*1.1. Small Wind Turbines—Interest and Research*

Nordic Folkecenter for Renewable Energy regularly publishes *The Small Wind Turbine* catalogue, the 8th edition of which brought together 104 companies from 28 countries, a total of 302 models of the rated power below 50 kW [5]. Publications such as the *Small Wind Guidebook* by WINDExchange (supported by the US Department of Energy and National Renewable Energy Association (NREL)) give guidelines about how to estimate whether or not an SWT is fit for a particular location and application and how to choose a proper solution for particular demands and needs [6]. SWTs in urban

applications are one of the key interests of programs like the Intelligent Energy–Europe and Horizon 2020 Energy Efficiency.

Stathopoulos et al. [7], in their review of urban SWT technologies, argue that, although Vertical-Axis Wind Turbines (VAWTs) tend to be quieter and visually pleasant, the Horizontal-Axis Wind Turbines (HAWTs) remain a preferred choice for urban applications. This is because VAWTs remain commercially less cost-efficient than HAWTs. The authors also stress the importance of reliable data on urban aerodynamics the more that these wind conditions tend to be more capricious and characterized by a high level of turbulence intensity due to obstructions. The latter is extensively discussed by Anup et al. [8], who stress the need to conceive particular standards pertaining to SWTs, as those referring to big-scale machines may not reproduce the adverse wind conditions correctly. The authors discuss the influence of stochastic flow phenomena on the power outcome and wind turbine loading, which leads to increased fatigue load and underline the need for the structural analysis of wind turbine rotors by means of numerical codes such as Fatigue, Aerodynamics, Structures and Turbulence (FAST). Mechanical analysis of wind turbine blades is also important from the point of view of their inertia. Pourrajabian et al. [9] optimized the wooden blade geometry using genetic algorithms, in order to maximize rotor efficiency while preserving blade loadings in a safe range and ensure low blade inertia for low cut-in wind speed. The authors concluded that not every timber may be successfully used over a wide spectrum of velocity and identified alder as a preferable choice for wooden SWT blades.

Contemporary computational methods offer the possibility to couple high-order simulation of fluid flow and structural response in the Fluid–Structure Interaction (FSI) approach. A one-way FSI is an operation of checking deformation once the whole fluid flow simulation is executed. In a two-way approach, in each coupling iteration of fluid flow simulation, the deformations of the structure are being calculated and according to it—fluid mesh is changing its shape [10,11]. In either case, the simulation requires a significant computational effort, hence its main interest is in case of large-scale wind turbines (see, e.g., [12]). Lee et al. [13] used a one-way FSI model in their NREL Phase VI [14] small HAWT structural studies. The authors claim that the deformation of the tested rotor blades is mainly due to operating conditions (stall, etc.) and not elevated wind speed. FSI also proved important input and validation data for simpler, Blade Element-Momentum theory-based computations. FSI computations are also crucial in the process of developing completely new SWT designs, such as VAWT with morphing blades by MacPhee and Beyene [15]. The authors claimed that the controllably deformable blades enabled an increase in efficiency by as much as 9.6% with respect to fully rigid ones.

SWTs, studied at Institute of Turbomachinery of Lodz University of Technology, incorporate both experimental [16] and numerical [17] research. The increasing use of new manufacturing technologies in SWT studies and prototype manufacturing [18] makes it essential to ensure rotor blade integrity and safe operation. In the current article, the authors summarize the outcomes of a one-way FSI case study of a Generative Urban Small Turbine (GUST) horizontal-axis SWT prototype (see Chapter 2). The research was performed in order to check the blade behavior and performance in different wind conditions, ranging from normal operation to extreme working and static loads, hoping to see if the resulting deformations (twisting, axial displacement) are a serious threat to blade performance. It is also important to find the blade regions most susceptible to load concentrations and compare them with the material strength parameters (see Table 1). To the knowledge of the authors, this kind of analysis is rarely performed for SWTs in general, and for the unorthodox selected material in particular.


