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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Xinqiang Xu EP-13-1103 2
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
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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Xinqiang Xu EP-13-1103 3
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
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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Xinqiang Xu EP-13-1103 4
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
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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Xinqiang Xu EP-13-1103 5
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
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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Xinqiang Xu EP-13-1103 6
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.
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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Xinqiang Xu EP-13-1103 7
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)
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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Xinqiang Xu EP-13-1103 8
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)
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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Xinqiang Xu EP-13-1103 9
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)
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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Xinqiang Xu EP-13-1103 10
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
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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Xinqiang Xu EP-13-1103 11
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.
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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Xinqiang Xu EP-13-1103 12
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
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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Xinqiang Xu EP-13-1103 13
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.
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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Xinqiang Xu EP-13-1103 14
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,
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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Xinqiang Xu EP-13-1103 15
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
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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Xinqiang Xu EP-13-1103 16
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.
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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Xinqiang Xu EP-13-1103 17
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
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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Xinqiang Xu EP-13-1103 18
d dielectric layer
g glass cover
m module
PV photovoltaic
t thermal clad
TE thermoelectric
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Xinqiang Xu EP-13-1103 19
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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Xinqiang Xu EP-13-1103 20
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.
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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Xinqiang Xu EP-13-1103 21
[27] Yang, R. G., Chen, G., 2005, “Nanostructured Thermoelectric Materials: From Superlattices to Nanocomposites,” Materials Integration. 18 (33).
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Xinqiang Xu EP-13-1103 22
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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Xinqiang Xu EP-13-1103 23
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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Xinqiang Xu EP-13-1103 24
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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Xinqiang Xu EP-13-1103 25
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
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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Xinqiang Xu EP-13-1103 26
Fig. 1(a)
Fig. 1(b)
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Xinqiang Xu EP-13-1103 27
Fig. 2
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Xinqiang Xu EP-13-1103 28
Fig. 3
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Xinqiang Xu EP-13-1103 29
Fig. 4
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Fig. 5
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Xinqiang Xu EP-13-1103 31
Fig. 6
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Xinqiang Xu EP-13-1103 32
Fig. 7
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Xinqiang Xu EP-13-1103 33
Fig. 8
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Xinqiang Xu EP-13-1103 34
Fig. 9
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Xinqiang Xu EP-13-1103 35
Fig. 10
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Fig. 11
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Fig. 12
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