1203_ewea2012presentation

8/13/2019 1203_EWEA2012presentation

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A new vertical axis w ind turbine design, integrated

in the urban environment

Dora IONESCU

University Transilvania of Brasov, [email protected]

Ion VISA

University Transilvania of Brasov, [email protected]

Abstract

The paper presents a new rotor design for asmall power, vertical axis wind turbine andfocuses on the design requirements/issuesraised by the urban environment integration.Considering these we propose a set ofinnovative, nature inspired rotor designconcepts for vertical axis wind turbines,

developed considering specific criteria. Aconceptual design model was developed as afirst step in the virtual prototyping platformworkflow, followed by the CAD model andmechanism analysis. Finally the paperdiscusses aspects related to the optimumimplementation location of the turbine withrespect to an urban implementation site.

Keywords: vertical axis wind turbines, rotordesign, urban integration, small power windturbines.

1. Introduction

The integration of small power wind turbinesinto the built environment is one of the biggestchallenges when considering renewableenergies in cities, a space that is more familiarto solar PVs and collectors. According to theAWEA (2010) [1], the market for small windenergy applications will continue to showrobust growth for the next decade.

In the built environment, due to thearchitectural objects, the air flow (wind) hasparticular properties and low wind speeds areexpected, thus aesthetics and the turbinedesign has to fulfil specific requirements.

The urban wind regime has two maincharacteristics [8]:

1) Lower Annual Mean Wind Speed (AMWS)- compared to rural areas - caused by the"rough uneven ground" created by buildings,street furniture and the other features from theurban landscape. The effect is the fact that the

wind speed increases more slowly with height,above the ground.

2) In urban areas the wind profiles tend to bemore turbulent and not along a single axis. Theturbulent flow results from the interactionbetween wind and the buildings or other urbanobstacles. The turbulence of the flow and thewind flowing over the buildings is acceleratedby the buildings architecture with sharp forms(corners, edges), thus, changing the directionof the wind from horizontal to slightly upward.

These imply enhanced performance for theturbines, with cut-in speed below 2.5 m/s,lower noise and vibrations, as well asaesthetics supporting the societal acceptance,thus innovative and appealing rotor designsthat could be harmoniously implemented in thebuilt environment.

The vertical axis wind turbines (VAWTs)industry has bloomed in the last few years,being more suitable for the urban environmentthan the horizontal axis ones, as they are

taking wind from any direction (Fig. 1). Newdesigns are proposed, aiming at obtainingimproved efficiency and safety, less noise, anda better performance-cost ratio.

Figure 1: Horizontal axis vs. Verical axis

The survey on small power VAWTs todayshowed that there are still important aspectsthat need to be solved in order to integrateharmoniously the wind turbines into the cities.Among the disadvantages of the todays offerthere are the relatively high cut-in speed (3-4m/s), their rated power and experimental basewhich is uncertain, the overrated specifications

which cannot be rigorously checked due tolack of standards; most of the solutions are toolarge for implementing in the street furniture


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and also the aesthetical aspect is usuallyneglected in favour of efficiency.

2. VAWTs design criteria

In order to define the new design concepts, amulti criteria analysis algorithm was developedconsidering both functionally and aesthetics:efficiency in low or turbulent winds, noise level,vibrations, cost, aesthetics, environment-friendly solutions (aiming to overcome theNIMBY statement), integration into hybrid PV-wind systems.

In terms of technical data, the turbines shouldbe silent and efficient (to start at low windspeeds). This means that the cut-in speedshould be lower than 2 m/s and the tip speed

ratio should be around 1.

Another highly discussed aspect regarding theturbines, the pay-back time, is important butcan be surpassed by the aesthetics, if/whenthe turbine can be perceived as an art object.These turbines are designed for public spaces,and in less extent for residences or houses,therefore, the main customers are the publicinstitutions and, in a larger approach thecommunity. Thus, the investment combines thequest for green energy (renewable sources)with those for well-being, for a public, kinetic,

work of art on the city sky, that transmit anidea, a state of mind, emotions, throughshapes, colours, lights and textures. Theturbines are designed for supplying either thestreet furniture with electricity especially forstreet lights, but also for bus stations, roadsigns, or to be implemented into the concept ofBuilding augmented wind turbines (BAWTs) wind turbines that are using the building insuch a way as to increase the performance ofthe wind turbines, for a specific place.

Thus, the most important criteriasupplementary formulated for urban integratedVAWT are:

1. Overall dimensions (aspect ratio, H/D) forobtaining 150W at 5m/s wind speed.

2. Rated power at a wind speed of 5m/s (forthe tested turbines) or the Reynolds number atthe same wind speed (for the untestedturbines).

3. Cut-in speed lower than 2 m/s and tip speedratio closer to 1.

4. Manufacturing cost.

5. Other aesthetic requirements (colour,texture, graphic).

6. Unconventionality/ degree of novelty.

7. Environmental impact (impact on birds,noise level etc).

8. The costs of the prototype.

9. Aesthetics. The ability to be easily integratedin any type of build environment (modern orhistorical) as kinetic sculpture.

Figure.2: Multi criteria analysis. Criteriaimportance - Level.

Obviously, the 19 criteria are not fullyindependent. Their inter-connection ispresented in Figure 2 and allows deciding onthe importance of each criterion.

The results show that the main aspect that hasto be considered, regarding the new VAWT

designs, is the aesthetical one (score 7.5),considering that the turbines are to beimplemented in urban areas, and the visualimpact is very high, with strong influence ontheir acceptance.

The following two criteria are also related toaesthetics: colour, texture, graphic (criterion 5)and Unconventionality/ degree of novelty(criterion 6).

The next criterion, in order of importance is theone related to lower cut-in speed and tip speed

ratio value (criterion 3), which, besidesaesthetics is one of the main design objectiveswhen designing an urban wind turbine.

Related to the previous criterion is the next one(criterion 2) which considers the rated power ofthe turbine at a wind speed of 5 m/s.

The criteria 1 and 7 have the same values, andare about: 1 - the needed overall dimensionsof the wind turbine (depending on the bladesdesign) in order to obtain 150w at 5 m/s wind

speed; 7 - the environmental impact of thewind turbine design.


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The manufacturing cost (criterion 4) is amongthe least important criteria that were taken intoconsideration in the early stages of our newdesign, as will be shown further in this paper.

The last criterion is the one related to

manufacturing costs of the real prototype.

3. New VAWT rotor design

Based on the multi-criteria analysis results anew VAWT rotor design is presented,considering both the efficiency of the windturbine and the aesthetics.

The designs correspond to a three bladed,Savonius type vertical axis wind turbinedesign, the Poppy turbine. Mimicking nature

represents a pre-requisite for societalacceptance, therefore nature was chosen asan inspiration source for the vertical axisturbine design, considering the fluidity of theshapes, the colours, the movement; the noveldesign is inspired by the shape of the poppypods and by the colour of the poppy flower.

The Poppy Turbine is part of a series ofinnovative, organic shaped VAWT rotor designconcepts, implementable in urban areas thatwe developed to answer the requirementswhich were neglected so far; this set of newrotors further include the Diamond Turbine, theGinkgo Turbine and the Maple Turbine. Thedesign process started from standard verticalaxis wind turbines rotors (Darrieus andSavonius), and considered the recent researchresults from several groups and the existingwind turbines on the market.

The following aspects are considered for thenew design:

a) Aesthetics: the products should makepeople aware and also gain their acceptance

towards renewable energies. It isnt just amatter of functionality, but the product has tobe also beautiful, to express more than itsfunctional role, to create a modern andcolourful atmosphere, a graphic play withcolours and shapes, to become part of theurban culture and, in particular to give identityto the urban open space. Wind turbines, onceimplemented in a city, are not single volumesthat spin above our heads, but dynamicstructures that create a volume made of formand emptiness, thus by placing a row ofturbines, along a street or in a park, an urban

rhythm and identity is created. Wind suggests,and usually is inducing movement - movement

that, at a city level, is not only related tomobility itself, but it comprises the whole ideaof innovation, of modernization, the need ofchange and embracement of modern solutionsand sustainable development.

b) The turbines are designed for supplying thestreet furniture with electricity especially forstreet lights, but also for bus stations, roadsigns etc.

c) To have low cut-in speed and low noiselevel.

Figure 3: Poppy turbine concept

We developed several design concepts: withthree or five blades (Figure 3). and twodifferent aspect ratios; this paper presents theresults for the three blades design.

A conceptual design model was created (seeFigure 4) before creating the CAD model,explaining the parts and their connectionswithin the mechanical system.

The CAD model, (Figure 5), was defined using

the CATIA software and it comprises thedetailed design for all the turbine assemblyparts (Rotor blades and shaft, alternatorassembly, bearings, mast), considering thatthe turbine rotor has a diameter of 1m and isdirectly coupled to the alternator.

In Figure 6 a realistic rendering of the Poppyturbine is presented, while in Figure 8 the CADmodels of the mast and of the alternatorassembly are shown. The overall dimensionsof the new design are shown in Figure 7.


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Figure 4: Conceptual design of the windturbine assembly.

Figure 5: CAD model of the wind turbine

Figure 6: Poppy turbine - CAD model

Figure 7: Poppy turbine rotor overalldimensions

Figure 8. CAD model of the mast, alternator

and bearings

Regarding the rotor aerodynamics of the, andthe similarities with traditional VAWT rotors,


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the blades of the Poppy turbine (made of glassfibre reinforced resin) are a combination ofscoops with the traditional Savonius rotor. Asmall scale model was developed in the earlydesign stages, allowing optimising the shape ofthe blades and the performance of the new

rotor was compared with that of an equivalentSavonius rotor. The results were encouraging,as the Poppy turbine provided betterperformance (see Table 1).

Table 1: Poppy turbine vs. Savonius rotor

4. Multi-body analysis of thenew VAWT

The aim of the Multi-body System (MBS)modelling is to simplify the mechanical modelin order to accomplish the virtual testing,

dynamic analysis and the optimisation of themodel.

In MBS, the multibody systems are defined ascontaining bodies subjected to geometricalrestrictions and kinematical restrictions [9].

The analysis is made in a planar system(spatiality S being 3) or in a spatial system(S=6).

The geometric restrictions are of two types [9]:simple (a kinematical joint, as the R-rotation) or

compound (with a linking element betweenbodies that has at both ends kinematic joints,as the RR rotation-rotation).

The kinematic restrictions are cancelling theremaining degrees of freedom after imposingthe geometrical restrictions between bodies. Ittakes into consideration the imposed relativemovement between bodies. The total numberof kinematical restrictions is equal with themobility of the mechanism associated to themultibody system.

The algorithm for determining the minimumnumber of bodies consists by the followingsteps: 1) fixed body; 2) input bodies, 3) output

bodies, 4) bodies with more than twoconnections and 5) bodies with applied forces.

The vertical axis Poppy Wind Turbinemay bemodelled as an MBS system with two bodies:one fixed and the other linked to it by a

rotational joint. Even if the real system is threedimensional (S=6), we may consider it, as well,as a planar system, with the (S=3). The MBSmodel contains two bodies with a rotationaljoint between them (Figure 9 and Figure 10).

The first body (1) which is the fixed bodycomprises the mast, outer rings of thebearings, alternator case and stator.

The second body (2) comprises the rotorblades, rotor shaft, inner rings of the twobearings and the alternator rotor.

Figure 9. MBS sketch model.

Figure 10. 3D MBS model exploded with thetwo parts and rotation joint.

2

1


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Therefore, the mobility of the MBS system is:

( 1) 3 1 2 1M S n r= = = (1) [10]

Where:

S spatiality of the system

n number of bodies

r restrictions

The mechanisms simulation was developedusing MD Adams (see Figure 11) and the CADcorresponding files of the blades, and of therotor shaft.

Considering the wind speed and the windturbine assembly geometry and materials, a

simulation of the previously discussedmechanism was developed, in order to selectthe appropriate alternator cogging andresistant torque for the wind turbine.

Figure 11: MSC ADAMS analysis

The aerodynamic torque was first calculatedwith the following formula [3], if the Cp isknown:

( )2 3

, /2turb p t T R v c

=

(2)

Where:

= is the air density [kg/m3] (1.1kg/m

3, at 15

oC

at 500m height)

R= rotor radius (0.5m)

v=2 m/s and 15 m/s wind speed

aempC = ,

Where:

mrepresents the efficiency of the mechanicaltransmission (m=0.95 0.97);

e efficiency of the electric components

(e=0.97 0.98);

a aerodynamic efficiency (depends upon theenvironmental characteristics of the testingarea and has the maximum value: a=0.59).

0.96 0.97 0.59 0.549p

C = =

For cut-in speed:

If t= 0.72 rad/sec = 6.8 RPM then, accordingto equation 2:

=0.5 1.1 0.52 1.83 0.549

0.72

= 1.9 [ ]

If t= 1.5 rad/sec and v=2m/s then, accordingto equation 2: = 0.47 [ ]

If t= 34 rad/sec and v=15 m/s then, accordingto equation 2: = 23.5 [ ]

We chose an alternator which has the resistanttorque (Tg): for 650 RPM = 11.2 [Nm], for 325RPM = 8.9 [Nm], cogging torque = 0.2 [Nm].

Based on these data and the selectedalternator we obtain a self-starting turbine, as

the cogging torque is very low.

In order to test the theory, were conducted aseries of CFD analyses for determining theinput values for our mechanism simulation. InFigure 12 are shown the results for the CFDsimulations when considering an air velocity of15 m/s.

Figure 12: CFD analysis, v=15 m/s

Firstly the turbine was tested in a virtual windtunnel with 2 m/s air velocity and consideringthe alternator resistant torque of 0.2 Nm. Thevalues for the angular velocity and torque ofthe turbine rotor are shown in Figure 13.


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Figure 13: Angular velocity and torque at v=2m/s

The average value of the torque, in this case(2m/s air speed and 0.2 Nm resistive torque) is0.4 Nm.

Secondly, the input values for the CFDanalysis were changed to v=15 m/s andresisant torque 8.9 Nm (Figure 14).

Figure 14: Angular velocity and torque at v=2m/s

The average value of the torque, for the

second case is 14 Nm.

5. Urban wind turbines

The output of any wind turbine depends on thefollowing main factors: rotor swept area, windspeed (as the power output is proportional tothe cube of wind speed), overall systemreliability and total power conversion efficiencyfrom wind to electricity.

In the built environment average wind speedsare generally low, of 2-4 m/s. When urban sitesare considered, the low wind speeds should beconsidered in relation to the start-up and cut-inwind speeds of the turbine - the wind speeds atwhich the rotor begins turning and,respectively, when the turbine begins togenerate power. For most small turbines, thestart-up wind speed is less than that of its cut-

in wind speed. So, the cut-in speed of theturbine is the one that must be below 2 m/swind speed [7, 8, 12].

The great advantage of urban wind turbines isthat they produce energy near the place whereit is used, consequently, needed most. Buildingaugmented wind turbines (BAWTs) - Figure 15[1, 5, 6, 11]- are wind turbines integrated intothe buildings, in such a way as to use thebuilding architecture as a concentrator of wind,thus increasing the wind speed at the turbinehub.

Figure 15: BAWTs

The Brasov area has relatively low windspeeds and implementation of wind turbines isa real challenge. Before implementing a windturbine, a complex study of the wind behaviourwas done, allowing to select the appropriateimplementation location.

The location site is represented by the R&DInstitute of the Transilvania University ofBrasov (Figure 16). The Institute consists of 12buildings with three levels (half-ground, groundand 1

st floor), with a regular height of 10 m

above ground and rectangle prismatic shape

(14 m x 31 m).


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Figure 16: The Implementation site the R&D

Institute of the Transilvania University of

Brasov

The buildings are linked by two spines startingfrom the central point represented by the

Atrium (a building with an egg-shape ceilingand a height of 13 m).

The CAD model of the buildings wasdeveloped using CATIA V5 software (Figure17). It is a simplified model of the buildings andit was used to determine the wind flow patternsin a Computational Fluid Dynamics (CFD)software [4]. The CFD analysis also took intoconsideration the position of the Students Hall.

Figure 17: CAD model of the R&D Institute ofthe Transilvania University of Brasov buildings

The small power, vertical axis wind turbineswill be implemented as part of the urbanfurniture and street lighting system, at a heightof 8 m; the velocity vectors profile at this heightare presented in Figure 18. The areashighlighted in red (with the highest wind speed)

are thus recommended. Too high wind speedmay be temporarily exhibited near the studenthall faade therefore, this location should becarefully analysed if selected forimplementation.

Figure 18. CFD analysis of the wind pattern

Figure 19. Streamlines along central axis

6. Conclusions

This paper brings up a new approach on smallpower, vertical axis wind turbines design. Theturbines, beside their functional role (providingelectricity for the street furniture, and makingpeople aware of the sustainability), are kineticworks of art on the city sky. They transmit anidea, a state of mind, emotions, throughshapes, colours, lights and textures. ThePoppy turbine concept is a design that issuitable for urban environment integration onits own or integrated into the street furniture.

The relatively small overall dimensions (1m

rotor diameter) of the VAWT rotor allows theimplementation in the street furniture.


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The CFD results and mechanism analysisshow that the turbine is self-starting and thesmall number of components (it doesnt have agear box) increases the Poppy turbinereliability.

In the future we will develop a detailed study ofthe new turbine design models - by means of avirtual prototyping platform workflow which hasbeen developed to evaluate the performanceof the wind turbine (complex mechanicalsimulationsand CFD analysis etc) - in order toobtain the virtual prototypes for technologicaltransfer. Also after optimising the design will beperformed several wind tunnel testings.

References

[1] *** AWEA Small Wind Turbine GlobalMarket Study. Year ending 2009.www.awea.org/smallwind

[2] G. Muller, M. F. Jentsch, E. Stoddart,"Vertical axis resistance type wind turbines foruse in buildings", Renewable Energy 34, pp1407-1412, ELSEVIER, 2009

[3] B. Neammanee et al, Development of awind turbine simulator for Wind Generatortesting, International Energy Journal 8, 21-28,2007

[4] L. Ledo, P.B. Kosasih, P. Cooper, "Roofmounting site analysis for micro-wind turbines",Renewable Energy 36, pp 1379-1391, 2011

[5] D. Ayhan, . Saglam, "A technical review ofbuilding-mounted wind power systems and asample simulation model", renewable andSustainable Energy Reviews 16, pp 1040-1049, 2012

[6] S. Mertens, Wind energy in urban areas:Concentrator effects for wind turbines close tobuildings, Refocus Vol. 3, pp 22-24, 2002

[7] E. Dayan, Wind energy in buildings : Powergeneration from wind in the urban environment- where it is needed most, Refocus Vol. 7, pp33-34, 36, 38, 2006

[8] ***Wineur. Urban wind turbines. Technologyreview. www.urbanwind.net/downloads.html

[9] Visa I., Antonya Cs.: Structural Modelling of

Planar Linkages as Multibody Systems,Proceedings of the 11th World IFToMMCongress, Tianjin, China, 2003, p. 1194 1198

[10] Visa I., Diaconescu D., et al., NewLinkage with Linear Actuator for Tracking PVSystems with Large Angular Stroke, ChineseJournal of Mechanical Engineering, 24, 2011,pp. 744-751

[11] S. Mertens, Urban Wind Energy, Round

table San Francisco, Ingreenious, March, 2011

[12] S. L. Walker, "Building mounted windturbines and their suitability for the urbanscaleA review of methods of estimatingurban wind resource", Energy and Building 43,pp 1852-1862, 2011.

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