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For the purposes of this feature, we will look at building-integrated and co-located wind turbine installations, featuring grid-connected systems that are integrated with the host building's electrical infrastructure. Commercial wind farms, and wind generation for direct space heating or water heating, will not be addressed.

But first, a little background information. In recognition of the likely causes and effects of global climate change, the Kyoto protocol was signed by the UK and other nations in 1992, with a commitment to reduce the emission of greenhouse gases relative to 1990 as the base year. The initial focus is on CO2 and the goals set by the UK government are 20% emission reduction by 2010; 60% emission reduction by 2050 and real progress towards the 60% target by 2020.

Many studies and reports published since Kyoto have drawn the conclusion that energy conservation measures alone, although important, will not enable these targets to be met and it is therefore imperative to make increasing use of renewable energy sources. It is against this background that local planning authorities now require developers to show how a proportion (typically 10%) of a site's electricity or heat would be generated from renewables.

Back to basics

All renewable energy, except tidal and geothermal, is ultimately derived from the sun which, as we know, radiates a huge amount of energy to the Earth. This solar radiation causes uneven heating of the earth's atmosphere. Hot air above the equator, for example, rises some 6 km above the earth and then spreads north and south towards the poles.

As a result of the earth's rotation and widely varying geographical features, the wind has very complex distribution patterns. In the UK, wind is widely spread, but is strongest and most consistent along coasts and at higher altitudes in the north and west. With the production of wind maps, it is widely acknowledged that the UK has the best wind regime in Europe, but with little take-up of its energy-generating potential to date compared, for example, with Germany, Spain and Denmark.

How much energy can be extracted from the wind?

A long-established and fairly simple formula may be used to estimate the theoretical power yield for a wind turbine, as follows:

Pw = 0.5 x Cp x d x A x v3 where:

Pw = Wind power converted by the rotor (W)

Cp = Power coefficient (a measure of the rotor efficiency)

d = density of the air (kg/m3)

A = rotor swept area (m2)

v = wind speed (m/s)

The two most significant variables are the blade length, affecting the rotor swept area according to the square law (fr2) and the wind speed, which affects the power output according to the cube law, so that a doubling of wind speed can cause an eight-fold increase in power.

Table 1 (attached) illustrates the dramatic effect of these two variables, while every other factor in the equation is kept constant (this is a hypothetical proposition for various technical reasons).

It can be seen that large turbines placed in good wind will produce far more power than small turbines placed in dubious wind. This demonstrates why the generation performance of wind farms is technically viable, and partly why the worthwhile integration of turbines onto buildings generally presents significant challenges.

Feasibility considerations

Any proposal for significant investment to mount one or more wind turbines on or near a building should be subjected to a feasibility study of the following general issues and probably take account of other particular local limitations or benefits.

Davis Langdon Mott Green Wall Wind maps and databases can give an indication of average wind speed at a particular location, but this is unlikely to give adequate resolution for a reliable estimate of the energy potential in urban or suburban settings - especially if the proposed hub height is different from the standard database wind height. The power in wind blowing over an open area may be reduced dramatically when it encounters obstructions such as buildings. On the other hand, it may be possible to take advantage of wind ‘speed-up' as it passes over a roof ridge or similar feature.

It may be necessary to commission a monitoring study to establish the likely wind speed frequency distribution: ie the number of hours over the year that the wind blows at 1 m/s, 2 m/s, 3 m/s, 4 m/s and so on, during a period of 12 months or more. Unfortunately, such data can be rendered invalid by any further construction developments that may arise later and consequently ‘block the wind' or create unpredicted turbulence.

Costs and benefits. The project costs will be affected by the following ten factors: 1) wind monitoring to obtain certified data; 2) how readily the available wind resource can be converted; 3) the number and size of turbines; 4) the number and type of mounting structures required; 5) the maintenance arrangements put in place; 6) electrical engineering works, gaining grid and local connections; 7) civil engineering works for foundations and cable trenches; 8) project management; 9) feasibility studies; 10) preparation of environmental statement and planning application.

The project benefits will be assessed under four categories. The first of these is the green energy generated and CO2 saved - and wind turbines can be very effective in the right setting. Second is energy balance or ‘embodied energy' - again, energy payback by wind turbines can be readily demonstrated in the right setting. Green ‘credentials and publicity' for the client will also be reviewed, although these may be difficult to evaluate. Finally, financial payback is assessed, and this is demonstrated by an investment appraisal - once again, this depends on the project's setting. A very good wind resource is needed for turbines retro-fitted to buildings.

Safety. The head weight of a 15 kW turbine is in excess of 1000 kg and it is likely to be mounted at a height of at least 15 m, so careful consideration must be given to its support structure or mast and foundations, as well as its maintenance arrangements. The possibility of blade strikes by birds, bats, blown debris, vandalism, and balls if near schools or recreation areas, should not be overlooked.

The mounting arrangement on a building, for any turbine larger than ‘micro' is likely to present problems, especially if the wind is gusty or turbulent, because these conditions result in additional structural loadings.

Ultimately, machines can fail and there is an unknown probability that such failure will be catastrophic. This consideration raises to potential insurance issues for design teams (professional indemnity) and for building developers/owners (employers and public liability).

n Audible noise and vibration and visible flicker. In rural areas, strong wind tends to create noise by disturbing trees and other vegetation and this ‘masking effect' can be taken into account when assessing the impact of a wind turbine. This effect is unlikely in an urban location, therefore the quietest possible turbines will be required, especially if there are sensitive residences or offices nearby. Noise from small turbines can exceed 60 dB(A) in moderate wind gusts of 20 m/s and this may be significant for occupants nearby, especially if they have their windows open.

Similarly, turbulent and gusty urban wind tends to create additional vibration that must be assessed and mitigated to prevent excessive stress on mountings, fixings and foundations, and a possible nuisance to building occupants.

Rotating turbine blades or their flickering shadows may be distracting and regarded as a nuisance by anyone in a situation where they normally see them, or perceive them in their peripheral vision - and this should be taken into account when choosing a suitable site.

A pre-application consultation based on the feasibility study is recommended before getting very far into a design, though if a turbine supplier had been selected, they would be able to supply a great deal of useful information - especially with regard to successful reference installations.

Projects likely to have a significant impact will necessitate an Environmental Statement, explaining the environmental aspects assessed and what mitigation strategies have been adopted.

Market conditions and costs

The micro turbine market is well-served by a number of UK suppliers and specialist installers. The main market is residential, agricultural and light industrial. A few turbines have been fitted to commercial buildings, mainly rooftop installations and electrically integrated. Energy export is technically possible if the site load pattern is suitable, but otherwise the only cost benefit will be the value of each unit generated. Micro turbines may also installed on ‘folding masts' and simple foundations. Overall project costs, ignoring planning issues costs and project management, are indicated in Figure 3 (attached graphic).

Small turbines. The market for wind turbines in the 10 kW to 100 kW range is currently very ‘thin', there being very few suppliers having equipment for sale in this range, and apparently little demand. One reason for this could be that installations larger than, say, 15 kW rating, require the use of a crane which adds to the cost. Other factors could be larger foundations and taller masts, which may be more difficult in terms of planning. They are not generally suitable for mounting on an existing building, and no existing examples of complete architectural integration were revealed by the research for this article. No representative cost information is therefore presented, and each proposal will be subject to its own feasibility study.

Large turbines. There is considerable potential at suitable industrial or brownfield sites and a number of installations have been successfully completed. This type of site will often have good access, be remote from residential areas and have substantial electrical demand. In many circumstances it is no more difficult to obtain planning permission for a 50 m mast than it is for a 20 m one, but a larger, higher turbine will often get better wind above local obstructions.

Very few manufacturers offer turbines in the 100-300 kW range, but a market exists in secondhand machines from Europe and these are very much in demand for their potential to reduce capital cost and improve payback.

Complete building integration by architectural design

Research has been carried out over a number of years in order to evaluate the potential for exploiting the prevailing wind on a building concept and, in particular, how it can be augmented or enhanced. One such concept is shown on page 71 - the conceptual design for a twin tower building with three 35 m diameter turbines.

Another concept under active research is ducted turbines, envisaged for incorporation into the building at roof level, where they could be screened with louvres.

Research has also been conducted to show how a roof can be constructed and shaped in accordance with computer modelling so as to gain maximum ‘speed-up' of the prevailing wind before directing it into a turbine.

Conclusions

At present, it does not appear technically feasible or economically worthwhile to mount wind turbines on large commercial buildings as a means of providing significant renewable energy. It is technically feasible to mount micro turbines on many types of roof, and to integrate their outputs with the building electrical system; a specific investment appraisal would be required to estimate the renewable energy generated energy and payback period.

It seems likely that fully-integrated architectural solutions will be developed and adopted in the future, adding some as yet unknown cost to new urban developments, but offering one of the best potential solutions to on-site renewable energy generation.

In the meantime, ‘merchant wind' green energy contracts with transmission via private wire or the grid appear to offer an incentive for the building of more wind farms - in favourable locations - as an effective means of reducing CO² emission.