New trends e.g. in architecture and urban planning are to reduce energy needs. Several technologies are employed to achieve this, and one of the technologies, not new as such, is wind energy. Wind turbines are installed in cities, both by companies and private persons on both old and new buildings. However, an overview of the energy content of the wind in cities and how consequently turbines shall be designed for such wind climates is lacking. The objective of the present work is to deliver an objective and fundamental overview of the social, practical and physical conditions relevant for the installation of wind turbines in cities, with Copenhagen, DK, as example. Focus is taken on turbine with a swept area of maximum 5m2, since turbines of this size are relatively easy to be integrated in the urban space and are in the financial range for small companies as well as for private persons. Elements important for the implementation of wind energy conversion systems are the macro and micro wind climate, the siting within a micro wind climate and the choice of a wind turbine model most appropriate for the selected site. In the frame of this work, all these important elements are analyzed and a row of conclusions are found. Not as a surprise, it can be concluded, that the average wind velocities and with that the wind energy available in a city is somewhat lower than at a rural site outside the city. Furthermore, the wind climate in cities is very dependent on the buildings character and higher turbulences are expected. Therefore, methods are developed in this work to estimate the wind energy in cities depending on the built-up character, with both, simple and advanced calculation models. The energy produced by a wind turbine is also dependent on the turbine design, but especially on the rotor design. An analysis of which rotor type is most effective is carried out. A result of this analysis is, that turbines for the installation in cities need to be designed very differently than e.g. turbines for off-shore sites and that an optimization of the design can increase the energy production remarkably. Despite the lower wind energy in cities other factors foster the attractiveness of urban wind energy application, like the demand or wish to reduce CO2 emissions and the possibility to produce energy directly to ones household.
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Danmarks Tekniske Universitet, Risø Nationallaboratoriet for Bæredygtig Energi, 2011