When you think about wind energy, you might think of those large wind farms you’ve passed while driving along highways, on your train rides, or perhaps you think about those huge offshore installations near the coast. Large scale wind turbines are skyrocketing, becoming more and more efficient with today’s ever-evolving technology.
The current tallest wind turbine is only a few dozens of meters short of being the same size as the Eiffel Tower. Conversely, urban wind turbines (UWTs) still have trouble emerging in the market. Why is that so? They look good on paper: localized power generation (just like solar panels on a roof), technical advantages when compared to its larger counterpart such as reduced noise, attractive aesthetics, etc.… However, it is affected by different bottlenecks that need to be overcome before we can see them as a standard element of our cities. In this paper, we explore the characteristics of UWTs, delving into the reasons of UWTs’ difficulties to ramp up and other challenges.
UWTs, what’s that?
What we call urban wind turbines are wind turbines in urban or suburban areas. From obvious space constraints and limited wind speeds, their sizes are limited (up to 12-15m) as are their power (around 10 to 50 kW). Their lightweight and small structure offer the possibility to be installed on roofs, or in streets and be part of the urban landscape.
Different types of UWTs
Mechanically stable and efficiency
Size and sound
Can tackle multi-directional winds, low cut-in speed, reduced maintenance due to fewer rotating components
Dynamic instabilities at large scale, lower power efficiency
Other examples: O-shape wind turbine, building-integrated turbines
Aerodynamics – the smaller scale problem
The most important parameter for wind turbine energy production is wind speed. As the power of a wind turbine rises with the cube of the wind velocity, it is no surprise that the wind turbine industry is going offshore where winds are more steady and stronger. Onshore, wide and flat landscapes are most favourable as they allow a shorter atmospheric boundary layer .
However, wind behaviour is completely different in urban areas compared with large spaces without obstacles. Wind is generally weaker, and the wind direction is changing as it flows around buildings (see Figure 1). This results in a turbulent flow that is chaotic and very difficult to model with existing computing capabilities. In such an environment, vertical axis wind turbines seem to outperform the more classical ones , presenting the opportunity to enhance this market.
Figure 1 – Airflow CFD simulation showing the wind velocity (m/s) at 25m high through the Marina Bay area in Singapore.
To this date, few complete studies  have been carried out on this specific topic, which makes it complicated for an investor to opt for turbines in urban areas, without being fully able to predict the wind availability at a specific location, and therefore the power expected and return on investment. Unsteady environment characterizing urban areas is a cause of concern for wind availability and steadiness, but also for fatigue performance of blades and support structures.
Are they economically appealing?
In 2014, Brussels Capital launched a measurement campaign  on different sites of the city to analyse and shed light on the potential of UWTs, in collaboration with universities (ULB and VUB) and the renewable energy consultancy company 3E.
In their best-case scenario, the internal rate of return can reach 17.2% and the payback period is around 7 years with financial support, 10-12 years without, compared to the average 20-30 years lifespan of most UWTs . Contrary to regular size wind turbines, for which the more energy produced the better, an urban wind turbine must be chosen so that the building energy consumption matches the turbine’s expected energy production as much as possible. Positioned on a roof more than 100m high, the yearly production from a 7.5 kW UWT would be around 14 200 kWh, which is more than 4 times the average consumption for a family  – and represents a significant part of SMEs electricity needs.
Their main conclusion is that the potential for small wind turbines in the capital can be significant, but it depends on several factors which are different than the usual factors used for regular wind turbines. In other words, we are not facing a smaller-scale problem; we are facing a totally different issue.
The WINEUR project  (Wind Energy Integration in the Urban Environment) carried out in 2007 by 5 different organizations from France, UK and the Netherlands and supported by the European Program “Intelligent Energy Europe”, has shown that the capital investment for an UWT varies between 2 400 and 9 100 €/kW, compared with 1 000 €/kW – or even less  – for its larger sized competitor. A difference mostly explained by the fact that UWT technology is still under research and development, with many different models and characteristics due to the lack of norms, standards and market maturity while large turbines benefit from economies of scale. This large cost range for UWT can still be seen today with little evolution over the last decade , , .
Stronger financial subsidies could increase the investment’s profitability. A call heard by Walloon and Flemish governments as they are increasingly favouring small-scale projects and self-consumption, and at the same time, decreasing financial subsidies for large scale wind turbines with proven profitability , . Even though some efforts were made to promote pilot projects , , the installation procedure remains challenging in the Brussels where an environmental permit is required, while unclear in other Belgian regions as well.
UWTs also currently suffer from incentive programs that favour other mature technology such as solar panel installations, affecting their popularity. A lost opportunity, as their complementarity could be further studied , with both technologies being intermittent at different moments (wind usually blows stronger at night).
Some could also wonder if social acceptance is an additional barrier. From 2013 to 2017, another European project, the SWIP program, analysed and developed innovative solutions for small and medium-size wind turbines to improve their competitiveness and facilitate their integration and deployment into urban and peri-urban areas . In that perspective, a public survey was carried to understand public opinion on small wind turbines and to anticipate community acceptance issues . 76% of the 425 answers were favourable regarding the installation of a SWT in their environment, with young people being most interested. Although the representativeness of this sample is questionable, the main barriers seem to be technological (performance) and economical (investment) rather than social. Co-ownership may represent a major opportunity for those types of WTs by reducing the risk of capital investment and, at the same time, insuring an auto-consumption by all the shareholders.
Where will the wind blow next for UWTs?
At Greenfish, we believe that UWTs should not be seen as a competitor to regular wind turbines, but as a complementary solution. Its use in complementarity with PVs could be very interesting as it answers the specificities of urban architecture. Further R&D is to be carried out for better assessing and enhancing its potential. Pilot projects are essential for the development of this sector. And as it was the case for regular WTs, public funding will be necessary for a push-start. The ingenious O-Wind turbine able to capture wind from any direction had the merit of winning the 2018 James Dyson Award and highlighting the diverse possibilities to develop this sector.
To boost the integration of UWTs, the European Cooperation in Science and Technology (COST) collected the existing expertise and challenges on wind energy technology in built environment from 2014 to 2018. The project, called WINERCOST, aimed at enhancing the concept of Smart Future Cities .
Therefore, a new technology trend is to combine wind turbines within buildings allowing for a larger range of power than the one presented in the scope of the paper and for an architectural integration in the urban landscape.
The idea is to design a suitable building shape to increase the wind speed to generate a larger aerodynamic power output than in the case of a stand-alone wind turbine . Well-known examples of these “building-augmented wind turbines” that take part of their own electricity from the wind are shown in Figure 2. While this innovative design would of course not apply to existing buildings, how much can it leverage energy consumption within cities?
Ludwig Carton – Project Manager, Green Solutions at Greenfish
Claire Peters – Junior Consultant at Greenfish
Nassim Daoudi – Chief Executive Officer at Greenfish
 YUELANG G., A case study on optimization of urban design base on wind environment simulation, 2016
 MICALLEF, D., VAN BUSSEL, G. A Review of Urban Wind Energy Research: Aerodynamics and Other Challenges. Energies, 2018, vol. 11, no 9, p. 2204
 RUNACRES, M. C., VERMEIR, J. J., et DE TROYER, T. BIM E11-359 Final Report—Identificatie sites, opzetten windmetingscampagnes en uitvoering van haalbaarheidsstudies in het Brussels Hoofdstedelijk Gewest. Leefmilieu Brussel, 2014.
 CACE, J., TER HORST, E., SYBGELLAKIS, K., et al. Urban Wind Turbines: Guidelines for Wind Turbines for the Built Environment. Wineur Report, 2007
 IRENA, Renewable Power Generation Costs in 2017, International Renewable Energy Agency, Abu Dhabi, 2018
 Orrell, Alice C., et al. 2017 Distributed Wind Market Report. Pacific Northwest National Lab.(PNNL), USA, 2018
 Abdelhady, Suzan, Domenico Borello, and Simone Santori. “Economic feasibility of small wind turbines for domestic consumers in Egypt based on the new Feed-in Tariff.” Energy Procedia 75 (2015): 664-670
 PITTELOUD, Jean-Daniel et GSÄNGER, Stefan. Small wind world report. World Wind Energy Association, Bonn, 2016
 CWaPE – Communication sur les coefficients économiques (kECO) applicables pour les différentes filières de production d’électricité verte à partir du 1er janvier 2019 jusqu’à l’entrée en vigueur du mécanisme réformé – 29/09/2018
 GULAGI, A., RAM, M., BREYER, C. Solar-Wind Complementarity with Optimal Storage and Transmission in Mitigating the Monsoon Effect in Achieving a Fully Sustainable Electricity System for India
 SWIP, Benchmarking of small and mediums size wind turbine technologies and legal framework, Poland, 2013
 SWIP, Improving social acceptance of small wind technologies Simon Hunkin, Greenovate! Europe – PPT
 BANIOTOPOULOS, C. C., BORRI, C., BLOCKEN, B. J. E., et al.Trends and challenges for wind energy harvesting: workshop, March 30-31, 2015, Coimbra, Portugal. 2015
 HEO, Young Gun, CHOI, Nak Joon, CHOI, Kyoung Ho, et al.CFD study on aerodynamic power output of a 110-kW building augmented wind turbine. Energy and Buildings, 2016, vol. 129, p. 162-173.