The rush to please

Much of the infrastructure for electrical generation for cities and towns currently functions at a national level rather than at an urban level. But the push for distributed electricity is continuing. In Britain, for example, over the past four years the witnessing, testing and exploration of a number of innovative micro and meso-generation solutions (deemed to be in reach of individual households and small-scale industry and commerce) have become prime-movers in the creation of a new technology industry – the use of wind turbines in an urban and cityscape setting.

But as a result of this there has been a ‘rush to please’ in the wind industry, resulting in failures to achieve the capacity promised – due to ignorance of the way wind energy behaves in an urban landscape.

Matthew Rhodes of UK-based Encraft — a consulting engineering firm specialising in micro generation – observed in a recent seminar that from the data available the current technology practiced in the UK does not work on low rise urban sites but does on high rise urban sites, and that it was important to know which is which. This view is echoed in the USA where Tracy Twist of Mariah Power Inc. told us that the single biggest problem is the lack of standards for the design and testing of urban wind turbines: “Manufacturers can claim just about anything and too many do”, she added. And the same is true in Britain.

Urban wind turbines can work. But to achieve success manufacturers, suppliers and customers must design for real wind situations, and not just as a quick-solution for individual needs with ‘fantasy wind’ applications that provide hardly any benefits of power supply for the costs spent. And to achieve this requires a better understanding of the art and design of wind power generation.

There is an opportunity at urban level to capitalise on ‘First Mover’ advantages by research and development organisations, manufacturers and stakeholders, in synergy with both national and local Governments. In the UK this has been recognised by supermarket chain Tesco Plc, which over the past four years has undertaken a number of ad-hoc decisions to install urban wind turbines at a number of its sites – including its head offices in Welwyn Garden City, England.

The results, however, haven't proven entirely satisfactory, and as a result Tesco has undertaken to fully investigate a range of technologies and applications – with a view to analysing the urban wind turbine potential for its supermarkets and sites across the UK.

Tesco's final report took into account Information from manufacturers of both horizontal and vertical axis urban wind turbines in the UK, USA, Ireland, Netherlands, Belgium, Denmark, Italy and Finland, with variable (and not always comparable) specifications. In addition, outside the normal family of horizontal and vertical wind turbines, the report looked into the new developments of modular stacked turbines for mounting on the top of high rise buildings, as well as large-scale urban solutions such as the 1998 UK-Dutch Sun Mill vertical wind-turbine – which includes solar panels and the use of the stack effect.

We will examine the results of this investigation below.

Case study – small wind in the UK

The popularity of small urban wind turbines in the UK and other European countries is increasing. Most people live in cities, and the use of wind energy can be of benefit to many urban dwellers as a free energy source as well as offering independence from global energy companies.

The potential in the UK's urban areas is immense with over 22 million buildings. Based upon current academic studies in the UK and the Netherlands, the UK has a potential installed capacity of 2.2 GW. There are, however, critical observations that have to be taken on board across industry, planning authorities and by keen investors:

• Current planning frameworks are confused, and essential safety checks, such as product standards, certification, and installer accreditations are missing;
• Unlike the mapped wind regimes for coastal large-scale wind farms, there is still no universal, comprehensive design code of practice for the use of urban wind turbines, interfaced with other technologies that can be used for built-up areas;
• Turbines need to be installed at heights above the negative effects caused by buildings, so that the annual energy yield from available wind in a particular location is maximised;
• Location is critical, and how turbines are located, either externally to a built structure, mounted on a building or integrated with the architectural fabric, needs careful planning. In particular the determination of available wind speeds and (most important of all), the height above ground level where the most viable wind speeds are available, needs to be assessed.
• The use of vertical axis wind turbines are found to give more benefit than horizontal axis machines, which need to be turned towards the prevailing wind and make little use of changeable wind directions;
• Amongst the products now referred to as micro-generation technologies, urban wind turbines are a comparative newcomer to this market place, and therefore the same room for development must be given for urban wind turbines as has been afforded the 30-year development of solar panels through PV technology;
• It is important to understand the limits and constraints of urban wind. Poorly-deployed machines only create bad publicity for a technology that – if carefully designed and fully integrated into the built environment – can still produce a substantial amount of energy to use alongside other technologies, whatever their scale.

Art and design of wind generation

Wind is the result of pressure differences in the atmosphere, and wind speed and direction are determined by the ratio of the pressure differences and the distance between the centres of high and low pressure. At a height of over 100 metres wind speed and direction will be the same over a large area.

Closer to the ground the pattern changes due to resistance the wind meets from what is known as terrain roughness. At ground level wind speed is practically zero. And wind is also affected by the shape, height and relationships that buildings have with each other; the impediments of parks and streets; and the creation of dead-zones that alter according to higher-level wind flows. In short, the urban wind industry needs a much more thorough understanding of the physics of wind.

In addition to this, climate change predictions and recent computer modelling indicate that wind speeds will increase dynamically as the atmosphere heats up. This will be particularly true of the increasing ‘heat-island’ effect over cities and dense urban areas. This will be very noticeable in Europe this summer as urban temperatures rise higher than previous years.

A relatively small difference in average wind speed results in a big difference in the energy output of a turbine. This means that a wind turbine has to have the capability to adapt to the wind regime that occurs within the micro or meso climatic context of the building or group of buildings where it is installed – if it is to be effective. Such turbines should not only resist wind gusts and turbulences, but also be designed to visually integrate with the built environment in which they are to be placed. Currently there are a number of machines being marketed that do not meet such criteria.

Realistically, modern wind turbines or aero-generators designed for an urban environment are only just at the early stages of their development. Despite that fact, there are some 40 manufacturers worldwide selling urban wind turbines – with capacities ranging from 1 to 20 kW. Larger capacities are coming on stream with a new breed of special purpose, architecturally-integrated turbines suitable for high rise applications.

Western Europe, the UK and the Netherlands have 24 different models, whereas Italy, Denmark, Belgium and France have two or three and Finland one manufacturer of a vertical wind turbine.

China, Japan and the USA are becoming active in the European market. US manufacturers who have been selling to a rural market are now beginning to seek new products suitable for an urban environment.

There is an open window of opportunity for urban wind turbines. But to sow the seeds of energy success it is important that in the light of poor results from the trials that took place in the UK, investors, manufacturers and end-users utilise the present economic slow-down to rethink their business portfolios and to help create a better understanding of the physics of urban wind. A number of recent papers released by academic institutions such as Delft, Uppsala and Sheffield universities have a consensus that vertical axis wind turbines have the potential to improve on the efficiencies experienced by the more prolific – especially across central Europe and parts of the US – land-based multi-megawatt horizontal axis wind turbines.