DT-17
CONFORMIDAD DE DEPÓSITO DE TESIS DOCTORAL POR LA COMISIÓN ACADÉMICA DEL PROGRAMA
D/Dª. Angel Molina García, Presidente/a de la Comisión Académica del Programa de Doctorado en Energías Renovables y Eficiencia Energética,
INFORMA:
Que la Tesis Doctoral titulada, “Thermal and electrical characterization of thin film photovoltaic technologies for building integration (Caracterización térmica y eléctrica de tecnologías fotovoltaicas de lámina delgada para su integración arquitectónica en edificios)”, ha sido realizada, dentro del mencionado Programa de Doctorado, por D/Dª.
Carlos Alberto Toledo Arias, bajo la dirección y supervisión del Dr. Antonio Urbina Yeregui.
En reunión de la Comisión Académica del pasado 8 de Julio de 2019, visto que en la misma se acreditan los indicios de calidad correspondientes y la autorización del Director/a de la misma, se acordó dar la conformidad, con la finalidad de que sea autorizado su depósito por el Comité de Dirección de la Escuela Internacional de Doctorado.
X Evaluación positiva del plan de investigación y documento de actividades por el Presidente de la Comisión Académica del programa (RAPI).
La Rama de conocimiento por la que esta tesis ha sido desarrollada es:
Ciencias
Ciencias Sociales y Jurídicas X Ingeniería y Arquitectura
En Cartagena, a 8 de Julio de 2019
EL PRESIDENTE DE LA COMISIÓN ACADÉMICA
Fdo: Angel Molina García
COMITÉ DE DIRECCIÓN ESCUELA INTERNACIONAL DE DOCTORADO
ANGEL|
MOLINA|
GARCIA
Firmado digitalmente por ANGEL|MOLINA|GARCIA Fecha: 2019.07.17 19:32:11 +02'00'
Versión de Adobe Acrobat:
2019.012.20035
A mi abuela Asceneth, porque tus huellas no se olvidan
Acknowledgements
Quiero aprovechar estas l´ıneas para hacer p´ublico mi agradecimiento a mis directores de tesis, los profesores Antonio Urbina y Jos´e Abad, por todo lo que me han ense˜nado a lo largo de estos a˜nos, por su paciencia, apoyo y consejos. De ambos he aprendido que la ciencia requiere trabajo diario, paciencia, rigor y esmero para salir adelante, y a ambos quiero agradecerles su calidad humana y la confianza que han depositado en m´ı para llevar a cabo esta tesis. Mi especial gratitud a Antonio Urbina porque sin ´el esta tesis no habr´ıa sido posible. La ciencia no ser´ıa la misma sin ´el.
I would also like to express my gratitude to professor Sjoerd Veenstra from ECN-Solliance, who gave me the opportunity to visit and use the laboratories in addition to providing the samples, resources and support to develop the study about BIOPV glass-glass samples. Likewise, I would like to thank to Giorgio Bardizza from the Joint Research Centre, for providing me with the opportunity to learn in depth about ESTI laboratories and the chance to participate in research concerning electrical characterization of BIOPV modules. I would like to extent my gratitude to Ana Maria Gracia for her advice on solar radiation models and energy rating analysis. Thank you all for your warm welcome and hospitality.
Tambi´en quiero aprovechar para agradecer a mis compa˜neros doctorandos del d´ıa a d´ıa a lo largo de estos a˜nos: Guido, Vero, Rodo, Adela, Mar´ıa, Chencho y Chiraz por compartir conmigo esta experiencia y por brindarme apoyo cada vez que lo necesitaba. Todos vosotros sois luchadores y ten´eis un gran potencial. Mucho ´animo en esta ´ultima etapa.
Gracias tambi´en a la doctora Luc´ıa Serrano por sus consejos, amabilidad y apoyo. Tambi´en a la doctora Nieves Espinosa por ayudarme en la primera etapa de la tesis, durante la elaboraci´on de los PV-Cubes. Seguro dos referentes a seguir.
A mis padres, a los que nunca he tenido la oportunidad de hacer p´ublico mi agradecimiento, por todo el esfuerzo que han hecho a lo largo de estos a˜nos para darnos un buen porvenir a mis hermanos y a m´ı. Hace casi veinte a˜nos tomaron una dif´ıcil decisi´on pensando en nosotros; han pasado muchas cosas desde entonces. Ahora, cada uno ha encontrado su camino, o est´a a´un en ello, pero en cualquier caso nunca han dejado de anteponer el beneficio com´un al propio. Ahora mi familia ha crecido y a todos les debo mucho. Gracias de coraz´on.
A mi compa˜nera de viaje, la mujer m´as valiente y la persona que m´as quiero. Alguien que ha vivido esta tesis tanto como yo y nunca ha dejado de darme su apoyo y confianza. Gracias Paula.
Por ´ultimo, esta tesis ha sido posible gracias a la financiaci´on de la Fundaci´on S´eneca (Regi´on de Murcia, Espa˜na) a trav´es del contrato predoctoral 19768/FPI/15. La financiaci´on correspondiente a equipos y asistencia a congresos fue proporcionada a trav´es de los proyectos ENE2016-79282-C5-
5-R (MINECO Espa˜na, incluidos fondos FEDER) y 19882-GERM-15 (Fundaci´on S´eneca, Regi´on de Murcia, Espa˜na). As´ımismo, quiero agradecer a la Cost Action MP1307 por la financiaci´on conseguida a trav´es de STSM 1307-050916-079835 para realizar parte de mi investigaci´on en los laboratorios ECN-Solliance. Me gustar´ıa por ´ultimo, agradecer a mis centros de acogida, ECN- Solliance y Joint Research Centre, por proveer medios y recursos para la realizaci´on de parte de la investigaci´on llevada a cabo en esta tesis doctoral.
A todos, gracias.
Resumen
Esta tesis aborda un problema actual y de ´ambito global como es la mitigaci´on del cambio clim´atico.
Conscientes de que la prestaci´on de servicios energ´eticos va a seguir en aumento en a˜nos futuros, se trabaja en una alternativa para disminuir las emisiones de gases de efecto invernadero, en un contexto de desarrollo sostenible dentro del sector de la industria y la construcci´on.
Una buena penetraci´on de las energ´ıas renovables pasa por fortalecer la integraci´on de las mis- mas, solucionando los costes sist´emicos adicionales y mejorando sus modalidades de aplicaci´on. En el sector de la construcci´on, la integraci´on de sistemas fotovoltaicos, m´as conocidos como BIPV (Building Integrated PhotoVoltaics), cuenta con numerosas ventajas ya que, al generar electrici- dad en el punto de consumo se puede reducir la demanda pico, las p´erdidas en la distribuci´on de la energ´ıa, el coste en infraestructuras, etc. Adem´as, se cuida el dise˜no arquitect´onico y est´etico del edificio sin privarlo de identidad y respetando el entorno paisaj´ıstico urbano. Sin embargo, hay barreras que no permiten consolidar estos sistemas, algunas de ellas pasan por mejorar la estandarizaci´on en los elementos fotovoltaicos–constructivos como tama˜nos, formas, grosor de ma- terial o colores; y a la vez hacerla una opci´on competitiva frente en t´erminos de coste, funcionalidad y dise˜no en comparaci´on con los materiales utilizados convencionalmente en obra.
Actualmente, la tecnolog´ıa fotovoltaica basada en silicio cristalino todav´ıa domina en gran me- dida el mercado fotovoltaico. Sin embargo, algunas aplicaciones finalistas, como la integraci´on en edificios, presenta importantes barreras para el silicio. Las caracter´ısticas de sus m´odulos como su rigidez, alto peso y medidas est´andar rectangulares no modificables sobre demanda, hacen dif´ıcil una verdadera integraci´on arquitect´onica. Desde hace varios a˜nos se est´an desarrollando nuevas tecnolog´ıas basadas en dispositivos de l´amina delgada, algunas ya han alcanzado el mercado (con eficiencias de conversi´on de potencia a nivel de m´odulo de ∼12%, ∼18% y ∼19% para tecnolog´ıas de silicio amorfo, teluro de cadmio y CIGS), pero otras est´an todav´ıa en fase de investigaci´on o proyectos de demostraci´on (c´elulas polim´ericas o h´ıbridas, perovskitas, etc.). Las tecnolog´ıas emer- gentes de l´amina delgada destacan por su coste econ´omico reducido (materiales que se utilizan en muy poca cantidad, con grosores de c´elula solar hasta 200 veces menores que en el silicio) y por tener menor impacto ambiental en el proceso de fabricaci´on (menores temperaturas y ausencia de
requisitos de vac´ıo) lo que hace que sea mucho m´as f´acil integrarlas en fachadas y tejados como material de construcci´on.
El principal objetivo de la tesis es estudiar el uso de las tecnolog´ıas de l´amina delgada en sistemas BIPV caracterizando sus par´ametros el´ectricos y t´ermicos considerados puntos clave para identificar el potencial t´ecnico que tienen estas tecnolog´ıas. Bajo este enfoque, se ha dise˜nado y construido un sistema experimental para estudiar la respuesta t´ermica y fotovoltaica de diferentes tecnolog´ıas en condiciones reales de trabajo con un montaje que representa un sistema BIPV en su forma m´as simple. Cuatro tecnolog´ıas han sido consideradas: una tecnolog´ıa de referencia como es el silicio cristalino, dos tecnolog´ıas de l´amina delgada consolidadas a nivel de mercado como son las de silicio amorfo y teluro de cadmio y una de tercera generaci´on con gran potencial como es la basada en pol´ımeros conjugados (mejor conocida como org´anica). La par´ametros t´ermicos, el´ectricos y ambientales recogidos por el dise˜no experimental durante un largo periodo de tiempo permiten tener un conocimiento m´as profundo del comportamiento de cada una de estas tecnolog´ıas para identificar y determinar su viabilidad t´ecnica. Para ello, tambi´en se requiere de validaci´on de modelos que determinen la irradiaci´on en el plano de incidencia, ya que el recurso solar a diferentes orientaciones e inclinaciones es raramente medido por estaciones meteorol´ogicas locales, siendo de gran utilidad tanto para la comunidad cient´ıfica como para arquitectos e ingenieros, debido a que, en un contexto de integraci´on arquitect´onica, las orientaciones e inclinaciones vienen determinadas por la forma del edificio, sin poder utilizar el ´angulo ´optimo como criterio de dise˜no.
Esta tesis busca contribuir significativamente al desarrollo de los sistemas fotovoltaicos integra- dos estrechando la relaci´on entre investigaci´on y arquitectura. Cada cap´ıtulo discute diferentes perspectivas de este ´ambito; de manera que se d´e un mejor entendimiento de las tecnolog´ıas fo- tovoltaicas cuando se llevan a edificios, dando una mayor difusi´on a esta prometedora aplicaci´on.
La estructura de la tesis se organiza de la siguiente manera: el cap´ıtulo uno presenta el panorama de la energ´ıa, el papel de las tecnolog´ıas fotovoltaicas en la transici´on hacia edificios de bajas emisiones de carbono y los puntos clave establecidos para la evaluaci´on del problema, como son la necesidad de contar con modelos de radiaci´on solar fiables y de establecer una visi´on global que cubra tanto las necesidades arquitect´onicas–constructivas, como la respuesta fotovoltaica de los sistemas. El cap´ıtulo dos presenta el objetivo principal de la tesis junto con los objetivos parciales llevados a cabo. La metodolog´ıa y la descripci´on del sistema experimental, junto con las bases de datos utilizadas para examinar los modelos, se describen en el cap´ıtulo tres. Los modelos de radiaci´on solar basada en mediciones horizontales para obtener la irradiancia seg´un la inclinaci´on y orientaci´on deseada se estudian en el cap´ıtulo cuatro. Los cap´ıtulos cinco y seis se centran en la caracterizaci´on t´ermica y el´ectrica de cada tecnolog´ıa fotovoltaica considerada. Dos casos de estudio con m´odulos fotovoltaicos integrados en diferentes tipolog´ıas de edificios se presentan en el cap´ıtulo siete. Y finalmente, las conclusiones y el trabajo futuro se discuten en el cap´ıtulo ocho.
Abstract
This PhD thesis addresses a current and global problem such as mitigating Climate Change. Aware that the provision of energy services will continue to increase in future years, this work focuses on an attractive alternative to reduce greenhouse gas emissions within a sustainable development context in the industry and construction sector.
A good penetration of renewable energies, photovoltaics (PV) in particular, involves strengthen- ing their integration, solving the additional systemic costs and improving their functionality in different applications. In the construction sector, the integration of photovoltaic systems, better known as BIPV (Building Integrated Photovoltaics), has numerous advantages since, by generating electricity at the point of consumption, grid power peak demand can be reduced, as well as the losses in the distribution of energy and the infrastructure costs. In addition, the architectural and aesthetic design of the building is taken care of without depriving it of identity and respecting the urban landscape environment. However, there are barriers that do not allow to consolidate these systems, and therefore it is necessary to improve the standardization in the photovoltaic- constructive elements such as sizes, shapes, thickness or colors; and at the same time, converting it into a competitive option in terms of economic cost, functionality and design compared to the materials conventionally used in the building sector.
Currently, photovoltaic technology based on crystalline silicon still dominates the photovoltaic mar- ket. However, some finalist applications, such as building integration, present significant barriers to silicon. The characteristics of silicon PV, such as rigidity, high weight and standard rectangu- lar measurements, cannot be modified on demand, making the architectural integration difficult.
In the last decade, new technologies based on thin-film devices have been developed, some have already reached the market (with power conversion efficiencies on module level of ∼12%, ∼18%
and ∼19% for amorphous silicon, cadmium telluride and CIGS respectively), but others are still under investigation or demonstration projects (organic PV, hybrid, perovskites and so on). Third generation photovoltaics, such as organic photovoltaics (OPV), stand out by low-cost (fewer ma- terial in comparison to silicon PV) and low-weight (solar cell thickness up to 200 times lower than silicon PV) having less environmental impact in the manufacturing process (less temperatures in
the production and absence of vacuum requirements). This facts make it much easier to integrate on roof or walls as building material.
The main objective of the thesis is to assess the use of thin-film photovoltaic technologies in BIPV systems by characterizing its thermal and electrical parameters, which are needed to identify the technical potential of these technologies. For this purpose, an experimental system working in real outdoor conditions has been designed and built to study the thermal and photovoltaic response of four different technologies (crystalline silicon PV technology, two thin-film technologies already at market level: cadmium telluride and amorphous silicon; and a third emerging generation PV with great potential, such as organic PV). The system is a simple experimental model which reproduces real BIPV conditions. The monitoring and recording of electrical, thermal and environmental parameters of the experimental design after a long period of time allows us to have an in-depth knowledge to identify and determine the viability of a range of PV technologies for BIPV applica- tions. This also requires the validation of models used to calculate in-plane irradiation since solar resource at different inclination and orientations (for instance vertical surface in fa¸cades) is rarely measured by local meteorological stations being of great utility both for the scientific community and for architects and engineers since, in a context of building integration, the orientation and in- clination of the systems are determined by the shape of the building, without adopting the optimal angle as design criterion.
This PhD thesis seeks to make an important contribution to the subject of BIPV creating a closer relationship between research and architecture. Each chapter discusses BIPV from different perspectives, together providing knowledge for the further spread of this promising application.
The structure of the present dissertation is organised as follows: chapter one presents the energy panorama, the role of photovoltaic technologies in the transition towards low-carbon emission buildings and the key points established for the assessment of the issue, such as the need for reliable solar radiation models and for establishing a global vision of the problem that covers both architectural and construction needs, as well as the photovoltaic response of the system. Chapter two presents the main objective of the thesis and the partial objectives. The methodology and description of the experimental system together with the databases used to examine models are described in chapter three. Solar radiation models to obtain the in-plane irradiance based on horizontal measurements are studied in chapter four. Chapters five and six focus on the thermal and electrical characterization of each PV technology under consideration respectively. Building simulations with PV modules integrated in two different building types are presented and discussed in chapter seven. And finally, conclusions and future work are presented in chapter eight.
Contents
1 Introduction 1
1.1 World energy demand and climate change . . . 1
1.2 Photovoltaics and low-carbon energy buildings . . . 3
1.3 PV technologies and BIPV . . . 5
1.4 Thermal and electrical performance of PV in building envelopes . . . 8
1.5 The solar resource in the urban environment . . . 9
1.6 The PV output estimation of BIPV modules: Power matrix model . . . 12
1.7 Architecture, research and strategies . . . 13
2 Thesis objectives 17 2.1 Main objective . . . 18
2.2 Specific objectives . . . 18
3 Methodology 21 3.1 Experimental setup: PV-Cubes . . . 21
3.1.1 Monitoring System . . . 29
3.2 Irradiance and environmental data from PGVIS and NREL laboratories . . . 31
3.2.1 PVGIS . . . 31
3.2.2 NREL . . . 33
3.3 Electrical characteristics of the photovoltaic cell . . . 33
3.3.1 Solar cell equivalent circuit . . . 34
3.3.2 Efficiency . . . 37
3.3.3 Parasitic resistances . . . 38
3.3.4 Effects of irradiation and temperature . . . 39
3.4 BIOPV measurements at ECN-Solliance and ESTI laboratories . . . 40
4 Solar radiation 43 4.1 In-plane irradiance from horizontal components . . . 43
4.1.1 Beam component . . . 43
4.1.2 Diffuse component . . . 46
4.1.3 Reflected component . . . 50
4.2 Experimental validation . . . 50
4.2.1 Local measurements from ground station . . . 54
4.2.2 NREL Laboratories . . . 57
4.2.3 Results for clear sky conditions . . . 58
4.3 Comparison to PVGIS database . . . 60
4.4 Outdoor operating conditions: one-year dataset . . . 62
4.5 Conclusions . . . 65
5 Thermal analysis 67 5.1 PV-Cubes: Module temperature . . . 67
5.1.1 Empirical coefficients to calculate module temperature . . . 69
5.1.2 Thermal inertia . . . 85
5.1.3 Validation of results . . . 89
5.2 Thermal performance of PV modules as building envelope elements . . . 95
5.2.1 Indoor climatic conditions: thermal comfort . . . 95
5.2.2 Working conditions: thermal stress . . . 96
5.2.3 Thermal properties as building materials . . . 98
5.3 Conclusions . . . 101
6 Electrical characterization 103 6.1 Indoor measurements . . . 103
6.1.1 Glass-Glass mini BIOPV module at ECN-Solliance . . . 103
6.1.2 BIOPV module at ESTI JRC laboratories . . . 110
6.2 Outdoor measurements . . . 113
6.2.1 I-V characterization of BIOPV module . . . 113
6.2.2 I-V characterization of PV-Cubes . . . 115
6.3 Conclusions . . . 122
7 Study cases: BIPV potential 123 7.1 Industrial building . . . 123
7.2 School building . . . 129
8 Conclusions 135 References 140 Appendices 153 A Database structure 155 A.1 Data from PV-Cubes . . . 155
A.2 Data from pyranometers . . . 157
A.3 Data from weather station . . . 157
B Matlab code: In-plane solar irradiance transposition models 159
C Matlab code: Merge individual files from database 169
Nomenclature
Abbreviations
a-Si:H Amorphous silicon AM Air mass
Bc Beam or direct irradiance at plane of array Bh Beam or direct irradiance at horizontal plane
BAPV Building attached/added photovoltaics BE Building element
BIOPV Building integrated organic photovoltaics BIPV Building integrated photovoltaics
c-Si Crystalline silicon Ces Standard cloud ratio Ce Cloud ratio
Cle Cloudless index CdTe Cadmium telluride
CIGS Copper indium gallium selenide Dc Diffuse irradiance at plane of array Dh, DHI Diffuse horizontal irradiance DN Day number of the year (0 to 365) DNI Direct normal irradiance
DSF Double skin fa¸cade
EOT Equation of time
EPBT Energy pay back time
ESTI European solar test installation FF Fill factor
FS Forecast skill
Gc Global irradiance at plane of array
Ge,0 Global horizontal extraterrestrial irradiance
Ge Global extraterrestrial irradiance on the plane normal to the direct irradiance
Gh, GHI Global horizontal irradiance
Gs Standard global irradiance HISG Solar insulation solar glass
Impp Current at the maximum power point
Isc Short circuit current
J Current density K Ross coefficient
K-value Thermal insulation value of the building
Kc Clear sky index MBD Mean bias deviation
NOCT Nominal operating cell temperature
NZEB Nearly zero-energy building
OPV Organic photovoltaics PCE Power conversion efficiency
PCM Phase change materials
PV Photovoltaics
PVGIS Photovoltaic geographical information system
Rb Geometric factor, conversion factor for the beam irradiance
Rc Reflected irradiance at plane of array
Rsh Shunt resistance
Rs Series resistance RMSE Root mean square error
Si Sky index
SNL Sandia National Laboratories
STC Standard test conditions Ta Ambient air temperature
Tfront Outer superficial temperature
Tin Indoor air temperature Tm Module temperature
Trear Inner superficial temperature
U-value Thermal transmittance coefficient
Vmpp Voltage at the maximum power point Voc Open circuit voltage
Ws Wind speed
Symbols
αs Solar altitude or elevation
β Surface tilt (0◦to +90◦; towards Equator is positive)
∆ Brightness
∅ Latitude (0◦to +90◦, north is positive)
η Energy-conversion efficiency
γ Orientation angle, azimuth of surface (0◦ to 180◦; 0◦ is south, 90◦ is west) γs Solar azimuth (0◦ to 180◦; 0◦ is south, 90◦is west)
ω Hour angle (-180◦to 180◦, solar noon is 0◦, afternoon is positive)
ρ Ground reflectance, albedo
θ Angle of incidence between the Sun and the normal plane (0◦ to +90◦) θz Zenith angle (0◦ to +90◦)
ε Clearness
List of Figures
1.1 World electricity generation by 2040 according to the International Energy Agency in the New Policies Scenario . . . 2
1.2 Share of renewable energy by sector 2011-2023 . . . 2
1.3 Global evolution of total PV installed peak power capacity 2000-2018 . . . 3
1.4 PV system design with high level of integration and architectural quality. Freiburg Town Hall (Germany), Ingenhoven architects . . . 4
1.5 Global installed peak power capacity of BIPV systems and forecast from 2014–2020 5
1.6 Solar cell technology efficiencies from 1976 to the present compiled by National Renewable Energy Laboratory (NREL), USA . . . 6
1.7 Heliatek’s organic photovoltaic films installed on the fa¸cade of a warehouse of the Duisburger Hafen AG . . . 7
1.8 Dutch Solar Design: Solar bricks . . . 14
1.9 Novartis Campus Gehry Building, Basel (Switzerland), 1300 m2, 92.74 KWp, Monocrys- talline silicon PV modules . . . 15
1.10 Logistic Center V-Zug, Zug (Switzerland), 19.25 KWp, Multicrystalline silicon PV modules . . . 15
1.11 Specific building demonstrator: Active Office, at the Swansea university campus, is the UK’s first energy positive office . . . 16
3.1 PV modules from left to right: c-Si, a-Si:H, CdTe and OPV . . . 22
3.2 The BIPV test facility located at 38◦102400N - 1◦1003200W coordinates, on the roof of CIOyN building (∼12 m high) at University of Murcia (Spain) . . . 23
3.3 System to fix the PV modules to the aluminium frame in both cases: PV modules with frame and frameless . . . 24
3.4 Sketch c-Si PV-Cube and dimensions . . . 25
3.5 Sketch a-Si:H PV-Cube and dimensions . . . 26
3.6 Sketch CdTe PV-Cube and dimensions . . . 27
3.7 Sketch OPV PV-Cube and dimensions . . . 28
3.8 Monitor system. Hardware components installed inside of each PV-Cube (the pho- tographs correspond to the c-Si PV-Cube, others are similar) . . . 29
3.9 I-V curve tracer (model PVPM1000CX) for electrical characterization of the PV-Cubes 30
3.10 Architecture of the monitoring system using a wireless sensor network . . . 30
3.11 PVGIS: Photovoltaic solar electricity potential in European Countries . . . 32
3.12 Current–voltage characteristic of ideal diode in the light and the dark . . . 35
3.13 Ideal circuit of the solar cell . . . 36
3.14 J-V curve characteristic of the solar cells . . . 37
3.15 Equivalent circuit including series and shunt resistances . . . 38
3.16 Effect of parasitic resistances on J(V) characteristics . . . 39
3.17 Effects of irradiance and temperature on J(V) characteristics . . . 40
3.18 BIOPV samples under test . . . 41
4.1 Zenith angle (θz), slope (β), surface azimuth angle (γ), and solar azimuth angle (γs) for a tilted surface . . . 46
4.2 Ground reflected value (ρ). Median value of 0.16 (one day, 5-minute time steps records) . . . 55
4.3 Correlation between calculated (based on BF5 pyranometer data) and measured (with CMP3 pyranometer) applying Perez (PE) and Muneer (MU) transposition models. Figure (a) shows high deviation between estimated and measured values, indicated by a red rectangle, generated by shadows at dawn. . . 56
4.4 Calculated vs. measured irradiance considering Sun elevation angles above 15◦ . . 57
4.5 Ground reflected value (ρ). Median value of 0.21 (monthly statistic for a single year based on daily averages) . . . 58
4.6 Histogram of the sky type according Igawa classification . . . 59
4.7 Comparison of PVGIS and models outcomes as well as the beam, diffuse and re- flected components for a random day . . . 60
4.8 Monthly average of daily irradiation (kWh/m2/day). PVGIS-SARAH database and measured data for the year 2018 . . . 62
4.9 Sum of the average of daily irradiation (kWh/m2/day) separated by orientations, together with the total . . . 63
4.10 Comparison of the daily radiation profile for January from PVGIS-SARAH satellite- based database for a multi-year period and from experimental measurements from in-situ ground station registered in the year 2018 at the considered location . . . . 63
4.11 Combination of in-plane irradiance calculated by Muneer’s method from horizon- tal records and ambient temperature collected for weather station. Frequency of observations are indicated by the right bar, using a color scale . . . 65
5.1 Correlation between surface PV module temperatures of the a-Si technology for south and horizontal faces from Aug, 2017 to Sept, 2017. In red, the records rejected by the test . . . 70
5.2 PV-Cubes cross-section with the temperatures under consideration (upper face de- tail) to calculate empirical coefficients for NOCT and SNL models, together with the cell temperatures defined for each module technology (Tm) . . . 71
5.3 Ross coefficient for each face of the c-Si PV-Cube . . . 72
5.4 Ross coefficient for each face of the a-Si:H PV-Cube . . . 73
5.5 Ross coefficient for each face of the CdTe PV-Cube . . . 74
5.6 Ross coefficient for each face of the OPV PV-Cube . . . 75
5.7 Histogram of the sky type according to Igawa et al. classification for the period under analysis . . . 76
5.8 Ross coefficient for each face of the c-Si PV-Cube discriminating values between morning and afternoon . . . 77
5.9 Ross coefficient for each face of the a-Si:H PV-Cube discriminating values between morning and afternoon . . . 78
5.10 Ross coefficient for each face of the CdTe PV-Cube discriminating values between morning and afternoon . . . 79
5.11 Ross coefficient for each face of the OPV PV-Cube discriminating values between morning and afternoon . . . 80
5.12 Wind rose for the analysed period (wind speed units in m/s and counts by frequency of observations) . . . 83
5.13 Ambient conditions (GHI and Ta) on the days selected for analysis . . . 85
5.14 Correlation between module temperature and in-plane irradiance for a selected clear day (03/08/2017) discerning between morning and afternoon records for the four studied technologies . . . 86
5.15 Correlation between module temperature and in-plane irradiance for a selected cloudy day (08/08/2017) discerning between morning and afternoon records for the four studied technologies . . . 87
5.16 Temperature difference (∆T) between module temperature values at different times of the selected clear day at the same irradiance level . . . 88
5.17 ∆T sensitivity as a function of the sky condition. The passage of a cloud in the afternoon modifies the trend of the data, reducing ∆T at the same irradiance levels 89
5.18 Correlation between measured module temperature and calculated one by NOCT- linear and SNL model, for the four PV technologies under consideration at east and horizontal faces . . . 92
5.19 Comparison between the predicted PV module temperature of Ross and SNL model, and the measured ones for all faces of c-Si technology on a clear day . . . 94
5.20 Indoor ambient temperature of the PV-Cubes and outdoor ambient temperature during the analysed period . . . 95
5.21 Inside air temperature on each PV-Cube and ambient conditions for two different sky conditions days . . . 96
5.22 Correlation between front PV module temperature and rear one for the PV-Cubes in the analysed period, for the four PV technologies under consideration . . . 97
5.23 Correlation between irradiance and temperature difference between inner and outer surfaces of the PV modules normalized by its thickness to determine the thermal conductivity of each module for the four technologies under consideration . . . 99
5.24 Evaluation of the U-value through minimizing nRMSE of the predefined values . . 100
6.1 Sample HA16A21E0160 with thermocouple PT100 . . . 104
6.2 I-V curves for BIOPV glass-glass sample at different temperatures and irradiances 105
6.3 Temperature dependence of the electrical parameters extracted from the I-V curves of glass-glass BIOPV minimodule in the temperature range from 25◦C to 45◦C . . 106
6.4 Regression analysis. Efficiency (shown in vertical axis and additionally using a color code) vs. cell temperature and irradiance (horizontal plane) for OPV technology . 107
6.5 Energy production of the BIOPV module for Petten in 2011 . . . 109
6.6 Monthly energy production with same input (environmental data obtained from PVGIS) and different models for Petten (The Netherlands) as calculated with the efficiency matrix model and with PVGIS . . . 109
6.7 Multiplot of I-V measurements performed in single flash and multi flash modes and comparison between both measurements . . . 110
6.8 TK801 BIOPV module tested in Apollo solar simulator . . . 111
6.9 Module temperature dependence of the electrical parameters for BIOPV module TK801 @1000 W/m2 irradiance . . . 112
6.10 Power matrix of BIOPV TK801 module under steady-state conditions. (a) Maxi- mum power point and (b) normalized maximum power point are shown for temper- ature ranging from 15◦C to 55◦C and irradiance ranging from 100 to 1000 W/m2 . 113
6.11 Temperature dependence of Isc and Voc. Normalized to initial values . . . 114
6.12 PV-Cubes I-V characterization for horizontal face ∼800W/m2. . . 115
6.13 I-V curves for the PV-Cubes at different orientations over a short period of time between measurements in the same day . . . 116
6.14 Irradiance dependence of Isc and Vocparameters for the horizontal surface and one of the open face, allowing airflow inside the structures . . . 118
6.15 OPV PV-Cubes faces south and west after two year of exposure to environmental conditions . . . 119
6.16 OPV PV-Cube I-V west face Gc∼700 W/m2(Voc = 270 V, Isc= 40.9 mA) . . . . 119
6.17 PCE PV modules @STC and about 2 years later exposing under real operating conditions in a fully integrated system . . . 120
6.18 OPV PV-Cubes visual degradation of horizontal face after two year exposed to real outdoor working conditions in Murcia, Spain . . . 121
7.1 Study case: climatic zones (red zones) . . . 124
7.2 Schematic diagram a) open rack mounted b) BAPV c) BIPV . . . 126
7.3 Global irradiation per square meter as a function of inclination and azimuth (from PVGIS) . . . 126
7.4 Monthly electricity production of each system . . . 127
7.5 PR losses due to temperature . . . 128
7.6 Heating and cooling loads in the R&D room of the industrial building . . . 129
7.7 Laboretec’s facility (Belgium). Fa¸cade integrated OPV pilot (in front of the visitor parking) . . . 130
7.8 Glass structure for the glass fa¸cade . . . 131
7.9 3D view of the designed building and south-oriented glass fa¸cade details . . . 131
7.10 Front view of the building and south-west 3D view . . . 131
7.11 Yearly energy production split by orientation of the PV modules installed on the school building . . . 133
7.12 Heating and cooling energy demand of the building school, together with BIOPV energy production . . . 134
List of Tables
3.1 Technical specifications of the PV modules . . . 23
3.2 Covered area of PV module, polycarbonate and frame for each face and technology in percentage (east = E, west = W, north = N, south = S, upper horizontal face = H) . . . 24
3.3 Technical specifications of the monitoring system . . . 31
4.1 Brightness coefficients for Perez model . . . 50
4.2 Results of the comparison of calculated and measured irradiance data at four differ- ent orientations (E, S, W, N) in the period between Jan 25th, 2019 and April 11th, 2019 . . . 55
4.3 Results after filtering data by avoiding low Sun elevation angles. . . 57
4.4 Results of the comparison of calculated and measured irradiance data at four dif- ferent orientations (E, S, W, N) for hourly data collected by NREL laboratories in 2018 . . . 58
4.5 Statistic values under clear sky conditions for the experimental testing ground sta- tion in Murcia . . . 59
4.6 Statistic values under clear sky conditions for NREL data . . . 59
5.1 Ross coefficients of the PV-Cubes . . . 81
5.2 K (Ross coefficient) difference between the fit of the morning and the afternoon records 81
5.3 K (Ross coefficient) difference between the fit of the clear sky records and the cloudy ones . . . 82
5.4 mpirically determined coefficients a and b used to predict module back surface temperature provided by Sandia laboratories . . . 83
5.5 SNL coefficients for different orientations of the PV-Cubes . . . 84
5.6 Average for all orientations of the SNL coefficients of the PV-Cubes . . . 84
5.7 Error metrics nRMSE, RMSE, MBD, nMBD for NOCT-linear model . . . 90
5.8 Error metrics nRMSE, RMSE, MBD, nMBD for SNL model . . . 91
5.9 FS taking nMBD SNL model as reference . . . 94
7.1 Description of case study 1: industrial building . . . 124
7.2 Electrical parameters at standard test condition (STC) and dimensions of PV panels used in this case study . . . 125
7.3 Specifications of each PV system to be designed . . . 126
7.4 System losses simulation parameters to calculate energy output of the rack mounted, BAPV and BIPV systems . . . 128
7.5 Results for solar modules in each array for the BIPV system on the school building 132
7.6 Result for solar modules in each array of the BIPV system of the school building . 132
7.7 Yearly average electricity production and in-plane irradiance in each orientation . . 132
7.8 Summary of the estimation of the thermal demands of heating and cooling for the school building, together with the estimated values of electrical production by the BIOPV system . . . 134
Chapter 1
Introduction
1.1 World energy demand and climate change
Global energy demand is predicted to increase in the coming decades driven by the permanent development of economy and a constantly growing global population which is projected to reach about 10 billion in 2050 [1]. In fact, half of the growth in global energy demand in 2018 came from the power sector in response to higher electricity consumption mainly in cities. More than 55%
of the World population is living in cities (4.2 billion people) in 2018; this means cities accounted two-thirds of global energy demand [2, 3]. Moreover, between 2017 and 2040, global primary energy demand shall be expanded by over 25%, and electricity generation by 60% (15000 TWh, Figure 1.1) according to the International Energy Agency under the New Polices Scenario1 [4].
In addition, the production and use of energy is the largest source to the generation of greenhouse gas emissions (GHG). Even if the emissions levels does not increase above its current annual index, CO2levels may reach 550ppmby 2050, reaching double its pre-industrial level, which means going beyond the worldwide established 2◦C climate goal, and therefore, a serious impact not only for biodiversity and human security: water supply, food production, health, land use and the environment [5], but also an over-cost to the global economy that could amount to a decrease of at least 5% of global gross domestic product (GDP) each year [6].
In response to this scenario, the strong link between economic growth and global energy-related CO2-eq emissions is weakened through the transformation of the global power sector. The impacts
1New Policies Scenario broadens the scope of current policies to include targets announced by governments including the Nationally Determined Contributions made for the Paris Agreement.
CHAPTER 1. INTRODUCTION
Figure 1.1: World electricity generation by 2040 according to the International Energy Agency in the New Policies Scenario [4]
of the renewable energy in the electricity sector by the rise of renewable sources, such as wind and solar photovoltaics (PV), plays a critical role for lower emissions strategies. Renewable energies will have the fastest growth in the electricity sector, providing almost 30% of power demand in 2023 (Figure 1.2) [7]. Global renewable power capacity grew to around 2378 GW in 2018, and PV accounted for 55% of renewable power capacity additions (around 100 GW was installed), followed by wind power (28%) and hydro-power (11%). This trend shows the accelerated deployment of renewable energies in recent years as well as it promising future driven by improvements in technology, price reductions2 and support from government policies.
Figure 1.2: Share of renewable energy by sector 2011-2023 [7]
2Solar PV module prices have fallen by around 90% since 2009. At the end of 2018, module prices in Europe ranged from USD 0.22/W to USD 0.42/W [8]
1.2. PHOTOVOLTAICS AND LOW-CARBON ENERGY BUILDINGS
The contribution of renewable energies to strategic sectors such as building, which represents 36%
of global final energy consumption and nearly 40% of CO2-eq, emissions is the key to tackle climate change and fulfil the temperature limits goal set by the world community. In recent years, the installed PV World capacity has increased until crossing the 500 GW mark in 2018 (Figure 1.3) [9].
This growth of the PV market shows a clear commitment by scientists, governments and industry for the decarbonisation of the economy. The market for PV systems is gradually expanding from the niche-markets of large PV plants (often in the order of MW) to small PV systems (from hundreds of Wpto kWp). In Europe, one of the most important long-term objectives include decarbonisation of the building stock by 2050 [10]. Under this roadmap, in most European countries, new regulations on energy transition are being under way. Particularly, the Spanish Government has recently approved the technical and economic conditions for energy self-consumption, promoting the use of renewable energy systems in residential and industrial areas. Thanks to its modularity, decreasing cost, lifespan and efficiency improvements, PV technology is playing a key role in the transition to low-carbon energy buildings.
Figure 1.3: Global evolution of total PV installed peak power capacity 2000-2018 [9]
1.2 Photovoltaics and low-carbon energy buildings
In building and industry sectors, PV systems are very interesting because the generation of elec- tricity on site presents some advantages such as a reduction of transportation and distribution losses, improvements in quality and continuity of power supply in peak-hours and reduction of environmental impacts [11]. Moreover, PV modules can replace conventional building materials by integrating them into the building envelope and create a dual function: building material and power generation, contributing to the aesthetic of the building as architectural component. This concept is known as Building Integrated Photovoltaics (BIPV) and, although current BIPV tech-
CHAPTER 1. INTRODUCTION
nology has small market compared to large PV plants3, BIPV represents a strategic part of future building vision with a huge potential driven by the growing shift towards renewable energy sources and focus on energy efficiency in buildings. In fact, BIPV represents one of the five major tracks for large penetration of PV applied in the built environment4and it is in line with European Union Directives [14, 10], Horizon 2020 program [15] and International Energy Agency programmes [13].
The shift of the conventional conception of buildings towards nearly zero-energy buildings (NZEB), where the energy consumption and production are in balance, is the main driver behind the imple- mentation of BIPV systems [16] which presents an opportunity to use solar panels into the building skin replacing parts of conventional building elements, mainly placed on the roof or fa¸cades, and thus, transforming building surfaces in active electricity generators. BIPV is an attractive solution to architect and engineers because of aesthetic and architectural configuration, in terms of size, colour, or shape, can be adapted according to the specific needs of the project (Figure 1.4).
Figure 1.4: PV system design with high level of integration and architectural quality. Freiburg Town Hall (Germany), Ingenhoven architects
Moving PV to include architectural design considerations is an ongoing challenge since architectural design objectives sometimes conflict with energy performance. Moreover, the standard approaches used until now for traditional PV systems, such as open-rack mounted or building attached pho- tovoltaics (BAPV)5 cannot be applied in BIPV. New list of requirements must be addressed to ensure the sustainable development of BIPV. In this sense, the module properties have to be con- sidered not only focusing on energy generation, but also considering it as a building structure [17]: mechanical and structural integrity, primary weather protection (rain, snow, wind and hail), thermal insulation, fire and noise protection. Therefore, several aspects have to be considered and evaluated related to the integration of the PV into the building skin. This means that technological
3Between 1% - 3% of PV systems installed are BIPV in 2016. It is expected to reach 5 GWpby 2020 [12]
4The IEA Photovoltaic Power Systems Programme (PVPS) have identified five key developments towards large- scale introduction of PV into building environments: price decrease, efficiency increase, improved quality, and building integration [13]
5PV modules installed in overlap in parallel to the skin of the building. Also know as building-added
1.3. PV TECHNOLOGIES AND BIPV
improvements and research has to be conducted in this field, specially for emerging technologies with higher technical potential in terms of production costs and environmental impacts.
In the upcoming years, the BIPV market is expected to grow strongly mainly driven by developed countries, especially Europe and the United States, in terms of installed power capacity [18] (see Figure 1.5). The annual cumulated World installed capacity of BIPV is expected to surpass 11 GWp
by 2020 with a contribution 4.8 GW from Europe. Future growth prospects of BIPV is justified through positive evolution of the regulatory framework, combined with the efforts made to address requirements related to the flexibility in design and aesthetic considerations, demonstration of long-term reliability of the technology and cost effectiveness.
Figure 1.5: Global installed peak power capacity of BIPV systems and forecast from 2014–2020 [18]
1.3 PV technologies and BIPV
Even though PV market is still dominated by crystalline silicon (c-Si) technology due to high efficiency and well-established manufacture [19], there are barriers that silicon can not overcome.
The characteristics of the silicon modules (rigidity, high weight and standard rectangular size that can not be modified on demand) make it difficult to achieve true structural architectural integration, for example as a fa¸cade or covering material, on roofs or even in windows. Thin-film technologies, such as amorphous silicon (a-Si:H) or Cadmium Telluride (CdTe), offer performance advantages for BIPV systems thanks to thinner active layers which reduce manufacturing cost, allowing the use of flexible substrates that can be applied on curved surfaces, and additionally offer the possibility to make semi-transparent devices providing a great variety of options for windows, curtain wall fa¸cades and skylights. Going one step further, third-generation solar cells
CHAPTER 1. INTRODUCTION
which may be developed into BIPV such as Organic Photovoltaics (OPV) have also remarkable interest because they have easier manufacture, lower energy pack back times and the barely use of toxic components [20, 21, 22]. Moreover, despite the low efficiency compared to silicon solar cells, the efficiency of third generation6 PV has increased progressively in the past decade (Figure 1.6) which helps to optimize the relationship cost production and efficiency/power generation of BIPV. Additionally, the possibility of tuning the band gap of the light absorbing material opens the possibility of designing OPV modules adapted to indoor illumination (natural or artificial), with different intensity and spectral distribution; this can be also considered for BIPV technology [23, 24].
Figure 1.6: Solar cell technology efficiencies from 1976 to the present compiled by National Re- newable Energy Laboratory (NREL), USA [25]
Commercial OPV for use as building element are already available at market level. One of the most outstanding companies in the field is Heliatek (Germany)7, who has installed the World’s largest fa¸cade, approximately 185 m2, with BIOPV (Building Integrated Organic Photovoltaic) modules in Germany in August 2018 (Figure 1.7). Other well-known company is Infinity PV (Denmark)8 which offers a showcase of organic solar cell for use in a multitude of applications, including BIPV.
There are studies showing the technical potential of OPV modules configurated for BIPV appli- cation highlighting its flexibility to be laminated onto a variety of substrates and its performance
6Devices based on crystallite silicon wafers and the thin film inorganic semiconductors are defined as first and second generation PV respectively
7https://www.heliatek.com
8https://infinitypv.com
1.3. PV TECHNOLOGIES AND BIPV
Figure 1.7: Heliatek’s organic photovoltaic films installed on the fa¸cade of a warehouse of the Duisburger Hafen AG [26]
under diffuse solar irradiance [27]; as well as its energy yield and outdoor performance in North Eu- ropean countries [28]. In fact, the potential for spectral selectivity through the choice of OPV ma- terials shows improved performance in applications such as greenhouses in comparison to opaque, flexible thin-film modules such as copper indium gallium selenide (CIGS) [24].
More recently, another emerging PV technology is perovskite-based solar cells (PSC) which has revolutionized the field with power conversion cell efficiencies up to 23.7% [29]. Although PSC has not demonstrated the same level of stability as conventional cells and extended its lifetime continues to be a key issue for the scientific world, there are research that points out the significant potential benefits for building applications in terms of energy production and visual comfort [30].
Furthermore, there are strategic materials such as graphene with excellent electronic, optical, thermal and mechanical properties which have became an interesting option to take into account in the future in photovoltaic engineering since it should improve the durability and simplify the technology of potential optoelectronic devices. In addition, it has been shown the possibility for mass production of graphene with a low environmental impact [31]. In this sense, Iwan and Cuchmala [32] completed a review analysing the use of graphene as addition to acceptor material, transparent electrode and separate layer for organic photovoltaics cells showing the most promising results with the use of graphene as an electrode, in replacement to the most used Indium and Tin Oxide (ITO), thus reducing manufacturing costs and avoiding the use of a scarce material such as Indium.
In the specific context of BIPV, second and third generation of PV might compete seriously with traditional c-Si. However, research to evaluate global energy performance (active role as solar panel turning building’s skin into energy generators and, at the same time, passive role in relation to create a thermal envelope ensuring indoor comfort) between different technologies in realistic outdoor conditions is important. The double functionality (thermal and electrical) of solar panels
CHAPTER 1. INTRODUCTION
used in BIPV must be studied in detail since they interact and have a strong impact on the performance of the systems.
1.4 Thermal and electrical performance of PV in building envelopes
The thermal behaviour of PV panels for BIPV applications affects, on the one hand, to the energy generation influenced by the photovoltaic function (via the thermal dependence of voltage and cur- rent photogeneration9), and on the other, to the indoor comfort of the building [33]. In this sense, the thermal performance of the BIPV modules assumes a relevant role since it is very different from conventional free-mounted systems. Generally, BIPV implies poor rear ventilation, therefore, high temperatures are reached more easily which results in loss in electrical performance, a reduction of PV modules lifetime and increasing cooling load of the building especially in summertime.
To overcome this issue, some authors have taken advantage of building systems, where natural ventilation can remove passively the heat generated through air gaps behind PV modules. Gan [34, 35] studied the effect of gap size for a range of roof pitches and PV panels lengths and found that the mean and maximum PV temperatures decrease with the increase of the air gap and pitch angle. He established, using a computational fluid dynamics method (CFD), a minimum air gap of 0.12m to reduce the overheating of PV panels. Peng et al. [36] developed a PV double skin fa¸cade (PV-DSF) system with a-Si:H modules and inward opening windows to analyse it under different operation modes. They concluded ventilated conditions gives the best performance in terms of solar heat gains coefficient, thus more suitable for subtropical climates. Gunawan et al.
[37] quantified differences in annual yield from different roof configurations taking into account the effect of ventilation and level of insulation for each system, they found changes of ∼ 3% comparing a cold roof to an on roof module.
Alternately, PV panels also can be mounted with thermal insulation systems. Wang et al. [38]
compared the performance between energy performance of PV-DSF and PV insulating glass unit (PV-IGU) found PV-DSF reduces the solar heat gain and increases power generation more than PV-IGU while the latter performs better in thermal insulation and visual comfort, which translated into a reduction in air conditioning consumption. Young et al. [39] developed a heat insulation solar glass (HISG) comprising multilayers materials together with a-Si:H semi-transparent module.
9The temperature sensitivity of PV converter from the temperature dependence is related to fundamental con- version losses and the bandgap of the material which is related to PV technology. Temperature coefficients differs from temperature dependence and affects to the open-circuit voltage (Voc), short-circuit current (Isc) and fill factor (FF)
1.5. THE SOLAR RESOURCE IN THE URBAN ENVIRONMENT
They highlight its potential in power generation, thermal insulation, energy saving and optical performance. From analysis of the experimental results, they obtained energy saves up to 40%
for air conditioning consumptions. Cuce et al. [40] investigated HIGS experimentally showing thermal resistance value of 1.1 W/m2K and 40 W of electrical power under solar irradiance of 850 W/m2 and 0.7 m2 of glass area. Olivieri et al. [41] analysed different window-to-wall ratios and degree of transparency of a-Si:H modules in order to study the energy saving potential compared to standard glass obtaining values between 18% and 59% for intermediate and large windows.
In other studies, the use of phase change materials (PCM) in PV systems for thermal regulation and electrical efficiency improvement has achieved great interest owing to high potential to have reductions up to 18◦C for 30 minutes and 10◦C for 5 hours at 1000 W/m2 [42]. Therefore, they have gained special attention recently and have been examined in performance, material selection, heat transfer improvement and model simulations [43]. However, the use of PCM materials in BIPV systems is still in research state mainly because it is not economically viable and causes over weight to the building element compared to passive cooling building systems.
Consequently, great attention must be paid to temperature since it is a critical parameter to assess the performance of BIPV systems and therefore to accurately predict the energy yield, evaluate future investments and find out the impact on the global energy performance of the building.
It is also equally important to the estimation of energy production of photovoltaic system to have reliable data about incident solar irradiance since it is directly linked with the electricity output.
Because of the usual lack of measured data at the plane of interest, specially for tilt and orientations used in urban environment, the validation of transposition models in BIPV, which calculate the global irradiance on a tilted plane through the analysis of light received components independently (direct beam, the sky-diffuse and ground-reflected), plays a crucial role in the development of this systems.
1.5 The solar resource in the urban environment
PV systems integrated in urban structures usually do not follow classical PV system design prac- tices where, in order to maximise the annual energy production, the tilt angle is approximately equal to the latitude and the orientation is facing towards the Equator. For a more integrated system, adapted to the building architectural design, non-optimal inclinations and orientations should be considered since fa¸cades and roofs will define the orientation and tilt of the modules comprising the BIPV system.
In major urban areas, PV fa¸cades have a significant impact on the solar energy potential because of their large surface area compared to roofs where structures and appliances, such as air-conditioning
CHAPTER 1. INTRODUCTION
units, ventilation systems or elevator engines, are commonly placed. In a building with 4 floors, for example, the area of the fa¸cades is about 4 times the area of the roof [44]. PV fa¸cades can produce, compared to rooftop systems, relatively more power in winter and less in summer, and more in the early and late hours of the day, when the Sun is lower, due to the more favourable inclination. In addition to this, the different fa¸cades of a building can produce electricity during different times of the day widening the peak of production and allowing a closer match to the consumption profile. Despite the annual irradiation on vertical surfaces is lower than on horizontal surface, several studies have shown that the solar potential of fa¸cades is relevant. When quantifying the solar potential of fa¸cades, it is also important to take into account the existence of windows which are normally considered for passive uses only. However, they may be used as solar active area by the integration of semi-transparent PV modules whose average efficiencies could be half of opaque standard PV modules [44]. D´ıez-Mediavilla et al. [45] analysed the potential of vertical fa¸cades using experimental data from Burgos (Spain) finding that the energy collected by four vertical fa¸cades facing cardinal points are almost double the collected by the horizontal plane over the year, and it would be almost three times compared to horizontal surface in winter. Redweik et al. [46] applied a digital surface model to a case study of the campus of the University of Lisbon and found that adding the potential of the fa¸cades to the roof area (with a potential of 34 GWh/year), the production increases and almost doubles with a total of 53 GWh/year. Vulkan et al. [47] estimated the solar potential using 3D modelling in an urban area in Rishon LeZion, Israel. Their results remark the substantial contribution of high-rise apartments blocks (8 to 13 floors) with south and east fa¸cades.
The potential of vertical PV arrays has been studied in the literature from different points of view.
For instance, analysing the energy yield losses caused by dust deposition at different tilt angles [48]. In this sense, the results of Elminir et al. [49] showed that the reduction depends strongly on the dust deposition density, together with tilt angle and the orientation of the surface. Their study shown maximum reduction in light transmission for a glass sample of 27.6% for horizontal surface and 4.9% at vertical surface facing southeast tested in outdoor environment in Minia, Egypt. Furthermore, Lu and Yang [50] address this work from the environmental point of view.
Environmental indicator, energy pack back time (EPBT), was used to measure the sustainability of a 22 kWp PV system in Honk Kong. The EPBT results ranged from 7.1 years for optimal orientation, to 20 years for vertical PV west-facing fa¸cade showing how the sustainability of the system is affected significantly by the system orientation. Suri et al. [51] showed that the seasonal variability of the solar resource in Europe is lower for vertically mounted PV modules than at optimal angles, which means better contribution to satisfying peak load demand in buildings and allows better adjustment to the load diagrams.
Whatever approach is chosen, reliable solar radiation data at a given orientation and tilt is essential
1.5. THE SOLAR RESOURCE IN THE URBAN ENVIRONMENT
to estimate the real potential of the considered system. Modelling the potential for BIPV systems depends on the exposure to solar radiation and weather conditions which vary with location.
However, solar resource at vertical surface is rarely measured. Usually, the most common solar radiation data measured is the global horizontal irradiance (GHI). Over large geographical areas, solar radiation data are available in several forms, from satellite-based dataset to ground stations distributed across different locations. In general, ground stations are used to validate satellite- based methods which have reached a high degree of maturity with global coverage and resolutions up to 15 minutes and a few kilometres [52]. For this reason, these databases are integrated within some of the most popular online PV simulation tools such as PVGIS [53] or PVWATTS [54].
Additionally, there are also approaches based on estimating solar radiation by correlating it with available meteorological parameters. Many models have been proposed and developed on this basis [55, 56]. Boca et al. [57] proposed a multi-regression model to estimate the yearly solar radiation only taking into account geographical factors (latitude and elevation of the site) and average temperature as input parameters. However, these approaches have some limitations since they do not consider, for instance, the horizon height profile or the daily variability. Moreover, they are commonly used to estimate monthly average daily or daily and hourly global radiation since error increases on smaller time intervals [56]. Therefore, for precise estimations, real-time measurements will be needed and, thus, a reliable monitoring system to gather long-term time series.
Photovoltaic system monitoring is an useful site-level analysis tool which provides information about the performance of a PV module subjected to a wide range of real operation conditions.
Measuring solar radiation is one of the most important parameters to be monitored since it can be used to determinate the optimal system size (installed capacity) and location of the PV installation.
Small changes in the geography or climate can affect the energy output of the system, and thus, future investment decisions. Furthermore, adding other meteorological, thermal and electrical parameters, such as ambient temperature, wind speed, PV module temperature and I-V curves, gives an indication of the change of system efficiency and performance ratio which are directly related to lifetime and reliability of the installation. However, due to cost and the difficulties of maintaining, calibrating and operating monitoring systems, data regarding global solar radiation received by each face (in-plane global irradiance) is not usually available as measured data. It is therefore necessary to use estimated values using as input available data in the most accurate possible way. For this purpose, it is necessary to validate the models used to calculate the solar radiation at different orientations from the measured data at a given orientation. The models should provide reliable solar radiation estimates which can be then used in simulations of energy production. Architects and engineers will use these results of solar radiation estimation from the point of view of passive architecture, providing flexibility for the structural design of the building, and at the same time promoting the use of BIPV systems.
CHAPTER 1. INTRODUCTION
In this sense, transposition models which take horizontal irradiance components as input are fre- quently used because measurements at this plane are commonly taken or can be retrieved for a specific location from satellite-based datasets (like those available at PVGIS). Many attempts have been made to validate and compare transposition models [58, 59]. However, no universal model has been found yet. There are cases where a specific model performs better than others. Yang [59]
inter-compared different transposition models and established four clusters based on its predictive accuracy, being the first two clusters those which are expected to provide better results. The first one, includes all Perez family models [60, 61, 62, 63] and the second groups well-known models such as Muneer [64, 65], Gueymard [66] or Hay [67, 68]. Gracia and Huld [69] analysed differ- ent anisotropic models for the validation of the transposition procedure used in PVGIS tool [53].
Muneer’s model [64] is applied to estimate global irradiance in PVGIS, and it was compared with two components [70, 66, 67, 71, 72] and three components [73, 60, 74, 75] anisotropic models. The report concludes that there is not a particular model that outperforms the others in terms of mean bias difference (MBD) and root mean square difference (RMSD). It is interesting to note that both studies agree on most transposition models struggling to perform in vertical planes which means a barrier to provide the most accurate results when they are used for architecture purposes.
1.6 The PV output estimation of BIPV modules: Power matrix model
The efficiency values of a photovoltaic module, defined under standard conditions10, are hardly ever meet outdoor in real working conditions for non-conventional installations such as BIPV. There are studies that show that the electrical and thermal behavior of an integrated module varies considerably compared to conventional open-rack systems [37, 76, 77]. Even if they are integrated in the same way, there are changes in the response of different PV technologies [78].
The temperature that the cell can reach at certain environmental conditions is one of the fun- damental variables that help to know the behaviour of the photovoltaic module. The increase in temperature in the solar cells means a slight increase in the short-circuit current, but also an important decrease in the open circuit voltage, so that the power of the panel decreases, and thus the power conversion efficiency. The nominal operating cell temperature (NOCT) is one of the pa- rameters generally used to model the thermal response of the photovoltaic panel. This parameter is defined as the temperature reached by the open circuit panel cells under ambient conditions11
10Standard test conditions (STC) 1000 W/m2, AM 1.5, Tm=25◦C, wind speed <1 m/s
11It should be emphasized that the ambient conditions to define NOCT are different from STC defined above
1.7. ARCHITECTURE, RESEARCH AND STRATEGIES
of solar radiation 800 W/m2, room temperature 20◦C, wind speed <1 m/s, exposing both sides of the panel to the same conditions of ambient temperature and wind speed (open-rack mounted systems). However, when a module is fully integrated into the skin of the building, the mounting conditions affect the heat transfer, since the rear area of the panel cannot exchange heat with the environment (see Chapter 5). There is therefore a feedback between the main functionality (gen- eration of electricity) and the secondary (thermal cover) so that the change in indoor temperature modifies the operating temperature of the module, and therefore its electrical production which generally decreases if the temperature increases.
Therefore, the efficiency of the photovoltaic module can differ substantially from the values given by the manufacturer, since its response depends on factors such as the temperature of the module, the incident solar radiation, the angle of incidence and the spectral response [79]. In addition, it is important to note that, for some technologies, the continuous exposure to high values of irradiance and temperature can increase the degradation rate, and therefore affect the long-term performance of the panel (ageing effect). All this makes the prediction of this behaviour is of outmost importance for the design of BIPV systems and their viability within the construction sector.
The overall influence of irradiance (G) and temperature (T) in the energy production of the module can be obtained by fitting a set of measurements (indoor or/and outdoor) at different temperatures and irradiance levels to produce an output power (P) function which depends on both parameters, well-known as the power matrix P (G, T ) [80]. The model stands out by its simplicity and robust- ness. In fact, it has been implemented as PV estimator in the online tool, PVGIS [53]. The model was initially developed based on data of c-Si modules. However, it has been used for technology assessment in new PV materials [79]. From their results, it is concluded that a better adjustment is required complementing indoor measurements with long-term outdoor data which is not as easy task since outdoor long-term data of new technologies are not usually available.
Reliable models, which allow users to assess the technical potential of each PV technology in BIPV applications, and thus to know the role that could play BIPV systems in the deployment of NZEB, requires a deep knowledge of the thermal behaviour of the PV devices, together with accurate models to estimate the energy production in the whole range of irradiance and temperature (power matrix) in order to cover all the possibilities of operating conditions.
1.7 Architecture, research and strategies
For architects, the last decades have brought significant changes to the design of the profession.
Now, solar modules transform roofs and fa¸cades into functional building envelopes, and therefore,
