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Keywords: Urban wind energy; Ducted turbine; Building integration; Pressure coefcients

in an urban environment;

clear fetch of wind to reach them.Project WEB concentrated largely on the third strategy,

producing designs and scale models of fully integrated

Again issues of noise, vibration and structural integrity

Small wind turbines of any type inevitably have highercosts per unit of electricity produced than wind-farmmachines, and so are unable at present to compete with

ARTICLE IN PRESSconventional sources of energy. But the same may be saidof photovoltaic systems, and these are nding widespreadapplication in urban environments throughout the world.

0960-1481/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.renene.2007.08.005

Corresponding author. Tel.: +44 141 548 2083; fax: +44 141 552 5105.E-mail address: andy@esru.strath.ac.uk (A. Grant). retro-tting wind turbines onto existing buildings; and integration of wind turbines into buildings which arespecially designed for the purpose.

The rst of these could perhaps be dismissed on semanticgrounds, as the word urban by denition suggests largebuildings in close proximity, with no space for free-standing wind turbines and certainly not for a reasonably

arise, along with concerns over visual impact. Somecommentators [3] remain sceptical of the ability of roof-mounted wind turbines to ever make a signicantcontribution to energy supply. Actual installations onbuildings (admittedly at a fairly small scale) are appearingsteadily [4], but the future remains uncertain: the potentialconsequences of a single well-publicised accident can easilybe imagined.1. Introduction

Generating energy from the wind in an urban environ-ment is an attractive idea. It places a source of supply at asite of strong energy demand, the essence of embeddedgeneration. However, there are major problems over itspractical implementation at a signicant scale. The EC-funded Project WEB [1] identied three possible strategies:

simply siting conventional free-standing wind turbines

systems. From an aerodynamic point of view these seem tobe quite effective. Public concerns over safety, and issues ofnoise and vibration might present barriers to progress. Andof course implementation of this strategy requires newconstruction, so at best the growth of installed generatingcapacity would be very slow.Some progress has been made elsewhere with the second

strategy. Urban wind characteristics are now being studiedby the research community [2], and a number ofmanufacturers offer turbines for attachment to buildings.The prospects for urban wind power are discussed. A roof-moun

wind ow around a building, is proposed as an alternative to m

prototype scale are described, and a simple mathematical model is p

produce very high specic power outputs, going some way to offse

are made to assess annual energy output and seasonal variations,

comparators. It is concluded that ducted turbines have signic

advantages where visual impact and safety are matters of concern

r 2007 Elsevier Ltd. All rights reserved.AbstractAvailable onlinRenewable Energy 33

Urban wind energy conversion:

Andrew Grant, Camer

Energy Systems Research Unit, Department of Mechanical Engineering, U

Received 19 June 200708) 11571163

he potential of ducted turbines

Johnstone, Nick Kelly

rsity of Strathclyde, 75 Montrose Street, Glasgow G1 1XJ, Scotland, UK

epted 14 August 2007

October 2007

ducted wind turbine, which uses pressure differentials created by

e conventional approaches. Outcomes from tests at model and

ented. Predictions from the latter suggest that a ducted turbine can

g its directional sensitivity. Further predictions using climate les

h a conventional small wind turbine and a photovoltaic panel as

potential for retro-tting to existing buildings, and have clear

www.elsevier.com/locate/renene

2. The ducted wind turbine

The ducted wind turbine described here was envisagedfrom the outset as an alternative to conventional roof-mounted turbines, in the form of a building-integrated orretro-tted module. The ducting protects the turbine fromextremes of building-generated turbulence, at the expenseof directional sensitivity. The original concept (see Fig. 1)came from a patent by Webster [5].The curved duct and shaft drive of the original Webster

patent makes sense for small turbines, where a hub-mounted generator would tend to block the air ow. Singleunits of this type have been built and tested over lengthyperiods [6], and have proved effective and robust inoperation. Fig. 2 shows an early prototype on a test sitein Glasgow city centre. Six machines were subsequentlyinstalled in the Lighthouse Building in Glasgow as part ofan EC-funded demonstration project [7]; these had to bevery small (0.5m rotor diameter) for architectural reasons.The design adopted, seen in Fig. 3, used an angled spoiler(tted with a photovoltaic panel) to induce low pressures at

form CP P/(1/2rV2), where P is the static pressure, r isthe air density and V is conventionally the air speed in theatmospheric boundary layer at a level equal to the height ofthe building. It might be possible to position a ductedturbine so that it experiences a large pressure differentialfor a wide range of incident wind directions.Dannecker and Grant [8] have conducted wind tunnel

tests on curved and straight ducts (without turbines) in arectangular building model, measuring pressure coefcientsand velocities within the duct. For certain congurations,the latter signicantly exceeded the velocity of theapproaching airstream. Importantly, these velocities weremaintained over a wide range of incident angles, 7601 inthe best cases. So the potential (at least in free ducts) hasbeen demonstrated.

3. Mathematical model

For ducted wind turbines, there is a crucial inter-dependence between the turbine resistance, the massowrate through the duct and the pressure differentialacross its ends. In order to determine the require-

ARTICLE IN PRESS

Fig. 2. Early ducted turbine prototype at University of Strathclyde,

Glasgow.

A. Grant et al. / Renewable En1158the duct exit.The underlying principle of operation of the device is the

use of pressure differentials produced by the wind owaround and over a building to drive air through the ductedturbine. High pressures will be experienced on verticalwalls facing the on-coming wind, and relatively low oneson the sides and rear. Locally, particularly around the roof,ow separation can induce extreme low pressures, widelyrecognised from their ability to lift tiles or other forms ofcladding. These pressure differentials are a function of bothwind speed and direction; the effect of speed may becompensated by referring to pressure coefcients of theFig. 1. Original ducted wind turbine from patent by Webster [5].ergy 33 (2008) 11571163ments for optimum performance, and to predict poweroutputs for typical operating conditions, a mathematical

ARTICLE IN PRESSEnA. Grant et al. / Renewablemodel was developed.

In a uniform unobstructed duct, the induced air velocitywill be

V 2 Cv2P01 P2

r

s,

where P01 is the stagnation pressure at duct inlet and P2 isthe static pressure at duct outlet. The air density is r and Cvis the velocity coefcient for the duct. If we dene adifferential pressure coefcient as

d P01 P2=1=2rV 21,where VN is the approaching wind velocity, then

V 2 Cvd

pV1. (1)

Values of the velocity ratio V2/VN are plotted in Fig. 4,for a range of values of Cv and d. It will be argued later thatd values of 2 and more should be achievable in practice, inwhich case strong velocity amplication can take place

Fig. 3. Ducted wind turbine module as installed on the Lighthouse

Building, Glasgow.even when there are signicant losses in the duct.

0.4

0.6

0.8

1

1.2

1.4

1.6

0.5 1 1.5 2 2.5

Differential pressure coefficient, Delta

Duct velo

city r

atio

0.6 0.7 0.8 0.9 1

Fig. 4. Velocity augmentation in a free duct, for a range of pressure

differentials. A number of duct Cv values are considered (see legend).

ergy 33 (2008) 11571163 1159If a turbine is placed in the duct, causing a pressure dropDPT, the equation for the induced velocity becomes

V2 Cv2 P01 P2 DPT

r

s Cv

dV 21

2DPTr

s.

The power extracted by the turbine

DPT=rrAV 2 rAV 2dV 21=2 V22=2C2v,where A is the duct cross-sectional area. Differentiatingwith respect to V2 gives a maximum power condition,

dV212

3V22

2C2v 0; or V 2 Cv

d3

rV1. (2)

So when extracting power using a turbine, the optimumvelocity ratio V2/VN is reduced by a factor O3 from thevalue given in Eq. (1) for an unobstructed duct.For maximum power,

DPT 1=2rdV21 d=3V 21 1=3rdV 21,and the power extracted from the airstream is

1=3rdV 21ACvd=3

pV1 Cv=3

3

prAd3=2V 31.

ARTICLE IN PRESS

Fig. 6. Three-dimensional view of 301 straight duct with wide spoiler atthe inlet.

1.3

1.4

1.5

plificati

on

ratio

EnThe power coefcient

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.5 1

Differential pressure coefficient, Delta

1.5 2 2.5

0.6 0.7 0.8 0.9 1

Tu

rbin

ce p

ow

er

coe

ffic

ien

t

Fig. 5. Predicted turbine power coefcients, for a range of pressure

differentials. A number of duct Cv values are considered (see legend).

A. Grant et al. / Renewable1160CP 2

33

p Cvd3=2. (3)

Fig. 5 shows the relationship between CP, Cv and d. Thissuggests that in practical cases, CP values in excess of theBetz limit for free turbines might be expected. Values ashigh as 1 might be attained in some cases.The CP values quoted here are of course gross ones (as in

the Betz analysis for free wind turbines). The actual CP forany system must incorporate corrections for the rotor andgenerator efciencies. In applying this theoretical model, itis necessary to make the assumption that values of thedifferential pressure coefcient d are unaffected by thepresence of the turbine duct: reasonably accurate for asingle unit, more questionable for several grouped alongthe roof edge.

4. Calibration of mathematical model

Some aspects of the mathematical model may be checkedagainst experimental measurements obtained by Danneck-er [9]. He carried out a series of wind tunnel tests on arectangular building model, tted with ducts in variouscongurations. The case presented here had a circular ductangled at 301 to the horizontal linking the front fac-ade tothe roof, and a detachable arched spoiler above theentrance (see Fig. 6). Velocities were measured in the ductclose to the exit, and compared with the reference windspeed: the ratio between the two is shown in Fig. 7, for21mm

86mm

ergy 33 (2008) 11571163cases with and without spoiler. He also measured staticpressures on the roof of the model, in the vicinity of theduct outlet. The duct was open, with no attempt tosimulate a turbine by restricting the ow.Looking rst at the case where the spoiler was tted,

signicant enhancement of velocity in the duct wasobserved: the velocity ratio V2/VN, averaged over thecross-sectional area, was in the range 1.31.35. The meanpressure coefcient at duct outlet was 0.8.According to Eq. (1), the velocity ratio should equal

Cvd

pand if it is assumed that Cv 1, the d value required

to match the observed ratio lies in the range 1.691.82. Astagnation pressure coefcient at the duct inlet close tounity (which with a spoiler tted is quite feasible) wouldgive a d value close to 1.8 and therefore within the requiredrange. So the model appears to give an accuraterepresentation in this case. It also indicates that thevelocity coefcient Cv for the duct must be high, certainly

1

1.1

1.2

0 0.2 0.4 0.6 0.8 1

d/D ratio

Velo

cit

y a

m

spoiler plain duct

Fig. 7. Measured velocity distributions for the duct shown in Fig. 5.

The best opportunities for producing large pressuredifferentials seem to occur on at-roofed buildings, wherethe upwind fac-ade might be linked to the separation bubbleon the roof. Large pressure differentials may also beobtained on buildings with low-pitch roofs [13]. Steeperpitches, as investigated by Ginger and Holmes [14] atmodel scale, are best exploited near the ridge line.However, the tting of ducted wind turbines onto pitchedroofs is complicated by the fact that the ducts willsignicantly alter the appearance of the building, whichmay be unacceptable. Also, their attachment would involvethe disturbance of cladding materials and might require thestrengthening of roof timbers.For a at-roofed building, it is clear from the literature

that a high-pressure differential can be maintained up to atleast 451 of misalignment with the approaching ow. Thereduction in positive pressure on the fac-ade of the building

edges of at roofs. A straight-duct module might be used,

ARTICLE IN PRESSEnergy 33 (2008) 11571163 1161in excess of 0.9, for the observed values of velocity ratio tobe attained.In the case where the spoiler was removed, a reduced

velocity ratio V2/VN was observed (in the range 1.01.1,averaged over the cross-sectional area). The mean pressurecoefcient at the duct outlet was 0.62. For Cv 1 in theduct, the d value required to produce these velocity ratioslies in the range 1.01.21. So now the stagnation pressurecoefcient at duct inlet is between 0.38 and 0.59.Certainly some reduction from the previous case would

be expected: without the spoiler, the pressure coefcient onthe front fac-ade of a rectangular building will fall quiterapidly over its uppermost 20%. It is also probable that theow into the duct would be less well controlled, withstronger separation at the edges and hence greaterdissipation of energy in the ow. So the most plausibleexplanation lies in some diminution of inlet pressurecoefcient, and a reduced value of Cv. There are moreuncertainties in this second case, but the mathematicalmodel emerges unscathed.

5. Pressure coefcients on buildings

For best performance, the duct should link a high-pressure (stagnation) zone on the building with a region ofvery low pressure, caused perhaps by ow separation.Opportunities are likely at the sides of tall buildings, at theedges of at roofs and near the ridge lines of pitched roofs.Determination of the likely pressure coefcient differ-

entials for these cases is not a straightforward matter.Building codes and guidelines are (quite understandably)primarily concerned with the structural integrity of roongand cladding panels, and concentrate on peak CP values,both positive and negative. But for energy production froma ducted wind turbine, time-averaged values are moreappropriate. Also it is becoming recognised that tests onmodels in wind tunnels do not give an accurate representa-tion of pressure coefcients at full scale, particularly inregions of separated ow.Hoxey et al. [10] investigated a building with a low-pitch

roof at model and full scale, and concluded that patterns ofow separation on the roof and the position of thestagnation point on the front wall were both affected byscale. The widely held belief that ow around objects withsharp edges is immune from Reynolds number effectsappears to be erroneous. Similar conclusions from model-ling the decks of suspension bridges are reported by Laroseand DAuteuil [11].Richards et al. [12] compared time-averaged CP mea-

surements over the surfaces of a cube from a number ofsources, with their own data for a 6m3 as a full-scalereference. They concluded that CP values in separatedregions are strongly inuenced by scale: the larger themodel, the lower the minimum pressure recorded on theroof. At full (6m) scale, the minimum time-averaged value

A. Grant et al. / Renewableof CP was about 1.2. For oblique ows where vorticeswere generated, scale effects appeared to be less signicant.angled downward to intercept the air ow rising up the faceof the building as shown in Fig. 8. Here, the visual impactis similar to the adding of a parapet, and xing should bestraightforward. The ducts could be grouped in a row, orspaced at intervals within an otherwise solid wall.is more than compensated by the effect of edge vortices inreducing the static pressure on the roof. This is consistentwith Danneckers ndings [9] on ducts in buildings atmodel scale. So the inherent directional sensitivity of aducted turbine with xed orientation is somewhat reduced.

6. Ideas on exploitation

The congurations investigated by Dannecker, whileeasy to reproduce in models, are only possible at full scalein a purpose-built structure. For widespread exploitationof the technology, designs to t onto existing buildings arerequired.Once more a promising application occurs around theFig. 8. Angled straight duct to intercept air ow at the edge of a at roof.

Studies by Stathopoulos et al. [15] at model scale foundthat solid parapets had little effect on mean pressurecoefcient values on a at roof. This is in contrast to short-duration extreme values, which could be strongly affected.Interestingly, tests by Kopp et al. [16] suggest thatdiscontinuous parapets can affect vortex formation overat roofs, altering the pressure distribution accordingly.Porous parapets as investigated by Pindado and

Meseguer [17], again at model scale, seem to reduceextreme negative pressures. The ducted turbine module inFig. 8 would behave like a porous parapet, so this nding issignicant. There is clearly scope for more research,particularly at full scale.The optimum size for a ducted turbine module will be a

compromise between economies of scale (larger windturbines are generally more cost-effective), ease of installa-tion on existing buildings and visual impact (both of which

maintained over at least 7451 variation in wind direction.But inevitably, there will be a wide range of angles overwhich a ducted turbine will produce zero output, so itsuffers in comparison with a conventional roof-mountedwind turbine. But the latter must also experience direc-tional effects to some extent: turbulence from adjacentbuildings and local roof elements will vary in nature andseverity with the wind direction, inuencing the turbinesperformance and perhaps also its longevity.

7. Energy capture

The relative performance of a number of building-integrated renewable energy converters has been investi-gated for climatic conditions in Glasgow, Scotland, UK.The devices considered were: a conventional small windturbine mounted on the roof (WTG); two ducted turbines,

ARTICLE IN PRESS

Jun

(S

A. Grant et al. / Renewable Energy 33 (2008) 115711631162would tend to limit size). Rotor diameters of 1m or moreshould be practical: in this case, a streamlined duct andhub-mounted generator could be employed.Ducted turbines at the edge of a at roof should

experience d values of around 2 when the wind blowsnormal to the roof edge. With duct Cv values close to 1,aerodynamic power coefcients in excess of unity arepossible (Fig. 5). When losses in the generator andelsewhere are considered, overall power coefcients around0.7 may be expected, about twice the gure for aconventional small wind turbine. A turbine module witha 1m2 duct and a somewhat smaller rotor diameter mightbe rated at around 250W in a wind speed of 10m/s. A roof-mounted array 20m wide might therefore produce 5 kW.But there is still some uncertainty about how the presenceof a substantial ducted air ow might affect the pressuredistribution around the building, and hence the effectivevalue of d.Vortex generation above the edges of a at roof in

oblique ows should ensure that turbine performance is

0

5

10

15

20

25

30

35

40

Jan Feb Mar Apr May

Energ

y c

aptu

re, kW

h

WTG DWTFig. 9. Energy capture predictions for ducted wind turbines in two orientations

climatic data).facing South and West, respectively; and a photovoltaicpanel (PV), xed at the optimum orientation for thelatitude of Glasgow. Rotor diameter for all turbines was1m, and the area of the photovoltaic panel was normal-ised to equal the turbine rotor swept area. Hourlyaveraged data for wind speed and direction and solarradiation over a typical year were used to compute energycapture per month. The results are displayed in Fig. 9.There are a number of factors which might affect the

accuracy of these predictions. Since power is proportionalto the cube of wind velocity, the use of hourly averages islikely to be unkind to the wind turbines. No directionalsensitivity has been attributed to the conventional turbine.Finally, assumptions have inevitably been made aboutturbine power coefcients and photovoltaic panel ef-ciency, which may or may not be accurate.Fig. 9 shows large differences in output for the two

ducted turbine orientations, so determining the optimumalignment is clearly important. Over the year, the expectedcomplementary nature of wind and solar energy is clearly

Jly Aug Sep Oct Nov Dec

) DWT(W) PV, compared with a conventional turbine and a photovoltaic panel (Glasgow

demonstrated. The annual totals for the four systems, inthe order in which they appear in the gure, are 156, 147,219 and 85 kWh, corresponding to mean outputs (in W) of17.8, 16.8, 25.0 and 9.7, respectively.The West-facing ducted turbine produces roughly 50%

more energy over the year than its South-facing counterpart,and also out-performs the conventional wind turbine.However, the capacity factors for the ducted turbines willbe comparatively low as a result of their higher rated power(250W as against 100W). The conventional turbine emergeswith the highest capacity factor of 0.178 although as stated,

References

[1] Campbell N, et al. Wind energy for the built environment (Project

WEB). In: Proceedings of the European wind energy conference,

Copenhagen, Denmark, July 2001.

[2] Mertens S. The energy yield of roof mounted wind turbines. Wind

Eng 2003;27:50718.

[3] Gipe P. Roof-top mounting. /http://www.wind-works.org/articles/RoofTopMounting.htmlS, 2003.

[4] Knight J. Urban wind power: breezing into town. Nature

2004;430:123.

[5] Webster GW. Devices for utilizing the power of the wind. Glasgow,

Scotland. USA Patent No. 4154556, 1979.

[6] Grant AD, Nasr el-Din SA, Kilpatrick J. Development of a ducted

wind energy converter. Wind Eng 1994;18(6).

[7] RE-Start (Renewable Energy Strategies and Technologies for

ARTICLE IN PRESSA. Grant et al. / Renewable Energy 33 (2008) 11571163 1163and highlights a key difculty for urban wind exploitation:the quality of the wind regime makes cost-effectiveexploitation very difcult to accomplish.

8. Conclusions

It is claimed that ducted turbine modules can be a viablealternative to the practice of attaching small conventionalmachines to the roofs of existing buildings. Ducted windturbines are protected from extremes of building-generatedturbulence and have small visual impact. In the commercialand industrial sectors particularly, the potential scope is large.A mathematical model has been developed to predict the

performance of a building-integrated, ducted wind turbine.From the experimental evidence presently available, itseems to give an accurate representation. In the mostpromising applications, power coefcients well in excess ofthe conventional Betz limit should be attainable. However,the turbines are necessarily fairly small and are direction-ally sensitive.At present, the combination of high cost per unit of rated

power output and low capacity factor makes it difcult forurban wind energy to compete with other sources. But thesame might be said about solar photovoltaics. In the longerterm, energy cost convergence should bring about morewidespread exploitation.framework 1996. Commission of the European Communities, DG

XVII, Directorate-General for Energy and Transport.

[8] Dannecker RKW, Grant AD. Investigations of a building-integrated

ducted wind turbine module. Wind Energy 2002;5:5371.

[9] Dannecker RKW. Wind energy in the built environment: an

experimental and numerical investigation of a building-integrated

ducted wind turbine module. PhD thesis, University of Strathclyde,

Glasgow, UK; 2001.

[10] Hoxey RP, Robertson AP, Richardson GM, Short JL. Correction of

wind tunnel pressure coefcients for Reynolds number effect. J Wind

Eng Ind Aerodyn 1997;6971:54755.

[11] Larose GL, DAuteuil A. On the Reynolds number sensitivity of the

aerodynamics of bluff bodies with sharp edges. J Wind Eng Ind

Aerodyn 2006;94:36576.

[12] Richards PJ, Hoxey RP, Short JL. Wind pressures on a 6m cube.

J Wind Eng Ind Aerodyn 2001;89:155364.

[13] Kumar KS, Stathopoulos T. Wind loads on low building roofs: a

stochastic perspective. J Struct Eng 2000;126:94456.

[14] Ginger JD, Holmes JD. Effect of building length on wind loads on

low-rise buildings with a steep roof pitch. J Wind Eng Ind Aerodyn

2003;91:1377400.

[15] Stathopoulos T, Saathoff P, Du X. Wind loads on parapets. J Wind

Eng Ind Aerodyn 2002;90:50314.

[16] Kopp GA, Surry D, Mans C. Wind effects of parapets on low

buildings: Part 1. Basic aerodynamics and local loads. J Wind Eng

Ind Aerodyn 2005;93:81741.

[17] Pindado S, Meseguer J. Wind tunnel study on the inuence of

different parapets on the roof pressure distribution of low-rise

buildings. J Wind Eng Ind Aerodyn 2003;91:11339.Regenerating Towns). Targeted project under the RUE (Rational

Use of Energy) in the building sector; THERMIE programme, 4thno allowance was made for degradation of its performancein certain wind directions. This gure is still only about halfthe value routinely achieved in Scottish onshore wind farms,

Urban wind energy conversion: The potential of ducted turbinesIntroductionThe ducted wind turbineMathematical modelCalibration of mathematical modelPressure coefficients on buildingsIdeas on exploitationEnergy captureConclusionsReferences