Why even try?

With increasing popular consciousness of environmental responsibility and sustainable development the energy sector is changing its profile, seemingly coming back to its roots and natural energy sources, as wind. A Windmill, a sail, a wind-powered water pump are all examples of milestones in the human history, traced back to the ancient Persia, Roman Empire and kingdoms of Middle-ages. The relatively simple techniques used in those solutions were mastered throughout the centuries to maximize their performance and adapt them to the needs of a modern society. Today they become a means to decrease pollution and improve the quality of the environment [1]. In 2015 1.3 GW in wind turbine power was installed in Poland, 9.9% of all power installed in wind turbines in the EU in this period – a number surpassed only by Germany [2]. Thus the wind turbines remain the most important Renewable Energy Source in the Polish market, with a share equal to almost 66%. The structure of the Polish wind market concentrates on the multi-megawatt machines condensed in wind parks. In the meantime Polish wind resources [3] seem to address the localized power harvesting, where each energy consumer may become a prosumer the minute they start feeding the national grid with electricity produced by Small Wind Turbines (SWT). Unfortunately, those available on the Polish market rarely address the regional meteorological conditions and are too expensive (per kWh). Thus the interest of the projects conducted currently at the Institute of Turbomachinery (IMP) to develop SWT adapted for the Polish market.

GUST (Generative Urban Small Turbine) is a student project established in 10/2015 at IMP. It combines a handful of talented and hard-working mechanical engineering students from BSc and MSc studies cycle. They were brought together by the idea of working on the up-to-date, real-life problem concerning wind energy, as well as further developing their engineering skills and gain practical experience. The object is to design and manufacture an SWT for the purpose of participation in an International SWT Contest, an international competition grouping teams from the leading European research centres. They race to propose a solution, efficient both energetically and commercially, that may change the future of wind energy. IMP proposes its design as a classic Horizontal-Axis Wind Turbine (HAWT) of diameter 1.6 m and nominal power 350W. Visualisation of the designed prototype is presented in Fig. 1.

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Fig. 1. CAD 3D model of the designed GUST turbine

Aerodynamics – it’s all about the wind

The first element in optimizing the HAWT performance is the rotor. Numerous theories deal with this aspect, of differing complexity and assumptions. Possibly the most commonly used is the Blade Element Momentum theory (BEM). It predicts the turbine power outcome on the basis of the rotor geometry and the flow character. Numerous ready-made programs use this algorithm to calculate the wind turbine characteristics – the team has chosen to work in FAST software, a code developed by the National Wind Technology Center, a division of the US Department of Energy. The blade geometry is based on the NREL S834/S826 aerofoil, which shows a promising aerodynamic performance at the considered Reynolds number (Re = 100’000). The characteristics of an optimized wind turbine rotor are visible in Fig. 2, where the rotor power is plotted as a function of wind speed for different tip-speed ratio (TSR = ωR/V, where ω – rotation velocity, R – radius of wind turbine, V – free-stream velocity). It is noticeable how the power harvested by the wind turbine changes with rotational velocity – increasing ω is interesting only up to some certain value of TSR, in this case equal to 5. Thus the need to perform the aerodynamic studies of the rotor and determine the optimal conditions for its control. Additional set of simulations was made to see the influence of the number of blades and the rotor solidity (amount of wind stream surface covered by the blades). The resultant conclusion was that increasing the number of blades may improve the wind turbine performance, but they have to be thinner. This, on the other hand, could cause them to bend (decreasing performance) or ultimately – break (causing even further decrease in performance). Thus the team decided to stick to the 3-blade design and perform an eventual further optimization in the years to come.

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Fig. 2. Wind turbine performance at different wind speed for various tip-speed ratio

Mechanics – after all, it’s not only about the wind

Even if the wind turbine aerodynamics was perfectly optimised it wouldn’t work if it wasn’t for the mechanical system behind it. The systems consists of the blades which are mounted to the hub in a way that provides secure connection and also facilitates their assembly. The hub is connected the shaft which at the other end makes a part of a generator. Generator is the main device which makes all of the previous worth-doing. It is there, were the kinetic energy of the rotor is converted into electric energy and could be later on transferred to the network. One of the things to be considered in the design process is what the designed parts would be made of. It is important to minimize rotors weight (its moment of inertia to be more precise) in order to increase the rotor’s performance. However, the material has to withstand the external loads, mainly aerodynamic and centrifugal forces exerted on the blades. The feasibility of our design was performed basing on several materials: two types of ABS used for 3D printing, Polyamide 66 and aluminum alloy as a point of reference. Most likely Polyamide 66 will be used for manufacturing of blades for GUST. It is a semi-crystalline polymer with very good mechanical characteristics: it’s hard, has good fatigue strength and wear resistance, suitable for machining and most of all very light. In order to verify the strength of the designed model two approaches were used. Firstly Simple Load Model was used. It is an analytical approach which allows to calculate the stresses regarding to designed conditions. The second is Finite Element Method which is based on computer simulations which are helpful for finding weak points and evaluating strength of an element with a particular geometry. Several simulations were performed in order to verify if chosen materials and rotor geometry withstand loads caused by different wind conditions, an exemplary simulation outcome is visible in Fig. 3.

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Fig. 3. Contour plots showing Von Mises equivalent stress distribution for two rotational speeds:
a) ω = 322 rpm and b) ω = 800 rpm

Controls – because neither aerodynamics nor mechanics is enough

Even the best design would be nothing if it couldn’t be controlled. Broadly understood safety of the proposed SWT design is a crucial part of the construction and a lot of effort was exerted in order to devise a solution that would meet the requirements stated by the organizers of the ISWT Contest 2016. The safety system should:

  • be independent and serve as a superior automatic protection for the turbine from single failures or faults in any component within control system and from faults in the load connection,
  • • prevent the rotational speed and produced power from exceeding design limits for all load cases,
  • • allow the turbine to be manually stopped (remotely from at least 10m distance) in case of emergency and keep the rotor blocked
  • • be integrated within the interface boundaries.

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Fig.4. Flow chart showing the proposed operational procedure for microcontroller

At first the microcontroller is checking whether all the sensors are active and work properly. Simultaneously, the controller would investigate if the manual emergency button is pressed and if there is enough energy stored in order to keep electronics running. If any of the above conditions is not fulfilled the emergency brake will be activated preventing the turbine from starting to spin. This action would be signaled by turning on an adequate LED (for example red) in order to inform the users about the fault. The same would happen in case the maximum rotational speed of 800 RPM is exceeded or voltage monitoring sensor indicate lack of load. The rotor can be unblocked only manually, using for instance remote control system. Additionally, the data acquired by the control system could be used to improve and optimize the design.

Further reading:

[1] P. Leconte i M. Rapin, „Éoliennes,” Techniques de l’Ingénieur.
[2] EWEA, „Wind in power – 2015 European statistics,” 2016.
[3] H. Lorenc, Atlas klimatu Polski, Warszawa: Instytut Meteorologii i gospodarki Wodnej, 2005.