Energy Conversion and Management 76 (2013) 634–644
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Energy Conversion and Management
journal homepage: www.elsevier.com/locate/enconman
Experimental and numerical results from hybrid retrofitted photovoltaic
panels
Cecilia Rossi, Luca A. Tagliafico, Federico Scarpa ⇑, Vincenzo Bianco
University of Genoa, DIME/TEC – Division of Thermal Energy and Environmental Conditioning, Via All’Opera Pia 15 A, 16145 Genoa, Italy
a r t i c l e
i n f o
a b s t r a c t
Article history:
Received 14 May 2013
Accepted 30 July 2013
The aim of present study is to investigate different methodologies to achieve a better contact between a
photovoltaic panel and a thermal plate, in order to cool the PV panel by means of water in the perspective
of coupling it with a heat pump. It is believed that this kind of system allows to obtain a higher energy
efficiency. The analysis is developed both experimentally and numerically, testing different kinds of configurations in different operating conditions. Simulations are employed to analyze the effect of the variations of the contact resistance between the panel and the thermal plates, demonstrating that the use of a
conductive paste increases the overall performance of the panel. Results show interesting possibilities in
terms of retrofitting of existing photovoltaic panels by employing very simple solutions, such as to fix the
thermal plate on the rear of the panel by means of wood ribs.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Solar energy
Hybrid collector
Photovoltaic thermal panel (PV/T)
Thermal characterization
1. Introduction
A hybrid photovoltaic and thermal panel (PV/T) is a particular
system able to generate heat and power at the same time, therefore providing a higher energy conversion rate of solar radiation
in more suitable forms. A hybrid panel is built by coupling a photovoltaic panel on the top of a thermal one, thus converting solar
radiation both in heat and electric power. These kinds of devices
result to be particularly effective at latitudes where the solar radiation is relevant.
The concept of hybrid panel is based on the observation that the
electrical efficiency of the photovoltaic modules decreases when
the cell temperature increases, therefore it might result to be convenient to cool the panel in order to increase its productivity. This
effect was shown by Evans [1], who proposed the following equation to estimate the efficiency of photovoltaic modules as a function of the temperature:
gel ¼ gref 1 bðT pv T ref Þ þ c log
G
1000
ð1Þ
where gref is the electrical efficiency at the reference temperature
Tref = 25 °C; Tpv is the cell temperature; G is the solar radiation,
G = 1000 W m2; b is the efficiency correction coefficient for temperature and c is the efficiency correction coefficient for solar radiation. Material properties have typical values of the order
gref = 0.12–0.14, b = 0.0045 °C1 and c ffi 0 for crystalline silicon
modules. The relation underlines the linear decrease of the electri⇑ Corresponding author. Fax: +39 010311870.
E-mail address: fscarpa@ditec.unige.it (F. Scarpa).
0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.enconman.2013.07.088
cal efficiency with increasing panel temperature, Tpv. In any case, up
to more than 50% of the incoming solar radiation is converted into
heat.
From the above mentioned observation, it is shown the possibility to extract heat from the panel, reducing the temperature of the
photovoltaic modules, thus improving their electrical efficiency,
and making heat available to other purposes.
In the last years, due to the increasing cost of primary energy
sources, many researchers focused their attention on the hybrid
photovoltaic panels, cooled by air and or by a refrigerant liquid,
which appear to be a promising technology for the combined generation of heat and power, therefore a large number of studies on
these devices have been carried out.
Garg and Agarwal [2] studied, by means of simulations, the
influence of the flow rate on the cooling capacity, and therefore
on the cell efficiency. Since an increased flow rate also increases
the parasitic load, an optimal flow rate has to be found to maximize the combined performance of the system.
Hegazy [3] provided a comprehensive comparison of four PV/T
air collectors. The investigation, based on extensive simulations,
analyzed various aspects of the systems; thermal, electrical,
hydraulic and in particular deepened the influence of mass flow
rate, flow channel ratio and selective absorber plate.
Tripanagnostopoulos [4] investigated some possible arrangement to enhance the heat transfer for a dual hybrid panel, cooled
either with air or water by means of low cost modifications. He
proposed a simple system suitable for building integration, providing hot water or air, depending on the season and the thermal
needs of the building. Kalogirou and Tripanagnostopoulos proposed a feasibility analysis of a PV/T system for industrial [5] and
C. Rossi et al. / Energy Conversion and Management 76 (2013) 634–644
635
Nomenclature
A
D
G
h
I
k
_
m
P
q
R
R00
s
std.dev
T
v
V
surface area of the solar panel, m2
diameter, mm
solar irradiation, W m2
heat tranfer coefficient, W m2 K1
current, A
thermal conductivity, W m1 K1
mass flow rate, kg s1
power, W
heat power, W
thermal resistance, K W1
unit area thermal resistance, m2 K W1
thickness, mm
standard deviation
temperature, °C
wind velocity m s1
voltage, V
Greek symbols
b
temperature correction coefficient
c
radiation correction coefficient
DT
temperature difference
g
efficiency
domestic [6] applications. They performed extensive simulations
showing that a considerable amount of thermal and electrical energy can be produced thus demonstrating that the economic viability of the systems improves, as positive life cycle savings are
obtained in the case of hybrid systems. The savings are increased
for higher load temperature applications.
Some researchers investigated the thermal and fluid dynamic
problems connected with PV/T panels. Particularly, Ibrahim et al.
[7] performed some simulations to investigate the influence of
different mass flow rates of the coolant on the electrical and thermal yield of a water cooled tube-on-plate hybrid panel. The
tube-on-plate configuration is preferable as it represents the simplest and the easiest to be manufactured. The study identify the
optimal mass flow rate for the configuration proposed, in order
to reach the maximum global efficiency of the system. Karava
et al. [8] performed a numerical investigation on forced convective
heat transfer from the inclined windward roof of an isolated
low-rise building, in order to estimate the external heat transfer
coefficient, with the aim to improve the design of buildingintegrated photovoltaic/thermal (PV/T) systems. They proposed
different dimensionless correlations useful for the calculation of
the convection coefficient.
The electrical and thermal yield of a tube-on-plate hybrid panel
was also investigated by means of numerical simulations by Santbergen et al. [9] in order to detect the main causes of efficiency
losses in domestic application and to propose the use of a lowemissivity coating to increase the global efficiency.
Gang et al. [10] presented a study about integrating heat pipes
and a PV/T flat-plate collector in a single unit. Results of a detailed
simulation was presented which put in evidence that these systems can be used in cold climates without risk of freezing.
Chow [11], instead, proposed a review on the trend of development of the technology, particularly he focused on the advancements achieved in the recent years and on the future work
required. Also Charalambous et al. [12] prepared a review on the
subject, highlighting the main achievements obtained up today
in the field of PV/T panels.
Subscripts
ca
cell-absorber
conv
convective
e
environmental
el
electric
forced
forced convection
in
inlet
ins
insulation
int
internal
mpp
maximum power point
natural natural convection
oc
open circuit
out
outlet
plate
absorber layer
PV
photovoltaic
PV_Al
between the rear of the PV panel and the Al plate
PV_rear backside of the photovoltaic panel
ref
reference
sc
short circuit
t
total
w
water
It can be observed that experimental literature about PV/T panels is still scarce and only in the last years some researches were
presented. An important contribution was given by He et al. [13],
who proposed some experimental tests to measure and analyze
the performance of a PV/T system. An experimental rig was set
up in order to compare and evaluate the global efficiency of a
PV/T solar collector, a traditional solar collector and a mono-crystalline photovoltaic panel, all having the same collecting area. The
results showed that the primary energy saving efficiency of a hybrid system is much higher than that of the traditional systems. Instead, Xu et al. [14] presented an experimental investigation to
study the performances of a low-concentrating solar photovoltaic/thermal integrated heat pump system. Their system achieved
an averaged COP of 4.8 for heating water from 30 °C to 70 °C on
a sunny summer day, whereas the electrical efficiency results to
be 1.36 times higher than that of the low concentrating solar photovoltaic standalone solution.
Teo et al. [15] reported about an experimental study on a water
cooled hybrid system and compared the behavior of the panel with
and without an active cooling. They found a linear relation between the efficiency and temperature, showing an interesting increase in the electrical efficiency with decreasing temperatures.
The experimental results were compared to a panel simulation
and they found a good agreement between numerical and experimental data.
Yang et al. [16]proposed a prototype of hybrid solar panel made
using a substrate of functionally graded material, in order to
improve its electrical and thermal yield. The panels were tested
at different water flow rates and solar irradiation intensities. They
also proposed a numerical model in order to gain a better understanding of heat transfer in the hybrid solar panel.
Calise et al. [17] presented an interesting solar trigeneration
system which also includes PV/T elements and simulate it dynamically in a Mediterranean climate. Instead Ashhab [18] suggested
the application of artificial neural network (ANN) to optimize the
performance of a photovoltaic integrated solar system. He used
experimental data to train the ANN, in order to obtain a reliable
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C. Rossi et al. / Energy Conversion and Management 76 (2013) 634–644
model of the system under investigation useful to identify the relevant parameters to pursue the optimization of the system.
Valeh-E-Sheyda et al. [19] reported an experimental analysis of
the performance of two-phase flows in a small hybrid microchannel solar cell. The influence of two-phase working fluid on PV cell
cooling was compared with single-phase and the great potential of
slug flow for heat removal enhancement in PV/T panel was investigated. Their data showed the proposed hybrid system could
substantially increase the output power of PV solar cells.
Zhao et al. [20] proposed the design optimization of a photovoltaic/thermal (PV/T) system using both non-concentrated and
concentrated solar radiation. First, the working fluid of the thermal
unit absorbs the solar infrared radiation. Then, the remaining
visible light is transmitted and converted into electricity by the
solar cell. This arrangement prevents excessive heating of the solar
cell which would otherwise negatively affects its electrical
efficiency. Their results indicate that an optimal system can
effectively and separately use the visible and infrared part of solar
radiation.
Rosell et al. [21] proposed the coupling of a linear Fresnel concentrator with a channel photovoltaic/thermal collector. An analytical model to simulate the thermal behavior of a prototype is
proposed and validated. Their theoretical analysis confirm that
thermal conduction between the PV cells and the absorber plate
is a critical parameter.
Recently, Dubey and Tay [22] executed different tests on commercially available hybrid solar panels under tropical climatic conditions. They performed different experiments by changing the
water mass flow rate. The electrical efficiency of the PV modules
was also compared with and without the thermal collector, and
it was found that the average PV efficiency of the PV/T modules
is about 0.4% higher than the normal PV module.
The object of the present study is to analyze different possible
methods to transform existing PV panels in PV/T panels, therefore
the main idea is to propose a retrofitting to increase the global energy efficiency.
To this aim, various experimental tests were executed by
retrofitting a commercially available PV panel with a thermal
plate, made up of an aluminum plate with a water cooled serpentine. Different experiments were performed in various climatic conditions in order to understand the behavior of the
panel in realistic circumstances. A simple numerical model was
also proposed in order to understand the effect of a possible contact resistance between the PV panel and the thermal plate. This
resistance is due to the presence of an air gap which has to be
reduced in order to maximize the thermal performance of the
hybrid panel. Simulations showed that by adding a conductive
thermal paste the performance of the PV/T panel noticeably
increases.
The present paper is supposed to give a useful contributions to
the developing literature about hybrid panels by pointing out some
practical issues on the construction of such devices.
2. Experimental set-up
In order to investigate the behavior of a hybrid panel, some prototypes were set up and tested. Three different water cooled hybrid
panels (W_PVT) were proposed varying the backside of the panels
(Fig. 1). All the experimental hybrid panels are composed of a commercial monocrystalline silicon photovoltaic panel and two water
cooled tube-on-plate aluminum plates located at the rear. The
main characteristics of the photovoltaic panel are: surface area
A = 1.31 m2, maximum power point Pmpp = 180 W, open circuit
voltage Voc = 30.0 V, short current Isc = 8.37 A, and a module efficiency of 13.7% in Standard Test Conditions (G = 1000 W m2, light
spectrum AM 1.5 and a cell temperature of 25 °C). A typical experimental outcome in terms of V, I and power, obtained in case of
G = 400 W m2 and Te = 20 °C and 30 °C is reported in Fig. 2.
The tube-on-plate panels are made of a 1 mm thick aluminum
thermal plate to uniformly distribute the heat to be collected by
the serpentines (Dint = 6 mm) placed on the back, within which
the cooling water flows. The internal diameter has been chosen
not too large to avoid a consequently large distance between the
tubes (see also Fig. 3, where the technologically possible minimum
curvature radius has been applied) in order to guarantee a more
uniform temperature distribution over the surface of the panel.
On the other hand it cannot be too small, in order to limit the pressure drop for the given mass flow rate. The select diameter is largely utilized in industry for these applications.
The serpentines are glued at the rear of the thermal panel by
means of a thermal conductive paste, while the thermal contact
between the photovoltaic panel and the thermal plate is ensured
only by a direct contact, in order to investigate the possibility of
traditional PV panels retrofitting and to minimize the manufacturing costs. The thermal and photovoltaic plates are the same for
each experimental set-up, while the backside of each set-up is
different.
As reported in Fig. 1, the hybrid panels are arranged in three different ways using the frame of the solar panel as a containment
structure. The different configurations directly impact on the coupling between the back aluminum plate and the rear of the PV panel and thus to the thermal resistance along the heat transfer path.
The first two panels, (a) and (b), are arranged with two different
types of backside insulation. The first is insulated by a polyurethane panel, while the second one is filled by a polyurethane foam
enclosed by using an aluminum plate at the rear. On the contrary,
in the last panel, (c), simple wood ribs are used to guarantee the
contact between the backside of the photovoltaic panel and the
thermal aluminum plate. No conductive paste is added between
the PV panel and the thermal plate.
The choice of using or not a backside insulation mainly depends
on the panel use. When the main purpose is the thermal output, a
good back insulation is recommended; whereas to improve the
electrical performance of the photovoltaic panel or in the perspective of using this kind of panels coupled with solar assisted heat
Fig. 1. Experimental hybrid W_PVT panels insulated with (a) polyurethane panel, (b) polyurethane foam, and (c) hybrid panel without insulation, the thermal contact is
ensured by means of a simple wood structure (ribs).
C. Rossi et al. / Energy Conversion and Management 76 (2013) 634–644
637
panel and the thermal plate (Fig. 3, symbols without subscript),
whereas the ones with subscript ‘‘1’’ reside on the back side of
the thermal plate, i.e. where the serpentine is placed. Water inlet
and outlet temperatures are also measured (respectively by the
thermocouple T3(inlet) and T2, T4 in Fig. 3).
To investigate the effect of the presence of an air gap between
the solar cells and the thermal absorber on the panel performance,
a FEM numerical model was also set up as reported later.
3. Results and discussion
3.1. Error analysis
Fig. 2. Characteristic I–V and P–V curves of the hybrid panel. The curves were
verified experimentally for the first and the third test done on the hybrid panel
W_PVT(b). The environmental data of the tests are respectively G = 400 W m2 and
Te = 20 °C and G = 400 W m2 and Te = 30 °C. (see also Table 2).
The confidence of the experimental results reported in the following paragraph was estimated using standard error propagation
techniques. We refer to the simpler, back insulated case for the
sake of clarity.
Under this assumption the heat transfer from the surface of the
collector into the fluid is given by:
_ w cp ðT w
qw ¼ m
pumps (which usually result in panel temperatures near the environmental one), the backside insulation is useless. However, the
aim of this work is to compare different kinds of experimental
W_PVT panel in order to focus on the main issues related to the
construction technologies of this kind of panels in view of retrofit
purpose.
The experimental tests on the hybrid panels were done from
January 2011 to November 2012 in Genoa, Italy. The prototypes
were tested under different environmental conditions, by means
of indoor and outdoor tests. To cool down the hybrid panels the
serpentines were connected to an hydraulic system, sketched in
Fig. 4, consisting of a water distribution system with pressure
and flow control and a thermostatic bath to control inlet water
temperature.
A data acquisition system was also set. The temperature distribution across the different layers of the hybrid panel was measured thanks to a set of T-type thermocouples connected to a
digital multimeter. Some thermocouples were placed on the rear
side of the photovoltaic panel, in the gap between the photovoltaic
out
T w in Þ
ð2Þ
Therefore, the total thermal resistance Rt of the W_PVT, that is the
resistance between the external side of the PV panel at temperature
TPV and the water at average temperature Tw = (Tw_in + Tw_out)/2 and
the more interesting thermal resistance RPV-Al between the backside
of the PV panel at TPV_rear and the aluminum plate at temperature
Tplate were calculated using the definition of thermal resistance,
respectively, as:
Rt ¼
RPV
DT t T pv ðT w out þ T w in Þ=2
¼
_ w cp ðT w out T w in Þ
m
qw
Al
¼
DT PV
qw
Al
¼
T pv rear T plate
_ w cp ðT w out T w in Þ
m
ð3Þ
ð4Þ
where DTt and DT PV Al are the temperature differences between the
external side of the PV panel and the fluid, and the one from the
back side of the PV panel and the plate, respectively. Eqs. (3) and
(4) represent our measurement model. In any case, we use a differential thermocouple arrangement to measure, with higher accuracy,
Fig. 3. Thermocouple distribution inside the hybrid panel (top view). The thermocouples without subscript are placed on the back side of the photovoltaic panel, whereas the
ones with subscript h1i are placed on the back side of the thermal panel. Water inlet and outlet temperatures are also measured (respectively T3(inlet) and T2, T4).
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C. Rossi et al. / Energy Conversion and Management 76 (2013) 634–644
Fig. 4. Hydraulic system consisting of the water distribution system with pressure and water flow control and a thermostatic bath for inlet temperature control.
the outlet-inlet temperature difference ðT w
that our formulas will become
Rt ¼
RPV
out
Tw
in Þ
¼ DT w , so
T pv T w in DT w =2
_ w c p DT w
m
ð3aÞ
T pv rear T plate
_ w c p DT w
m
ð4aÞ
Al
¼
Errors affecting the measured variables (temperatures and mass
flow rate) will obviously propagate to the calculated ones in case
of both random noise and biases. Since we have no knowledge
about possibly present biases, we utilize standard Gaussian noise
assumption to propagates the error covariance. Our measurement
model can be expressed by the vector function
y ¼ gðxÞ
ð5Þ
If Px is the (known) diagonal error covariance matrix of the
measurement vector x, the covariance of the error associated to
the calculated y vector will be
Py ¼ J Px J t
ð6Þ
where
2 @g
1
6 @x1
@g i
6
¼ 6 ...
J¼
4
@xj
@g m
@x1
..
.
@g 1
@xn
3
7
.. 7
. 7
5
ð7Þ
@g m
@xn
is the 2 6 Jacobian matrix of g with respect to x, that is the
sensitivity matrix of g with respect to x. In our case
_ w t and gðÞ ¼ ½ Rt Rpv Al t
x ¼ T pv T pv rear T plate T w in DT w m
By the proper derivatives, expression (7) will turn in
2
T T
1 0 0 1
pvDT ww in
1
4
J¼
_ p DT w 0 1 1 0 T pv rear T plate
mc
DT w
T pv T w
in DT w =2
_
m
T pv
rear T plate
3
5
_
m
ð8Þ
A preliminary analysis of the quality of the temperature signal indicated that, using a sampling of 100 readings for each measurement,
the errors superimposed on the mean value were characterized by a
sample standard deviation rT = 0.05 °C. In addition, the mass flow
rate measurements are known affected by a typical error with
std.dev of about 0.005 kg/s.
However, taking also into account possible biases affecting the
measurement chain and the errors arising from the non-uniform
distribution of temperature on the panel surface, we prudentially
assign 95% confidence bounds to the measured quantity as follows
rear ; T plate ; T w in ! 2rT ¼ 0:6 K
DT w ! 2rD ¼ 0:2 K
_ w ! 2rm_ ¼ 0:01 kg s1
m
T pv ; T pv
Under these assumption the 95% confidence (2r) associated to the
calculated values of the thermal resistances resulted to be in the
range from ±12% up to about ±35%. Indeed, as it will be seen, these
values noticeably depends on the particular value of the working
conditions during the test.
3.2. Experimental results
The first series of tests was done indoor on the prototype
W_PVT(a), insulated with the polyurethane panel (see Fig. 1) and
without any artificial radiation or ambient temperature control,
see Table 1. The tests estimated the thermal resistances inside
the panel by heating and cooling the panel in steady state, with a
water inlet temperature approximately of 45 °C, 11 °C and 6 °C,
and evaluating the total net heat transferred to the water (water
_ ¼ 90 l=h). The measurement of the temperatures
mass flow rate m
at different locations for each layer of the panel made possible an
evaluation of the temperature distribution across the panel while
measuring temperatures between the different layers allows also
the evaluation of the thermal resistances.
A scheme of the thermal resistances in the hybrid panel is
shown in Fig. 5, where the measured temperatures TPV, TPV_rear,
Tplate, Tw corresponding respectively to the temperature on the
front and the rear side of the photovoltaic panel, the temperature
on the rear side of the thermal plate and the average water temperature are indicated. The heat flows represented in Fig. 5 are the
environmental one, q2, the total heat flow collected by the cooling
fluid, qW, and the net incoming flow at the front side of the panel,
q1. The aim of the experimental tests is to provide a rough estimate
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C. Rossi et al. / Energy Conversion and Management 76 (2013) 634–644
Table 1
Indoor test on the hybrid panel (a). The measures are referred to the average values of the measuring points and interest more than 80% of the panel surface.
Tamb (°C)
Tw_in (°C)
Tw_out (°C)
Tw (°C)
q00w (W m2)
Tplate-rear (°C)
TPV-rear (°C)
TPV (°C)
R00t (m2 K W1)
R00PVAl (m2 K W1)
sPV-Al (mm)
23.5
21.2
20.3
44.76
10.71
5.60
43.05
12.14
7.00
43.9
11.4
6.3
-136.5
114.5
111.7
42.42
11.38
6.65
38.93
13.54
8.99
39.20
15.10
10.30
0.034
0.032
0.036
0.026
0.019
0.021
0.66
0.49
0.54
0.034
0.022
0.57
G
q ext
Tamb
TPV
q1
q1
Te
q2
q
TPV_rear TPlate
TW
Fig. 5. Equivalent electrical diagram of a generic water cooled hybrid panel.
of the average global thermal resistance (per unit area) R00t of the
W_PVT panels, i.e. the resistance between the external side of
the photovoltaic panel at temperature TPV and the water at average
temperature Tw, and the thermal resistance R00PV Al between the
backside of the photovoltaic panel at TPV_rear and the thermal plate
at temperature Tplate. For the first two panels, W_PVT(a) and
W_PVT(b), the environmental incoming heat flow, q2, can be neglected and therefore the heat power collected from the water corresponds to the net incoming heat flow, q1. On the contrary, for the
non-insulated prototype W_PVT(c), q2, linked to the temperature
difference between the water and the environment, has to be accounted for. The results of the tests are shown in Table 1.
An average per unit area (panel surface 1.31 m2) thermal resistance of about 0.034 m2 K W1 ±25% (95% confidence) between the
external surface temperature and mean inlet–outlet water
temperatures was measured. An average rear contact resistance
between the backside of the photovoltaic panel and the thermal
plate of 0.022 m2 K W1 ±35% corresponding to an air gap of
0.57 ± 0.2 mm was estimated, considering a thermal air conductivity kair = 0.026 W m1 K1. Details about the confidence bound estimation are given in the next paragraph.
A thermography of the panel, reported in Fig. 6, clearly highlights the difficulty to stick the photovoltaic and thermal panels
perfectly along the whole panel surface and a large temperature
difference along the panel surface was detected. These working
conditions are clearly detrimental regarding the panel performance both from the electric and from the thermal point of view.
Fig. 6. Thermal imaging of the W_PVT(a) panel during the heating indoor tests.
A new W_PVT panel was then built. Its back face was insulated
with a polyurethane foam in order to exploit the foam expansion to
push the aluminum plate against the rear of the photovoltaic panel
(see W_PVT(b) panel in Fig. 1). In the meantime, a simple experimental set up, shown in Fig. 7, was assembled to test hybrid modules and to estimate, in more controllable conditions, the thermal
characteristic of the tube-and-plate W_PVT. This set up is composed by a set of lamps which produces a mix of radiation similar
to natural sunlight with a relevant spectral radiation emission also
in the U.V. region. This blend of radiation is generated by a quartz
discharge tube and a tungsten filament. The set of lamp is composed by 12 lamps (total installed electrical power 3.6 kWe) in order to have a radiation as uniform as possible, over the panel
surface, of approximately 400 W m2. The main purpose of the
irradiation system is not to investigate the electrical behavior of
the hybrid panel, but to have an almost constant heating source
over the panel in order to study the thermal properties of the panel. The main advantage of this experimental set-up is therefore
the presence of a constant radiation, even if not perfectly uniform
over the surface. The constant radiation, coupled to a control of the
water inlet and of the ambient temperature, makes precise and
repeatable tests possible.
The temperature through the different layers of the panel was
then measured for different water inlet and ambient temperatures
(see Table 2). The results of the test with ambient temperature of
20 °C, cooling water inlet temperature of 14 °C and a constant irradiation of about 400 W m2 are shown in Fig. 8. In these conditions
the net incoming heat flow is about 247 W m2.
The results showed a high temperature difference between the
rear of the photovoltaic and the rear of the aluminum plate (see
Fig. 8 and Table 2). The evaluation of the temperature distribution
pointed out then presence of a 2 mm air gap between the photovoltaic panel and the thermal aluminum plate due to bad thermal
contact between the two layers.
The thermal resistances were then evaluated in case of various
environmental conditions. An average thermal resistance R00t of
about 0.108 m2 K W1 ±12% between external surface temperature
and mean inlet–outlet water temperatures was measured. An average rear contact resistance between the backside of the photovoltaic panel and the thermal plate of 0.079 m2 K W1 ±13%
corresponding to an air gap of 2.05 ± 0.21 mm. The presence of
an air gap was due to a poor foam expansion probably caused by
unsuitable environmental working conditions (such as low temperature, high humidity) during the setup phase and a consequent
separation of the aluminum plate from the photovoltaic panel. This
experience underlines the difficulty of using this kind of solution in
the perspective of retrofitting existing PV panels and therefore the
use of polyurethane foam was then discarded and a new prototype
was built (W_PVT panel c, see Fig. 1). This new panel was assembled with wooden ribs in order to improve theadhesion between
the two panels without adding too much load. The panel was
constructed without insulation in order to test it with a different
set-up for further comparison between insulated and non-insulated panels. It was tested both indoor and outdoor and the results
are shown in Table 3.
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C. Rossi et al. / Energy Conversion and Management 76 (2013) 634–644
Fig. 7. Experimental set-up system composed by lighting system (set of 12 Osram Vitalux lamps); Hydraulic system (consisting of the water distribution system and
thermostatic bath Thermo Haake DC50k41), Data acquisition system (computer, digital multimeter Keithley DMM 2000 and IV Tracer FTV200). Room temperature control is
also present.
Table 2
Indoor test on the hybrid panel (b). The measures are referred to the average values of the measuring points and interest more than 80% of the panel surface.
G (W m2)
Tamb (°C)
Tw_in (°C)
Tw_out (°C)
Tw (°C)
q00w (W m2)
Tplate-rear (°C)
TPV-rear (°C)
TPV (°C)
R00t (m2 K W1)
R00PVAl (m2 K W1)
sPV-Al (mm)
400
400
400
400
20
20
30
30
14.17
24.53
24.72
12.67
17.26
26.71
27.75
17.50
15.7
25.6
26.2
15.1
246.5
174.0
241.9
385.3
22.26
30.14
31.95
22.96
39.51
47.06
51.83
48.43
40.74
47.93
53.03
50.34
0.102
0.128
0.111
0.092
0.070
0.097
0.082
0.066
1.82
2.53
2.14
1.72
0.108
0.079
2.05
Fig. 8. Steady state of W_PVT(b) for constant mean irradiation of 400 W m2,
constant ambient temperature of 20 °C and water inlet temperature of 14 °C. Open
circuit with qw = 246.5 W m2 being the total heat transfer rate to the cooling water
(90 l h1).
An average thermal resistance, R00t , of about 0.032 ± 16%
m2 K W1 between external surface temperature and mean inlet–
outlet water temperatures is detected. An average rear contact
resistance between the backside of the photovoltaic panel and
the thermal plate of 0.018 ± 18% m2 K W1 is evaluated, corresponding to an air gap of 0.48 ± 0.09 mm. During the outdoor tests
the behavior of the W_PVT panel has been compared to a
traditional photovoltaic panel in order to evaluate the decrease
of temperature that could be obtained.
From a comparison of the different tests (see Table 4), the panel
configurations (a) and (c) appear to be the best solutions.
A thermal imaging of the photovoltaic panel and the hybrid
panel W_PVT(c) during the outdoor tests (Fig. 9) highlights the
presence of a hotter area in the middle of the panel as seen
during the heating test of the panel W_PVT(a) (Fig. 6). The
presence of this small area (around 4% of the total surface) in
the same position even for this configuration, is probably due
to a little overlap of the thermal plates and a consequent non-flat
surface and could be easily overcome by using new or more
accurately assembled thermal plates. Despite the presence of this
hotter area, Fig. 9 shows the hybrid panel to be clearly colder.
The average temperature difference is around 10 °C, corresponding to an increase in electrical efficiency of about 5%.The
presence of an area of non-adherence could be also settled by
using some thermal conductive paste, as will be discussed
further.
Table 6 reports a comparison of the hybrid panel considered in
the present work with similar panels analyzed in other works
[14,23]. The comparison is qualitative, because it is extremely difficult to find the same configuration, in particular the various
investigations propose different arrangements of the heat recuperator. Anyway, the increase of the efficiency varies between 3% and
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C. Rossi et al. / Energy Conversion and Management 76 (2013) 634–644
Table 3
Indoor and outdoor test on the hybrid panel (c). The measures are referred to the average values of the measuring points and interest more than 80% of the panel surface.
G
(W m2)
Tamb
(°C)
Tw_in
(°C)
Tw_out
(°C)
Tw
(°C)
q00w
(W m2)
q001
(W m2)
Tplate-rear
(°C)
TPV-rear
(°C)
TPV
(°C)
R00t
(m2 K W1)
R00PVAl
(m2 K W1)
sPV-Al
(mm)
400
200
890
21.5
20.8
15.7
14.39
14.24
14.05
19.98
17.36
20.20
17.2
15.8
17.1
446.6
249.0
491.4
403.5
199.0
506.1
21.07
18.38
23.89
28.78
21.83
33.50
29.71
21.99
35.30
0.030
0.028
0.036
0.019
0.017
0.019
0.50
0.45
0.49
0.032
0.018
0.48
Table 4
Experimental values of thermal resistances and equivalent air gap thickness of the
tested water cooled hybrid panels (W_PVT(a), W_PVT(b) and W_PVT(c)) along with
associated 95% confidence bounds.
R00t
R00t
R00PVAl
sPV-Al
(m2 K W1)
(m2 K W1)
(m2 K W1)
(mm)
W_PVT(a)
W_PVT(b)
W_PVT(c)
0.034 ± 0.0085
0.034 ± 0.0085
0.022 ± 0.0075
0.57 ± 0.20
0.108 ± 0.013
0.108 ± 0.013
0.079 ± 0.010
2.05 ± 0.21
0.032 ± 0.005
0.032 ± 0.005
0.018 ± 0.003
0.48 ± 0.09
5% and, according to Eq. (1), it is closely linked to the temperature
of the panel, therefore it is largely influenced by the effectiveness
of the heat exchanger utilized.
3.3. FEM analysis results
Fig. 9. Thermal imaging of the traditional PV panel and W_PVT panel (c) during the
outdoor tests. Measured temperatures range from 25 °C up to 50 °C.
As said, the possible presence of an air gap between the solar
cells and the thermal absorber would greatly affects the involved
thermal resistance, with a detrimental effect on both the thermal
and electrical performance of the panel since a higher heat transfer
coefficient corresponds to a better cooling of the photovoltaic panel and to an increase in electrical efficiency. To investigate the
influence of a potential air gap, a numerical model was set up. A
simulation of the hybrid panel, based both on design parameters
and literature data [24,25], was then performed with the aid of a
commercial FEM software in order to assess the impact of both
the thickness of the air gap and of the introduction of conductive
pastes. We tested more and more refined meshes until the solution
temperature field was no more affected (temperature variation less
than 1e4 K). The chosen mesh is composed by about 3500 elements of which about 1500 quadrilateral elements, utilized to discretize the domain of prevalent interest (i.e. the air gap underneath
the panel), whereas the remaining part of the domain is discretized
by utilizing triangular elements, which are finer near the tube and
air gap/thermal paste layer, where the temperature gradients are
relevant while a coarser grid is considered far from this zone, as
shown in Fig. 10.
Further details about the fem method applied to heat conduction can be found in [26].
Quadrilateral elements are used where thermal gradients are
strong, because they allow to describe in a more accurate way
the resulting heat flux (i.e. linear interpolation within each single
elements), whereas triangular elements are considered appropriate
for regions with moderate thermal gradients, because they furnish
Table 6
Comparison of different hybrid panels available in the literature.
Present
work
[14] (A)
[14] (B)
[23] (A)
[23] (B)
Kind of PV
panel
Module
area
(m2)
Heat
exchanger
Variation of
electrical efficiency
PV/T vs. PV
Monocrystalline
silicon solar
cells
Monocrystalline
silicon solar
cells
Multicrystalline
silicon solar
cells
Polycrystalline
silicon solar
cells
Polycrystalline
silicon solar
cells
1.31
Tube on plates
aluminum
panels
+5.0%
1.267
Tube and
sheet copper
plate
+3.5%
1.47
Aluminum
parallel plates
+3.6%
0.65
Spiral coil
+4.8%
0.65
Single pass
rectangular
tunnel
+3.1%
Table 5
Comparison between the experimental and numerical simulation results for the hybrid panel (c). The data are referred to the outdoor test environmental conditions.
Experimental (Table 3)
Numerical
G (W m2)
Tamb (°C)
Tw (°C)
q00w (W m2)
q001 (W m2)
Tplate-rear (°C)
TPV-rear (°C)
TPV (°C)
890
890
15.7
15.7
17.1
17.1
491.4
474.1
506.1
488.1
23.89
24.18
33.50
32.56
35.30
34.80
642
C. Rossi et al. / Energy Conversion and Management 76 (2013) 634–644
Fig. 10. Mesh of the Finite Element numerical model. On the right, some characteristic of the FEM mesh are reported.
Fig. 11. Modeling of the photovoltaic panel based on design parameters and literature data ([19,20]).
a more approximate description of the heat flux (i.e. constant heat
flux within each single elements).
The thermal characteristics of the photovoltaic panel has been
evaluated assuming it is layered as in Fig. 11 according, as said,
to design parameter and literature data [24,25].
The heat transfer in the hybrid panel was modeled considering
the thermal energy conservation equation and the Fourier’s Law.
As initial condition, the panel is assumed at ambient temperature,
while radiative and convective heat flux boundary conditions are
imposed at the upper and lower surfaces (i.e. the external surfaces), whereas only convection is considered for the inner side
of the pipe. The convection heat transfer coefficient is evaluated
as the cubic mean between the natural and forced convection heat
transfer coefficient [27], that is
hconv ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
3
3
3
hnatural þ hforced
ð9Þ
The natural convection heat transfer coefficient is determined for a
rectangular plate tilted to the horizontal at a specified angle using
the correlation proposed by Hollands et al. [28], which was developed for a heated or cooled tilted plate. The forced convection heat
transfer coefficient is estimated as a function of the wind velocity, v,
by means of the correlation proposed by Watmuff et al. [29]
hforced ¼ 2:8 þ 3:0v
ð10Þ
Some thin thermally resistive layers were introduced to take into
account the presence of air between the thermal and the photovoltaic panel and the uneven distribution of the thermal paste. The val-
Fig. 12. Simulation of W_PVT(c) for the experimental outdoor test conditions (see
Table 3): G = 890 W m2, Te = 16 °C, Tw = 17 °C. On the right, the layer structure is
reported.
idation of the model was performed by using the outdoor tests
condition of the W_PVT(c) and it resulted to be satisfactory
(Fig. 12), as it is shown Table 5.
A simulation of the hybrid panel with a perfect thermal contact
is developed for the same average water and ambient temperature
Tw = 17.1 °C, Te = 15.7 °C and solar radiation G = 890 W/m2.
C. Rossi et al. / Energy Conversion and Management 76 (2013) 634–644
643
el
Fig. 13. Trend of thermal resistance R00ca and heat transfer coefficient (hca) between
the solar cells and the thermal plate corresponding to the variation of the air gap
thickness, s, from 0 up to 2 mm. Our best average result corresponding to the hybrid
panel W_PVT(c) is also highlighted.
A perfect adhesion between the thermal plate and the photovoltaic panel could lead to a decrease of the thermal resistance down
to 0.0015 m2 K W1. One of the most important key points of the
W_PVT is to detect a solution to get closer to a perfect adherence
between the layers with a consequent evident increase in the system efficiency. To this aim some manufacturer introduced a thin
layer of thermal paste between the two panels in order to improve
the heat transfer and the cooling effect of the refrigerated thermal
plate. Usually the manufacturer opts for thermal pastes with a
thermal conductivity kpaste around 0.7 W m1 K1, although products with higher thermal conductivity are available on market, in
order to keep the production costs relatively low.
In order to evaluate the effect of the presence of an air gap and
the possible advantages linked to the addition of a thermal paste
layer and above all to investigate the consequent impact on the
cells electrical efficiency, the thermal resistance is estimated for
different values of air gap and different thickness of thermal paste
from 0 to 2 mm. Thanks to the panel modeling, it is possible to
evaluate approximately the effective thermal resistance R00ca , and
its reciprocal heat transfer coefficient hca, from the silicon cells of
the photovoltaic panel and the aluminum thermal absorber. These
parameters are generally used for modeling the hybrid panel
Fig. 14. Trend of thermal resistance R00ca and heat transfer coefficient (hca) between
the solar cells and the thermal plate as a function of the thermal paste thickness
from 0 up to 2 mm (kpaste = 0.7 W m1 K1).
Fig. 15. Trend of electrical efficiency as a function of the gap (from 0 up to 2 mm)
filled with air (lower line) or a thermal conductive paste (kpaste = 0.7 W/m K – upper
line). The dashed lines refer to a panel without insulation, while the solid lines refer
to a panel with a backside insulation (sins = 3 cm). Operating conditions; mean
_ ¼ 90 l h1, G = 890 W m2. See also
water temperature, Tw = 17.1 °C, Te = 15.7 °C, m
Table 3.
according to the Hottel–Whillier model adapted for PV/T panels
[30–32]. This is the reason for which we will now refer to these
parameters.
The values of R00ca and hca for different air gap (from 0 up to
2 mm) are shown in Fig. 13, where also our best result is
highlighted, whereas the results for different thickness of thermal
paste kpaste are shown in Fig. 14. From an analysis of Fig. 13 it can
be deduced that, above an air gap of 0.8 mm, the rate of change of
hca is negligible, while as much as the gap approaches zero, the
more rapidly the heat transfer coefficient grows. This highlights
even more the importance of a good adhesion between the two
panels.
Thanks to the panel simulation and Evans relation [1], the electrical efficiency of the hybrid panel for different gaps has been
evaluated for the same environmental data and the same average
water temperature for a W_PVT panel without thermal paste (simple retrofitting of an existing photovoltaic panel and a W_PVT panel with a layer of thermal paste). The simulations were repeated
for a water cooled hybrid panel non insulated or insulated on the
rear side with 3 cm of polyurethane and the results were compared
to the simple photovoltaic panel working in the same weather conditions. The results, shown in Fig. 15, underline that the presence
of a back rear insulation does notinfluence too much the electrical
efficiency of the photovoltaic panel, as long as the water cooling
temperature is kept constant. Another interesting result is that
even the use of a thermal paste with a relatively low thermal conductivity may lead to a noticeable increase in the electrical efficiency of the hybrid panel, especially if compared to a traditional
photovoltaic panel. However the largest part of the enhancement
of electrical efficiency with respect to a traditional photovoltaic panel is achieved by simply refrigerating the panel and thermal paste
can be added to further increase the performances and to get a
more uniform temperature distribution over the panel. This make
the retrofitting of the existing photovoltaic panels extremely interesting, as it enables an improvement in the electrical efficiency
compared to low production costs. In fact in the retrofitting solution the costs consist essentially in the installation of the thermal
panel and the thermal panel itself, without any additional costs
compared to the thermal paste hybrid panels.
However for both the manufacturing solutions the improvement is effective only if the water temperature is kept lower than
the ambient temperature. This implies that the hybrid panels make
644
C. Rossi et al. / Energy Conversion and Management 76 (2013) 634–644
sense only if connected to a thermal user in order to lower the
temperature of the cooling water before starting to circulate again
under the photovoltaic panel.
4. Conclusions
The present paper reports a study about water cooled retrofitted solar hybrid panels achieved by means of experimental and
numerical techniques.
Three identical prototypes with different backside insulation
are proposed and experimentally analyzed. The investigation demonstrated that the wood ribs configuration guarantees the best
performances, because it helps to stick the thermal plate against
the photovoltaic panel in the most effective way among the tested
ones. This solution is very attractive for retrofitting of existing PV
panels as it represents a cheap and ‘‘easy to implement’’ solution.
Furthermore, a numerical analysis was conducted to evaluate
the possible benefits achievable by means of the utilization of a
thermal paste. The results have shown that the thermal resistances
can be effectively decreased improving the performance of the
system. However, in our investigations, it is shown that the highest
increase of efficiency is guaranteed by the simple presence of the
water cooled thermal plate, if an acceptable thermal contact is
ensured (by using wood ribs in our case). A further increase might
be surely achieved by adding a thermal paste, but the economical
convenience should be assessed.
Acknowledgements
This work was supported by the University of Genoa (PRA2012
Grant No. CUP D31J13000000005) and by Regione Liguria (Grant
No. 533/May 2009).
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