Aerogeneratori eolici ad asse verticale: analisi numerica, verifica sperimentale e messa a punto di un campo di prova per prototipi full-scale

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1 !! Doctoral School on Engineering Sciences! Università Politecnica delle Marche Extended summary Aerogeneratori eolici ad asse verticale: analisi numerica, verifica sperimentale e messa a punto di un campo di prova per prototipi Curriculum: Energy Author Roberto Romagnoli Tutor(s) Ch.mo Prof. Renato Ricci February 12 th 2013 Abstract. This work is focused on drag driven vertical axis wind turbines. The study involved both experimental measurements and numerical analyses. The experiments have been performed in a closed circuit wind tunnel. They investigate the configurations that could improve the performance of a given rotor. The numerical analyses were conducted to improve the flow field knowledge and to design new rotor geometries. Both approaches started from a classic Savonius rotor equipped with semi-circular straight blades S-shaped and End-Plates to replicate the two-dimensional effects of the numerical model. The performance were evaluated with C P -λ plots, where C P is the power coefficient and λ is the tip speed ratio. An external test station was build up to perform experimental measurements on wind turbines models. It is composed by a test station for prototypes, a 10 meters meteorological mast and a Sodar-Rass station. The external station is operational and an urban lamppost prototype supplied by wind energy and is currently the subject of experimental studies. The results of this research will be published in the scientific journal, therefore only a first part of them will be reported in this paper. Keywords. Savonius, Vertical axis wind turbine, Computational fluid-dynamics, Wind tunnel testing, Sodar 1

2 Introduction The work was focused on the investigations of drag driven wind turbines of Savonius type [1]. The choice to use a Savonius wind rotor derives from architectonic and wind availability needs: the rotor has a vertical long shape that allows to integrate this device inside urban environments. Besides Savonius rotor is a drag wind turbine able to start at low velocities and it is substantially not sensitive with respect to the wind direction; these two abilities well integrate with the typical wind resources available inside urban areas (low velocities and high wind direction irregularities) and so the lower power coefficient, with respect to a lift wind turbine, goes on a background layer. The Savonius rotor is vertical axis wind turbine having semi-circular straight blades. The rotor movement is due to the torque deriving from the drag unbalance between a concave advancing blade and a convex returning one (Fig. 1). The approach has involved simultaneously both the experimental evidence and the numerical analyses, in order to exploit the strength of each methodology. The reference model for experimental and numerical analyses is a Savonius rotor with end plates: those are circular plates fixed at the bottom and the top of the rotor to obtain a confined flow field. This allows the comparison of the experimental results to the numerical computational data from a 2D domain. The two methods are complementary: CFD analyses allow us to gain an insight into the rotor fluid dynamics that lead to energy production. The experimental measurements allow us to evaluate the elements that are difficult to put inside a numerical model. For this reason the two approaches have different aims: the experimental tests want to determine the configurations and elements able to improve the energy production of a given rotor. The numerical analyses are used to design new rotors shapes. Fig.1 Scheme of Savonius rotor Meanwhile, a set-up for external tests of prototypes in 1:1 scale has been realised: with this tests site is possible to perform measurements in real applications. Two kinds of Doctoral School on Engineering Sciences 2

3 meteorological data acquiring systems were used to characterise the site. The first one is a meteorological mast equipped with traditional sensors such as a cup anemometer and a wind vane. The second is a remote-sensing system, namely a Sodar - Rass system. The test site is currently working and holds a street lamp prototype powered with wind and solar energy; the system is equipped with a rotor of the studied ones. 1. Experimental tests The experimental measurements were carried out in the environmental wind tunnel at the Università Politecnica delle Marche. This facility is a single return closed circuit tunnel [1] having overall dimensions of 29 [m] x 9 [m]; The test section is 1.8 [m] high x 1.8 [m] large and 9 [m] long. It has three subsections dedicated to different measurements types. The former is immediately downstream the contraction cone and is used for aeronautical tests involving a uniform velocity distribution and low turbulence values. The second locates in the middle and is employed for interference measurements among elongated bodies. The latter and more relevant is at the end of the test section and is dedicated to environmental studies. The Savonius rotor model is 1 [m] high and has an external diameter of 0.6 [m]. The structural body is formed by modular elements axially connected; each element can be displaced with respect the previous one, on an azimuthal direction, in order to obtain a vertical twist from the base up to the top (geometric twist). It was realised a geometry having both aerodynamic and geometric twist too. This geometry has been made in two and three blades version. The tested rotor was placed in the first test subsection of the wind tunnel in order to ensure incoming wind profiles having uniform velocities and low turbulence values. The tests were performed to investigate the effects of several geometric parameters, aerodynamic appendages and structural elements, as (see also Fig. 2): - Aerodynamic Parameters: 1. Reynolds number Re = V D/υ, where D = rotor diameter; υ = air cinematic viscosity; V = air speed. So, as the rotor and the fluid do not change, the Re is determined by air speed only. - Geometric Parameters 1. the presence of a gap between blades roots and rotor shaft. (i.e. blades root extend beyond the rotor shaft so to form a flow duct that allows a flow exchange between the advancing blade and the returning one); 2. the rotor twist angle; 3. the numbers of blades (2 or 3); - Aerodynamic Appendages 1. circular end plates fixed at the bottom and the top of the rotor to obtain a confined flow field; Doctoral School on Engineering Sciences 3

4 1. a rotating conveyor applied on the rotor lateral side in correspondence with the advancing blade; 2. a rotating deflector; - Structural Elements 1. tubular posts needed to ensure a structural stiffness; 2. intermediate ribs. They are small screens fixed at various levels on the concave side of the blades; they reduce the vertical component of the air speed on the twisting blades; 3. external metal mesh used to protect the rotor blades. A list of the test performed (which does not include, however, the experiments with the metallic mesh), can be found in Table 1. The tests are named with the following codes: S0deg = Savonius rotor with straight blades; S90deg = Savonius rotor with geometrical twist of 90 degrees; S105deg = Savonius rotor with geometrical twist of 90 degrees; BS0deg = Savonius rotor with straight blades (a second version); TdV = 3 blades rotor with aerodynamical and geometrical twist; BdV = 2 blades rotor with aerodynamical and geometrical twist; BdVc = 2 blades rotor with aerodynamical and geometrical twist without internal gap; n/s = no/yes S = End plates; C = Conveyor; D = Deflector; M = Post; CE = Intermediate ribs; The mechanical torque is measured by means of a braking disk, a lever arm and a load cell; the braking disk is fixed to the rotor shaft while the brake caliper is fixed to the lever arm that transfer the mechanical torque to an axial load cell. The measurement procedure contemplates tests at different braking torque for a fixed velocity and several velocity runs are carried out for each wind rotor assembly. In this way are obtained torque and power curves as function of rotating velocity and wind velocity. Tests were also performed with rotor blocked to measure the static torque in various angular positions on a whole revolution. Doctoral School on Engineering Sciences 4

5 tests perfmoed 2 blades bi-twisted 3 blades bi-twisted straight 90 geom. twist 105 geom. twist BdV_nS_nCE TdV_nS_nCE S0deg_sS_nC_nD_4M S90deg_nS_nC_nD_nM S105deg_nS_nC_nD BdV_nS_sCE TdV_nS_sCE S0deg_nS_nC_nD_nM S90deg_sS_nC_nD_nM S105deg_nS_nC_sD BdV_sS_nCE TdV_sS_nCE S0deg_sS_nC_nD_nM S90deg_sS_nC_nD_sM3 S105deg_nS_sC_nD BdV_sS_sCE TdV_sS_sCE S0deg_sS_nC_nD_sM3 S90deg_sS_nC_nD_sM4 S105deg_nS_sC_sD BdVc_nS_nCE S0deg_sS_nC_nD_sM4 S90deg_sS_nC_sD_nM S105degc_sS_nC_nD BdVc_sS_nCE S0deg_sS_nC_sD_nM S90deg_sS_nC_sD_sM4 S105deg_sS_nC_nD_nM BdVc_sS_sCE S0deg_sS_nC_sD_sM4 S90deg_sS_sC_nD_nM S105deg_sS_nC_nD_sM4 S0deg_sS_sC_nD_nM S90deg_sS_sC_nD_sM4 S105deg_sS_nC_sD_nM S0deg_sS_sC_nD_sM4 S90deg_sS_sC_sD_nM S105deg_sS_sC_nD_nM S0deg_sS_sC_sD_nM S90deg_sS_sC_sD_sM4 S105deg_sS_sC_sD_nM S0deg_sS_sC_sD_sM4 S105deg_sS_sC_sD_sM4 BS0deg_nS_nC_nD_nM S105deg_sS_sC_sD_sM4s Table 1 Rotor combinations tested in the wind gallery. Fig. 2 Scheme and image of the main external elements. 2. Numerical model Previous numerical analyses have been performed on Savonius rotor aerodynamic using both DVM (Discrete Vortex Method) and CFD methods. A crucial issue in the analysis of the flow field around the Savonius rotor is the treatment of the fluid-solid coupling and its modelling. The model developed in DIISM department of University of Ancona is a twoway coupling type [2]; the structure (rotor) was treated as a rigid body while the fluid was Doctoral School on Engineering Sciences 5

6 modelled as incompressible and viscous. To take into account the motion of the solid body in the solution of the fluid-dynamics equations is used a SMM (Sliding Mesh Model) approach while Navier-Stokes equations were solved using the finite volume code Fluent. The computational 2D domain is divided in a fluid portion and in the solid portion: this one is a rotating frame around a fixed axis normal to the represented plane. The solid body motion was treated solving the second cardinal equation of dynamics by means of a custom MatLab numerical algorithm able to import CFD data, calculate the rotor angular velocity and export this variable as input to the CFD code (see scheme at Fig. 3). Time marching of the solution of the second cardinal equation has been executed using an Euler method in the initial steps and a 4-stage Runge-Kutta scheme in the following steps. Fig. 3 Flow chart of fluid-rigid body coupling algorithm. The RANS equations, closed using the v 2 -f turbulence model, swere solved in order to obtain accurate information about the flow field. In the numerical simulations the inflow wind velocity has been fixed to 9 m/s and the turbulent intensity at the inflow section was fixed to 0.2 % as in the experimental set-up. 1. Performance evaluation Wind rotor, as other operational machines, can be characterised with maps of Power or Momentum Vs Rotational speed. This is used to represent the single machine, but it is not useful when you want to compare different rotors with each other. In order to evaluate the effect of the various parameters and to extrapolate rotor performances at velocities different from the tested ones, a dimensionless representation of the acquired data was performed. These kind of maps represent relative performance and are independent of the speed of the wind (then from Reynolds number); so much so that the tests at various speeds are concentrated in a single curve. The dimensionless parameter C P, C m and λ are defined in Eq. (1); the first is the power coefficient (i.e. the ratio between the obtained mechanical power and the available wind Doctoral School on Engineering Sciences 6

7 power), the second is the torque coefficient and the latter the tip speed ratio (i.e. the ratio between the blades tip and the incoming wind velocities). P C P = ; C 1 m = 2 ρv 3 S m M 1 2 ρv ; λ = ω R 2 RS V (1) m In Eq. (1) P = power [W]; M = momentum [Nm]; ρ = air density [kg/m 3 ]; V = air speed [m/s]; S m = rotor front section [m 2 ]; R = rotor radius [m]; ω = rotational speed [rad/s]. In numerical simulations, the rotor mean performance at the operating point, was evaluated making a one-loop average of the main parameters (C P and C m ) once the asymptotic behaviour was reached. 2. First experimental results The angular trends of the static torque coefficient showed that: the torque distribution is highly irregular (sinusoidal trend); the number of peaks is equal to the number of blades; twisted rotors and 3-blades rotors have more regular behaviour (less amplitude of the sinusoidal trend). In Fig. 4 is shown as example the static tests result for the twisted rotor BdVc (without internal gap) performed at three different air speed. Fig. 4 Static momentum coefficient of the rotor BdVc for experimental tests at different wind speed. Fig. 5 is an example of the dynamic tests: it shows the effect of the end plates on the regression curves obtained from the experimental data (rotor BdV). The end plates Doctoral School on Engineering Sciences 7

8 increase the C P : they are useful to confine the flow field inside the rotor allowing to elaborate more fluid and to reduce the trailing edge vortexes. Fig. 5 The positive effect of the end plates on the Power coefficient. Regression curves obtained from the experimental data. 3. First numerical results Numerical and experimental data are reported in Fig. 6. Aerodynamic performance, obtained by the numerical simulations, shows a very good agreement with experimental data. This allows to use the computational model for a fluid-dynamic analysis of a Savonius wind rotor. Fig. 6 Comparison between numerical and experimental curves: CP-λ on the left plot, Cm-λ on the right plot. The fluid dynamic analyses have shown, in spite of the rather simple conception of the rotor, a particularly complex flow field. In Fig. 7 is represented the flow field near Savonius Doctoral School on Engineering Sciences 8

9 PhD Thesis of Roberto Romagnoli rotor blades at λ = Is visible the presence of an overlap jet, that increase the maximum CP power coefficient of about 20%. This jet starts from the concave side of the advancing blade and develops toward the returning blade. The increment of the pressure on the impinging side reduces the negative contribution of the returning blade to the overall torque. It was found also that these rotors are not purely drag driven: in some conditions it is possible to overcome the λ = 1 limit. This is mainly due to the curvature of the convex part when the blade is beginning to receive the incoming wind. For the definition of new geometries were used different approaches that led to useful insights to understand the fluid nature of this particular problem. In particular, were developed: a. Variations of the classic Savonius (adding internal gap); b. Rotors aimed to the increase the lift zone; c. Rotor aimed to the reduce the resistant zone. Best results in therms of maximum CP, equal to 0.29, were found in the first (a) and latter (c) types. Fig. 7 Flow pattern near rotor blades (Experimental value: λ = 0.735). 4. The external test station The external test site was realised inside the Engineering Faculty properties in Montedago, Ancona. The selected area is on the hill behind the Sciences buildings and it can be seen in Fig. 8. Doctoral School on Engineering Sciences 9

10 PhD Thesis of Roberto Romagnoli Fig. 8 The site for external tests and position of the facilities. The facilities are a test site for external models and a meteorological mast to record environmental variables. The mast is a 10 m lattice tower of aluminium, triangle based with a side of 30 cm provided of the main sensors for meteorological data measurement. It is located on this small plateau on the highest part of the hill of Montedago (175 m a.s.l.), near the southern border of the Faculty property. The test site is located along the road behind the south side of the Science buildings, at an altitude of about 160 m a.s.l. This area is located just below the top of the hill that houses the meteorological tower. The selected area is subject to constraint landscaped, so it was necessary to carry out considerable paperwork before making the facilities. The area represents a typical urban site where the energy systems tested may actually be used. It was also set up a remote-sensing measurement station with Sodar-Rass sensors on the terrace of the DIISM department: this system is very different from the traditional systems mounted on the meteorological mast. These latter are a three cup anemometer and a wind vane: they are direct devices that examine a small area and are the standard in wind resource assessment studies. The Sodar-Rass, as many other remote-sensing instruments, are statistical and volumetric devices. The use of these two systems for monitoring environmental data allowed us to make a wind analysis of the site, characterised by local breezes most of the year and the prevailing direction of the wind from 330. It was possible to compare the two systems in short-term and long-term measurement campaigns. In the first case, i.e. one-day measurement, is extremely difficult to find an good Doctoral School on Engineering Sciences 10

11 PhD Thesis of Roberto Romagnoli match between the wind speeds recorded by the two systems. In this case it is better to use Sodar system to evaluate local phenomena, such as orographic accelerations, wind shear and so on. In long-term campaigns, statistical evaluations obtained from both systems, such the prevailing winds and directions, are very close each other. Fig. 9 The meteorological mast and the concrete foundation of the test site. 5. Conclusion and remarks The present paper is mainly a description of the activities carried within the Doctoral School of Engineering. The work involved both experimental and numerical investigations on vertical axis, drag driven wind rotors. These kind of rotors are very interesting because they are particularly suitable for energy systems using wind power inside urban areas. From the experimental tests were individuated the elements that may improve or decrease the rotor performances. Some of them have a relevant role on structural design, such as the external posts or the central shaft. From the numerical analyses were defined new rotor geometries in order to increase the power coefficient. A set-up for external tests of prototypes in 1:1 scale has been realised: with this tests site is possible to perform measurements in real applications. From meteorological devices it was possible to make a wind analysis of the site, characterised by local breezes most of the year and the prevailing direction of the wind from 330. The test site is currently used to study a first prototype of an urban lamppost supplied by wind energy. The main results has not been reported because they will be published in the scientific journals. Doctoral School on Engineering Sciences 11

12 References [1] S. J. Savonius. The wing rotor in theory and practice. Savonius Co., Finland, [2] D Alessandro V., Montelpare S., Ricci R., Secchiaroli A. Unsteady Aerodynamics of a Savonius wind rotor: a new computational approach for the simulation of energy performance, Energy (2010), doi: /j.energy Doctoral School on Engineering Sciences 12