Feasibility analysis of a Darrieus vertical-axis wind turbine installation in the rooftop of a building Francesco Balduzzi, Alessandro Bianchini, Ennio Antonio Carnevale, Lorenzo Ferrari ⇑ , Sandro Magnani ‘‘Sergio Stecco’’ Department of Energy Engineering, University of Florence, Via di Santa Marta 3, 50139 Florence, Italy article info Article history: Available online 28 December 2011 Keywords: Darrieus VAWT Urban context Skewed flow Microeolic turbine abstract The renewed interest that is being paid by architects, project developers and local governments to small- size wind turbines is mainly connected to the attractive prospects of future applications in the urban environment; the delocalized power production of these systems could indeed provide an effective answer to both the growing demand for renewable energy and the increased attention in buildings with a sustainable and low-energy design. In particular, Darrieus vertical-axis wind turbines (VAWTs) are being considered as one of the most attractive solutions due to their low visual impact, the reduced acoustic emissions and their better response to a turbulent and skewed oncoming flow. The feasibility of this scenario has, however, to be proved yet; in particular, doubts are still connected to the real pro- ducibility in a complex terrain like the urban one and to the compatibility of microeolic machines with a densely populated area. On these assumptions, the aim of this work is to critically evaluate the energetic suitability of a Darrieus VAWT installation in the rooftop of a building in a reference European city. With this goal in mind, a numerical CFD analysis was carried out to characterize the flow field in the rooftop area of buildings with different shapes and geometrical proportions: the flow velocity modulus and direc- tion were calculated for different oncoming wind profiles and the results were projected into a net avail- able wind distribution in the rooftop of each building. As a second step, in order to provide a reliable estimation of the real functioning of the turbine in the investigated environment, a specific numerical model has been developed to account for the effects of a skewed flow on the power performance of the Darrieus rotor. The results of these analyses were finally combined and synthesized in an energy- oriented study to evaluate the feasibility of a rooftop installation. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Increasing interest is being paid to understand where small-size wind turbines can effectively be exploited to provide delocalized power in the built environment [1]. The main prospects of a similar installation context can be readily understood; in particular, small rotors positioned at the top of a tall building could theoretically ex- ploit a higher zone of the wind profile with respect to that usually exploitable by means of the only turbine tower. Moreover, the en- ergy could be produced directly where it is needed, with a notable contribution to a sustainable design of new buildings in terms of energy consumption. Due to the high roughness length of the terrain and the pres- ence of obstacles characterized by different shapes and permeabil- ity along the flowpath, the wind conditions in urban locations are, however, very complex and the real adaptability of wind turbines to this environment is not yet tested both in terms of real produc- ibility and of structural compatibility with the buildings. As a re- sult, the wind profile in urban locations is quite different from the classical log-law based profile [1], with the zero-velocity height shifted up to a peculiar value (displacement, d) which is a function of the average height of the surrounding buildings (Fig. 1). Notwithstanding this aspect, some general principles to identify the main requirements of a suitable installation site are provided in technical literature (e.g. [1–4]). In detail, from a theoretical point of view, wind turbines in the urban environment require buildings that are reasonably higher than the average height of the surround- ing constructions, in order to take advantage from the local flow deflection and acceleration [1], but only on condition that peculiar geometric proportions between the buildings are fulfilled [5]. Within this context, Darrieus vertical-axis wind turbines (VAWTs) are increasingly appreciated and often considered as the most promising solution in the built environment, due to their very low noise levels and to their reduced sensitivity to a turbulent oncoming wind [5–7]. In addition, recent studies [6,8] put in evidence that some benefits in terms of power increase can be obtained from a Darrieus functioning under skewed flow, mainly 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.12.008 ⇑ Corresponding author. Tel.: +39 055 4796 570; fax: +39 055 4796 342. E-mail addresses: balduzzi@vega.de.unifi.it (F. Balduzzi), bianchini@vega.de.u nifi.it (A. Bianchini), ennio.carnevale@unifi.it (E.A. Carnevale), ferrari@vega.de.uni fi.it (L. Ferrari), magnani@vega.de.unifi.it (S. Magnani). Applied Energy 97 (2012) 921–929 Contents lists available at SciVerse ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy