• Energy Research

Abstract Obtaining electrical energy from water streams with velocities below one meter per second using hydrokinetic turbines presents an important challenge as it is significantly lower than the range of operations (higher than two meters per second) offered by units currently commercialized. This research goes one step further than the state-of-the-art studies regarding the degree to which energy extraction is influenced by the water flow blockage caused by turbines in water flows; presenting that blockage as a key factor in the procurement of electrical energy using hydrokinetic micro turbines from low-velocity flowing water. Primarily, it describes the design and characterization of a small vertical-axis cross-flow hydrokinetic turbine whose dimensions maximize the blockage of the water circulating in a channel at very low velocity. For the turbine characterization, a model has been tested experimentally in a water tunnel with different flow rates and complemented by simulations under the same conditions using an adjusted computational fluid dynamics model. The experimental tests contemplate the measuring of the turbine’s electrical power, hydraulic coefficient and rotational speeds. The simulations of the numerical model facilitate the procurement of the corresponding water pressure and velocity fields. Further analysis has allowed for a closer examination of the different type of forces exerted on the faces of the blades as well as their position. Upon even closer inspection there appear to be a variety of possible scenarios contingent on the turbine load.

Smart cities will have a strong impact on the future of renewable energies as terms like sustainability and energy saving will be more common. In this sense, both of wind and hydrokinetic compact-size turbines, can play an important role in urban communities by providing energy to nearby consumption points in an environmentally suitable way. This work presents the experimental evaluation for a vertical-axis turbine Darrieus type, operating in an open-field wind tunnel and a confined water channel. Power and characteristic curves have been obtained for all test conditions, also the effect of turbine blockage has been evaluated under blockage values ranging from 6.8% to 35%. The peak power coefficient for the confined flow condition reached a value of 0.31 which is 1.5 times higher than the peak one for the experimental open field condition at the same Reynolds number and a blockage of 20%. Finally, two blockage correction equations have been applied to the water channel tests, which gave values quite similar to the results obtained from the wind tunnel.

La quantification du rayonnement solaire incident sur une surface est une tâche complexe qui nécessite la connaissance des caractéristiques géométriques, géographiques, astronomiques, physiques et météorologiques de l'emplacement. Le but de cet article est d'analyser les processus d'atténuation du rayonnement solaire et de passer en revue les travaux scientifiques dans ce domaine, en particulier les modèles analytiques pour le calcul de l'irradiance solaire, ainsi que d'établir une méthode alternative pour calculer l'ampleur de la transmittance atmosphérique globale. Des modèles analytiques ont été développés depuis 1940 et leur précision et leur complexité se sont améliorées. Jusqu'à présent, le modèle Bird & Hulstrom est le plus complet et le plus précis de tous. Le principal inconvénient de ce modèle est qu'un grand nombre d'équations et de paramètres tels que la température, les heures d'ensoleillement, l'humidité, etc. sont nécessaires. Dans cet article, une nouvelle méthode très rapide et précise est développée pour quantifier les irradiations solaires sur n'importe quel site. L'analyse montre que les paramètres requis ne sont que le type de climat, l'altitude et l'état de l'atmosphère. Cette méthode permet également de quantifier l'influence du degré de turbidité dans les irradiations directes et diffuses. Ces informations sont essentielles pour sélectionner les technologies solaires qui conviennent à chaque endroit. En tant qu'application, la nouvelle méthode a été mise en œuvre et caractérisée au Mexique. L'énergie solaire est une ressource abondante au Mexique, et il existe des études sur le potentiel de l'énergie solaire dans ce pays, mais l'influence des facteurs physiques et météorologiques sur le rayonnement solaire n'a pas été liée. Dans cette étude, les informations météorologiques de 74 stations météorologiques situées dans différents climats du pays ont été utilisées pour déterminer les paramètres requis. Les résultats ont été validés avec des données expérimentales disponibles pour différents sites. Cuantificar la radiación solar incidente sobre una superficie es una tarea compleja que requiere el conocimiento de las características geométricas, geográficas, astronómicas, físicas y meteorológicas de la ubicación. El objetivo de este trabajo es analizar los procesos de atenuación de la radiación solar y revisar los trabajos científicos en este campo, específicamente los modelos analíticos para el cálculo de la irradiancia solar, así como establecer un método alternativo para calcular la magnitud de la transmitancia atmosférica global. Los modelos analíticos se han desarrollado desde 1940 y han ido mejorando en precisión y complejidad. Hasta ahora, el modelo Bird & Hulstrom es el más completo y preciso de todos. La principal desventaja de este modelo es que se requiere un gran número de ecuaciones y parámetros como temperatura, horas de sol, humedad, etc. En este documento, se desarrolla un nuevo método muy rápido y preciso para cuantificar las irradiancias solares en cualquier sitio. El análisis muestra que los parámetros requeridos son solo el tipo de clima, la altitud y el estado de la atmósfera. Este método también permite cuantificar la influencia del grado de turbidez tanto en irradiaciones directas como difusas. Esa información es esencial para seleccionar qué tecnologías solares son adecuadas en cada lugar. Como aplicación, el nuevo método ha sido implementado y caracterizado en México. La energía solar es un recurso abundante en México, y existen algunos estudios sobre el potencial de energía solar en ese país, pero no se ha relacionado la influencia de factores físicos y meteorológicos en la radiación solar. En este estudio se utilizó la información meteorológica de 74 estaciones meteorológicas ubicadas en diferentes climas del país para determinar los parámetros requeridos. Los resultados se han validado con datos experimentales disponibles para diferentes ubicaciones. Quantifying incident solar radiation on a surface is a complex task that requires the knowledge of geometric, geographical, astronomical, physical and meteorological characteristics of the location. The aim of this paper is to analyze the attenuation processes of the solar radiation and to review the scientific works in this field, specifically the analytical models for solar irradiance calculation, as well as to establish an alternative method to compute the magnitude of the overall atmospheric transmittance. Analytical models have been developed since 1940 and they have been improving in precision and complexity. Up until now, the Bird & Hulstrom model is the most complete and accurate of them all. The main disadvantage of this model is that a great number of equations and parameters such as temperature, sunshine hours, humidity, etc. are required. In this paper, a very fast and accurate new method is developed to quantify solar irradiances at any site. The analysis shows that the parameters required are only the type of climate, altitude and state of the atmosphere. This method also allows to quantify the influence of the turbidity degree in both direct and diffuse irradiances. That information is essential to select which solar technologies are suitable in each place. As an application, the new method has been implemented and characterized in Mexico. Solar energy is an abundant resource in Mexico, and there are some studies about the solar energy potential in that country, but the influence of physical and meteorological factors on the solar radiation have not been related. In this study, the meteorological information of 74 weather stations located in different climates of the country were used to determine the parameters required. The results have been validated with experimental data available for different locations. يعد القياس الكمي للإشعاع الشمسي الساقط على السطح مهمة معقدة تتطلب معرفة الخصائص الهندسية والجغرافية والفلكية والفيزيائية والأرصاد الجوية للموقع. الهدف من هذه الورقة هو تحليل عمليات توهين الإشعاع الشمسي ومراجعة الأعمال العلمية في هذا المجال، وتحديدًا النماذج التحليلية لحساب الإشعاع الشمسي، وكذلك إنشاء طريقة بديلة لحساب حجم النفاذية الجوية الإجمالية. تم تطوير النماذج التحليلية منذ عام 1940 وتحسنت من حيث الدقة والتعقيد. حتى الآن، يعد نموذج Bird & Hulstrom الأكثر اكتمالًا ودقة من بينها جميعًا. العيب الرئيسي لهذا النموذج هو أن هناك حاجة إلى عدد كبير من المعادلات والمعلمات مثل درجة الحرارة وساعات أشعة الشمس والرطوبة وما إلى ذلك. في هذه الورقة، تم تطوير طريقة جديدة سريعة ودقيقة للغاية لقياس الإشعاع الشمسي في أي موقع. يوضح التحليل أن المعلمات المطلوبة هي فقط نوع المناخ والارتفاع وحالة الغلاف الجوي. تسمح هذه الطريقة أيضًا بتحديد تأثير درجة التعكر في كل من الإشعاعات المباشرة والمنتشرة. هذه المعلومات ضرورية لاختيار تقنيات الطاقة الشمسية المناسبة في كل مكان. كتطبيق، تم تنفيذ الطريقة الجديدة وتميزها في المكسيك. الطاقة الشمسية مورد وفير في المكسيك، وهناك بعض الدراسات حول إمكانات الطاقة الشمسية في ذلك البلد، لكن تأثير العوامل الفيزيائية والأرصاد الجوية على الإشعاع الشمسي لم يكن مرتبطًا. في هذه الدراسة، تم استخدام معلومات الأرصاد الجوية لـ 74 محطة أرصاد جوية تقع في مناخات مختلفة من البلاد لتحديد المعلمات المطلوبة. تم التحقق من صحة النتائج من خلال البيانات التجريبية المتاحة لمواقع مختلفة.

Abstract Tidal energy has significant potential yet to be developed. One of the key areas of the research lines being developed is the ability to perform accurate computational models in order to make a full appraisal of kinetic energy potential in different areas through tide movements. So far, due to the limitations of computational resources and the extension of the geographical domains, existing models have manly used simplifications such as low resolution mesh sizes and velocity profile approximations in the vertical dimension. The investigation put forward includes a high resolution two dimensional equivalent model of the zone studied using a longitudinal section with depth and length dimensions, as well as a simulation methodology to study the velocity field in different sections. The model can be used in cases of channels of high width compared to the depth. With such data, kinetic power and energy of tidal currents can be predicted. Both, model and methodology have been applied and validated to the specific case of the estuary of Aviles port, thereby obtaining a new perspective in the hydrodynamic study as well as a tidal current energy potential assessment.

The growing interest on low-carbon energy systems has been essential to develop new electricity devices based on renewable resources. In this context, channel turbines are an excellent alternative to supply demands that are isolated from power mains. These devices, which harness the kinetic term of the water current, can be installed in hydraulic channels, rivers, or estuaries. This article carries out an experimental characterization of a hydrokinetic turbine in a hydrodynamic water tunnel under low velocity conditions, so its behavior under open field conditions can be obtained. The results show the relevance of the blockage effect made by the turbine during the power stage. Keywords: hydrokinetic turbines, blockage, water channels, low velocity

Tides can be a vast and predictable source of renewable energy. Due to the solar and lunar influx on our planet, they move large amounts of water periodically, and this energy can be harnessed using devices designed and positioned adequately, such as current turbines. However, the relation between the energy obtained with actual devices and the economic and environmental cost of their installation limits the practical application of these solutions. In order to optimize the design of this technology and achieve its successful installation and use, a detailed knowledge about the energy potential of tides at the specific location is necessary. This calculation is not easy and requires the use of specialized software tools. Currently, there is no specific software to evaluate the tidal currents energy potential, but there are more than a few codes able to calculate the hydraulic flow in rivers, estuaries and coastal regions. These programs are usually used for the calculation of pollutant dispersion and floods, but they can be adapted with more or less success. This paper reviews the available 1D, 2D, and 3D software tools with the aim of analyzing their functionality and their validity to evaluate the energy potential of tidal currents.

Abstract This paper sets out to explore and promote the use of a hydrokinetic microturbine module designed to obtain energy from water streams. It is made up of a hydrokinetic rotor, a permanent magnet generator, power electronic stages and a microcontroller to regulate power production. It is further complemented by a control strategy focused on maximizing the electrical power produced when varying different water velocities. This method is based on three operating zones defined by the electrical frequency produced by the generator, and where different values of direct current set points are defined. It is set in motion in a boost converter power stage, using a maximum current control, with constant off time in the power switch, and continuous or discontinuous mode of operation depending on the current value required. The control strategy has been validated though the use of dynamic simulations. The microturbine modules are suitable for alignment in rows to increase the power produced. The modules connected in smart grids define the hydrokinetic smart grid concept. A global control strategy for hydrokinetic smart grids has also been proposed and tested.

The enormous inroads made by renewable energy in recent years have been the key to the development of new technologies designed to obtain energy from a range of resources. Hydrokinetic microturbines used to harness kinetic energy from rivers, tidal and marine currents epitomize such developments. As the reservoir is dispensed with, the water footprint normally associated with conventional hydroelectric generation is minimized. The new prototypes being developed require laboratories with water tunnel infrastructures where they can be accurately reproduced under controlled conditions. However, the construction of a water tunnel demands considerable investment, which prevents many research groups from completing their prototype design work. This paper charts the design of a low-cost hydrodynamic water tunnel at the University of Oviedo, indicating the mechanical and electronic elements as well as the software developments that make up the facility. This construction is a part of a research strategy focused on making the study of new hydrokinetic microturbines designs economically feasible. Moreover, it includes a description of a special software application used to perform the characterization of a hydrokinetic microturbine model in the water tunnel and a demonstration of the scope of the facility in the experimental study of a unit with a Darrieus rotor.