In urban areas, municipal governments, business owners, and residents generally have limited options when it comes to renewable energy. Distributed solar generation, popular in suburbs and rural areas, shows some promise in certain urban applications. Another option for city-dwellers is to select a renewable electricity rate from their utility. These plans are largely backed by the renewable energy credits purchased by the electricity companies from wind generating facilities in rural areas. This leads to the challenge of developing wind generation in urban settings.
For many years, the infrastructure required to harness wind has not lent itself to urban environments. The most prevalent issue has been the size of conventional wind turbines. Wind turbines take up a large amount of real estate and are considered by some to be eyesores and obstructions to the visual landscape. In addition, they can be noisy, cause disconcerting shadow flickers, and require consistent, non-turbulent winds of certain speeds that are uncharacteristic of urban environments. As a result, photovoltaic solar panels are the logical choice for renewable energy generation in urban areas. Fortunately, technology and design have teamed up to advance the state of wind generation in our cities.
Vertical Axis Wind Turbines Advantages
One of the main technological advances increasing the viability of wind energy in urban applications is the improvement of the vertical-axis wind turbine (VAWT) design. The bladed towers that most people envision when thinking of wind energy are horizontal-axis wind turbines (HAWTs). HAWTs do have advantages including more mature technology, higher rotation speeds, and lower cost. As such, HAWTs have been the favored wind generation technology.
However, VAWTs have attributes that could position them as being preferable to HAWTs in urban applications. Those attributes include quieter operation and their ability to better deal with variable wind currents. VATWs do not need to be pointed into the wind to be effective. This is advantageous given the oft-variable nature of city winds. Unlike HAWTs, the generator and gearbox do not need to be mounted near the blades and can be situated at the base, improving both the profile of the turbine and maintenance accessibility. As the technology progresses, new designs and installation techniques have mitigated the shortcomings of VAWTs such as low rotational speeds and lower power coefficients.
In working towards feasible urban applications, the focus has largely been on VAWT micro wind turbines. These smaller turbines vary from the truly micro-sized installations with as little as 1 kW output to larger models with outputs exceeding 20 kW. Ideally, newer turbines should be self-starting, ultra-quiet, provide smooth torque, and be highly efficient so usable energy can be delivered at modest wind speeds. Until recently, most urban micro turbines have been of the well-recognized Darrieus or Savonius types. However, in the quest to improve performance, new designs are emerging. Some of these designs are modifications of VAWT designs while others are completely new.
The Giromill is a variation of the Darrieus or ‘eggbeater’ turbine. Rather than the Darrieus’ curved airfoils, the Giromill is outfitted with vertical blades. An advanced version of this is the Cycloturbine. It features blades that are each able to rotate around their respective vertical axis and thus always have an angle of attack relative to the wind. As a result, maximum torque is achieved and delivered smoothly. These movable blades also allow the Cycloturbine to self-start at very low wind speeds. Disadvantages include the need for a wind direction sensor and a complex blade pitching mechanism that adds weight to the turbine. The wind direction sensor need has been addressed with the development of a self-actuating pitch control system.
Helical turbines are another variation of the Darrieus turbine. In this design, the blades are curved into a helix so that the turbine’s torque is spread evenly. This is an important modification as Darrieus turbines are known to suffer from torque ripple and cyclical stress. The twisted Savonius, meanwhile, is a Savonius modified to have long, helical scoops for the same reason.
While these designs exhibit features that lend them to urban use, the greatest advances have come from turbines developed from the ground up for use in urban spaces. The McCamley MT01 Mk2, for example, exhibits nearly all the traits ideally suited for city use. Its direct drive design can withstand strong, irregular wind gusts and it fits easily on rooftops with a sturdy eight-legged support structure. A rooftop wind farm could easily be created with these turbines. Rated at 12 kW, this technology is scalable up to 24 kW.
Technology Meets Design
French company New Wind has created an ingenious design named Tree Vent that cleverly solves not only the problem of where to place micro turbines in an urban environment but also addresses the issue of them being considered an eyesore by some. Their “turbine trees” are exactly what they sound like. The 36-foot tall steel structures are designed to look like trees with their “leaves” being tiny green VAWTs. The turbines operate almost silently and the cables, generators, and other components are hidden away, integrated into the tree structure. Each unit puts out a modest 3.1 kW, however, their design could be the perfect one for some urban installations. This is an example of how renewable energy does not have to look bad.
One of the most exciting new micro turbines purpose-designed for urban environments is the Liam F1 Urban Wind Turbine. One of the few non-VAWT examples, the Liam F1 is based on the Archimedes screw principle. Long used as highly efficient water turbines, the application to wind energy has yielded similarly impressive results. With a claimed efficiency of 80%, this design provides the same power as considerably larger units. It creates very little operational noise and is designed to always face into the wind.
To offset the inability of micro wind turbines to produce large power outputs, some manufacturers have developed hybrid solar/wind units. One example is the SolarMill by Windstream Technologies. The SolarMill consists of Savonius VAWTs mated to solar panels. This rooftop-mounted system is scalable to the energy needs of the user. Additional turbines and solar panels can be added to increase the power output.
The performance and cost effectiveness of wind turbines in urban environments is still being evaluated. Reliable performance data is too scarce to provide a conclusive answer, as many of the case studies have been done by manufacturers rather than independent parties. A lack of standardization and certification could lead to dissatisfied customers or technology that simply is not viable. Conversely, if the quickly advancing technology is combined with more field trials, the technology can be refined.
Studies confirm that the current technology is suitable for small-scale energy generation in locations where there is no space for large turbines. In addition, complementing solar technology with micro wind turbines can boost electricity yield and ensure power can be generated when weather conditions are unfavorable to one technology or the other. Aside from the energy produced, building and business owners can use wind generation to enhance their renewable energy profile. Public perceptions of urban wind technology can be further enhanced through successful installations and education.
As for the turbines themselves, they have the proven ability to provide on or off-grid energy. Bringing the technology to a level where the return on investment is attractive to a larger demographic is necessary to optimize the design and operation of turbines for urban wind generation. With technological advancements, the likelihood of increased usage of urban wind turbine installations seems very promising.