Heure d'été | EPFL Graph Search

vignette|Printemps : passer de l'heure d'hiver à l'heure d'été – l'horloge est avancée d'une heure (le dernier dimanche de mars à , pour l'Europe). vignette|Automne : passer de l'heure d'été à l'heure d'hiver – l'horloge est retardée d'une heure (le dernier dimanche d'octobre à , pour l'Europe). L’heure d'été est un système consistant à ajuster l'heure locale officielle, en ajoutant une heure à l'heure normale (appellée alors « heure d'hiver ») pour une période allant généralement du début du printemps au milieu de l'automne, ce qui a pour effet de retarder l'heure à laquelle le Soleil se lève et se couche. L'heure d'été est principalement utilisée dans les régions tempérées, où les variations saisonnières de luminosité rendent cette mesure pertinente. Pour l'Europe, la période s'étend du dernier dimanche de mars au dernier dimanche d'octobre. Pour l'Amérique du Nord, depuis 2007, cette période estivale débute dans la nuit du deuxième dimanche de mars, tandis que le retour à l'heure

A contribution to energy saving in induction motors

Electric motors consume over half of the electrical energy produced by power stations, almost the three-quarters of the electrical consumption in industry and almost the half of commercial electrical consumption in developed countries. Motors are by far the most important type of electric charges, and so constitute the main targets to achieve energy saving. Owing to their simple and robust construction, the asynchronous motors and especially those of squirrel-cage types, represent about 90-95% of the electrical energy consumption of electric motors, which is equivalent to about 53% of total electrical energy consumption. They are widely used as electrical drives in industrial, commercial, public service, traction and domestic applications. Owing to the importance of induction motors, this thesis is aimed at contributing to energy saving efforts, more specifically in the field of low power induction motors. A contribution is kept in perspective by taking into consideration the energy saving potential during the motor design stage as well as during its operation. Every effort to save energy in motor application can be made by always attempting to use energy only as much as what needed during its operation. The best way is to exploit the saving potential during motor design, while at the same time taking into account its intended application. It can be achieved either through the improvement of motor design or through the reduction of its input electrical energy when the motor has already been built. These two efforts are studied, elaborated and worked out thoroughly in this thesis. To attain this objective, a synthesis has been started with the description of how to model an induction motor. To obtain a better model, an improvement is proposed by using the Schwarz-Christoffel mapping to calculate the slot leakage inductance in induction motor. With such method, slot-leakage inductance can be determined more precisely, resulting in more accurate prediction of motor characteristics. It is based on the stored magnetic energy calculation using two-directional field distribution in the slot. The air gap influence can be observed easily, so that a reasonable slot leakage definition can be adopted. Unlike the conventional method, which is only suitable for rectangular slots (otherwise empirical corrections are required), the proposed general slot form can be extended to any desired polygonal slot form. Consideration of saturation is also indispensable because ignoring it could result in inaccuracy in motor performance prediction. Considering the saturation is essential owing to its important role in self-excitation phenomenon to establish voltage build-up in induction generator. However, the self-excitation phenomenon is undesirable in certain group of capacitor motors as it may hinder the switching-off process and mechanical braking at a desired moment. The undesirable switching-off failure condition is to be avoided by properly designing the capacitor motor. Like in this capacitor motor special application, where a proper design is useful from the point of view of operation safety, designing properly a motor is also very important in energy saving efforts. Motor design and optimization to minimize losses as well as to make possible wide speed-range motor operation are some of the efforts. However, when induction motor has already been built, saving energy is only possible by managing its supplying electrical energy. Various strategies are possible and a particular emphasis on the use of triac to reduce motor input voltage is presented. Besides, a brief economic saving evaluation is given to draw attention to the energy saving potential.

EPFL2005

Density currents driven by differential cooling in lakes: occurrence, dynamics and implications for littoral-pelagic exchange

Tomy Doda

Cross-shore flows exchange water laterally in lakes, with ecological implications for the ecosystem. One example is the convective circulation induced by differential cooling, also known as the thermal siphon. This lateral flow forms when the sloping sides of lakes experience surface cooling. Shallow areas cool faster, which generates lateral density gradients and drives a cold downslope gravity current and an onshore surface flow. The role of this two-layer-circulation in lateral exchange in lakes is poorly understood, due to the lack of long-term and high-resolution measurements of thermal siphons. This thesis aims at filling this gap by investigating the occurrence and dynamics of thermal siphons from extensive in situ observations in a wind-sheltered Swiss lake (Rotsee).We first quantified the seasonality of thermal siphons from one-year-long observations in Rotsee. Thermal siphons occurred on a daily basis from late summer to winter and flushed the littoral region in ~ 10 h. Their duration increased but their intensity decreased over the same period. We linked this seasonality to the change in forcing conditions by testing scaling relationships from theoretical and laboratory-based studies.We then focused on the short-term temporal variability of the transport by quantifying the dynamics of thermal siphons over a diurnal cycle and their interaction with convective plumes. Our results reveal that convective plumes penetrated into the gravity current at night and eroded its upper interface. This vertical mixing generated vertical interface fluctuations and reduced the lateral transport at night. The maximal transport was delayed to daylight conditions when radiative heating weakened penetrative convection.After having quantified the physical water transport, we assessed the role of thermal siphons in the lateral exchange of dissolved gases. We found that both branches of the circulation were capable of transporting gases laterally. The downslope current brought littoral gases to the base of the mixed layer in the stratified region whereas the surface flow transported gases towards the shore. We quantified this exchange for oxygen and methane.Finally, we generalized our observations to other lakes with different bathymetry and forcing conditions. The frequent occurrence of thermal siphons observed in six lakes confirmed that this lateral transport process is ubiquitous in lakes with shallow littoral regions. We showed that our results from Rotsee were applicable to other systems, where the same scaling relationships predicted the formation and intensity of thermal siphons. Based on these results, we proposed a procedure to predict the contribution of thermal siphons for lateral transport in any lake.This thesis provides a comprehensive understanding of the formation and dynamics of thermal siphons and paves the route for integrating this lateral transport process in lake ecosystem research.

EPFL2022

Solar potential in early neighborhood design

Émilie Nault

In light of the acknowledged need for a transition toward sustainable cities, neighborhoods and buildings, urban planners, architects and engineers have to comply with evermore demanding energy regulations. These decision-makers must be supported early-on in their process by adequate methods and tools. Indeed, early-design decisions, which concern parameters linked to the building form and urban layout, strongly dictate the solar exposure levels of buildings, in turn influencing their energy need (e.g. for heating and cooling) and production potential (e.g. through on-site active solar systems). Despite the spread of existing digital tools, limitations remain, withholding their integration into the early design process. These considerations lay down the context within which this doctoral research was carried out. The main objective of this thesis is the development of a performance-based workflow to support decision-making in early-design neighborhood projects. The performance is here defined through three criteria: (i) the daylight potential, quantified by the spatial daylight autonomy, (ii) the passive solar potential, quantified by the annual energy need for space heating and cooling, and (iii) the active solar potential, quantified by the annual energy production. The research process consisted of two main phases. First, the development of a performance assessment engine allowing real-time evaluation of an ensemble of buildings. Second, the integration of this method into a decision-support workflow, taking the form of a digital prototype that was tested among practitioners. For the first phase, a metamodeling approach was adopted to circumvent the limitations associated to simulations involving solving physics-based equations. Mathematical functions were obtained to predict the daylight and energy performance of a neighborhood, from a series of geometry- and irradiation-based parameters, easily computable at the early-design phase. To derive these functions (or metamodels), a neighborhood modeling and simulation procedure was executed to acquire a dataset of reference cases, from which the metamodels were trained and tested. The resulting multiple-linear regression functions, combined to an algorithm for quantifying the active solar potential from the irradiation data, formed our performance assessment engine. To assess its usability and relevance, the workflow was implemented as a prototype, supported by existing 3D modeling and scripting tools. Inspired by the emerging performance-driven and non-linear design paradigms, a multi-variant approach was adopted for this implementation; from the space of possible designs defined by a small set of user-inputs, a series of neighborhood variants are generated through a random sampling algorithm. Results of their evaluation by the core engine are displayed to allow a comparative assessment of the variants in terms of their morphology and solar potential. Having been tested among practitioners during workshops, the prototype appears promising for providing design decision-support. Direct feedback gathered from participants support the relevance of the approach and reveals multiple avenues for further improvement. Results collected during the workshops also allowed probing the validity boundaries of the metamodels: the prediction accuracy achieved attests the potential of the approach as an alternative to more complex methods, less adequate for exploring early-phase design alternatives.

EPFL2016