AbstractThe objectives of this research were introduced the methods of the
modeling and building the PV/T collectors for reducing excessive temperatures. Theeffect of excessive temperatures on photovoltaic (PV) modules causes decreasingefficiency.
In this study, we used PV model BP 253 installed with 2 cooling systems:photovoltaic/thermal-water (PV/T-Water) system and photovoltaic/thermal-fin (PV/TFin) system. For the PV/T-Fin system, we applied ambient air in the system. For the PV/T-Water system, we used 2.88×10−6 ,3.60×10−6 ,4.24×10−6 and 5.88×10−6
m3/s and of flow rate.According to our result, we found that the PV/T-Water system at the flow
rate 4.24×10−6 m3/s gave us the best result and it caused the excessive temperatureson the PV module decreasing 23.2˚C and 0.5 % of the gain efficiency, while thePV/T-Fin system could be caused the excessive temperatures on the PV moduledecreasing 10.0˚C and 0.3% of the gain efficiency, when compared with the PVmodule alone. The increasing of electrical yield of the PV/T-Water and the PV/T-Finsystem were equaled to 7.3 % and 5.3 %, respectively.
Key words: Solar cell, Photovoltaic, PV/T Collectors, Cooling Systems
1. IntroductionPhotovoltaic (PV) cell used the intensity solar radiation on the surface area
of PV to produce electricity power. The excessive PV temperature was causingelectrical power output (electrical yield) decreased 0.4 - 0.5 %/ ˚C for mono- andmulti-crystalline silicon solar cells [7]. Thus, the electrical efficiency was decreased0.06%/˚C [6]. The photovoltaic/thermal (PV/T) technologies are introduced todecrease excessive PV temperature and prevent the PV efficiency loss. Both of thePV/T water (PV/T-Water) system and air (PV/T-Air) system could apply fortemperature reduces on the PV modules. Moreover, the PV/T-Water system helped tokeep more efficiency than the PV/T-Air system [8].
Sopian et al. (1996) had presented that the double-pass PV/T-Air systemcould reduce more PV temperature and gain the PV efficiency than the single-passPV/T-Air system. Stefan Krauter (2004) had reduced excessive PV temperature by thecooling system with flowing water over the module front at 17 m3/h of the flow rateand found that the system could decrease excessive temperature up to 22˚C andcalculated that increase 9 % of the electricity yield.
Tonui and Tripanagnostopoulos (2007) developed PV/T-Air system whichconsist of glazed PV/T with fin (PV/T-Fin + GL) system and unglazed (PV/T-Fin +UNGL) systems. They showed clearly that the PV/T-Fin + UNGL system couldincrease more the PV efficiency than the PV/T + Fin + GL system.
Nomenclature fins area ( ) fins temperature ( ) 2m finTfinA K
TAA ( )2m inT water temperature inlet ( ) area of TA sheet K
pc hest capacity of water ( )KJ/kg ⋅ PVT PV module temperature
solar irradiance
( )K
iI ( )2W/m outT water temperature outlet maximum electric current
( )K
maxI ( )A TAT TA sheet temperature short-circuit electric current
( )K
scI ( )A surT surround temperature ( )K k thermal conductivity ( )KW/m ⋅ m& mass flow rate )
heat convection from fins to air (kg/s
surfinc,q → ( )W
surTAc,q → heat convection from TA sheet to surround ( )W
tubec,q heat convection with water ( )W
surfinr,q → heat radiation from fins to surround ( )W
surTAr,q → heat radiation from fins to surround ( )W
heat radiation per area unit rq ′′ ( )2W/m
ocV open-circuit electric voltage ( )V
maxV maximum electric voltage ( )V Δx thickness of materiel ( )m Greek symbols
emissivity of materiel Stefan-Boltzmann constant
εσ
( )28 W/m1067.5 −×
2. The objectives 2.1. To study the method of modeling designs and development the PV/T collectors and investigate the gain efficiency of the PV module. 2.2. To construct the PV/T-Water system and the PV/T-Fin system to reduce the excessive temperature on the PV modules. 2.3. To compare the influence of the flow rate on reducing the excessive temperature. 3. The modeling developments and methods
In Fig.1, we showed the cross-section views of our 2 studied models: the PV/T-Water system (see Fig.1a) and the PV/T-Fin system (see Fig.1b).
The PV/T-Water system composed of a thin aluminum (TA) sheet which was about 2 mm of thickness and water tubes (made from copper tube) were 1 mm of thickness, 0.15 mm of diameter and 42 cm of length. In this work, we installed 7 tubes and the free space between each tube was 10 cm.
Fig.1. Show cross-section view of the PV/T collector model Both of the PV/T-Water collector and the PV/T-Fin collector were
attached to the rear surface of each PV module (see in Fig.2). The PV/T-Fin system composed of TA sheet and the fins made from aluminum sheet. The TA sheet was about 4 mm of thickness and the fins were 2 mm of thickness, 6 cm of height and 49 cm of length. In our designed system, we installed 30 pieces of the fins and each fin had separated by 2 cm. The surface area of the TA sheets for the PV/T collectors was equaled to . 2m0.395
(a) The PV/T-Water system
(b) The PV/T-Fin system
Fig.2. The structure and installation of the PV/T systems for tested out door of the PV/T-Water system (a) and the PV/T-Fin system (b), respectively.
3.1. Conditions of the experiment study
In our experiment, we used 2 mono-silicon PV modules (model BP 253) and their electrical power outputs were equivalence.
The system was installed at a tilt angle of (approximate tilt angle of Songkla Province, Thailand) on a mobile structure and a front of PV modules turned to the north. Irradiance was measured by a DL-204 of TENMARS luxmeter at the
same incidence plane of the PV modules. Data Logger model DL 2100 (K type of sensors) was used to record temperature. Ambient temperature was measured in the shade (under PV modules), while PV temperature was measured on the rear of the 2 PV modules. In addition, we measured temperature at inlet and outlet of water and at the middle position of fin.
Tracking of the maximum power point was done manually by a variation of an ohmic load and measuring electrical power output. Data was recorded every 10 min from 8:00 AM to 5:00 PM. Fig.3 illustrated system installations and structures of the PV/T-Water system and the PV/T-Fin system. In the PV/T-Water system, we provided the flow rates of water at , , and
from water tank. For the PV/T-Fin system, we used free air force from the ambient air.
61088.2 −× 61060.3 −× 61024.4 −×61088.5 −× /sm3
Fig.3. Show location and structure of the PV/T-Water system in tested outdoor.
3.2. Energy Balance Equations for Modeling Designs
The equations describing of energy balance for the energy flows and designing models could be defined from the theoretical models. Fig.4 illustrated heat convection coefficient and heat radiation transfer of the energy flows on the PV/T-Fin and PV/T-Water collector. The energy balance consists of input energy and output energy. The input energy was obtained from the optical incidences in front of the PV surface and the output energy was obtained from the heat loss on the PV/T collectors. The assumptions of the energy balance equations for the PV/T collectors are given by [1], [2], [3] and [6]. In addition, we shall define from the heat achieve and the heat loss in Fig.4 as follows: 3.2.1. PV/T-Fin system
Fig.4. Schematic diagrams of described the energy flows on The PV/T-Fin
collector (a) and the PV/T-Water collector (b), respectively. 3.2.2. PV/T-Water system
( ) ( )surTAsurTAc,ATTA
TAPVTAAT TThA
ΔxTT
kA −=−
→
( ) ( )inoutp4
sur4TATAAT TTcmTTσεA −+−+ & (2)
In Eq.1 and Eq.2, The left term occurs from the PV/T collectors and it was achieved the heat current from the PV module and the right terms occurs from it’s transfer heat to the ambience air that occur on the PV/T collectors as follow Fig.4. 3.3. Thermal energy The thermal efficiency ( )thη could be defined from [2], [3] and [6] as follows: 3.3.1. The thermal efficiency of the PV/T-Water collector ( )wth,η was obtained from
iPV
tubec,surTAr,surTAc,wth, IA
qqqη
++= →→ (3)
where , and could define from Eq.2 surTAc,q → surTAr,q → tubec,q
3.3.2. The thermal efficiency of the PV/T-Fin collector ( )fth,η was obtained from
iPV
surTAr,surTAc,surfinr,surfinc,fth, IA
qqqqη →→→→ +++
= (4)
where , , and can define from Eq.1 surfinc,q → surfinr,q → surTAc,q → surTAr,q →
3.4. Electrical energy The electrical efficiency ( )elη of the PV module is varied as a function of the irradiance intensities in front of the PV surface, from [4] and [5] is given by
iPV
ocscel IA
VIFFη
⋅= (5)
Where is a fill factor of the PV module. ocscmaxmax V/IVIFF = 4. Results and Discussion
Due to the various weather conditions in each day, the characteristic electricity yields of the PV modules shall be changing follow the irradiance of the sunshine as incidence in front of the PV surfaces and the operator temperature on the PV modules. We could obtain the effect of the reduction of excessive temperature on the PV modules by utilization from the PV/T-Water collector and the PV/T-Fin collector.
Fig.5. Comparison between the PV module and the PV/T-Water system at of the flow rate on April 09, 2008. /sm104.24 36−× 4.1. The effects of The PV/T systems
In Fig.5, we presented the result for the case of the PV/T-Water system at the flow rate . Similarly, in Fig.6, we presented the result for the case of the PV/T-Fin system compared with the PV module.
Fig.6. Comparing between the PV module and the PV/T-Fin system on April 28, 2008. 4.2. The electrical efficiency and the thermal efficiency The values of the electrical efficiency of the PV/T-Water and the PV/T-Fin system could be calculated from Eq.5 and the thermal efficiency of the PV/T-Water and the PV/T-Fin system could be calculated from Eq.3 and Eq.4, respectively. In Table 1, we summarized the best values of each condition. According to this result, we suggested that the PV/T-Water system at the flow rate could offer less power loss than the other system (see Table1).
/sm104.24 36−×
Table 1. Summaries list of the best results from the experiment in this research
Fig.7. Showing comparison the electrical power output between the PV module and the PV/T-Water system at the flow rate on April 09, 2008. /sm104.24 36−×
Fig.8. Showing comparison the electrical power output between the PV module and the PV/T-Fin system on April 28, 2008. In Fig.7 and Fig.8, we illustrated the results of the electric power obtained from the PV/T systems comparing with the PV module. In Fig.9, we presented the influence of the flow rate for the case of the PV/T-Water system and fins of the PV/T-Fin system. Clearly, we obtained the maximum electrical yield from the PV/T-Water system when the system was supplied at the flow rate . /sm104.24 36−×
Fig.9. Illustrating of the relationship between the flow rates of the water system with the electric power and effect of the fin system
5. Conclusion The results of this experiment could confirm that the PV/T collectors could help on increasing the electricity yield more than using the PV module alone. Clearly, our results presented that the PV/T-Water system could prevent more the energy loss than the PV/T-Fin system. However, we suggested that in each application, the appropriate system must be concerned. 6. Suggestion
We would like to increase the performance of the PV/T-Fin system to be better efficiency than the present work by developing other fins modeling.
In the case of the PV/T-Water system, we will study other parameter such as the effect of controlling temperature of flow water before supplying to the PV/T-Water system and study the influence of other flow form of the water.
We also suggest that warm water obtained from this system could be reused for the related application to save electrical energy.
We would like to suggest to focus on the effect of material of fins in the PV/T-Fin system. For instance, material alloys could offer better potential on transferring heat from the source to the surroundings better than the pure materials.
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