Roof mounting site analysis for micro-wind turbines
Introduction
Growing public awareness of the raising level of greenhouse gas emissions has led to significant efforts to adopt renewable energy technologies in suburban environments. Building-integrated micro-wind turbines are potential low-cost renewable energy devices that could be adopted in these environments. Micro-wind turbines are defined as wind turbines with capacity less than 2.5 kW [1] or with rotor diameter less than 1.25 m [2]. Commercial micro-wind turbines are capable of generating power from 0.4 kW to 1.5 kW at 12.5 m/s wind speed [3], [4]. Despite of their obvious potential, micro-wind turbines installations in residential suburban dwellings have not yet been widely accepted due to a number of issues including low wind speeds, high levels of turbulence and relatively high aerodynamic noise levels generated by the turbines. Makkawi et al. [5] who had measured the performance of micro-wind turbine pointed out the need of improving micro-wind turbine energy capture efficiency and ability to adjust to changes in the wind direction.
Wind speed and turbulence intensity in built environments are affected to a large extent by the ground topography which in these environments are influenced by the shape of the buildings; how the buildings are laid out [6] and strongly on the roof profiles [7]. The mounting site for any turbine on a building should therefore be assessed so that the optimum annual amount of energy can be obtained. Achieving this will involve a detailed analysis of factors such as the direction of the mean wind speed, the interaction of the building envelope with the wind flow and the level of turbulence the turbine is exposed to. Siting a turbine where the turbulence intensity is high, for example, could cause early fatigue failure of the turbine blades [8] and placing it in a location with substantial separated zone will make the turbine subject to very low wind speed. Thus, numerical modelling of the wind flow above the building roof is important for the design of residential suburban landscapes. It is expected that more and more houses with integrated wind turbines will be built as sustainability becomes an increasing design driver for new houses in the future [1].
There have been limited previous studies [7], [9], [10] that have addressed the effect of different roof profiles on the wind flow above buildings in suburban built environments. Wind tunnel experiments by Rafailidis [7] demonstrate that the wind flow and turbulence intensity at the roof level are strongly dependant on the roof shape but less on the areal building density. Mertens [9] analyzed flow over a flat roof with a view to developing small wind turbine siting guidelines focusing on the mounting height, the wind speed and the probability distribution of the wind speed above the roof. The development of such guideline will enable houses to be designed with roof profiles that effect lower turbulence intensities, and have reduced separation/recirculation zone size. A study by Heath et al. [10] shows significant differences in the flow field around isolated houses to that around houses in adjacent to other buildings.
In the present work, local wind flow characteristics above three typical suburban roof profiles are considered, namely pitched roof, pyramidal roof and flat roof. These houses represent dwellings in three suburbs in Spain.
Section snippets
Reynolds-averaged Navier–Stokes equations and turbulence model
Several previous studies have used CFD methods to model wind flow around buildings [9], [10], [11]. A number of different techniques have been employed in CFD simulations including: Direct Numerical Simulation (DNS) [12]; Large Eddy Simulation (LES) [13], [14], or the Reynolds-Averaged Navier–Stokes (RANS) method with various turbulence models [15]. The choice is generally made based on the details of the flow to be obtained and the computing resources available. If only quasi-steady data is of
Wind flow above suburban houses
Wind flow above the roofs is complex and cannot be predicted from the wind data or the wind atlas alone. Because of the proximity of the roofs in densely populated suburban residential houses, the flow is highly turbulent and the wind velocity field is very different from the freestream velocity due to the bluff body effects of the buildings and the evolution of separated regions. It is therefore important that local wind characteristics such as the flow pattern, turbulence intensity and wind
Conclusion
Wind flow simulations above houses with different roof profiles have been performed. The simulations looked into the wind flow characteristics in terms of turbulence intensity, wind velocity and wind flow pattern. Based on the computational results, site for micro-wind turbine installation above these roofs has been assessed. It has been found that turbulence intensity strongly depends on the roof profiles as well as the wind direction. The wind above flat roofs has not only lower turbulence
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