

(18)

Where, m is the mass flow rate; pC is the water heat capacity; outfT , is the water outlet

temperature; infT , is the water inlet temperature, A is the total top surface area of the

system, and G is incident solar energy.

TE Model Evaluation

In this section, the hybrid collector described in Fig. 1 is simulated. Three

different meshes (with a total number of finite elements equal to 268191, 330885 and

477171) are employed to assess the grid independence of the results. Fig. 4 shows cells'

average temperature in the module at the inlet fluid velocity being 0.01m/s computed

using three different finite element meshes. The figure shows the very high convergence

behavior and accuracy of the computations.

While the simulation is a common engineering practice, the validity of the

proposed methodology becomes very important. The experiments conducted by Niu et

al. [25] are referred to demonstrate the applicability of the coupled-field model to

predict the conversion capability. The variations of power generation with Tin_hot from

both simulations and experiments are shown in Fig. 5.

The power output obtained by the thermal-based model is generally higher as

compared to that of the experiment, which may due to the electric field is neglected. As

clearly indicated by the comparisons, the coupled-field model shows better

performance and is then used in the succeeding simulations.


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RESULT AND DISCUSSION

The multiphysics model shown above is solved to determine the electrical and

thermal performance of the HCPV-TE/T system. Four levels of mesh resolution (coarse,

normal, fine, and extra fine) are tested to check the dependence of the solutions on grid

design. The variation of the pressure distribution along the centerline of the straight

channel at the identical velocity inlet condition (Vin=0.04m/s) for the different meshes is

compared to confirm the high convergence behavior and accuracy of the computations.

Thus, considering the balance between the computational efficiency and the accuracy of

the results, the third level of resolution (fine mesh) is used.

HCPV-TE/T System Performance

The open voltage and maximal powers generated for a single couple of TE

modules are presented in Fig. 6 corresponding to the different values of ∆T (the

temperature difference across the TE module). Whereas voltage with respect to ∆T is in

a linear manner, the power is approximately a quadratic function. It clearly can be

drawn that the TEGs work at higher temperature differences more efficiently.

In the PV-TE hybrid system, the efficiency of the germanium PV module at

20000W/m2 is 17.2% at the reference temperature TPV = 28oC, and the temperature

coefficient is -0.16%/K. For the TE layer, the thickness of δTE = 1.2mm and a specific

figure of merit of Z = 0.00275K-1 are used as a baseline case. For a fixed inlet water

temperature of 20oC, Figs. 7 and 8 present the numerically predicted electrical

efficiencies, and thermal efficiencies with respect to different water inlet velocities for


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the PV-TE/T system and the PVT system, respectively. In Fig. 7, the electrical efficiencies

of the PV-TE/T system and the PVT system increase with the flow rate until the flow rate

reaches 0.02m/s, and then approach to relatively constant values. The electrical

efficiency curve of the TE modules from the PV-TE/T system keeps flat. That is because

the electric efficiency is mainly determined by the temperature difference between the

hot junction and the cooled surface of TE modules’, which maintains 41oC as the flow

rate changes. For the curve of the PV module from the PV-TE/T system, as the flow rate

increases, the decreased module temperature improves the electrical efficiency.

Compared with the PVT system, the PV-TE/T system leads to an increase of about 8%

efficiency.

The thermal efficiencies are plotted as a function of flow rate in Fig. 8. It is

apparent, as expected, that as the water flow rate increases, the thermal efficiencies

increase. For high water flow rates, the system operating temperature is lowered,

resulting in lower heat losses and subsequently higher thermal efficiencies. Also, the

thermal efficiencies of the heat extractors taper off to reach a constant level when the

velocity exceeds 0.05m/s, which demonstrates that the quantity of heat extracted by

the cooling fluid has a limit and cannot be increased further.

In Fig.9, with the identical convective surface area and cooling flow rate in both

configurations, the PV-TE/T hybrid system yields higher PV cell temperatures but more

uniform temperature distributions across the cell array, and eliminates the current

matching problem. However, the higher cell temperature lowers the PV module’s

fatigue life, which is one of the biggest challenges in the PV-TE hybrid system.


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Effect of the TE Material's Figure of Merit

One of the physical properties, the figure of merit, of the TE materials is an

important factor that affects the thermoelectric performance of the PV-TE/T system.

The maximum value of the figure of merit of ZT = 2.4 at room temperature reported by

Venkatasubramanian et al. [24], showed the value of Z can reach 0.008K-1. In this study,

two figure of merit values Z1 = 0.00275K-1 and Z2 = 0.00534K-1 [25] are chosen for this

analysis.

The effect of the figure of merit on electrical efficiencies is plotted in Fig. 10. The

larger Z generates the higher electrical efficiency. With respect to the PVT system

electrical efficiency of 16.9% (at the water inlet velocity of 0.02m/s), the PV-TE/T system

with Z = 0.00534K-1 gives an efficiency of 25%, and 48% larger than that for a PVT

system. For Z = 0.00275K-1, the PV-TE electrical efficiency of 18.2% is reached, which is

28% less than that of Z = 0.00534K-1. Therefore, more attention needs to be drawn on

exploring and explaining the increase in figure of merit values, especially for new

nanomaterials such as superlattices and nanowires, in order to improve the overall

performance of the PV-TE hybrid system and decrease the cost per watt.

Effect of the Thickness of the TE Layer

In the PT-TE/T system, the thickness of the TE layer θTE influences not only the

electrical power but the temperature distribution of the system, thus affects the

electrical efficiency of the PV module.

As shown in Fig. 11, with the thickness of the TE layer increasing from 0.8mm to

2mm in the hybrid system, the electrical efficiency of the PV module decreases,


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nevertheless, the electrical efficiencies of the TE material and the whole PV-TE hybrid

system increase. As the thickness of the TE layer keeps increasing, the thermal

resistance between the PV module and the heat extractor becomes larger, and the

system temperature rises, which decreases the PV module's efficiency. However, the

temperature difference between the TE layer's hot and cool surfaces is improved in

Fig.12, and the TE layer's efficiency is increased. Since the increase of the TE's efficiency

is larger than the amount of the PV's decreased, the overall efficiency of the PV-TE/T

system is improved.

In addition, although the overall electrical efficiency of the hybrid system

improves as the thickness of the TE layer increases, it is important to highlight that the

higher system temperature causes significant higher thermal stress, decreasing the

operational life and reliability of the system. Therefore, a balance between the system

temperature and the electrical efficiency is necessary.

CONCLUSION

The hybrid concentrating PV-TE/T systems can be considered useful, economic

and clean, especially as global warming and air pollution have become serious issues in

recent years. A multiphysics model is developed to determine the efficiency of the

hybrid system. Water is used to extract the heat from the PV-TE hybrid module and

improve the thermal efficiency of the solar hybrid system. The results indicate that the

thermal and electrical efficiencies increase with the increased water-flow rate. The

comparison of the PV-TE/T system with the PVT system shows that the PV-TE/T system


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has a comparable thermal efficiency and a much higher overall electrical efficiency.

Adding a TE converter between the PV module and the heat extractor can lead to an

increase of 8% on the electrical efficiency.

The results derived from the simulated PV-TE/T system are reported for the

different figure of merit values and TE layer's thicknesses. Current studies in TE

materials make Z enhanced to 0.00534K-1, or even greater. Compared to the lower

value, the high-Z material allows an electrical efficiency increase of at least 40%.

Additionally, in the hybrid system, the electrical efficiency of the PV module decreases,

but that of the TE material and the overall efficiency of the system increases, as the

thickness of the TE layer increases.

Finally, although the overall electrical efficiency of the PV-TE/T system is higher

than that of the PVT system, the PV cell operating temperatures in the PV-TE/T system is

also much higher than those in the PVT system at the same cooling conditions. The

higher system temperature causes significant higher thermal stress, thus decrease the

operational life and reliability of the system. Therefore, a balance between the system

temperature and the electrical efficiencies is necessary.

ACKNOWLEDGMENT

This work was supported by the Integrated Electronics Engineering Center at the

State University of New York at Binghamton.


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NOMENCLATURE

A total top surface area of the system, m2

Ac top surface area of PV cell, m2

E energy, W/m2

G incident solar energy, W/m2

H height, m

L length, m

T temperature, K

W width, m

Greek symbols

σ electrical conductivity, S/m

δ seebeck coefficient, V/K

θ thickness of TEG, m

η efficiency, %

κ thermal conductivity, W/(m∙K)

ρ mass density, kg/m3

Subscripts

c cell

ce cell electrical

ct cell thermal

ch channel


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d dielectric layer

g glass cover

m module

PV photovoltaic

t thermal clad

TE thermoelectric


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REFERENCES [1] Radziemska, E, 2003, “The effect of temperature on the power drop in crystalline silicon solar cells,” J. Renewable Energy, 28(1), pp. 1-12. [2] Xu, X, et al., 2013, “Thermal Modeling and Life Prediction of Water-Cooled Hybrid Concentrating PVT Collectors,” J. Solar Energy Engineering, 135, pp. 011010-1~8. [3] O’leary, M. J., Clements, L. D., 1980, "Thermal–electric performance analysis for actively cooled, concentrating photovoltaic systems," Sol Energy, 25, pp. 401-406. [4] Mbewe, D. J., Card, H. C., Card, D. C., 1985, "A model of silicon solar cells for concentrator photovoltaic and photovoltaic/thermal system design," Sol Energy, 35(3), pp. 247-258. [5] Garg, H. P., Adhikari, R. S., 1999, "Performance analysis of a hybrid photovoltaic/thermal (PV/T) collector with integrated CPC troughs," Int J Energy Res., 23, pp. 1295-1304. [6] Akbarzadeh, A., Wadowski, T., 1996, "Heat pipe-based cooling systems for photovoltaic cells under concentrated solar radiation," Appl Therm Eng., 16(1), pp. 81-87. [7] Brogren, M., Karlsson, B., 2001, "Low-concentrating water-cooled PV–thermal hybrid systems for high latitudes," Proc. 17th EUPVSEC. [8] Coventry, J. S., 2005, "Performance of a concentrating photovoltaic/thermal solar collector," Solar Energy, 78 (2), pp. 211-222. [9] Chen, J. C., 1996, “Thermodynamic analysis of a solar-driven thermoelectric generator,” J. Appl. Phys, 79, pp. 2717. [10] Gunter, R., et al., 1999, “PV-hybrid and thermoelectric collectors,” Sol. Energy, 67, pp. 227. [11] Omer, S.A., Infield, D.G., 1998. “Design optimization of thermoelectric devices for solar power generation,” Sol. Energy Mater. Sol. Cells, 53, pp. 67-82. [12] Maneewan, S., Hirrunlabh, J., Khedari, J., Zeghmati, B., Teekasap, S., 2005, “Heat gain reduction by means of thermoelectric roof solar collector,” Sol. Energy, 78, pp. 495. [13] Lertsatitthanakorn, C., Khasee, N., Atthajariyakul, S., Soponronnarit, S., Therdyothin, A., Suzuki, R. O., 2008, “Performance analysis of a double-pass


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thermoelectric solar air collector,” Solar Energy Materials & Solar Cells, 92, pp. 1105-1109. [14] Peng, L., Lanlan, C., Pengcheng, Z., Xinfeng, T., Qingjie, Z., Niino, M., 2010, “Design of a concentration solar thermoelectric generator,” J.Electron. Mater, 39, pp. 1522–1530. [15] COMSOL, version 4.1, COMSOL Inc., 2008 [16] Spectrolab Solar, GaAs/Ge Single Junction Solar Cells, www.spectrolab.com [17] The Bergquist Company, Thermal Clad Substrate, http://www.bergquistcompany.com/thermal_substrates/t-clad-product-overview.htm [18] Jaegle, M., 2008, “Multiphysics Simulation of Thermoelectric Systems - Modeling of Peltier-Cooling and Thermoelectric Generation,” Proceeding of the COMSOL Conference, 2008, Hannover, German. [19] Smolec, W., Thomas, A., 1993, “Theoretical and experimental investigations of heat transfer in a Trombe wall,” Energy Conversion and Management 34(5), pp. 385–400. [20] Sarhaddi, F., at al., 2010, “An improved thermal and electrical model for a solar photovoltaic thermal (PV/T) air collector,” Applied Energy 87, pp. 2328–2339. [21] Xu, X., Sammakia, B.G., Murray, B.T. and Meyers, M.M., 2012, "Thermal Modeling of Hybrid Concentrating PV/T Collectors with Tree-shaped Channel Nets Cooling System", accepted, Proceeding of IEEE, ITherm Conference, San Diego, CA. [22] Chow, T.T., He, W., Ji, J., 2006. “Hybrid photovoltaic-thermosyphon water heating system for residential application,” Solar Energy, 80, pp. 298-306. [23] Rowe, D. M, editor. CRC handbook of thermoelectrics. London, NY, USA: CRC Press; 1995. [24] Topal, E. T., 2011, “A Flow Induced Vertical Thermoelectric Generator and its Simulation Using COMSOL Multiphysics,” Proc. 2011 COMSOL Conference, Boston, MA. [25] Niu, X., and Yu, J.L., 2009, “Experimental Study on Low-Temperature Waste Heat Thermoelectric Generator,” J. Power Sources, 188, pp. 621-626. [26] Venkatasubramanian, R., Siivola, E., Colpitts, T., O’Quinn, B., 2001, “Thin-film thermoelectric devices with high room-temperature figures of merit,” Nature, 413, pp. 597–602.


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http://www.spectrolab.com/

http://www.bergquistcompany.com/thermal_substrates/t-clad-product-overview.htm



[27] Yang, R. G., Chen, G., 2005, “Nanostructured Thermoelectric Materials: From Superlattices to Nanocomposites,” Materials Integration. 18 (33).


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Figure Captions List

Fig. 1 (a). Section of the Hybrid System; (b). Schematic of the TEG Panel

Fig. 2 Layout of the Proposed TEG

Fig. 3 Surfaces with Boundary Condition for Electric Part

Fig. 4 PV Cells’ Temperature Distribution along Flow Direction for Different

Mesh Refinement at a Fixed Inlet Velocity

Fig. 5 Model Validations with Maximum Power Output at the Reference

Condition (Tin_cold = 293K, Gin_Hot = 0.4m3/hr, Gin_Cold = 0.3m3/hr)

Fig. 6 Single TE Module’s Open Voltage and Maximal Power Generated Upon

the Temperature Difference

Fig. 7 Effect of Flow Rate on Electrical Efficiencies

Fig. 8 Effect of Flow Rate on Thermal Efficiencies

Fig. 9 PV Cell Temperature at Solar Heat Flux G = 20kW/m2 and Inlet Velocity

u=0.01m/s

Fig. 10 Effect of Figure of Merit on the Electrical Efficiency of PV-TE/T System

(Thickness of TE Layer: 1.2mm)

Fig. 11 Electrical Efficiency of Hybrid System as a Function of the TE Layer's

Thickness (Fixed Water Inlet Velocity of 0.02m/s)

Fig. 12 Effect of TE Layer's Thickness on Temperature Difference Between the TE

Layer's Hot and Cool Surfaces (Fixed Water Inlet Velocity of 0.02m/s)


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Table Caption List

Table 1 Dimension of the System and the Physical Properties

Table 2 Material Properties of TE Modules [18]


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Table 1

HCPV-TE/T WATER COLLECTOR TECHNOLOGY

GaAs/Ge PV Cell

Specification

Water Properties (at 298K)

Wc 10 mm ρw

1000 kg/m3

Lc 20 mm κw 0.6 W/(m∙K)

δg 1 mm CP

4200 J/(kg∙K)

δpv 0.5 mm Single Row of The Hybrid Module

Specification

δd 0.4 mm Lm

200 mm

δt 1.5 mm Wm

10 mm

Single Cooling Channel

Dimension

TE Generator Properties (Bismuth

Telluride)

W0

6 mm θTE 1.2 mm

H0

6 mm δ p: 2x10-4 V/K

n: -2x10-4 V/K

Concentrator

Specification

κTE 1.5 W/(m∙K)

Ratio 20 σ 1.1x105 S/m


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Table 2

SYMBOL Bi2Te3 ELECTRODE

(COPPER)

δ, [V/K] P: 200X10-6

N: -200X10-6

6.5X10-6

σ, [S/m] 1.1X105 5.9X108

κ, [W/(m*K)] 1.6 350


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Fig. 1(a)

Fig. 1(b)


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concentratin collector

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