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Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Built-Environment Wind Turbine 

Roadmap

J. Smith, T. Forsyth, K. Sinclair, and F. Oteri

Technical Report

NREL/TP-5000-50499

November 2012
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Contract No. DE-AC36-08GO28308

Built-Environment Wind 

Turbine Roadmap

J. Smith, T. Forsyth, K. Sinclair, and F. Oteri

Prepared under Task No. WE11250

Technical Report

NREL/TP-5000-50499

November 2012
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iii

Abbreviations and Acronyms

BWT built-environment wind turbine

CFD computational fluid dynamics

HAWT horizontal-axis wind turbine

IEA International Energy Agency

IEC International Electrotechnical Commission

kW kilowatts

MCP measure-correlate-predict

NREL National Renewable Energy Laboratory

NWTC National Wind Technology Center

SWT small wind turbine

TKE turbulent kinetic energy

VAWT vertical-axis wind turbine

WINEUR Wind Energy Integration in the Urban Environment
iv

Executive Summary

For the United States to generate 20% of its electricity requirements from wind 

technology by 2030, strong support from the general public will be needed. The majority 

of this production will come from large commercial wind projects installed throughout 

the United States, both land-based and offshore. To date, many of the easily developable 

sites have already been utilized, and future sites could face a range of potential barriers, 

including resistance from the general public.

Although only a small contributor to total electricity production needs, built-environment 

wind turbines (BWTs) nonetheless have the potential to influence the public’s perception 

of renewable energy, and wind energy in particular. Higher population concentrations in 

urban environments offer greater opportunities for project visibility and an opportunity to 

acquaint large numbers of people to the advantages of wind projects. However, turbine 

failures will be equally visible. High-profile installations, many of which have failed to 

produce electricity as advertised, could have a negative effect on public safety and 

perception of wind technology because the general public cannot differentiate between 

emerging technology and proven technology used in the commercial wind industry.

The market currently encourages BWT deployment before the technology is ready for 

full-scale commercialization. To address this issue, industry stakeholders convened a 

Rooftop and Built-Environment Wind Turbine Workshop on August 11 - 12, 2010, at the 

National Wind Technology Center, located at the U.S. Department of Energy’s National 

Renewable Energy Laboratory in Boulder, Colorado. Workshop attendees adopted the 

following vision statement:

“To provide current, state-of-the-science recommendations for 

optimization (reliable and safe) of wind turbine design and placement in 

the built environment, assessment of potential challenges unique to the 

built environment, a list of barriers, and priorities for addressing those 

knowledge gaps with data/observations and modeling tools.”

Workshop attendees identified barriers to BWT deployment in five key areas. 

• Safety is considered the most critical issue for BWTs. Sub-areas include fatigue 

resistance, braking redundancy, fail-safe mechanisms, and ice- and part-shedding 

containment. 

• Understanding the wind resource (including annual averages, turbulence, and 

extremes) and developing better wind resource maps are also considered high 

priorities to support BWTs.

• Improvements to the turbine technology, such as using control strategies to reduce 

vibration and noise, understanding loads measurements and yaw rates, and 

developing design and testing standards, will move the BWT industry toward 

stronger customer acceptance. 
v

• At the same time, in terms of building-mounted systems, understanding building

interactions will be pivotal. Concerns exist regarding resonance frequencies, and 

an understanding of how the building-turbine vibrations are coupled is needed. 

BWT system designs must comply with building codes as well as integrate with 

the building’s mechanical and electrical systems.

• Non-technical obstacles, such as concerns regarding safety hazards during 

installation, operations and maintenance, and inspections must be understood. 

Consumer outreach and education, along with overcoming economic barriers, 

must also be addressed.

The BWT roadmap also outlines stakeholder actions to overcome the barriers identified. 

The actions are categorized as near-term (0 - 3 years), medium-term (4 - 7 years), and 

both near- and medium-term. The BWT industry is evolving rapidly, so long-term actions 

cannot be projected.

Workshop attendees developed a strategic approach to accomplish these actions that 

identifies two focus areas: understanding the built-environment wind resource and 

developing testing and design standards. In this report, the authors summarize the 

expertise and resources needed in these areas. A wide variety of domestic and 

international stakeholders are currently engaged with BWTs. Existing wind tunnels, wind 

measurement data, and models could be utilized and enhanced to expedite the 

development and deployment of BWTs.

This roadmap identifies key barriers to the development and deployment of BWTs and 

outlines a strategic approach to addressing these barriers. 
vi

Table of Contents

Introduction................................................................................................................................... 1

State of the BWT Industry........................................................................................................... 4

State of BWT Technology ............................................................................................................ 6

BWT Industry Barriers................................................................................................................ 8

BWT Stakeholder Actions.......................................................................................................... 17

Strategy ........................................................................................................................................ 31

References.................................................................................................................................... 37

Bibliography ................................................................................................................................ 39

Appendix I: Types of Built-Environment Wind Turbines...................................................... 41

Appendix II: Rooftop and Built-Environment Wind Turbine Workshop Attendees .......... 43

Appendix III: Workshop Agenda.............................................................................................. 44

Appendix IV: Roadmap Reviewers........................................................................................... 46

Appendix V: Contacts Summary............................................................................................... 47
vii

List of Tables

Table 1. Summary of BWT Barriers........................................................................................... 9

Table 2. Near-Term Actions and Barriers Addressed............................................................. 21

Table 3. Medium-Term Actions and Barriers Addressed....................................................... 25

Table 4. Near-Term and Medium-Term Actions and the Barriers Addressed..................... 29

Table 5. Strategy Summary ....................................................................................................... 36

Table 6. Summary of Contacts, Affiliation, Expertise, and Interests .................................... 47

List of Figures

Figure 1. Examples of various BWT installations. Left to right: Side mounted to 

building, roof mounted, ground mounted in the built environment, building 

integrated................................................................................................................................ 1

Figure 2. Students at the University of California - Davis work on a roof-mounted 

Bergey wind turbine. Photo from University of California – Davis, NREL/PIX 

17997 ....................................................................................................................................... 4

Figure 3. Quiet Revolution vertical-axis wind turbines at Cleveleys Promenade, 

United Kingdom. Photo from Quiet Revolution, NREL/PIX 18054 ................................... 6

Figure 4. Example of roof-mounted BWTs at the Museum of Science, Boston, 

Massachusetts. Photo from Boston Museum of Science, NREL/PIX 18005 .................... 10

Figure 5. Skystream turbine in San Francisco, California. Photo from Danielle 

Murray, Department of the Environment, San Francisco, NREL/PIX 18445 .................. 12

Figure 6. Swift wind turbines at Meijer Inc.’s headquarters in Walker, Michigan. 

Photo from Cascade Engineering, NREL/PIX 17895......................................................... 13

Figure 7. Windspires at Adobe Systems Inc., in San Jose, California. Photo from 

Windspire Energy, NREL/PIX 18000.................................................................................. 15

Figure 8. A SWIFT wind turbine at a municipality's Board of Public Works 

building. Photo from Cascade Engineering, NREL/PIX 17889 ........................................ 16

Figure 9. BWTs at the Boston Museum of Science. Photo by Joe Smith, NREL/PIX 

18461 ..................................................................................................................................... 18

Figure 10. A Windspire vertical-axis wind turbine in the built environment. Photo 

from Windspire Energy, NREL/PIX 18464......................................................................... 19

Figure 11. Windspire vertical-axis wind turbine at the Boston Museum of Science. 

Photo from the Boston Museum of Science, NREL/PIX 18007......................................... 22

Figure 12. University of California - Davis Bergey wind turbine. The university is 

conducting BWT research. Photo from the University of California – Davis, 

NREL/PIX 17899.................................................................................................................. 24

Figure 13. An example of a roof-mounted vertical-axis wind turbine in San 

Francisco. Photo from Danielle Murray, Department of the Environment, San 

Francisco............................................................................................................................... 26

Figure 14. Bergey turbine outfitted with two three-dimensional sonic anemometers 

that measure wind speeds upwind and in the tail wake. Photo by Dave Corbus, 

NREL/PIX 13175.................................................................................................................. 32
viii

Figure 15. An example of a vertical-axis wind turbine in San Francisco. Photo from 

Danielle Murray, Department of the Environment, San Francisco, NREL/PIX 

18447 ..................................................................................................................................... 34

Figure 16. The Boston Museum of Science currently has five turbines in its Wind 

Turbine Lab. Photo from the Boston Museum of Science, NREL/PIX 18001................. 35

Figure 17. Building-mounted (side) wind turbine at the Boston Museum of Science. 

Photo by Joe Smith, NREL/PIX 18463................................................................................ 41

Figure 18. Building-mounted (roof) wind turbine at the Boston Museum of Science. 

Photo by Joe Smith, NREL/PIX18460................................................................................. 41

Figure 19. Building-integrated wind turbine at the Bahrain World Trade Center. 

Photo from iStock/6924031 .................................................................................................. 41

Figure 20. This Aeroturbine at the Randall Museum in San Francisco, California, is 

an example of a vertical-axis wind turbine in the built environment. Photo from 

Danielle Murray, Department of the Environment, San Francisco, NREL/PIX 

18443 ..................................................................................................................................... 42
1

Introduction

What are built-environment wind turbines (BWTs)? In this roadmap, BWTs are defined 

as wind turbines located in an urban or suburban environment (built environment). Most 

BWTs are also classified as small wind turbines (SWTs), which are 100 kilowatts (kW)

or less. While the terms “BWT” and “SWT” are interchangeable in many cases, this 

roadmap uses the term “SWT” when referring to turbines 100 kilowatts and less and the 

term “BWT” when referring to SWTs in the built environment. “SWT” refers to a 

category of turbines, and “BWT” refers to a specific application or market niche. See 

Appendix I for a detailed description of BWTs.

To date, most wind turbines installed in the built environment have been sited with 

limited understanding of or regard for the unique challenges of BWTs (Encraft 2009). 

Most SWTs were designed for rural areas, not the built environment with its high

turbulence, lower average wind speed, more frequent wind direction changes, and 

potentially higher vertical inflow. Nor were turbines designed to be in close proximity to 

people, businesses, and other property. Poor siting and improper use of BWTs could lead 

to turbine failure, possibly resulting in injury, property damage, and potential liabilities. 

These liabilities extend to not only BWT owners but also to the industry, which would 

suffer from general negative perceptions of wind technology.

Figure 1. Examples of various BWT installations. Left to right: Side mounted to building, 

roof mounted, ground mounted in the built environment, building integrated

Recent research on wind energy in urban areas demonstrates that there are promising 

opportunities to extend the use of wind energy in the built environment. However, 

developers must pay careful attention to the micro or local wind conditions produced by 

the stochastic wind interactions with localized structures. Turbine efficiency is highly 

sensitive to the rapid variations in wind conditions that prevail in the built environments

(Kooiman and Tullis 2010). Other difficulties include transfer of vibration and loads to a 

building structure, potentially causing noise and structural failures (Encraft 2009).
2

Understanding the loads, dynamics, yaw rate, and other technical specifications is critical 

in designing or modifying existing commercial products.

The number of BWT installations is increasing as consumers have easier access to 

relatively inexpensive SWTs (James et al. 2010). In 2010, BWT units experienced 

substantial sales growth to more than 1,700 kW, or 7% of 2010 U.S. SWT capacity sales. 

This represents a remarkable 430% growth from 2009. In terms of units, 1,074 roof-top 

units were sold (American Wind Energy Association 2011). Many people are motivated 

by a desire to be environmentally responsible, and they want clean, renewable energy to 

help power their homes or businesses. While the increased visibility of a BWT can be 

used to enhance a “green” image, a poorly sited turbine will not produce much electricity

and may not even spin, which implies that “turbines don’t work.” Moreover, poor siting 

will likely increase fatigue issues and may drastically shorten a turbine’s life span. This 

perception of BWT underperformance introduces a risk that the public will become 

disillusioned with the greater wind energy industry (Encraft 2009).

By developing the “Built-Environment Wind Turbine Roadmap,” representatives from 

industry, government, academics, and those with an interest in BWTs have produced a 

document that addresses the critical needs of safety, technology, and non-technical 

obstacles in the built environment. Although this is a U.S.-centric document, it includes 

contributions from international stakeholders. Further, this work will be coordinated 

through the International Energy Agency (IEA) so that a variety of international entities

can pursue this research area. This work is intended to aid in the crafting of public and 

business BWT policy by providing current, state-of-the-science recommendations.

This roadmap delves into the background of BWTs, including the current state of the 

BWT industry and the current state of BWT technology. Furthermore, this document 

describes the five categories of BWT industry barriers: safety, wind resource, turbine 

technology, building interactions, and non-technical obstacles. An action section 

addresses these barriers. One action may address more than one barrier, so these actions 

are grouped into three categories defined by their urgency: near-term (0-3 years), 

medium-term (4-7 years), and both. Because BWTs are a new wind technology and are 

evolving rapidly, long-term actions cannot be projected for the current BWT industry.

The document concludes with a strategy section that identifies resources to help carry out 

the actions and provides a plan to remove BWT barriers.

This document is based on presentations and the ensuing discussions from the Rooftop 

and Built-Environment Wind Turbine Workshop hosted at the National Wind 

Technology Center (NWTC) at the U.S. Department of Energy’s National Renewable 

Energy Laboratory (NREL) on August 11-12, 2010. Workshop participants are 

experienced in SWT modeling and data collection. Some of the key points from the

workshop include:

• Approximate correlations exist between wind tunnel measurements and 

computational fluid dynamics (CFD) models for specific sites.
3

• More sophisticated CFD wind resource modeling tools exist, and there is a 

substantial body of existing work pertaining to other wind propagation models.

• Countries of focus on the topic of built-environment wind turbines (see

www.urbanwind.net) exist, and the International Energy Agency Task 27 recently 

proposed new work.

• As a result, novel approaches to wind energy harvesting may emerge as important 

players in the effort to increase wind energy use in urban areas. 

During the workshop, the participants agreed to the following vision statement:

“To provide current, state-of-the-science recommendations for 

optimization (reliable and safe) of wind turbine design and placement in 

the built environment, assessment of potential challenges unique to the 

built environment, a list of barriers, and priorities for addressing those 

knowledge gaps both with data/observations and modeling tools.”

This roadmap is a step toward that vision.
4

State of the BWT Industry

This section describes the state of the BWT industry in Europe and the United States. The 

Built-Environment Workshop team could not identify experts in this topic area from 

other regions of the world; therefore, this report focuses on Europe and the United States. 

Europe

The European Union has moved forward with the development and deployment of BWTs 

through several programs supported by the European Commission. The United Kingdom, 

the Netherlands, and France participated in a large multi-country activity called Wind 

Energy Integration in the Urban Environment (WINEUR). Most of this work was 

completed in 2006 and 2007.

This program began by building the European Cities Urban Wind Network, a network of 

cities in which turbines were installed in the urban environment. Other activities 

followed, including the United Kingdom’s Warwick Wind Trials1

. Units were installed in 

The Hague, and researchers conducted a structured test program in Zeeland.

United States

The market for BWTs in the United States is 

expected to grow and develop in the coming years. 

SWTs appeal to consumers wishing to generate 

renewable energy and display environmental 

responsibility. “Urban turbines” experienced a 

430% sales growth between 2009 and 2010, which 

resulted in 1,700 kW of installed capacity

(American Wind Energy Association 2011).

Historically, specialized distributors and installers 

provided the link between the SWT manufacturer 

and consumer, but increasingly SWT 

manufacturers market directly to consumers.2

Easier consumer access to SWTs appears to have 

increased sales, but without professional 

installation services, site suitability and 

installation quality will suffer. 

The poor quality of SWT installations completed 

without professional consultation is particularly 

apparent in BWT installations (Encraft 2009). 

These installations are often located in areas 

where the wind is blocked or diverted by 

 1 http://www.warwickwindtrials.org.uk/

2 Lowes Home Improvement Centers sell Bergey 10-kW wind turbines, and Home Depot sells the 

Southwest Windpower Air-X 400-Watt wind turbine.

Figure 2. Students at the 

University of California - Davis 

work on a roof-mounted Bergey 

wind turbine. Photo from 

University of California – Davis, 

NREL/PIX 17997
5

upstream obstacles and may present issues related to safety, turbine durability, effects on 

the building, and energy production.

Although people may intuitively believe that the wind resource on their roofs is adequate 

for SWT installations, in fact it may not be. Underperformance is widely reported for 

many BWT installations. Although the causes are diverse, and some are not well 

understood, a poor understanding of the wind resource is a consistent issue. An example 

of this situation is a turbine at the Boston Museum of Science, which was installed inside 

a roof re-circulation eddy. Completion of a CFD analysis revealed that the winds flow on 

the bottom part of the turbine in opposition to the prevailing winds and the wind through 

the top part of the turbine (Viti et al. 2010).

In some documented cases, few or no studies were conducted before a BWT installation 

(Encraft 2009). In another case, the structural analysis and public safety concerns were 

considered in addition to the wind resource, and the final BWT placement was a 

compromise among all three (Museum of Science et al. 2006). 

Besides the intangible “green” value, BWTs have economic value. The economic value, 

however, is diminished due to poor turbine production and the likely additional costs for 

custom-designed and fabricated mounting systems (when compared to conventional 

ground-mounted SWTs with open access to the wind).

It is also worth noting that the economics of BWTs are not limited to electricity 

production. Some people simply want to make a statement, and in certain cases a good 

economic argument exists even when the electricity generated does not create revenue

(Beller 2011). A recent BWT installation on the roof of a Portland apartment building is a 

good example of this scenario. The turbines serve as a message to the community about 

environmental consciousness and, as a result, the building has become quite popular.
6

State of BWT Technology 

SWTs can be safe and reliable; installing 

them on towers tall enough to place them 

well above any nearby obstacles 

increases production and reduces 

turbulence-induced loads. In the built

environment, the wind resource is more 

turbulent than conventional locations, 

which can lead to increased loads. 

Placing an SWT into a built environment 

may exceed the design limitations of the 

SWT and present issues related to safety, 

durability, and performance.

The current International Electrotechnical 

Commission (IEC) design standard 

designates a maximum occurrence of turbulence intensity of 18% (IEC 2006) for siting 

an SWT, but this is well below a documented incident of a turbulence intensity of 41%

measured at a 10-m height over complex terrain in a forest (Carpman 2010). Scale effects 

cause small turbines to be affected by turbulence in a different way than large turbines. 

Turbulent vortices can fully engulf a small turbine, whereas similar vortices impacting 

larger turbines are distributed across the large turbine and therefore have a less significant 

effect.

The most common sizes of BWTs are between 1 and 3 kW of rated output, which 

correspond to a rotor diameter of approximately 2 to 4 m (7 to 13 ft) for horizontal-axis 

wind turbines (HAWTs). SWTs of this size may employ a variety of governing 

mechanisms to control rotor speed. One of the most common governing mechanisms is 

passive overspeed control, such as furling, mechanical, and electrical control through 

design stall and braking. Due to differing response and recovery time, some methods of 

governing will prove more effective in the higher turbulence and rapidly changing wind 

directions of the built environment.

The HAWTs in this size range are free-yaw machines, employing a tail or blade coning to 

orient to the wind without the aid of a yaw motor. Due to the vertical orientation of 

vertical-axis wind turbine (VAWT) shafts, they do not need to orient into the wind to 

generate power. Thus VAWTs may have an advantage in this regard over HAWTs in the 

built environment because the wind direction may change more frequently than in a 

conventional SWT site.

VAWTs have two general topologies. They generate power through drag, such as the 

Savonius design, or they generate power through aerodynamic lift, such as the Darrieus 

design. It should be noted that the Savonius design is not purely a drag-driven machine 

because it is based on both drag and suction forces. The Savonius design has higher 

Figure 3. Quiet Revolution vertical-axis 

wind turbines at Cleveleys Promenade, 

United Kingdom. Photo from Quiet 

Revolution, NREL/PIX 18054
7

efficiency than a pure drag-driven machine but a relatively low efficiency compared with 

lift-driven machines. 

The Darrieus design has a higher maximum efficiency than the Savonius, yet efficiencies 

of Darrieus designs are generally lower than conventional HAWTs (Mertens 2006, Beller 

2011). However, a Darrieus design is only self-starting when the blade chord lengths are 

large (high solidity), which generally reduces overall aerodynamic efficiency. Most 

Darrieus designs require a push-start, which is usually accomplished by the generator 

consuming power and acting as a motor. It can be difficult to use aerodynamic methods 

to regulate turbines from over-speeding, and this is particularly true of Darrieus designs.
8

BWT Industry Barriers

One of the near-term actions proposed in this roadmap is to conduct a BWT market 

assessment. It is understood that BWTs will support a specific market niche; however, 

the market potential in the United States has not yet been quantified. Although the 

capacity additions from BWTs toward meeting overall U.S. installed capacity objectives

may be small, the contribution toward acquainting the public to wind technology may be 

invaluable.

Industry stakeholders attending the Rooftop and Built-Environment Wind Turbine 

Workshop identified barriers to BWT deployment in the following five key areas:

• Safety

• Wind resource

• Turbine technology

• Building interactions

• Non-technical obstacles.

The challenge for BWTs can be summarized as a need to understand the wind resource in 

the built environment, combined with a lack of measurements and model results to assist 

in the development of international design and test standards. Specific design guidelines

are lacking in IEC61400-2. 

The following section summarizes input from workshop attendees regarding these five 

key barrier areas. 
9

Table 1. Summary of BWT Barriers

Safety Wind Resource Turbine 

Technology

Building 

Interactions

Non-Technical 

Obstacles

The effect of a 

high-fatigue 

environment on 

BWT life is poorly 

understood.

BWTs lack the 

following safety 

features:

-Braking 

redundancy

-Fail-safe 

features

-Ice- and partshedding 

containment.

The following 

aspects of the 

wind resource in 

the built 

environment are 

poorly 

understood:

-Turbulence and 

directional 

variability

-Wakes, eddies, 

and separation 

zones

-Threedimensional wind 

speed profile and 

distribution.

Existing wind 

resource maps

do not translate 

to the built 

environment.

The following 

aspects of turbine 

technology in the 

built environment 

are poorly 

understood:

-Control 

strategies to 

reduce vibration 

and noise

-Loads 

measurements 

and yaw rates.

Design and test 

standards for 

BWTs (especially 

for high-fatigue 

environments)

are non-existent.

Resonance 

frequencies

(linked buildingturbine 

vibrations) are 

poorly 

understood.

Code compliance

is difficult (most 

codes do not 

address BWTs; 

existing codes 

add great 

uncertainty; 

additional zoning 

and permitting 

may apply).

Mechanical and 

electrical 

integration are 

costly and 

difficult.

Hazards exist for

personnel 

installing and 

servicing BWTs.

Outreach and 

education are 

required as 

credible BWT 

information is 

limited.

Economics

(project costs and 

return on 

investment) are 

unpredictable.

Safety

Safety is the most critical BWT barrier area. BWTs are installed on or in close proximity 

to buildings, people, and other property, so a catastrophic failure could damage property, 

injure people, and tarnish the wind industry's image.

This report outlines several important safety aspects. The barriers are characterized by a 

need for better understanding of the wind resource and turbines designed for that 

resource. Turbines must be developed with:

• Fatigue resistance

• Braking redundancy

• Fail-safe features

• Strategies for ice- and part-shedding containment.
10

Fatigue Resistance

There is a lack of understanding regarding how the built-environment inflow conditions 

impact the fatigue life of BWTs. Lower average wind speeds in the built environment 

may reduce one aspect of fatigue loads on BWTs. However, increased turbulence 

intensity and directional variability will increase another aspect of fatigue loads on a 

turbine, reducing its design life. A better understanding of the fatigue issues in the built

environment is required to remedy safety concerns. Once data exist, they must be refined 

to develop conservative safety factors found in IEC standards, as noted above.

Braking Redundancy 

BWTs require a redundant braking method to stop the rotor even if the turbine has 

partially failed, leaving the primary brake inoperable. A BWT must not be allowed to 

operate without control or load (a condition in which the rotor spins uncontrollably fast 

and may lead to catastrophic failure). If an SWT in a rural environment enters rotor 

overspeed, the owner typically protects himself and others by staying away until the 

winds calm and the rotor can be stopped by other means. In a built environment, waiting 

for calm winds is not an option. 

Fail-Safe Features

If a BWT were to fail, it must not be allowed to do so in a catastrophic manner. The 

BWT must remain intact if the turbine suffers damage from an external event or from 

failure. BWTs should incorporate fail-safe mechanisms and design philosophies.

Ice- and Part-Shedding 

Containment

Ice-shedding and part-shedding 

incidents have been infrequent, but risks 

are present, and increased numbers of 

built-environment installations also 

increase the magnitude of these 

concerns. It is worth noting that parts 

and ice can be tossed long distances, and 

it is difficult to predict where they may 

land. Part-shedding cannot be predicted, 

but there must be a way to contain this 

safety hazard. Ice-shedding can be 

expected after an icing event, but no 

SWTs on the market automatically 

detect or respond to ice build-up.

Wind Resource

The wind resource is site specific, and in the built environment, large differences exist 

among sites with small vertical or horizontal separation. Information on and 

understanding of the wind resource in the built environment is critical for designing 

BWTs, micrositing, and estimating the energy production. However, the builtFigure 4. Example of roof-mounted BWTs at 

the Museum of Science, Boston, 

Massachusetts. Photo from Boston Museum 

of Science, NREL/PIX 18005 
11

environment wind resource is not well understood. Unlike rural environments with few 

obstructions and adequate estimates for average wind speed and turbulence, limited 

knowledge exists that can be applied to wind resources in the built environments.

As with other BWT barriers, the wind resource barrier is primarily a lack of information 

and understanding. Important areas to address include:

• Turbulence and directional variability in the built environment

• Wakes, eddies, and separation zones

• Three-dimensional wind speed profile and distribution.

In addition to increasing existing knowledge in each of these areas, an effort to use an 

integrated and validated approach is required.

Turbulence and Directional Variability in the Built Environment

Besides average wind speed, turbulence and directional variability are also important. 

Because of the surrounding structures, the built environment has higher turbulence and 

directional variability than rural environments. Unfortunately, only rough estimates exist 

for turbulence intensity, and even fewer exist for directional variability. Additionally, a 

better metric for turbulence may be turbulent kinetic energy (TKE),

3 which requires highfrequency wind measurements in three dimensions. Few sites are currently instrumented 

for this measurement type. However, the use of a single number cannot communicate the 

range of turbulent phenomena that could occur. For example, gustiness is a form of 

turbulence in which a large vortex travels with the wind and passes a measurement 

location or wind turbine (Mertens 2006). In the built environment, in addition to ambient 

turbulence (large vortices traveling with the wind), turbulence is also generated locally by 

shear and wake-inducing building features (Mertens 2006). An investigation into the 

most appropriate turbulence parameters and standard for data collection is required.

Wakes, Eddies, and Separation Zones

Most SWT designs and experience pertain to relatively low-inflow turbulence, which is

common to open areas in rural settings (Beller 2011). Wakes, eddies, and separation 

zones mean increased turbulence in the built environment. These are complex turbulent 

phenomena, and the lack of information on them impedes BWT design criteria, 

micrositing, and production estimates.

Wakes behind cylinders and other bluff body shapes studied in the laboratory are 

generally well understood, but it is not trivial to apply those results to the built

environment because of large wind resource variations (in speed and direction) and urban 

topography (Yersel and Gobel 1986, Hurley 1997). Computer models of wakes, eddies, 

and separation zones are generally inadequate for the small scale of interest for BWTs.

 3 TKE is the mean kinetic energy per unit mass associated with eddies in the wind. TKE = 0.5 (u’2 + v’2 + 

w’2

) where u’2, v’2, and w’2 are the mean square velocity fluctuations in x, y, and z direction, respectively.
12

High-resolution models can provide useful information regarding the larger-scale 

structure of flows in urban environments, such as flow channeling and separation, and 

gross characteristics of wakes. However, details of these flow structures are difficult to 

simulate due to limited representations of important small-scale structures within the built 

environment. Limitations on computational resources and a lack of data to validate 

models further limit simulations. Consequently, BWT designers do not have many tools 

or adequate guidance to help them design to and account for wakes, eddies, and 

separation zones (Kalmikov et al. 2010).

High-resolution, three-dimensional wind measurements at multiple locations within built 

environments are required to provide data to improve computational models. Such model 

simulations are extremely time-consuming to set up, execute, and analyze. Therefore,

simplifying computational parameters, applying appropriate boundaries, and developing 

data analysis tools for built environments are required to develop usable BWT toolkits.

Three-Dimensional Wind Speed 

Profile and Distribution

Estimates exist for wind speed 

distributions and profiles in rural 

environments, but their application to 

the built environment is not 

appropriate. Standard models of 

vertical wind profiles (e.g., the log or 

power laws) do not apply at BWT 

locations, and alternative approaches 

are needed for predicting the wind 

fields above buildings at BWT heights 

(Beller 2011). Additionally, in the 

rural environment, the wind 

distribution is approximately twodimensional: The wind increases with 

height, but the vertical component to 

the wind is limited (the IEC standard 

requires up to 8 deg vertical inflow) 

(IEC 2006). On the contrary, the built environment has large vertical components as wind 

moves over and around buildings. Winds in built environments will have different 

probability distributions (mostly non-Weibull distribution) than rural environments, and 

these distributions will be further modified due to building blockages and diversions. 

Understanding the horizontal and vertical wind speed profiles and distributions is critical 

to BWT design and predicting production.

Wind Resource Maps

Wind resource maps provide an estimate of the wind resource at different heights. 

Resource maps work reasonably well at heights well above the ground and far from 

obstacles. For SWT sites with good exposure to the prevailing wind directions and 

without major obstacles (such as groves of trees) within at least several kilometers, an 

Figure 5. Skystream turbine in San Francisco, 

California. Photo from Danielle Murray, 

Department of the Environment, San Francisco, 

NREL/PIX 18445
13

onsite measurement campaign may not be necessary. However, this is not true for built 

environments. Existing wind resource maps may provide an initial estimate of the highlevel and unobstructed wind resource, but they must be translated to lower heights and 

obstructed areas of the built environment (a map is only representative for heights above 

approximately 20 times the roughness length z0) (Mertens 2006).

Modeling of simple obstacles has been performed, and knowledge exists that can be used 

to roughly translate existing wind maps to the built environment. However, buildings and 

obstacles block and alter the wind around them, so built environments are much more 

complicated than solitary or simplified obstacles. Data exist regarding flow around 

buildings, and it has been used to validate models. Although the approach for

understanding the wind resource at a given site remains the same, much of this modeling 

work was performed in the context of Atmospheric Transport and Dispersion of 

contaminants in cities and not in the context of wind energy (Beller 2011). Without an 

initial assessment of the wind resource, BWT owners must invest in and complete an 

onsite measurement campaign or install a BWT with no knowledge of the wind resource. 

Both options are financially risky. A wind resource map for built environments or an 

equivalent tool for estimating the wind resource should be developed.

Turbine Technology

Most SWTs were designed for the open 

areas common to rural environments. 

However, as discussed above, the built 

environment has significantly higher 

turbulence and wind direction variability. 

This means the turbine extreme, 

deterministic, and stochastic loads will be 

different and probably higher in the built 

environment. Historical and derived 

design guidelines, testing methods, and 

control strategies developed for SWTs 

must be revised for BWTs.

Turbine technology barriers to BWTs 

consist of a lack of knowledge as to how 

the built-environment wind resource 

affects the turbine. After enough 

measurements are conducted and the resulting information is analyzed and disseminated, 

appropriate BWT designs, controls, and tests can be developed. The following areas are 

characterized by a lack of knowledge and are therefore BWT technology barriers:

• Control strategies to reduce vibration and noise

• Loads measurements to validate dynamic models and yaw rates

• A standard for BWT design and testing.

Figure 6. Swift wind turbines at Meijer 

Inc.’s headquarters in Walker, Michigan.

Photo from Cascade Engineering,

NREL/PIX 17895
14

Control Strategies to Reduce Vibration and Noise

Vibration and noise issues will impede BWT acceptance and deployment. Due to the 

close proximity of people, acoustic emissions will be even more important in the built 

environment than for SWTs in rural environments. Strategies and controls to mitigate 

noise must be developed for BWTs to encourage acceptance and to conform to 

community noise ordinances.

Similarly, vibrations and noise issues will also be more important for BWTs than for 

conventionally mounted SWTs because BWTs are often mounted on buildings. Most 

vibration control and mitigation strategies for BWTs are custom solutions and therefore 

expensive and uneconomical. In most cases, the vibration is damped by installing rubber 

pads on the tower. When BWTs are installed without knowing the potential effects, they 

may have to be removed if the vibrations prove to be a nuisance. This issue poses an 

economic risk for the owner.

Loads Measurements and Yaw Rates

The aforementioned increased turbulence and directional variability could result in higher 

(peak) loads on BWTs. Gusts will cause large forces on the blades and increases in rotor 

speed. Stronger and sharper gusts from the built environment will increase this effect. For 

HAWTs, direction variability will cause the turbine to yaw, and if the directional wind 

change occurs quickly, large gyroscopic forces will be exerted on a moving rotor, tail (if 

applicable), and yaw system. It should be noted that for the few SWTs that fail, yaw 

forces are sometimes the cause. With the increased directional variability of the builtenvironment wind resource, extreme yaw changes will be more important to BWTs.

BWT Design and Testing Standard

As with SWTs, there is a growing demand for BWT certifications. Consumers want to 

know that they are investing in a safe product with proven performance, as demonstrated 

through standardized testing and review. Testing standards exist for large turbines and 

SWTs, but standards specific to BWTs do not exist.

BWTs need to be tested and reviewed for the increased loads (due to increased 

turbulence) and not to current standards, which are inadequate. Additional modifications 

to existing standards may be necessary to ensure that certified BWTs have the same 

reliability and quality as properly installed and certified SWTs.

Testing to standards is important for product safety, but it must also be affordable. If 

testing is too expensive, the added cost could undermine the economics of BWTs.

Building Interactions

Many BWTs are mounted to buildings, so interactions with buildings are a major design 

and siting concern. Furthermore, whether they are attached to or detached from the 

building structure, BWT systems have electrical integration considerations. The barriers 

regarding building interactions are further complicated by the multitude of building types 

and locations. Concerns include not only mounting the BWT on buildings but also:

• Resonance frequencies
15

• Code compliance

• Mechanical and electrical integration.

Resonance Frequencies

BWT vibrations will in general be much higher than natural frequencies of buildings. 

Concerns remain regarding excitation of resonance frequencies for buildings of different 

construction types and heights. Conversely, there is interest in the effect of building 

vibrations on a BWT and its tower. Besides whole-building resonance frequencies, 

individual building components may be excited by BWT vibrations. An increased

understanding of linked building-turbine vibrations is needed to remove this barrier.

Code Compliance

Most, if not all, building 

codes do not specifically 

address BWTs. Searching for 

and complying with the 

existing applicable building 

codes could be daunting and 

expensive, and the outcome 

may not be favorable. The 

uncertainty of this process 

adds an additional barrier to 

the process of BWT code 

compliance.

Proximity to people and 

property may create 

additional zoning and 

permitting issues. If these 

policies are crafted well, they 

will reduce hazards to 

personnel installing and servicing BWTs and will facilitate BWT installations. However, 

poorly informed zoning and permitting policies will create a barrier for BWTs.

Mechanical and Electrical Integration

Integrating BWTs with a building’s mechanical and electrical systems can present a 

barrier. Besides a mounting system that mitigates vibration transfer to or from the BWT, 

aesthetics, function (particularly service and maintenance access), and cost are other 

barriers. In some cases, the design and materials for the BWT mount can cost as much as

the turbine and the rest of the installation.

Electrical integration of BWTs also presents a barrier. Besides connecting to the 

building’s electrical system and ensuring proper grounding, radio frequency interference 

presents concerns. Additionally, lights may be required by the Federal Aviation 

Administration if the BWT is significantly above the roof level. BWTs may be connected 

Figure 7. Windspires at Adobe Systems Inc., in San 

Jose, California. Photo from Windspire Energy, 

NREL/PIX 18000
16

to the electric grid or used to charge the battery on site. While policy barriers are 

common to grid-connected BWTs, both of these options have financial and technical 

limitations (Chiras 2009).

Non-Technical Obstacles

The fifth and final area of BWT barriers is non-technical obstacles: 

• Hazards to personnel installing and servicing BWTs

• Outreach and education

• Economics.

Hazards to Personnel Installing and Servicing BWTs

Many of the safety issues pertaining to 

BWTs were addressed in the safety 

section, but additional concerns exist 

regarding hazards during BWT operation, 

installation, maintenance, and inspections. 

Servicing BWTs requires additional 

procedures due to the limited space in 

built environments, particularly rooftops. 

These can be addressed with knowledge 

of best practices and enforced through 

procedures.

Outreach and Education

Interest in renewable energy has increased 

dramatically in the past few years, and 

many people want to generate their own 

electric power. Potential benefits need to 

be weighed against the costs and hazards 

of BWTs. Little knowledge currently 

exists about BWTs, and finding and understanding credible information often requires 

help from an expert. The lack of readily accessible, public information is a barrier to 

BWTs.

Economics

While initial cost is not the driver for some investing in BWTs, it still presents a 

significant barrier to others. Another part of the economic barrier is the return on 

investment. In many cases, the BWT’s production will offset some electricity purchases, 

but if production is not predictable, then savings will not be predictable either. Other 

economic incentives will depend on the utility provider and local, state, or federal policy. 

Many questions regarding BWT economics must be addressed at the outset of the BWT 

project. The lack of knowledge about financial cost and benefits of BWTs presents a 

major barrier, although many BWT projects have little to do with turbine economics and 

more to do with using renewables to market green values.

Figure 8. A SWIFT wind turbine at a 

municipality's Board of Public Works 

building. Photo from Cascade 

Engineering, NREL/PIX 17889
17

BWT Stakeholder Actions

This section outlines actions to address the five areas of barriers presented in the previous 

section. The action categories are organized by timelines:

• Near-term: should be implemented during the next 3 years

• Medium-term: should be implemented in 4 to 7 years

• Both near-term and medium-term: require immediate to medium-term effort.

Regardless of when each action takes place, all of them should help increase the BWT 

knowledge base to aid BWT owners, industry, and government. Local and state 

governments in particular should benefit from increased knowledge as they establish 

BWT policy.

Near-Term Actions (0-3 Years)

Near-term actions require immediate attention to overcome BWT barriers. In some cases, 

these actions are needed without delay. In other cases, the actions will be a precursor to 

medium-term actions. In all cases, actions should increase the knowledge of BWTs and 

improve the industry. 

Produce a Consumer Guide and Fact Sheets

A consumer guide and fact sheets based on the best available information about BWTs 

are needed. These publications should:

• Help educate potential BWT owners and stakeholders

• Answer basic installation questions, including best methods for estimating wind 

resource, energy production, and cost

• Provide information on existing BWT installations and where to find additional 

information.

Produce Risk- and Hazard-Focused Fact Sheets

Creating risk- and hazard-focused fact sheets would address concerns of installers and 

permitting and planning agencies. They should provide information on the following 

topics:

• Applicability of existing building codes

• Building-integration barriers related to mechanical, electrical, and control systems

• Mounting methods

• Grid interconnection

• Radio frequencies interference

• Ways to reduce vibrations associated with BWTs
18

• Safety barriers, such as installation risks, fail-safe strategies, and actions to reduce ice 

shedding if appropriate.

Create Standardized Resource Data Assessment Protocols

Creating wind resource data assessment tools and methods should help standardize data 

collection and provide a more efficient means of conducting research and comparing 

wind resources and BWTs. In terms of wind resource, consistent, high-quality, threedimensional data must be gathered. This wind speed and turbulence data should be 

collected from multiple locations to address wind resource questions. An example for the 

application to turbine technology is gathering load and yaw rates across multiple turbines 

in multiple test locations.

Survey and Analyze Existing 

Data

BWT costs and a range of installation 

and maintenance costs should be 

compiled. Some BWT owners may not 

want their costs published, so the costs 

data could be presented anonymously 

as averages and ranges. Cost data 

should help potential owners decide 

whether BWTs are right for them.

Analysis of existing data can provide a 

picture of the BWT market as a whole. 

This could be useful for policymakers, 

manufacturers, and investors to 

understand where and why BWTs are 

being installed.

Although limited in quantity and quality, existing turbine performance data should be 

analyzed. Of particular interest is the power performance of BWTs. Power performance 

measurements require high correlation between anemometer-measured wind speed and 

the wind experienced by the turbine, so care must be taken to verify the validity of 

existing BWT power performance data. The BWT power performance data should be 

compared to the same model's IEC power performance tests. Understanding the 

differences between power curves should also help establish better estimates of BWT 

power production.

If vibration measurements exist, they would be very useful in defining best-practice 

installation and design protocols.

Other measurements of interest are the wind resource of built environments. Average 

speed measurements are important for verifying wind resource models of built 

environments. If the data are three-dimensional and have a high enough sampling rate, it 

would be useful for calculating turbulence parameters.

Figure 9. BWTs at the Boston Museum of 

Science. Photo by Joe Smith, NREL/PIX 

18461 
19

Investigate and Compare Wind Resource Modeling Methods

There are several methods for modeling and predicting built-environment wind resources.

CFD and wind tunnel testing are often cited. Of particular interest are field experiments 

to study flow in an urban environment in the context of Atmospheric Transport and 

Dispersion of contaminants.

Measurements as well as models exist for cities such as Oklahoma City, Salt Lake City, 

New York City (Manhattan), and various European cities. Those data could be used to 

model and compare the wind energy resource.

Besides CFD and wind tunnels, models using measure-correlate-predict (MCP) 

procedures and analytical methods exist.

Each modeling technique has some success in the built environment, as well as 

limitations. Investigations should be conducted to identify the strengths and weakness of 

each and how their predictions compare to measured wind data. Comparisons include:

• Accurate prediction of annual average wind

Figure 10. A Windspire vertical-axis wind turbine in the built environment. 

Photo from Windspire Energy, NREL/PIX 18464 
20

• Ability to provide a wind probability distribution, including extremes and nonstandard wind conditions (wind speeds and turbulence)

• Accounting for turbulence parameters

• Accuracy of energy production estimates

• Cost to implement.

These comparisons should help to establish rules for providing initial estimates of wind 

resource and turbulence parameters. Knowledge of these should help to estimate the cost 

of energy.

Summary of Near-Term Actions

Table 2 summarizes the actions needed immediately and through approximately 3 years 

and the BWT barriers addressed.

Note: Safety is a primary concern, and many actions that address safety barriers are 

addressed in the Both Near-Term and Medium-Term Actions section. Note that there are 

no near-term actions that quantifiably address safety due to the need to collect 

measurements and model results first and then build safety into the standards and their 

resulting designs.

Medium-Term Actions (4-7 Years)

This section outlines the actions that need to be taken in approximately 4 to 7 years to 

develop testing and design standards for the BWT industry. In most cases, the 

prerequisite data and experience do not exist, so these actions cannot be implemented 

now.

Create Best-Practice Recommendations

Best-practice recommendations should be created based on existing knowledge and 

research findings. Turbine manufacturers, installers, stakeholders, policymakers, and the 

general public should utilize these actions. Through these recommendations, all BWT 

barriers should be addressed.

• Safety: Recommendations should address issues of ice- and parts-shedding and 

braking redundancy. Towers should be accessible from the roof without endangering 

workers and provide access to BWTs. Setbacks from buildings and rooftop 

limitations should also be addressed.

• Wind resource: Recommendations should include how to avoid wakes, eddies, and 

separation zones, as well as where to place BWTs for the highest average wind speed. 

It should also address the issues of turbulence and directional variability and their 

impacts on BWTs.

• Turbine technology: Draft IEC standard language should be created to define the 

design envelope using information on turbulence and directional variability. 

Understanding of yaw rates, loads, and fatigue in comparison to other SWTs will be 

gained. Fail-safe BWT designs and testing methods will be developed.
21

Table 2. Near-Term Actions and Barriers Addressed

Actions Safety Wind 

Resource

Turbine 

Technology

Building 

Interactions

NonTechnical 

Obstacles

Produce a 

consumer 

guide and fact 

sheets

Lack of credible 

BWT

information

Produce riskand hazardfocused fact 

sheets

Mechanical and 

electrical 

integration

issues

Lack of credible 

BWT 

information

(installation, 

planning, 

permitting)

Create 

standardized 

resource data 

assessment 

protocols

Poor 

understanding 

of:

-Turbulence and 

directional 

variability in the 

built 

environment

-Threedimensional 

wind speed 

profile and 

distribution

Poor 

understanding of:

-Load 

measurements

-Yaw rates

Survey and 

analyze 

existing data

-Costly and 

difficult 

mechanical and 

electrical 

integration

-Poor 

understanding 

of resonance

frequencies

Unpredictable 

economics 

(project costs 

and return on 

investment)

Investigate 

and compare 

wind resource 

modeling 

methods

Poor 

understanding 

of:

-Wakes, eddies, 

and separation 

zones

-Turbulence and 

directional 

variability
22

• Building interactions: Approaches should include mounting, integration, and 

vibration mitigation. Building codes for different building types should be reviewed,

and recommendations should be made regarding areas of applicability to BWTs.

• Non-technical obstacles: Barriers of cost and hazards to personnel installing and 

servicing BWTs should be addressed. Cost estimates should be provided in terms that 

can be useful to most potential BWT installations.

Adapt Data Assessment Tools

Data assessment tools already exist for 

some aspects of wind energy, and some 

of these could be adapted for BWTs. As 

discussed previously, wind resources are 

assessed for horizontal winds, but in 

built environments the winds have a 

vertical component. This presents the 

opportunity to adapt existing wind 

resource assessment tools for three

dimensions. Wind maps are widely 

used, but their application and 

effectiveness need to be evaluated and 

adapted for the built environment.

Cost of energy calculators exist and are 

widely used; however, they perform 

poorly in terms of predicting the cost of 

energy of turbines in complex terrain. 

Hence, their predictive capability in 

built environments is highly dubious. 

The existing cost of energy calculators 

must be modified and adapted for 

BWTs.

Instrument Existing BWTs

By instrumenting existing BWTs with measurement devices, all the identified BWT 

barrier areas should be addressed.

• Safety: A good understanding of the events immediately prior to any failures should 

be known.

• Wind resource: Emphasis should be placed on three-dimensional wind speed profiles, 

turbulence, directional variability, and wind probability distributions.

• Turbine technology: Load, yaw rates (for HAWTs), and vibration frequencies should 

be measured. Load cells, strain gauges, and yaw encoders will correlate BWT 

response to wind speed and direction data. Incorporating knowledge of turbulence 

Figure 11. Windspire vertical-axis wind 

turbine at the Boston Museum of Science. 

Photo from the Boston Museum of Science, 

NREL/PIX 18007
23

parameters and directional variability should also increase a model’s ability to predict 

BWT performance. Additional actions should include applying instruments and 

condition monitoring for any common failure modes identified using the reliability 

database (near-term action).

• Building interactions: Building resonance due to turbine vibrations should be better 

understood.

• Non-technical obstacles: Barriers to be addressed include actual energy and power 

production. Down-time and repairs should be documented, and this should relate to 

the cost of operating and maintaining a BWT.

Conduct Model Validation at Demonstration Sites

Wind resource models should be compared to each other as a near-term action. As a 

medium-term action, researchers would follow up by comparing model results (CFD, 

wind tunnel, MCP, and analytical methods) to field measurements for various 

demonstration sites. These sites should be highly instrumented and representative of 

potential BWT installation sites. Ideally the location of a demonstration site would be 

within one of the Atmospheric Transport and Dispersion field test cities or another city 

with extensive anemometry data for model validation. Models should be checked for 

their ability to predict wind speed distribution and the location of wakes, eddies, and 

separation zones. Other items of interest that may be compared are turbulence parameters 

and directional variability.

The critical component for this action is the use of sonic anemometers or other types of 

three-dimensional wind measurement devices placed in areas appropriate for BWTs. This 

will also require some locations with multiple instruments to characterize and compare 

the special variation of the wind in close proximity to buildings – an array of 

anemometers across height and length to capture eddies, wakes, and separation zones.

This validation should further improve rules for built-environment wind resources and 

cost of energy estimates.

Provide Recommendations to Governing Bodies and Standards

Wind turbines are certified to a standard, such as the IEC's set of wind turbine standards, 

designated as IEC-61400. These standards require turbines to be tested in ways that 

should reflect their actual use and demonstrate actual performance. At present, BWTs are 

not included in any standard.

Recommendations must be made regarding appropriate BWT testing. Additionally, the 

IEC-61400 standard, which is the basis for most other wind turbine standards, does not 

provide design criteria appropriate to the wind loads a BWT could experience. This 

action should modify or add tests for BWTs to ensure their safe and reliable operation.
24

Besides providing 

recommendations to the IEC, 

information and recommendations 

could be provided for building 

codes and municipal governments 

contemplating BWT regulations.

Conduct Turbine Research 

and Development

Research and development should 

accelerate the deployment of safe, 

durable, and effective BWTs. This 

action can answer specific concerns 

and develop better methods for 

future integration. New control 

methods can be tested to reduce 

noise and vibrations (reduced noise 

and vibrations will be important for 

BWT acceptance). While some 

research has been conducted on 

SWT noise reduction, these issues 

are not as important for rural 

installations as for built

environments, and consequently, 

much more research is required. 

Accelerated fatigue testing due to gusts and yawing can be researched. Additionally, 

other mechanical and electrical building integration barriers may be overcome.

Examples of areas in which research and development can be applied include:

• Comparison of yaw mechanisms

• Containment of failures (e.g., by using tethers on blades, nacelle, base)

• No single point of failure and redundant, independent braking mechanisms

• Rotor-speed control through inverter loading for vibration mitigation; tuning BWTs 

to avoid exciting building frequencies

• Tip-speed control to reduce noise (e.g., at night)

• Resonance.

Summary of Medium-Term Actions

The following table summarizes medium-term actions that will require effort in 

approximately 4 to 7 years and the BWT barriers addressed.

Figure 12. University of California - Davis Bergey 

wind turbine. The university is conducting BWT 

research. Photo from the University of California 

– Davis, NREL/PIX 17899
25

Table 3. Medium-Term Actions and Barriers Addressed

Actions Safety Wind 

Resource

Turbine 

Technology

Building 

Interactions

NonTechnical 

Obstacles

Create bestpractice 

recommendations

BWTs lack 

braking 

redundancy, 

partsshedding 

and icethrow

containment 

Poor 

understanding 

of wakes, 

eddies, and 

separation 

zones

Mechanical 

integration 

issues 

(develop and 

refine 

mounting 

strategies)

Hazards to 

personnel 

installing and 

servicing 

BWTs

Adapt data 

assessment tools

Poor 

understanding 

of 3-D wind 

resource

-Existing wind 

resource maps

do not 

translate to the 

built 

environment

Unpredictable 

economics 

(create 

economic 

assessment 

tool)

Instrument 

existing BWTs

Poor 

understanding 

of:

-3-D wind 

resource

-Turbulence 

and directional 

variability

Poor 

understanding 

of loads and 

yaw rates

(measure and 

correlate to 3-

D wind)

Poor 

understanding 

of resonance 

(measure and 

correlate to 3-

D wind)

Conduct model 

validation at 

demonstration 

sites

Poor 

understanding 

of:

-Wakes, 

eddies, and 

separation 

zones

-Turbulence 

and directional 

variability

(measure)

Poor 

understanding 

of loads and 

yaw rates 

(measure)
26

Actions Safety Wind 

Resource

Turbine 

Technology

Building 

Interactions

NonTechnical 

Obstacles

Provide 

recommendations 

to governing 

bodies and 

standards

Poor 

understanding 

of 3-D wind 

resource

Non-existent 

BWT design 

class

Model effects 

of building 

codes

Conduct turbine 

research and 

development

BWTs lack 

braking 

redundancy, 

fail-safe 

turbine

features

Vibrations and 

noise (address 

control 

strategies to 

reduce)

Both Near-Term and Medium-Term Actions

The actions in this section require immediate to medium-term effort to reduce BWT 

barriers and improve the industry.

Create a Reliability 

Database

A BWT reliability database 

should provide a central location 

for logging all BWT failures. By 

tracking failures, this database 

could be used to identify issue 

patterns, minimize repeated

similar mistakes, and improve 

designs and design requirements. 

This database could be made 

public, limited access could be 

granted, or a combination of both 

could be implemented. It is 

expected that it will be used by 

owners, policymakers, installers, 

and manufacturers. Although 

each group will have its own 

database need, the goal is to 

improve BWTs' safety and quality.

Measure Loads and Yaw Rates

Turbulence and directional variability experienced by BWTs will be higher than for most 

SWTs designed for a rural environment, and this will probably result in higher loads on 

BWTs. Gusts will create large forces on the blades and increases in rotor speed. Stronger 

Figure 13. An example of a roof-mounted verticalaxis wind turbine in San Francisco. Photo from 

Danielle Murray, Department of the Environment, 

San Francisco
27

and sharper gusts will increase this effect. For HAWTs, direction variability will cause 

the turbine to yaw, and if the directional wind change occurs quickly, large forces will be 

exerted on a moving rotor, tail (if applicable), and yaw axis. It is critical to measure and 

therefore better understand the extreme loads and yaw rates that BWTs experience.

Produce Case Studies

Case studies of existing and future BWT installations, including lessons learned, should 

be produced to help develop best practices. They should show trends of installation cost 

and energy generation, thus helping to show BWT cost of energy and payback. 

Additionally, case studies should be useful in reviewing the applicability of existing 

building codes and ordinances to BWTs, therefore helping to develop future codes and 

ordinances.

Conduct Site Wind Resource Assessment through Measurement and

Validate Analytical Models of the Wind Resource

The most likely instrument for wind resource assessment in an urban environment is the 

sonic anemometer because it measures wind in three dimensions and is durable enough 

for field measurements, although SODAR4 and LIDAR5 also remain possible. An array 

of anemometers at a single location combined with high sampling rates can provide 

detailed measurements of the wind’s structure (turbulence, directional variability, and 

shear) at that location. Measuring three-dimensional winds should reduce the barriers of 

unknown wind speed, turbulence parameters, and directional variability in the built 

environment. Detailed knowledge of the wind resource is necessary for understanding the 

wind loads on a BWT and to avoid wakes, eddies, and separation zones. Measurements 

should also improve predictions of energy output and therefore answer questions about 

cost.

These measurements can be used to validate numerical models such as CFD models and 

stochastic models (e.g., TurbSim). With advancements in computer performance, many 

CFD packages are available today, and more important, these packages are widely used 

as a tool for wind flow analysis. There are few commercially available CFD packages,

such as UrbaWind and PlayBox, that are especially made for wind flow analysis in the 

urban environment (Beller 2010, Fahssis et al. 2010). In the past, the CFD wind flow 

analyses have mostly been used for dispersion studies, but now they are increasingly 

being used to assess the wind resource for wind turbine technology. However, the use of 

CFD in the area of wind resource assessment for the urban environment is emerging and 

needs validation through site measurement. Similarly, TurbSim is an NREL-developed 

computer code that outputs a turbulence inflow field to be used in aero-elastic models for 

wind turbine loads response calculation, such as ADAMS and FAST (also developed by 

NREL). TurbSim uses several spectral models, including two IEC models, the Riso 

smooth-terrain models, and several NREL site-specific models (Jonkman 2009). Among 

these models, the IEC models might be used for the built environment, but their

applicability must be ascertained through site measurement and validation. A new 

 4 Sonic Detection and Ranging; used for remote sensing of wind speed and direction.

5 Light Detection and Ranging; used for remote sensing of wind speed and direction.
28

stochastic model specifically for the built environment can also be integrated into 

TurbSim to expand its relevancy in the built environment.

High-resolution wind measurements can be the basis for rules for the built-environment 

wind resource, which should improve energy production and cost of energy estimates,

resulting in increased deployment of BWTs.

Build Demonstration Sites

BWT demonstration sites should have strong and dynamic winds. They should display 

proper installations and offer information about installation costs and energy production. 

Demonstration sites should also be used to increase public understanding and acceptance.

Besides displaying a BWT's operation, some additional uses of a demonstration site 

include displaying and explaining:

• Compliance with building codes and local ordinances

• Mechanical and electrical integration of BWTs and their towers to the building

• Ice build-up potential and actions to mitigate ice throw

• Resonance and vibration mitigation

• When and how to perform maintenance.

Conduct Turbine Testing

Rigorous field testing should find flaws quicker than display and demonstration sites. 

Additionally, procedures can be established to protect the workers from a turbine failure

at a test site; public safety would not be an issue.

Some tests may be conducted on the complete wind turbine system, allowing for 

comparison of different BWTs. Conversely, tests may be designed to answer specific 

questions about turbine components or operational details. Tests length may vary, but it is 

important to control as many variables as possible. A dedicated test site can provide that 

control, whereas demonstration sites cannot.

Tests at dedicated test sites may be developed to answer questions about:

• Loads

• Yaw rates

• Wear

• Fatigue

• Condition monitoring and frequency for scheduled maintenance 

• Resonance frequencies.
29 

Test sites should provide input on how standardized tests are conducted safely and 

efficiently. 

Summary of Both Near-Term and Medium-Term Actions 

The following table summarizes actions that require immediate and sustained effort and 

the BWT barriers addressed. 

Table 4. Near-Term and Medium-Term Actions and the Barriers Addressed 

Actions Safety Wind 

Resource 

Turbine

Technology 

Building 

Interactions 

NonTechnical 

Obstacles 

Create a 

reliability 

database 

-BWTs lack 

containment 

features for 

parts-shedding 

and ice-throw 

-BWTs 

experience 

shortened 

lifetime in highfatigue 

environment 

 Mechanical and 

electrical 

integration 

issues 

Hazards to 

personnel 

installing and 

servicing BWTs 

Conduct BWT 

test 

measurement 

campaign for 

loads and yaw 

rates 

 Poor 

understanding of 

loads 

measurements 

and yaw rates 

 

Produce case 

studies 

 -Mechanical and 

electrical 

integration 

issues 

-Poor 

understanding of 

resonance 

-Hazards to 

personnel 

installing and 

servicing BWTs 

-Unpredictable 

economics 

Conduct site 

wind resource 

assessment 

through 

measurement 

and validate 

analytical 

model(s) 

 Poor 

understanding 

of: 

-3-D wind 

resource 

-Wakes, eddies, 

and separation 

zones 

-Turbulence and 

directional 

variability 

Poor 

understanding of 

loads and yaw 

rates 

 -Unpredictable 

economics 
30 

Actions Safety Wind 

Resource 

Turbine

Technology 

Building 

Interactions 

NonTechnical 

Obstacles 

Build 

demonstration 

sites 

-BWTs lack 

containment 

features for 

parts-shedding 

and ice-throw 

-BWTs 

experience 

shortened 

lifetime in highfatigue 

environment 

Poor 

understanding 

of: 

-3-D wind 

resource 

-Wakes, eddies, 

and separation 

zones 

-Turbulence and 

directional 

variability 

Poor 

understanding of 

loads and yaw 

rates 

-Mechanical and 

electrical 

integration 

issues 

-Poor 

understanding of 

resonance 

-Hazards to 

personnel 

installing and 

servicing BWTs 

-Unpredictable 

economics 

Conduct turbine 

testing 

BWTs 

experience 

shortened 

lifetime in highfatigue 

environment 

 Poor 

understanding 

of: 

-Loads and yaw 

rates 

-BWT class 

design 

-Fatigue 

-Mechanical and 

electrical 

integration 

issues 

-Poor 

understanding of 

resonance 

Lack of outreach 

and education 

(noise and 

permitting 

issues) 
31

Strategy

This section outlines the strategy to implement the actions outlined in the previous 

section and identifies necessary expertise and resources. The strategy focuses on two 

main areas:

• Understand the built-environment wind resource

• Develop testing and design standards.

Understand the Built-Environment Wind Resource

Investigating the built-environment wind resource should be a closely coordinated effort 

between modeling and measurements. This process will probably be iterative as data 

from different anemometer locations are compared to model-based simulations and 

experiments of the flow around buildings in areas of high turbulence. The model-based 

methodologies should then be updated to better predict for diverse sites.

Validate and Develop Models

Actions to understand the built-environment’s wind resource should start with sites that 

have already been modeled, preferably in the wind tunnel and through CFD. Results 

would be compared to each other and with three-dimensional field measurements. The 

anemometers should be sited in multiple locations where BWTs would likely be installed. 

Wind modeling and validation efforts are already underway at the University of 

California-Davis (UC-Davis) and Massachusetts Institute of Technology (MIT). 

Researchers at UC-Davis have studied wind turbine siting by verifying wind tunnel data 

with site measurements (Strataridakis et al. 1998). A similar effort was carried out at MIT 

by comparing the site measurement data with a CFD model (WEPA 2010). 

Besides city-wide modeling, smaller areas have been modeled. For the Twelve West 

building in Portland, Oregon, Zimmer Gunsul Frasca Architects LLP used Cermak 

Peterka Petersen, Inc. for wind tunnel modeling of the firm’s roof-mounted Skystream 

turbines. 

Some university campuses, such as the Massachusetts Institute of Technology, George 

Mason University, and Penn State, have computer models of the wind resource on 

campus. 

Penn State has a wind tunnel model of part of its campus and a meteorological 

department, which may prove useful as well. The Massachusetts Institute of Technology

has several anemometer locations and sets of LIDAR measurement data. Working with 

these and other universities to validate and further develop models could greatly enhance 

the understanding of the built-environment wind resource.

Modeling software is increasingly available to end users; some products include 

UrbaWind (software developed by Meteodyne) and Wind Analytics (software developed 

by Wind Products, Inc.). Some modeling companies sell their software to other 
32

companies or individuals who can then model their built environment and predict the 

wind resource. Validation of commercial CFD software could be performed at any 

location with sonic anemometers. Ideally, these would be compared to areas with 

anemometers that have already been modeled, such as San Francisco, New York City, 

and various university campuses.

Besides commercial software, models are being developed at Lawrence Livermore 

National Laboratory. The laboratory has not used its CFD software for BWTs, but it has 

been used in built environments. It could be modified to be directly applicable to BWTs.

The IEA is working on Task 27, which has developed the Recommended Practices for 

Consumer Small Wind Labeling and is beginning to develop a recommended practice for 

the built environment. Research conducted under IEA Task 27 on the built environment 

will help to develop an IEC load case specifically for the built environment turbine and 

will be discussed in the fourth revision of the IEC standard. Some of the universities 

involved with the IEA work are analyzing three-dimensional built-environment data for 

turbulence intensity, wind speed, and wind direction characteristics. Comparing BWT 

models with the IEA team should be beneficial to all parties. The IEA also has access to a 

large set of wind speed 

measurements.

The IEC is modeling turbine 

loads. Because the load models are 

directly related to the wind 

resource model, collaboration with 

the IEC could benefit both. Plans 

have been discussed to develop a 

built-environment wind class and 

loads methodology requirements.

This effort will follow the IEA 

work, as well as work in countries 

to better understand inflow and 

turbine response.

NREL developed the software 

TurbSim, which simulates the 

turbulent inflow to a turbine. This 

is then used to simulate turbine response to wind loads and forces in other NRELdeveloped software. For reasons mentioned earlier, modifying TurbSim for the built

environment would be useful for BWT designers.

Conduct Measurements

Wind speed and direction measurements should be in three dimensions, so sonic 

anemometers will probably be utilized. Any group with sonic anemometers and threedimensional wind data in the built environment would be valuable. Coordination with 

built-environment modelers could identify optimum locations to deploy anemometers. 

Figure 14. Bergey turbine outfitted with two threedimensional sonic anemometers that measure 

wind speeds upwind and in the tail wake. Photo by 

Dave Corbus, NREL/PIX 13175
33

Ideally arrays of anemometers would be deployed to investigate the special variability of 

the wind resource.

Measurements must be taken for validating TurbSim. Validation of TurbSim is necessary 

before it is used to design BWTs. NREL should provide guidance for evaluating 

TurbSim’s capabilities and application for modeling the wind resource in the built 

environment.

Develop Testing and Design Standards

BWT testing falls into two categories:

• Initial and high-risk testing (should only be performed where risk to the public and 

property is eliminated)

• Tests on BWT installations (conducted after the high-risk testing to continue to gather 

and improve knowledge).

Test BWTs at an Established Test Site

BWTs should be tested at an established test site with staff members who have expertise 

in safely testing wind turbines that can be applied to testing BWTs.

Buildings at the test site could be inventoried for potential BWT installations and testing

and then modified for various types of BWT testing. Along with this effort, flow models 

could be employed to understand wind flow around the building with and without a BWT 

installed. BWTs can be moved so that optimum turbine locations can be determined. This 

should be of great value in providing guidance and best-practice advice.

In addition to testing BWTs, three-dimensional measurements of the wind resource can 

be made and used to validate models. Additionally, multiple anemometer measurements, 

upstream and downstream of buildings, would improve knowledge of wake, eddies, 

separation zones, turbulence parameters, and directional variability. These highresolution measurements would complement wind resource measurements listed in the 

previous section.

NREL and some universities have expertise in building science. This can be applied to 

investigate interactions between the turbines and buildings, especially vibrations. Highly 

instrumented BWTs and buildings, in addition to fully characterized winds, would 

provide much valuable data to the BWT industry.

A site dedicated to testing has several advantages over an urban location. A test site could 

have buildings that move (e.g., adjustable pitch roofs, modular building layouts, etc.) to 

measure flow blockage and turbulence, thus providing additional control for simulating 

turbulence in the built environment. The general difficulty of testing in an urban area, 

where anemometer placement is restricted and ambient noise level is high, makes it less 

attractive than a test site. Also, comparing results from different cities may be 

problematic, and results from a test site may be no more different from one urban site 

than urban sites are from each other. 
34

One such wind turbine test site is 

NREL’s NWTC. Independent 

testing of BWTs at the NWTC 

would provide information to 

consumers and guidance to other 

organizations as they develop 

their own BWT test sites.

The NWTC is also an ideal place 

for research and development. 

This is important in researching 

BWT safety concerns such as failsafe design, braking redundancy, 

ice throw and parts-shedding, and 

shortened turbine life due to high 

fatigue.

In addition to testing, NREL 

personnel are involved in standards development. Knowledge gained during testing at the 

NWTC could be incorporated into future standards. This may include developing a BWT 

classification or a modification of existing standards to accommodate BWT designs and 

to adequately test them.

Test BWTs in the Built Environment

Testing in the built environment requires a partnership with the owners and neighbors. 

These tests could have more restrictions than tests at a turbine test site, but testing BWTs 

that are installed in the built environment will be more realistic. Regardless of the 

location of BWT testing, safety is the main concern, especially when people and property 

are in close proximity. The following are examples of BWT partnerships that could be 

pursued.

• Several individuals and institutions have expertise that can be used for testing 

BWTs in the built environment. Sander Mertens of Ingreenious is an example of a 

leading expert on BWTs, and the knowledge, advice, and contacts of such 

individuals would be beneficial.

• The Warwick Wind Trials in the United Kingdom demonstrated SWTs in the built 

environment. This work increased the level of BWT knowledge, and it should be 

used for case studies and best-practice guidance.

• Several other groups and individuals have installed and tested BWTs; for 

example, the University of California at Davis; TUV-NEL’s6 Meyers Hill test site 

near Glasgow, Scotland; and Spain’s CIEMAT7 facility. All have valuable BWT 

experience that can provide input for best-practice guidelines. In addition, their 

 6 TUV-NEL is a for-profit multinational test and certification agency. 7 Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas

Figure 15. An example of a vertical-axis wind 

turbine in San Francisco. Photo from Danielle 

Murray, Department of the Environment, San 

Francisco, NREL/PIX 18447
35

experience can be the foundation for a reliability database to track BWT 

weaknesses and potential liabilities, as well as best practices.

• The Boston Museum of Science has established its Wind Turbine Lab with five 

types of turbines installed on the museum’s roof. This is an ideal institution for 

publishing findings of tests conducted on the roof (Museum of Science 2011).

Additionally, museum staff members have experience and are able to offer advice 

on best practices.

• Several federal facilities are interested in BWTs as part of the Federal Energy 

Management Program. BWTs installed on federal buildings as part of the program 

could be instrumented to provide power performance and other test data. Ideally, 

flow models and wind tunnel validation would be accomplished before BWT 

placement. The test anemometer and BWT location should be calibrated to 

provide a high 

correlation between 

measured wind speed 

and what is 

experienced by the 

BWT. These rigorous 

and methodical power 

performance tests are 

greatly needed.

• Southbank University 

has experience with 

roof-mounted turbines 

in central London (Day

and Dance 2011).

• The University of 

Salford in the U.K. has experience in acoustic testing SWTs in the built

environment (Moorhouse 2011) and should be engaged for many types of BWT 

acoustic tests.

In addition, BWT manufacturers can provide great value. For example, Cascade Swift 

and Quiet Revolution have knowledge of the wind resource in the built environment and 

how it affects their products. These and other manufacturers can provide input into best 

practices and provide partnerships for advanced turbine testing.

Strategy Summary

There are four strategic areas for accomplishing the actions designed to address barriers 

to BWT deployment. Each area is summarized below, including the most pressing actions 

that they address.

Figure 16. The Boston Museum of Science currently 

has five turbines in its Wind Turbine Lab. Photo from 

the Boston Museum of Science, NREL/PIX 18001
36

Note: Actions for “Understanding the Built-Environment Wind Resource” are highly 

iterative; models require data for validation, and measurements are generalized in 

models. Both provide a basis for BWT design and testing.

Table 5. Strategy Summary

Understand the Built-Environment Wind 

Resource

Develop Testing and Design Standards

Validate and 

Develop Models

Conduct 

Measurements

Test BWTs at 

Established Turbine 

Test Site (i.e., the 

NWTC)

Test BWTs in the Built 

Environment

Model comparison: 

CFD & tunnel (near

term)

Create or adapt data 

assessment protocols 

(medium term)

Make best-practice 

recommendations (medium

term)

Produce a consumer guide and 

fact sheets (near term)

Validate model at 

installations (medium

term)

Conduct 

measurements at 

demonstration sites: 

CFD & tunnel to 

installations (medium

term)

Conduct model validation at 

demonstration sites: CFD 

and wind tunnel to 

installations (medium term)

Produce a risk- and hazardfocused fact sheet (near term)

Make best-practice 

recommendations 

based on existing 

knowledge (medium

term)

Make 

recommendations to 

governing bodies and 

standards (medium

term)

Make recommendations to 

governing bodies and 

standards (medium term)

Analyze existing data for actual 

turbine performance (near term)

Validate turbine inflow 

models with 3-D 

measurements (both)

Conduct sonic 

anemometer 

measurements (both)

Conduct turbine research 

and development (medium

term)

Create a reliability database 

(both)

Build and instrument 

demonstration sites 

and validate flow 

models (both)

Conduct turbine testing 

(both)

Make best-practice 

recommendations (medium

term)

Conduct sonic anemometer 

measurements and validate 

TurbSim (both)

Create or adapt data 

assessment tools (medium

term)

Instrument existing BWTs 

(medium term)

Build demonstration sites (both)

Conduct turbine testing (both)

Produce case studies (both)
37

References

American Wind Energy Association. (2011). 2010 U.S. Small Wind Turbine Market 

Report. Washington, DC.

Beller, C. (2010). "Development of a simulation tool to predict urban wind potential."

Sustainability in Energy and Buildings, SIST 7, pp. 111-120.

Beller, C. (2011). Urban Wind Energy. Ph.D. thesis. Riso National Laboratory for 

Sustainable ENERGY, Technical University of Denmark, Denmark.

Carpman, N. (2010). “Turbulence intensity binned with mean wind speed.” Uppsala 

University. Uppsala, Sweden.

Chiras, D. (2009). Power from the Wind. New Society Publishers, Canada.

Day, T.; Dance, S. (2011). Ashend House Wind Turbine Trial. Centre for Efficient and 

Renewable Energy in Buildings. {http://static.cereb.org.uk/documents/10-47-ashendenturbine-trial.pdf}.

Encraft. (2009). Warwick Wind Trials Project: Final Report. UK: Encraft. 

Fahssis, K.; Dupont, G.; Leyronnas, P. (2010). "Urbawind, a Computational Fluid 

Dynamics tool to predict wind resource in urban area." International Conference of 

Applied Energy, April 2010, Singapore.

Hurley, P.J. (1997). "An evaluation of several turbulence schemes for the prediction of 

mean and turbulent fields in complex terrain." Boundary-Layer Meteorology (83), pp. 43-

73.

International Electrotechnical Commission. (2006). Wind Turbines – Part 2: Design 

Requirements for Small Wind Turbines, second edition. International Electro Technical 

Commission: Geneva, Switzerland.

James, P.A.B.; et al. (2010). "Seawater Implications of the UK Field Trial of Building 

Mounted Horizontal Axis Micro-Wind Turbines." Energy Policy (38); pp. 6130-6144.

Jonkman, B.J. (2009). TurbSim User’s Guide: Version 1.5. NREL/TP-500-46198. 

Golden, CO: National Renewable Energy Laboratory. 

Kalmikov, A.; et al. (2010). "Wind Power Resource Assessment in Complex Urban 

Environments." AWEA Windpower Conference, May 23-26, 2010, Dallas, Texas.

Kooiman, S.J.; Tullis, S.W. (2010). “Response of a Vertical Axis Wind Turbine to Time 

Varying Wind Conditions Found within the Urban Environment.” Wind Engineering

(34).
38

Mertens, S. (2006). Wind Energy in the Built Environment – Concentrator Effects of 

Buildings. Brentwood, UK: Multi-Science.

Moorhouse, A. (2011). Structure-borne sound from wind turbines. University of Salford, 

Manchester. {http://www.acoustics.salford.ac.uk/profiles/moorhouse/}.

Museum of Science; Boreal Renewable Energy Development; Gross, R.C.; Phelan, P.; 

AeroVironment. (2006). Feasibility Study for Wind Turbine Installations at Museum of 

Science Boston, MA. 

{http://www.mos.org/media/docs/MOS_Boreal_Renewable_Wind_Study.pdf}.

Museum of Science. (2011). Wind Turbine Lab. The Museum of Science, Boston. 

{http://www.mos.org/WindTurbineLab}.

Strataridakis, C.J.; White, B.R.; Greis, A. (1998). Turbulence Measurements for Wind

Turbine Siting on a Complex Terrain. American Institute of Aeronautics and 

Astronautics.

Viti, V.; DesAutels, C.; Schulman, L. (2010). “Computational Model of the Flowfield 

around the Museum of Science, Boston.” Conference on Small-Scale Urban Wind 

Turbines: Lessons from the Museum of Science Wind Turbine Lab. Boston, MA. 19 May 

2010. {http://www.mos.org/media/docs/Conference_Materials_2010.zip}.

Wineur. (2005). Urban wind turbines: Technology review. Work performed by Wineur. 

Europe: Intelligent Energy Europe.

Wind Energy Projects in Action (WEPA). (2010). Project Full Breeze, Massachusetts 

Institute of Technology.

Yersel, M.; Goble, R. (1986). "Roughness Effects on Urban Turbulence Parameters." 

Boundary Layer Meteorology (37), pp. 271-284.
39

Bibliography

Blanch, M.J. (2002). “Wind Energy Technologies for use in the Built Environment.” 

Wind Engineering (26); pp. 125-143.

Cace, J; et al. (2007). Urban wind turbines: Guidelines for small wind turbines in the 

built environment. Work performed by Wineur. Europe: Intelligent Energy Europe.

Carbon Trust. (2008). Small-scale wind energy: Policy insights and practical guidance. 

UK: Carbon Trust.

Chapman, N. (2011). Turbulence Intensity in Complex Environments and its Influence on 

Small Wind Turbines. Uppsala, Sweden: Uppsala University.

Dales, C.; Tangler, J. (August 1992). Aerodynamic Performance Prediction and the 

Effect of Turbulence on Wind Turbine Power. Work performed as summer internship at 

National Wind Technology Center. Golden, CO: National Renewable Energy Laboratory.

Dayan, E. (March-April 2006). “Wind Energy in Buildings.” REFocus; pp. 33-38.

Faure, X.; Sabre, M.; Palier, P. Small Wind Turbines in Urban Environment: 

Qualification Steps. France: Centre Scientifique et Technique du Bâtiment.

International Energy Agency. (forthcoming). Recommended Practices for Consumer 

Small Wind Labeling in the Built Environment. 

Lissaman, P.B.S. (2005). "Preliminary Analysis of Windwall Vane Configuration: Final 

Report." Prepared for NREL. Santa Fe, NM: Da Vinci Ventures, 16 pp.

Lubitz, W.D.; Hakimi, R. (January 2008). "Effects of Roof Slope on a Building-Mounted 

Wind Turbine." Prepared for the 46th AIAA Aerospace Sciences Meeting and Exhibit, 

January 2008. Reno NV: AIAA, 10 pp.

Mithraratne, N. (2009). “Roof-top Wind Turbines for Microgeneration in Urban Houses 

in New Zealand.” Energy and Buildings (41); pp. 1013-1018.

Moriarty, P.; Butterfield, S. (2004). Analysis of Wind Turbine Installation on EPA 

Building. Golden, CO: National Renewable Energy Laboratory.

Oppenheim, D. “Outside the square: Integrating wind into urban environments.” 

REFocus (May-June 2004); pp. 32-35.

RenewableUK. (2010). Small Wind Systems: UK market report. RenewableUK.

Robichaud, R. (2005). Building-Mounted Wind Turbine Study for EPA Building in 

Denver, Colorado. Golden, CO: National Renewable Energy Laboratory.
40

Ruran, S.; Peacock, A.D.; Newborough, M. (May 2007). “Micro and Small Wind Turbine 

Applications in the Built Environment.” ISESCO Science and Technology Vision (3:3); 

pp. 106-110.

Small Wind Systems: UK Market Report. London, UK: Renewable UK, April 2010.

Stankovi, S.; Campbell, N.; Harries, A. (2009). Urban Wind Energy. UK: Earthscan.

Walker, S. L. (2011). "Building Mounted Wind Turbines and Their Suitability for the 

Urban Scale –A Review of Methods of Estimating Urban Wind Resource.” Energy and 

Buildings (43); pp. 1852-1862.

Westerholm, G. “The Wind Energy Frontier: Urban Retail Markets.” REFocus (JulyAugust 2004); pp. 42-45.

Wiser, R.; Bolinger, M. (2011). 2010 Wind Technologies Market Report. U.S. 

Department of Energy.
41

Appendix I: Types of Built-Environment Wind Turbines

There are four types of BWT installations: 

• Building-mounted on the side of a building (Figure 17)

• Building-mounted on the roof (Figure 18) 

• Building-integrated wind turbine (Figure 19)

• Ground-mounted in a built environment (Figure 20).

The majority of BWTs are roofmounted SWTs (Figure 18) with a 

rated power of 10 kW or less (Wineur, 

2005). For a conventional HAWT, this 

means a rotor diameter of less than 

approximately 7 meters (23 feet). 

Besides HAWTs, several VAWTs 

have gained public interest, and some 

manufacturers market their VAWTs 

for urban and suburban installations.

The building-integrated wind turbine

(Figure 19) employs an 

unconventional design to allow for 

Figure 19. Building-integrated wind turbine at 

the Bahrain World Trade Center. Photo from 

iStock/6924031

Figure 17. Building-mounted 

(side) wind turbine at the 

Boston Museum of Science. 

Photo by Joe Smith, NREL/PIX 

18463

Figure 18. Building-mounted (roof) wind 

turbine at the Boston Museum of 

Science. Photo by Joe Smith, 

NREL/PIX18460
42

wind turbine placement within the structure. Building-integrated wind turbines are 

architecturally noteworthy and require extensive engineering expertise and added cost. 

With most building-integrated wind turbines, issues of aerodynamics, vibration, and 

building interaction have been studied, and potential effects have been mitigated. 

Building-integrated wind turbines are custom designed, and their installations typically 

do not represent Universal Building Code practices that influence structural resonance 

frequencies. Therefore, this roadmap does not address them.

Ground-mounted turbines (Figure 

20) require consideration for 

installing and raising the turbine as 

well as decreased production due to 

the poor wind resource found in the 

urban environment.

Figure 20. This Aeroturbine at the Randall 

Museum in San Francisco, California, is an 

example of a vertical-axis wind turbine in the 

built environment. Photo from Danielle Murray, 

Department of the Environment, San Francisco, 

NREL/PIX 18443
43

Appendix II: Rooftop and Built-Environment Wind Turbine 

Workshop Attendees

The following people attended the Rooftop and Built-Environment Wind Turbine 

Workshop on August 11-12, 2010, at the National Wind Technology Center in Boulder, 

Colorado:

John Breshears, ZGF Architects LLP

Craig M. Briscoe, ZGF Architects LLP

Brad Cochran, CPP Inc.

John Dunlop, American Wind Energy Association

Guillaume Dupont, Meteodyn America Inc.

Graham Eastwick, Encraft

Fort Felker, National Renewable Energy Laboratory

Trudy Forsyth, National Renewable Energy Laboratory

Sue Ellen Haupt, Penn State

Feitau Kung, National Renewable Energy Laboratory

Michael Lenartowicz, Intertek

Dave Lubitz, University of Guelph

Becki Meadows, National Renewable Energy Laboratory

Sander Mertens, Ingreenious BV

Jeff Mirocha, Lawrence Livermore National Laboratory

Pat Moriarty, National Renewable Energy Laboratory

Guy R. Nelson, Western Area Power Administration contractor

Rich Peek, Cascade-Swift

Karin Sinclair, National Renewable Energy Laboratory

Joe Smith, National Renewable Energy Laboratory

Susan Stewart, Penn State

Russell Tencer, Wind Products Inc

Marian Tomusiak, Museum of Science, Boston

Case van Dam, University of California - Davis

Dawn White, Accio Energy 
44

Appendix III: Workshop Agenda

Wednesday, August 11, 2010

9 – 9:30

Trudy Forsyth

Welcome and introductions

9:30 – 11:30

Sander Mertens, Ingreenious BV

“Urban Wind Turbines: Identifying a New Branch”

Data and observations presentations moderator: Case van Dam

12:30 – 1:30

Graham Eastwick, Encraft

“Lesson Learned during the Warwick Wind Trials: A Study of 23 Roof- and BuildingMounted Turbines” 

1:30 – 2:15

Craig Briscoe and John Breshears, ZGF Architects

“A Portland Case Study: Predicted and Measured Results from a Small Wind Installation 

on an Urban Highrise” 

2:15 – 2:45

Marian Tomusiak, Museum of Science, Boston

“Lessons from the Boston Museum of Science Wind Turbine Lab”

3 – 3:35

Case van Dam, University of California – Davis

“Research on Wind Power in the Built Environment”

3:35 – 4:10

David Lubitz, University of Guelph

“Measurement and Assessment of the Wind Environment near Low-Rise Buildings”

4:10 – 4:55

Patrick Moriarty, National Renewable Energy Laboratory

“Aeroacoustics and Vibration of Wind Turbine Systems”

4:55 – 5:40

Rich Peek, Cascade – Swift

“Manufacturer’s Perspective” 

5:40 – 5:50

Remarks 
45

Thursday, August 12, 2010

8:30 – 8:45

Welcome back, recap

Modeling presentations moderator: Jeff Mirocha

8:45 – 10

Brad Cochran, CPP Wind

“Modeling Airflow over Buildings Using Wind Tunnel Data and CFD Models”

10 – 10:45

Guillaume Dupont, Meteodyne

“Use of a CFD Model for Wind Assessment in the Built Environment: Introduction to the 

UrbaWind Model”

11 – 11:30

Russell Tencer, Wind Products

“Wind Analytics: Modeling Software for Small Wind Turbine Siting”

11:30 – 12

Susan Stewart, Penn State

“Mapping Wind Power Resources around Buildings”

12 – 12:30

Jeff Mirocha, Lawrence Livermore National Laboratory

“Development of Multi-Scale Mesoscale/CFD Modeling Approaches”

Parallel Focus Group Discussions

1:30 – 2:30

Moderator: Jeff Mirocha 

“Strategies for Model Improvement”

2:30 – 3:30

Moderator: Case van Dam 

“Strategies for Data Improvement”

Whole Group Discussions

3:45 – 4:45

Moderator: Trudy Forsyth 

“Strategic Outline of Research Priorities”

4:45 – 5

Closing remarks
46 

Appendix IV: Roadmap Reviewers 

Bret Barker, U.S. Department of Energy 

Michael Derby, U.S. Department of Energy 

Guillaume Dupont, Meteodyn America Inc. 

Katherine Dykes, Massachusetts Institute of Technology 

Graham Eastwick, Encraft 

Fort Felker, National Renewable Energy Laboratory 

Michael Jason Fields, National Renewable Energy Laboratory 

Sue Ellen Haupt, Penn State 

Feitau Kung, National Renewable Energy Laboratory 

Michael Lenartowicz, Intertek 

Dave Lubitz, University of Guelph 

Sander Mertens, Ingreenious BV 

Jeff Mirocha, Lawrence Livermore National Laboratory 

Danielle Murray, City of San Francisco 

Jason Roadman, National Renewable Energy Laboratory 

Susan Stewart, Penn State 

Russell Tencer, Wind Products Inc. 

Marian Tomusiak, Museum of Science, Boston 

Case van Dam, University of California - Davis 

Jeroen van Dam, National Renewable Energy Laboratory 
47 

Appendix V: Contacts Summary 

Table 6. Summary of Contacts, Affiliation, Expertise, and Interests 

Point of 

Contact 

Affiliation Email Expertise Interests 

Case van 

Dam 

University of 

California - 

Davis 

cpvandam@ 

ucdavis.edu 

Rooftop BWT; 

atmospheric 

boundary layer 

wind tunnel used 

for urban wind 

resource 

measurements 

Continuing BWT work in wind 

tunnel 

Danielle 

Murray 

City & County of 

San Francisco 

danielle.murray@sf

gov.org 

San Francisco 

urban wind and 

energy planning, 

community 

outreach 

Refining model of San 

Francisco urban wind 

resource; developing a small 

wind turbine test, certification, 

and demonstration facility; 

looking for partners 

Dave Lubitz University of 

Guelph 

wlubitz@ 

uoguelph.ca 

Sonic anemometry 

to assess the flow 

field above a 

peaked-roof 

building; 

anemometer arrays 

to assess obstacle 

wakes; 

atmospheric 

boundary layer 

wind tunnel used to 

assess BWT 

placement 

Installing rooftop-mounted 

BWT in 2011; developing 

wind resource assessment 

tools and characterizations; 

developing design guidelines 

and testing procedures 

Graham 

Eastwick 

Encraft Graham.Eastwick@

encraft.co.uk 

Warwick Wind 

trials; noise and 

vibration studies; 

site selection, 

permitting, and 

project 

management 

Advising clients on the best 

renewable energy system 

options for their projects; 

participating in studies to 

increase practical knowledge 

Guillaume 

Dupont 

Meteodyn 

America Inc. 

guillaume.dupont@

meteodyn.com 

CFD development 

(UrbaWind 

software) 

Generating 1-m resolution 

wind maps of major U.S. 

cities for which wind data are 

available (San Francisco, 

New York City), also including 

TKE comparisons 
48 

Point of 

Contact 

Affiliation Email Expertise Interests 

Katherine 

Dykes, Alex 

Kalkimov, Cy 

Chan 

Massachusetts 

Institute of 

Technology 

dykesk@mit.edu 

kalex@mit.edu 

cychan@ 

csail.mit.edu 

Assess MIT 

campus wind 

resource for 

suitability for smallscale wind turbines 

within a built 

environment 

Improving CFD results with 

LiDAR data and GIS models 

of MIT campus 

Jeff Mirocha Lawrence 

Livermore 

National 

Laboratory 

jmirocha@llnl.gov Urban CFD 

modeling for 

atmospheric 

dispersion 

purposes; 

simulation of 

boundary layer flow 

over complex 

terrain 

CFD modeling of urban 

landscapes for turbine siting; 

modeling of turbulent stress 

loading; incorporating 

mesoscale inflow boundary 

conditions for CFD 

simulations 

Marian 

Tomusiak 

Boston Museum 

of Science 

mtomusiak@ 

mos.org 

Rooftop Wind 

Turbine Lab data 

analysis 

Communicating data and 

experiences with rooftop wind 

turbines 

Michael 

Lenartowicz 

Intertek michael.lenartowicz

@intertek.com 

 Measuring and simulating 

load 

Russell 

Tencer 

Wind Products 

Inc. 

rtencer@ 

wind-products.com 

BWT manufacture; 

developer of wind 

energy modeling 

software designed 

for the built 

environment 

Establishing guidelines to 

protect industry from bad 

turbine and poor installations; 

working with partners to 

educate potential consumers 

Sander 

Mertens 

Ingreenious BV sandermertens@ 

ingreenious.com 

Fluid dynamics; 

wind resource 

assessments; 

measurement 

analysis; 

Measurement 

Correlate Predict 

procedures; CFD 

simulations of the 

built environment; 

wind tunnel 

measurements of 

BWTs; open-air 

BWT tests 

All theoretical and empirical 

BWT issues; certifying BWTs 

Sue Ellen 

Haupt 

Penn State seh19@psu.edu Wind power density 

mapping in built 

environments; 

wake impact 

Defining the flow interaction 

with the BWT; leveraging air 

pollution knowledge to form 

CFD models of various scales 
49 

Point of 

Contact 

Affiliation Email Expertise Interests 

Susan 

Stewart 

Penn State sstewart@psu.edu Detached Eddy 

Simulation CFD 

analysis of cubeshaped building 

and a pitched-roof 

building; building 

VAWTs and testing 

in wind tunnel and 

dynamometer; oncampus HAWT with 

2-Hz data; BWT 

design concept 

review 

TKE and yaw variability in the 

built environment and their 

impact on design and 

standards; advancing 

knowledge to the point at 

which IEC recommendations 

can be made; refining and 

improving Detached Eddy 

Simulation model