- Email: [email protected]
T) system
Accepted Manuscript Exergy analysis of a naturally ventilated Building Integrated Photovoltaic/Thermal (BIPV/T) system Rafaela A. Agathokleous, Soteris A. Kalogirou, Sotirios Karellas PII:
S0960-1481(17)30589-X
DOI:
10.1016/j.renene.2017.06.085
Reference:
RENE 8952
To appear in:
Renewable Energy
Received Date: 21 December 2016 Revised Date:
21 June 2017
Accepted Date: 22 June 2017
Please cite this article as: Agathokleous RA, Kalogirou SA, Karellas S, Exergy analysis of a naturally ventilated Building Integrated Photovoltaic/Thermal (BIPV/T) system, Renewable Energy (2017), doi: 10.1016/j.renene.2017.06.085. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Exergy analysis of a Naturally Ventilated Building Integrated Photovoltaic/Thermal (BIPV/T) System
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Rafaela A. Agathokleousa, Soteris A. Kalogiroub and Sotirios Karellasc
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4 5 6 7
Cyprus University of Technology, Kitiou Kyprianou 36, 3041 Limassol, Cyprus, [email protected] b Cyprus University of Technology, Limassol, Cyprus, [email protected] c National Technical University of Athens, Athens, Greece, [email protected]
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Abstract: The efficiency of Building Integrated Photovoltaic/Thermal (BIPV/T) systems depends on various
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parameters such as the location, amount of incident radiation, orientation of the collector surface,
11
slope of the system and the type of ventilation of the air gap between the Photovoltaic (PV) panels and
12
the secondary skin of the building. However, in order to examine the performance of the system, apart
13
from the energy efficiency, the exergy efficiency needs to be estimated as well. There are numerous
14
studies about energy and exergy efficiency of PV systems, however, most of them are based on PV/T
15
systems, water systems and mechanically ventilated air systems. This paper examines theoretically
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and experimentally the energy and exergy analysis of a naturally ventilated BIPV/T system.
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Experimental procedure is carried out to record the temperature distribution of a naturally ventilated
18
BIPV/T system. The results from the experimental procedure are used to estimate the energy
19
efficiency and exergy efficiency of the system. It is proved that the energy efficiency of the system
20
varies from a minimum of 26.5% to a maximum of 33.5%, and the exergy efficiency varies from a
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minimum 13% to a maximum of 16%. It is also observed that the exergy input to the system is much
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higher than the exergy output of the system.
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Keywords: BIPV/T, exergy, photovoltaics, thermal behaviour, natural ventilation.
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1 Introduction
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Photovoltaics use has significantly increased the last years reaching the total installed capacity of 227
27
GW at the end of 2015 and it is expected to grow more in the next years. However, despite this growth,
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photovoltaics produce only 1.3% of the worlds electricity [1].
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During the last few years Building Integrated Photovoltaic (BIPV) systems are being increasingly
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popular in sustainable design. BIPV systems can be used mainly to produce electricity but in some
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applications, they can also provide hot air for space heating. PVs can be integrated directly to other
32
parts of the building’s envelope to form a part of the building e.g. replace a wall with PV panels and
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create an opaque wall or a wall with shadings, or integrated on a skin of the building e.g. façade or
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roof. In both cases PVs take the place of conventional construction materials.
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The integration of PV panels on a second surface generates heat behind the PVs, which can either be
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discarded to the environment (Figure 1) or be used to heat the interior of the building (Figure 2).
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When the heated air is used to heat the building, then the system is called Building Integrated
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Photovoltaic/Thermal (BIPV/T).
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Figure 1. A general schematic diagram of a naturally ventilated BIPV system driving the hot air to the environment.
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Figure 2. A general schematic diagram of a BIPV/T system driving the hot air into the building or outside, with the use of fan.
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In both BIPV and BIPV/T systems, when PVs are integrated in front of the outer skin of the building,
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they create an air gap between the two layers. The air passes through the air gap between the two
45
skins and is heated because of the hot PV surface. If the heated air is not removed from the duct, it
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increases the temperature of the PV and lowers its efficiency. Additionally, the excess heat increases
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the cooling loads of the building. In BIPV/T systems the heated air is additionally driven into the
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building to provide space heating.
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The ventilation of the air gap can be natural or mechanically driven (fan) depending on the needs of
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the building or the use of the system. Natural ventilation has a number of advantages, the most
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important of which are the avoidance of energy to power the fans, the operation with no noise and the
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avoidance of overheating which can happen when the fan stops in an active system. On the other
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hand, mechanical ventilation can be more effective to remove excess heat from the gap.
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This study investigates experimentally a naturally ventilated single PV unit BIPV/T system operated in
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real sunshine conditions. The air enters the air duct from the bottom and exits from the top and the
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aim of the experimental procedure is to record the temperature distribution of the system as well as
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the velocity of the air in the duct formed between the PV and the wall. The outcomes of the
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experimental procedure are used to estimate the energy and exergy efficiencies of the system.
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There are various studies about the performance of Photovoltaic/Thermal (PV/T) systems and BIPV/T
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systems. There are many researchers who agree that to examine the performance of those systems,
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energy and exergy analyses should be done simultaneously since energy describes the quantity of
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energy while exergy represents the quality of energy.
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A review of exergy analysis of solar thermal systems is presented by Kalogirou et al. [2] which includes
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various types of solar collectors and applications of solar thermal systems. Additionally, Kalogirou et
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al. [3] presented a comprehensive review on the exergy analysis of solar thermal collectors and
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processes including PV/T systems and the use of phase change materials. It is concluded that exergy
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analysis is a valuable method to evaluate and compare possible configurations of the solar thermal
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systems. Moreover, a review on exergy analysis of various solar energy systems, is carried out by
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Saidur et al. [4] summarizing that comparing the thermal efficiency and exergetic efficiency of the
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systems, it can be concluded that the thermal efficiency is not sufficient to choose the desired system.
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The systems discussed in this study are solar photovoltaic, solar heating devices, solar water
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desalination systems, solar air conditioning and refrigerators, solar drying processes and solar power
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generation. Another review on exergetic analysis and performance evaluation of a wide range of
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renewable energy resources is presented by Hepbasli [5].
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Chow et al. [6] made an energy and exergy analysis of a PV/T collector with and without glass cover.
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The energetic efficiency of the glazed collector was found to be always better than the unglazed
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collector. The exergetic efficiency of the unglazed collector has been found to be better than the glazed
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one in the specific range of the tested parameters. It is concluded that if the target is to have either
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more electrical energy or overall energy output, the second law is more appropriate to assess the
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system.
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Sarhaddi et al. [7] carried out a study to evaluate the exergetic performance of a PV/T air collector. It is
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concluded that the thermal efficiency of the PV/T air collector is about 17.18%, the electrical efficiency
83
is 10%, the overall energy efficiency is 45% and the exergy efficiency is 10.75% for sample climatic
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operating and design parameters.
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Saloux et al. [8] studied the analysis of photovoltaic systems and PV/T systems using the exergy
86
method by developing explicit electrical and thermal models in order to characterize each system.
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Ceylan and Gurel [9] performed exergetic analysis of a new design PV/T system under specific
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temperatures 45ºC and 55ºC. The results showed that the exergy efficiency obtained for 45ºC and 55ºC
89
was 17% and 21% respectively.
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Jafarkazemi and Ahmadifard [10] studied the effect of the entire design parameters on the
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performance of flat plate solar collectors with a theoretical and comprehensive model for energy and
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exergy analysis. More studies on the energetic and exergetic aspects of solar air collectors is presented
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by Oztop et al. [11] and Bahrehmand and Ameri [12].
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Nayak and Tiwari [13] carried out an energy and exergy analysis of a PV/T integrated system with a
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solar greenhouse in India. In order to maintain the movement of the air inside the greenhouse, a fan is
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operated continuously. The exergy analysis calculations of the PV/T integrated greenhouse system
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show an exergy efficiency of 4% approximately. The exergy efficiency is defined as:
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× 100
(1)
Where is the exergy output of greenhouse (kWh) is the exergy input for PV modules (kWh)
SC
=
Fujisawa and Tani [14] studied the annual exergy on PV/T hybrid collector consisting of a liquid
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heating flat plate solar collector with mono Si PV on substrate of non-selective aluminium absorber
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plate. From the experimental evaluation, they concluded that the PV/T collector can produce higher
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output energy than a unit PV module or liquid heating flat plate solar collector. Assuming that the
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initial temperature of the fluid medium is equal to the ambient temperature, the overall exergetic
106
efficiency ( ) of a PV/T system is defined by:
, = , + ,
109 110 111
Where , is the exergetic efficiency of the PV
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, is the exergetic efficiency of the thermal collector
Saitoh et al. [15] calculated the exergy of a hybrid solar collector considering that the electrical energy
is equivalent to the exergy. Exergy efficiency equation is given in Eq. (3) where the exergy of heat and exergy from the global solar irradiance sun are given by the Eqs. (4) and (5). , =
112 113
(2)
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∙ + sun
(3)
Where is the conversion efficiency is the global irradiance (W/m2) 5
is the exergy of heat (W/m2) shown in Eq. (4)
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115 =
sun is the exergy from the global solar irradiance (W/m2) shown in Eq. (5)
!"# $ − !& ' !"# $
(4)
sun = 0.95
117 118
Where Ta is the ambient temperature (K) !fluid is the supply temperature of the collector fluid (K)
' is the collected heat amount per unit time per panel area (W/m2)
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(5)
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The exergy input of solar radiation is determined by different methods. According to Chow et al. [6],
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the three most commonly used calculation methods are those suggested by [16]–[18]shown by
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Equations (6), (7) and (8) respectively. According to Shahsavar et al. [19], the differences in the results
122
comparing the three equations are less than 2%. sun = /1 −
!&01 2 !
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(7)
(8)
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4 !&01 sun = /1 − 2 3 !
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1 !&01 7 4 !&01 sun = 31 + 5 6 − 9 3 !
3 !
(6)
Where Tamb is the ambient environment temperature (K)
124
Tsun is the temperature of the sun taken as 5777K
125
G is the solar radiation per unit area (W/m2)
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Shahsavar et al. [19] analysed the energy and exergy performance of a naturally ventilated PV/T air
127
collector designed, manufactured and tested in Iran. The tested system had a wooden structure and an
128
inclination angle of 30°. The total exergy efficiency of the studied PV/T system is calculated by:
6
, = ,# + ,
(9)
Where ,# is the electrical exergy efficiency calculated from Eq. (10) , is the thermal exergy efficiency calculated from Eq. (11)
130 ,# =
, =
:el <in
(10)
:th <in
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(11)
Where <in is the sun presented in Eq. (7) by [17], :el is equal to the electrical energy (Eel=I V) and :th
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is the thermal exergy as defined by Dubey et al. [20]:
133 134
!&01 9 !",
(12)
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Where '? is the rate of useful energy transfer (kW) !", is the outlet fluid temperature (K)
The analysis by [19] showed that the total energy efficiency of the system increases with increasing
136
solar radiation intensity but the total exergy efficiency decreases. There is also an optimum channel
137
depth at which total energy and exergy efficiencies of the system are maximum. Finally, it is observed
138
that the total energy and exergy efficiencies of the system increase with the increase of the PV cell
139
efficiency.
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Joshi and Tiwari [21] made an attempt to evaluate exergy analysis of a hybrid PV/T parallel plate air
141
collector for cold climatic conditions in India. The energy and exergy efficiencies of a PV/T air collector
142
were estimated. It is observed that an instantaneous energy efficiency of a PV/T air heater varies
143
between 55-65% and exergy efficiency 12-15%. The results obtained are in agreement with the
144
results predicted by Bosanac et al. [22] who studied the potential of PV/T solar collectors in Denmark.
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The exergy efficiency of the PV/T air collector is determined by:
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(!&01 + 273) = @ A1 − BCDE + /1 − 2 293 + CD
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Where β is the packing factor of solar cell ηο is the electrical efficiency under standard test conditions
148
ηth is the thermal efficiency
149
ΔΤ is the difference between the ambient temperature and collector outlet temperature
150
Park et al. [23] presented a comprehensive literature review on energy and exergy analyses of
151
renewable energy conversion systems including solar air heater, solar water heater, solar photovoltaic
152
and cooking devices. The authors recommended to use PV/T collectors than PVs alone for better
153
performance and economic benefits of these systems. Regarding the PVs, the exergy efficiency was
154
determined by: =
J0 K0 − L1 − (!&01 /! )Nℎ PQR (! − !&01 ) L1 − (!&01 /! )N PQR
(14)
Where Vmp is the voltage at the maximum power point (V)
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Imp is the current at the maximum power point (Amps)
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Tc is the temperature of the cell (K)
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Tamb is the ambient temperature (K)
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hc is the convective heat transfer coefficient; hc =5.7+3.8v where v is the wind speed
160
Tsun is the temperature of the sun taken as 5777K
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APV is the area of the module (m2)
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The literature review presents various studies on the exergy analysis of PV/T systems and the various
163
correlations obtained for the estimation of the exergy efficiency of the PV/T systems but none was
164
found on naturally ventilated vertical BIPV/T systems and very few found on inclined air PV/T
165
systems. Our objective in this paper is to present an experimental procedure carried out for a naturally
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ventilated vertical BIPV/T system tested outside under the sun, in order to record the temperature
167
distribution of the system and estimate its energy and exergy efficiency and evaluate its benefits.
168
2 Energy and Exergy Analysis
169
The most appropriate way to discuss the performance of a BIPV/T system is to estimate the energy
170
and exergy efficiency of the system. Generally, the exergy is the amount of energy available to be used.
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When an energy source is at equilibrium with the environment, its exergy is zero. This section
172
presents the energy and exergy analysis of a naturally ventilated BIPV/T system.
173
The energy efficiency is based on the energy balance, which is the first law of thermodynamics. The
174
exergy efficiency is the ratio of the maximum theoretical work that can be produced by utilizing a heat
175
source. The exergy efficiency is related to the second law of thermodynamics known as the exergy
176
efficiency law.
177
2.1 Energy Analysis
178
The actual power output of a PV module is shown in Eq. (15) as given by Kalogirou [24]:
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S0& = JTU KVU WW
(15)
Where Voc is the open circuit voltage (V)
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Isc is the short circuit current (Amps)
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FF is the fill factor
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Fill factor describes the quality of solar cells and is defined as the ratio of the maximum power from
183
the solar cell to the product of VOC and ISC.: WW =
184 185
K0 J0 S0& = KVU JTU KVU JTU
(16)
Where Imp is the current at maximum power point (Amps) as shown in Figure 3 Vmp is the voltage at maximum power point (V) as shown in Figure 3 9
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--------------------------
--------------
Isc
6
1
0
5
10
15
20
Voltage (V)
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40 20
25
Voc
30
0
35
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Figure 3. Current -Voltage Curve and Current Power Curve for PV module.
The nominal energy efficiency of solar cells and PV module can be defined by: =
S0& WW KVU JTU = PQR PQR
(17)
Where G is the incident solar radiation (W/m2)
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60
Vmp
0
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100
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Current (A)
4
2
188
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Mpp
5
187
140
--------------------------------Imp
160
Pmax
Power (W)
7
APV is the area of the PV module (m2)
191
The nominal efficiency is always specified under the Standard Test Conditions (STC) at a temperature
192
of 25°C and solar radiation of 1000 W/m2. The electrical efficiency (ηel) at particular irradiance or
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temperature is the result of the nominal efficiency (ηn) minus the change in efficiency (C) due to the temperature effect which is expressed by the cells temperature coefficient (B ):
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# = − C
(18)
C = −B (25℃ − !U ) 195
(19)
Accordingly, the electrical conversion efficiency can be defined by: # = Y1 − B L!U − ! " NZ
196
(20)
Where Tref is the reference temperature at STC (25°C) 10
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197
Tc is the cell temperature (K)
198
βc is the cells temperature coefficient, shows the drop of efficiency with temperature (%/ºC) The cell temperature can be defined by: [\]! − 20°] !U = !&01 + 5 6 800 `/a2
200
(21)
Where Tamb is the temperature at the ambient environment (ºC)
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NOCT is the Nominal Operating Cell Temperature reached by open circuit cells in a module
202
under standard operating conditions (°C)
The useful heat gain induced to the system by the air flow is defined by: '? = a ] L!", − !", N = PQR
204
208 209
(23)
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Where ṁ is the fluid mass flow rate (kg/s)
!", is the temperature of the fluid at the outlet (K) !", is the temperature of the fluid at the inlet (K) ] is the specific heat of the fluid (kJ/kg K)
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a ] L!", − !", N PQR
The total energy efficiency can be defined by:
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(22)
Thus, the thermal PV efficiency can be defined by: =
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= + #
(24)
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The air mass flow rate for the induced air in the air duct between the PVs and the wall, can be defined
211
by the Eq. (25) given by Tonui and Tripanagnostopoulos [25] for the induced mass flow rate of a PV/T
212
collector.
11
2 c B (P d)e P f sin g ] hi
j kl
+ 2 B !", m
p
n
(25)
Where g is the acceleration due to gravity (m/s2)
214
β is the thermal expansion of air
215
Ach is the cross section area of the channel (m2)
216
ρ is the fluid density (kg/m3)
217
is the thermal efficiency of the PV
218
L is the length of the panel (m)
219
θ is the inclination angle of the system
220
f is the friction factor
221
DH is the hydraulic diameter of the channel (m)
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Where Gr is the Grashof number
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Pr is the Prandtl number 4 P u
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(26)
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a = b
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(27)
Where p is the perimeter of the channel (m)
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226
2.2 Exergy Analysis
227
The exergy analysis is based on the second law of thermodynamics which includes an account of the
228
total exergy inflow, exergy outflow and exergy destructed from the system. The general exergy balance
229
of a BIPV/T system in steady state conditions can be written as: v − v = v w
232
∑ is the rate of overall exergy outlet, given by Eq. (35)
∑ w is the exergy destruction within the system
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Where ∑ is the rate of overall exergy inlet, given by Eq. (29)
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(28)
The rate of the exergy inlet to the system is the rate of thermal exergy inlet plus the rate of the
234
electrical exergy inlet as given by: = ,& + ,
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(29)
Where ,& is the rate of thermal exergy inlet to the PV module from the air flow, given by Eq. (30)
236
as defined by Ceylan et al. [9]
237
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,
is the rate of electrical exergy inlet to the PV module from the sun radiation, given by Eq. (35) as defined by Petela [17].
239
The flow rate of the exergy transferred from the fluid in the inlet that is heated while crossing the duct
240
may be defined by:
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,& = a y" = a YLℎ", − ℎ&01 N − !&01 Lz", − z&01 NZ 241
Where the variation of specific enthalpy is given by: ℎ", − ℎ&01 = ] L!", − !&01 N
242
(30)
(31)
And the variation of specific entropy from:
13
z", − z&01 = ] ln h
{|,}
{~
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m
243
Where !", is the temperature of the fluid in the inlet.
244
Thus, Eq. (30) becomes: !", ,& = a ] /L!", − !&01 N − !&01 ln 5 62 !&01
(33)
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(32)
Considering the temperature of the fluid at the inlet equal with the ambient temperature, the term ,& becomes zero.
247
The exergy inlet to the system from the sun radiation given by Petela [17] which is an extension of Eq.
248
(7) is given by: 4 !&01 1 !&01 7 + 5 6 9 3 !
3 !
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= PQR 31 −
SC
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Where Tsun is the temperature of the sun taken as 5777K
250
The rate of the exergy outlet from the system is defined by: = ,& + ,
251 252
TE D
249
(34)
(35)
Where ,& is the rate of thermal exergy outlet of PV module, given by Eq. (36)
,
is the rate of electrical exergy outlet of PV module, given by Eq. (37)
By using again Eqs. (31) and (32), ,& may be defined by Eq. (36) and ,
by employing ηel by
254
Eq. (37):
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!", ,& = a ] /L!", − !&01 N − !&01 ln 5 62 !&01 ,
= # PQR 31 −
(36)
4 !&01 1 !&01 7 + 5 6 9 3 !
3 !
(37)
255
Where # is the electrical conversion efficiency defined earlier by Eq. (20). As can be observed, Eq.
256
(34) is very similar to the Eq. (37) with the only difference that it is multiplied by the electrical 14
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257
conversion efficiency. The electrical exergy outlet of the system depends on the efficiency of the PV
258
module to convert the sunlight to electricity.
259
Accordingly, the exergy balance of a BIPV/T system from Eq. (28) can be written as: w = L ,& + ,
N − ( ,& + ,
) Using above equations, the exergy efficiency of the system can be defined by: = 1 −
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(38)
w
(39)
3 Experimental Analysis
262
Experimental procedure is carried out in mid-September in real outside conditions at the Archimedes
263
Solar Energy Laboratory (ASEL) in Limassol, Cyprus using a custom made single PV unit BIPV/T
264
apparatus.
265
System test rig is shown in Figure 4 showing also various devices and wires connected. The BIPV/T
266
apparatus comprises a 250 W polycrystalline PV panel with dimensions 1640 x 992 x 45 mm, a brick
267
wall with 20 mm width and 2 plexiglass sides with dimensions 1640 x 100 x 5 mm. The system is in
268
vertical position to represent a façade application, and has two openings at the top and bottom of the
269
duct formed between the PV, the wall and the plexiglass sides. The ventilation of the duct was natural
270
since no fan was employed to drive the air through the duct.
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Figure 4. Experimental test rig facing east.
273
Figure 5 shows the experimental set up and the position of the thermocouples on the system. As
274
shown, nine thermocouples were placed in the back side of the PV panel (in order to avoid shading in
275
the front side), three thermocouples installed on the top, three in the middle and three on the bottom
276
of the PV, 6 thermocouples in the air gap in two positions, bottom and top, and 3 thermocouples on the
277
wall, at the top, bottom and middle of its height. All the thermocouples were connected to DaqPro data
278
logger devices to record the temperature at every point during the experiment every 30 seconds. A
279
pyranometer is also connected to a data logger in order to measure the solar radiation in the form of
280
voltage (mV). A PVPM (Peak Power Measuring Device and Curve Tracer for Photovoltaic Modules)
281
device is connected to the PV panel to measure the electrical characteristics of the PV in the form of
282
the Current-Voltage (I-V) curve. For the velocity of the air in the duct, a hot wire anemometer is used
283
to measure the air velocity in the middle of the air gap. The technical data of the instruments used are
284
shown in Table 1 in Appendix I. At the end of the experiment, all data from the equipment used are
285
downloaded to a computer.
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AIR
WALL
Section AIR OUT
Probe
PV AIR IN
PC
DaqPro data Loggers
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WALL
SC
Anemometer
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PVPM
Figure 5. The experimental set up, showing the place of the thermocouples and other instruments, on the three parts of the
288
system, PV panel, air gap and brick wall.
289
The BIPV/T apparatus is set to a vertical position facing east in order to exploit the highest radiation.
290
East orientation is selected after observing the graphs of the solar radiation measured in east and
291
south orientation for vertical surfaces as shown in Figure 6, for September 15th in Limassol, Cyprus
292
(34.70ºN, 33.02ºE). The highest range of radiation is observed in east orientation and this is selected
293
for the experimental procedure in order to be more representative for the highest range of incidence
294
radiation (the sun is almost perpendicular). The measurements were performed during the effective
295
hours of the east orientation which are from 7:00 am to 12:00 noon.
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East 700
South 2
Radiation (W/m )
600 500
Clouds
400 300 200
07:00 08:00
09:00
10:00 11:00 12:00 13:00 14:00 15:00
Time (hh:mm)
296 297
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100
Figure 6. Solar radiation recorded in east and south orientation on vertical PV panel on 15 September 2016 in Limassol, Cyprus.
4 Results Discussion
299
As it is already mentioned, the system investigated is a naturally ventilated BIPV/T system forming an
300
air gap (duct) between the PV and a wall, with air inlet and outlet. The system is tested experimentally
301
from 7:00 am to 12:00 noon on September 15th, with the use of a custom made experimental
302
apparatus. The results obtained from the experimental procedure were used in Matlab to solve the
303
equations presented in Section 3 to estimate the energy and exergy efficiencies of the system. This
304
section will present the results from the experimental procedure as well as the results obtained for the
305
energy and exergy efficiency of the BIPV/T system.
306
The ambient temperature (Tamb) and the solar radiation (Rad) from the pyranometer measured on
307
vertical surface from 7:00 am to 12:00 noon, are shown in Figure 7. The temperature line appears to
308
be noisy because of the short sample recording time which was 30 seconds and the thermocouple that
309
left free in the ambient air was apparently affected by the wind. However, the trend of the line is clear
310
and the range of the ambient temperature can be seen clearly.
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800 Rad Tamb
700
36
2
Radiation (W/m )
32
500
30
400
28 300 26
Temperature (deg C)
34
600
200
07:00
08:00
09:00
10:00
Time (hh:mm)
311
11:00
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24 100
22 12:00
Figure 7. Solar Radiation and Ambient Temperature during experiment from 7:00 am to 12:00 noon.
313
Regarding the temperature of the PV panel, nine thermocouples were used to record its temperature
314
on the back side. Instantaneous measurements of the temperature of the PV in the front side and back
315
side showed that the temperature in the front was 1-2ºC higher than the back side. Despite this, in
316
order to avoid shading of the PV panel, the thermocouples were installed on the back side to the PV as
317
shown in Figure 5.
318
Figure 8 shows the average temperature of the nine thermocouples at the back side of the PV (TPV), in
319
comparison with the ambient air temperature (Tamb). As can be observed, the PV gets hot mainly at the
320
time when the solar radiation is highest. The maximum temperature obtained is 57ºC.
321
In order to be able to understand the relationship between the PV temperature and the air velocity in
322
the duct, Figure 9 shows the air velocity measured with a hot wire anemometer probe which was
323
placed in the middle of the air gap between the PV panel and the back wall. The measurements were
324
taken every 30 minutes during the experimental procedure. The range of measurements is shown in
325
the boxes.
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TPV
Temperature (deg C)
50 45 40
Tamb
35 30
20 07:00
08:00
09:00
10:00
Time (hh:mm)
326
12:00
Figure 8. Temperature of the PV panel and the ambient air temperature.
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1.8
1.4
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Air Velocity in the Duct (m/s)
1.6
1.2 1.0 0.8 0.6 0.4 0.2 0.0
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7:00 7:30 8:00 8:30 9:00 9:30 10:0010:3011:0011:3012:00
Time (hh:mm)
328
Figure 9. Air velocity measured in the middle of the duct between the PV panel and the brick wall, including its range indicated
330
by the boxes.
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From Figures 8 and 9 it can be observed that while the temperature of the PV panel was increasing,
332
the air velocity was also increasing. According to Zogou and Stapountzis [26], in a case with buoyancy
333
flow experiment, the air velocity in the duct was from 0.0025 – 0.02 m/s which is very low in
334
comparison with the cases that the air is driven in the duct with the use of fan. As can be observed in
335
this study, although no fan is used to drive the air in the duct, the air velocity recorded is higher from
336
the values observed in [26].
337
Besides this, there was another issue we had to clarify, which concerns the temperature changes of the
338
air in the duct. It is expected that after the air enters the duct from the bottom, it will be heated from
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the hot PV surface and move upwards due to buoyancy. Thus, the flowing air is expected to exit the
340
duct at higher temperature than in the entrance. Figure 10 shows the temperature of the air in the
341
inlet of the duct (Tfi) and the outlet (Tfo). As can be observed, from 8:00 am to 12:00 noon the air in the
342
outlet was higher that the air in the inlet. Figure 11 shows the temperature difference (DT) between
343
the inlet and outlet of the air in the duct, which shows a similar trend as the input solar radiation.
344
Regarding the temperature of the brick wall, it was stable at about 25-27ºC during the experiment. As
345
mentioned by the authors in [27], for the analysis of BIPV/T systems the wall behind the PV panel can
346
be considered as an isothermal surface and the PV an isoflux surface.
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Tfo
34
Temperature (deg C)
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32 30 28
Tfi
26 24
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22 07:00
08:00
09:00
10:00
11:00
12:00
Time (hh:mm)
347
Figure 10. Temperature of the air inlet and outlet of the duct between the PV panel and the brick wall.
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DT
Temperature (deg C)
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-1 07:00
08:00
09:00
10:00
11:00
12:00
Time (hh:mm)
349 350
Figure 11. Temperature difference of the air inlet and outlet.
21
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351
The estimation of the energy and exergy efficiency of the system is made based on some assumptions
352
in order to simplify the analysis: •
355
(Figure 8). •
356 357
over time (Figure 7). •
358 359
The solar radiation on the PV surface is considered to be the same in all its area but changing
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The temperature of the PV is considered to be the same in all its area but changing over time
The temperature of the wall is considered to be the same in all its area remaining constant over time at 26ºC.
•
For the calculation of the convective heat transfer coefficients, the equations proposed in [27]
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are used. The range of the calculated values are from 3.4 W/m2K to 5.1 W/m2K depending on
361
the variation of the temperature of the PV.
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As already mentioned all equations are solved by using the experimental data in order to estimate the
363
energy and exergy efficiency of the system under natural outdoor conditions and natural air gap
364
ventilation.
365
Figure 12 shows the comparison of the PV temperature (Tpv) and the electrical conversion efficiency of
366
the PV (ηel). As can be observed, the hotter the PV gets, the lower is electrical conversion efficiency.
367
The electrical efficiency of the system is minimum when the temperature of the PV is maximum. As
368
shown, the electrical conversion efficiency varies between 24%-28.5%.
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TPV η el
27.5
Τ PV
27.0
45 26.5 40
η el (%)
50
28.0
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26.0 25.5
35
ηel
30 07:00
08:00
09:00
10:00
Time (hh:mm)
369
11:00
25.0
24.5 12:00
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Temperature (deg C)
55
Figure 12. The Photovoltaic panel temperature over time in comparison with the electrical efficiency of the PV
371
The thermal efficiency (ηth) which is affected by the temperature of the fluid inlet and outlet and the
372
electrical efficiency are shown in Figure 13. As can be observed, the electrical efficiency is more
373
dependent on the radiation and the PV temperature because it drops when solar radiation and
374
temperature of the PV is maximum. On the other hand, the thermal efficiency is following an upward
375
trend through time since it is mainly dependent on the temperature difference between the fluid inlet
376
and outlet. This relationship can be seen in Figure 14.
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27.5
ηel
9
η th
8 7 6
η th
27.0
η el (%)
η el
5
26.5
4
26.0
3
25.5
2
25.0 24.5 07:00
1 08:00
09:00
10:00
11:00
0 12:00
Time (hh:mm)
377 378
Figure 13. The thermal and electrical efficiencies of the system.
23
ηth (%)
28.0
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6
DT
8
4
2
4
η th (%)
6
3
1
ηth
0 -1 07:00
08:00
09:00
10:00
2
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Temperature (deg C)
5
10
0 12:00
11:00
Time (hh:mm)
379
Figure 14. Thermal efficiency of the system in comparison with the temperature difference of the fluid inlet and outlet of the air
381
gap.
382
Examining the I-V and P-V curves collected by the PVPM device in the graphs of Figure 15, it can be
383
observed that short circuit current drops due to the reduction of radiation, which can be observed
384
from Figure 7 at the times shown in Figure 15. The change in the open circuit voltage is almost zero.
385
Thus, the temperature of the PV surface did not affect the overall performance of the PV, which mainly
386
affects the open circuit voltage. This means that the 0.10 m air gap was adequate to create sufficient
387
air speed to remove the excess heat behind the PV panel, to prevent overheating and loss of efficiency.
388
The best curve is observed at 9:00 am where solar radiation was at maximum (perpendicular) which
389
also lead to higher PV temperature, which is shown in Figure 12.
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Current (A)
5 4 3 2
9:00 10:00 11:00 12:00
160
9:00 10:00 11:00 12:00
140 120
Power (W)
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100 80 60 40
1
20 0
0 0
5
10
15
20
25
30
0
35
5
10
15
20
Voltage (V)
Voltage (V)
390
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Figure 15. Current – Voltage and Power - Voltage curves during experiment.
24
25
30
35
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Earlier in Section 2 it was stated that the amount of energy flowing in and out of a system is the same
392
under thermally steady-state conditions according to the law of energy conservation. This is not
393
happening in exergy since the amount of exergy flowing out (Exo) is smaller than that flowing in (Exi),
394
because exergy is consumed within the system to produce entropy. This is shown in Figure 16.
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700
Exi
500 400
SC
Exergy (kWh/m2)
600
300
Exo
100 0 07:00
08:00
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09:00
10:00
11:00
12:00
Time (hh:mm)
Figure 16. The amount of exergy flowing in and out of the system.
397
Following the equations for the exergy analysis presented earlier in section 3.2, the exergy efficiency
398
of the system (nex) is calculated as shown in Figure 17 in comparison with the outlet air temperature
399
(Tfo). As can be observed, the exergy efficiency of the system varies from 13% to 16%. Although, the
400
data do not follow a specific trend, the exergy efficiency shows an increase from 9:30 am - 12:00 noon
401
where the radiation and PV temperature are high. The range of the exergy efficiency is in agreement
402
with the results obtained by Joshi and Tiwari [21] who estimated the exergy efficiency of a hybrid air
403
PV/T collector in India. Additionally, as shown in Figure 18, the energy efficiency is higher than the
404
exergy efficiency during the whole measurement period. The overall energy efficiency (nen) of the
405
system varies between 25.5-33.5%.
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Τf o
18
32
28
14
26
ηex
24
12
22 09:00
09:30
10:00
10:30
Time (hh:mm)
406
10 12:00
11:30
Figure 17. The exergy efficiency of the system related to the temperature of the air at the exit of the duct.
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08:30
ηex (%)
16
30
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Temperature (deg C)
34
34 32 30
26 24 22 20 18 16 14
TE D
Efficiency (%)
28
ηen η ex
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12 10
408 409
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08:30
09:00
09:30
10:00
10:30
11:00
11:30
12:00
Time (hh:mm)
Figure 18. The energy and exergy efficiencies.
410
Unfortunately, no other paper is found on the same system (facade-building integrated naturally
411
ventilated system) to compare the results. These however are of the same order of magnitude as other
412
similar but forced circulation systems, which proves that the penalty paid in terms of exergy spent by
413
the fan (in forced circulation systems) is counterbalanced by the slightly lower performance of the
414
naturally operated system (lower heat transfer values), so the two systems, naturally and forced 26
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systems, behave similarly without the disadvantages of the forced system as outlined earlier in the
416
paper.
417
5 Conclusions
418
In this paper the energy and exergy analysis of a naturally ventilated BIPV/T system is studied
419
theoretically and experimentally. Based on the temperature distribution of the system, energy and
420
exergy analyses are carried out and the correlations for the energy and exergy efficiencies are
421
presented. The energy efficiency of the system is estimated to be up to 26.5-33.5% while the exergy
422
efficiency is estimated to be between 13-16%.
423
Based on the results obtained, the following general conclusions can be drawn:
425
The exergy efficiency of the system examined increases by increasing the temperature of the fluid at the outlet.
•
426
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•
424
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415
Vertical BIPV/T systems in façade applications (vertical position) performs better when facing east, due to the higher incident radiation due to the morning hours (the sun is almost
428
perpendicular). •
429
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The air gap of 0.1 m in a vertical BIPV/T system is enough to prevent PV panel overheating which will lead to significant loss of electrical efficiency. Maximum PV temperature observed
431
from the experiment is 57ºC. •
432
The electrical efficiency of the system varies between 24% and 28.5% when the temperature of the PV panel varies between 30-57ºC.
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434
•
The exergy input to the system is bigger than the exergy output of the system.
435
•
The energy efficiency of the system is higher than the exergy efficiency as in solar systems
436 437
exergy of solar radiation is very high because of the high temperature of the sun.
Nomenclature PU
Area of the collector (m2)
L
27
Length (m)
a
Fluid mass flow rate (kg/s)
Cross section area of the channel (m2)
P
Power output (W)
Specific heat of the fluid (J/kg K)
Pr
Prandlt number
Hydraulic diameter of the duct (m)
p
Perimeter (m)
Exergy rate (W)
Rate of useful energy transfer (W)
'
Energy rate (W)
sf
Specific Entropy (J/kg K)
f
Friction factor
!&01
Ambient temperature (°C)
FF
Fill factor
Gr
Grashof number
G
Solar radiation (W/m2)
g
Acceleration of the gravity (m/s2)
hf
Specific enthalpy of the fluid (J/kg)
ℎ
Convective heat transfer coeff. (W/m2 K)
KVU
K0
NOCT
439
!", !", !
Short circuit current (A)
Final fluid temperature (°C)
Temperature of the sun (K)
J0
Outlet fluid temperature (°C) Inlet fluid temperature (°C)
Cell temperature (°C) Voltage at maximum power point (V)
JTU
Open circuit voltage (V)
PV/T
Photovoltaic/Thermal
Current at maximum power point (A)
Abbreviations BIPV/T
!
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!e
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or :
SC
t
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Area of the PV (m2)
EP
PQR
Building Integrated Photovoltaic/Thermal Nominal Operating Cell Temp. (°C)
Greek symbols β
ηen
Packing factor of solar cells
28
Energy efficiency
Cells temperature coefficient
θ
Inclination angle of the system (°)
Exergy efficiency
ρ
Fluid density (kg/m3)
Subscripts and superscripts average
i
inlet or inside
pvt
photovoltaic thermal
en
energy
max
maximum
ref
reference conditions
ex
exergy
n
nominal
sys
system
el
electrical
o
outlet or outside
tot
total
f
fluid
pv
photovoltaic
SC
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av
th
thermal
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βc
Acknowledgements
442
The research presented in this paper was presented in the 9th International conference ECOS 2016, in
443
June 19-23rd, 2016 in Portorož, Slovenia
444
The work is carried out under the research program Building-integrated fibre-reinforced solar
445
technology (BFirst), funded by the EU Seventh Framework Programme FP7/2007-2013, under grant
446
agreement n° 296016.
447
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Appendixes
508
Appendix I
509
Table 1. Characteristics of the devices used in the experimental procedure.
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Pyranometer: Eppley Radiometer PSP Spectral Range:
295-2800 nm
Oper. Temperature:
-50ºC to 80ºC
Output:
0-10 mV analog
Temp. Response:
0.5% (-30ºC to 50ºC)
Response time:
95%, 5 s
Tilt Response:
0.5%
Data Loggers: Daq Pro 8-channel Data Logger, Omega
Voltage
-250ºC to 1200ºC
Range:
Accuracy:
±0.5ºC
Accuracy:
Resolution:
0.1°C (1 μV)
Accuracy:
-250 to 1200°C:±0.5 %
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Range:
SC
Thermocouples: K- type
Resolution:
0 to 50 mV ±0.5 % 3 μV
Photovoltaic Panel - Polycrystalline 1640 x 992 x 45 mm
Power (W):
250
Impp(A):
8.16
NOCT:
47 ± 2ºC
Efficiency at STC: 15.46 %
TE D
Area:
Isc(A):
8.61
Vmpp (V):
30.83
Voc(V):
37.41
Temp. Coefficient:
-0.45%/ºC
No of cells:
6 x 10, 3 strings in a row
Hot Wire Anemometer: HD 2303.0 Delta Ohm, Probe: AP471S2, Pt100 Air speed, flow rate, air Speed Resolution: temp.
0.01 m/s
Speed Range:
0.1 to 5 m/s
Temp. Resolution:
0.1°C
Temp. Range:
0 to 80°C
Cable length:
∼2 m
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Type of meas.
PVPM 2540C
Resolution:
0.01V-0.25V, 0.005A – 0.01A
Single Meas. Durat.:
20 ms up to 2 s
Voltage Range:
25V / 50V / 100V / 250V
Irradiance Range:
0 - 1300W/m2
Current Range:
2A / 5A / 10A / 40A
Temperature Range:
-40°C to +100°C
510 511 512 33
S0960-1481(17)30589-X
DOI:
10.1016/j.renene.2017.06.085
Reference:
RENE 8952
To appear in:
Renewable Energy
Received Date: 21 December 2016 Revised Date:
21 June 2017
Accepted Date: 22 June 2017
Please cite this article as: Agathokleous RA, Kalogirou SA, Karellas S, Exergy analysis of a naturally ventilated Building Integrated Photovoltaic/Thermal (BIPV/T) system, Renewable Energy (2017), doi: 10.1016/j.renene.2017.06.085. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Exergy analysis of a Naturally Ventilated Building Integrated Photovoltaic/Thermal (BIPV/T) System
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Rafaela A. Agathokleousa, Soteris A. Kalogiroub and Sotirios Karellasc
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Cyprus University of Technology, Kitiou Kyprianou 36, 3041 Limassol, Cyprus, [email protected] b Cyprus University of Technology, Limassol, Cyprus, [email protected] c National Technical University of Athens, Athens, Greece, [email protected]
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Abstract: The efficiency of Building Integrated Photovoltaic/Thermal (BIPV/T) systems depends on various
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parameters such as the location, amount of incident radiation, orientation of the collector surface,
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slope of the system and the type of ventilation of the air gap between the Photovoltaic (PV) panels and
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the secondary skin of the building. However, in order to examine the performance of the system, apart
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from the energy efficiency, the exergy efficiency needs to be estimated as well. There are numerous
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studies about energy and exergy efficiency of PV systems, however, most of them are based on PV/T
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systems, water systems and mechanically ventilated air systems. This paper examines theoretically
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and experimentally the energy and exergy analysis of a naturally ventilated BIPV/T system.
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Experimental procedure is carried out to record the temperature distribution of a naturally ventilated
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BIPV/T system. The results from the experimental procedure are used to estimate the energy
19
efficiency and exergy efficiency of the system. It is proved that the energy efficiency of the system
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varies from a minimum of 26.5% to a maximum of 33.5%, and the exergy efficiency varies from a
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minimum 13% to a maximum of 16%. It is also observed that the exergy input to the system is much
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higher than the exergy output of the system.
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Keywords: BIPV/T, exergy, photovoltaics, thermal behaviour, natural ventilation.
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1 Introduction
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Photovoltaics use has significantly increased the last years reaching the total installed capacity of 227
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GW at the end of 2015 and it is expected to grow more in the next years. However, despite this growth,
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photovoltaics produce only 1.3% of the worlds electricity [1].
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During the last few years Building Integrated Photovoltaic (BIPV) systems are being increasingly
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popular in sustainable design. BIPV systems can be used mainly to produce electricity but in some
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applications, they can also provide hot air for space heating. PVs can be integrated directly to other
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parts of the building’s envelope to form a part of the building e.g. replace a wall with PV panels and
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create an opaque wall or a wall with shadings, or integrated on a skin of the building e.g. façade or
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roof. In both cases PVs take the place of conventional construction materials.
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The integration of PV panels on a second surface generates heat behind the PVs, which can either be
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discarded to the environment (Figure 1) or be used to heat the interior of the building (Figure 2).
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When the heated air is used to heat the building, then the system is called Building Integrated
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Photovoltaic/Thermal (BIPV/T).
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Figure 1. A general schematic diagram of a naturally ventilated BIPV system driving the hot air to the environment.
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Figure 2. A general schematic diagram of a BIPV/T system driving the hot air into the building or outside, with the use of fan.
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In both BIPV and BIPV/T systems, when PVs are integrated in front of the outer skin of the building,
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they create an air gap between the two layers. The air passes through the air gap between the two
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skins and is heated because of the hot PV surface. If the heated air is not removed from the duct, it
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increases the temperature of the PV and lowers its efficiency. Additionally, the excess heat increases
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the cooling loads of the building. In BIPV/T systems the heated air is additionally driven into the
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building to provide space heating.
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The ventilation of the air gap can be natural or mechanically driven (fan) depending on the needs of
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the building or the use of the system. Natural ventilation has a number of advantages, the most
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important of which are the avoidance of energy to power the fans, the operation with no noise and the
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avoidance of overheating which can happen when the fan stops in an active system. On the other
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hand, mechanical ventilation can be more effective to remove excess heat from the gap.
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This study investigates experimentally a naturally ventilated single PV unit BIPV/T system operated in
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real sunshine conditions. The air enters the air duct from the bottom and exits from the top and the
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aim of the experimental procedure is to record the temperature distribution of the system as well as
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the velocity of the air in the duct formed between the PV and the wall. The outcomes of the
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experimental procedure are used to estimate the energy and exergy efficiencies of the system.
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There are various studies about the performance of Photovoltaic/Thermal (PV/T) systems and BIPV/T
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systems. There are many researchers who agree that to examine the performance of those systems,
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energy and exergy analyses should be done simultaneously since energy describes the quantity of
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energy while exergy represents the quality of energy.
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A review of exergy analysis of solar thermal systems is presented by Kalogirou et al. [2] which includes
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various types of solar collectors and applications of solar thermal systems. Additionally, Kalogirou et
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al. [3] presented a comprehensive review on the exergy analysis of solar thermal collectors and
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processes including PV/T systems and the use of phase change materials. It is concluded that exergy
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analysis is a valuable method to evaluate and compare possible configurations of the solar thermal
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systems. Moreover, a review on exergy analysis of various solar energy systems, is carried out by
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Saidur et al. [4] summarizing that comparing the thermal efficiency and exergetic efficiency of the
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systems, it can be concluded that the thermal efficiency is not sufficient to choose the desired system.
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The systems discussed in this study are solar photovoltaic, solar heating devices, solar water
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desalination systems, solar air conditioning and refrigerators, solar drying processes and solar power
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generation. Another review on exergetic analysis and performance evaluation of a wide range of
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renewable energy resources is presented by Hepbasli [5].
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Chow et al. [6] made an energy and exergy analysis of a PV/T collector with and without glass cover.
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The energetic efficiency of the glazed collector was found to be always better than the unglazed
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collector. The exergetic efficiency of the unglazed collector has been found to be better than the glazed
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one in the specific range of the tested parameters. It is concluded that if the target is to have either
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more electrical energy or overall energy output, the second law is more appropriate to assess the
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system.
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Sarhaddi et al. [7] carried out a study to evaluate the exergetic performance of a PV/T air collector. It is
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concluded that the thermal efficiency of the PV/T air collector is about 17.18%, the electrical efficiency
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is 10%, the overall energy efficiency is 45% and the exergy efficiency is 10.75% for sample climatic
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operating and design parameters.
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Saloux et al. [8] studied the analysis of photovoltaic systems and PV/T systems using the exergy
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method by developing explicit electrical and thermal models in order to characterize each system.
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Ceylan and Gurel [9] performed exergetic analysis of a new design PV/T system under specific
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temperatures 45ºC and 55ºC. The results showed that the exergy efficiency obtained for 45ºC and 55ºC
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was 17% and 21% respectively.
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Jafarkazemi and Ahmadifard [10] studied the effect of the entire design parameters on the
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performance of flat plate solar collectors with a theoretical and comprehensive model for energy and
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exergy analysis. More studies on the energetic and exergetic aspects of solar air collectors is presented
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by Oztop et al. [11] and Bahrehmand and Ameri [12].
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ACCEPTED MANUSCRIPT 1.1 Theoretical Background and Literature
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Nayak and Tiwari [13] carried out an energy and exergy analysis of a PV/T integrated system with a
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solar greenhouse in India. In order to maintain the movement of the air inside the greenhouse, a fan is
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operated continuously. The exergy analysis calculations of the PV/T integrated greenhouse system
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show an exergy efficiency of 4% approximately. The exergy efficiency is defined as:
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× 100
(1)
Where is the exergy output of greenhouse (kWh) is the exergy input for PV modules (kWh)
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=
Fujisawa and Tani [14] studied the annual exergy on PV/T hybrid collector consisting of a liquid
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heating flat plate solar collector with mono Si PV on substrate of non-selective aluminium absorber
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plate. From the experimental evaluation, they concluded that the PV/T collector can produce higher
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output energy than a unit PV module or liquid heating flat plate solar collector. Assuming that the
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initial temperature of the fluid medium is equal to the ambient temperature, the overall exergetic
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efficiency ( ) of a PV/T system is defined by:
, = , + ,
109 110 111
Where , is the exergetic efficiency of the PV
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, is the exergetic efficiency of the thermal collector
Saitoh et al. [15] calculated the exergy of a hybrid solar collector considering that the electrical energy
is equivalent to the exergy. Exergy efficiency equation is given in Eq. (3) where the exergy of heat and exergy from the global solar irradiance sun are given by the Eqs. (4) and (5). , =
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(2)
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∙ + sun
(3)
Where is the conversion efficiency is the global irradiance (W/m2) 5
is the exergy of heat (W/m2) shown in Eq. (4)
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115 =
sun is the exergy from the global solar irradiance (W/m2) shown in Eq. (5)
!"# $ − !& ' !"# $
(4)
sun = 0.95
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Where Ta is the ambient temperature (K) !fluid is the supply temperature of the collector fluid (K)
' is the collected heat amount per unit time per panel area (W/m2)
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(5)
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The exergy input of solar radiation is determined by different methods. According to Chow et al. [6],
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the three most commonly used calculation methods are those suggested by [16]–[18]shown by
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Equations (6), (7) and (8) respectively. According to Shahsavar et al. [19], the differences in the results
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comparing the three equations are less than 2%. sun = /1 −
!&01 2 !
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(7)
(8)
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4 !&01 sun = /1 − 2 3 !
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1 !&01 7 4 !&01 sun = 31 + 5 6 − 9 3 !
3 !
(6)
Where Tamb is the ambient environment temperature (K)
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Tsun is the temperature of the sun taken as 5777K
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G is the solar radiation per unit area (W/m2)
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Shahsavar et al. [19] analysed the energy and exergy performance of a naturally ventilated PV/T air
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collector designed, manufactured and tested in Iran. The tested system had a wooden structure and an
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inclination angle of 30°. The total exergy efficiency of the studied PV/T system is calculated by:
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, = ,# + ,
(9)
Where ,# is the electrical exergy efficiency calculated from Eq. (10) , is the thermal exergy efficiency calculated from Eq. (11)
130 ,# =
, =
:el <in
(10)
:th <in
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(11)
Where <in is the sun presented in Eq. (7) by [17], :el is equal to the electrical energy (Eel=I V) and :th
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is the thermal exergy as defined by Dubey et al. [20]:
133 134
!&01 9 !",
(12)
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Where '? is the rate of useful energy transfer (kW) !", is the outlet fluid temperature (K)
The analysis by [19] showed that the total energy efficiency of the system increases with increasing
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solar radiation intensity but the total exergy efficiency decreases. There is also an optimum channel
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depth at which total energy and exergy efficiencies of the system are maximum. Finally, it is observed
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that the total energy and exergy efficiencies of the system increase with the increase of the PV cell
139
efficiency.
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Joshi and Tiwari [21] made an attempt to evaluate exergy analysis of a hybrid PV/T parallel plate air
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collector for cold climatic conditions in India. The energy and exergy efficiencies of a PV/T air collector
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were estimated. It is observed that an instantaneous energy efficiency of a PV/T air heater varies
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between 55-65% and exergy efficiency 12-15%. The results obtained are in agreement with the
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results predicted by Bosanac et al. [22] who studied the potential of PV/T solar collectors in Denmark.
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The exergy efficiency of the PV/T air collector is determined by:
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(!&01 + 273) = @ A1 − BCDE + /1 − 2 293 + CD
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Where β is the packing factor of solar cell ηο is the electrical efficiency under standard test conditions
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ηth is the thermal efficiency
149
ΔΤ is the difference between the ambient temperature and collector outlet temperature
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Park et al. [23] presented a comprehensive literature review on energy and exergy analyses of
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renewable energy conversion systems including solar air heater, solar water heater, solar photovoltaic
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and cooking devices. The authors recommended to use PV/T collectors than PVs alone for better
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performance and economic benefits of these systems. Regarding the PVs, the exergy efficiency was
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determined by: =
J0 K0 − L1 − (!&01 /! )Nℎ PQR (! − !&01 ) L1 − (!&01 /! )N PQR
(14)
Where Vmp is the voltage at the maximum power point (V)
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Imp is the current at the maximum power point (Amps)
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Tc is the temperature of the cell (K)
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Tamb is the ambient temperature (K)
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hc is the convective heat transfer coefficient; hc =5.7+3.8v where v is the wind speed
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Tsun is the temperature of the sun taken as 5777K
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APV is the area of the module (m2)
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The literature review presents various studies on the exergy analysis of PV/T systems and the various
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correlations obtained for the estimation of the exergy efficiency of the PV/T systems but none was
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found on naturally ventilated vertical BIPV/T systems and very few found on inclined air PV/T
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systems. Our objective in this paper is to present an experimental procedure carried out for a naturally
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ventilated vertical BIPV/T system tested outside under the sun, in order to record the temperature
167
distribution of the system and estimate its energy and exergy efficiency and evaluate its benefits.
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2 Energy and Exergy Analysis
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The most appropriate way to discuss the performance of a BIPV/T system is to estimate the energy
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and exergy efficiency of the system. Generally, the exergy is the amount of energy available to be used.
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When an energy source is at equilibrium with the environment, its exergy is zero. This section
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presents the energy and exergy analysis of a naturally ventilated BIPV/T system.
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The energy efficiency is based on the energy balance, which is the first law of thermodynamics. The
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exergy efficiency is the ratio of the maximum theoretical work that can be produced by utilizing a heat
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source. The exergy efficiency is related to the second law of thermodynamics known as the exergy
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efficiency law.
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2.1 Energy Analysis
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The actual power output of a PV module is shown in Eq. (15) as given by Kalogirou [24]:
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S0& = JTU KVU WW
(15)
Where Voc is the open circuit voltage (V)
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Isc is the short circuit current (Amps)
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FF is the fill factor
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Fill factor describes the quality of solar cells and is defined as the ratio of the maximum power from
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the solar cell to the product of VOC and ISC.: WW =
184 185
K0 J0 S0& = KVU JTU KVU JTU
(16)
Where Imp is the current at maximum power point (Amps) as shown in Figure 3 Vmp is the voltage at maximum power point (V) as shown in Figure 3 9
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--------------------------
--------------
Isc
6
1
0
5
10
15
20
Voltage (V)
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40 20
25
Voc
30
0
35
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Figure 3. Current -Voltage Curve and Current Power Curve for PV module.
The nominal energy efficiency of solar cells and PV module can be defined by: =
S0& WW KVU JTU = PQR PQR
(17)
Where G is the incident solar radiation (W/m2)
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60
Vmp
0
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100
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Current (A)
4
2
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Mpp
5
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--------------------------------Imp
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Pmax
Power (W)
7
APV is the area of the PV module (m2)
191
The nominal efficiency is always specified under the Standard Test Conditions (STC) at a temperature
192
of 25°C and solar radiation of 1000 W/m2. The electrical efficiency (ηel) at particular irradiance or
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temperature is the result of the nominal efficiency (ηn) minus the change in efficiency (C) due to the temperature effect which is expressed by the cells temperature coefficient (B ):
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# = − C
(18)
C = −B (25℃ − !U ) 195
(19)
Accordingly, the electrical conversion efficiency can be defined by: # = Y1 − B L!U − ! " NZ
196
(20)
Where Tref is the reference temperature at STC (25°C) 10
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197
Tc is the cell temperature (K)
198
βc is the cells temperature coefficient, shows the drop of efficiency with temperature (%/ºC) The cell temperature can be defined by: [\]! − 20°] !U = !&01 + 5 6 800 `/a2
200
(21)
Where Tamb is the temperature at the ambient environment (ºC)
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NOCT is the Nominal Operating Cell Temperature reached by open circuit cells in a module
202
under standard operating conditions (°C)
The useful heat gain induced to the system by the air flow is defined by: '? = a ] L!", − !", N = PQR
204
208 209
(23)
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Where ṁ is the fluid mass flow rate (kg/s)
!", is the temperature of the fluid at the outlet (K) !", is the temperature of the fluid at the inlet (K) ] is the specific heat of the fluid (kJ/kg K)
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a ] L!", − !", N PQR
The total energy efficiency can be defined by:
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(22)
Thus, the thermal PV efficiency can be defined by: =
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= + #
(24)
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The air mass flow rate for the induced air in the air duct between the PVs and the wall, can be defined
211
by the Eq. (25) given by Tonui and Tripanagnostopoulos [25] for the induced mass flow rate of a PV/T
212
collector.
11
2 c B (P d)e P f sin g ] hi
j kl
+ 2 B !", m
p
n
(25)
Where g is the acceleration due to gravity (m/s2)
214
β is the thermal expansion of air
215
Ach is the cross section area of the channel (m2)
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ρ is the fluid density (kg/m3)
217
is the thermal efficiency of the PV
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L is the length of the panel (m)
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θ is the inclination angle of the system
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f is the friction factor
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DH is the hydraulic diameter of the channel (m)
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Where Gr is the Grashof number
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Pr is the Prandtl number 4 P u
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(26)
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a = b
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(27)
Where p is the perimeter of the channel (m)
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2.2 Exergy Analysis
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The exergy analysis is based on the second law of thermodynamics which includes an account of the
228
total exergy inflow, exergy outflow and exergy destructed from the system. The general exergy balance
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of a BIPV/T system in steady state conditions can be written as: v − v = v w
232
∑ is the rate of overall exergy outlet, given by Eq. (35)
∑ w is the exergy destruction within the system
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Where ∑ is the rate of overall exergy inlet, given by Eq. (29)
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(28)
The rate of the exergy inlet to the system is the rate of thermal exergy inlet plus the rate of the
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electrical exergy inlet as given by: = ,& + ,
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(29)
Where ,& is the rate of thermal exergy inlet to the PV module from the air flow, given by Eq. (30)
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as defined by Ceylan et al. [9]
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,
is the rate of electrical exergy inlet to the PV module from the sun radiation, given by Eq. (35) as defined by Petela [17].
239
The flow rate of the exergy transferred from the fluid in the inlet that is heated while crossing the duct
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may be defined by:
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,& = a y" = a YLℎ", − ℎ&01 N − !&01 Lz", − z&01 NZ 241
Where the variation of specific enthalpy is given by: ℎ", − ℎ&01 = ] L!", − !&01 N
242
(30)
(31)
And the variation of specific entropy from:
13
z", − z&01 = ] ln h
{|,}
{~
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m
243
Where !", is the temperature of the fluid in the inlet.
244
Thus, Eq. (30) becomes: !", ,& = a ] /L!", − !&01 N − !&01 ln 5 62 !&01
(33)
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(32)
Considering the temperature of the fluid at the inlet equal with the ambient temperature, the term ,& becomes zero.
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The exergy inlet to the system from the sun radiation given by Petela [17] which is an extension of Eq.
248
(7) is given by: 4 !&01 1 !&01 7 + 5 6 9 3 !
3 !
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= PQR 31 −
SC
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Where Tsun is the temperature of the sun taken as 5777K
250
The rate of the exergy outlet from the system is defined by: = ,& + ,
251 252
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(34)
(35)
Where ,& is the rate of thermal exergy outlet of PV module, given by Eq. (36)
,
is the rate of electrical exergy outlet of PV module, given by Eq. (37)
By using again Eqs. (31) and (32), ,& may be defined by Eq. (36) and ,
by employing ηel by
254
Eq. (37):
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!", ,& = a ] /L!", − !&01 N − !&01 ln 5 62 !&01 ,
= # PQR 31 −
(36)
4 !&01 1 !&01 7 + 5 6 9 3 !
3 !
(37)
255
Where # is the electrical conversion efficiency defined earlier by Eq. (20). As can be observed, Eq.
256
(34) is very similar to the Eq. (37) with the only difference that it is multiplied by the electrical 14
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conversion efficiency. The electrical exergy outlet of the system depends on the efficiency of the PV
258
module to convert the sunlight to electricity.
259
Accordingly, the exergy balance of a BIPV/T system from Eq. (28) can be written as: w = L ,& + ,
N − ( ,& + ,
) Using above equations, the exergy efficiency of the system can be defined by: = 1 −
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(38)
w
(39)
3 Experimental Analysis
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Experimental procedure is carried out in mid-September in real outside conditions at the Archimedes
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Solar Energy Laboratory (ASEL) in Limassol, Cyprus using a custom made single PV unit BIPV/T
264
apparatus.
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System test rig is shown in Figure 4 showing also various devices and wires connected. The BIPV/T
266
apparatus comprises a 250 W polycrystalline PV panel with dimensions 1640 x 992 x 45 mm, a brick
267
wall with 20 mm width and 2 plexiglass sides with dimensions 1640 x 100 x 5 mm. The system is in
268
vertical position to represent a façade application, and has two openings at the top and bottom of the
269
duct formed between the PV, the wall and the plexiglass sides. The ventilation of the duct was natural
270
since no fan was employed to drive the air through the duct.
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Figure 4. Experimental test rig facing east.
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Figure 5 shows the experimental set up and the position of the thermocouples on the system. As
274
shown, nine thermocouples were placed in the back side of the PV panel (in order to avoid shading in
275
the front side), three thermocouples installed on the top, three in the middle and three on the bottom
276
of the PV, 6 thermocouples in the air gap in two positions, bottom and top, and 3 thermocouples on the
277
wall, at the top, bottom and middle of its height. All the thermocouples were connected to DaqPro data
278
logger devices to record the temperature at every point during the experiment every 30 seconds. A
279
pyranometer is also connected to a data logger in order to measure the solar radiation in the form of
280
voltage (mV). A PVPM (Peak Power Measuring Device and Curve Tracer for Photovoltaic Modules)
281
device is connected to the PV panel to measure the electrical characteristics of the PV in the form of
282
the Current-Voltage (I-V) curve. For the velocity of the air in the duct, a hot wire anemometer is used
283
to measure the air velocity in the middle of the air gap. The technical data of the instruments used are
284
shown in Table 1 in Appendix I. At the end of the experiment, all data from the equipment used are
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downloaded to a computer.
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AIR
WALL
Section AIR OUT
Probe
PV AIR IN
PC
DaqPro data Loggers
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WALL
SC
Anemometer
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PVPM
Figure 5. The experimental set up, showing the place of the thermocouples and other instruments, on the three parts of the
288
system, PV panel, air gap and brick wall.
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The BIPV/T apparatus is set to a vertical position facing east in order to exploit the highest radiation.
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East orientation is selected after observing the graphs of the solar radiation measured in east and
291
south orientation for vertical surfaces as shown in Figure 6, for September 15th in Limassol, Cyprus
292
(34.70ºN, 33.02ºE). The highest range of radiation is observed in east orientation and this is selected
293
for the experimental procedure in order to be more representative for the highest range of incidence
294
radiation (the sun is almost perpendicular). The measurements were performed during the effective
295
hours of the east orientation which are from 7:00 am to 12:00 noon.
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17
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East 700
South 2
Radiation (W/m )
600 500
Clouds
400 300 200
07:00 08:00
09:00
10:00 11:00 12:00 13:00 14:00 15:00
Time (hh:mm)
296 297
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Figure 6. Solar radiation recorded in east and south orientation on vertical PV panel on 15 September 2016 in Limassol, Cyprus.
4 Results Discussion
299
As it is already mentioned, the system investigated is a naturally ventilated BIPV/T system forming an
300
air gap (duct) between the PV and a wall, with air inlet and outlet. The system is tested experimentally
301
from 7:00 am to 12:00 noon on September 15th, with the use of a custom made experimental
302
apparatus. The results obtained from the experimental procedure were used in Matlab to solve the
303
equations presented in Section 3 to estimate the energy and exergy efficiencies of the system. This
304
section will present the results from the experimental procedure as well as the results obtained for the
305
energy and exergy efficiency of the BIPV/T system.
306
The ambient temperature (Tamb) and the solar radiation (Rad) from the pyranometer measured on
307
vertical surface from 7:00 am to 12:00 noon, are shown in Figure 7. The temperature line appears to
308
be noisy because of the short sample recording time which was 30 seconds and the thermocouple that
309
left free in the ambient air was apparently affected by the wind. However, the trend of the line is clear
310
and the range of the ambient temperature can be seen clearly.
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800 Rad Tamb
700
36
2
Radiation (W/m )
32
500
30
400
28 300 26
Temperature (deg C)
34
600
200
07:00
08:00
09:00
10:00
Time (hh:mm)
311
11:00
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24 100
22 12:00
Figure 7. Solar Radiation and Ambient Temperature during experiment from 7:00 am to 12:00 noon.
313
Regarding the temperature of the PV panel, nine thermocouples were used to record its temperature
314
on the back side. Instantaneous measurements of the temperature of the PV in the front side and back
315
side showed that the temperature in the front was 1-2ºC higher than the back side. Despite this, in
316
order to avoid shading of the PV panel, the thermocouples were installed on the back side to the PV as
317
shown in Figure 5.
318
Figure 8 shows the average temperature of the nine thermocouples at the back side of the PV (TPV), in
319
comparison with the ambient air temperature (Tamb). As can be observed, the PV gets hot mainly at the
320
time when the solar radiation is highest. The maximum temperature obtained is 57ºC.
321
In order to be able to understand the relationship between the PV temperature and the air velocity in
322
the duct, Figure 9 shows the air velocity measured with a hot wire anemometer probe which was
323
placed in the middle of the air gap between the PV panel and the back wall. The measurements were
324
taken every 30 minutes during the experimental procedure. The range of measurements is shown in
325
the boxes.
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TPV
Temperature (deg C)
50 45 40
Tamb
35 30
20 07:00
08:00
09:00
10:00
Time (hh:mm)
326
12:00
Figure 8. Temperature of the PV panel and the ambient air temperature.
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1.8
1.4
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Air Velocity in the Duct (m/s)
1.6
1.2 1.0 0.8 0.6 0.4 0.2 0.0
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7:00 7:30 8:00 8:30 9:00 9:30 10:0010:3011:0011:3012:00
Time (hh:mm)
328
Figure 9. Air velocity measured in the middle of the duct between the PV panel and the brick wall, including its range indicated
330
by the boxes.
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From Figures 8 and 9 it can be observed that while the temperature of the PV panel was increasing,
332
the air velocity was also increasing. According to Zogou and Stapountzis [26], in a case with buoyancy
333
flow experiment, the air velocity in the duct was from 0.0025 – 0.02 m/s which is very low in
334
comparison with the cases that the air is driven in the duct with the use of fan. As can be observed in
335
this study, although no fan is used to drive the air in the duct, the air velocity recorded is higher from
336
the values observed in [26].
337
Besides this, there was another issue we had to clarify, which concerns the temperature changes of the
338
air in the duct. It is expected that after the air enters the duct from the bottom, it will be heated from
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the hot PV surface and move upwards due to buoyancy. Thus, the flowing air is expected to exit the
340
duct at higher temperature than in the entrance. Figure 10 shows the temperature of the air in the
341
inlet of the duct (Tfi) and the outlet (Tfo). As can be observed, from 8:00 am to 12:00 noon the air in the
342
outlet was higher that the air in the inlet. Figure 11 shows the temperature difference (DT) between
343
the inlet and outlet of the air in the duct, which shows a similar trend as the input solar radiation.
344
Regarding the temperature of the brick wall, it was stable at about 25-27ºC during the experiment. As
345
mentioned by the authors in [27], for the analysis of BIPV/T systems the wall behind the PV panel can
346
be considered as an isothermal surface and the PV an isoflux surface.
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Tfo
34
Temperature (deg C)
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32 30 28
Tfi
26 24
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08:00
09:00
10:00
11:00
12:00
Time (hh:mm)
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Figure 10. Temperature of the air inlet and outlet of the duct between the PV panel and the brick wall.
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Temperature (deg C)
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-1 07:00
08:00
09:00
10:00
11:00
12:00
Time (hh:mm)
349 350
Figure 11. Temperature difference of the air inlet and outlet.
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351
The estimation of the energy and exergy efficiency of the system is made based on some assumptions
352
in order to simplify the analysis: •
355
(Figure 8). •
356 357
over time (Figure 7). •
358 359
The solar radiation on the PV surface is considered to be the same in all its area but changing
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The temperature of the PV is considered to be the same in all its area but changing over time
The temperature of the wall is considered to be the same in all its area remaining constant over time at 26ºC.
•
For the calculation of the convective heat transfer coefficients, the equations proposed in [27]
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are used. The range of the calculated values are from 3.4 W/m2K to 5.1 W/m2K depending on
361
the variation of the temperature of the PV.
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As already mentioned all equations are solved by using the experimental data in order to estimate the
363
energy and exergy efficiency of the system under natural outdoor conditions and natural air gap
364
ventilation.
365
Figure 12 shows the comparison of the PV temperature (Tpv) and the electrical conversion efficiency of
366
the PV (ηel). As can be observed, the hotter the PV gets, the lower is electrical conversion efficiency.
367
The electrical efficiency of the system is minimum when the temperature of the PV is maximum. As
368
shown, the electrical conversion efficiency varies between 24%-28.5%.
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TPV η el
27.5
Τ PV
27.0
45 26.5 40
η el (%)
50
28.0
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26.0 25.5
35
ηel
30 07:00
08:00
09:00
10:00
Time (hh:mm)
369
11:00
25.0
24.5 12:00
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Temperature (deg C)
55
Figure 12. The Photovoltaic panel temperature over time in comparison with the electrical efficiency of the PV
371
The thermal efficiency (ηth) which is affected by the temperature of the fluid inlet and outlet and the
372
electrical efficiency are shown in Figure 13. As can be observed, the electrical efficiency is more
373
dependent on the radiation and the PV temperature because it drops when solar radiation and
374
temperature of the PV is maximum. On the other hand, the thermal efficiency is following an upward
375
trend through time since it is mainly dependent on the temperature difference between the fluid inlet
376
and outlet. This relationship can be seen in Figure 14.
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27.5
ηel
9
η th
8 7 6
η th
27.0
η el (%)
η el
5
26.5
4
26.0
3
25.5
2
25.0 24.5 07:00
1 08:00
09:00
10:00
11:00
0 12:00
Time (hh:mm)
377 378
Figure 13. The thermal and electrical efficiencies of the system.
23
ηth (%)
28.0
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6
DT
8
4
2
4
η th (%)
6
3
1
ηth
0 -1 07:00
08:00
09:00
10:00
2
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Temperature (deg C)
5
10
0 12:00
11:00
Time (hh:mm)
379
Figure 14. Thermal efficiency of the system in comparison with the temperature difference of the fluid inlet and outlet of the air
381
gap.
382
Examining the I-V and P-V curves collected by the PVPM device in the graphs of Figure 15, it can be
383
observed that short circuit current drops due to the reduction of radiation, which can be observed
384
from Figure 7 at the times shown in Figure 15. The change in the open circuit voltage is almost zero.
385
Thus, the temperature of the PV surface did not affect the overall performance of the PV, which mainly
386
affects the open circuit voltage. This means that the 0.10 m air gap was adequate to create sufficient
387
air speed to remove the excess heat behind the PV panel, to prevent overheating and loss of efficiency.
388
The best curve is observed at 9:00 am where solar radiation was at maximum (perpendicular) which
389
also lead to higher PV temperature, which is shown in Figure 12.
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Current (A)
5 4 3 2
9:00 10:00 11:00 12:00
160
9:00 10:00 11:00 12:00
140 120
Power (W)
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100 80 60 40
1
20 0
0 0
5
10
15
20
25
30
0
35
5
10
15
20
Voltage (V)
Voltage (V)
390
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Figure 15. Current – Voltage and Power - Voltage curves during experiment.
24
25
30
35
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Earlier in Section 2 it was stated that the amount of energy flowing in and out of a system is the same
392
under thermally steady-state conditions according to the law of energy conservation. This is not
393
happening in exergy since the amount of exergy flowing out (Exo) is smaller than that flowing in (Exi),
394
because exergy is consumed within the system to produce entropy. This is shown in Figure 16.
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700
Exi
500 400
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Exergy (kWh/m2)
600
300
Exo
100 0 07:00
08:00
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09:00
10:00
11:00
12:00
Time (hh:mm)
Figure 16. The amount of exergy flowing in and out of the system.
397
Following the equations for the exergy analysis presented earlier in section 3.2, the exergy efficiency
398
of the system (nex) is calculated as shown in Figure 17 in comparison with the outlet air temperature
399
(Tfo). As can be observed, the exergy efficiency of the system varies from 13% to 16%. Although, the
400
data do not follow a specific trend, the exergy efficiency shows an increase from 9:30 am - 12:00 noon
401
where the radiation and PV temperature are high. The range of the exergy efficiency is in agreement
402
with the results obtained by Joshi and Tiwari [21] who estimated the exergy efficiency of a hybrid air
403
PV/T collector in India. Additionally, as shown in Figure 18, the energy efficiency is higher than the
404
exergy efficiency during the whole measurement period. The overall energy efficiency (nen) of the
405
system varies between 25.5-33.5%.
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Τf o
18
32
28
14
26
ηex
24
12
22 09:00
09:30
10:00
10:30
Time (hh:mm)
406
10 12:00
11:30
Figure 17. The exergy efficiency of the system related to the temperature of the air at the exit of the duct.
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08:30
ηex (%)
16
30
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Temperature (deg C)
34
34 32 30
26 24 22 20 18 16 14
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Efficiency (%)
28
ηen η ex
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12 10
408 409
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08:30
09:00
09:30
10:00
10:30
11:00
11:30
12:00
Time (hh:mm)
Figure 18. The energy and exergy efficiencies.
410
Unfortunately, no other paper is found on the same system (facade-building integrated naturally
411
ventilated system) to compare the results. These however are of the same order of magnitude as other
412
similar but forced circulation systems, which proves that the penalty paid in terms of exergy spent by
413
the fan (in forced circulation systems) is counterbalanced by the slightly lower performance of the
414
naturally operated system (lower heat transfer values), so the two systems, naturally and forced 26
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systems, behave similarly without the disadvantages of the forced system as outlined earlier in the
416
paper.
417
5 Conclusions
418
In this paper the energy and exergy analysis of a naturally ventilated BIPV/T system is studied
419
theoretically and experimentally. Based on the temperature distribution of the system, energy and
420
exergy analyses are carried out and the correlations for the energy and exergy efficiencies are
421
presented. The energy efficiency of the system is estimated to be up to 26.5-33.5% while the exergy
422
efficiency is estimated to be between 13-16%.
423
Based on the results obtained, the following general conclusions can be drawn:
425
The exergy efficiency of the system examined increases by increasing the temperature of the fluid at the outlet.
•
426
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•
424
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Vertical BIPV/T systems in façade applications (vertical position) performs better when facing east, due to the higher incident radiation due to the morning hours (the sun is almost
428
perpendicular). •
429
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The air gap of 0.1 m in a vertical BIPV/T system is enough to prevent PV panel overheating which will lead to significant loss of electrical efficiency. Maximum PV temperature observed
431
from the experiment is 57ºC. •
432
The electrical efficiency of the system varies between 24% and 28.5% when the temperature of the PV panel varies between 30-57ºC.
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434
•
The exergy input to the system is bigger than the exergy output of the system.
435
•
The energy efficiency of the system is higher than the exergy efficiency as in solar systems
436 437
exergy of solar radiation is very high because of the high temperature of the sun.
Nomenclature PU
Area of the collector (m2)
L
27
Length (m)
a
Fluid mass flow rate (kg/s)
Cross section area of the channel (m2)
P
Power output (W)
Specific heat of the fluid (J/kg K)
Pr
Prandlt number
Hydraulic diameter of the duct (m)
p
Perimeter (m)
Exergy rate (W)
Rate of useful energy transfer (W)
'
Energy rate (W)
sf
Specific Entropy (J/kg K)
f
Friction factor
!&01
Ambient temperature (°C)
FF
Fill factor
Gr
Grashof number
G
Solar radiation (W/m2)
g
Acceleration of the gravity (m/s2)
hf
Specific enthalpy of the fluid (J/kg)
ℎ
Convective heat transfer coeff. (W/m2 K)
KVU
K0
NOCT
439
!", !", !
Short circuit current (A)
Final fluid temperature (°C)
Temperature of the sun (K)
J0
Outlet fluid temperature (°C) Inlet fluid temperature (°C)
Cell temperature (°C) Voltage at maximum power point (V)
JTU
Open circuit voltage (V)
PV/T
Photovoltaic/Thermal
Current at maximum power point (A)
Abbreviations BIPV/T
!
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!e
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or :
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t
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Area of the PV (m2)
EP
PQR
Building Integrated Photovoltaic/Thermal Nominal Operating Cell Temp. (°C)
Greek symbols β
ηen
Packing factor of solar cells
28
Energy efficiency
Cells temperature coefficient
θ
Inclination angle of the system (°)
Exergy efficiency
ρ
Fluid density (kg/m3)
Subscripts and superscripts average
i
inlet or inside
pvt
photovoltaic thermal
en
energy
max
maximum
ref
reference conditions
ex
exergy
n
nominal
sys
system
el
electrical
o
outlet or outside
tot
total
f
fluid
pv
photovoltaic
SC
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av
th
thermal
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βc
Acknowledgements
442
The research presented in this paper was presented in the 9th International conference ECOS 2016, in
443
June 19-23rd, 2016 in Portorož, Slovenia
444
The work is carried out under the research program Building-integrated fibre-reinforced solar
445
technology (BFirst), funded by the EU Seventh Framework Programme FP7/2007-2013, under grant
446
agreement n° 296016.
447
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448
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Appendixes
508
Appendix I
509
Table 1. Characteristics of the devices used in the experimental procedure.
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Pyranometer: Eppley Radiometer PSP Spectral Range:
295-2800 nm
Oper. Temperature:
-50ºC to 80ºC
Output:
0-10 mV analog
Temp. Response:
0.5% (-30ºC to 50ºC)
Response time:
95%, 5 s
Tilt Response:
0.5%
Data Loggers: Daq Pro 8-channel Data Logger, Omega
Voltage
-250ºC to 1200ºC
Range:
Accuracy:
±0.5ºC
Accuracy:
Resolution:
0.1°C (1 μV)
Accuracy:
-250 to 1200°C:±0.5 %
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Range:
SC
Thermocouples: K- type
Resolution:
0 to 50 mV ±0.5 % 3 μV
Photovoltaic Panel - Polycrystalline 1640 x 992 x 45 mm
Power (W):
250
Impp(A):
8.16
NOCT:
47 ± 2ºC
Efficiency at STC: 15.46 %
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Area:
Isc(A):
8.61
Vmpp (V):
30.83
Voc(V):
37.41
Temp. Coefficient:
-0.45%/ºC
No of cells:
6 x 10, 3 strings in a row
Hot Wire Anemometer: HD 2303.0 Delta Ohm, Probe: AP471S2, Pt100 Air speed, flow rate, air Speed Resolution: temp.
0.01 m/s
Speed Range:
0.1 to 5 m/s
Temp. Resolution:
0.1°C
Temp. Range:
0 to 80°C
Cable length:
∼2 m
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Type of meas.
PVPM 2540C
Resolution:
0.01V-0.25V, 0.005A – 0.01A
Single Meas. Durat.:
20 ms up to 2 s
Voltage Range:
25V / 50V / 100V / 250V
Irradiance Range:
0 - 1300W/m2
Current Range:
2A / 5A / 10A / 40A
Temperature Range:
-40°C to +100°C
510 511 512 33