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12СОЛНЕЧНАЯ ЭНЕРГЕТИКА
SOLAR ENERGY
Статья поступила в редакцию 06.11.10. Ред. рег. № 872 The article has entered in publishing office 06.11.10. Ed. reg. No. 872
ТЕПЛОВАЯ ПРОИЗВОДИТЕЛЬНОСТЬ ГИБРИДНОГО ФОТОГАЛЬВАНИЧЕСКОГО/ТЕПЛОВОГО КОЛЛЕКТОРА, СПРОЕКТИРОВАННОГО ДЛЯ ЕСТЕСТВЕННОЙ ВЕРТИКАЛЬНОЙ ТЯГИ ОХЛАЖДАЮЩЕГО ВОЗДУХА В ЗДАНИЯХ,
НАХОДЯЩИХСЯ В ТРОПИКАХ
1 12 Я. Нугблега , М. Банна , К. Напо
'Лаборатория солнечной энергетики / Группа явлений переноса энергии - Университет Ломе BP 1515 Ломе-Того, Тел +228 225 50 94, Факс: + 228 221 85 95 2Кафедра ЮНЕСКО возобновляемых источников энергии (CUER) BP 1515 Ломе-Того, Тел +228 2255094, Факс: + 228 221 85 95, e-mail: magbanna@yahoo.fr
Заключение совета рецензентов: 26.11.10 Заключение совета экспертов: 30.11.10 Принято к публикации: 05.12.10
Во всем мире, и особенно в развивающихся странах энергетический кризис и экологические проблемы приводят к снижению потребления энергии, связанного с использованием систем принудительного нагрева и кондиционирования воздуха путем применения пассивных технологий кондиционирования. Доказано, что комбинация различных технологий пассивного кондиционирования значительно уменьшает нагрузку на энергосети и улучшает комфорт в зданиях. В частности, фотогальванические панели в солнечных коллекторах на крышах могут обеспечивать здания электричеством и теплом одновременно.
В этой работе проводится анализ гибридного фотогальванического/теплового коллектора (PV/T), интегрированного в здания и спроектированного в виде дымохода для пассивного тока воздуха в зданиях, находящихся в тропиках. Исследуются потенциально полезные характеристики конструкции PV/T для определения эффективности и взаимодействия с различными элементами. Исследование характеристик сфокусировано на двух основных областях: увеличение тепловой эффективности и уменьшение температуры солнечных элементов для улучшения электрической эффективности. Созданы квазидвумерные модели переноса тепла PV/T, интегрированные в здание в виде дымохода. Разработана модель на основе конечно-разностного метода и итеративного метода Гаусса-Зейделя. Проведенный численный анализ подтверждается экспериментами с воздушным коллектором PV/T. В работе оценивается и представляется падение давления в дымоходе и температуры компонентов PV/T системы.
Ключевые слова: фотогальваническое элементы, гибридный солнечный коллектор, теплопередача, моделирование, симуляция.
THERMAL PERFORMANCES OF HYBRID PHOTOVOLTAIC/THERMAL COLLECTOR DESIGNED FOR NATURAL AIR UPDRAUGHT COOLING INTO
BUILDINGS IN TROPICS
Ya. Nougblega1, M. Banna1, K. Napo2
'Laboratoire Sur l'Energie Solaire / Groupe Phénomène de Transfert et Energétique - Université de Lomé BP 1515 Lome-Togo, Tel +228 225 50
94, Fax: + 228 221 85 95 2Chaire Unesco sur les Energies Renouvelables (CUER) BP 1515 Lome-Togo, Tel +228 2255094, Fax: + 228 221 85 95, e-mail: magbanna@yahoo.fr
Referred: 26.11.10 Expertise: 30.11.10 Accepted: 05.12.10
In the world and more particularly in the development countries, energy crisis and environmental problems lead to reduce the great electrical energy consumption related to the use of the heating ventilating and air conditioning processes by introducing passive air-conditioning techniques. It has been proved that the combination of various techniques passives air-conditionings decrease the energy loads considerably and improves comfort into the buildings. Particularly, the inserting of photovoltaic panels into roof solar collectors can provide electricity and thermal heat simultaneously.
International Scientific Journal for Alternative Energy and Ecology № 11 (91) 2010
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This paper analyses of hybrid Photovoltaic/Thermal collector (PV/T) integration into buildings designed as chimney for passive air updraught into buildings in tropics. Potentially useful features in the design of PV/T are explored in order to determine the effectiveness and interacting of different elements. Features explored center on two main areas: increasing thermal efficiency and reducing solar cells temperature in order to improve electrical efficiency. Nearly bi-dimensional heat transfer models of PV/T integrated into buildings as chimney are established. A simulation model based on implicit finite-differences and Gauss-Seidel iterative methods has been developed. The numerical analysis developed is supplemented by an experimental work on a PV/T air collector. The pression drops in the chimney and temperatures of PV/T system components are evaluated and presented.
Keywords: photovoltaic cells; hybrid solar collector; heat transfer; modelling; simulation.
Organization: University of Lome Togo, Faculty of Science, Department of Physic, Solar Energy Laboratory.
Education: I received my Bachelor of Science degree in physic in 2002 and my master's degree in Material science in 2006 at the University of Lome. I am preparing my Doctorate Thesis at University of Lome.
Experience: Scientific research project, member of Laboratory of Solar Energy of University of Lome, ATER (Attaché Temporaire d'Enseignement et de la Recherche) of University of Lome since January 2007.
Main range scientific interest: Modeling, Heat transfer, Passive cooling.
Yawovi Nougblega
Magolmèèna Banna
Organization: University of Lome Togo, Faculty of Science, Department of Physics, Solar Energy Laboratory.
Education: At University of Perpignan (France), I received successively my Bachelor of Science degree of physics in 1985, my master's degree in Thermodynamics/Energetic system in 1987 and Unique Doctorate Thesis in 1990.
Experience: Lecturer at University of Lome since 1993, Assistant Professor since 2003 and Head of Phenomena Transfer and Energetic Group at Solar Energy Laboratory. I am Head of Division in charge of academics inscriptions in master and doctorate programs at University of Lome since 2007.
Main range scientific interest: Modeling, Heat and Mass transfer in porous materials (crops and vegetation). Measure and identification of mechanics and thermal property of material. Solar radiation and systems. Low energy buildings designing and passive cooling. Waste combustion.
Publications: 12 Papers in international and national scientific journal and five several oral communications.
Kossi Napo
Organization: University of Lome Togo, Faculty of Sciences, Department of Physics, Solar Energy Laboratory.
Education: master degree in physic in 1982 at the University of Abidjan (Côte d'Ivoire), DEA in material sciences in 1983 at University of Nantes, First Doctorate Thesis in 1986 at University of Nantes, Second Doctorate Thesis in 1998 at University of Nantes and University of Lome.
Experience: Lecturer at University of Lome since 1988, person in charge of Doctorate training in material science at Faculty of Sciences, National coordinator of UNITWIN of UNESCO, Senior associate of ICTP(Italy), member of scientific committee of University of Lome.
Main range of scientific interest: Renewable Energy, solar energy, thin film research, optoelectric and electronic thin films applications.
Publications: 20 papers in international and national scientific journal, 10 oral communications and 5 posters.
Nomenclature Greek symbols
а: absorptivity
Letters P: air density (kg-m"3)
Cp: heat capacity J-kg-1-°C-1 e : Inclination of the collector (°)
e: thickness (m) s: emissivity (")
h: heat transfer coefficient (Wm-2-K-1) Ф: Energy flux per unit area on 8 inclined plate W-m"2
m a : air flow rate (m3s-1) £ transparency coefficient (")
R: S: radiosity (W) PV module surface (m2) т: X: transmittance (") Thermal conductivity (W-m"'-K"1)
t: time (s) о: Boltzmann constant (5.6697-10"8 W-m"2-K"4)
T: temperature (°C)
У: axis coordinate (m)
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Subscripts
a: air
f fluid (air)
v: glass
pv: pv cells
as: air up the PV plate
ai: air under back cover
g: ground
w: back cover
r: radiative coefficient
Introduction
The best strategy when considering a photovoltaic (PV) module is to integrate it into the initial design of the building. The inserting of semi transparent PV panels in a building can provide natural ventilation by adopting it as a roof or chimney solar collector. In this way the ventilation will be dominated by thermal buoyancy because the outdoor air will enter the room through the opening vents to the rooms, and exit through a hybrid photovoltaic/thermal chimney. Hybrid Photovoltaic/ Thermal (PV/T) systems are new type of solar energy devices that provide electricity and heat simultaneously. They consist of PV panels and thermal units mounted together as an integral unit. The unit, employing low temperature circulating fluid, is used to extract the absorbed sunlight that is not converted into electricity by the PV cells.
Several studies have been done and reported on PV/T solar collectors. Extensive studies on water and air cooled PV/T collectors have been presented by Tripanagnostopoulos et al [1-3] who gave extensive experimental results of their studies and detailed references. A review of measurements and theoretical studies [4] made concerning the performance of PV/T collectors, in both electrical and thermal energy
production, suggests two general conclusions: that air PV/Ts are less efficient than liquid, and that both electrical and thermal efficiencies need improvement. Tripanagnostopoulos et al [5] also did experimental and theoretical study of a modified PV/T air system employing a low cost Thin Flat Metallic Sheet (TFMS) for heat extraction improvement from the PV panel. Regarding the performance during hot season, Khedari [6] observed that the simple roof solar collector will not meet the required ventilation level and recommended the use of PV driven fan to enhance air flow rate.
The interest in this study is to investigate the possibility of using the heat recovered from PV incorporated in solar flat-plate collector and used as chimney (and intended to meet its electrical loads) to provide natural air updraught cooling in tropics with minimum additional cost. The study focus on the modelling of the complex PV/T designed as chimney by joining classic flat-plate and flat-plate PV/T collectors end to end to enhance the natural flow, giving ventilation rate required for effective cooling and therefore reducing the PV cells temperatures.
Theory and methodology
Theoretical analysis: Collector design and PV/T's temperatures models The schematic diagrams of flat-plate PV/T air collectors are illustrated in Fig. 1. Two designs PV/T are carried out and referred as CHPVTC (Complex Hybrid Photovoltaic/Thermal air Collector) and ICHPVTC (Inversed Complex Hybrid Photovoltaic/Thermal air Collector). The CHPVTC is obtained by joining Simple Hybrid Photovoltaic/Thermal air Collector in zone 1 (SHPVTC) and Simple Classic flat-plate collector SCC in zone 2 end to end in order to enhance the natural flow and therefore reduce the PV cells temperatures.
Рис. 1. Схема гибридных фотогальванических/тепловых моделей Fig. 1. Schematic view of Hybrid Photovoltaic/Thermal design models
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The PV/T modelling is based on nearly bi-dimensional model which explains the essential thermal transfers. This model is composed of a serial assembling of many one dimensional elementary models. Each model is based on a nodal discretization of a hybrid photovoltaic/thermal. The studied domain is broken up into elementary volumes supposed isothermal, and for each node, thermal balance equations are written. These equations are elaborated from a modelling based on an electrical analogy where temperatures, flows, flow sources and imposed temperatures are respectively assimilated to potentials, currents, current generators and voltage generators. In this model thermo-physical properties in solids are considered constant. The simulation is based on a one-dimensional model function of the time and uses the following assumptions:
• All material properties are presumed to be independent of temperature and equal on both sides.
These equations can be easily rewritten when considering ICHPVTC (Inversed Complex Hybrid Photovoltaic/Thermal Collector). The net radiative-heat flux Qri and Q,- are expressed by the approximated expressions:
Q„ =°[T4 - T4]/(l/e, +1 /e, -1) and Qri=-Q„ . (9)
• The part of solar radiation which is not converted into electrical energy is absorbed by the PV cells as thermal energy.
• The multiple reflections and transmissions between the components (particularly between the photovoltaic cells and the front glass) and the radiation exchange of the PV cells to the glass are considered as negligible. Taking into account these effects introduce numerous terms difficult to determine and to measure [7]. A very good study of these various effects was performed by Krauter and Hanitsch [8].
• The ambient temperature is postulated as equal on all sides of the module.
The simulation is done by dividing the collector into four isothermal regions: the front glass cover, the semitransparent photovoltaic cells (PV), the air flow and the back wall. For each node, thermal energy balance equations are written as following:
' (1)
(2)
(З)
(4)
(5)
(6)
(7)
(8)
The pression drop in channel due the buoyant pressure head can be calculated from the equation given by Ekechukwu and Norton [9] and considering the inclination of the PV/T system:
AP = g(L sin 0)(pfl -pch), (10)
First zone: Simple Hybrid Photovoltaic/Thermal air Collector (SHPVTC).
■ Front glass:
djv dt
Photovoltaic cells:
ev (PCP)y-T- = avФ + GV,PV (TPV - TV ) - has (TV - Tas) - К,Sky (TV ~ TSky ) - К,g (TV - Tg
ef (PCP)f I + Uf -у I = hal (Tpy - Tf ) - ha2 (Tf - Tw ) with hna = Ufef ;
dT
epv(1 -D(pCp)= [(-npv))1 4) + «vQri00-GvpvTv-Tv)-hal(Tpv - Tf);
Air flow :
dT, dT, f + U^—f dt f dy
■ Back wall or cover :
dT
ew (P Cp)^~dtL = ^TV aw Ф + ha 2(Tf - TW ) - kai (TW - Tai) + Qri (У) - hrw 'Sky (TW " ^ ) - ^ (Tr - ^ ) '
Second zone: Simple Classic air Collector (SCC)
■ Front glass:
dTv
ev (PCP)v -jf = avФ - Ks (Tv - Tas ) - ha5 (Tv - Tf ) - Qri - К,Sky (Tv - TSky ) - К,g (Tv - Tg) ;
■ Air flow in the channe :
idT dT ^
~dt~ + UfI = ha5 (Tv - Tf) - ha6 (Tf - Tp) with ma = Ufef ;
Absorber plate:
T
ep (P CP)^~dP = TV а P ^ + Qrj - GP,W (TP - TW ) - ha 4(TP - Tf ) ;
For back wall:
dTw dt
ew (PCP)w^T- = GP,W (TP - TW ) - К (TW - Tai ) - К,g (TW - Tg ) - К,Sky (TW - TSky ) .
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where g is the gravitational constant, L is the length of air channel, 8 is the inclination angle, pa and pch are average densities of the ambient air and the air in the channel. Expressing equation (10) in terms of air channel and ambient temperatures, Tch and Ta respectively gives [9]:
AP = ßgpaL sin Q(Tch - Ta ),
(11)
where p is the coefficient of volume expansion of the air and is equal to the reciprocal of absolute temperature for an ideal gas. Natural convective and radiative-heat transfer coefficients and others various thermal coefficients used in the equations are deduced from the classic relations.
Transfer equations are discretized using the full implicit finite-differences method. The resolution of the equations is made using Gauss-Seidel iterative method. Thermal performances of PV/T collectors are investigated using simulation programs written in FORTRAN. The density and heat capacity of the air depending of temperature, natural convective and radiative heat transfer coefficients used in the model are deduced from the classic relations [2, 3, 10, 11, and 12].
Experimental setup and procedure
Experimental measures on PV/T collectors are carried out under meteorological conditions at the Laboratory in Lome (Togo) in November 2009. Fig. 2 is showing the schematic view of instruments and test structures.
Рис. 2. Экспериментальное устройство: схематическое изображение приборов и экспериментальных устройств Fig. 2. Experimental device: schematic view of instruments and test structures
The collectors are equipped with a network of type E thermocouples placed in different location in order to measure temperatures of PV cells, air flow and absorber plate with time and along the length of the collector. The thermocouples were placed and handled with care to ensure proper temperature data. These thermocouples make it possible to measure a range of temperature ranging from -200 °C to +1000 °C with an absolute uncertainty lower than 1 °C, for the gap of temperature going from 0 °C to +70 °C. The thermocouples are calibrated at the Laboratory using a bath of melting ice at 0 °C and a bath controlled in temperature. The acquisition chain included a network of thermocouples, 20 channel multiplexer 7700, KEITHLEY DM2700/E data acquisition system integrating the IEEE card, GPIB interface using KUSB-488A port and a micro computer. The temperature measurement is carried out with an accuracy of 0.5 degree. Data processing is carried out using Excelinx acquisition software. Global solar radiation on a horizontal plan is recorded using a pyranometer and data Logger. Wind velocity is measured by a hot wire anemometer TESTO 425 which provides values with an accuracy of 5%.
Results and discussions
Numerical calculations have been performed corresponding to the hourly variation of global solar radiation and ambient air temperature on typical dry season day (November) in Lome. These meteorological data are used as inputs to the program. Solar energy reaches a maximum value of 750 W/m2 at 12 noon whereas the maximum air ambient temperature reaches a maximum of 307 K at 1 p.m, with one hour delay time. The wind speed and air humidity remain nearly stable with average values respectively of 3 m/s and 68% during the experiment days.
The interest in this study was to investigate the possibility of using the heat recovered from PV incorporated in solar flat-plate collector designed as chimney (and intended to meet its electrical loads) to provide natural air updraught cooling in tropics with minimum additional cost. The parameters that were monitored are the temperatures of the system components and as well as the pression drops along the channel.
The results obtained from the SCC and SHPVTC indicated that in spite of the fluctuation of the values with time, theoretical and experimental temperatures curves have the same pattern. As shown on the Fig. 3, they are correlated to the ambient air temperature and solar radiation variations.
In fact, the temperatures are changing with irradiance as expected. From 10:00 pm to 16:00 pm, high values are generally observed (Fig. 4). As the radiation start decreasing in the afternoon, the air output temperature starts to decrease. The output airflow temperatures depend strongly on the instantaneous irradiance and thus its flow rate is itself regulating with irradiance level. In
International Scientific Journal for Alternative Energy and Ecology № 11 (91) 2010
© Scientific Technical Centre «TATA», 2010
other words, the flow rate is high during high isolation period, resulting in the required high ventilation rate and vice versa.
12 14
Solar Time (h)
Рис. 3. Дневное колебание температуры воздуха: эксперимент Fig. 3. Daily variation of the air temperatures: Experimental
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Solar time (h) b
Рис. 4. Дневное колебание температур PV элементов (a) и потока воздуха (b) Fig. 4. Daily variation of Pv's cells (a) and air flow (b) temperatures
Otherwise, as expected, the SCC is thermally more performed than SHPVTC. The maximal air temperature arise 41 °C in the first collector against 50 °C in second with the inlet air temperature equal to 28 °C. The need of the improvement of thermal and electrical performances of SHPVTC simultaneously, involved the proposal of the new designs of collectors and the study of the thermal characteristics.
p
w 5C-
дДдааааааадл
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PV cells Fluid (air)
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0.6
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Lenght of the colloctor y (m)
a
—Д— Absorber -O- PV cells —□— Fluid (air)
—I—1—I—1—I—1—I—1—r^—I—1—I—1—I—1—I—1—I—1—I—1—
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1
Lengt of the collector ; y r (m)
b
Рис. 5. Изменение температуры вдоль CHPVTC: a - модель; b - эксперимент Fig. 5. Variation of the temperatures along CHPVTC length: a - model; b - experimental
The variation of the system components' temperatures against different collector length are plotted respectively in Fig. 5 and 6. Naturally, the absorber temperature of the SCC is higher than the temperatures of PV cells whatever the configurations considered. It can be seen that the temperature recorded experimentally is higher than the numerical results. The raison may probably be related to PV cells thermal and optical properties values used. In fact they are not available and were estimated in this study.
65-
SC
55-
45-
4C-
35-
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0.В
б
1В
50-
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35-
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e а Е
e
65- ' ' ' Лд 1 1 1 1
60-
55- /
50- у
45-
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—Л— Absorber -О- PV cells —□— Fluid (air)
It can be seen that the ICHPVTC induces higher pressure drop at the exit of channel and by consequently high airflow and ventilation rates, required for an effective cooling.
The results indicate that high thermal and electrical efficiencies can not be obtained simultaneously in any collector proposed. However, electrical performances and the ventilation rate may be improved simultaneously by designing CHPVTC system in its second zone as jet pipe.
Air flow rate = 5.10"3m3.s"1 Time = 14 p.m.
ra
er о.
mte 47 to
о <л
Lenght of collecor : y (m)
Рис. 7. Изменение температуры PV элементов вдоль коллектора
Fig. 7. Variation of PV's cells temperatures along the collector
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 Lenght of the collector : y(m) b
Рис. 6. Изменение температуры вдоль ICHPVTC: a - модель; b - эксперимент Fig. 6. Variation of the temperatures along ICHPVTC length: a - model; b - experimental
Comparing the thermal performances, Fig. 7 shows the variation of the solar cells temperatures along the length of the SHPVTC, CHPVTC and ICHPVTC, whereas the figures 4a and 4b show respectively daily variation of the solar cells and air average temperatures.
The results on Fig. 7 indicate that solar cells temperatures of CHPVTC system are lower than those obtained from SHPVTC and ICHPVTC system. As shown by the curves in Fig. 4, b the thermal efficiency of ICHPVTC system is higher than CHPVTC system indicating that the heat transfer to air in the ICHPVTC system is increased, hence high heat gain by airflow.
The buoyancy force causes air flow in solar chimney and this force is proportional to the difference between the mean air density within the chimney and the ambient air density. The pression drops for each collector are calculated. For CHPVTC and ICHPVTC, pressure drops decrease progressively along the first zone. In the second zone the pression drops continue to decrease for CHPVTC system even though it increases for ICHPVTC system (Fig. 8).
7,0' 6,96,8 6,7-
- 6,M
CL
W 6,5 O.
< 6,4 4
6,З 6,26,1
1 i 1 i 1 1 1 1 1
-□- CHPVTC 1
-o- ichpvtcI
^ :
0,0 0,2 0,4 0,6 0,8 1,0 1,:
Lenght of the colloctor (m)
Рис. 8. Изменение давления вдоль коллектора Fig. 8. Variation of pression drops along the collector
Conclusion
Thermal performances of Hybrid Photovoltaic/ Thermal Collector have been evaluated. A numerical model based on an implicit finite-differences method was developed for calculating the time-dependent temperature variation of PV/T components. Gauss-Seidel's iterative method was used to resolve linear equations. The models were applied for the simulation
50
0,2
1,0
1,2
49
100
O
International Scientific Journal for Alternative Energy and Ecology № 11 (91) 2010
© Scientific Technical Centre «TATA», 2010
and comparison of the thermal characteristics of two design PV/T air collectors to be integrated into buildings as chimney in tropics. The investigation was carried out under variable climatic conditions. The numerical analysis developed is supplemented by an experimental work on a PV/T air collector. The results indicate that ICHPVTC system offers a good thermal efficiency even though a good electrical efficiency is given by CHPVTC system. The integration of Hybrid Photovoltaic/Thermal Collector into building seems to be appropriated in tropics for passive air updraught. Therefore, the ventilation rate may be improved by using CHPVTC system and design its second zone as jet pipe or/and by taking part of electricity generated by the PV module to drive a low power fan to enhance airflow during hot seasons in tropics.
References
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