## 1 Introduction  
Mounting wind turbines in a built environment is estimated to have huge potential not only to satisfy the energy demands of the urban environment and provide decentralized generation but also to help tackle fuel poverty and to achieve the reduction in emissions [1]. Over the past decades, the significant progress made in the conversion of wind energy to electrical power is a result of considerable engineering research and development of wind machines with the emphasis on improved design, aerodynamics, structural and systems characteristics [2]. Research has shown that the mounting of wind turbines on urban buildings is affected by positioning (height above the roof ridge) and position relative to the prevailing wind direction [3]. The influence of the buildings on the wind turbines distinguished between the wind energy conversion in rural and urban areas [4]. However, Wind resource in the urban environment is complicated where the resource is proportionate to the surface topography, temperature influences and the dynamic nature of the urban environment [5]. Because of the high roughness length of the environment and the presence of obstacles characterized by different shapes and porousness along the flow stream path, the wind profile in urban areas is entirely not quite the same as the classical log-law based profile with the zero-speed elevation shifted up to a peculiar value (displacement, \(d\)) which is a function of the average height of the neighboring buildings (Figure 1) [6]. The wind speed increases logarithmically with height. The natural wind profile extrapolates to zero wind speed at zero displacement height (\(d\)) when above vegetation or buildings as shown in Equation (1). \[\overline{u}(\mathrm{z})=\ \frac{U_{*}}{k}\ \ln\left(\frac{z-d}{z_{o}}\right) \tag{1}\]  
where \(U\)- is the friction velocity, \(k\) is the von [PERSON]'s constant, \(z_{o}\) is the roughness length, \(d\) is the displacement height and \(\overline{u}(\mathrm{z})\) is the average wind speed at height \(z\) above ground [3]. The wind profile in the internal boundary layer in an urban environment is shown in Figure 1. Other factors includes the orientation of the buildings to each other, the rooftop layout, street canyons, and trees [7]. Wind turbines on rooftops operate at about 20% higher wind speed than the undisturbed wind [4]. The slope of the roof has an effect on the wind speed over the buildings [8]. The significant improvement in the operating efficiencies of wind power systems makes them more economically competitive to other energy generation techniques [9]. At present, two main categories of wind turbines have been used for electricity generation based on the axis about which the wind turbines rotate; the horizontal axis wind turbines (HAWTs) and the vertical axis wind turbines (VAWTs) [10; 11]. Both turbines have their merits and demerits. The HAWT can achieve higher energy efficiencies and is capable to self-start at low wind speed but it only operates under one wind direction and thus requires a yaw mechanism to direct the turbine into the wind. It also has a high cost of maintenance and repairs due to the tower structure and the placement of the transmission and electrical generation equipment at the hub [9]. Furthermore, it is dangerous to surrounding birds and has a high noise level so it is not suitable for the urban environment [12]. Owing to these disadvantages, the VAWT has attracted, much attention recently, in comparison to the HAWT. The VAWT is omni-directional and hence averts the need for a yawing mechanism. The transmission and electrical generation equipment can be located at the ground level, making the installation, operation and maintenance much easier and suitable for the urban environment [12]. Although Darrieus wind turbine was previously known for its difficulty in self-start [13; 14; 15], recent studies have shown that there are certain conditions that can make it otherwise.