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Studio di dispositivi solari fotovoltaici per produzione di energia elettrica e recupero di calore (in lingua Inglese)

Un'ndagine sperimentale e numerica su dispositivi fotovoltaici-termici (PV/T) per la produzione di energia elettrica e calore. I dispositivi studiati consistono di un modulo in silicio cristallino e di una piastra canalizzata roll-bond in alluminio, applicata sul retro del modulo e utilizzata come scambiatore di calore. L'efficienza di alcuni prototipi è misurata. Simulazioni di producibilità  annua e mensile di energia elettrica e calore sono riportate e discusse.

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Articoli tecnico scientifici o articoli contenenti case history
La Termotecnica aprile 2013

Pubblicato
da Alessio Rampini




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Estratto del testo
INTRODUCTION
The installation of photovoltaic (PV) modules has exhibited a rapid
growth in the last years. During normal operation, the efficiency of
crystalline silicon PV cells decreases with increasing the temperature
of the cell. This penalization is significant especially in summer
months, when the availability of solar radiation energy and the
ambient temperature are higher. The efficiency decrease can be
reduced by cooling the PV module. Moreover, heat can be recovered
and used for low temperature applications, such as heating of water
for swimming pools and pre-heating of water for domestic use.
In this work three prototypes of photovoltaic-thermal (PV/T) module
have been characterized and compared. They consist of a PV
crystalline silicon module and a roll-bond canalized panel, made
of aluminium, which is connected to the back side of the module
and used as heat exchanger. The three PV/T devices differ for the
connection of the roll-bond heat exchanger to the module. In the
first prototype, the roll-bond panel is glued to the back side of
the PV module and an insulation layer of 1,5 cm of polyurethane
is added for reducing the heat losses from the heat exchanger to
ambient air in the rear part. In the second prototype, the roll-bond
heat exchanger is mechanically connected to the back side of the
PV module by means of springs. No insulation layer is used to
cover the roll-bond panel. The third prototype is built by a unique
lamination process, in which the roll-bond sheet is added to the
multilayer components of the PV module (glass, EVA, PV cells, EVA,
backsheet, EVA, roll-bond). The laminated device is not insulated
in the rear part. All the PV modules have an aperture area of 1,6
m2. A scheme of the devices is reported in Figure 1.
In the roll-bond panel the channels for the liquid are integrated in
the plate. Del Col et al. [1] showed that in the case of solar thermal
collectors the roll-bond absorber provides a higher performance
than the sheet-and-tube absorber. Besides, the roll bond panel has
a configuration adequate to be laminated with the PV module. An example of lamination of PV cells with a roll-bond panel is reported
by Dupeyrat et al. [2].
Experimental measurements of thermal and electrical efficiency are
discussed and used to verify a numerical model for the prediction
of the performance of PV/T devices.
Simulations of monthly and annual production of electrical energy
and heat are reported. Different operating strategies are analyzed:
no cooling of the PV module, cooling of the PV module without heat
recovery and cogeneration of electrical energy and heat. EXPERIMENTAL TESTS
Tests have been run for the three PV/T prototypes to measure the
thermal and electrical efficiency. The experimental apparatus is
located at the solar energy laboratory of the University of Pado-
va, Italy (Zambolin and Del Col [3]). The steady-state method, as
described by the standard EN 12975-2 [4], has been followed for
the tests. Efficiency data have been measured at different values
of inlet water temperature. The thermal efficiency is measured as: Tecnica Energie Rinnovabili LA TERMOTECNICA aprile 2013 di Davide Del Col, Matteo Bortolato, Andrea Padovan, Marco Dai Prè Davide Del Col, Matteo Bortolato, Andrea Padovan, Marco Dai Prè, Dipartimento di Ingegneria Industriale, Università degli Studi di Padova Investigation of pv solar devices
for production of electricity and heat recovery This paper reports an investigation of flat plate photovoltaic-thermal (PV/T) devices for production of electric energy and heat. The present devices
consist of a crystalline silicon PV module and a roll-bond canalized panel, made of aluminium, which is applied to the back side of the module and
used as heat exchanger. The efficiency of some prototypes is measured. Simulations of monthly and annual production of electric energy and heat
are reported and discussed. 63 STUDIO DI DISPOSITIVI SOLARI FOTOVOLTAICI PER PRODUZIONE DI ENERGIA ELETTRICA E
RECUPERO DI CALORE
Questa memoria riporta un''indagine sperimentale e numerica su dispositivi fotovoltaici-termici (PV/T) per la produzione di energia elettrica e calore.
I dispositivi studiati consistono di un modulo in silicio cristallino e di una piastra canalizzata roll-bond in alluminio, applicata sul retro del modulo
e utilizzata come scambiatore di calore. L''efficienza di alcuni prototipi è misurata. Simulazioni di producibilità annua e mensile di energia elettrica
e calore sono riportate e discusse. FIGURE 1 - Energy fluxes in the PV/T devices ηth = qu G '' A = mw ''cp '' Tout ''Tin ( ) G '' A (1) Tecnica Energie Rinnovabili 64 LA TERMOTECNICA aprile 2013 where q u is the useful heat flow rate, G is the irradiance on the plane of the module, A is the aperture area of the module, m w is the water mass flow rate, T in and Tout are the inlet and outlet water temperatures.
Efficiency data points are presented in graphs as a function of the
reduced temperature difference, defined as: T*m = Tin +Tout 2 '' Tamb G where T amb is the ambient air temperature. Each experimental point of thermal efficiency is measured during a
time interval of 10 minutes. The experimental uncertainty (with 95%
confidence interval) of thermal efficiency and reduced temperature
difference are determined following the procedure described in ISO
Guide to the Expression of Uncertainty in Measurement [5] and in
Kratzenberg et al. [6].
The electrical efficiency has been measured using a current-voltage
curve tracer. This instrument provides the current-voltage curve of the
module, so it is possible to determine the maximum electrical power
produced, P max. Each measurement requires about 3 seconds, and two measurements every minute are taken. The electrical efficiency is: ηel = Pmax G '' A NUMERICAL MODEL
The model is based on a lumped capacitance scheme under steady-
state conditions. The energy fluxes, considered in the energy balan-
ce of the PV/T device, are shown in Figure 1. The model calculates
the useful heat flow rate gained by the fluid q u, the electrical power P el and the average temperature of each layer of the PV/T module. The input parameters are: irradiance on the plane of the module
G, ambient air temperature T amb, water inlet temperature Tin, water mass flow rate m w and sun spatial coordinates. The heat balance in the PV/T module gives the thermal power ab-
sorbed by the roll-bond plate: qcell''abs = G'' A'' 'α ( ) eff '' qloss_top where ('α) eff is calculated as: 'α ( ) eff = ' g '' αc '' p +αb '' 1'' p ( ) ' ' ' '''ηel The glass transmittance ' g is calculated as reported in Duffie and Beckman [7], neglecting the effect of the absorptance of the glass
and assuming a refractive index of 1.53. The terms α c and αb are the absorptance of PV cells and backsheet respectively, p is the
packing factor of the module, which is the ratio of total surface of
PV cells to aperture area. In open circuit conditions, the electrical
efficiency, η el, is set equal to zero, while when the electric load is connected it is calculated as reported in Zondag [8]: ηel =ηref '' 1'' b'' Tcell ''Tref ( ) ' ' ' ' where η ref is the electrical efficiency of the module at the standard test conditions (25 °C PV cell temperature, 1 kW/m2 irradiance
and 1.5 air mass), b is the temperature coefficient of the module,
T cell is the average temperature of the PV cell and Tref is the reference temperature equal to 25 °C. The heat flow rate lost from the glass
cover to the ambient air is calculated as: qloss_top = A'' hrad_top +hconv_top ( )'' Tcover ''Tamb ( ) with hrad_top =εg ''' '' Tcover 4 ''T sky 4 ( ) Tcover ''Tamb hconv_top =1.247'' Tcover ''Tamb ( )''cosθ ' ' ' ' 1/3 + 2.658'' v where ε g is the emittance of the glass cover, ' is the Stefan-Boltzmann constant, T cover is the average temperature of the glass cover of the PV module, T amb is the ambient air temperature, Tsky is the equivalent sky temperature, calculated using the Whillier equation for clear sky
(T sky = Tamb - 6); the heat transfer coefficient between the glass cover of the PV module and the ambient air is calculated with Equation
(9) as reported in Stultz and Wen [9], where θ is the tilt angle of
the module and v is the wind speed.
The heat flow rate transferred from the PV cells to the absorber plate
is equal to the sum of the heat flow rate gained by the fluid and the
dissipations towards the back side of the device: qcell''abs = qu +qloss_back The heat loss from the absorber plate towards the back of the module
is calculated as: qloss_back = A'' hrad_back +hconv_back ( )'' Tabs ''Tamb ( ) in the case of laminated module and mechanically connected mo-
dule, and: qloss_back = A''hcond_ins '' Tabs ''Tamb ( ) in the case of glued and insulated module. The heat transfer coeffi-
cients in Equation (11) and Equation (12) are calculated as: hrad_back =εabs ''' '' Tabs 4 ''T amb 4 ( ) Tabs ''Tamb hconv_back =1.247'' Tabs ''Tamb ( )''cosθ ' ' ' ' 1/3 + 2.658'' v hcond_ins = λins sins where T abs is the average temperature of the absorber plate, εabs is the emittance of the absorber plate, λ ins is the thermal conductivity of the back insulation layer, s ins is the thickness of the insulation layer. Conduction heat transfer occurs between the glass cover and the
absorber plate and the thermal resistance of the different layers
is considered in the model. In the case of the glued and thermally
insulated PV/T module the layers consist of: glass cover, a first EVA
layer, a second EVA layer, polyester based backsheet, glue, roll- (2) (3) (4) (5) (6) (8) (9) (10) (11) (12) (13) (14) (15) (7) bond plate. The thermal resistance of the PV cells is neglected due
to the higher value of the silicon thermal conductivity as compared
to the other terms. In the case of the mechanically connected module
the layers are the same as the glued and thermally insulated device
with the exception of the glue layer; a small air gap is assumed
between the backsheet and the absorber plate, in order to take into
account the non-uniform contact between the plate and the module.
In the case of the laminated module the layers are the same as the
glued and thermally insulated device with the exception of the glue
layer, which is replaced with an EVA layer to ensure the contact
between the backsheet and the roll-bond plate.
The outlet water temperature is obtained from the useful heat flow
rate transferred to the fluid: qu = mw ''cp '' Tout ''Tin ( ) while the electrical power produced is: Pel = G'' A''ηel EXPERIMENTAL AND NUMERICAL RESULTS
The results are reported in graphs that show the electrical and ther-
mal efficiency as a function of the reduced temperature difference.
The calculated efficiency is obtained with 1 kW/m2 irradiance on
the plane of the module and ambient temperature and air speed
equal to the average conditions observed during the tests. The graphs of Figure 2 show the results for the glued and insulated
PV/T module. The thermal efficiency has been measured in open cir-
cuit conditions and with electric load set at maximum power point.
High values of reduced temperature difference are associated with
low irradiance or high fluid to ambient temperature difference. The
thermal efficiency is higher in open circuit condition than on load
condition: looking at Equation (4), Equation (5) and Equation (10),
if η el = 0, the heat flow rate transferred from the PV cells to the absor- ber plate is higher and the useful heat flow rate q u is higher too. The graph of electrical efficiency reports the experimental points and the
calculated curve with electric load set at the maximum power point.
The results for the laminated PV/T module are reported in Figure 3.
Also in this case the thermal efficiency has been measured in open
circuit condition and with electric load at maximum power point. The graph of electrical efficiency reports the experimental data points
and the model prediction obtained with the temperature of PV cells
under the assumption of open circuit. In fact, due to the short time
of the electrical measurement, for most of the time the module is in
open circuit conditions; from the thermal point of view, the short time
when the circuit is closed for the measurement is not long enough to
reach the operating cell temperature. The electrical efficiency cal-
culated by the model is related to the cell temperature, as reported
in Equation (6), and the cell temperature is different if calculated
with or without the electric load connected to the module. The lower
electrical efficiency observed in the laminated module as compared
to the glued and insulated module is explained by the lower number
of PV cells due to design reasons of the tested prototype. Figure 4 reports the calculated thermal efficiency at the maximum
electric power point, for the three prototypes and for the laminated
PV/T module with a rear thermal insulation of 1,5 cm of polyure-
thane. When reduced temperature difference is small, the laminated
devices exhibit the higher thermal efficiency than the other types of
PV/T modules, because of the low thermal resistance between PV
cells and absorber plate.
At high reduced temperature difference, the rear insulation plays
a significant role and the laminated device with insulation has the
higher efficiency. The laminated modules also provide the better
cooling of the cells. The lower performance of the module with
mechanical connection of the heat exchanger is confirmed by
experimental results. Tecnica Energie Rinnovabili 37 LA TERMOTECNICA aprile 2013 65 (16) (17) FIGURE 2 - PV/T module with glued roll-bond plate:
thermal efficiency (left) and electrical efficiency (right)
FIGURE 4 - Calculated thermal efficiency (left) and PV cell
temperature (right) for different PV/T modules
FIGURE 3 - PV/T module with roll-bond applied during lamination
process: thermal efficiency (left) and electrical efficiency (right)
ASSESSMENT OF ANNUAL PRODUCTION
OF ELECTRICAL ENERGY AND HEAT
Simulations of monthly and annual yield of electrical energy and
heat have been run for Padova (45.4° N, 11.9 °E), starting from a
database of solar radiation on the horizontal plane and ambient
air temperature. The Climate-SAF database, by PVGIS [10], has
been used. This database provides the daily profile of global and
diffuse irradiance and ambient air temperature, with a time step of
fifteen minutes, for a representative day of each month of the year.
The inclination of the module is 34°, which is the optimal tilt for
Padova. The global irradiance on the tilted plane of the module is
calculated with the HDKR model by Reindl et al. [11]. This model
has a good prediction accuracy, as shown by Padovan and Del Col
[12]. In the present simulation, the glued and thermally insulated
PV/T device has been considered.
A parameter, named OCT (Operating Cell Temperature) is intro-
duced in the calculations. This parameter is the estimated working
temperature of the PV cells under the same boundary conditions as
defined for NOCT (800 W/m2 irradiance, 20 °C ambient tempe-
rature, 1 m/s wind speed), but considering different heat transfer
modes in the back side of the module. The OCT is calculated from
an energy balance on the photovoltaic module. Two cases are con-
sidered: natural convection heat transfer, which provides an OCT
equal to around 50 °C, and no heat loss, which provides an OCT
equal to around 60 °C. When occurring forced convection heat
transfer with a wind speed of 1 m/s on the front and back sides of
the photovoltaic module, the estimated temperature is equal to the
NOCT (45 °C for the present module). From OCT, it is possible to
determine the real operating temperature of the PV module and thus
the electrical efficiency corresponding to each database condition,
when convection heat transfer or adiabatic behaviour occurs in the
back side of the module.
Water is sent in the roll-bond panel at 20 °C temperature when
only cooling of the PV module is required and at 40 °C when heat
recovery is required. The water mass flux is 0,02 kg/(m2 s). In the
two different cooling strategies the water is pumped to the roll-bond
heat exchanger only when the PV cell temperature is higher than
the inlet water temperature and if the global irradiance on the tilted
plane of the module is at least 100 W/m2. Provided that the PV cell
is hotter than the inlet water, the thermal efficiency is calculated from
the corresponding curve of the PV/T module under investigation.
Once the heat flow rate transferred to water is known, the water
outlet temperature is computed and compared with the water inlet
temperature: if the temperature gain is less than 1 K, no water flow
is considered. When cooling occurs, the total daily amount of heat
produced by the unitary surface of the module, the daily number
of hours of heat production and the daily electrical energy yield
are obtained. When there is no cooling, only the electrical energy
yield is calculated.
The results of the simulations report four different cases: PV module
with OCT = 50 °C, PV module with OCT = 60 °C, hybrid PV/T de-
vice cooled by water at 20 °C inlet temperature and hybrid PV/T
device cooled by water at 40 °C inlet temperature. In the cases of
PV module, with OCT 50 °C or 60 °C, the PV cell temperature is calculated as: Tcell =Tamb + OCT '' 20 800 '' G In the case of hybrid PV/T device, the cell temperature is calculated
from the heat balance of the module.
Table 1 reports the results of the simulation.
When water is sent at 20 °C, the annual yield of electrical energy
increases, depending on the value of OCT, but there is not heat
recovery. When water is sent at 40 °C, the yield of electrical energy does not
change significantly as compared to the PV module, but there is
heat recovery, which is considerable from June to August as shown
in Figure 5. Tecnica Energie Rinnovabili 66 LA TERMOTECNICA aprile 2013 (18) FIGURE 5 - Heat production at 40 °C by the PV/T device
with glued roll-bond panel in Padova: average daily heat
production (left) and average number of daily hours with heat
production (right)
TABLE 1 - Production of electrical energy in Padova by PV/T
module with glued roll-bond panel (monthly and yearly values)
CONCLUSIONS
The results have shown that the lamination process of the photovol-
taic module with the roll-bond canalized panel allows to achieve the
higher thermal performance, because it ensures the lower thermal
resistance between PV cells and aluminium plate: in this case the
thermal efficiency is the highest and the temperature of PV cells is
the lowest among the three solutions investigated.
The simulation analysis has shown the capability of PV/T devices to
cover the heat demand at low temperature. For example, one square
meter of solar thermal collector with an average daily efficiency of
50%, installed in Padova with optimal orientation and inclination,
provides a daily heat production of around 3 kWh during summer
months. To produce the same amount of heat at 40 °C it is necessary
a surface of PV/T modules around 3 to 5 times higher. NOMENCLATURE
A area of the module [m2] b temperature coefficient of the module [1/K] c p specific heat at constant pressure [J/(kg K)] G global irradiance on the module plane [W/m2] h cond_ins conduction heat transfer coefficient of the insulation layer [W/(m2 K)]
h conv_back convection heat transfer coefficient between absorber plate and ambient air [W/(m2 K)] h conv_top convection heat transfer coefficient between glass cover and ambient air [W/(m2 K)] h rad_back radiation heat transfer coefficient between absorber plate and ambient air [W/(m2 K)] h rad_top radiation heat transfer coefficient between glass cover and ambient air [W/(m2 K)] m w water flow rate [kg/s] NOCT normal operating cell temperature [°C]
OCT operating cell temperature [°C] p packing factor [-] P el electrical power [W] P max maximum electrical power [W] q cell-abs heat flow rate from PV cells to absorber plate [W] q loss_top heat flow rate loss from glass cover to ambient air [W] q loss_back heat flow rate from absorber plate to ambient air [W] q u useful heat flow rate to water [W] s ins thickness of insulation [m] T* m reduced temperature difference [(K m2)/W] T abs temperature of absorber plate [K] T amb temperature of ambient air [K] T cell temperature of PV cells [K] T cover temperature of glass cover [K] T in inlet temperature of water [K] T out outlet temperature of water [K] T ref reference temperature at standard test conditions [K] T sky equivalent sky temperature [K] v wind speed [m/s] Greek symbols
α b absorptance of backsheet [-] α c absorptance of PV cells [-] ε abs emittance of absorber plate [-] ε g emittance of glass cover [-] η el electrical efficiency of the module [-] η ref electrical efficiency of the module at standard test conditions [-] η th thermal efficiency of the module [-] θ tilt angle of the module [°] λ ins thermal conductivity of the insulation [W/(m K)] ' Stefan-Boltzmann constant [W/(m2 K4)] ('α) eff effective trasmittance absorptance product [-] ' g trasmittance of the glass cover [-] ACKNOWLEDGEMENTS
The authors would like to acknowledge the financial support of the
Regione Veneto through the project FSE 2105/1/6/1102/2010.
The support of CGA Technologies Spa is also acknowledged. REFERENCES
1. D. Del Col, M. Dai Prè, M. Bortolato, A. Padovan, Experimental characterization of thermal performance of flat plate solar
collectors with roll-bond absorbers, Proc. ISES Solar World
Congress, Kassel, Germany, 2011. 2. P. Dupeyrat, C. Ménézo, M. Rommel, H.-M. Henning, Efficient single glazed flat plate photovoltaic-thermal hybrid collector for
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