Low-cost mounting arrangements for building-integrated wind turbines

Low Cost Mounting Arrangements for Building Integrated Wind Turbines

1 3/04/2013

Low Cost Mounting Arrangements for Building Integrated Wind

Turbines

David Udell1, David Infield

2 and Simon Watson

3

1PDL Solutions (Europe), 1 Tanners Yard, Hexham, NE46 3NY, UK

2University of Strathclyde, Glasgow, G1 1XW, UK

3Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK

Abstract: Micro-generation is being widely promoted as a way for householders in the UK and

elsewhere to take part in ‘the Green Revolution’. Building integrated wind turbines (BIWTs)

provide a way to do this, enabling people to reduce their contribution to the problems of both

climate change and decreasing fossil fuel availability. Although energy yields from BIWTs for

many householders have been shown to be low, there are still situations where such turbines can

make a useful contribution to electricity generation, e.g. in windier areas and for isolated detached

buildings. The standards for the installation of BIWTs are still being developed including those for

the safe mounting of turbines on domestic buildings. This paper investigates the current trend for

mounting small wind turbines on the walls of domestic premises and compares this with an

approach which uses roof timbers. It identifies the main characteristics of building construction

which affect the integrity of such installations. European and British Standards have been used to

calculate wind and gravitational loads. Finite element models are used to derive working stresses

and hence some basic principles of good design. The likely costs of wall and roof mounting are then

compared. Installation and health and safety issues are also examined briefly.

Keywords: Building Integrated Wind Turbine, Wind loads, Finite element modelling


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1. Introduction

Micro-generation is being promoted by politicians, media and commercial organisations alike as a

way for householders in the UK and elsewhere to take part in ‘the Green Revolution’. Building

integrated wind turbines (BIWTs) provide a way to do this, enabling people to reduce their

contribution to the problems of both climate change and decreasing fossil fuel availability. Recent

trials of BIWTs have indicated that energy yields have been disappointing [1] and simulations of

expected yields in urban areas have predicted low capacity factors [2]. Nonetheless, there are

situations where such turbines have made a reasonable contribution to on-site electricity generation,

e.g. on isolated detached buildings in high wind speed areas and on high-rise buildings. A number

of commercial organisations in the UK already sell small wind turbines designed for wall mounting.

These installations are best suited to gable end attachment although this approach is not universally

available for domestic housing. There are concerns that not enough has been done to assess the

structural integrity of such installations. This paper aims to address some of these concerns and to

examine alternative mounting arrangements. In particular, it looks at using the timbers of pitched

roofs. Horizontal axis machines rated between 400W and 1.5kW are considered in the context of

UK wind conditions.

2. Methodology

The approach adopted here was firstly to understand the loads which buildings and small wind

turbines are normally subject to. A datum building geometry was defined and has been used

throughout the work; in addition, the effects of changing some of the geometric parameters have

been evaluated. Then an understanding of material capabilities was developed in order to assess

how close to the limits building stresses are with and without a turbine attached. And finally, finite

element (FE) analysis has been used to assess different mounting arrangements and to set practical

limits on wind turbine size and mounting position.

https://www.researchgate.net/publication/235746988_Estimating_the_potential_yield_of_small_building-mounted_wind_turbines?el=1_x_8&enrichId=rgreq-8fd9228d-62ba-4155-97e8-ace18b3b0eb7&enrichSource=Y292ZXJQYWdlOzIyOTg5MjQxMTtBUzo5OTU5NDgzMDYxNDUzMEAxNDAwNzU2NjY2Mzcz


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2.1 Typical Building Loads

A selection of European and British Standards were used to calculate the main loads:

2.1.1 Wind Loads

BS 6399-2 [3] describes the methods for calculating wind loads on buildings and EN 61400-2 [4]

defines the wind speed that should be used for the design of small wind turbines. It states an

extreme wind speed of 35 m/s should be used but combines this with a gust factor of 1.4 to give a

design wind speed of 49 m/s. By selecting particular parameters it is also possible to calculate a

speed of 49 m/s using [3] which is desirable in that it gives consistency between the two Standards.

The value of 49 m/s will be referred to as design speed 1. However, this wind speed, according to

[3] can only occur in exposed coastal regions of the UK where there are no upstream obstructions

and where in addition the local topography causes speed-up effects. This is not typical of an urban

environment in the UK or elsewhere in Europe so a second design speed has also been considered.

This has been taken as an urban location in Preston, Lancashire, UK. The speed as calculated in [3]

is dependent on the height of the building. For the geometry defined in section 2.2 for the datum

building, the resulting wind speed is 26.4 m/s. If, in addition the wind turbine hub is assumed to be

2m above the ridge of the roof, the wind speed increases to 28.6 m/s. This is taken to be design

speed 2. Both of these wind speeds are consistent with a 50 year return period.

Both these design wind speeds exceed those at which most small wind turbines cut out, these being

typically around 15 m/s. For design speed 1, this means that the highest thrust experienced by the

turbine is when it is in its parked position. For the 1.5 kW and 400W machines, with estimated

swept areas of 6 m2 and 1.9 m

2 respectively, the loads were estimated to be 1600N and 600N

respectively. These areas were estimated from a survey of small wind turbine swept areas versus

thrust conducted by the New and Renewable Energy Centre (NaREC) in the UK. However, the


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thrust calculated for a machine operating at a wind speed of 15 m/s exceeds that for a parked

machine at design speed 2. For the large and small wind turbines, the peak loads were thus

estimated to be 800N and 300N respectively. These loads were used for the Preston location. Drag

loads due to the support pole and attachment structure were also estimated using [4] and included in

the FE modelling. Note that the thrust on the rotor, Trotor, was calculated using Equation 1:

Trotor = ½ ρ A CT U2 Eq. 1

where A is the area of the rotor, U is the wind speed, ρ is the air density and CT is the thrust

coefficient. Trotor was calculated for two cases: 1) parked, where CT was assumed to be 0.18 for a

large machine and 0.21 for a small machine; and 2) operating, where CT was assumed to be 1.0 for

both a large and small machine.

The thrust on the pole, Tpole, was calculated assuming that the pole was cylindrical with a drag

coefficient, CD, of 1.3, as specified in [4], and using Equation 2:

Tpole = ½ ρ Afrontal CD U2 Eq. 2

where Afrontal is the frontal area of the pole given by the product of the pole diameter and pole

height.

Wind loads perpendicular to the gable end and perpendicular to the non-gable wall were calculated

for design speeds 1 and 2 (see Fig.4 for definitions of gable end and non-gable wall). The pressures

were applied to the whole area of the walls even though, in an urban environment, this may not be

realistic due to upstream obstructions. This approach ensures the calculations remain conservative.


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For a detached house, [3] was used to calculate the loads on the walls parallel to the wind direction.

These side loads on the building were shown to have a minimal effect on the stresses in the walls

facing the wind so were not considered further. Wind loads on the lee side of the building are less

than those on the windward side so the former were not considered further.

For masonry structures designed using BS 5628-1 [5] a partial safety factor (PSF), for loads, of 1.4

should be applied. BS 1995-1 [6] gives the main calculations for designing timber structures but

does not contain a PSF for loads. However, a value of 1.4 is given in BS 1990 [7], matching the

figure for masonry structures.

The focus thus far has been on extreme wind speeds. However, fatigue and gyroscopic loads also

need to be considered. Average urban wind speeds in the UK, at a height of 10m above ground, are

estimated to be less than 5.5 m/s [8] or even less than 4.0 m/s [9]. Because the thrust loads are

proportional to the square of the wind speed, the turbine load at 6 m/s is only 5% of what it is at

28.6 m/s. For a more conservative approach [4] recommends that cycling loads should be estimated

on the basis of the wind speed varying between 0.5 and 1.5 times rated speed. Since rated speeds

are typically up to 12 m/s this is equivalent to varying the speed between 6 m/s and 18 m/s. Even at

18 m/s the thrust is less than 40% of what it is at 28.6 m/s. Providing the material fatigue

capabilities exceed 40% of their ultimate capabilities, fatigue should not be a major influence on the

design, see section 2.4. The gyroscopic yaw moment was shown, using [4], to cause reactions an

order of magnitude lower than those caused by the thrust loading.

2.1.2 Dead Loads and Imposed Loads

BS 6399-1 [10] describes the calculation of roof, floor and ceiling loads. The main concern in

buildings is the avoidance of tensile stresses because these are not well resisted by mortar joints, see

https://www.researchgate.net/publication/30415094_The_Feasibility_of_Building-MountedIntegrated_Wind_Turbines_BUWTs_Achieving_their_potential_for_carbon_emission_reductions_Final_Report_of_Carbon_Trust_Contract_2002-07-028-1-6?el=1_x_8&enrichId=rgreq-8fd9228d-62ba-4155-97e8-ace18b3b0eb7&enrichSource=Y292ZXJQYWdlOzIyOTg5MjQxMTtBUzo5OTU5NDgzMDYxNDUzMEAxNDAwNzU2NjY2Mzcz


6 3/04/2013

section 2.3. Wind loads tend to cause tensile bending stresses at the bottom of the wall which may

not be overcome by the compressive stresses due to gravity. Wind turbines induce localised tensile

stresses around their support attachments.

For wall mounted turbines, in order to be conservative, floor, roof and ceiling loadings were

calculated at their minimum and snow loads were assumed to be zero. However, for roof mounted

turbines, larger ceiling and roof loads were considered, as this gives the more conservative case. In

designing a timber roof structure BS5268-3 [11] recommends combining the wind load calculated

in section 2.1.1 with a ceiling load (for a loft used for storage only) of 0.55 kN/m2 and half of the

maximum imposed load calculated in BS 6399-3 [12], which accounts for snow loads.

2.2 Datum Building Geometry

A small building with internal dimensions 5m x 5m x 5m, excluding the roof has been selected

since it is the largest practical size where a close couple roof (i.e. one that does not require purlins)

can be made using readily available timbers [13], and hence the timber stresses are relatively high.

The foundations were modelled by extending the walls to 0.5m below ground level. To keep the

model simple no windows or doors were included. This is not unreasonable as apertures are

generally remote from the highly stressed regions.

Each wall used a cavity construction with two leaves 100 mm thick, separated by a 50 mm gap.

These dimensions were chosen because of their common use [13]. The leaves are assumed to be

joined by wall ties spaced as recommended in [5] i.e. 2.5 ties per square metre.


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Using guidance from [13] the ceiling joists and rafters were spaced 0.5m apart and measured 100

mm deep by 50 mm thick. The ceiling joists were joined by a binder of section 75 mm x 50 mm

which was joined to a ridge board of section 175 mm x 32 mm by hangers of section 50 mm x 38

mm, see Fig. 1. Note the noggings (green), which attach the end rafters to the gable ends via steel

straps [12] and the wind turbine support (cyan).

This roof model represents the style of construction used before trussed rafters became the norm.

Most of the analysis work was carried out using this kind of roof but trussed rafter construction is

considered in section 3.2.3.

In common with most domestic buildings [14], roof loads are transmitted to the inner leaf of the

wall cavity only. This makes the outer leaf more susceptible to tensile stresses so it is tied to the

roof structure to minimise this. A roof pitch angle of 30o was chosen as this is typical of UK

construction.

2.3 Finite Element Meshing Detail

The full house FE mesh used 20 node brick elements of side length 100 mm. This was sufficient to

assess the bulk effects of wind and gravity. Loads and displacements from this model were applied

to a more finely meshed sub-model which was used to assess the local stresses due to the thrust and

weight of the wind turbine around the attachment points. Bolt fixing loads were applied at the mid

height of the bricks causing localised peak stresses there. However, the region of interest is the

mortar line where the greatest material weakness exists. This location was two elements away from

the point load so that stresses should be accurately predicted there. In assessing the stress around a

particular attachment point, an average value along the length of the brick (215mm) was calculated,

before comparing against the known material limits, see Fig. 2.


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2.4 Material Properties

Many kinds of brick, mortar and timber are used in the construction of buildings. This is further

complicated by their often highly variable densities, stiffnesses and strengths. In this work, typical

densities and stiffnesses and minimum reasonable strengths were assumed.

2.4.1 Wall Materials

Wall and roof material densities were obtained from either BS 648 [15] or Illston and Domone [16].

The compressive stiffness of walling materials is well documented but it is the flexural stiffness

which is the main concern here. In [16], the modulus of elasticity for such materials is estimated to

be 1 to 2 GPa so this range of values was investigated. The flexural strength of masonry made with

clay bricks with medium water absorption and type (iii) mortar is given in [5] as 0.4 MPa. However,

[5] recommends applying a PSF for material strength of 3, giving a design limit of 0.13 MPa.

Masonry fatigue stress limits are not given in [5]. However, concrete seems to perform well in

fatigue, at least up to 106 cycles. A survey in [17] shows a number of data sets where the 10

6

strength for plain concrete is around 65% of its ultimate strength whilst [18] records a 2 x 106

strength of 58% of the same. Hence for masonry materials, following the argument of section 2.1.1,

fatigue loading from a wind turbine is expected to be significantly less severe than that caused by

the extreme once in 50 years wind.

2.4.2 Roof and Floor Materials

The roof timbers were considered to be made from UK grown Douglas Fir as this is the weakest of

the wood types given in BS EN 338 [19]. The strength grade is C14 and its density is 350 kg/m3.

The characteristic bending strength for short term loading of a 100 mm deep beam, parallel to the

grain, is 16.7 MPa. A PSF of 1.3 was applied to bring the limit to 12.8 MPa. The equivalent


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strength across the grain is only around 0.65 MPa so any significant loading in this direction should

be avoided.

Timber fatigue stress limits are not given in [19]. However, timber also performs well in fatigue,

with a 107 strength of 60% of the ultimate strength recorded in [16]. This is largely species

independent. Hence for timber too, fatigue loading from a wind turbine is a less severe load case

than that due to the 50 year gust.

The ground floor is normally supported by sleeper walls built up from the foundations and does not

interface with the walls. The first floor was modelled with 225 x 50mm timber joists at 0.5m

centres.

2.5 Wind Turbine Installations

To begin with, the stresses in buildings without wind turbines were investigated. Vertical tensile

stresses were found to be the main issue because of mortar’s weakness under this regime. Next,

loads deriving from typical wind turbine installations were assessed. As already mentioned, the

initial height chosen for the hub was 2m above the ridge of the roof. This is slightly arbitrary as

there are many opinions on the subject. Renewable Devices [20] advise a minimum roof to blade

height of 0.5m for their 1.5 kW rated machine whilst [8] recommends 10m, see section 4.

For a hub height of 2m above the ridge, the attachment point reactions due to the support pole drag

loads represented around 20% of those caused by the 1.5kW turbine itself. It was also estimated that

the weight of the support pole and brackets would be around 1.5 to 2 times the weight of the

turbine. Obviously much depends on the chosen materials but for this research it was assumed that

the support poles were made of hollow steel tubing of diameters as follows: 100 mm and 62.5 mm


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for the non-gable mounted large and small turbines respectively and 80 mm and 50 mm for the

gable mounted large and small turbines respectively.

2.5.1 Wall Mounting

Wall mounting is the current state of the art but is most suited to gable end attachment as the

support pole length can be minimised. Non-gable wall attachment was also evaluated. There are

currently three main types of attachment. These are depicted in Fig. 3 and described below.

a) Poles supported using two or three tripods. The feet of a tripod are fixed to the wall whilst its

head supports the pole. One tripod supports the lowest point on the pole whilst a second is located

as high up the wall as possible to minimise the length of unsupported pole. Other tripods can be

added in between. The ‘height’ of the tripod (in the horizontal plane) is set such that the support

pole avoids the roof overhang. The effects of varying the distances between the pole and the wall

and the distance between the tripods were considered. Two tripod arrangements can be seen in Fig.

3 and Fig. 4 showing a typical installation.

b) Poles supported using three or four ‘C’ shaped brackets attached directly to the wall such that the

pole breaks through the roof overhang. In Fig. 3 these brackets are represented by short cross

beams.

c) Poles supported at two positions using flat ‘H’ shaped structures which are close to the wall.

Once again, the support pole has to break through the roof overhang.

An important aspect in the mounting of wind turbines on buildings is the minimisation of structure

borne vibration. The configurations detailed above are those most commonly used and vibration

isolation devices such as rubber strips are often used to minimise the transmission of vibrations to a


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building. The majority of domestic installations have not experienced major problems with structure

borne vibrations using the above types of mounting arrangements but there was noted to be a

vibration problem for 29% of the installations which formed part of the UK Energy Savings Trust’s

domestic scale wind trial [21]. There has been very little work to date in this area and a proper study

of structure borne vibration is outside the scope of this paper. This paper does not deal with the

possible transmission of vibrations to a domestic building, but it is noted that measures must be

taken to minimise this potential problem. In this paper, the emphasis is on static loading in order to

make the analysis tractable.

2.5.2 Roof Mounting

Mounting from the ridge board and an additional horizontal member supported by two rafters is

discussed in detail. Fig. 1 shows the mounting close to the gable wall but the same mounting was

also considered for a mid roof position because gable end access may be restricted by a chimney or

television aerial.

3. Results

3.1 Wall Mounting

Unless otherwise stated, ‘stress’ refers to the vertical tensile stress. The results for the wall

mounting arrangements are summarised in Table 1, at the end of section 3.1.2.

For design speed 2 with the wind load applied to the non-gable wall the peak stress in the lower

wall was 0.165 MPa after gravity effects were accounted for. For the wind load applied to the gable

wall the equivalent stress was 0.235 MPa, as shown in Fig. 4. These exceed the limit given in

section 2.4.1 suggesting that the PSF for material strength given in [5] is rather pessimistic and


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perhaps not strictly adhered to in practice. Therefore, for the purposes of this report, non-gable and

gable limits of 0.165 MPa and 0.235 MPa were set, respectively, for design speed 2. Providing the

stresses in the wall due to the wind turbine pole support feet do not exceed these values then the

wall will not experience stresses higher than it would if there were no wind turbine.

Note that the stress due to gravity at the top of the wall is virtually zero whilst the stress in the lower

wall due to the turbine supports is virtually zero. Therefore the stresses induced by the turbine are

effectively independent of what is going on at the bottom of the wall. This means that providing the

local stresses due to the pole supports are less than those in the bottom of the wall (under the same

wind conditions), the former must be acceptable. The limits of 0.165 MPa and 0.235 MPa can be

said to be relative because they depend on what the peak stress in the wall is without the wind

turbine present.

For design speed 1, tensile stresses due to the wind increase in proportion to the wind speed

squared. The tensile stresses due to the wind are offset by the effect of gravity which is clearly

greater at the bottom of the wall than it is at the top. Relative to design speed 2 the effect of gravity

is reduced which means that wall local stresses due to the pole support feet increase by a smaller

amount than stresses in the lower wall. Therefore, because relative limits are being used for the wall

mounting studies, design speed 2 gives similar but slightly more conservative limits than design

speed 1 so only results for the former are detailed.

At this point it is worth noting the importance of wall ties and floor bracing. If the first floor were

removed the stress at the bottom of the non-gable wall would increase from 0.165 MPa to 0.20 MPa

and if there were no wall ties it would be 0.42 MPa. These results are for equal leaf stiffnesses of 2

GPa but if the outer leaf stiffness was half that of the inner leaf the stress at the bottom of the wall


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would decrease by around 30%. These results emphasise the importance of obtaining a detailed

understanding of the building construction in assessing its suitability for a wind turbine installation.

3.1.1 Non-Gable Wall Attachment

The first option considered was the dual tripod with the supports 750 mm apart and with the support

pole 450 mm from the wall. For the large turbine with the wind acting perpendicular to the outside

surface of the wall, the average mortar line stress across the worst brick was 0.58 MPa which is well

in excess of the limit and cracking is predicted at four of the six attachment points. With the wind

applied parallel to the wall, the worst brick stress was 0.68 MPa with cracking at three other

attachment points also likely, see Fig. 5.

When the distance of the support pole to the wall was decreased to 250 mm, the peak stress

increased by about 5%. When three tripods were used instead of two, spaced at 600 mm, the worst

brick stresses for perpendicular and parallel winds were 0.52 MPa and 0.57 MPa respectively; not

much lower than with twin tripods.

For the small (400 Watt) turbine installation, with two supports, the worst brick stresses reduced to

0.16 MPa and 0.31 MPa for the perpendicular and parallel winds respectively. For the latter case,

four attachment points failed the criterion.

Only the tripod mounting systems have been considered here because, as the next section shows,

the other systems cause higher stresses. This means that none of the mounting systems provides an

acceptable solution for attachment to the non-gable wall.


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3.1.2 Gable Wall Attachment

3.1.2.1 Tripod Type Attachment

For the large (1.5 kW) turbine supported by dual tripods 750 mm apart and with the support pole

450 mm from the wall the worst brick stresses were 0.34 MPa and 0.36 MPa for the perpendicular

and parallel winds respectively.

When the distance between the tripods was increased to 1200 mm the worst brick stresses were 0.21

MPa and 0.32 MPa for the perpendicular and parallel winds respectively, with cracking predicted at

two points. For the small turbine installation, the worst brick stresses reduced to 0.11 MPa and 0.14

MPa for the perpendicular and parallel winds respectively.

Using the widely spaced supports and a hub height of 5m above the ridge, loads from the small

machine gave worst brick stresses of 0.37 MPa and 0.46 MPa for the perpendicular and parallel

winds respectively. It is estimated that the small turbine installation would be acceptable at hub

heights up to 3.2m above the ridge.

3.1.2.2 Close To Wall ‘C’ Bracket Type Attachment

For the large turbine supported by four brackets 0.9m apart and with the turbine hub height 2.6m

above the top bracket, the worst brick stresses were 0.84 MPa and 0.85 MPa for the perpendicular


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and parallel winds respectively. For the small turbine, these stresses reduced to 0.16 MPa and 0.36

MPa respectively, with two attachment points predicted to crack in this latter case.

3.1.2.3 Close To Wall ‘H’ Type Attachment

For the large turbine supported by brackets 1.3m apart and with the turbine hub height 2.6m above

the top bracket, the worst brick stresses were 0.82 MPa and 0.59 MPa for the perpendicular and

parallel winds respectively. For the small turbine installation, both these stresses reduced to 0.23

MPa, this being marginally acceptable from a structural point of view.

Turbine

Power

(kW)

Height

Above

Ridge (m)

Description

Support

Spacing

(m)

Wall Stress for

Perpdr Wind

(MPa)

Stress for

Parallel Wind

(MPa)

Acceptable?

1.5 2 Tripods x 2 0.75 Non-

gable

0.58 0.68 No


gable

0.52 0.57 No


gable

0.16 0.31 No

1.5 2 Tripods x 2 0.75 Gable 0.34 0.36 No


0.4 2 Tripods x 2 1.2 Gable 0.11 0.14 Yes


1.5 2 C Brakt x 4 0.9 Gable 0.84 0.85 No

0.4 2 C Brakt x 4 0.9 Gable 0.16 0.36 No

1.5 2 H Brakt x 2 1.3 Gable 0.82 0.59 No


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0.4 2 H Brakt x 2 1.3 Gable 0.23 0.23 Yes

Table 1 – Wall Mounting assessments for Design Speed 2

3.2 Roof Timber Attachment

For a roof without a turbine, the stresses due to wind and gravity were found to be well below the

limits given in 2.4.2. Absolute stress limits could therefore be used here rather than relative stress

limits which had to be used for the brick wall mounting (see section 3.1). The results for the roof

timber mounting arrangements are summarised in Table 2, at the end of section 3.2.3.

Mounting from the ridge board and ceiling joists might be desirable in that it minimises the amount

of support required, and thus the potential cost. However, any vibration would be transmitted

directly to the ceiling which would be unacceptable to the occupants, particularly at night. All

options therefore have the bottom of the pole supported by an additional horizontal beam, see Fig.

1, itself supported by rafters.

3.2.1 Assessment at Design Speed 2

With the wind approaching the non-gable end, the arrangement in Fig. 1, with the large turbine,

gave a peak bending stress of 5.8 MPa in the ridge board, see Fig. 6. This was the average stress

across the ridge board and includes both an allowance for the bolt hole and the PSF for loads of 1.4.

The final stress was well below the 12.8 MPa target.

For a turbine mounted at the roof mid-span, the peak stress for the non-gable wind increased by

around 10% due to the lack of support from the gable end wall.


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Repeating the assessment with the small turbine mounted 5m above the ridge line gave a peak stress

of 7.9 MPa. Note that 30% of this stress was due to the aerodynamic drag on the support pole.

With the wind approaching the gable end, the peak stress occurred in the rafters at the attachment

points, see Fig 7. When the bolt hole and the PSF of 1.4 were accounted for, a realistic peak stress

of 4.4 MPa was calculated for the large machine mounted 2m above the ridge line.

Repeating the assessment with the small turbine mounted 5m above the ridge gave a peak stress of

6.1 MPa. This is approximate only as much depends on the diameter of the support pole.

The stresses have so far been assessed against the limit parallel to the grain. This is reasonable at

the ridge board but the rafters are at such an angle that cross grain stresses become more important

and these would increase as the pitch angle of the roof increased. It is therefore recommended that

some rig testing is carried out to establish accurate limits. Alternatively, a significant reduction in

rafter stresses can be obtained by adding an additional angled brace from the bottom of the support

pole to a position on the ridge board between the 3rd

and 4th

rafters. This ensures that the non-gable

wind is the limiting case. This additional brace also markedly decreases the deflection of the wind

turbine and thereby keeps the axis of blade rotation closer to the horizontal giving better yawing

behaviour.

For the same heights above the ridge line, the roof is capable of supporting turbines around twice

the sizes of the ones considered above. It is estimated that the large machine could be mounted up

to 4.2 m above the ridge line with acceptable stresses. For the pole, this would necessitate the use of

either a stronger material or a larger diameter: the former increasing cost and the latter drag.


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FE models were created with both fully fixed and pinned joints at the rafter to ridge board and rafter

to wall plate connections. The models with pinned connections gave stresses about 19% higher. As

the real joints are considered to be somewhere between the two, average stresses are given here.

3.2.2 Assessment at Design Speed 1

Design speed 1 gives stresses just over twice those for design speed 2. This means that the large

machine at 2m and the small machine at 5m produce peak stresses of 12 MPa and 17 MPa

respectively, for wind approaching the non-gable end. So, because relative stresses were used for

the brick wall limits, the roof capability could be said to be only a little better than for the wall.

However, this only applies to extremely exposed and windy locations where dwellings may well

suffer damage on a once in 50 year basis. If so, this would invalidate the use of relative stress limits.

In any case, design speed 1 is not realistic for urban locations so the superiority of roof mounting is

clear.

3.2.3 Modern Trussed Rafter Roofs

Unfortunately, because modern roofs do not usually have a ridge board, the arrangement in Fig. 1 is

not possible. It is not even possible to add supports between trusses, at ridge level, because the

timber close to the ridge level is hidden by the nail plates used to join the various parts of the truss,

Fig 8. The trusses are attached to each other using braces at ceiling height and below the ridge. The

braces are typically fairly thin at 100 mm x 25 mm, but butt up against the brick walls where

available, for extra support.

None of these bracing elements provides the necessary vertical face and, if they were used (in

conjunction with wedges to create a vertical face), significant cross grain stresses could be induced.

The trussed rafter alone would therefore need to provide all of the support. Unfortunately, trussed


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rafters tend to have smaller sections. For a roof of the size considered here, sections 72 mm x 35

mm are typical. The bending and shear stresses therefore tend to be greater than in the traditional

roof and the effect of the bolt holes is more marked.

However, this is partly offset by the strength of the timber. The wood used for the truss is grade

TR26 which is similar to grade C27 and much stronger than C14, in particular the bending stress

limit along the grain is 24.7 MPa rather than 12.8 MPa.

For the large machine mounted at 2m from the ridge, wind approaching the gable end and design

speed 2, the arrangement in Fig. 8 gives a peak stress over 40 MPa when all effects are accounted

for (18.5 MPa for turbine loading alone, excluding the effect of the bolt hole). Including an

additional angled brace drops this peak stress to 34 MPa and moves the peak stress position to the

apex of the truss. This suggests that for a turbine mounted 2m above the roof line the size limit is

around 2/3 of the large machine i.e. a swept area of 4m2.

Roof

Description

Turbine

Power (kW)

Height

Above

Ridge (m)

Wind

Direction

Approx Peak

Stress (MPa)

Design

Speed

Acceptable?

Traditional 1.5 2 Non-gable 5.8 1 Yes

Traditional 0.4 5 Non-gable 7.9 1 Yes

Traditional 1.5 2 Gable 4.4 1 Yes

Traditional 0.4 5 Gable 6.1 1 Yes

Traditional 1.5 2 Non-gable 12 2 Yes

Traditional 0.4 5 Non-gable 17 2 No

Modern 1.5 2 Gable 34 1 No

Table 2 – Roof Mounting assessments for Design Speeds 1 & 2


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4. Discussion

Computational fluid dynamics (CFD) modelling from Renewable Devices, cited in [20], predicts

the wind speed over a 4m pitched roof building where the pitch angle is around 15o and the wind

approaches perpendicular to the building. This shows a maximum speed-up effect of approx. 2 m/s

but also high wind shear for 2m above the ridge height. However, it should be noted from [1] that

this speed-up effect is reduced if the building is within an urban area as opposed to an isolated

location. High wind shear generates high cyclic stresses which reduce component life and this is in

addition to turbulence effects. As the pitch of the roof increases, wind shear effects and turbulence

are likely to increase but more work is required to quantify these effects. All that can be said here is

that to maximise energy capture and turbine life, turbines should be mounted well above the ridge.

Manufacturers recommend that wind turbines be mounted greater than 1m above the roof line and

the height of 2m used in this paper is commonly used. The energy that can be captured will increase

as the turbine is mounted further above the ridgeline, but there are often planning restrictions to

limit the mounting height to minimise visual impact.

For brickwork, the PSFs of 3 for material strength and 1.4 for load give a total safety factor of 4.2

which is rather conservative. In aerospace applications, where the loads and material properties are

well understood, equivalent factors might be 1.2 and 1.1 giving a total of just 1.3. It would therefore

be prudent, given the large potential energy resource, for the wind and construction industries to

undertake further testing so that more accurate limits can be established.

4.1 Comparison of Mounting Arrangements

Mounting the turbine on the non-gable wall did not produce a workable solution. The peak stress

was almost twice that for the equivalent gable end mounting. Gable wall mounting only produced


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solutions for the small 400 Watt turbine. The best arrangement was the wide spaced dual tripod

gable mounting.

It would be possible to reduce the mortar line stresses by mounting the turbine closer to the corner

of the building rather than at the mid-span of the wall but then windows and drain pipes would have

to be accounted for. This would also require a longer pole to maintain sufficient height above the

ridge line for the purposes of maintaining energy yield.

It would be worth giving some further consideration to the design wind speeds used. For a house, a

50 year return period seems entirely reasonable as excessive stresses could destabilise an entire

wall. However, for a wind turbine, only mortar line cracking around a few bricks may result,

requiring some re-pointing after a heavy storm. All the arrangements considered here had multiple

attachment points which should prevent the mounting arrangement failing completely. It may then

be considered acceptable to use a shorter return period of, say, 20 years. [3] then shows that the

basic wind speed for Preston would be reduced by 17% in this case which would reduce the loads

by about one third. However, it would be difficult to build this into a Standard because of the

multitude of possible mounting arrangements which would have to be considered.

It is worth noting that the wall integrity test carried out by Windsave (www.windsave.com –

December 2007 [22]) using a point load of 4kN perpendicular to the wall, is a good test for

assessing the capability of a wall as it introduces an average vertical tensile stress of 0.61 MPa

across a brick. This exceeds the stresses for gable end tripod type installations (see section 3.1) for

design speed 2.

Unless cleverer ways of wall mounting are developed it seems that there is more potential in roof

mounting. It may be slightly more expensive in the first place, see section 5, but it offers both the

http://www.windsave.com/


22 3/04/2013

potential of capturing more energy and shorter payback times. For design speed 2 the capability of

the conventional roof was estimated to exceed that of the wall by a factor of around 4.

The capability of the trussed rafter mounting was much less than that of the conventional roof and

only a little better than the best of the wall mountings. However, because the trussed rafter is a

single self contained unit it would be possible to design a special truss capable of withstanding the

turbine loads and to brace this to the standard trusses in order to spread the load. As the UK

government intends to build 3 million new homes over the next 20 years, one could envisage a

relatively cheap mass produced trussed rafter designed to mount BIWTs.

4.2 Health and Safety and Installation Issues

Wall mounted turbines are reasonably well insulated from the internal space of a house. Their only

connection to the inside wall is via wall ties and any vibration in these may be damped, to some

extent, by cavity wall insulation. Roof mounted machines have a more direct connection with the

inner wall via the rafters and wall plates. However, between the turbine and the inner skin of the

wall there are a large number of timber-to-timber and timber to masonry joints which will provide

damping. Wood also has more internal damping capability than most other construction materials

because of its multicellular construction and this limits the transmission of vibration energy. [23]

quotes noise reduction coefficients for brick as 0.02 to 0.05 and wood as 0.11 to 0.28. Some kind of

anti-vibration mounting is still likely to be required but this could be of a similar specification for

roof mounted and wall mounted machines alike.

A disadvantage of the roof mounted machine is that installation is likely to be more difficult,

certainly for retrofits. Internal and external access to the roof will be required and some form of

scaffolding to do the necessary roof modifications. Clearly the wall mounted machines do not break


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through the external skin of the house and it maybe possible to install, at least the small machine,

without the use of scaffolding.

5. Wall and Roof Mounting Relative Costs

Most of the support structure for a roof mounted machine is internal so, compared to a wall

mounting, appearance and corrosion are less important issues. For the supports, much use could be

made of conventional scaffolding. Besides being relatively cheap and readily available, the

mechanical integrity of scaffolding tubes and couplings is covered by BS1139 [24] and [25].

Overall costs for roof and wall mounting are expected to be similar.

The main difference will be in the roof modifications which involve the cost of replacing a ridge tile

with a ridge vent [26]. This is likely to add only 5-10% to the total cost of installation of a roof-

mounted turbine compared to a wall-mounted device.

6. Conclusions

From the research presented a number of conclusions can be drawn:

1. Both the 400W and the 1.5kW wall-mounted turbines, mounted 2m above the ridge line,

cause mortar stresses in excess of those suggested by BS5628 [5] but so do wind alone loads

calculated by BS6399 [3].

2. Of the various wall mountings considered, the dual widely spaced tripod arrangement was

the best but gave an acceptable solution for the small (1.9 m2 swept area) turbine only. It

could support the small machine to a maximum height of 3.2m above the ridge line.


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3. A traditional close couple roof was shown to be four times as capable of absorbing the loads

as the brick wall. It could support the large (6m2 swept area) turbine at 4.2m above the ridge.

4. A typical trussed rafter roof was capable of supporting a machine with a 4m2 swept area 2m

above the ridge. However, it would be possible to design a special truss capable of

supporting larger machines and fix this to standard trusses in the usual way.

5. More work is needed to understand the wind strength above domestic housing but mounting

from roof timbers is both viable and, at a small extra initial cost, likely to provide solutions

with more structural integrity and shorter payback times than those using brick walls.

7. Acknowledgements

The author would like to express his gratitude to Mr A. Wilson of NaREC for his helpful advice.

Thanks also to Mr J. Gardiner of J G Design Services for his development of the FE code

throughout the project.

8. References

[1] Encraft 2009, Warwick Wind Trials Final Report,

http://www.warwickwindtrials.org.uk/resources/Interim+Report+August+2008.pdf, last accessed

12/2/09.

[2] M. A. Heath, J. D. Walshe and S. J. Watson (2007). “Estimating the potential yield of small

building-mounted wind turbines”, Wind Energy 10 pp 271-287.

[3] BS 6399-2:1997, Loading for buildings – Part 2: Code of practice for wind loads.

[4] BS EN 16400-2:1996, Wind turbine generator systems – Part 2: Safety of small wind turbines.

http://www.warwickwindtrials.org.uk/resources/Interim+Report+August+2008.pdf


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[5] BS 5628-1:2005, Code of practice for the use of masonry – Part 1: Structural use of

unreinforced masonry.

[6] BS 1995-1-1:2004, Eurocode 5 – Design of timber structures.

[7] BS 1990:2002, Eurocode – Basis of structural design.

[8] N. Martin, “Can we harvest useful wind energy from the roofs of our buildings?”, Building for a

Future, Winter 05, pp. 46-52.

[9] A. Dutton, M. Halliday and M. Blanch, “The Feasibility of Building-Mounted/Integrated Wind

Turbines (BUWTs): Achieving their potential for carbon emission reductions”, Energy Research

Unit, CCLRC, May 2005, pp. 77-83.

[10] BS 6399-1:1996, Loading for buildings – Part 1: Code of practice for dead and imposed loads.

[11] BS 5268-3:2006, Structural use of timber – Part 3: Code of practice for trussed rafter roofs.

[12] BS 6399-3:1988, Loading for buildings – Part 3: Code of practice for imposed roof loads.

[13] J. S. Foster, Mitchell’s Structure and Fabric Part 1 6th

Edition, Longman, 2000, pp. 78-153.

[14] S. Emmitt and C. Gorse, Barry’s Introduction to Construction of Buildings, Blackwell, 2005,

p. 6-187.

[15] BS 624:1964, Schedule of Weights of building materials.

[16] J. M. Illston and P. L. J. Domone, Construction Materials Their Nature and Behaviour, Spon

Press, 2001, pp. 281-510.

[17] M. K. Lee and B. I. G. Barr, “An overview of the behaviour of plain and fibre reinforced

concrete”, Cement and Concrete Composites, Nov 2002, pp. 301 – 303.

[18] S. P. Singh, Y. Mohammadi and S. K. Madan, “Flexural fatigue strength of steel fibrous

concrete containing mixed steel fibres”, Journal of Zhejiang University, March 2006, p. 1332.

[19] BS EN 338:2003, Structural timber – Strength classes.

[20] Renewable Devices, “Swift Rooftop Wind Energy System: Technical and Planning

Information – Rev F1”, June 2005.


26 3/04/2013

[21] Energy Saving Trust 2009, Location, Location, Location. Domestic Small-Scale Wind Field

Trial Report, http://www.energysavingtrust.org.uk/cym/Global-Data/Publications/Location-

location-location-The-Energy-Saving-Trust-s-field-trial-report-on-domestic-wind-turbines, last

accessed 4/8/09.

[22] Wind Save 2007, Wall Strength Testing, http://windsave.com, last accessed December 2007.

[23] C. Hornbostel, Construction Materials: Types, Uses and Applications, Wiley-IEEE, 1991, pp.

9-10.

[24] BS1139-1.1:1990, Metal scaffolding – Part 1: Tubes – Section 1.1 Specification for steel tube.

[25] BS1139-2.2:1991, Metal scaffolding – Part 2: Couplers – Specification for couplers, fittings

and accessories for use in tubular scaffolding.

[26] BCIS, the Property Makeover Price Guide, RICS, 2007, p. 72.

http://www.energysavingtrust.org.uk/cym/Global-Data/Publications/Location-location-location-The-Energy-Saving-Trust-s-field-trial-report-on-domestic-wind-turbines

http://www.energysavingtrust.org.uk/cym/Global-Data/Publications/Location-location-location-The-Energy-Saving-Trust-s-field-trial-report-on-domestic-wind-turbines

http://windsave.com/


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Figure 1 - Roof Construction with Cladding Suppressed

Figure 2 - Fine Mesh Showing Brick and Mortar Outline

Rafter

Ceiling Joist

Binder

Hanger

Wind Turbine Pole

Ridge Board

Wall Plate,

(underside fixed to the inside skin of the non-gable wall)


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Figure 3 – Support Arrangements for Wall Mounting

Figure 4 – Stress Distribution Due to Wind and Gravity

4 off ‘C’, 2 off ‘H’, Dual Tripod, Triple Tripod

Brackets Brackets

1.2m 1.3m

0.9m

Gable End Wall

Non Gable Wall


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Figure 5 – Sub-model Stresses for Dual Tripod Mount Excluding Bulk Effects of Wind and Gravity

Figure 6 – Uncorrected Ridge Board Bending Stresses for Design Speed 2


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Figure 7 – Unmodified End Rafter Bending Stresses for Design Speed 2

Figure 8 – Modern Trussed Rafter Construction: Peak Stress Position Due to Turbine Load Only


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This is the pre-peer reviewed version of the following article: Udell D, Infield D

and Watson S (2010). ‘Low Cost Mounting Arrangements for Building

Integrated Wind Turbines’. Wind Energy 13 pp 657-669, which has been

published in final form at

http://onlinelibrary.wiley.com/doi/10.1002/we.386/abstract .

http://onlinelibrary.wiley.com/doi/10.1002/we.386/abstract

Low-cost mounting arrangements for building-integrated wind turbines

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