Climate change and its impacts are constantly triggering and initiating research and new solutions for sustainable and climate resilient urban environments. One of the topics attracting more and more attention are urban wind electricity generation which has not been addressed much for years due to high infrastructure required to exploit wind resource (substantial size of the conventional wind turbines, visual impacts, noise etc.).
Urban settings and the accompanying conditions are quite complex and can vary not only between cities but also within one (e.g. highly developed city centre with many buildings vs city’s riverbank). There are many factors influencing the wind regime upon which electricity generation is primarily dependent thus making urban wind turbine design a real challenge. Two most important characteristics of the urban wind regime are:
- lower annual mean wind speeds due to presence of buildings and
- more turbulent flow resulting from the interaction of the wind with the buildings and other obstacles.
Higher turbulent flow is especially tricky since it sets the need for turbine’s quick reaction to the changing wind directions, otherwise the output power is significantly reduced. Hence, the answer may lie in either turbine topology handling turbulence well or in finding the least turbulent areas of the urban environment. Considering the latter, promising areas are building-tops (such as one installed on a Chicago apartment building) and open areas on the ground such as parks, sport fields or flood defences.
In addition to aforementioned demands, an important aspect is structural and visual integration into the urban settings as well as maintenance and noise which is particularly relevant in densely populated areas.
When it comes to turbines’ design, there are two main groups which differentiate based on the orientation of their axis of rotation, namely:
- horizontal axis wind turbine (HAWTs) and
- vertical axis wind turbines (VAWTs).
HAWTs are advantageous since they represent more mature technology with higher rotation speeds, lower cost and higher efficiency. However, improvements in the VAWT design increased the viability of wind energy in urban applications. In general, VAWTs perform better with urban turbulence conditions, they are less noisy, low rotational speed and lower power coefficients have been improved.
Feasible urban applications mostly entail VAWT micro wind turbines from 1 kW output to larger models with outputs exceeding 20 kW. In the EU, urban wind power units have been installed in Netherlands, UK and France.
Some of these designs are very innovative and well-integrated into the urban surroundings. One example is the so called “Wind Tree”, a product of a French startup which already deployed a few units generating electricity in Paris.
The “Wind Tree” is a wind installation actually imitating a tree with leaves being represented by the small wind turbines. Average height is around 9 meters with diameter of about 8 meters. There are 54 “leaves” per “tree”. Each “tree” represents an installed capacity of 3.5 kW which can generate electricity in wind speeds as low as 4.5 mph. Currently, these installations are marketed for civil engineering and landscaping projects in cities but could be promising with regards to residential use. Some of the current challenges are the requirement of significant installation area, efficiency issues as well the costs.
Overall, urban turbines are not fully developed, mature technologies and many challenges are still ahead. Feasibility and viability of such projects is yet to be determined and verified. Additional research will show how promising these solutions could be in tackling climate change in urban environments.