Urban wind energy: empirical optimization of high-rise building roof shape for the wind energy exploitation

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(1)URBAN WIND ENERGY: EMPIRICAL OPTIMIZATION OF HIGH-RISE BUILDING ROOF SHAPE FOR THE WIND ENERGY EXPLOITATION. Ph.D. THESIS. Francisco Toja Silva Escuela Técnica Superior de Ingenieros Aeronáuticos Universidad Politécnica de Madrid 2015.

(2) TEXiS v.1.0.. This document is prepared to be printed on both sides..

(3) URBAN WIND ENERGY: EMPIRICAL OPTIMIZATION OF HIGH-RISE BUILDING ROOF SHAPE FOR THE WIND ENERGY EXPLOITATION. A Thesis presented for the Doctor in Aerospace Engineering degree by Francisco Toja Silva. Supervisors. Dr. Oscar López García Dr. Jorge Navarro Montesinos. Escuela Técnica Superior de Ingenieros Aeronáuticos Universidad Politécnica de Madrid 2015.

(4) Copyright c Francisco Toja Silva.

(5) To you, and nobody else..

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(7) Acknowledgements If you're not having fun, you're doing something wrong. Groucho Marx. The author wants to acknowledge all the people cited in this Chapter. The present Thesis is dedicated to: - The reader. You are the actual purpose of this document. I hope it will be interesting and useful for you. - The Energy and Environment Research Center (CIEMAT) for the PhD fellowship that made possible the development of the present Thesis.. A. sincere acknowledgement for making true my dream of progressing in the research career towards a new stage.. I am aware of the eort that the. society made funding my career, and I must deserve it bringing all the best with the highest motivation and ethical principles in my present and future work. - The School of Aeronautics of the UPM for giving me the opportunity to defend this PhD Thesis in Aerospace Engineering. - My supervisors Oscar and Jorge for the support given. Also to Ignacio, the Director of the Wind Energy Unit of the CIEMAT. I had freedom during the Thesis development. It was a privilege because I could focus on the topics really interesting and motivating for me.. It carried out hard diculties,. but thanks to these diculties I have improved not only my scientic and technical skills but also my management and transferable skills. I want to really acknowledge them, they where there when I needed them. - Mari and Lulila for being on my side in my life.. The hard eort to. reach the excellence in research is much easier in the best company. - My work mates at the CIEMAT Anne, Carmen, Fernando, Luis, Manuel, Pablo, Pedro and Vadim.. A special mention to Dani, for the knowledge. transferred, his patience and the support given. Also a very special mention to Alfredo and Julien for the excellent knowledge transferred and for giving me the opportunity of leaning from their high-level scientic experience. - My work mates and personal friends at the Fraunhofer IWES Bastian, vii.

(8) viii. Acknowledgements. Bernhard, Michael, Jose, Thais, etc.; for the very good experience of working with them and the very good moments together. I felt at home in Oldenburg. A very special mention to Carlos for the very much signicant and appreciated support given, decisive for the success of this Thesis. - My family and my personal friends Alba, Alberto, Andrea, Candelarias, Carles, Concepción, David, Eduardo, Eric, Franciscos, Isabel, Ismaeles, Julieta, Julio, Laura, Lluís, Luis, María Jesús, Mariela, Marc, Mirta, Queralt, Raúl, Rosa, Ruben, Tomás, etc. I apologize the rest of the people not explicitly mentioned above. This work is also dedicated to them..

(9) Abstract The clear and present danger of climate change means we cannot burn our way to prosperity. We already rely too heavily on fossil fuels. We need to nd a new, sustainable path to the future we want. We need a clean industrial revolution. Ban Ki-moon. Resumen en Castellano El programa Europeo HORIZON2020 en Futuras Ciudades Inteligentes establece como objetivo que el 20% de la energía eléctrica sea generada a partir de fuentes renovables. Este objetivo implica la necesidad de potenciar la generación de energía eólica en todos los ámbitos.. La energía eólica reduce. drásticamente las emisiones de gases de efecto invernadero y evita los riesgos geo-políticos asociados al suministro e infraestructuras energéticas, así como la dependencia energética de otras regiones. Además, la generación de energía distribuida (generación en el punto de consumo) presenta signicativas ventajas en términos de elevada eciencia energética y estimulación de la economía. El sector de la edicación representa el 40% del consumo energético total de la Unión Europea. La reducción del consumo energético en este área es, por tanto, una prioridad de acuerdo con los objetivos 20-20-20 en eciencia energética. La Directiva 2010/31/EU del Parlamento Europeo y del Consejo de 19 de mayo de 2010 sobre el comportamiento energético de edicaciones contempla la instalación de sistemas de suministro energético a partir de fuentes renovables en las edicaciones de nuevo diseño. Actualmente existe una escasez de conocimiento cientíco y tecnológico acerca de la geometría óptima de las edicaciones para la explotación de la energía eólica en entornos urbanos. El campo tecnológico de estudio de la presente Tesis Doctoral es la generación de energía eólica en entornos urbanos.. Especícamente, la opti-. mización de la geometría de las cubiertas de edicaciones desde el punto de vista de la explotación del recurso energético eólico. Debido a que el ujo ix.

(10) x. Abstract. del viento alrededor de las edicaciones es exhaustivamente investigado en esta Tesis empleando herramientas de simulación numérica, la mecánica de uidos computacional (CFD en inglés) y la aerodinámica de edicaciones son los campos cientícos de estudio. El objetivo central de esta Tesis Doctoral es obtener una geometría de altas prestaciones (u óptima) para la explotación de la energía eólica en cubiertas de edicaciones de gran altura. Este objetivo es alcanzado mediante un análisis exhaustivo de la inuencia de la forma de la cubierta del edicio en el ujo del viento desde el punto de vista de la explotación energética del recurso eólico empleando herramientas de simulación numérica (CFD). Adicionalmente, la geometría de la edicación convencional (edicio prismático) es estudiada, y el posicionamiento adecuado para los diferentes tipos de aerogeneradores es propuesto.. La compatibilidad entre el aprovechamiento de. las energías solar fotovoltaica y eólica también es analizado en este tipo de edicaciones. La investigación prosigue con la optimización de la geometría de la cubierta. La metodología con la que se obtiene la geometría óptima consta de las siguientes etapas: - Vericación de los resultados de las geometrías previamente estudiadas en la literatura. Las geometrías básicas que se someten a examen son: cubierta plana, a dos aguas, inclinada, abovedada y esférica. - Análisis de la inuencia de la forma de las aristas de la cubierta sobre el ujo del viento. Esta tarea se lleva a cabo mediante la comparación de los resultados obtenidos para la arista convencional (esquina sencilla) con un parapeto, un voladizo y una esquina curva. - Análisis del acoplamiento entre la cubierta y los cerramientos verticales (paredes) mediante la comparación entre diferentes variaciones de una cubierta esférica en una edicación de gran altura: cubierta esférica estudiada en la literatura, cubierta esférica integrada geométricamente con las paredes (planta cuadrada en el suelo) y una cubierta esférica acoplada a una pared cilíndrica. El comportamiento del ujo sobre la cubierta es estudiado también considerando la posibilidad de la variación en la dirección del viento incidente. - Análisis del efecto de las proporciones geométricas del edicio sobre el ujo en la cubierta. - Análisis del efecto de la presencia de edicaciones circundantes sobre el ujo del viento en la cubierta del edicio objetivo. Las contribuciones de la presente Tesis Doctoral pueden resumirse en: - Se demuestra que los modelos de turbulencia RANS obtienen mejores resultados para la simulación del viento alrededor de edicaciones empleando los coecientes propuestos por Crespo y los propuestos por Bechmann y Sørensen que empleando los coecientes estándar. - Se demuestra que la estimación de la energía cinética turbulenta del.

(11) xi. Abstract. ujo empleando modelos de turbulencia RANS puede ser validada manteniendo el enfoque en la cubierta de la edicación. - Se presenta una nueva modicación del modelo de turbulencia Durbin. k−ε. que reproduce mejor la distancia de recirculación del ujo de acuerdo. con los resultados experimentales. - Se demuestra una relación lineal entre la distancia de recirculación en una cubierta plana y el factor constante involucrado en el cálculo de la escala de tiempo de la velocidad turbulenta.. Este resultado puede ser empleado. por la comunidad cientíca para la mejora del modelado de la turbulencia en diversas herramientas computacionales (OpenFOAM, Fluent, CFX, etc.). - La compatibilidad entre las energías solar fotovoltaica y eólica en cubiertas de edicaciones es analizada. Se demuestra que la presencia de los módulos solares provoca un descenso en la intensidad de turbulencia. - Se demuestran conictos en el cambio de escala entre simulaciones de edicaciones a escala real y simulaciones de modelos a escala reducida (túnel de viento).. Se demuestra que para respetar las limitaciones de similitud. (número de Reynolds) son necesarias mediciones en edicaciones a escala real o experimentos en túneles de viento empleando agua como uido, especialmente cuando se trata con geometrías complejas, como es el caso de los módulos solares. - Se determina el posicionamiento más adecuado para los diferentes tipos de aerogeneradores tomando en consideración la velocidad e intensidad de turbulencia del ujo. El posicionamiento de aerogeneradores es investigado en las geometrías de cubierta más habituales (plana, a dos aguas, inclinada, abovedada y esférica). - Las formas de aristas más habituales (esquina, parapeto, voladizo y curva) son analizadas, así como su efecto sobre el ujo del viento en la cubierta de un edicio de gran altura desde el punto de vista del aprovechamiento eólico. - Se propone una geometría óptima (o de altas prestaciones) para el aprovechamiento de la energía eólica urbana.. Esta optimización incluye:. vericación de las geometrías estudiadas en el estado del arte, análisis de la inuencia de las aristas de la cubierta en el ujo del viento, estudio del acoplamiento entre la cubierta y las paredes, análisis de sensibilidad del grosor de la cubierta, exploración de la inuencia de las proporciones geométricas de la cubierta y el edicio, e investigación del efecto de las edicaciones circundantes (considerando diferentes alturas de los alrededores) sobre el ujo del viento en la cubierta del edicio objetivo. Las investigaciones comprenden el análisis de la velocidad, la energía cinética turbulenta y la intensidad de turbulencia en todos los casos..

(12) xii. Abstract. Abstract in English The HORIZON2020 European program in Future Smart Cities aims to have 20% of electricity produced by renewable sources. This goal implies the necessity to enhance the wind energy generation, both with large and small wind turbines. Wind energy drastically reduces carbon emissions and avoids geo-political risks associated with supply and infrastructure constraints, as well as energy dependence from other regions. Additionally, distributed energy generation (generation at the consumption site) oers signicant benets in terms of high energy eciency and stimulation of the economy. The buildings sector represents 40% of the European Union total energy consumption.. Reducing energy consumption in this area is therefore a pri-. ority under the 20-20-20 objectives on energy eciency.. The Directive. 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings aims to consider the installation of renewable energy supply systems in new designed buildings. Nowadays, there is a lack of knowledge about the optimum building shape for urban wind energy exploitation. The technological eld of study of the present Thesis is the wind energy generation in urban environments. Specically, the improvement of the building-roof shape with a focus on the wind energy resource exploitation. Since the wind ow around buildings is exhaustively investigated in this Thesis using numerical simulation tools, both computational uid dynamics (CFD) and building aerodynamics are the scientic elds of study. The main objective of this Thesis is to obtain an improved (or optimum) shape of a high-rise building for the wind energy exploitation on the roof. To achieve this objective, an analysis of the inuence of the building shape on the behaviour of the wind ow on the roof from the point of view of the wind energy exploitation is carried out using numerical simulation tools (CFD). Additionally, the conventional building shape (prismatic) is analysed, and the adequate positions for dierent kinds of wind turbines are proposed. The compatibility of both photovoltaic-solar and wind energies is also analysed for this kind of buildings. roof optimization.. The investigation continues with the building-. The methodology for obtaining the optimum high-rise. building roof shape involves the following stages: - Verication of the results of previous building-roof shapes studied in the literature. The basic shapes that are compared are: at, pitched, shed, vaulted and spheric. - Analysis of the inuence of the roof-edge shape on the wind ow. This task is carried out by comparing the results obtained for the conventional edge shape (simple corner) with a railing, a cantilever and a curved edge. - Analysis of the roof-wall coupling by testing dierent variations of a spherical roof on a high-rise building: spherical roof studied in the litera-.

(13) xiii. Abstract. ture, spherical roof geometrically integrated with the walls (squared-plant) and spherical roof with a cylindrical wall. The ow behaviour on the roof according to the variation of the incident wind direction is commented. - Analysis of the eect of the building aspect ratio on the ow. - Analysis of the surrounding buildings eect on the wind ow on the target building roof. The contributions of the present Thesis can be summarized as follows: - It is demonstrated that RANS turbulence models obtain better results for the wind ow around buildings using the coecients proposed by Crespo and those proposed by Bechmann and Sørensen than by using the standard ones. - It is demonstrated that RANS turbulence models can be validated for turbulent kinetic energy focusing on building roofs. - A new modication of the Durbin. k−ε. turbulence model is proposed. in order to obtain a better agreement of the recirculation distance between CFD simulations and experimental results. - A linear relationship between the recirculation distance on a at roof and the constant factor involved in the calculation of the turbulence velocity time scale is demonstrated.. This discovery can be used by the research. community in order to improve the turbulence modeling in dierent solvers (OpenFOAM, Fluent, CFX, etc.). - The compatibility of both photovoltaic-solar and wind energies on building roofs is demonstrated.. A decrease of turbulence intensity due to. the presence of the solar panels is demonstrated. - Scaling issues are demonstrated between full-scale buildings and windtunnel reduced-scale models. The necessity of respecting the similitude constraints is demonstrated. Either full-scale measurements or wind-tunnel experiments using water as a medium are needed in order to accurately reproduce the wind ow around buildings, specially when dealing with complex shapes (as solar panels, etc.). - The most adequate position (most adequate roof region) for the different kinds of wind turbines is highlighted attending to both velocity and turbulence intensity. The wind turbine positioning was investigated for the most habitual kind of building-roof shapes (at, pitched, shed, vaulted and spherical). - The most habitual roof-edge shapes (simple edge, railing, cantilever and curved) were investigated, and their eect on the wind ow on a highrise building roof were analysed from the point of view of the wind energy exploitation. - An optimum building-roof shape is proposed for the urban wind energy exploitation. Such optimization includes: state-of-the-art roof shapes test, analysis of the inuence of the roof-edge shape on the wind ow, study of the.

(14) xiv. Abstract. roof-wall coupling, sensitivity analysis of the roof width, exploration of the aspect ratio of the building-roof shape and investigation of the eect of the neighbouring buildings (considering dierent surrounding heights) on the wind ow on the target building roof. The investigations comprise analysis of velocity, turbulent kinetic energy and turbulence intensity for all the cases..

(15) Contents Acknowledgements. vii. Abstract. ix. Nomenclature. xxxi. 1 Introduction. 1. 1.1. Scientic and technological elds of study. . . . . . . . . . . .. 1. 1.2. Objectives and methodology . . . . . . . . . . . . . . . . . . .. 1. 1.3. Why empirical? . . . . . . . . . . . . . . . . . . . . . . . . .. 3. 1.4. Research scope. 1.5. Justication of this investigation. 1.6. A review of urban wind energy. . . . . . . . . . . . . . . . . . . . . . . . . . .. 3. . . . . . . . . . . . . . . . .. 5. . . . . . . . . . . . . . . . . .. 7. 1.6.1. A basic concept of the wind ow on building roofs. . .. 9. 1.6.2. Wind energy exploitation in large structures . . . . . .. 11. 1.6.3. Wind energy exploitation in buildings. . . . . . . . . .. 13. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 26. Bibliography notes. 2 CWE applied to building aerodynamics. 31. 2.1. Basic aspects. 2.2. The atmospheric boundary layer. 2.3. Governing equations of the ow and turbulence modeling. . .. 40. 2.4. Building aerodynamics . . . . . . . . . . . . . . . . . . . . . .. 44. 2.5. Hardware used for the simulations. . . . . . . . . . . . . . . .. 49. 2.6. Validation of RANS turbulence models . . . . . . . . . . . . .. 51. 2.6.1. Flat roof building model in wind tunnel. 51. 2.6.2. Curved-roof building model in wind tunnel. 2.7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .. 31 36. . . . . . .. 73. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 78. Bibliography notes. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3 Compatibility of solar and wind energy systems. 80. 81 xv.

(16) xvi. Index. 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3.2. Complementary validation . . . . . . . . . . . . . . . . . . . .. 82. 3.3. Description of the cases and simulation details . . . . . . . . .. 84. 3.4. Results and discussion. 88. 3.5. . . . . . . . . . . . . . . . . . . . . . .. 3.4.1. Reynolds number similarity constraints . . . . . . . . .. 93. 3.4.2. Solution verication. . . . . . . . . . . . . . . . . . . .. 96. 3.4.3. Wind energy exploitation and wind turbine positioning. 97. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100. Bibliography notes. . . . . . . . . . . . . . . . . . . . . . . . . . . . 102. 4 Building-roof shape optimization 4.1. Methodology. 4.2. State-of-the-art roof shapes. 4.3. 4.4. 4.5. 4.6. 4.7. 4.8. 81. 103. . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 . . . . . . . . . . . . . . . . . . . 104. 4.2.1. Description of the cases. 4.2.2. Simulation results. 4.2.3. Conclusion. . . . . . . . . . . . . . . . . . 105. . . . . . . . . . . . . . . . . . . . . 105. . . . . . . . . . . . . . . . . . . . . . . . . 110. Inuence of the roof-edge shape on the wind ow . . . . . . . 112 4.3.1. Description of the cases. 4.3.2. Simulation results. 4.3.3. Conclusion. . . . . . . . . . . . . . . . . . 112. . . . . . . . . . . . . . . . . . . . . 114. . . . . . . . . . . . . . . . . . . . . . . . . 119. Wall-roof coupling analysis. . . . . . . . . . . . . . . . . . . . 120. 4.4.1. Description of the cases. 4.4.2. Simulation results. 4.4.3. Solution verication. 4.4.4. Conclusion. . . . . . . . . . . . . . . . . . 120. . . . . . . . . . . . . . . . . . . . . 121 . . . . . . . . . . . . . . . . . . . 125. . . . . . . . . . . . . . . . . . . . . . . . . 125. Sensitivity analysis of the roof width . . . . . . . . . . . . . . 127 4.5.1. Description of the cases. 4.5.2. Results and discussion . . . . . . . . . . . . . . . . . . 128. . . . . . . . . . . . . . . . . . 127. 4.5.3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 131. Analysis of the building aspect ratio. . . . . . . . . . . . . . . 132. 4.6.1. Description of the cases. . . . . . . . . . . . . . . . . . 132. 4.6.2. Results and discussion . . . . . . . . . . . . . . . . . . 133. 4.6.3. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . 136. Inuence of the surrounding buildings. . . . . . . . . . . . . . 136. 4.7.1. Description of the cases. 4.7.2. Results and discussion . . . . . . . . . . . . . . . . . . 139. . . . . . . . . . . . . . . . . . 136. 4.7.3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 144. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145. Bibliography notes. . . . . . . . . . . . . . . . . . . . . . . . . . . . 150. 5 Concluding remarks. 151.

(17) Index. xvii. 5.1. Final conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 151. 5.2. 5.1.1. Contributions of this Thesis . . . . . . . . . . . . . . . 151. 5.1.2. Conclusions from the investigations . . . . . . . . . . . 153. Suggestions for further works. . . . . . . . . . . . . . . . . . . 156. Bibliography. 159. A RBF-based immersed boundary method. 179. A.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179. A.2. RBF interpolation from an arbitrarily scattered set of nodes . 182. A.3. Imposition of Dirichlet boundary conditions . . . . . . . . . . 185. A.4. A.5. A.3.1. Methodology. . . . . . . . . . . . . . . . . . . . . . . . 187. A.3.2. Results and discussion . . . . . . . . . . . . . . . . . . 189. Treatment of Neumann boundary conditions . . . . . . . . . . 192 A.4.1. Methodology. . . . . . . . . . . . . . . . . . . . . . . . 193. A.4.2. Results and discussion . . . . . . . . . . . . . . . . . . 195. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198. Bibliography notes. . . . . . . . . . . . . . . . . . . . . . . . . . . . 199. B Curriculum Vitae. 201. B.1. Education . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201. B.2. Publications and patents . . . . . . . . . . . . . . . . . . . . . 202. B.3. Conferences, congresses and seminars . . . . . . . . . . . . . . 204. B.4. Research experience, fellowships and awards . . . . . . . . . . 204. B.5. Complementary education . . . . . . . . . . . . . . . . . . . . 205. B.6. Complementary work experience. B.7. Computational skills. B.8. Languages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208. . . . . . . . . . . . . . . . . 207. . . . . . . . . . . . . . . . . . . . . . . . 208.

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(19) List of Figures. 1.1. Diagram of the methodology (milestones) followed for obtaining and analysing an optimum building-roof shape for the urban wind energy exploitation.. Deliverables:. article 1 is. Toja-Silva et al. (2015d), article 2 is Toja-Silva et al. (2015b), article 3 is Toja-Silva et al. (2015c) and article 4 is Toja-Silva et al. (2015a). . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. bines (Pantazopoulou (2009)). . . . . . . . . . . . . . . . . . . 1.3. 2. Example of sound levels vs. wind speed for small wind tur8. Diagram of the simulation case (left) and a symmetric building with a large length with respect to its height (right). The simulation comprises the central length of the building. . . . .. 1.4. 10. Instantaneous velocity (m/s) maps obtained in the simulation for incident wind velocities of 1, 2, 4 and 10 m/s (from left to right and from the top to the bottom). . . . . . . . . . . . . .. 1.5. 11. Wind turbines at the Bolte Bridge in Melbourne (Australia). (Oppenheim (2004)). . . . . . . . . . . . . . . . . . . . . . . .. 1.6. Examples of building-augmented wind turbines (BAWT).. 1.7. Horizontal axis wind turbine (HAWT) in the urban environment.. . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1.8. Diuser-augmented wind turbine (DAWT).. . . . . . . . . . .. 1.9. Superposition of a HAWT over the air ow for an incident velocity of 1 m/s. . . . . . . . . . . . . . . . . . . . . . . . . .. 1.10 Hybrid VAWT generator (Darrieus and Savonius). 1.11 Power coecients. CP. vs. the specic velocity. . . . . . .. 12 14. 15 15. 16 17. λ for three tur-. bines: HAWT, Darrieus and H-rotor (Eriksson et al. (2008a)).. 18. 1.12 Giromill wind turbine with ve blades. . . . . . . . . . . . . .. 19. 1.13 Power coecient of a Giromill wind turbine with two (left) and three (right) blades (Howell et al. (2010)). . . . . . . . . . 1.14 Power coecients. Cp. (in %) vs.. the specic velocity. λ. 19. for. three types of blades, Giromill wind turbine with four blades (El-Samanoudy et al. (2010)). . . . . . . . . . . . . . . . . . .. 20 xix.

(20) xx. List of figures. 1.15 Superposition of a VAWT over the air ow for an incident velocity of 1 m/s. . . . . . . . . . . . . . . . . . . . . . . . . .. 21. 1.16 Helical Darrieus wind turbine. . . . . . . . . . . . . . . . . . .. 21. 1.17 Power coecient for various solidities, helical Darrieus wind turbine (Kirke and Lazauskas (2011)).. . . . . . . . . . . . . .. 22. 1.18 Flexible Darrieus wind turbine in both horizontal (left) and vertical (right) positions (Sharpe and Proven (2010)). . . . . .. 23. 1.19 Superposition of a horizontal Darrieus wind turbine for an incident velocity of 1 m/s. . . . . . . . . . . . . . . . . . . . .. 24. 1.20 Power vs. rotation speed for various wind velocities, exible Darrieus wind turbine (Sharpe and Proven (2010)). . . . . . . 1.21 Power coecient. 24. Cp of the Savonius wind turbine (D'Alessandro. et al. (2010)). . . . . . . . . . . . . . . . . . . . . . . . . . . .. 25. 1.22 Extremely low-cost Savonius wind turbine (left), and a diagram of a Savonius wind turbine with two blades and a deector sheet (right) (Mohamed et al. (2010)).. . . . . . . . . .. 25. 1.23 Power coecient of a Savonius wind turbine with two (left) and three (right) blades with and without a deector sheet (Mohamed et al. (2010)). . . . . . . . . . . . . . . . . . . . . . 1.24 Helical Savonius wind turbine with three blades.. . . . . . . .. 26 26. 1.25 Coecients of power, torque and static torque for a two-blade Savonius wind turbine with both helical and conventional blades (Kamoji et al. (2009)). . . . . . . . . . . . . . . . . . .. 27. 1.26 Vertical-axis resistance-type wind turbine (Müller et al. (2009)). 28 1.27 Eciency of vertical-axis resistance-type wind turbines (Müller et al. (2009)). . . . . . . . . . . . . . . . . . . . . . . . . . . .. 28. 1.28 Ducted wind turbine on the edge of the building roof (Grant et al. (2008)). . . . . . . . . . . . . . . . . . . . . . . . . . . .. 29. 1.29 Power coecient vs. the dierential pressure coecient of a ducted wind turbine for various values of the speed coecient (Grant et al. (2008)). . . . . . . . . . . . . . . . . . . . . . . .. 29. 1.30 Superposition of a ducted wind turbine on the edge of the building roof for an incident velocity of 1 m/s. . . . . . . . . . 2.1. Representation of turbulence in water ows, Leonardo da Vinci 1508-1509 (Werne (2015)).. 2.2. . . . . . . . . . . . . . . . .. 33. Transition from laminar to turbulent ow at the cigarette smoke (adapted from Tompitak (2015)).. 2.3. 30. . . . . . . . . . . . .. Flow over an obstacle inside a channel with. Re = 800.. 34. Note. that velocity is appreciated inside the obstacle, because an immersed boundary method has been used in a DNS CFD code. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 34.

(21) xxi. List of figures. 2.4. Transition from laminar to turbulent ow in a boundary layer. The characteristic sublayers are shown: viscous sublayer, buer layer and turbulent region. (Comsol (2015)) . . . . . . . . . .. 2.5. 35. Diagram of the layers of Earth's atmosphere, showing heights of characteristic atmospheric phenomena (Encyclopædia Britannica (2015)). . . . . . . . . . . . . . . . . . . . . . . . . . .. 2.6. 37. Diagram of the meteorological phenomena and their respective time and length scale, and the most usual classication of the meteorogical scales (Geostationary Operational Environmental Satellites R Series (2015)). . . . . . . . . . . . . . .. 2.7. Diagram of the time-evolution of the atmosphere (Garratt (1994)).. 2.8. 38. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 39. Schematic diagram showing the three regimes of wind ow over an obstacle array, and the proportions to which each regime applies. (Oke (1988)) . . . . . . . . . . . . . . . . . . .. 2.9. 45. Wind-ow pattern around an isolated building in 2D and 3D. Legend: (1) ow over building, (2) oncoming ow, (3) ow from stagnation point over building, (4) ow from stagnation point around vertical building edges, (5) downow from stagnation point, (6) standing vortex (base vortex or horseshoe vortex), (7) stagnation ow in front of building near ground level, (8) corner streams (vortex wrapping around corners), (9) ow around building sides at ground level (adding to corner streams), (10) recirculation ow behind the building, (11) stagnation region behind building at ground level, (12) restored ow direction, (13) large vortices behind building and (16) small vortices in shear layer. . . . . . . . . . . . . . . . .. 46. 2.10 Instantaneous vortex generated by a slender prismatic obstacle: (a) top view and (b) lateral view. (Joubert et al. (2015)). 47. 2.11 Diagram of the corner roof vortices appearing when the incident wind direction is oblique to the walls. Cernak (1976)). (Peterka and. . . . . . . . . . . . . . . . . . . . . . . . . . .. 47. 2.12 Examples of pressure elds in recirculation regions on the roof. 48 2.13 Diagram of the case of study. All dimensions are in m. . . . .. 53. 2.14 Inlet proles: mean streamwise velocity (a), turbulent kinetic energy (b) and turbulent dissipation (c).. The points repre-. sent the inlet proles used at the experiment of Meng and Hibi (1998), and the solid lines are the numerical inlets of the simulations. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 54. 2.15 Vertical section of the rened mesh obtained using snappyHexMesh with close to 3.1M cells. . . . . . . . . . . . . . . . .. 55.

(22) xxii. List of figures. 2.16 Diagram of the axes (V1-V6) at the vertical section of the central part of the domain, for the comparison of the results. All lengths are in meters.. . . . . . . . . . . . . . . . . . . . .. 2.17 Sensitivity analysis for the recirculation distance ing the constant factor in the denition of. TD. XR. 56. by vary-. (Eq. (2.20)).. .. 58. . . . . . . . . . . . . . . . . . . . . . . . .. 61. 2 2 2.18 Comparison of the turbulent kinetic energy [m /s ] at the vertical section at the center of the domain, using the models with. HRk ≥ 66%. .. 2.19 Detail of the rened meshes obtained for the convergence analysis, using the. snappyHexMesh. application of OpenFOAM.. 62. 2.20 Diagram of the regions of the building roof for the ow analysis. All lengths are in meters. 2.21 Vertical proles comparison for. . . . . . . . . . . . . . . . . .. U. (left) and. k. 64. (right) at the. upstream region of the building, using the RANS models that successfully pass the validation. . . . . . . . . . . . . . . . . . 2.22 Vertical proles comparison for. U. (left) and. k. 65. (right) at the. central region of the building roof, using the RANS models that successfully pass the validation. 2.23 Vertical proles comparison for. U. . . . . . . . . . . . . . .. (left) and. k. 67. (right) at the. downstream region of the building, using the RANS models that successfully pass the validation. 2.24 Wind turbine positioning diagrams:. . . . . . . . . . . . . . .. 68. (A) Most appropriate. wind energy exploitation systems at the dierent regions of the building roof. The vector eld is the velocity, the background colormap is turbulence intensity (T I ) and the bold line (in magenta) is an isocontour of the isoline corresponding to. T I = 0.15.. (B) Ducted wind turbine at the upstream. corner of the roof into the pressure eld. . . . . . . . . . . . . 2.25 Isosurfaces of. T I = 0.15. 70. (in grey colour) for a normal inci-. ◦. ◦. dent wind direction (0 ) and an oblique wind direction (45 ). HAWT can be placed above the isosurface. Below this region, VAWT must be considered. Dark red represents the building, green the ground and blue the sky.. . . . . . . . . . . . . . . .. 71. 2.26 Diagram of the wind-tunnel geometry and the axes V1-V3 for the validation of the results. All lengths are in meters.. . . . .. 73. 2.27 Inlet wind proles for the curved-roof validation case: mean streamwise velocity (U ), turbulent kinetic energy (k ) and turbulent dissipation (ε). The points represent the inlet proles used at the experiment of Ntinas et al. (2014), and the solid lines are the numerical inlets of the simulations. . . . . . . . .. 75. 2.28 Vertical section of the rened mesh obtained using snappyHexMesh.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 76.

(23) xxiii. List of figures. 2.29 Vertical section at the center of the domain of the. U. and. k. elds around the building. . . . . . . . . . . . . . . . . . . . . 2.30 Comparison of of the domain. 3.1. U. and. k. at the vertical section at the center. . . . . . . . . . . . . . . . . . . . . . . . . . .. 77. Diagram of the validation of the innite-span array of solar panels.. The AXIS indicates the points where the data is. compared for the validation.. All values are dimensionless,. expressed as multiples of the width of the plate 3.2. 76. W =. 1 m. . .. 83. Comparison between numerical and experimental values for validation using the experimental data of Fage and Johansen (1927). All values are dimensionless: distances with respect to the innite array width free-stream velocity. 3.3. U∞. W. and velocity with respect to the. (inlet velocity).. . . . . . . . . . . . . . . . . . .. ◦. ◦. 86. . . . . . . . . . . . . . .. 87. Vertical section of the rened meshes obtained using snappyHexMesh.. 3.7. . . . . . . . . . . . . . .. Diagram of the geometry of the 30.36 kW photovoltaic facility with a tilt angle of 30 , at full scale.. 3.6. 85. Diagram of the geometry of the 41.75 kW photovoltaic facility with a tilt angle of 10 , at full scale.. 3.5. 84. Diagram of the computational domain of the full-scale building. All values are in meters.. 3.4. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 88. Diagram of the axes (V1-V4) at the vertical section of the central part of the domain, for the comparison of the results. All lengths are dimensionless.. 3.8. . . . . . . . . . . . . . . . . . .. 89. Comparison of the velocity at the vertical section at the center of the domain, for the full-scale model. Note that some series. ◦ in V2 and V3) do not have values close to the roof,. (10. because the solar panels (including the support structure) ll this space. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9. 91. Comparison of turbulent kinetic energy at the vertical section at the center of the domain, for the full-scale model.. . . . . .. 92. 3.10 Streamlines that show the recirculation vortices on the roof for the 30. ◦ raised panels, at the full-scale model.. . . . . . . .. 93. 3.11 Comparison of the velocity at the vertical section at the center of the domain, for the reduced-scale model.. . . . . . . . . . .. 94. 3.12 Comparison of turbulent kinetic energy at the vertical section at the center of the domain, for the reduced-scale model. . . .. 95. 3.13 Streamlines that show the recirculation vortices on the roof for raised panels in unfavourable position, at the reduced-scale model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 96.

(24) xxiv. List of figures. 3.14 Comparison of velocity at the vertical section at the center of the domain using 3 dierent meshes. Case of raised panels in unfavourable position.. . . . . . . . . . . . . . . . . . . . . . .. 98. 3.15 Comparison of turbulence kinetic energy at the vertical section at the center of the domain using 3 dierent meshes. Case of raised panels in unfavourable position. . . . . . . . . .. 99. 3.16 Vertical proles of turbulence intensity up and downstream of the roof considering dierent incident wind directions for the raised solar panels, until the threshold. T I = 0.15. .. . . . . 100. 3.17 Turbulence intensity eld around the building and detail of a VAWT in horizontal position upstream. The grey line represents the threshold. T I = 0.15. for the installation of HAWT. (above) and VAWT (below), and the vectorial eld is the velocity. 4.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101. Central vertical section detail of the dierent roof shapes investigated.. The red AXIS indicates the points where the. data is compared between the dierent cases. 4.2. 4.3. . . . . . . . . . 106. Vertical section detail of the rened meshes obtained using snappyHexMesh for the state-of-the-art analysis.. . . . . . . . 107. Comparison of speed-up, nondimensional TKE and. TI. for the. state-of-the-art roof shapes analysis at the vertical axis on the center of the roof. . . . . . . . . . . . . . . . . . . . . . . . . . 108 4.4. 4.5. Comparison of speed-up (U/Uref ) and. TI. elds on the roof. for the state-of-the-art cases: sharp roofs.. . . . . . . . . . . . 109. Comparison of speed-up (U/Uref ) and. elds on the roof. TI. for the state-of-the-art cases: curved roofs. . . . . . . . . . . . 110 4.6. Examples of the dierent roof-edges tested.. . . . . . . . . . . 112. 4.7. Central vertical section detail of the dierent roof-edge shapes investigated. The red axes V1-V4 indicate the points where the data is compared between the dierent cases. Note that the axes V1 and V4 start from the normal height of the roof also for the curved edge, although both upstream and downstream edges of the roof are 1 m below in this case. Additionally, note that the axes V1 and V4 start from 1 m above the normal roof height for the railing due to the presence of this element. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113. 4.8. Vertical section detail of the rened meshes obtained using snappyHexMesh.. 4.9. . . . . . . . . . . . . . . . . . . . . . . . . . 114. Comparison of the speed-up (U/Uref ) at the vertical section on the center of the domain. . . . . . . . . . . . . . . . . . . . 115.

(25) xxv. List of figures. 2. 4.10 Comparison of the nondimensional TKE (k/Uref ) at the vertical section on the center of the domain. . . . . . . . . . . . . 116 4.11 Comparison of. TI. below the limit of. section on the center of the domain.. T I < 0.15 at the vertical . . . . . . . . . . . . . . 117. 4.12 Comparison of speed-up (U/Uref ) and 4.13 Comparison of speed-up (U/Uref ) and. TI TI. elds on the roof. . 118 elds on the roof. . 119. 4.14 Vertical section detail of the rened meshes obtained using snappyHexMesh for the additional spheric roofs.. . . . . . . . 121. 4.15 Comparison of speed-up, nondimensional TKE and. TI. for the. wall-roof coupling analysis at the vertical axis on the center of the roof.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122. 4.16 Comparison of speed-up (U/Uref ) and. TI. elds for the wall-. roof coupling analysis of the spheric roofs. . . . . . . . . . . . 123 4.17 Transversal elds of speed-up (U/Uref ) and and cylindrical wall-spheric roof.. TI. for vaulted. . . . . . . . . . . . . . . . . 124. 4.18 Detail of the 3 dierent meshes used for the solution verication.125 4.19 Comparison of speed-up and nondimensional TKE on the center of the roof using 3 dierent meshes for the solution verication. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 4.20 Diagram of the dierent roof-shapes investigated for the sensitivity analysis of the roof width (r ). . . . . . . . . . . . . . . 127 4.21 Vertical section detail of the rened mesh obtained using snappyHexMesh for the sensitivity analysis of the roof width. 4.22 Comparison of speed-up, nondimensional TKE and. TI. 128. for the. roof width analysis at the vertical axis on the center of the roof.129 4.23 Maximum values of speed-up (U/Uref ) and nondimensional. 2. TKE (k/Uref ) for the dierent roof widths investigated. 4.24 Comparison of speed-up (U/Uref ) and. TI. . . . 130. elds for the roof. width analysis: thinner width shapes. . . . . . . . . . . . . . . 131 4.25 Comparison of speed-up (U/Uref ) and. TI. elds for the roof. width analysis: wider width shapes. . . . . . . . . . . . . . . . 132 4.26 Diagram of the dierent aspect ratios (AR) investigated. . . . 133 4.27 Vertical section detail of the rened mesh obtained using snappyHexMesh for the dierent aspect ratios tested.. The. aspect ratio (AR) and the number of mesh cells are also indicated.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134. 4.28 Comparison of speed-up, nondimensional TKE and. TI. for the. aspect ratio analysis at the vertical axis on the center of the roof.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135. 4.29 Maximum values of speed-up (U/Uref ) and nondimensional. 2. TKE (k/Uref ) for the dierent aspect ratios investigated.. . . 136.

(26) xxvi. List of figures. 4.30 Comparison of speed-up (U/Uref ) vector eld and isosurface for the dierent aspect ratios.. T I = 0.15. . . . . . . . . . . . . 137. 4.31 Diagram of the computational domain for the surroundingbuildings eect analysis. All values are in meters. The conguration shown corresponds to. h/H = 0.5,. as an example.. . 138. 4.32 Diagram of the dierent aspect ratio buildings. The conguration shown corresponds to. h/H = 0.5,. as an example.. . . . 139. 4.33 Vertical section detail of the rened mesh obtained using snappyHexMesh for the surrounding buildings analysis (short surrounding buildings). . . . . . . . . . . . . . . . . . . . . . . 140 4.34 Vertical section detail of the rened mesh obtained using snappyHexMesh for the surrounding buildings analysis (tall surrounding buildings). . . . . . . . . . . . . . . . . . . . . . . 141 4.35 Comparison of the speed-up (U/Uref ) for the surroundingbuildings inuence analysis at the vertical axis on the center of the roof.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142. 2. 4.36 Comparison of the nondimensional TKE (k/Uref ) for the surroundingbuildings inuence analysis at the vertical axis on the center of the roof.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142. 4.37 Comparison of. TI. below the limit of. T I < 0.15. for the. surrounding-buildings inuence analysis at the vertical axis on the center of the roof. . . . . . . . . . . . . . . . . . . . . . 143 4.38 Comparison of the maximum speed-up (U/Uref ) and the. TI. threshold height for the surrounding-buildings inuence analysis at the vertical axis on the center of the roof.. . . . . . . . 144. 4.39 Maximum values of speed-up (U/Uref ) and nondimensional. 2. TKE (k/Uref ) for the surrounding-buildings inuence analysis considering the dierent aspect ratios investigated.. . . . . 145. 4.40 Comparison of speed-up (U/Uref ) and. 4.43. T I elds for h/H = 0.25.146 Comparison of speed-up (U/Uref ) and T I elds for h/H = 0.5.147 Comparison of speed-up (U/Uref ) and T I elds for h/H = 0.75.148 Comparison of speed-up (U/Uref ) and T I elds for h/H = 1. 149. A.1. Norm of the interpolation error at the Lagrangian points for. 4.41 4.42. the case described in 3.1. Red squares: interpolation error; solid line:. ∆x. (1st order); dashed line:. ∆x2. (2nd order). . . . 185. A.2. Diagram of the interpolation support.. A.3. Mean velocity and vorticity contours at. . 190. A.4. Instantaneous velocity and vorticity contours at. . 191. A.5. Shape parameters of the wake formed al. (2010)).. . . . . . . . . . . . . . 187. ReD = 30. . . . . . ReD = 185. at Re = 30 (Pinelli et. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.

(27) xxvii. List of figures. A.6. Norm of the interpolation error at the immersed surface for Red squares: error u; blue v ; solid line: ∆x (1st order); dashed line: ∆x2. a Dirichlet boundary condition. asterisks: error. (2nd order). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 A.7. T (x, y). External boundary condiTN = 100, TS = 25, TW = 75 and the right hand side. Final temperature eld tions:. wall is considered adiabatic. . . . . . . . . . . . . . . . . . . . 196 A.8. Streamwise section of the temperature eld at the center of the domain. Red line: without considering the adiabatic embedded surface. Black line: considering the adiabatic embedded surface. Blue line: body conformal case. . . . . . . . . . . 196. A.9. Normal derivative of the closest temperature eld outside the vertical wall, in front of the height.. Red circles:. without. considering the adiabatic embedded surface. Black squares: considering the adiabatic embedded surface. . . . . . . . . . . 197 A.10 Norm of the interpolation error of the derivative at the immersed surface. Red squares: derivative error; solid line:. ∆x. 2 (1st order); dashed line: ∆x (2nd order). . . . . . . . . . . . 198. T (x, y). External boundary conditions: = 100, TS = 25, TW = 75 and TE = 50. . . . . . . . . .. A.11 Temperature eld. TN. . 198.

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(29) List of Tables. 1.1. Water consumption of various energy technologies (Saidur et al. (2011)).. 1.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 22. Main dierences between the ABL and the free atmosphere (Blocken (2014b)).. 2.2. 18. Improvements to increase the performance of Savonius wind turbines (D'Alessandro et al. (2010)). . . . . . . . . . . . . . .. 2.1. 8. Water consumption of various energy technologies (Saidur et al. (2011)).. 1.3. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. 37. Boundary conditions imposed at each boundary of the domain following Architectural Institute of Japan (2013) and Tominaga et al. (2008). Nomenclature: iP= Inlet prole, zG = zeroGradient, C = Calculated, fV = xedValue, wF = wall function, sP = Symmetry plane.. . . . . . . . . . . . . . . . .. 53. 2.3. RANS turbulence models tested.. . . . . . . . . . . . . . . . .. 56. 2.4. Tested coecients of the linear. 2.5. Comparison of the results using dierent RANS models: Reat-. k−ε. models.. . . . . . . . . .. 56. tachment distance relative to the roof length (XR ) of the recirculation vortex on the building roof and hit rate (HR) for the variables. U. and. k.. The values that do not pass the valida-. tion process are in red colour, and the results obtained with the modication of the Durbin turbulence model proposed in this Thesis are in blue colour. . . . . . . . . . . . . . . . . . . 2.6. Main parameters of the mesh renement using the. 59. snappyHexMesh. application of OpenFOAM, and values obtained for the hit rates (HR). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. 62. Boundary conditions imposed at each boundary of the domain for the curved roof validation. Nomenclature: C = Calculated, fV = xedValue, iP = Inlet prole, sl = slip, sP = Symmetry plane, wF = wall function, zG = zeroGradient. . .. 74 xxix.

(30) xxx. List of tables. 2.8. Hit rates (HR) for the variables. U. and. k. RANS models tested for the curved roof.. at the dierent The results ob-. tained with the modication of the Durbin turbulence model proposed in this Thesis are in blue colour. . . . . . . . . . . . A.1. 78. Comparison of the main parameters of the wake and the drag coecient at. ReD = 30. with other works and experimental. data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 A.2. Comparison of the Strouhal number and the drag coecient at. ReD = 185. with other works and experimental data. . . . . 192.

(31) Nomenclature ABL . . . . . . . . . .. Atmospheric Boundary Layer. AD . . . . . . . . . . . .. Absolute maximum admissible deviation from the experimental data. p̄ . . . . . . . . . . . . . .. Mean pressure [Pa]. BAWT . . . . . . . .. Building-Augmented Wind Turbine. C . . . . . . . . . . . . . . Calculated CAD . . . . . . . . . .. Computer-Aided Design. CBL . . . . . . . . . .. Convective Boundary Layer. cP. Mean pressure coecient [-]. .............. Cε1 . . . . . . . . . . . .. Closure constant. k−ε. model [-]. Cε2 . . . . . . . . . . . .. Closure constant. k−ε. model [-]. CFD . . . . . . . . . .. Computational Fluid Dynamics. CIEMAT . . . . .. Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas. Cµ . . . . . . . . . . . . .. Model coecient turbulence model (proportional number) [-]. CP. ............. Power coecient [-]. CPU . . . . . . . . . .. Central Processing Unit. CWE . . . . . . . . .. Computational Wind Engineering. DAWT . . . . . . . .. Diuser-Augmented Wind Turbine. DDN . . . . . . . . . .. DataDirect Networks. DDR . . . . . . . . . .. Double Data Rate xxxi.

(32) xxxii. Nomenclature. δij . . . . . . . . . . . . .. Kronecker Delta function. D. Building-base diameter [m]. .............. DIC . . . . . . . . . . .. Diagonal Incomplete-Cholesky. DILU . . . . . . . . .. Diagonal Incomplete LU. DNS . . . . . . . . . .. Direct Numerical Simulation. ε ............... Turbulence dissipation [m /s ]. EU . . . . . . . . . . . .. European Union. EXPi. Experimental value. .......... 2. 3. FC . . . . . . . . . . . .. Fibre Channel. fV . . . . . . . . . . . .. xedValue. GAMG . . . . . . .. Generalised Geometric-Algebraic Multi-Grid. GCI . . . . . . . . . . .. Grid convergence index [%]. H. Building height [m]. .............. HAWT . . . . . . . .. Horizontal-Axis Wind Turbine. h ............... Height of the surrounding buildings [m]. HPC . . . . . . . . . .. High Performance Cluster. HR . . . . . . . . . . . .. Hit rate [%]. HRk . . . . . . . . . . .. Hit rate for. k. HRU. ........... Hit rate for. U. iP . . . . . . . . . . . . .. Inlet prole. k ............... Turbulent kinetic energy [m /s ]. κ ............... Von Karman constant [-]. KL . . . . . . . . . . . .. Kato-Launder. KLY . . . . . . . . . .. Kato-Launder-Yap. L............... Characteristic length of the problem [m]. λ............... Tip-speed ratio [-]. [%] [%]. 2. 2.

(33) xxxiii. Nomenclature. LES . . . . . . . . . . .. Large-Eddy Simulation. LU . . . . . . . . . . . .. Lower Upper (factorization). M .............. Millions. MMK . . . . . . . . .. Murakami-Mochida-Kondo. MPI . . . . . . . . . . . Message Passing Interface. µ............... Dynamic viscosity [Pa·s]. NBL . . . . . . . . . .. Nocturnal Boundary Layer. N. Total number of cells. .............. 2. ν ............... Kinematic viscosity [m /s]. νt . . . . . . . . . . . . . .. Kinematic eddy viscosity [m /s]. ω............... Specic rate of dissipation (of. 2. k). 2. 2. [m /s ]. PBiCG . . . . . . . . Preconditioned Bi-Conjugate Gradient. p ............... Convergence rate [-]. P∞ . . . . . . . . . . . .. Free-stream pressure [Pa]. Pk . . . . . . . . . . . . .. Production of. P ............... Pressure eld [Pa]. k. 2. 3. [m /s ]. RAM . . . . . . . . . . Random-Access Memory RANS . . . . . . . . . Reynolds-Averaged Navier-Stokes. RD . . . . . . . . . . . .. Relative maximum admissible deviation from the experimental data. Re . . . . . . . . . . . . .. Reynolds number [-]. u0i u0j. Reynolds stresses [m /s ]. ............ 2. 2. 3. ρ ............... Fluid density [kg/m ]. RNG . . . . . . . . . .. Re-Normalisation Group. r ............... Building roof height [m]. S ............... Modulus of the rate of strain tensor [-]. SATA . . . . . . . . .. Serial AT (Advanced Technology) Attachment.

(34) xxxiv. Nomenclature. SFS . . . . . . . . . . .. Self-certifying File System. σε . . . . . . . . . . . . .. Dissipation Prandtl number [-]. σk . . . . . . . . . . . . .. Kinetic energy Prandtl number [-]. SIMi . . . . . . . . . .. Simulation value. SKE . . . . . . . . . .. Standard. sl . . . . . . . . . . . . .. Slip (boundary condition). k−ε. sP . . . . . . . . . . . . . Symmetry plane SST . . . . . . . . . . .. Shear Stress Transport. STL . . . . . . . . . . . Stereolithography. 4. Source of. TD. Turbulence velocity time scale adopted for the Durbin tur-. ............. ε. 2. Sε . . . . . . . . . . . . .. [m /s ]. bulence model [s]. TI.............. Turbulence intensity [-]. T ............... Turbulence velocity time scale [s]. TKE . . . . . . . . . .. Turbulent kinetic energy [m /s ]. U ............... Streamwise velocity [m/s]. U∗ . . . . . . . . . . . . .. Frictional velocity [m/s]. U∞ . . . . . . . . . . . .. Free-stream velocity (reference velocity) [m/s]. URANS . . . . . . .. Unsteady Reynolds-Averaged Navier-Stokes. Uref. Reference velocity [m/s]. ............ U/Uref. 2. 2. ......... Speed-up ratio [-]. Ω............... Vorticity scale [-]. VAWT . . . . . . . .. Vertical-Axis Wind Turbine. wF . . . . . . . . . . . .. Wall function. W. Innite solar array width for validation [m]. .............. XR . . . . . . . . . . . .. Recirculation (or reattachment) distance on the roof [-]. yn . . . . . . . . . . . . .. Normal distance to the nearest wall [m].

(35) xxxv. Nomenclature. z ............... Independent variable measuring the height above ground [m]. z0. .............. Roughness height [m]. zG . . . . . . . . . . . .. zeroGradient. zref . . . . . . . . . . . .. Reference height [m].

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(37) Chapter 1. Introduction Discovery is seeing what everybody else has seen, and thinking what nobody else has thought. Albert Szent-Györgyi. 1.1. Scientic and technological elds of study. The technological eld of study of the present Thesis is the wind energy generation in urban environments.. Specically, the improvement of the. building-roof shape with a focus on the wind energy resource exploitation. Since the wind ow around buildings is exhaustively investigated using numerical simulation tools, both computational uid dynamics (CFD) and building aerodynamics are the scientic elds of study of this Thesis.. 1.2. Objectives and methodology. The main objective of this Thesis is to obtain an improved (or optimum) shape of a high-rise building for the wind energy exploitation on the roof, and the analysis of the behaviour of this shape in an urban environment. To achieve this nal objective, an analysis of the inuence of the building shape on the behaviour of the wind ow on the roof from the point of view of the wind energy exploitation is carried out using numerical simulation tools (CFD). Figure 1.1 shows the methodology followed for obtaining and analysing the optimum shape, and the deliverables (journal articles) derived from the milestones reached. Initially, the conventional building shape (prismatic) is analysed, and the adequate positions for dierent kinds of wind turbines are proposed.. A curved roof is also simulated with validation purposes.. The 1.

(38) 2. Chapter 1. Introduction. compatibility of both photovoltaic-solar and wind energies is also analysed for a conventional at roof. This compatibility is analysed for both a windtunnel reduced scale model and a full-scale building, and some scaling issues are observed and reported.. Figure 1.1: Diagram of the methodology (milestones) followed for obtaining and analysing an optimum building-roof shape for the urban wind energy exploitation. Deliverables: article 1 is Toja-Silva et al. (2015d), article 2 is Toja-Silva et al. (2015b), article 3 is Toja-Silva et al. (2015c) and article 4 is Toja-Silva et al. (2015a). The methodology for obtaining the optimum high-rise building roof shape continue with the analysis of a full-scale building by carrying out the following stages: - Verication of the results of previous building-roof shapes studied at the literature. The basic shapes that are compared are: at, pitched, shed, vaulted and spheric..

(39) 1.3. Why empirical?. 3. - Analysis of the inuence of the roof-edge shape on the wind ow. This task is carried out by comparing the results obtained for the conventional edge shape (simple corner) with a railing, a cantilever and a curved edge. - Analysis of the roof-wall coupling by testing dierent variations of a spherical roof on a high-rise building: spherical roof studied at the literature, spherical roof geometrically integrated with the walls (squared-plant) and spherical roof with a cylindrical wall. The ow behaviour on the roof according to the variation of the incident wind direction is commented. - Analysis of the eect of the building aspect ratio on the ow. The behaviour of the optimum shape previously obtained for an isolated building is analysed in an urban environment, by investigating the eect of the surrounding buildings (with dierent heights) on the wind ow on the target building roof.. 1.3. Why empirical?. The title of the thesis is Urban wind energy: empirical optimization of highrise building roof shape for the wind energy exploitation. The question here is: why the optimization must be empirical? Optimal design methods involving the solution of an adjoint system of equations (adjoint method) are often used in Computational Fluid Dynamics (CFD), particularly for aeronautical applications (Giles and Pierce (2000)). The most common use of the adjoint method is shape optimization problems related to the design of airfoils, wings, compressor-turbine blades, etc. The objective of the application of such method is lift maximization, drag minimization, control of ow separation, etc. (Kampolis et al. (2015)) In the present case, the objectives of the optimization are speed-up maximization and the turbulence intensity minimization. Both objectives are not related to the building surface but to the surrounding uid eld. This is one of the reasons why the adjoint method is not used. Additionally, there are many subjective restrictions to the optimum building shape (aesthetics, feasibility, habitability, etc.). Therefore, neither the adjoint method nor other deterministic mathematical optimization method known today (Thévenin and Janiga (2008)) can be used in the present case. The optimization must be based on empirical sensitivity analyses.. This empirical optimization is. carried out by means of numerical experimentation with accurately validated CFD tools.. 1.4. Research scope. Urban wind energy is a very large eld of study, and it is necessary to dene a limited scope for the investigations. Therefore, the constraints due.

(40) 4. Chapter 1. Introduction. to the assumptions done have to be considered. In the following, the global assumptions (concerning to the general outline) are commented, whereas the specic assumptions for each task are explained in their respective Chapter. The most important constraint is that this investigation focuses on the wind ow around buildings, excluding the consideration of the wind turbine eect on the ow. Only the available wind resource is obtained, and both amount and quality of the wind resource are analysed. The wind ow around the building may change due to the presence of the wind turbine but, nevertheless, the advantages of a building-roof shape with respect to the others should not be altered. This is, the advantages of a roof shape with respect to another will remain after considering the wind turbine. Note that the objective of this Thesis is to identify the best building-roof shape for the wind energy exploitation, but not to bring an energy production value because, among other things, it will depend on the dimensions of the real building and on the wind resource in the actual site.. It is recommended. for further investigations the simulation of wind turbines on the optimum roof obtained, in order to analyse the eect of the wind turbine on the ow. Due to the complex geometry involved in such kind of projects, actuator disk or actuator line wind turbine models can be used. Additionally, a new immersed boundary method has been developed in a parallel project within the framework of this Thesis. This new method has been implemented into a Direct Numerical Simulation (DNS) code and, in the medium term, it can be implemented into a LES or RANS code for dealing with complex geometries and/or moving boundaries associated to exible wind turbine blades. This immersed boundary method is presented in Annex A. The wind turbines positioning on the roof is recommended according to the European Wind Turbine Standards II (Pierik et al. (1999)). The atmospheric wind turbulence is one of the main eects causing fatigue damage on wind turbine components (Mouzakis et al. (1999)), and Pierik et al. (1999) stat that when the turbulence intensity (T I ) exceeds 15% the fatigue loads on the conventional wind turbines (HAWT) have to be re-evaluated based on the actual conditions at the site. Therefore, it is assumed in this Thesis that a new concept of Horizontal Axis Wind Turbine (HAWT) or a Vertical Axis Wind Turbine (VAWT) should be used in this situation (T I. > 0.15).. The. terminology in this Thesis for referring to these wind turbines is VAWT, although it may include new designs and conceptual HAWT specially designed for high turbulence environments. Another very important assumption is a neutrally stratied atmospheric boundary layer (explained in detail in Chapter 2). Other states of the atmospheric boundary layer befall, but this state is the most frequent when dealing with high wind phenomena. This is, the wind does not use to be high when the atmospheric boundary layer is not neutrally stratied and vice versa.. Unstable atmospheric conditions are associated with thermal.

(41) 1.5. Justication of this investigation. 5. processes (such natural convection) that do not take place with high wind conditions. Therefore, it is convenient to consider a neutrally stratied atmospheric boundary layer from the wind energy exploitation point of view. Additionally, this is the assumption on which almost all wind tunnel testing and most of the CFD simulations in computational wind engineering rely (Blocken (2014b)). The turbulence modelling and the computational settings are validated using wind-tunnel experimental data.. The Reynolds number obtained in. wind-tunnel experiments is two orders of magnitude lower than the obtained in full scale buildings. It implies that the similitude constraints are not adequately satised. Actually, scaling issues are reported and analysed in Chapter 3. However, the research community accept such validations because the large amount of the CFD simulations are validated using wind-tunnel experimental data.. Nevertheless, full-scale experimental measurements are. recommended as further works in order to conrm the results of the present investigations. Two-equation steady-state RANS turbulence modelling is used to perform the CFD simulations. This is deeply discussed and justied in Chapter 2. The main reason is that DNS of the wind ow around a real-scaled building geometry is unapproachable, and Large-Eddy Simulation (LES) presents an agreement with experimental data better than RANS but its computational cost is very high for real-scaled geometries, especially in the case of the wind ow around buildings (Franke et al. (2007)). Regarding the directional sensitivity, the at roof building (with and without solar panels) is analysed considering 8 dierent incident wind directions, although most of the attention is focused on the normal-wall direction. The state-of-the art roof shapes and the edges analyses are carried out only considering an incident wind direction normal to the main plane, according to the most advantageous position.. This is because it is an intermediate. task. However, the eect of dierent incident wind directions in the most interesting shapes is commented.. The optimum building-roof shape has. the same behaviour for all the incident wind directions and, therefore, it is not necessary to analyse them. The surrounding buildings (for the optimal building-roof shape analysis) can show dierent results for dierent wind directions. Since a general pattern is used, the most disadvantageous position is used for these buildings.. 1.5. Justication of this investigation. The HORIZON2020 (European Commission (2015)) in Future Smart Cities aims to have 20% of electricity produced by renewable sources. This goal implies the necessity to enhance the wind energy generation, both with large.

(42) 6. Chapter 1. Introduction. and small wind turbines. Wind energy drastically reduces carbon emissions and avoids geo-political risks associated with supply and infrastructure constraints, as well as energy dependence from other regions. Large wind turbines are very ecient when they are installed in big wind farms but small wind turbines in the urban environment are still unused, what supposes a waste of an important energy resource (Walker (2011)). Additionally, distributed energy generation (generation at the consumption site) oers signicant benets in terms of high energy eciency, lower emission of pollutants, reduced energy dependence and stimulation of the economy (Chicco and Mancarella (2009)). The main reasons of the waste of the urban wind energy resource are the lack of research works related to the resource availability assessment and to a certain lack of societal acceptance. The COST Action TU1304 (WINERCOST (2015)) has as a principal objective to collect the existing expertise on Building-Integrated-Wind Energy Technology and to investigate eective adoption methods for enabling the concept of Smart Future City. The dissemination is focused in particular on the societal acceptance. One of the most signicant aspects of this lack of societal acceptance is due to the customers disappointment regarding the difference between the expected and the real energy generated. This dierence occurs because the performance of the wind turbines are calculated under ideal conditions in at terrain, conditions very dierent than the real conditions in the urban environment. Another important aspect for the social acceptance is the visual impact, avoided by considering the wind turbines during the architectural design of the buildings. The buildings sector represents 40% of the European Union (EU) total energy consumption.. Reducing energy consumption in this area is there-. fore a priority under the 20-20-20 objectives on energy eciency (European Commission (2008)).. The Directive 2010/31/EU of the European. Parliament and of the Council of 19 May 2010 on the energy performance of buildings (European Union (2010)) contributes to achieving this aim by proposing guiding principles for Member States regarding the energy performance of buildings. It implies the setting of minimum requirements for the design of new buildings.. They shall comply with these requirements. and undergo a feasibility study before construction starts, looking at the installation of renewable energy supply systems and other sustainable systems (European Union (2010)). To the best of the author's knowledge, the only precedent of an exhaustive analysis of the most appropriate building shape for the wind energy exploitation on building roofs is the work of Abohela (2012), who studied simple geometric roof shapes: at, domed, gabled, pyramidal, barrel vaulted and wedged.. Therefore, there is a lack of knowledge about the optimum. building shape. One of the aims of this Thesis is to extend studies like the one mentioned above, and to study more complex shapes..

(43) 1.6. A review of urban wind energy. 7. The buildings aspect ratio is studied in the literature mainly for pollutant dispersion (Tong and Leung (2012)) and thermal performance (Inanici and Demirbilek (2000); Memon et al. (2010)) purposes. There are some illustrative estimations of the wind resource (wind energy maps) in real cities such Barcelona (Ajuntament de Barcelona (2015)). These maps can bring an estimation of the resource, but they do not bring feasible wind resource data for a specic building roof region.. Therefore, an additional analysis. of the target building must be carried out considering the surroundings. Some authors have analysed a target building considering the surroundings for natural ventilation problems. These studies included specic surrounding buildings for a particular case (van Hoo and Blocken (2010b,a)) and a generic pattern of surrounding buildings (Ramponi et al. (2015)), as in the present case. In the present Thesis, an investigation of the both building aspect ratio and surrounding buildings eects for the optimum shape obtained is presented.. 1.6. A review of urban wind energy. The largest amount of the wind energy power growth comes from at-terrain installations. However, the urban environment has a great potential for wind power that has not been harnessed (Walker (2011)). In urban areas, there is a multiplication factor of the wind speed because of the presence of buildings, but the turbulence intensity and the multidirectionality severely increase, which is an aspect that requires special attention (Grautho (19901991); Ledo et al. (2011); Lu and Ip (2009); Bahaj et al. (2007)).. Additionally,. these installations increase the protability of the external surfaces, i.e., the roof and the walls, which currently serve only to enclose the building. Another advantage of exploiting wind energy in urban environments is its proximity to the consumption points (distributed electric power generation) that entails signicant benets in terms of high energy eciency, lower emissions (of pollutants), reduced energy dependence and stimulation of the economy (Chicco and Mancarella (2009)). The optimization of distributed generation involves voltage prole improvements, the reduction of electric energy ow in power lines (with the associated reduction of energy losses in power lines and electric devices), and the increase of the energy source availability (El-Ela et al. (2010)). The reduction of greenhouse gas emissions is another signicant factor (Ackermann et al. (2001)). Water consumption, a limited resource in some regions, is reduced with the exploitation of wind power.. Table 1.1 shows a comparison of water. consumption associated with various energy technologies. Despite the great positive impact of wind power, this energy source has disadvantages.. One of the shortcomings is the visual impact.. However,.

(44) 8. Chapter 1. Introduction. Technology. Liters/kWh. Nuclear. 2.30. Coal. 1.90. Oil. 1.60. Combined cycle gas. 0.95. Wind. 0.004. Solar. 0.110. Table 1.1: Water consumption of various energy technologies (Saidur et al. (2011)).. urban buildings and their auxiliary facilities (e.g., chimneys and aerials) share the visual impact with the wind turbines, minimising it. Additionally, the wind generators can be architecturally integrated. Noise emissions, both audible and infrasound, are a signicant environmental factor to consider. Most of the noise pollution comes from conversion and generation machinery, although the blades of horizontal-axis wind turbines (HAWT (Horizontal-Axis Wind Turbine)) also cause noise when they interact with the tower structure, especially with leeward working conditions (Grautho (19901991)). At high wind velocities, the noise due to the forced circulation of the wind around the building and its associated facilities is higher than the noise generated by the wind turbine (Pantazopoulou (2009)), gure 1.2.. Figure 1.2: Example of sound levels vs. wind speed for small wind turbines (Pantazopoulou (2009))..

(45) 1.6. A review of urban wind energy. 9. Wind-powered generators also generate infrasound (with frequencies above 16 Hz) and low frequency vibrations that can be transmitted to the building structure.. These vibrations can be tolerated by industrial buildings, but. they can cause problems in residential buildings (Grautho (19901991)). This aspect highly varies depending on both the generator and the building characteristics, and it must be analysed case by case. The impact of wind turbines on birds is very important in at-terrain wind facilities (Saidur et al. (2011); Bansal et al. (2002)), but its repercussions are smaller in urban environments because of other anthropogenic factors that have a greater impact. Wind power can also aect TV and radio reception (Saidur et al. (2011); Bansal et al. (2002); Dabis and Chignell (1999)). This is due to the periodic modulation of the electromagnetic elds by means of reection, absorption and dispersion by the blades (Saidur et al. (2011)). In urban environments, this impact is lower because of the building dimensions, which are usually larger than wind turbines. Any mobile or stationary structure generates interference with electromagnetic signals (Bansal et al. (2002)). However, both the low power and size of urban wind turbines lessen this impact because the intensity of the interference has a direct relationship with the obstacle size (Dabis and Chignell (1999)). Following Grautho (19901991), mechanical safety is also a fundamental aspect to consider. For each wind turbine, an analysis of the resistance to fatigue of both the structural (including the building structure) and mobile components (especially the blades) must be conducted.. The detachment. of a blade (or a part of it) can cause a very serious accident because of the substantial momentum. However, the probability of a blade breaking, striking a person and causing injuries (independent of the distance it covers) is extremely low, and the author did not advise establishing a safety perimeter around the wind turbine outside of the facility. Grautho (1990 1991) advises establishing a safety perimeter only in the case of hazardous industries.. 1.6.1 A basic concept of the wind ow on building roofs As a prelude of the more detailed review of building aerodynamics done in Chapter 2 (Section 2.4), a qualitative 2D simulation of the wind circulation around a vertical section of a long-short building is conducted using the computational uid dynamics software Ansys Fluent-Workbench (using URANS (Unsteady Reynolds-Averaged Navier-Stokes)), in order to comment the inuence of the multidirectional urban wind on the dierent types of wind turbines. A at roof is chosen since, according to Ledo et al. (2011), the power density available in a at roof is the highest of the most common roof types (pitched, pyramidal, etc.). A concentration factor of the wind is.