**1. Introduction**

To fulfill global energy needs, manufacturers and most of the wind turbine industry have concentrated their efforts on large utility-scale machines [1]. The standard design for horizontal-axis turbines consists of a three-blade, upwind rotor featuring an active yaw and pitch regulation. Such machines benefit from large levels of aerodynamic optimization, often using purposely developed airfoils featuring twisted and tapered blades and large resources for development and testing. On the other hand, small wind turbines (SWTs) often do not feature the same level of optimization, with low power coefficients often resulting from unoptimized designs [2]. Such sub-optimal aerodynamic designs have been identified amongst the issues that hamper the diffusion and economic feasibility of SWTs [3,4], with larger SWTs suffering the most from the often used simplistic approaches [4]. This type of turbine, which marked the dawn of wind energy, is still used in a variety of applications, from rural areas to off-grid applications [5]; notwithstanding this, their high levelized cost of energy [6] has thus far hampered an effective diffusion. On the other hand, interest has been rising lately again, as testified by the creation of a dedicated technical committee for SWTs by the European Academy of Wind Energy (EAWE) [7]; this is mainly due to the role that distributed production, even with small rated power, could have in the transition towards smart energy systems [8]. In doing so, the "old generation" of turbines seems unsuitable in terms of efficiency and flexibility, and so better designs are about to be explored.

**Citation:** Papi, F.; Nocentini, A.; Ferrara, G.; Bianchini, A. On the Use of Modern Engineering Codes for Designing a Small Wind Turbine: An Annotated Case Study. *Energies* **2021**, *14*, 1013. https://doi.org/10.3390/ en14041013

Academic Editor: David Wood Received: 13 January 2021 Accepted: 10 February 2021 Published: 15 February 2021

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The present article aimed to analyze the main issues causing low power output in an SWT while also detailing how an effective preliminary design can be achieved by using and properly integrating current industry best-practices and open-source tools. In particular, even though the latter are indeed familiar to the wind energy community, their conscious use is not trivial, and organic design guidelines are often not available. Effectively and economically designing an SWT is not a trivial task, and many hurdles must be overcome in terms of aerodynamics, materials, structural resistance, and economics.

While a good overview of these issues can be found in [4], the aim of the present study was twofold. On the one hand, it aspired to provide the reader with an organic overview of the steps that need to be followed for a first turbine design, suggesting how to integrate existing engineering open-source tools and how to tune them, especially in cases of the realistic turbulent inflow conditions that are required by standards (summarized in the chart in Appendix A). This, while not completely novel from a scientific point of view, is thought to be of industrial relevance and significance for newcomers. Guidelines and general indications on blade design can be in fact found in the available literature [9,10]. For instance, various aspects of aerodynamic design and optimization were discussed in [11]. Such studies, however, do not account for control or dynamic inflow conditions [12]. The study instead specifically focused on the implication of control for small wind turbines. In particular, it is shown that using modern control strategies, which have rarely been applied to SWTs, can lead to much more efficient design and more convenient loading. The importance of making an early decision regarding control in the design phase was assessed, as this aspect significantly influences aerodynamic design, and controller tuning and optimization should go hand in hand with aerodynamic optimization. Most small wind turbines indeed use a stall as their main power-limiting strategy. This involves controlling the rotor speed so that, as the wind speed increases, the turbine gradually enters the stall, the lift decreases, and the drag increases, thus effectively regulating the power output. Fixed-speed stall-controlled turbines were the de-facto standard in the nineties [2], and successful applications of this design can be found [13]; however, most stallregulated turbines, including commercially available products, now feature variable speed generators [14]. By adopting variable-speed control, a turbine is able to operate at or near the design tip–speed ratio (TSR) at a low wind speed, greatly improving energy capture. Even when adopting variable speed control, however, significant compromises must be made in order to ensure good stall regulation, from setting the blades to a manual fixed pitch angle to varying the twist and chord distributions of the blade. Such compromises can be avoided if pitch regulation is employed. Two kinds of pitch regulation strategies are possible: pitch-to-stall and pitch-to-feather. As noted in [15], the pitch-to-stall strategy is able to provide effective regulation, though it increases most design loads. Moreover, given that the pitch-to-feather strategy is the most widely adopted control method in modern utility-scale turbines, this kind of pitch regulation is discussed in this paper. While the benefits of this control strategy are apparent, it does not come without drawbacks, mainly connected to its added complexity and, especially, cost. However, examples of manufacturers proposing this kind of solution can be found, as is the case with the line of products by Tozzi Nord [16]. For all of these reasons combined, two SWT designs are compared in this paper—a variable-speed stall-regulated turbine and a variable-speed, pitch-regulated turbine.

The selected testcase for the entire analysis was a 50 kW machine with a 200 m2 swept rotor area, which is in line with the definition of an SWT according to International Electrotechnical Commission (IEC) 61400-2 [12]. The authors indeed had a direct industrial experience with this size of machine, and this experience mainly drove the present study. Due to the industrial non-disclosure agreement with the partner, however, all analyses were repeated on a purely theoretical case study. Notwithstanding this, the results are fully representative of those found in reality.

## **2. Methods**

In this section, the numerical tools used throughout the study are briefly presented. The methods to determine a preliminary chord and twist distribution are explained. Then, the airfoils considered in the design process are discussed, and the modifications done to the ideal design in order to meet the desired targets in terms of power output are analyzed in detail.
