Urban wind energy - Development of methods for the assessment of small wind turbines in urban areas
Guaranteeing a sustainable robust and safe urban energy supply that does not depend on energy production from rural regions, requires the optimal exploitation of existing energetic resources in cities. Alongside photovoltaic systems, small wind turbines (SWT) represent one of the few feasible ways to generate electric energy in urban areas with a high building density and in urban areas, thus contributing to the EU building directive with the claim for "nearly zero energy" buildings. Due to high turbulent wind conditions especially in urban areas, small wind turbines face additional challenges – especially when it comes to installations on and near by buildings. The resulting complexity hinders the selection and evaluation of a potential site as well as the selection of a suitable small wind turbine. Furthermore, the interaction with its immediate surroundings holds several uncertainties in terms of planning and security of small wind turbines.
Contents and Objectives
An interdisciplinary team of researchers lead by the UAS Technikum Wien addressed the following topics among others in the research project "urban wind energy" carried out in the energy research park Lichtenegg:
- How can strongly turbulent wind flows be characterized in complex urban areas? Which measurements and modelling approaches are suited for resource assessment for a small wind turbine at a specific urban site, taking the energy yield potential into account?
- How does a strongly turbulent wind flow affect the energy yield of a SWT as well as the quality of the supplied electric power. How is the lifetime of a SWT affected by high levels of turbulence?
- Which amount of stress does vibration of SWT cause for the ambient infrastructure? What potential hazards does vibration cause for people in the surroundings of the turbine?
- Which type of SWT is suited for applications in the built environment?
- How should a site assessment for SWT be performed in the built environment?
In order to answer these questions and to evaluate the usage of SWT in the urban environment, the following assessments were performed:
- Development and validation of suitable methods for a characterization of urban sites and turbulent wind flow (e.g. wind measurement, CFD simulation, ...) as well as the assessment of the related influences on the performance (energy yield, lifetime, ...) of SWTs.
- Investigation of several SWTs on rural and urban sites in the course of an impact analysis and implementation of metrological registrations of selected and relevant operating parameters (power, yield, vibration, noise, ...)
- Analysis of planning and safety aspects (e.g. investigation of risk potential) with regard to the application of SWT in urban areas, including an experimental investigation of ice throw and ice fall as well as probabilistic safety assessment to evaluate the risk of fire
Results, conclusion and prospect
The following results of the project represent significant fundamentals for the technical assessment of small wind turbines operating in urban areas:
- The analysis of the operation behavior revealed that no SWT investigated in the project had a technical availability greater than 95 % due to maintenance service, disruption, failures and other incidents. Two out of three SWTs had fatal failures with high risk potential before the official end of the project.
- The studies have shown that the wind resource over the roof of a building may vary a lot with increased turbulence. Besides that, obstructions and buildings in environment of a SWT, especially in the prevailing wind direction, influence the site substantially.
- CFD models are well suited to estimate and assess these interferences. During the investigation on the scientific problem, the use of complex LES models has not shown significant advantages over more simple stationary models.
- All power curves measured according to the standard in the course of the project deviated from the specifications of the manufacturer and were not able to fully reach the specified power output.
- During the operation of SWTs mass imbalances, aerodynamic imbalances, electro-mechanic interactions and gusts lead to vibration and oscillation of the plant. The results show that eigenmodes of the entire system (mast and rotor), which are excited by the rotor at specific rotational speeds are the main cause for alternated vibration and oscillation.
- Probabilistic safety assessments revealed that falling parts and thrown ice fragments are a safety risk of SWTs in urban areas that should not be underestimated. It is important to reduce this risk with appropriate safety measures. In contrast, fire constitutes a significant lower risk potential.
Technikum Wien GmbH, Institut für Erneuerbare Energie